Pharmacology 4th semester

1. PHARMACOKINETICS
SATYAJIT GHOSH
B. PHARM 4TH
SEMESTER

2.  Introduction:
- Pharmacokinetics is a branch of pharmacology dedicated to determine the fate of substances administrated to a living organism. The substance includes
any chemical xenobiotics such as pharmaceutical drugs, pesticides, food additives, cosmetics, etc.
- It is the process of the uptake of drugs by the body, the biotransformation they undergo, the distribution of drugs and their metabolites in the tissues, and
the elimination of drugs and from the body over a period of time.
- ADME, the absorption, distribution, metabolism, and elimination of drugs, are the processes of pharmacokinetics.
 Passage of drugs across membrane barriers:
 The plasma membrane:
- The plasma membrane consists of a bilayer of amphipathic lipids with their hydrocarbon chains oriented inward to the centre of the bilayer to form a
continuous hydrophobic phase, with their hydrophilic heads oriented outward.
- Individual lipid molecules in the bilayer vary according to the particular membrane and can move laterally and organize themselves into microdomains
(e.g., regions with sphingolipids and cholesterol, forming lipid rafts), endowing the membrane with fluidity, flexibility, organization, high electrical
resistance, and relative impermeability to highly polar molecules.
- Membrane proteins embedded in the bilayer serve as structural anchors, receptors, ion channels, or transporters to transduce electrical or chemical
signalling pathways and provide selective targets for drug actions.
- Far from being a sea of lipids with proteins floating randomly about, membranes are ordered and compartmented, with structural scaffolding elements
linking to the cell interior. Membrane proteins may be associated with caveolin and sequestered within caveolae, be excluded from caveolae, or be
organized in signalling domains rich in cholesterol and sphingolipid not containing caveolin or other scaffolding proteins.

3.  Drugs transport or types of absorption:
- Drug transport through cell membrane by following ways:
A- Passive Diffusion:
- In passive transport, the drug molecule usually penetrates by diffusion along a concentration gradient by virtue of its solubility in the lipid bilayer.
Such transfer is directly proportional to the magnitude of the concentration gradient across the membrane, to the lipid: water partition coefficient of
the drug, and to the membrane surface area exposed to the drug.
- At steady state, the concentration of the unbound drug is the same on both sides of the membrane if the drug is a nonelectrolyte. For ionic
compounds, the steady-state concentrations depend on the electrochemical gradient for the ion and on differences in pH across the membrane, which
Drug Transport
Passive Diffusion Carrier mediated transport
Facilitated transport Active transport
Primary
Secondary
Vesicular transport
Endocytosis
Exocytosis

4. will influence the state of ionization of the molecule disparately on either side of the membrane and can effectively trap ionized drug on one side of
the membrane.
Influence of pH on Ionizable Drugs:
- Many drugs are weak acids or bases that are present in solution as both the lipid-soluble, diffusible non-ionized form and the ionized species that
is relatively lipid insoluble and poorly diffusible across a membrane.
- The transmembrane distribution of a weak electrolyte is influenced by its pKa and the pH gradient across the membrane. The pKa is the pH at
which half the drug is in its ionized form.
- The ratio of nonionized to ionized drug at any pH may be calculated from the Henderson-Hasselbalch equation:
𝐥𝐨𝐠
[𝐩𝐫𝐨𝐭𝐨𝐧𝐚𝐭𝐞𝐝 𝐟𝐨𝐫𝐦]
[𝐮𝐧𝐩𝐫𝐨𝐭𝐨𝐧𝐚𝐭𝐞𝐝 𝐟𝐨𝐫𝐦]
= 𝐩𝐊𝐚 − 𝐩𝐇
- This equation relates the pH of the medium around the drug and the drug’s acid dissociation constant (pKa) to the ratio of the protonated (HA or
BH+
) and unprotonated (A−
or B) forms, where
𝐇𝐀 ↔ 𝐇 + 𝐀 , 𝐰𝐡𝐞𝐫𝐞 𝐤𝐚 =
[𝐇 ] [𝐀 ]
[𝐇𝐀]
describes the dissociation of an acid, and
𝐁𝐇 ↔ 𝐁 + 𝐇 , 𝐰𝐡𝐞𝐫𝐞 𝐊𝐚 =
[𝐁] [𝐇 ]
[𝐁𝐇 ]
describes the dissociation of the protonated form of a base.
- At steady state, an acidic drug will accumulate on the more basic side of the membrane and a basic drug on the more acidic side. This
phenomenon, known as Ion Trapping, is an important process in drug distribution with potential therapeutic benefit.
- In the kidney tubules, urine pH can vary over a wide range, from 4.5 to 8. As urine pH drops (acidic), weak acids (A–
) and weak bases (B) will
exist to a greater extent in their protonated forms (HA and BH+
).

5. - Thus, alkaline urine favours excretion of weak acids; acid urine favours excretion of weak bases. Elevation of urine pH (by giving sodium
bicarbonate) will promote urinary excretion of weak acids such as aspirin (pKa ~ 3.5) and urate (pKa ~ 5.8).
B- Carrier-Mediated Membrane Transport:
- Proteins in the plasma membrane mediate transmembrane movements of many physiological solutes; these proteins also mediate transmembrane
movements of drugs and can be targets of drug action.
- Mediated transport is broadly characterized as facilitated diffusion or active transport.
Facilitated Diffusion:
- Facilitated diffusion is a carrier-mediated transport process in which the driving force is simply the electrochemical gradient of the transported
solute; thus, these carriers can facilitate solute movement either in or out of cells, depending on the direction of the electrochemical gradient.
- The carrier protein may be highly selective for a specific conformational structure of an endogenous solute or a drug whose rate of transport by
passive diffusion through the membrane would otherwise be quite slow.
- For instance, the organic cation transporter OCT1 (SLC22A1) facilitates the movement of a physiologic solute, thiamine, and also of drugs,
including Metformin, which is used in treating type 2 diabetes.
Active Transport:
- Active transport is characterized by a direct requirement for energy, capacity to move solute against an electrochemical gradient, saturability,
selectivity, and competitive inhibition by cotransported compounds.
- Na+
, K+
-ATPase is an important example of an active transport mechanism that is also a therapeutic target of Digoxin in the treatment of heart
failure.

6. - A group of primary active transporters, the ABC family, hydrolyse ATP to export substrates across membranes. For example, the P-glycoprotein,
also called ABCB1 and MDR1, exports bulky neutral or cationic compounds from cells; its physiologic substrates include steroid hormones such as
testosterone and progesterone.
- P-glycoprotein in the enterocyte limits the absorption of some orally administered drugs by exporting compounds into the lumen of the GI tract
subsequent to their absorption. ABC transporters perform a similar function in the cells of the BBB, effectively reducing net accumulation of
some compounds in the brain. By the same mechanism, P-glycoprotein also can confer resistance to some cancer chemotherapeutic agents.
- Members of the SLC superfamily can mediate secondary active transport using the electrochemical energy stored in a gradient (usually Na+
) to
translocate both biological solutes and drugs across membranes. The Na+
–Ca2+
exchange protein (SLC8) uses the energy stored in the Na+
gradient established by Na+
, K+
-ATPase to export cytosolic Ca2+
and maintain it at a low basal level.
- SLC8 is thus an antiporter, using the inward flow of Na+
to drive an outward flow of Ca2+
; SLC8 also helps to mediate the positive inotropic
effects of digoxin and other cardiac glycosides that inhibit the activity of Na+
, K+
-ATPase and thereby reduce the driving force for the extrusion of
Ca2+
from the ventricular cardiac myocyte.
- Other SLC cotransporters are symporters, in which driving force ion and solute move in the same direction. The CNT1(SLC28A1), driven by the
Na+
gradient, moves pyrimidine nucleosides and the cancer chemotherapeutic agent gemcitabine and cytarabine into cells.
- DAT, NET, and SERT, transporters for the neurotransmitters dopamine, norepinephrine, and serotonin, respectively, are secondary active
transporters that also rely on the energy stored in the transmembrane Na+
gradient, symporters that coordinate movement of Na+
and
neurotransmitter in the same direction (into the neuron); they are also the targets of CNS-active agents used in therapy of depression.
Examples:
Symporters: Na+
- glucose symporters, Na+
- choline symporters, Na+
/K+
/2Cl-
- symporters in loop of henle
Antiporters: Na+
- H+
antiporters, Na+
/Ca2+
antiporters

7. C- Vesicular transport:
- A binding protein located on membrane complexes with the substances and initiate vesicle formation. The vesicle then detaches from the membrane
and may remain stored within the cell or may release the substances in the cytoplasm or may move to the opposite membrane fuse with it to release
the substances across the cell.
- The taking in of matter by a living cell by invagination of its membrane to form a vacuole known as endocytosis. Endocytosis includes pinocytosis
(cell drinking) and phagocytosis (cell eating).

8. ABSORPTION, BIOAVAILABILITY, AND ROUT OF ADMINISTRATION
 Absorption & bioavailability:
- Absorption is the movement of a drug from its site of administration into the central compartment. For solid dosage forms, absorption first requires
dissolution of the tablet or capsule, thus liberating the drug. Except in cases of malabsorption syndromes, the clinician is concerned primarily with
bioavailability rather than absorption.
- Bioavailability describes the fractional extent to which an administered dose of drug reaches its site of action or a biological fluid (usually the systemic
circulation) from which the drug has access to its site of action.
- A drug given orally must be absorbed first from the GI tract, but net absorption may be limited by the characteristics of the dosage form, by the drug’s
physicochemical properties, by metabolic attack in the intestine, and by transport across the intestinal epithelium and into the portal circulation.
- The absorbed drug then passes through the liver, where metabolism and biliary excretion may occur before the drug enters the systemic circulation.
Accordingly, less than all of the administered dose may reach the systemic circulation and be distributed to the drug’s sites of action.
- If the metabolic or excretory capacity of the liver and the intestine for the drug is large, bioavailability will be reduced substantially (first-pass effect). This
decrease in availability is a function of the anatomical site from which absorption takes place; for instance, intravenous administration generally permits all
of the drug to enter the systemic circulation.
- Other anatomical, physiological, and pathological factors can influence bioavailability, and the choice of the route of drug administration must be based on
an understanding of these conditions.
- We can define bioavailability F as:
𝐅 =
𝐐𝐮𝐚𝐧𝐭𝐢𝐭𝐲 𝐨𝐟 𝐝𝐫𝐮𝐠 𝐫𝐞𝐚𝐜𝐡𝐢𝐧𝐠 𝐬𝐲𝐬𝐭𝐞𝐦𝐢𝐜 𝐜𝐢𝐫𝐜𝐮𝐥𝐚𝐭𝐢𝐨𝐧
𝐐𝐮𝐚𝐧𝐭𝐢𝐭𝐲 𝐨𝐟 𝐝𝐫𝐮𝐠 𝐚𝐝𝐦𝐢𝐧𝐢𝐬𝐭𝐫𝐚𝐭𝐞𝐝
, 𝐰𝐡𝐞𝐫𝐞 𝟎 < 𝐅 ≤ 𝟏

9. Ineffective
Concentration
Plasma
drug
concentration
Time
A
B
C
Therapeutic
Concentration
- Factors modifying bioavailability apply as well to prodrugs that are activated by the liver, in which case availability results from metabolism that produces
the form of the active drug.
- The plasma concentration – time curve can be obtained by plotting plasma concentration of the drug against time. Bioavailability of drug can be calculated
by comparing the area under the curve (AUC) after oral administration of a single dose with AUC obtained when the same dose is given by intravenous
route.
𝐁𝐢𝐨𝐚𝐯𝐚𝐢𝐥𝐚𝐛𝐢𝐥𝐢𝐭𝐲 =
𝐀𝐔𝐂 (𝐨𝐫𝐚𝐥)
𝐀𝐔𝐂 (𝐢. 𝐯)
× 𝟏𝟎𝟎
- The bioavailability of drug from its dosage form depends on following factors:
a- Administration route, Parenteral > oral > rectal > topical
b- Patient related factor
c- Physiochemical factors
d- Characteristics of properties of dosage form

10.  Routes of Administration:
- Some characteristics of the major routes employed for systemic drug effect are compared in following table:
Route & Bioavailability (F) Absorption patterns Special utility Limitation and precautions
Intravenous, F = 1
Absorption circumvented Valuable for emergency use Increased risk of adverse effect
Potentially immediate effect Permits titration of dosage Must inject solution slowly as a rule
Suitable for large volumes and for
irritating substances or complex
mixture, when diluted
Usually required for high molecular
weight protein and peptide drugs
Not suitable for oily solutions or
poorly soluble substances
Subcutaneous, 0.75<F<1
Prompt from aqueous solution Suitable for some poorly soluble
suspensions and for instillation of
slow released implants
Not suitable for large volume
Slow and sustained from repository
preparation
Possible pain or necrosis from
irritating substances
Intramuscular, 0.75<F<1
Prompt from aqueous solution Suitable for moderate volumes, oily
vehicles and some irritating
substances
Precluded during anticoagulant
therapy
Slow and sustained from repository
preparation
Appropriate for self-administration
(e.g., Insulin)
May interfere with interpretation of
certain diagnostic tests (e.g.,
creatine kinase)
Oral ingestion, 0.05<F<1
Variable, depends on many factors Most convenient and economical Requires patient compliance
Bioavailability potentially erratic
and incomplete

11.  Enteral route of Administration:
- Enteral route of drug administration is the drug administration via the human GI tracts. Enteral administration involves the oesophagus, stomach, and
intestines and rectal.
- Methods of administration includes oral, sublingual, and rectal.
 Oral route of administration:
- Oral ingestion is the most common method of drug administration. It is the safest, most convenient, and most economical.
- Its disadvantages include limited absorption of some drugs because of their physical characteristics (e.g., low water solubility or poor membrane
permeability), emesis as a result of irritation to the GI mucosa, destruction of some drugs by digestive enzymes or low gastric pH, irregularities in
absorption or propulsion in the presence of food or other drugs, and the need for cooperation on the part of the patient.
- In addition, drugs in the GI tract maybe metabolized by the enzymes of the intestinal microbiome, mucosa, or liver before they gain access to the
general circulation.
- Absorption from the GI tract is governed by factors such as surface area for absorption; blood flow to the site of absorption; the physical state of the
drug (solution, suspension, or solid dosage form); its aqueous solubility; and the drug’s concentration at the site of absorption.
- For drugs given in solid form, the rate of dissolution may limit their absorption. Because most drug absorption from the GI tract occurs by passive
diffusion, absorption is favoured when the drug is in the nonionized, more lipophilic form.
- Based on the pH-partition concept, one would predict that drugs that are weak acids would be better absorbed from the stomach (pH 1–2) than
from the upper intestine (pH 3–6), and vice versa for weak bases.
- However, the surface area of the stomach is relatively small, and a mucus layer covers the gastric epithelium. By contrast, the villi of the upper
intestine provide an extremely large surface area. Accordingly, the rate of absorption of a drug from the intestine will be greater than that from the
stomach even if the drug is predominantly ionized in the intestine and largely nonionized in the stomach.

12. - Thus, any factor that accelerates gastric emptying will generally increase the rate of drug absorption, whereas any factor that delays gastric
emptying will have the opposite effect. The gastric emptying rate is influenced by numerous factors, including the caloric content of food; volume,
osmolality, temperature, and pH of ingested fluid; diurnal and interindividual variation; metabolic state (rest or exercise); and the ambient
temperature.
- Gastric emptying is influenced in women by the effects of estrogen (i.e., compared to men, emptying is slower for premenopausal women and those
taking estrogen replacement therapy).
- Drugs that are destroyed by gastric secretions and low pH or that cause gastric irritation sometimes are administered in dosage forms with an
enteric coating that prevents dissolution in the acidic gastric contents.
A- Merits:
i. Safe, convenient, painless method.
ii. Economical, sterilisation is not required.
iii. Any assistance is not required during drug administration.
iv. Less chances of acute drug reaction.
B- Demerits:
i. Complete drug is not absorbed.
ii. First-pass metabolism takes place n liver where drug reaches through the portal vein.
iii. Gastric mucosa irritation may lead to nausea and vomiting.
iv. Route is not preferred in unconscious and uncooperative patients.
v. Drug could not be administrated during emesis.

13.  Sublingual Administration:
- Absorption from the oral mucosa has special significance for certain drugs despite the surface area available is small. Venous drainage from the
mouth is to the superior vena cava, thus bypassing the portal circulation.
- As a consequence, a drug held sublingually and absorbed from that site is protected from rapid intestinal and hepatic first-pass metabolism. For
example, sublingual nitro-glycerine is rapidly effective because it is non-ionic, has high lipid solubility, and is not subject to the first-pass effect
prior to reaching the heart and arterial system.
A- MERITS:
i. Rapid absorption of drugs due to highly vascularised site.
ii. Drugs do not undergo first pass metabolism.
iii. Portal circulation is by-passed.
iv. Less chances of infection.
B- DEMERITS:
i. Sometime inconvenient to keep drug in mouth.
ii. Drugs having high molecular weight cannot be absorbed.
iii. Unpleasant, tasteless, irritant drugs cannot be administrated.
 Rectal Administration:
- Approximately 50% of the drug that is absorbed from the rectum will bypass the liver, thereby reducing hepatic first-pass metabolism. However,
rectal absorption can be irregular and incomplete, and certain drugs can cause irritation of the rectal mucosa.
- Drug absorption occur directly through thin highly vascularised wall of rectum. Rectal administration may be desirable, as in the use of opioids in
hospice care.

14. - Suppository drugs are administrated through rectal route.
A- MERITS:
i. This route avoids destruction of drugs by intestinal enzyme or by low pH of stomach.
ii. Useful when patient is suffering with nausea and vomiting.
iii. Gastric irritant drugs administrated through this route.
B- DEMERITS:
i. Rectal inflammation
ii. Irregular absorption.
Controlled-Release Preparations.
- The rate of absorption of a drug administered as a tablet or other solid oral dosage form is partly dependent on its rate of dissolution in GI fluids. This is
the basis for controlled-release, extended-release, sustained-release, and prolonged-action pharmaceutical preparations that are designed to produce slow,
uniform absorption of the drug for 8 h or longer.
- Potential advantages of such preparations are reduction in the frequency of administration compared with conventional dosage forms, maintenance of a
therapeutic effect overnight, and decreased incidence and intensity of undesired effects and nontherapeutic blood levels of the drug that often occur after
administration of immediate-release dosage forms.
- Controlled-release dosage forms are most appropriate for drugs with short half-lives (t1/2 < 4 h) or in select patient groups, such as those receiving
antiepileptic or antipsychotic agents.
 Parenteral Injection:
- Parenteral (i.e., not via the GI tract) injection of drugs has distinct advantages over oral administration. In some instances, parenteral administration is
essential for delivery of a drug in its active form, as in the case of monoclonal antibodies.

15. A- MERITS:
i. Availability usually is more rapid, extensive, and predictable when a drug is given by injection;
ii. the effective dose can be delivered more accurately to a precise dose;
iii. this route is suitable for the loading dose of medications prior to initiation of oral maintenance dosing (e.g., digoxin).
iv. In emergency therapy and when a patient is unconscious, uncooperative, or unable to retain anything given by mouth, parenteral therapy may
be necessary.
B- DEMERITS:
i. Asepsis must be maintained, especially when drugs are given overtime (e.g., intravenous or intrathecal administration);
ii. Pain may accompany the injection; and
iii. It is sometimes difficult for patients to perform the injections themselves if self-medication is necessary.
- The major routes of parenteral administration are intravenous, subcutaneous, and intramuscular. Absorption from subcutaneous and intramuscular
sites occurs by simple diffusion along the gradient from drug depot to plasma.
 Intravenous:
- The drug is injected as a bolus or infused slowly over hours in one of the superficial veins. The drug reaches directly into the blood stream and
effects are produced immediately.
- The intima of vein is insensitive and drug et diluted with blood, thus, highly irritant drug can be injected.
- Factors limiting absorption are circumvented by intravenous injection of drugs in aqueous solution because bioavailability is complete (F = 1.0)
and distribution is rapid. Also, drug delivery is controlled and achieved with an accuracy.
- Certain irritating solutions can be given only in this manner because the drug, when injected slowly, is greatly diluted by the blood.
- Unfavourable reactions can occur because high concentrations of drug may be attained rapidly in plasma and tissues.

16. - Intravenous administration of drugs warrants careful determination of dose and close monitoring of the patient’s response; once the drug is
injected, there is often no retreat. Repeated intravenous injections depend on the ability to maintain a patent vein.
- Drugs in an oily vehicle, those that precipitate blood constituents or haemolyze erythrocytes must not be given intravenously.
 Subcutaneous:
- Injection into a subcutaneous site can be done only with drugs that are not irritating to tissue; otherwise, severe pain, necrosis, and tissue
sloughing may occur. The rate of absorption in subcutaneous injection of a drug is constant and slow to provide a sustained effect.
- The incorporation of a vasoconstrictor agent in a solution of a drug to be injected subcutaneously also retards absorption. Absorption of drugs
implanted under the skin in a solid pellet form occurs slowly over a period of weeks or months; some hormones (e.g., contraceptives) are
administered effectively in this manner.
- Some special forms of this route are:
a- Dermojet:
A high velocity jet of drug solution is projected from a microfine orifice using a gun like implement. The solution is passes through the
superficial layers and get deposited in the subcutaneous tissue. It is painless and suited for mass inoculation.
b- Pellet implantation:
The drug in the form of solid pellet is introduced with a trocar and cannula. This provide a sustained release of drug over weeks and month, e.g.
DOCA, testosterone.
c- Silastic (nonbiodegradable) and biodegradable implants:
Crystalline drugs are packed in tubes or capsules made of suitable materials and implanted under skin. Slow and uniform leaching of drug
occurs over months providing constant blood levels.
The nonbiodegradable implants has to be removed later but not the biodegradable one.

17.  Intramuscular:
- The drug is injected in one of the large skeletal muscles, i.e. deltoid, triceps, gluteus maximus, rectus femoris, etc.
- Absorption of drugs in aqueous solution after intramuscular injection depends on the rate of blood flow to the injection site and can be relatively
rapid. Absorption may be modulated to some extent by local heating, massage, or exercise.
- Generally, the rate of absorption following injection of an aqueous preparation into the deltoid or vastus lateralis is faster than when the injection is
made into the gluteus maximus.
- The rate is particularly slower for females after injection into the gluteus maximus, a feature attributed to the different distribution of subcutaneous
fat in males and females and because fat is relatively poorly perfused.
- Slow, constant absorption from the intramuscular site results if the drug is injected in solution in oil or suspended in various other repository
(depot) vehicles.
 Intra-arterial:
- Drug is injected directly into an artery to localize its effect in a particular tissue or organ, such as in the treatment of liver tumours and head and
neck cancers. Diagnostic agents sometimes are administered by this route (e.g., technetium-labelled human serum albumin).
- Inadvertent intra-arterial administration can cause serious complications and requires careful management.
 Intrathecal:
- The BBB and the blood-CSF barrier often preclude or slow the entrance of drugs into the CNS, reflecting the activity of P-glycoprotein (MDR1)
and other transporters to export xenobiotics from the CNS.
- Therefore, when local and rapid effects of drugs on the meninges or cerebrospinal axis are desired, as in spinal anaesthesia, drugs sometimes are
injected directly into the spinal subarachnoid space.

18. - Brain tumours or serious CNS infections also may be treated by direct intraventricular drug administration, increasingly through the use of
specialized long-term indwelling reservoir devices.
Parenteral route Advantages Disadvantages
Subcutaneous Slow on-set, may be used to administer oil-based drugs Slow onset, small volumes
Intramuscular Intermediate onset, may be used to administer oil-based drugs Can affect lab test, intramuscular haemorrhage, painful.
Intravenous Rapid onset, controlled drug delivery Peak – related rug toxicity
Intrathecal Bypass BBB Infection
 Topical Application:
 Mucous Membranes:
- Drugs are applied to the mucous membranes of the conjunctiva, nasopharynx, oropharynx, vagina, colon, urethra, and urinary bladder primarily for
their local effects.
- Absorption from these sites is generally excellent and may provide advantages for immune therapy because vaccination of mucosal surfaces using
mucosal vaccines provides the basis for generating protective immunity in both the mucosal and systemic immune compartments.
 Eye:
- Topically applied ophthalmic drugs are used primarily for their local effects. The use of drug-loaded contact lenses and ocular inserts allows drugs
to be better placed where they are needed for direct delivery.
 Skin: Transdermal Absorption:
- Absorption of drugs able to penetrate the intact skin is dependent on the surface area over which they are applied and their lipid solubility.
Systemic absorption of drugs occurs much more readily through abraded, burned, or denuded skin.
- Absorption through the skin can be enhanced by suspending the drug in an oily vehicle and rubbing the resulting preparation into the skin.
Hydration of the skin with an occlusive dressing may be used to facilitate absorption.

19. - Controlled-release topical patches are increasingly available, with nicotine for tobacco-smoking withdrawal, scopolamine for motion sickness,
nitro-glycerine for angina pectoris, testosterone and estrogen for replacement therapy, various oestrogens and progestins for birth control, and
fentanyl for pain relief.
A- Merits of topical route:
i. Drugs can be applied easily.
ii. Less complication
iii. First action on application sites.
B- Demerits:
i. Skin irritation.
ii. Improper absorption of certain drugs.
iii. Longer duration of action due to sticky and oily texture.
 Pulmonary Absorption:
- Gaseous and volatile drugs may be inhaled and absorbed through the pulmonary epithelium and mucous membranes of the respiratory tract. Access to
the circulation is rapid by this route because the lung’s surface area is large.
- In addition, solutions of drugs can be atomized and the fine droplets in air (aerosol) inhaled. Advantages are the almost instantaneous absorption of a
drug into the blood, avoidance of hepatic first-pass loss, and in the case of pulmonary disease, local application of the drug at the desired site of action.
A- MERITS:
i. Surface area of the respiratory epithelium is large causing rapid absorption.
ii. Bronchodilators and inhaled steroids affect lungs with less systemic absorption and minimum side effects.
iii. Instant absorption of drug.

20. B- DEMERITS:
i. Specialised equipment required for drug delivery.
ii. Due to use of inhalers dose regulation is difficult.
 Novel Methods of Drug Delivery:
- Drug-eluting stents and other devices are being used to target drugs locally to maximize efficacy and minimize systemic exposure. Recent advances in
drug delivery include the use of biocompatible polymers and nanoparticles for drug delivery.
 Factors affecting drug absorption:
Following factors affects the drug absorption:
a- Aqueous solubility:
- Drugs given in solid form must dissolve in the aqueous biophase before they are absorbed. For poorly water-soluble drugs (aspirin, griseofulvin)
rate of dissolution governs rate of absorption. Ketoconazole dissolves at low pH: gastric acid is needed for its absorption.
- A drug given as watery solution is absorbed faster than when the same is given in solid form or as oily solution.
b- Concentration:
- Passive diffusion depends on concentration gradient; drug given as concentrated solution is absorbed faster than from dilute solution.
c- Area of absorbing surface:
- Larger is the surface area. faster is the absorption.
d- Vascularity of the absorbing surface:
- Blood circulation removes the drug from the site of absorption and maintains the concentration gradient across the absorbing surface. Increased
blood flow hastens drug absorption just as wind hastens drying of clothes.
e- Influence of pH and ionisation:

21. - Drugs are mostly weak electrolytes, i.e. their ionisation is pH dependent. Only the unionised fraction of drug, which is sufficiently lipid soluble can
penetrate the membrane via passive transport.
- The ionisation of weak acid (HA) is given by following equation:
HA ⇌ H + A
pH = pK + log
[ ]
[ ]
------------------- (1)
→At 50% drug ionisation: [HA] = [A-
] = [H+
]
Then, pH=pKa
A- Case-1 (If pH is increased by 1 scale):
pH = pKa +1 = pKa + log 10--------(2)
→ Comparing equation (1) & (2):
[A ]
[HA]
= 10 ⇒ [A ] = 10 × [HA]
It concludes that weak acidic drugs, ionise at alkaline pH and the absorption decrease. 1 scale increases in pH cause 10-fold increases in
ionisation.
B- Case-2 (if pH decreased by 1 scale):
pH=pKa +(-1) = pKa + log ( )--------(3)
→ Comparing equation (1) & (3):
[𝐴 ]
[𝐻𝐴]
=
1
10
⇒ [𝐻𝐴] = [𝐴 ] × 10
It concludes that weak acidic drugs, less ionise at acidic pH and the absorption increases. 1 scale decreases in pH cause 10-fold decreases in
ionisation.
- Similarly, weakly basic drugs are ionises at acidic pH and get less absorbed, and less ionises at basic pH and absorption increases.

22. DISTRIBUTION
 Introduction:
- Following absorption or systemic administration into the bloodstream, a drug distributes into interstitial and intracellular fluids as functions of the
physicochemical properties of the drug, the rate of drug delivery to individual organs and compartments, and the differing capacities of those regions to
interact with the drug.
- Cardiac output, regional blood flow, capillary permeability, and tissue volume affect the rate of delivery and amount of drug distributed into tissues.
Kidney Heart Liver Brain Skeletal muscle Fat Remainder ∑
Blood flow
(ml / min)
1100 250 1700 800 900 250 500 5500
Mass
(kg)
0.3 0.3 2.6 1.3 34 10 21.5 70
Flow / Mass
(ml / min / kg)
3667 833 654 615 26 25 23
% Cardiac output 20 4.5 31 14.5 16.4 4.5 9.1 100
- Initially, liver, kidney, brain, and other well-perfused organs receive most of the drug and delivery to muscle, most viscera, skin, and fat is slower. This
second distribution phase may require minutes to several hours before the concentration of drug in tissue is in equilibrium with that in blood.
- With exceptions such as the brain, diffusion of drug into the interstitial fluid occurs rapidly because of the highly permeable nature of the capillary
endothelium. Thus, tissue distribution is determined by the partitioning of drug between blood and the particular tissue.
 Binding to Plasma Proteins:
- Many drugs circulate in the bloodstream bound to plasma proteins. Albumin is a major carrier for acidic drugs and basic drugs to α1-acid glycoprotein. The
binding is usually reversible.

23. - In addition, certain drugs may bind to proteins that function as specific hormone carrier proteins, such as the binding of estrogen or testosterone to sex
hormone–binding globulin or the binding of thyroid hormone to thyroxin-binding globulin. The fraction of total drug in plasma that is bound is determined
by the drug concentration, the affinity of binding sites for the drug, and the concentration of available binding sites.
- The clinically significant implications of plasma protein binding are:
a- Highly plasma protein bound drugs are largely restricted to the vascular compartment because protein bound drug does not cross membranes (except
through large paracellular spaces, such as in capillaries). They tend to have smaller volumes of distribution.
Drug highly bound to plasma proteins
To albumin To α1-acid glycoprotein
Barbiturates B-blockers
Benzodiazepines Bupivacaine
NSAIDs Lidocaine
Valproic acid Disopyramide
Phenytoin lmipramine
Penicillin Methadone
Sulfonamide Prazosin
Tetracyclines Quinidine
Tolbutamide Verapamil
Warfarin
b- The bound fraction is not available for action. However, it is in equilibrium with the free drug in plasma and dissociates when the concentration of the
free drug is reduced due to elimination. Plasma protein binding thus tantamount to temporary storage of the drug.
c- High degree of protein binding generally makes the drug long acting, because bound fraction is not available for metabolism or excretion, unless it is
actively extracted by liver or by kidney tubules.
Glomerular filtration does not reduce the concentration of the free form in the efferent vessels, because water is also filtered. Active tubular secretion,
however, removes the drug without the attendant solvent → concentration of free drug fall → bound drug dissociates and is eliminated resulting in a
higher renal clearance value of the drug than the total renal blood flow.

24. The same is true of active transport of highly extracted drugs in liver. Plasma protein binding in this situation acts as a carrier mechanism and hastens
drug elimination, e.g. excretion of penicillin (elimination t½ is 30 min); metabolism of lidocaine.
Highly protein bound drugs are not removed by haemodialysis and need special techniques for treatment of poisoning.
d- The generally expressed plasma concentrations of the drug refer to bound as well as free drug. Degree of protein binding should be taken into account
while relating these two concentrations of the drug that are active invitro, e.g. MIC of an antimicrobial.
e- One drug can bind to many sites on the albumin molecule. Conversely, more than one drug can bind to the same site. This can give rise to
displacement interactions among drugs bound to the same site.
The drug bound with higher affinity will displace that bound with lower affinity and tend to increase the concentration of its free form. This, however,
is often transient because the displaced drug will diffuse into the tissue as well as get metabolized or excreted.
Two highly bound drugs do not necessarily displace each other-their binding sites may not overlap, e.g. Probenecid and Indomethacin are highly bound
to albumin but do not displace each other. Similarly, acidic drugs do not generally displace basic drugs and vice versa.
f- In hypoalbuminemia. binding may be reduced and high concentrations of free drug may be attained, e.g. phenytoin and furosemide. Other diseases
may also alter drug binding, e.g. phenytoin and pethidine binding is reduced in uraemia; propranolol binding is increased in pregnant women and in
patients with inflammatory disease.
 Tissue Binding:
- Many drugs accumulate in tissues at higher concentrations than those in the extracellular fluids and blood. Tissue binding of drugs usually occurs with
cellular constituents such as proteins, phospholipids, or nuclear proteins and generally is reversible.
- A large fraction of drug in the body maybe bound in this fashion and serve as a reservoir that prolongs drug action in that same tissue or at a distant site
reached through the circulation. Such tissue binding and accumulation also can produce local toxicity (e.g., renal and ototoxicity associated with
aminoglycoside antibiotics).

25.  CNS, the BBB, and CSF:
- The brain capillary endothelial cells have continuous tight junctions; therefore, drug penetration into the brain depends on transcellular rather than
paracellular transport. The unique characteristics of brain capillary endothelial cells and pericapillary glial cells constitute the BBB.
- The BBB is a highly selective semipermeable border of endothelial cell that prevents solutes in the circulating blood from crossing into the
extracellular fluid of the CNS.
- At the choroid plexus, a similar blood-CSF barrier is present, formed by epithelial cells that are joined by tight junctions. The lipid solubility of the
nonionized and unbound species of a drug is therefore an important determinant of its uptake by the brain; the more lipophilic a drug, e.g. Levodopa,
Propranolol, Physostigmine, the more likely it is to cross the BBB.
- In general, the BBB’s function is well maintained; however, meningeal and encephalic inflammation increase local permeability. Drugs may also be
imported to and exported from the CNS by specific transporters.
- Some enzymes like MAO, cholinesterase present in capillary wall do not allow catecholamine, 5-HT, acetyl choline, etc. enter into brain in active form.
 Placental Transfer of Drugs:
- Placental membrane is lipoidal and allow free passage of lipophilic drug while restricting hydrophilic drugs. The transfer of drugs across the placenta is
of critical importance because drugs may cause anomalies in the developing foetus.
- Lipid solubility, extent of plasma binding, and degree of ionization of weak acids and bases are important general determinants in drug transfer across
the placenta. The placenta functions as a selective barrier to protect the foetus against the harmful effects of drugs.
- Members of the ABC family of transporters limit the entry of drugs and other xenobiotics into the foetal circulation via vectorial efflux from the
placenta to the maternal circulation. The foetal plasma is slightly more acidic than that of the mother (pH 7.0–7.2 vs. 7.4), so that ion trapping of basic
drugs occurs.
- Heparin cannot cross the placental barrier, hence, used as anticoagulant in pregnancy.

26.  Fat as a Reservoir:
- Many lipid-soluble drugs are stored by physical solution in the neutral fat. In obese persons, the fat content of the body may be as high as 50%, and
even in lean individuals, fat constitutes 10% of body weight; hence, fat may serve as a reservoir for lipid-soluble drugs.
- Fat is a rather stable reservoir because it has a relatively low blood flow.
 Bone:
- The tetracycline antibiotics (and other divalent metal-ion chelating agents) and heavy metals may accumulate in bone by adsorption onto the bone
crystal surface and eventual incorporation into the crystal lattice.
- Bone can become a reservoir for the slow release of toxic agents such as lead or radium; their effects thus can persist long after exposure has ceased.
Local destruction of the bone medulla also may result in reduced blood flow and prolongation of the reservoir effect as the toxic agent becomes sealed
off from the circulation; this may further enhance the direct local damage to the bone.
- A vicious cycle results, whereby the greater the exposure to the toxic agent, the slower is its rate of elimination. The adsorption of drug onto the bone
crystal surface and incorporation into the crystal lattice have therapeutic advantages for the treatment of osteoporosis.
Drug concentrated on tissue
Skeletal muscle, heart Digoxin, Emetine (bound to muscle proteins).
Liver Chloroquine, Tetracyclines, Emetine, Digoxin.
Kidney Digoxin, Chloroquine, Emetine.
Thyroid Iodine.
Brain Chlorpromazine, Acetazolamide, Isoniazid.
Retina Chloroquine (bound to nucleoproteins).
Iris Ephedrine, Atropine (bound to melanin).
Bone and teeth Tetracyclines, Heavy Metals (bound to mucopolysaccharides of connective tissue), Bisphosphonates (bound to hydroxyapatite).
Adipose tissue Thiopentone, Ether, Minocycline, Phenoxybenzamine, DDT dissolve in neutral fat due to high lipid-solubility; remain stored due
to poor blood supply of fat.

27.  Factors affecting distribution:
 Physio-chemical factors:
- The major physio-chemical properties determining the distribution of drugs in the body are lipid solubility and (if the drug is a weak electrolyte) the
degree of non-ionization in the tissues.
- If the drug is highly lipid-soluble, it is generally distributed through all fluid compartments and, since it can readily cross lipid membranes, it is usually
promptly distributed to most organs of the body, including the brain.
- If the drug is poorly lipid-soluble, it is normally still able to enter the interstitial fluid since the capillary endothelium does not restrict its movement
into this compartment. If the molecule is relatively small (e.g. urea) it can sometimes penetrate aqueous channels in weak electrolytes, only that
proportion which is non-ionized will be able to penetrate cells and then only in proportion to the lipid/water partition coefficient.
 Biological factors:
A- Blood flow:
- The rate of distribution of drugs to different tissues will depend on the regional blood flow to those tissues. The amount that is able to enter the
tissues will depend on the degree of protein binding in the blood, as well as the degree of binding to the particular tissue or the presence of active
uptake mechanisms into the tissue.
a- Plasma protein binding:
- Plasma constitutes 50% of the total blood volume of around 6 litre in the adult male. This plasma contains 180 g of protein, of which
approximately 50% is albumin.
- It is therefore the most abundant single protein in the plasma. Globulins α, β and γ make up the bulk of the remaining proteins. Because of
its abundance, as well as its physio-chemical characteristics, albumin is the most important drug binding protein in the plasma.

28. - At present, two major binding sites for drugs have been identified on the albumin molecule. A large number of compounds (some oral
anticoagulants, diuretics, sulphonamides, non-steroidal anti-inflammatory drugs, oral hypoglycaemics, phenytoin share the Site I binding
site with the endogenous compound bilirubin. This has been called the 'warfarin' site.
- Site II, also known as the 'diazepam' site, binds, for example benzodiazepines, some non-steroidal anti-inflammatory drugs, penicillin, oral
hypoglycaemic agents and the endogenous amino acid, tryptophan.
- Another plasma protein, α-1 -acid-glycoprotein (orosomucoid), is an important binding site for drugs, particularly for basic agents.
- The total amount of this protein in plasma is generally less than a fiftieth of the total amount of albumin, but its affinity for binding many
basic drugs is so high that a large proportion of the total bound drug in plasma may be bound to this protein.
- At therapeutic concentrations of lignocaine, for example, approximately 70% of the drug which is in the bound form, is bound to α-1-acid-
glycoprotein. Its binding capacity is, however, relatively limited, and several drugs which bind significantly at this site show concentration-
dependence in their plasma protein binding.
- Other plasma proteins may also bind drugs (e.g. thyroid-binding globulin and sex hormone-binding globulin), but their importance appears
to be limited.
- More plasma protein binding, less will be the drug distribution.
b- Tissue binding:
- Since the distribution volume of many drugs is greater than the total body water volume, however, these agents must be taken up in certain
tissues in higher concentrations than in plasma, and in many cases, this may be related to tissue binding.
- It is known that 60% of the total exchangeable body albumin is extravascular (1) Skin contains around 18% of the total and the muscle a
further 15%. Even if a drug is very tightly bound to albumin (e.g. warfarin), the steady-state volume of distribution may be larger than the
plasma volume.

29. - Thus, for warfarin the steady-state volume of distribution is approximately 9 litre in the adult, whereas the plasma volume is only 3 litres.
Another protein, ligandin, has been implicated in drug binding. It is found in relatively high concentrations in the hepatocytes, renal
tubular and intestinal mucosal cells.
- Ligandin and other macromolecules within the cell may act as a large reservoir for drug distribution., The lipid content of the tissue and its
pH relative to that of plasma will also determine the degree of uptake of the drug by the tissue.
B- Obesity:
- High adipose tissue results to low drug distribution and perfusion. High fatty acid content alters the binding property of acidic drugs.
 Redistribution:
- Termination of drug effect after withdrawal of a drug usually is by metabolism and excretion but also may result from redistribution of the drug from its site of action into
other tissues or sites.
- Highly lipid soluble drugs get initially distributed to organs with high blood flow, i.e. brain, heart, kidney, etc. Then drug is redistributed among less vascular, bulky
tissue like muscle, fat, and thus, plasma concentration falls and the drug is withdrawn from the highly perfused organ.
- Redistribution is a factor in terminating drug effect primarily when a highly lipid-soluble drug that acts on the brain or cardiovascular system is administered rapidly by
intravenous injection or inhalation.
- Such is the case of the intravenous anaesthetic Thiopental, a lipid-soluble drug. Because blood flow to the brain is high and thiopental readily crosses the BBB, thiopental
reaches its maximal concentration in brain rapidly after its intravenous injection.
- Subsequently, the plasma and brain concentrations decrease as thiopental redistributes to other tissues, such as muscle and, finally, adipose tissue. This redistribution is
the mechanism by which thiopental anaesthesia is terminated because its clearance is rather slow (elimination t1/2 after a single dose is 3–8 h).
- The concentration of the drug in brain follows that of the plasma because there is little binding of the drug to brain constituents. Thus, both the onset and the termination
of thiopental anaesthesia are relatively rapid, and both are related directly to the concentration of drug in the brain.

30. METABOLISM OF DRUGS OR BIOTRANSFORMATION
 Introduction:
- Biotransformation means chemical alternation of the drug in the body. During biotransformation nonpolar or lipid soluble drug is converted into polar or
lipid insoluble substances, that easily removed from the body.
- The many therapeutic agents that are lipophilic do not pass readily into the aqueous environment of the urine. The metabolism of drugs and other
xenobiotics into more hydrophilic metabolites is essential for their renal elimination from the body, as well as for termination of their biological and
pharmacological activity.
- Most hydrophilic drugs, e.g. Streptomycin, Neostigmine, Pancuronium, etc. are little biotransform ed and are largely excreted unchanged. The primary site
for drug metabolism is liver; others are-kidney, intestine, lungs and plasma.
- Biotransformation of drugs may lead to the following:
A- Inactivation:
- Most drugs and their active metabolites are become inactive or less active, e.g. ibuprofen, paracetamol, lidocaine, chloramphenicol, propranolol
and its active metabolite 4-hydroxypropranolol.
- Thus, biotransformation provides an alternative method of terminating drug action to excretion.
B- Active metabolite from an active drug:
- Many drugs are partially converted to one or more active metabolite; the effects observed are the sum total of that due to the parent drug and its
active metabolite.

31. Examples:
Active drug Active metabolites
Morphine Morphine-6-glucuronide
Cefotaxime Desacetyl cefotaxime
Allopurinol Alloxanthin
Procainamide N-acetyl procainamide
Primidone Phenobarbitone, phenylethylmalonamide
Diazepam Desmethyl-diazepam, oxazepam
Digitoxin Digoxin
lmipramine Desipramine
Amitriptyline Nortriptyline
Codeine Morphine
Spironolactone Canrenone
C- Activation of inactive drug:
- Few drugs are inactive as such and need conversion in the body to one or more active metabolites. Such a drug is called a prodrug. The prodrug
may offer advantages over the active form in being more stable, having better bioavailability or other desirable pharmacokinetic properties or less
side effects and toxicity.
- Some prodrugs are activated selectively at the site of action.
Examples:
Prodrug Active form
Levodopa Dopamine
Enalapril Enalaprilat
α-Methyl dopa α-Methyl norepinephrine
Dipivefrine Epinephrine
Proguanil Cycloguanil
Prednisone Prednisolone
Clopidogrel Thiol metabolite
Bacampicillin Ampicillin
Sulfasalazine 5-Aminosalicylic acid
Cyclophosphamide Aldophosphamide, Phosphoramide mustard, Acrolein
Fluorouracil Fluorouridine monophosphate
Mercaptopurine Methyl mercaptopurine ribonucleotide
Acyclovir Acyclovir triphosphate

32. - From the point of view of pharmacokinetics, the following are the three essential aspects of drug metabolism:
A- First-order kinetics:
- For most drugs in their therapeutic concentration ranges, the amount of drug metabolized per unit time is proportional to the plasma concentration
of the drug (Cp) and the fraction of drug removed by metabolism is constant (i.e., first-order kinetics).
B- Zero-order kinetics:
- For some drugs, such as ethanol and phenytoin, metabolic capacity is saturated at the concentrations usually employed, and drug metabolism
becomes zero order; that is, a constant amount of drug is metabolized per unit time.
- Zero-order kinetics can also occur at high (toxic)concentrations as drug-metabolizing capacity becomes saturated.
C- Inducible biotransforming enzymes:
- The major drug-metabolizing systems are inducible, broad-spectrum enzymes with some predictable genetic variations. Drugs that are substrates
in common for a metabolizing enzyme may interfere with each other’s metabolism, or a drug may induce or enhance metabolism of itself or other
drugs.
 The phases of drug metabolism:
- Drug-metabolizing enzymes have been categorized as:
a- Phase 1 reactions, which include oxidation, reduction, or hydrolytic reactions; or
b- Phase 2 reactions, in which enzymes catalyse the conjugation of the substrate (the phase 1 product) with a second molecule.
 Phase-1 reactions:
- The phase 1 enzymes lead to the introduction of functional groups, such as –OH, –COOH, –SH, –O–, or NH2. The addition of functional groups little to
increases the water solubility of the drug but can alter the biological properties of the drug.

33. - Reactions carried out by phase 1 enzymes usually lead to the inactivation of a drug. However, in certain instances, metabolism, usually the hydrolysis
of an ester or amide linkage, results in bioactivation of a drug.
- Inactive drugs that undergo metabolism to an active drug are called prodrugs. Examples of prodrugs bioactivated by CYPs are the antitumor drug
Cyclophosphamide, which is bioactivated to a cell-killing electrophilic derivative, and the anti-thrombotic agent Clopidogrel, which activated to 2-oxo-
clopidogrel, which is further metabolized to an irreversible inhibitor of platelet ADP P2Y12 receptors. The liver is important in phase-1 reaction.
- Phase-1 reactions are:
a- Oxidation
b- Reduction
c- Hydrolysis
d- Cyclization
e- Decyclization
 Oxidation:
- These reactions involve addition of oxygen/negatively charged radical or removal of hydrogen/positively charged radical. It is the most important
drug metabolising reaction. Various oxidation reactions are as:
a- Hydroxylation
b- Oxygenation at C, N, S atoms
c- N, or O dealkylation
d- Oxidative deamination
- In many cases initial insertion of oxygen atom into drugs molecules produces short lived highly reactive quinone/epoxide/superoxide intermediates,
which is then convert into more sable compound.

34. - Oxidative reactions are mostly carried out by a group of monooxygenases in the liver, which in the final step involves a cytochrome P450
haemoprotein, NADPH, cytochrome P450 reductase, and molecular oxygen (O2).
- Drugs oxidised by CYPs are:
Barbiturates Propranolol Steroids
Phenothiazine Ibuprofen Phenytoin
Imipramine paracetamol Benzodiazepine
- Some drugs are oxidised at their N, P, S atoms by a group of flavin-monooxygenase that are located in the hepatic endoplasmic reticulum:
Cimetidine
Ranitidine
Clozapine

35.  CYPs: The Cytochrome P450 Superfamily:
- The CYPs are a superfamily of enzymes, each of which contains a molecule of heme bound noncovalently to the polypeptide chain. Many enzymes that
use O2 as a substrate for their reactions contain heme, and heme is the oxygen-binding moiety in haemoglobin. Heme contains one atom of iron in a
hydrocarbon cage that functions to bind O2 in the active site of the CYP as part of the catalytic cycle of these enzymes.
- CYPs use O2, plus H+
derived from the cofactor-reduced NADPH, to carry out the oxidation of substrates. The H+
is supplied through the enzyme
NADPH-CYP oxidoreductase. Metabolism of a substrate by a CYP consumes one molecule of O2 and produces an oxidized substrate and a molecule of
H2O as a by-product.
- CYPs, depends on the nature of the substrate. The reaction is “uncoupled,” consuming more O2 than substrate metabolized and producing activated
oxygen or O2
-
. The O2
-
is usually converted to water by the enzyme superoxide dismutase.

36. - Among the diverse reactions carried out by mammalian CYPs are N-dealkylation, O-dealkylation, aromatic hydroxylation, N-oxidation, S-oxidation,
deamination, and dehalogenation.
- More than 50 individual CYPs have been identified in humans. As a family of enzymes, CYPs are involved in the metabolism of dietary and xenobiotic
chemicals, as well as the synthesis of endogenous compounds such as steroids and fatty acid signalling molecules such as epoxyeicosatrienoic acids.
- CYPs also participate in the production of bile acids from cholesterol. In contrast to the drug-metabolizing CYPs, the CYPs that catalyse steroid and
bile acid synthesis have specific substrate preferences. For example, the CYP that produces estrogen from testosterone, CYP19 or aromatase, can
metabolize only testosterone or androstenedione and does not metabolize xenobiotics. Specific inhibitors for aromatase, such as Anastrozole, have been
use in the treatment of estrogen-dependent tumours.
- The synthesis of bile acids from cholesterol occurs in the liver, where, subsequent to CYP-catalysed oxidation, the bile acids are conjugated with amino
acids and transported through the bile duct and gallbladder into the small intestine.
 The Naming of CYPs:
- CYPs are complex and diverse in their regulation and catalytic activities. Genome sequencing has revealed the existence of 102 putatively functional
CYP genes and 88 pseudogenes in the mouse and 57 putatively functional genes and 58 pseudogenes in humans.
- These genes are grouped, based on amino acid sequence similarity, into a superfamily composed of families and subfamilies with increasing sequence
similarity. CYPs are named with the root CYP followed by a number designating the family, a letter denoting the subfamily, and another number
designating the CYP form.
- Thus, CYP3A4 is family 3, subfamily A, and gene number 4.

37.  A Small Number of CYPs Metabolize the Majority of Drugs:
- A limited number of CYPs (15 in humans) that fall into families 1, 2, and3 are primarily involved in xenobiotic metabolism. Because a single CYP can
metabolize a large number of structurally diverse compounds, these enzymes can collectively metabolize scores of chemicals found in the diet,
environment, and pharmaceuticals.
- In humans, 12 CYPs (CYP1A1, 1A2,1B1, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1, 3A4, and 3A5) are important for metabolism of xenobiotics. The liver
contains the greatest abundance of xenobiotic-metabolizing CYPs, thus ensuring efficient first-pass metabolism of drugs.
- The most active CYPs for drug metabolism are those in the CYP2C, CYP2D, and CYP3A subfamilies. CYP3A4, the most abundantly expressed in liver,
is involved in the metabolism of over 50% of clinically used drugs.
- The CYP1A, CYP1B, CYP2A, CYP2B, and CYP2E subfamilies are not significantly involved in the metabolism of therapeutic drugs, but they do
catalyse the metabolic activation of many protoxins and procarcinogens to their ultimate reactive metabolites.
- Several human CYP genes exhibit polymorphisms, including CYP2A6, CYP2C9, CYP2C19, and CYP2D6. Allelic variants have been found in the
CYP1B1and CYP3A4 genes, but they are present at low frequencies in humans and appear not to have a major role in interindividual levels of
expression of these enzymes. However, homozygous mutations in the CYP1B1 gene are associated with primary congenital glaucoma.
CYP450 isoenzyme Drugs metabolized Inhibitors Inducers
CYP3A4, CYP3A5
Terfenadine, Astemizole, Cisapride, Losartan,
Carbamazepine, Hydrocortisone, Paracetamol,
Diazepam, Buspirone, Mifepristone, Ritonavir,
Saquinavir, Simvastatin, Quinidine, Verapamil,
Lidocaine, Dapsone, Nevirapine
Erythromycin, Clarithromycin,
Ketoconazole, ltraconazole,
Verapamil, Ritonavir,
Fluoxetine, Grape fruit juice
Barbiturates, Phenytoin, Carbamazepine,
Rifampin, Glucocorticoids, Nevirapine
CYP2D6
Metoprolol, Debrisoquine, Nebivolol,
Amitriptyline, Clomipramine, Fluoxetine,
Paroxetine, Venlafaxine, Haloperidol,
Clozapine, Risperidone, Codeine, Propafenone,
Flecainide
Quinidine, Fluoxetine,
Paroxetine
Phenobarbitone, Rifampin

38. CYP2C8, CYP2C9
Phenytoin, Carbamazepine, Warfarin,
Tolbutamide, Repaglinide, Pioglitazone,
Diclofenac, lbuprofen, Losartan
Fluvoxamine, Fluconazole,
Gemfibrozil, Trimethoprim
Phenobarbitone, Carbamazepine,
Rifampin
CYP2C19
Omeprazole, Lansoprazole, Amitriptyline,
Citalopram, Phenytoin, Diazepam, Propranolol,
Clopidogrel
Omeprazole, Fluconazole Carbamazepine, Rifampin
CYP1A1, CYP1A2
Theophylline, Caffeine, Paracetamol, Warfarin,
Carbamazepine
Fluvoxamine, Fluoxetine Polycyclic hydrocarbons, Cigarette
smoke, Charbroiled meat, Rifampin,
Carbamazepine
CYP2E1 Alcohol, Halothane, Paracetamol Disulfiram, Fomepizole Chronic alcoholism, lsoniazid
CYP286
Efavirenz, Nevirapine, Cyclophosphamide,
Methadone, Sertraline, Clopidogrel
Paroxetine, Sertraline,
Clopidogrel
Phenobarbitone, Cyclophosphamide
 Flavin-Containing Monooxygenases:
- The FMOs are another superfamily of phase 1 enzymes involved in drug metabolism. Similar to CYPs, the FMOs are expressed at high levels in the
liver and are bound to the endoplasmic reticulum, a site that favours interaction with and metabolism of hydrophobic drug substrates.
- There are six families of FMOs, with FMO3 the most abundant in liver. FMO3 is able to metabolize nicotine, as well as H2-receptor antagonists
(Cimetidine and Ranitidine), antipsychotics (Clozapine), and antiemetics (Itopride).
- Trimethylamine N-oxide (TMAO) occurs in high concentrations, up to15% by weight, in marine animals, where it acts as an osmotic regulator.
Inhumans, FMO3 normally metabolizes TMAO to TMA, but a genetic deficiency of FMO3 causes the fish-odour syndrome, in which unmetabolized
TMAO accumulates in the body and causes a socially offensive fish odour.
- FMOs are considered minor contributors to drug metabolism, and they almost always produce benign metabolites. In addition, FMOs are not readily
inhibited and are not induced by any of the xenobiotic receptors; thus, in contrast to CYPs, FMOs would not be expected to be involved in drug-drug
interactions.
- In fact, this has been demonstrated by comparing the pathways of metabolism of two drugs used in the control of gastric motility: Itopride and
Cisapride. Itopride is metabolized by FMO3; Cisapride is metabolized by CYP3A4.

39. - As predicted, itopride is less likely to be involved in drug-drug interactions than is Cisapride. CYP3A4 participates in drug-drug interactions through
induction and inhibition of metabolism, whereas FMO3 is not induced or inhibited by any clinically used drugs.
 Reduction:
- This reaction involves addition of hydrogen and removal of oxygen. It involves CYP450 enzymes working in the opposite direction. Drugs that
primarily reduced are:
Chloral hydrate
Chloramphenicol
Halothane
Warfarin
Methadone
 Hydrolysis:
- This is cleavage of drug molecule by taking up a molecule of water.
Easter + H O ⎯⎯⎯⎯⎯ Acid + Alocohol
- Amides and polypeptides are hydrolysed by amidase and peptidase. There are epoxide hydroxylases which detoxify epoxide metabolite of some drugs
generated by CYP oxygenase.
- Hydrolysis occurs in liver, plasma, intestine.
- Drugs that hydrolysed are:
Choline esters Aspirin
Procaine Indomethacin
Lidocaine Carbamazepine
Procainamide Pethidine
Succinyl choline Oxytocin

40.  Hydrolytic enzyme:
- Two forms of epoxide hydrolase (EH) carry out hydrolysis of epoxides, most of which are produced by CYPs. The soluble epoxide hydrolase (sEH) is
expressed in the cytosol; microsomal epoxide hydrolase (mEH) is localized to the membrane of the endoplasmic reticulum.
- Epoxides are highly reactive electrophiles that can bind to cellular nucleophiles found in protein, RNA, and DNA, resulting in cell toxicity and
transformation. Thus, EHs participate in the deactivation of potentially toxic metabolites generated by CYPs.
- The antiepileptic drug Carbamazepine is a prodrug that is converted to its pharmacologically active derivative carbamazepine-10,11-epoxide, by CYP.
This metabolite is efficiently hydrolysed to a dihydrodiol by mEH, resulting in inactivation of the drug.
- The tranquilizer Valnoctamide and anticonvulsant Valproate inhibit mEH, resulting in clinically significant drug interactions with carbamazepine. This
has led to efforts to develop new antiepileptic drugs, such as Gabapentin and Levetiracetam, that are metabolized by CYPs and not by EHs.
- The epoxides of arachidonic acid and docosahexaenoic acid reduce inflammation, hypertension, and pain but are normally degraded quickly by sEH to
vicinal diols that are generally less biologically active. Thus, by inhibiting sEH, one can obtain dramatic biological effects.
- The carboxylesterases comprise a superfamily of enzymes that catalyse the hydrolysis of ester- and amide-containing chemicals. These enzymes are
found in both the endoplasmic reticulum and the cytosol of many cell types and are involved in detoxification or metabolic activation of various drugs,
environmental toxicants, and carcinogens.
- Carboxylesterases also catalyse the activation of prodrugs to their respective free acids. For example, the prodrug and cancer chemotherapeutic agent
Irinotecan is a camptothecin analogue that is bioactivated by intracellular carboxylesterases to the potent topoisomerase inhibitor SN-38.

41.  Cyclisation:
- This is the formation of the ring structure from a straight chain compound. Examples of drugs are: Proguanil→Cycloguanil
 Decyclisation:
- This implies opening of the ring structure of the cyclic drug molecule. E.g.: - Phenytoin, Barbiturates.
 Phase-2 reaction:
- Phase 2 enzymes produce a metabolite with improved water solubility and thereby facilitate the elimination of the drug from the tissue, normally via
efflux transporters.
- Thus, in general, phase 1 reactions result in biological inactivation of a drug, and phase 2 reactions facilitate the drug elimination and the inactivation
of electrophilic and potentially toxic metabolites produced by oxidation.
- The phase 2 enzymes include several super families of conjugating enzymes. Among the more important are the GSTs, UGTs, SULTs, NATs, and MTs.
These conjugation reactions usually require the substrate to have oxygen (hydroxyl or epoxide groups), nitrogen, and sulphur atoms that serve as
acceptor sites for a hydrophilic moiety, such as glutathione, glucuronic acid, sulphate, or an acetyl group, that is covalently conjugated to an acceptor
site on the molecule.
Sulfotransferase (SULT) Addition of sulphate
UDP – glucuronosyl transferase (UGTs) Addition of glucuronic acid
Glutathione – S – transferase (GSTs) Addition of glutathione
N – Acetyl transferase (NATs) Addition of acetyl group
Methyl transferase (MTs) Addition of methyl group

42. - The oxidation by phase 1 enzymes either adds or exposes a functional group, permitting the products of phase 1 metabolism to serve as substrates for
the phase 2 conjugating or synthetic enzymes.
- In the case of the UGTs, glucuronic acid is delivered to the functional group, forming a glucuronide metabolite that is more water soluble and is
targeted for excretion in the urine or bile.
- Phase 2 reactions also normally terminate the biological activity of the drug, although there are exceptions: For Morphine and Minoxidil, glucuronide
and sulphate conjugates, respectively, are more pharmacologically active than the parent.
- Two of the phase 2 reactions, glucuronidation and sulfation, result in the formation of metabolites with a significantly increased water-to lipid partition
coefficient. Sulfation and acetylation generally terminate the biological activity of drugs, and the minor change in overall charge increases the aqueous
solubility of the metabolite.
- The enhanced hydrophilicity facilitates metabolite transport into the aqueous compartments of the cell and the body. The phase 2 reactions depend on
the catalytic reactions for cofactors (or, more correctly, cosubstrate): uridine diphosphate–glucuronic acid (UDP-GA) for uridine diphosphate–
glucuronosyltransferase (UGT) and 3′-phosphoadenosine-5'-phosphosulfate (PAPS) for sulfotransferase (SULTs), which react with available functional
groups on the substrates.
- The reactive functional groups are often generated by the phase 1 CYPs, although there are many drugs (e.g., Acetaminophen) for which
glucuronidation and sulfation occur directly without prior oxidative metabolism.

43. - The catalytic rates of phase 2 reactions are significantly faster than the rates of the CYPs. Thus, if a drug is targeted for phase 1 oxidation through the
CYPs, followed by a phase 2 conjugation reaction, usually the rate of elimination will depend on the initial (phase 1) oxidation reaction.
 Glucuronidation:
- Among the more important of the phase 2 reactions in drug metabolism is those catalysed by uridine diphosphate–glucuronosyltransferase (UGTs).
These enzymes catalyse the transfer of glucuronic acid from the cofactor uridine diphosphate–glucuronic acid (UDP-GA) to a substrate to form β-d-
glucopyranosiduronic acids (glucuronides), metabolites that are sensitive to cleavage by β-glucuronidase.
- Glucuronides can be formed via alcoholic and phenolic hydroxyl groups; carboxyl, sulfuryl, and carbonyl moieties; and primary, secondary, and tertiary
amines.
- UGT substrates include chemically unique pharmaceuticals; dietary substances; environmental agents; humoral agents such as circulating hormones
(androgens, estrogen, mineralocorticoids, glucocorticoids, thyroxine); bile acids; retinoids; and bilirubin, the end product of heme catabolism.
- The UGTs are found highest concentration in the GI tract and liver. Per tissue weight, there are a greater number and higher concentration of the
UGTs in the small intestine compared to the liver, so efficient first-pass metabolism plays a role in predicting bioavailability of many orally
administered medications.
- Formation of glucuronides and their increased polarity can result in their passage into the circulation, from which they are excreted into the urine.
Alternatively, as xenobiotics enter the liver and are absorbed into hepatocytes, glucuronide formation provides substrates for active transport into the
bile canaliculi and ultimate excretion with components of the bile.
- Many of the glucuronides that are excreted into the bile eventually become substrates for soluble microbial β-glucuronidase in the large intestine,
resulting in the formation of free glucuronic acid and the initial substrate. Depending on its solubility, the glucuronide or the original substrate may
be reabsorbed via passive diffusion or by apical transporters in the colon and re-enter the systemic circulation. This process, called Enterohepatic
Recirculation, can extend the half-life of a xenobiotic that is conjugated in the liver because the compound’s ultimate excretion is delayed.

44. - There are 19 human genes that encode the UGT proteins. Nine are encoded by the UGT1A locus on chromosome 2q37 (1A1, 1A3, 1A4, 1A5,1A6, 1A7,
1A8, 1A9, and 1A10), while 10 genes are encoded by the UGT2 family of genes on chromosome 4q13.2 (2B17, 2B15, 2B10, 2A3, 2B7, 2B11,2B2, 2B4,
2A1, 2A2, and 2A3).
- Of these proteins, the major UGTs involved in drug metabolism are UGT1A1, 1A3, 1A4, 1A6, 1A9, and 2B7. Although both families of proteins are
associated with metabolism of drugs and xenobiotics, the UGT2 family of proteins have greater specificity for the glucuronidation of endogenous
substances.
- From a clinical perspective, the expression of UGT1A1 assumes an important role in drug metabolism because the glucuronidation of bilirubin by
UGT1A1 is the rate-limiting step in ensuring efficient bilirubin clearance. Bilirubin is the breakdown product of heme, 80% of which originates from
circulating haemoglobin and 20% from other heme-containing proteins, such as the CYPs. Bilirubin is hydrophobic, associates with serum albumin,
and must be metabolized further by glucuronidation to ensure its elimination.
- The failure to efficiently metabolize bilirubin by glucuronidation leads to elevated serum levels and a clinical symptom called hyperbilirubinemia or
jaundice. Delayed expression of the UGT1A1 gene in new borns is the primary reason for neonatal hyperbilirubinemia.
- Examples of drugs that are metabolised by glucuronide conjugation are:
Chloramphenicol Diazepam
Aspirin Lorazepam
Paracetamol Morphine
Metronidazole Phenacetin

45.  Sulfation:
- The sulfotransferase (SULTs), located in the cytosol, conjugate sulphate derived from 3'-phosphoadenosine-5'-phosphosulfate (PAPS) to hydroxyl
and, less frequently, amine groups of aromatic and aliphatic compounds.
- In humans, 13 SULT isoforms have been identified; based on sequence comparisons, they are classified into the SULT1 (SULT1A1, SULT1A2,
SULT1A3/4, SULT1B1, SULT1C2, SULT1C3, SULT1C4, SULT1E1); SULT2 (SULT2A1, SULT2B1a, SULT2B1b); SULT4 (SULT4A1); and SULT6
(SULT6A1) families.
- SULTs play an important role in normal human homeostasis. For example, SULT2B1b is a predominant form expressed in skin, carrying out the
catalysis of cholesterol. Cholesterol sulphate is an essential metabolite in regulating keratinocyte differentiation and skin development.
- SULT 2A1 is highly expressed in the foetal adrenal gland, where it produces the large quantities of dehydroepiandrosterone sulphate that are required
for placental estrogen biosynthesis during the second half of pregnancy.
- SULTs 1A3 and 1A4 are highly selective for catecholamines, while oestrogens (in particular 17β-estradiol) are sulphated by SULT 1E1. In humans,
significant fractions of circulating catecholamines, oestrogens, iodothyronines, and DHEA exist in the sulphated form.
- Members of the SULT1 family are the major isoforms involved in xenobiotic metabolism, with SULT1A1 quantitatively and qualitatively the most
important in the liver. SULT1A1 displays extensive diversity in its capacity to catalyse the sulfation of a broad variety of structurally heterogeneous
xenobiotics with high affinity. The isoforms in the SULT1 family are recognized as phenol SULTs; they catalyse the sulfation of phenolic molecules
such as Acetaminophen, Minoxidil, and 17α-Ethinyl Estradiol.
- In rodents, SULT1C enzymes are capable of sulphating the hepatic carcinogen N-OH-2-acetylaminofluorene and are responsible for the bioactivation
of this and related carcinogens. SULT1C enzymes are expressed abundantly inhuman foetal tissues, yet decline in abundance in adults. SULT1E
catalyses the sulfation of endogenous and exogenous steroids and is localized in liver and in hormone-responsive tissues such as the testis, breast,
adrenal gland, and placenta.

46. - In the upper GI tract, SULT1A3/4 and SULT1B1 are particularly abundant. The conjugation of drugs and xenobiotics is considered primarily a
detoxification step, ensuring that the metabolites enter the aqueous compartments of the body and are targeted for elimination.
- However, drug metabolism through sulfation often leads to the generation of chemically reactive metabolites, wherein the sulphate is electron
withdrawing and maybe heterolytically cleaved, leading to the formation of an electrophilic cation.
- Examples of drugs that are metabolised by sulphate conjugation are:
Chloramphenicol Adrenaline
Methyldopa Sex corticoids
 Glutathione conjugation:
- The glutathione-S-transferase (GSTs) catalyse the transfer of glutathione to reactive electrophiles, a function that serves to protect cellular
macromolecules from interacting with electrophiles that contain electrophilic heteroatoms (–O, –N, and –S) and in turn protects the cellular
environment from damage.
- The cosubstrate in the reaction is glutathione, a tripeptide consisting of γ-glutamic acid, cysteine, and glycine. Glutathione exists in the cell in
oxidized (GSSG) and reduced (GSH) forms, and the GSH: GSSG ratio is critical in maintaining a cellular environment in the reduced state.
- In the formation of glutathione conjugates, the GST reaction generates a thioether linkage with drug or xenobiotic to the cysteine moiety of the
tripeptide. Characteristically, all GST substrates contain an electrophilic atom and are hydrophobic; by nature, they will associate with cellular
proteins.
- Because the concentration of glutathione in cells is usually high, typically 7 μmol/g of liver or in the 10-mM range, many drugs and xenobiotics can
react nonenzymatically with glutathione.

47. - However, the GSTs have been found to occupy up to 10% of the total hepatocellular protein concentration, a property that ensures efficient
conjugation of glutathione to reactive electrophiles. The cytosolic pool of GSTs, once identified as ligandin, binds steroids, bile acids, bilirubin,
cellular hormones, and environmental toxicants, in addition to complexing with other cellular proteins.
- There are in excess of 20 human GSTs, divided into two sub families: the cytosolic and the microsomal forms. The major differences in function
between the microsomal and cytosolic GSTs reside in the selection of substrates for conjugation; the cytosolic forms have more importance in the
metabolism of drugs and xenobiotics, whereas the microsomal GSTs are important in the endogenous metabolism of leukotrienes and prostaglandins.
- The cytosolic GSTs are divided into seven classes termed alpha (GSTA1 and 2), mu (GSTM1 through 5), omega (GSTO1), pi (GSTP1), sigma
(GSTS1), theta (GSTT1 and GSTT2), and zeta (GSTZ1). Those in the alpha and mu classes can form heterodimers, allowing for a large number of
active transferases to form. The cytosolic forms of GST catalyse conjugation, reduction, and isomerization reactions.
- Acetaminophen, normally metabolized by glucuronidation and sulfation, is also a substrate for oxidative metabolism by CYP2E1 and CYP3A4, which
generate the toxic metabolite N-acetyl-p-benzoquinone imine (NAPQI), which, under normal dosing, is readily neutralized through conjugation with
GSH. An overdose of acetaminophen can deplete cellular GSH levels and thereby increase the potential for NAPQI to interact with other cellular
components, resulting in toxicity and cell death. Acetaminophen toxicity is associated with increased levels of NAPQI and hepatic necrosis, although
it may be treated in a time- and drug concentration–dependent manner by administration of N-acetylcysteine.
- The mu (GSTM1*0) and theta (GSTT1*0) genotypes express a null phenotype; thus, individuals who are polymorphic at these loci are predisposed
to toxicities by agents that are selective substrates for these GSTs. For example, the mutant GSTM1*0allele is observed in 50% of the Caucasian
population and links genetically to human malignancies of the lung, colon, and bladder. Null activity in the GSTT1 gene associates with adverse side
effects and toxicity in cancer chemotherapy with cytostatic drugs; the toxicities result from insufficient clearance of the drugs via GSH conjugation.
- While the GSTs play an important role in cellular detoxification, their activities in cancerous tissues have been linked to the development of drug
resistance toward chemotherapeutic agents that are both substrates and non-substrates for the GSTs. Many anticancer drugs are effective because

48. they initiate cell death or apoptosis, which is linked to the activation of MAPKs such as JNK and p38. Investigational studies demonstrated that
overexpression of GSTs is associated with resistance to apoptosis and the inhibition of MAPK activity.
- In a variety of tumours, GSTs are overexpressed, leading to a reduction in MAPK activity and reduced efficacy of chemotherapy. Taking advantage of
the relatively high levels of GST in tumour cells, inhibition of GST activity has been exploited as a therapeutic strategy to modulate drug resistance
by sensitizing tumours to anticancer drugs.
- TLK199, a glutathione analogue, is a prodrug that plasma esterase converts to a GST inhibitor, TLK117, which potentiates the toxicity of different
anticancer agents. Alternatively, the elevated GST activity in cancer cells has been utilized to develop prodrugs that can be activated by the GSTs to
form electrophilic intermediates. For example, TLK286 is a substrate for GST that undergoes a β-elimination reaction, forming a glutathione
conjugate and a nitrogen mustard that is capable of alkylating cellular nucleophiles and resulting in antitumor activity.
- Examples of drugs that are metabolised by glutathione conjugation are: Paracetamol
 N-Acetylation:
- The cytosolic N-acetyltransferase (NATs) are responsible for the metabolism of drugs and environmental agents that contain an aromatic amine or
hydrazine group. The addition of the acetyl group from the cofactor acetyl coenzyme-A leads to a metabolite that is less water soluble because the
potential ionizable amine is neutralized by the covalent addition of the acetyl group.
- Pharmacogenetic studies led to the classification of “rapid” and “slow” acetylators, with the slow phenotype predisposed to toxicity. There are two
functional NAT genes in humans, NAT1 and NAT2. Over25 allelic variants of NAT1 and NAT2 have been characterized. In individuals in whom
acetylation of drugs is compromised, homozygous genotypes for at least two variant alleles are required to predispose a patient to slower drug
metabolism.

49. - Aromatic amine or hydrazine groups exist in many classes of clinically used drugs, and if a drug is known to be subjected to metabolism through
acetylation, determining an individual’s phenotype can be important in maximizing a positive therapeutic outcome. For example, hydralazine, a
once-popular orally active antihypertensive (vasodilator) drug, is metabolized by NAT2.
- The administration of therapeutic doses of hydralazine to a slow acetylator can result in extreme hypotension and tachycardia. Several known targets
for acetylation, such as the sulphonamides, have been implicated in idiosyncratic hypersensitivity reactions; in such instances, an appreciation of a
patient’s acetylation phenotype is particularly important.
NATs Substrate Therapeutic uses Adverse effect
Acebutolol Adrenal cortex carcinoma, breast cancer Drowsiness, weakness, insomnia
Aminoglutethimide β Blockade, arrhythmias, hypertension Clumsiness, nausea, dizziness, agranulocytosis
Amino salicylic acid Ulcerative colitis Allergic fever, itching, leukopenia
Amrinone Positive inotrope in heart failure Thrombocytopenia, arrhythmias
Benzocaine Local anaesthesia Dermatitis, itching, rash, methemoglobinemia
Caffeine Neonatal respiratory distress syndrome Dizziness, insomnia, tachycardia
Clonazepam Seizures, anxiety Drowsiness, ataxia, dizziness, slurred speech
Dapsone Leprosy, dermatitis Haemolysis, methemoglobinemia, nausea, dermatitis
Hydralazine Hypertension (acts via vasodilation) Hypotension, sympathetic baroreceptor reflex effects
Isoniazid Tuberculosis Peripheral neuritis, hepatotoxicity
Nitrazepam Insomnia Dizziness, somnolence
Phenelzine Depression (acts via MAO inhibition) Dizziness, CNS excitation, insomnia, orthostatic hypotension, hepatotoxicity
Procainamide Ventricular tachyarrhythmia Hypotension, bradycardia, lupus erythematosus
Sulphonamides As bacteriostatic agents
Hypersensitivity, acute haemolytic anaemia, reversible bone marrow suppression
(with AIDS or myelosuppressive chemotherapy)

50.  Methylation:
- In humans, drugs and xenobiotics can undergo O-, N-, and S methylation. Humans express two catechol-O-methyltransferase (COMTs), three N-
methyl transferases, a phenol-O-methyltransferase (POMT), a thiopurine methyltransferase (TPMT), and a thiol methyltransferase (TMT). All of the
MTs exist as monomers and use S-adenosylmethionine (Ado Met) as the methyl donor.
- Among the N-methyl transferases, nicotinamide N-methyltransferase (NNMT) methylates serotonin and tryptophan as well as pyridine-containing
compounds such as nicotinamide and nicotine. Phenyl ethanolamine N-methyltransferase (PNMT) is responsible for the methylation of the
neurotransmitter norepinephrine to form epinephrine; the histamine N-methyltransferase (HNMT) metabolizes drugs containing an imidazole ring.
- COMT, which exists as two protein isoforms generated by alternate exon usage, methylates neurotransmitters containing a catechol moiety, such as
dopamine and norepinephrine, as well as methyldopa and ecstasy (3,4-methylenedioxymethamphetamine, MDMA).
- From a clinical perspective, the most important MT may be a thiopurine methyltransferase (TPMT), which catalyses the S-methylation of aromatic
and heterocyclic sulfhydryl compounds, including the thiopurine drugs azathioprine (AZA), 6-mercaptopurine (6-MP), and thioguanine. AZA and
6-MP are used for the management of inflammatory bowel disease, as well as autoimmune disorders such as systemic lupus erythematosus and
rheumatoid arthritis. Thioguanine is used in the treatment of acute myeloid leukaemia, and 6-MP is used worldwide for the treatment of childhood
acute lymphoblastic leukaemia.
- Examples of drugs that are metabolised by methylation are:
Adrenaline Nicotinic acid Captopril
Histamine Methyldopa Mercaptopurine
 Drug metabolising enzymes:
- The drug metabolising enzymes are divided into two types:
a- Microsomal enzymes

51. b- Non-microsomal enzyme
 Microsomal enzyme:
- These are located on smooth endoplasmic reticulum, primarily in liver, also in kidney, intestinal mucosa and lungs. The monooxygenases, cytochrome
P450, UGTs, epoxide hydrolases, etc. are microsomal enzymes. They catalyse most of the oxidations. reductions, hydrolysis and glucuronide
conjugation.
 Microsomal Enzyme Induction:
- Many drugs, insecticides and carcinogens interact with DNA and increase the synthesis of microsomal enzyme protein, especially cytochrome P-
450 and UGTs. As a result, the rate of metabolism of inducing drug itself (autoinduction) and/or some other co-administered drugs is accelerated.
- Different inducers are relatively selective for certain cytochrome P-450 isoenzyme families, e.g.:
Isoenzymes Inducers
CYP 3A Anticonvulsants (phenobarbitone. phenytoin, carbamazepine), Rifampin, glucocorticoids
CYP 2D6 and CYP 2C8/9. Phenobarbitone and Rifampin
CYP 2C l9 and CYP 1A1/2. Carbamazepine and Rifampin
CYP 2E1 lsoniazid and chronic alcohol consumption
CYP 1A Polycyclic hydrocarbons like 3-methylcholanthrene and benzopyrene found in cigarette smoke, charcoal broiled
meat, omeprazole and industrial pollutants
 Consequences of microsomal enzyme induction (clinical significance):
a- Decreased intensity and/or duration of action of drugs that are inactivated by metabolism, e.g. failure of contraception with oral contraceptives and
loss of anti-HIV action of nevirapine due to Rifampin coadministration.
b- Increased intensity of action of drugs that are activated by metabolism. Acute paracetamol toxicity is due to one of its metabolites- toxicity occurs
at lower doses in patients receiving enzyme inducers.

52. c- Tolerance- if the drug induces its own metabolism (autoinduction), e.g. carbamazepine, rifampin; nevirapine dose needs to be doubled after 2
weeks.
d- Some endogenous substrates (steroids, bilirubin) are also metabolized faster.
e- Precipitation of acute intermittent porphyria, enzyme induction increases porphyrin synthesis by derepressing delta-aminolevulinic acid
synthetase.
- Drugs whose metabolism is significantly affected by enzyme induction are- phenytoin, carbamazepine, antidepressants, warfarin, tolbutamide. oral
contraceptives. chloramphenicol. doxycycline, theophylline, griseofulvin, nevirapine.
 Possible uses of enzyme induction:
a- Congenital non haemolytic jaundice: It is due to deficient glucuronidation of bilirubin: phenobarbitone hastens clearance of jaundice.
b- Cushing's syndrome: phenytoin may reduce the manifestations by enhancing degradation of adrenal steroids which are produced in excess.
c- Chronic poisonings: by faster metabolism of the accumulated poisonous substance.
d- Liver disease.
 Non-microsomal enzymes:
- These are present in the cytoplasm and mitochondria of hepatic cells as well as in other tissues including plasma. The esterases, amidases, some
flavoprotein oxidases are non-microsomal.
- The non-microsomal enzymes are not inducible but many shows genetic polymorphism (acetyltransferase, pseudocholinesterase) similar to the
microsomal enzymes.
 Both microsomal and non-microsomal enzymes are deficient in the new born, especially premature, making them more susceptible to many drugs, e.g.
chloramphenicol, opioids.

53.  This deficit is made up in the first few months, more quickly in case of oxidation and other phase I reactions than in case of glucuronide and other
conjugations which take 3 or more months to reach adult levels.
 Hofmann elimination: This refers to inactivation of the drug in the body fluids by spontaneous molecular rearrangement without the agency of any enzyme,
e.g. atracurium.
 Inhibition of Drug Metabolism:
- Azole antifungal drug, macrolide antibiotics and some other drugs bind to the heme iron in CYP450 and inhibit the metabolism of many drugs, as well as
some endogenous substances like steroids, bilirubin.
- One drug can competitively inhibit the metabolism of another if it utilizes the same enzyme or cofactors.
- A drug may be the substrate as well as inhibitor of the same CYP isoenzyme. In case the drug inhibits its own metabolism, the oral bioavailability is likely
to increase (for high first pass metabolism drugs), while systemic clearance is decrease, prolonging plasma half-life.
- On chronic dosing, the oral bioavailability of verapamil is nearly doubled and t½ is prolonged, because it inhibits its own metabolism. A drug may also
inhibit one isoenzyme while being itself a substrate of another isoenzyme, e.g. quinidine is metabolized mainly by CYP3A4 but inhibits CYP2D6.
- Clinically significant inhibition of drug metabolism occurs in case of drugs having affinity for the same isoenzyme, especially if they are metabolized by
saturation kinetics or if kinetics changes from first order to zero order over the therapeutic range (capacity limited metabolism).
- The ' boosted' HIV-protease inhibitor (Pl) strategy utilizes the potent CYP3A4 inhibitory action of low dose Ritonavir to lower the dose of other PIs like
atazanavir, lopinavir. saquinavir given concurrently.
 Drugs that inhibit drug metabolizing enzymes:
Allopurinol Amiodarone
Omeprazole Propoxyphene
Erythromycin lsoniazid
Clarithromycin Cimetidine
Chloramphenicol Quinidine
Ketoconazole Disulfiram

54. Ltraconazole Diltiazem
Metronidazole Verapamil
Ciprofloxacin MAO inhibitors
Fluoxetine (and other SSRls) Ritonavir and other HIV protease inhibitors)
 First-pass (Presystemic) Metabolism:
- This refers to metabolism of a drug during its passage from the site of absorption into the systemic circulation. All orally administered drugs are exposed to
drug metabolizing enzymes in the intestinal wall and liver (where they first reach through the portal vein).
- Presystemic metabolism in the gut and liver can be avoided by administering the drug through sublingual, transdermal or parenteral routes. However.
Limited presystemic metabolism can occur in the skin (transdermally administered drug) and in lungs (for drug reaching venous blood through any
route).
- The extent of first pass metabolism differs for different drugs (Table 3.2) and is an important determinant of oral bioavailability.
Extent of hepatic first pass metabolism of some important drugs
Low Intermediate
High
not given orally high oral dose
Phenobarbitone Aspirin lsoprenaline Propranolol
Phenylbutazone Quinidine Lidocaine Alprenolol
Tolbutamide Desipramine Hydrocortisone Verapamil
Theophylline Nortriptyline Testosterone Salbutamol
Pindolol Chlorpromazine Glyceryl trinitrate
Diazepam Pentazocine Morphine
lsosorbide mononitrate Metoprolol Pethidine
 Attributes of drugs with high first pass metabolism:
a- Oral dose is considerably higher than sublingual or parenteral dose.
b- There is marked individual variation in the oral do e due to differences in the extent of first pass metabolism.

55. c- Oral bioavailability is apparently increased in patients with severe liver disease.
d- Oral bioavailability of a drug is increased if another drug competing with it in first pass metabolism is given concurrently, e.g. chlorpromazine and
propranolol.
 Factors affecting drug metabolism:
- A number of factors may influence the metabolic rate of a drug. Some of them are:
a- Chemical factors:
i. Enzyme induction
ii. Enzyme inhibition
iii. Environmental chemicals
b- Biological factors:
i. Age
ii. Diet
iii. Sex difference
iv. Species difference
v. Strain difference
vi. Altered physiological factors
c- Physicochemical properties of the drug

56.  Chemical Factors:
A- Enzyme induction:
- The phenomenon of increased drug metabolizing ability of enzymes by several drugs and chemicals is called as enzyme induction and the agents
which bring about such an effect are called enzyme inducers.
- Mechanisms of enzyme induction:
i. Increase in both liver size and liver blood flow
ii. Increase in both total and microsomal protein content
iii. Increased stability of enzymes
iv. Increased stability of cytochrome P450
v. Decreased degradation of cytochrome P450
vi. Proliferation of smooth endoplasmic reticulum
- Consequences of enzyme induction include:
i. Decrease in pharmacological activity of drugs
ii. Increased activity where metabolites are active
iii. Altered physiological status due to enhanced metabolism of endogenous compounds such as sex hormones.
iv. Decreased intensity and/or duration of action of drugs that are inactivated by metabolism, e.g. failure of contraception with oral contraceptives
and loss of anti-HIV action of nevirapine due to Rifampin coadministration.
v. Increased intensity of action of drugs that are activated by metabolism. Acute paracetamol toxicity is due to one of its metabolites- toxicity
occurs at lower doses in patients receiving enzyme inducers.

57. Induction
(Rifampin)
vi. Tolerance- if the drug induces its own metabolism (autoinduction), e.g. carbamazepine, rifampin; nevirapine dose needs to be doubled after 2
weeks.
vii. Some endogenous substrates (steroids, bilirubin) are also metabolized faster.
viii. Precipitation of acute intermittent porphyria, enzyme induction increases porphyrin synthesis by derepressing delta-aminolevulinic acid
synthetase.
- Some examples of drug induction are:
Oral contraceptive steroids ⎯⎯⎯⎯ Inactive, Excreted
B- Enzyme inhibition:
- A decrease in the drug metabolizing ability of an enzyme is called as enzyme inhibition. The process of inhibition may be direct or indirect.
- Direct inhibition:
It may result from interaction at the enzyme site, the net outcome being a change in enzyme activity. Direct enzyme inhibition can occur by one of
the following mechanisms:
i. Competitive inhibition: it occurs when structurally similar compounds compete for the same sites on the enzyme
ii. Non-competitive inhibition: it occurs when a structurally unrelated agent interacts with the enzyme and prevents the metabolism of drugs.
iii. Product inhibition: it occurs when the metabolic product competes with the substrate for the same enzyme.
- Indirect inhibition:
It is caused by one of the following mechanisms:
i. Repression: it may be due to fall in the rate of enzyme synthesis or rise in the rate of enzyme degradation.
ii. Altered physiology: it may be due to nutritional deficiency or hormonal imbalance.

58. Inhibition
(Erythromycin, Ketoconazole)
- Some examples of enzyme inhibition are:
Terfenadine ⎯⎯⎯⎯ Active Antihistamine
Enzyme Enzyme inhibitors Drugs whose metabolism is inhibited
Hepatic Microsomal
Cimetidine Chlordiazepoxide, Diazepam, Phenytoin, Antidepressants, Theophylline,
Lidocaine, Propranolol, Quinidine, Testosterone, Warfarin
Erythromycin Theophylline, Warfarin, Cyclosporine, Carbamazepine
Sod. Valproate Phenytoin, Phenobarbital, Primidone
Quinolone Theophylline
Chloramphenicol Phenytoin, Tolbutamide, Warfarin
Verapamil, Diltiazem, Propoxyphene Theophylline, Carbamazepine, Cyclosporine, Carbamazepine.
Xanthine Oxidase Allopurinol 6-Mercaptopurine, Azathioprine
Monoamine Oxidase MAO Inhibitors Pethidine, Tricyclic Antidepressants
Aldehyde Dehydrogenase Disulfiram, Metronidazole Alcohol, Phenytoin, Some Benzodiazepines, Warfarin.
Cholinesterase Echothiopate Suxamethonium, Procaine, Propanidid.
Angiotensin Converting
Enzyme
Captopril, Enalapril Angiotensin I, Bradykinin
C- Environmental chemicals:
- Several environmental agents influence the drug metabolizing ability of enzymes. For example:
a- Halogenated pesticides such as DDT and polycyclic aromatic hydrocarbons contained in cigarette smoke have enzyme induction effect.
b- Organophosphate insecticides and heavy metals such as mercury, nickel, cobalt and arsenic inhibit drug metabolizing ability of enzymes
c- Other environmental factors that may influence drug metabolism are temperature, altitude, pressure, atmosphere, etc

59.  Biological factors:
A- Age:
- The drug metabolic rate in the different age groups differs manly due to variations in the enzyme content, enzyme activity and haemodynamic.
- In neonates (upto 2 months) and in infants (2 months to l year), the microsomal enzyme system is not fully developed So, many dugs are
metabolized slowly. For e.g., caffeine has a half-life of 4 days in neonates in comparison to 4 hrs in adults.
- Children (between 1 year and 12 years) metabolize several drugs much more rapidly than adults as the rate of metabolism reaches a maximum
somewhere between 6 months and 12 years. As a result, they require large mg/kg dose in comparison to adults.
- In elderly persons, the Liver size is reduced, the microsomal enzyme activity is decreased and hepatic blood flow also declines as a result of
reduced cardiac output, all of which contributes to decreased metabolism of drugs. For example, clomethiazole shows a high bioavailability within
the elderly, therefore they require a lower dose.
B- Diet:
- The enzyme content and activity are altered by a number of dietary components. Generally:
a- Low protein diet decreases and high protein diet increases the drug metabolizing ability as enzyme synthesis is promoted by protein diet and
also raise the level of amine acids for conjugation with drugs.
b- Fat free diet depresses cytochrome P450 levels since phospholipids, which are important components of microsomes become deficient.
c- Grapefruit inhibits metabolism of many drugs and improve their oral bioavailability.
d- Dietary deficiency of vitamins like Vitamin A, B2, B3, C and E) and minerals such as Fe, Ca, Mg, Zn retard the metabolic activity of enzymes.
e- Starvation results in decreased number of glucuronides formed than under normal conditions.

60. C- Sex difference:
- Since variations between male and female are observed following puberty. So, sex related differences in the rate of metabolism may be due to sex
hormones.
- Such sex differences are widely studied in rats where male rats have greater drug metabolizing capacity. Ln humans, women metabolize
benzodiazepines slowly than men.
- Several studies have shown that women on contraceptive pills metabolize a number of drugs at SLOW rate.
D- Altered physiological factors:
i. Pregnancy:
- Pregnancy is known to affect hepatic drug metabolism. Physiological changes during pregnancy are probably responsible for the reported
alteration in dug metabolism.
- These include elevated concentrations of various hormones such as estrogen, progesterone, placental growth hormones and prolactin. For
example: in women, metabolism of promazine and pethidine is reduced during pregnancy.
ii. Disease states:
- There are many disease states that affect the metabolism or drugs. Some of them are Cirrhosis of liver, alcoholic Liver disease, cholestatic
Jaundice, diabetes mellitus, acromegaly, malaria, various bacterial and viral infections, etc.
- The possible cause in the effect of metabolism due to diseases may be:
a- Decreased enzyme activity in liver
b- Altered hepatic blood flow
c- Hypoalbuminemia (leading to lower plasma binding of drugs).

61. iii. Hormonal imbalance:
- Higher level of one hormone may inhibit the activity of few enzymes while inducing that of others. Adrenalectomy, thyroidectomy and alloxan-
induced diabetes in animals showed impairment in the enzyme activity with subsequent fall in the rate of metabolism.
- A similar effect was also observed in the pituitary growth hormone and stress related changes in ACTH levels.
 Physicochemical properties of the drug:
- Molecular size and shape, pKa, acidity/basicity, lipophilicity and steric and electronic Characteristics of a drug influence in interaction with the active
sites of enzyme and the metabolism to which it is subjected.