Mechanism of drug action

1. MECHANISM OF DRUG ACTION
Name  Anurag Sarao
MSc Pharmacology

2. Pharmacodynamics
• It is the study of drug effects.
• It starts with describing what the drugs do, and goes on to explain
how they do it.

3. Pharmacodynamic Concepts
• Pharmacodynamics is the study of the:
Biochemical
Cellular
 Physiological effects of drugs and their mechanisms of action.
• The term drug receptor or drug target denotes the cellular
macromolecule or macromolecular complex with which the drug
interacts to elicit a cellular or systemic response.

4. PRINCIPLES OF DRUG ACTION
• Drugs do not impart new functions to any system, organ or cell; they
only alter the pace of ongoing activity.
• The basic types of drug action can be broadly classed as:
1. Stimulation It refers to selective enhancement of the level of
activity of specialized cells, e.g. adrenaline stimulates heart.
2. Depression It means selective diminution of activity of specialized
cells, e.g. barbiturates depress CNS

5. 3. Irritation This connotes a nonselective, often noxious effect and is
particularly applied to less specialized cells. Strong irritation results in
inflammation, corrosion, necrosis and morphological damage. This may
result in diminution or loss of function.
4. Replacement This refers to the use of natural metabolites,
hormones or their congeners in deficiency states, e.g. levodopa in
parkinsonism, insulin in diabetes mellitus, iron in anaemia.
5. Cytotoxic action Selective cytotoxic action on invading parasites
or cancer cells, attenuating them without significantly affecting the
host cells is utilized for cure of infections and neoplasms, e.g.
cyclophosphamide, etc.

6. The Site Of Drug Action
• Extracellular site of action For example, antacids neutralising
gastric acidity; chelating agents forming complexes with heavy metals;
or magnesium sulfate acting as osmotic purgative by retaining the
fluid inside the lumen of intestine and thus increasing the faecal bulk.
• Cellular site of action For example, action of acetylcholine on
nicotinic receptors of motor end plate, leading to contraction of
skeletal muscle; or inhibition of membrane bound ATPase by cardiac
glycosides

7. • Intracellular site of action For example, trimethoprim or sulfa drugs
act by interfering with the synthesis of folic aid which is an
intracellular component.

8. Physiological Receptors
• Many drug receptors are proteins that normally serve as receptors for
endogenous regulatory ligands. These drug targets are termed
physiological receptors.
• Drugs that block or reduce the action of an agonist are termed
antagonists.
• Antagonism generally results from competition with an agonist for
the same or overlapping site on the receptor but can also occur by
interacting with other sites on the receptor.

9. The overall drug effect is attributed to the
following two factors
• Affinity It means the capability of a drug to form the complex with
its receptor (DR complex), e.g. the key entering the key hole of the
lock has got an affinity to its levers.
• Intrinsic activity (IA) or Efficacy It means the ability of a drug to
trigger the pharmacological response after making the drug receptor
complex.

10. On the basis of affinity and efficacy, the drugs
can be broadly classified as:
• Agonist These have both the high affinity as well as high intrinsic
activity (IA=1) & therefore can trigger the maximal biological response
after combining with the receptor, e.g., methacholine is a
cholinomimetic drug (agonist) which mimics the effect of
acetylcholine on cholinergic receptors.
• Antagonist These have only the affinity but no intrinsic activity
(IA=0). These drugs bind to receptor but do not mimic, rather block or
interfere with, binding of an endogenous agonist. For example,
atropine blocks the effect of acetylcholine on the cholinergic
muscarinic receptor.

11. • Partial agonist these have full affinity to the receptor but with low
intrinsic activity (IA=0 to 1) & hence these are only partly as effective
as agonist. For example, pentazocine ( a narcotic analgesic) is a partial
agonist at the mu receptor.
• Inverse agonist (negative antagonist) these have full affinity
towards the receptor, but their intrinsic activity ranges between
0 to -1. For example, B-carbolines act as inverse agonists at
benzodiazepine receptor and produce effects like, anxiety, awakening
etc which are opposite of the effect of benzodiazepines.

13. Quantifying Agonism
M
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14. Additivity and Synergism: Isobolograms

15. • RECEPTORS It is defined as a macromolecule or binding site located
on the surface or inside the cell that serves to recognize the signal
molecule/drug and initiate the response to it, but itself has no other
function.
• TRANSDUCER MECHANISMS multistep processes that provide for
amplification and integration of concurrently received extra- and
intra-cellular signals at each step.

16. TYPES OF RECEPTOR

17. G Protein–Coupled Receptors
• The GPCRs comprise a large family of transmembrane receptors that
span the plasma membrane as a bundle of seven α helices.
• ligands – NTs - ACh, biogenic amines such as NE, all eicosanoids and
other lipid-signaling molecules, peptide hormones, opioids, amino
acids such as GABA, and many other
• important regulators of nerve activity in the CNS

18. • These are membrane bound receptor which are coupled to the
effector system through GTP binding proteins called G-protein.
• G-protein are bound to the inner (cytoplasmic) face of plasma
membrane. These are heterotrimeric molecules, i.e., having three
subunits designated as alpha, beta, delta units.
• In the resting state GDP is bound to the alpha subunit of alpha, beta,
delta trimer. When the receptor is activated by the binding of an
agonist, a conformational change occurs causing it to acquire a high
affinity for alpha, beta, delta trimer unit of G-protein

19. • During these process GDP dissociates from alpha, beta, delta subunit
while GTP associates in its place (GDP-GTP exchange).
• Binding of GTP activates the alpha subunit & ultimately dissociates as
alpha-GTP unit & remaining beta, delta subunit.
• The free alpha-GTP subunit then activates the effectors protein. This
is called as active state.
• The GTPase of alpha the alpha subunit quickly hydrolyses GTP to GDP
& resultant the trimer is formed again.

20. GPCR Subtypes
• There are multiple receptor subtypes within families of receptors.
• Ligand-binding studies initially identified receptor subtypes.
• The distinction between classes and subtypes of receptors, however, is
often arbitrary or historical.
• The α1, α2, and β adrenergic receptors differ from each other both in
ligand selectivity and in coupling to G proteins (Gq, Gi, and Gs,
respectively), yet α and β are considered receptor classes and α1 and α2
are considered subtypes

21. • Pharmacological differences amongst receptor subtypes are exploited
therapeutically through the development and use of receptor-
selective drugs.
• For example
β receptor
α receptor

22. GPCR

24. Second-Messenger Systems
• Cyclic AMP => cAMP is synthesized by the enzyme AC; stimulation is
mediated by the Gsα subunit, inhibition by the Giα subunit.
• In addition, cAMP directly opens a specific type of membrane Ca2+ channel
called cyclic nucleotide gated channel (CNG) in the heart, brain and kidney.
• The other mediators of cellular actions of cAMP are: cAMP response
element binding protein (CREB) which is a transcription factor, cAMP
regulated guanine nucleotide exchange factors called EPACs.
• The action of cAMP is terminated intracellularly by phosphodiesterase's
(PDEs) which hydrolyse it to 5-AMP.

25. • PKA => The PKA holoenzyme consists of two catalytic (C) subunits
reversibly bound to a regulatory (R) subunit dimer to form a
heterotetrameric complex (R2C2).
• When AC is activated and cAMP concentrations increase, four cAMP
molecules bind to the R2C2 complex, two to each R subunit, causing a
conformational change in the R subunits that lowers their affinity for
the C subunits, resulting in their activation.
• The active C subunits phosphorylate serine and threonine residues on
specific protein substrates.

26. • PKG=> Stimulation of receptors that raise intracellular cGMP
concentrations leads to the activation of the cGMP-dependent PKG
that phosphorylates some of the same substrates as PKA.
• Protein kinase G exists in two homologous forms, PKG-I and PKG-II.
PKG-I has an acetylated N terminus, is associated with the cytoplasm
and has two isoforms (Iα and Iβ) that arise from alternate splicing.
• PKG-II has a myristylated N-terminus, is membrane-associated, and can
be localized by PKG-anchoring proteins in a manner analogous to that
for PKA

27. • PDEs=> PDEs hydrolyze the cyclic 3′,5′-phosphodiester bond in cAMP and
cGMP, thereby terminating their action.
• The PDEs comprise a superfamily with more than 50 different proteins.
• The substrate specificities of the different PDEs include those specific for
cAMP hydrolysis and for cGMP hydrolysis and some that hydrolyze both
cyclic nucleotides.
• PDEs (mainly PDE3 forms) are drug targets for treatment of diseases such
as asthma, cardiovascular diseases such as heart failure, atherosclerotic
coronary and peripheral arterial disease, and neurological disorders.
• PDE5 inhibitors (e.g., sildenafil) are used in treating chronic obstructive
pulmonary disease and erectile dysfunction

28. • EPACs=>EPAC serves as a cAMP-regulated GEF for the family of small
Ras GTPases (especially the Rap small GTPases), catalyzing the
exchange of GTP for GDP, thus activating the small GTPase by
promoting formation of the GTP-bound form .
• Two isoforms of EPAC are known, EPAC1 and EPAC2; they differ in
their architecture and tissue expression.
• EPAC2 can promote incretin-stimulated insulin secretion from
pancreatic β cells.

29. • Gq-PLC-DAG/IP3-Ca2+ Pathway=> Calcium is an important messenger in
all cells.
• Ca2+ can enter the cell through Ca2+ channels in the plasma membrane or
be released by hormones or growth factors from intracellular stores.
• In keeping with its role as a signal, the basal Ca2+ level in cells is
maintained in the 100-nM range by membrane Ca2+ pumps that extrude
Ca2+ to the extracellular space and a SERCA in the membrane of the ER
that accumulates Ca2+ into its storage site in the ER.
• Hormones and growth factors release Ca2+ from its intracellular storage
site, the ER, via a signaling pathway that begins with activation of PLC, of
which there are two primary forms, PLCβ and PLCγ.

30. • GPCRs that couple to Gq or Gi activate PLCβ by activating the Gα
subunit and releasing the βγ dimer. Both the active, Gq-GTP–bound α
subunit and the βγ dimer can activate certain isoforms of PLCβ.
• PLCγ isoforms are activated by tyrosine phosphorylation, including
phosphorylation by receptor.
• The PLCs are cytosolic enzymes that translocate to the plasma
membrane on receptor stimulation. When activated, they hydrolyze a
minor membrane phospholipid, phosphatidylinositol-4,5-
bisphosphate, to generate two intracellular signals, IP3 and the lipid
DAG.

31. • DAG directly activates some members of the PKC family. IP3 diffuses
to the ER, where it activates the IP3 receptor in the ER membrane,
causing release of stored Ca2+ from the ER.
• Release of Ca2+ from these intracellular stores raises Ca2+ levels in
the cytoplasm many-fold within seconds and activates Ca2+-
dependent enzymes such as some of the PKCs and Ca2+/calmodulin-
sensitive enzymes such as one of the cAMP-hydrolyzing PDEs and a
family of Ca2+/calmodulin-sensitive PKs.

32. Ionotropic Receptors
• These receptors are localised on the cell membrane and are coupled
directly to the ion channel.
• These “agonist regulated ion channels” (known as ligand-gated ion
channels) open only when the receptor is occupied by an agonist.
• The subsequent flow of ions through these channels can elicit cellular
response in form of depolarisation or hyperpolarisation of cell
membrane.

33. Examples include
• Nicotinic-cholinergic receptor, GABAa receptor, glutamate receptor. Some
drugs are allosteric modifiers of channel gating, e.g., BZD allosterically
enhance the chloride transport through GABAa Cl channel.
• Other drugs the ion channel physically, e.g., blocking action of local
anaesthetics on voltage gated Na+ channel.

34. Agonist
 Antagonist

35. Ion Channels
• Changes in the flux of ions across the plasma membrane are critical
regulatory events in both excitable and nonexcitable cells.
• To establish the electrochemical gradients required to maintain a
membrane potential, all cells express ion transporters for Na+, K+, Ca2+,
and Cl–.
• The ions are move the antiport and symport process.
• They can also be classified as voltage-activated, ligand-activated and
temperature activated channels.

36. Voltage-Gated Channels
• Humans express multiple isoforms of voltage- gated channels for Na+, K+,
Ca2+, and Cl– ions.
• In nerve and muscle cells, voltage-gated Na+ channels are responsible for
the generation of action potentials that depolarize the membrane from its
resting potential of –70 mV up to a potential of +20 mV within a few
milliseconds.
• Na+ channels are composed of three subunits, a pore-forming α subunit
and two regulatory β subunits.

38. • Voltage-gated Ca2+ channels have a similar architecture to voltage-
gated Na+ channels.
• Ca2+ channels can be responsible for initiating an action potential (as
in the pacemaker cells of the heart) but are more commonly
responsible for modifying the shape and duration of an action
potential initiated by fast voltage-gated Na+ channels.
• These channels initiate the influx of Ca2+ that stimulates the release
of neurotransmitters in the central, enteric, and autonomic nervous
systems and that control heart rate and impulse conduction in cardiac
tissue

40. Ligand-Gated Channels
• Channels activated by the binding of a ligand to a specific site in the
channel protein.
• Major ligand-gated channels in the nervous system are those that
respond to excitatory neurotransmitters such as ACh or glutamate (or
agonists such as AMPA and NMDA) and inhibitory neurotransmitters such
as glycine or GABA.
• The nicotinic ACh receptor is an example of a ligand gated ion channel.

41. • The pentameric channel consists
of four different subunits (2α, β,
δ, γ) in the neuromuscular
junction.
• Each α subunit has an identical
ACh binding site.
• By blocking binding domain side,
this property is exploited to
provide muscle relaxation during
surgery

42. Transient Receptor Potential Channels
• The TRP cation channels are involved in a variety of physiological and
pathophysiological sensory processes, including heat and cold
sensation, sensation of chemicals such as capsaicin and menthol.
• The typical TRP channel structure consists of monomers predicted to
have six transmembrane helices (S1–S6) with a pore-forming loop
between S5 and S6 and large intracellular regions at the amino and
carboxyl termini.
• Agonists and antagonists are being developed and are in clinical trials
for a wide variety of indications, including pain, gastroesophageal reflux
disorder, respiratory disorders, osteoarthritis, skin disorders, and
overactive bladder.

43. Transmembrane Receptors Linked to Intracellular Enzymes
Receptor Tyrosine Kinases

44. Jak-STAT Receptor Pathway

45. Receptor Serine-Threonine Kinases
• Protein ligands such as TGF-β activate a family of receptors that are analogous to the receptor
tyrosine kinases except that they have a serine-threonine kinase domain in the cytoplasmic
region of the protein.
• In the basal state, these proteins exist as monomers; upon binding an agonist ligand, they
dimerize, leading to phosphorylation of the kinase domain of the type I monomer, which activates
the receptor.
• The activated receptor then phosphorylates a gene regulatory protein termed a Smad.
• Once phosphorylated by the activated receptor on a serine residue, Smad dissociates from the
receptor, migrates to the nucleus, associates with transcription.

46. Toll-like Receptors
• In a single polypeptide chain, these receptors contain a large
extracellular LBD, a short membrane-spanning domain, and a
cytoplasmic region termed the TIR domain that lacks intrinsic
enzymatic activity.
• Ligands for TLRs comprise a multitude of pathogen products, including
lipids, peptidoglycans, lipopeptides, and viruses. Activation of TLRs
produces an inflammatory response to the pathogenic
microorganisms.

47. • The first step in activation of TLRs by ligands is dimerization, which in
turn causes signaling proteins to bind to the receptor to form a
signaling complex.
• Ligand-induced dimerization recruits adaptor proteins.,MyD88
(myeloid differentiation protein 88) to the intracellular TIR domain;
these proteins in turn recruit the IRAKs (interleukin-1 receptor-
associated kinase).
• The IRAKs auto phosphorylate in the complex and subsequently form
a more stable complex with MyD88.

48. • The phosphorylation event also recruits TRAF6 (TNF receptor–
associated factor) to the complex, which facilitates interaction with a
ubiquitin ligase that attaches a polyubiquitin molecule to TRAF6.
• This complex can now interact with TAK1 (transforming growth factor
β–activated kinase 1) and the adaptor protein TAB1. TAK1 is a
member of the MAPK family, which activates the NF-κB kinases.
• Phosphorylation of the NF-κB (nuclear factor kappa B) transcription
factors causes their translocation to the nucleus and transcriptional
activation of a variety of inflammatory genes.

49. Receptors That Stimulate Synthesis of cGMP

50. In vascular smooth muscle, activation of PKG leads to vasodilation by
• Inhibiting IP3-mediated Ca2+ release from intracellular stores.
• Phosphorylating voltage-gated Ca2+ channels to inhibit Ca2+ influx.
• Phosphorylating and opening the Ca2+-activated K+ channel, leading
to hyperpolarization of the cell membrane, which closes L-type Ca2+
channels and reduces the flux of Ca2+ into the cell.

51. Nuclear Hormone Receptors

52. Non-receptor Mediated Mechanism
• Not all drugs actions are mediated by receptors. They may act by a
chemical action or by physical action or through other modes .

53. By chemical action
• Neutralisation antacids act by neutralising gastric hyperacidity.
• Chelation some drugs trap the heavy metals (Pb, Hg, Ca, Cu & Fe) in
their ring structure & form water soluble complexes which are then finally
excreted. E.g-> EDTA (chelates PB++), dimercaprol (chelates Hg++) ,
penicillamine (chelates Cu++), deferoxamine (chelates iron). All these drugs
are used to treat heavy-metal poisoning.
• Ion Exchanger For ex, anion exchange resin like cholysteramine exchange
Cl- ions from the bile salts. The resultant complex is not absorbed & is
excreted out.

54. By Physical action
• Osmosis Magnesium sulphate acts as a purgative by exerting osmotic
effect with in lumen of the intestine.
• Adsorption kaolin adsorbs bacterial toxins & thus acts as an
antidiarrheal agent.
• Demulcents these drugs coat the inflamed mucus membrane & provide
a soothing effect, e.g., pectin, vasaka.
• Astringents they precipitate & denature the mucosal proteins & thus
protect mucosa, e.g., tannic acid

55. • Saturation in the Biophase for example, general anaesthetics
simply saturate the cellular sites (called the biophase) of CNS.
• They get packed in between the membrane lipids and thus hinder
some metabolic functions or disrupt the membrane organisation.

56. By Counterfeit or False Incorporation Mechanism
• For ex Sulfa drugs & antineoplastic drugs, like methotrexate act by
this mechanism.
• Bacteria synthesise their own folic acid from PABA, for their growth
and development.
• Sulfa drugs resemble PABA in their chemical configuration & therefore
falsely enter into the synthetic process in place of PABA.

57. • The folic acid derivative now formed contains a sulfa drug moiety in
place of PABA & is therefore, non-functional & is no utility for
bacterial growth & development.
• As a result, the bacteria get deprived of the required folate & their
growth ceases (bacteriostatic action).

58. Through Formation of Antibodies
• Some drugs like vaccines produce their effects by inducing the
formation of antibodies & thus stimulate the defence mechanisms of
the body, e.g, vaccines against small pox & cholera (providing active
immunity ) & antisera against tetanus & diphtheria (providing passive
immunity).

59. Through Placebo Action
• It is pharmacodynamically inert & harmless substance which is
sometimes given to the patient in dosage form which resembles the
actual medicament exactly in size, shape, colour, smell & weight.
• It is generally used in psychological patient with the problems like
anxiety, headache, pain, insomnia, tremors & lack of appetite etc.
• Such a patient is called placebo reactor & the drug is called as
placebo.

60. • Usually starch or lactose are used as placebo in solid dosage forms.
• Placebo can bring relief in subjective symptoms only (mainly
psychogenic manifestations ) but not in objective responses , i.e., it
can not increase or decrease eosinophils or neutrophils etc.

61. Nocebo
• It is the converse of placebo, and refers to negative psychodynamic
effect evoked by the pessimistic attitude of the patient, or by loss of
faith in the medication and/or the physician. Nocebo effect can
oppose the therapeutic effect of active medication.

62. By Target Specific Genetic Changes
• The knowledge of altered gene function in cancer cells has allowed
the dream-designing of novel anticancer drugs that specifically target
these genetic changes. These include:
• 1) The inhibitors of ras-modifying-enzyme farnesyl transferase that
reverse the malignant transformation in cancer cells containing ras
oncogene.
• 2) The inhibitors of specific tyrosine kinase that block the activity of
oncogenes

63. Receptor desensitisation
• Receptor mediated responses to drugs & hormones often
‘desensitise’ with time.
• After reaching an initial high level, the response gradually diminishes
over sec or min even in the continuing presence of agonist. It is
usually a reversible process.
• There is evidence that the desensitisation is caused by the slow
conformational change in the receptor resulting in the tight binding of
agonist molecule.

64. Up- and Down-regulation of receptors
• Prolonged exposure to high concentration of agonist cause a
reduction in the number of receptor available for activation (down
regulation of the receptor).
• On the other hand, prolonged occupation of receptors by a blocker
(antagonist) leads to increase in the number of the receptor (up-
regulation) with subsequent increase in receptor sensitivity.

65. REFERENCES
Goodman & Gillman's The Pharmacological Basis Of Therapeutics 13th
edition.
 Bertram G. Katzung ,Basic and Clinical Pharmacology 14th Edition.
Rang & Dale's Pharmacology 8th Edition.
Sharma KK Pharmacology Principle 2nd Edition.

66. Thank you