1. Effect of
various environment/processing on stability of
the formulation and techniques for
stabilization of products against the same
Seminar submitted to:-
Mr.Afrasim Moin Khan,
Dept. of Pharmaceutics,
JSSCP, Mysore.
submitted by:-
T.Manoj Kumar,
Final year B.Pharm.

2. .
• All medical agents are to be investigated for their
decomposition before being marketed.
• Most drugs contain one or more functional groups and
therefore may undergo different chemical reactions.
• Ingredients present in dosage forms and environmental
factors affect the physical and chemical stability of of
drugs, such as –
–Moisture
–Heat
–Light
–Radiation etc.,

3. PHYSICAL DEGRADATION OF
PHARMACEUTICAL PRODUCTS
-preventive measures

4. Loss of volatile constituents :-
 Medicinal agents such as iodine, camphor, menthol, ethyl alcohol,
anesthetic ether, chloroform have a tendency to evaporate from the
product during storage.
 Similarly, nitroglycerine tablets may loose its potency owing to
volatilisation of the medicament.
The preventive measures include keeping the product in well-closed
containers, and storing it in cool place.
Loss of water :-
 Loss of vehicle (water) from the product leads to decrease in weight,
rises in concentration of drug and increase potency.
 Efflorescent substances, such as borax, caffeine and quinidine
sulphate, have a natural tendency to loose water.
 Products such as emulsions and semisolids exhibit cracking.
 Loss of water depends on temperature and humidity.
Preventive measures include preserving the product in a well-closed
container and storing it in a cool place.

5. Absorption of water :-
 Absorption of moisture from the atmosphere increase the weight of
the product, dilutes the dose, and decreases the potency.
 Deliquescent substances, such as calcium chloride and potassium
chloride have a natural tendency to absorb water. Gelatin capsules
will absorb moisture and become soft and sticky.
Preventive measures include storage of such products in well-closed
containers.
Crystal growth :- Fluctuations in the ambient temperature (day & night
or seasonal) cause crystal growth.
 Solutions- when temperature is lowered, the solution becomes
supersaturated. Hence, precipitation & crystal growth of the drug is
observed.
 For example, 10% W/V calcium gluconate in injection is a
supersaturated solution. But in order to reduce the risk of
crystallization, the I.P. suggests the use of calcium saccharate (More
soluble calcium salt) as a stabilizer. A part of calcium gluconate (not
more than 5%) is replaced by calcium saccharate.

6. .
 Suspensions –
 particles slowly become bigger in size and finally may form a hard
cake. These crystals, if present in the injection, may block the
hypodermic needle. These particles produce gritty texture when
applied as an ophthalmic preparation.
Preventive measures include :-
 Select suitable storage conditions to reduce fluctuations in ambient
temperature.
 Increase the viscosity of the product so that diffusion of solute
molecules onto the crystal surface will be hindered.
 Include surface active agents in formulations. These agents get
adsorbed on the surface of the crystal and inhibit the further
deposition of solute molecules.

7. Polymorphism :-
 Polymorphs exhibit significant differences in important
physicochemical properties such as solubility, dissolution rate and
melting pont.
 In general, more soluble metastable drug is employed in the
manufacture.
 For example, cortisone acetate, form II is more soluble (metastable)
and formulated as an aqueous suspension. During storage, it may be
converted into form IV (more stable form). Such changes lead to
caking of the cortisone acetate suspension.
Normally suspending agents such as methyl cellulose are added to
prevent the conversion owing to enhanced viscosity and limited
diffusion of molecules.

8. .
Colour changes :-
 Colour changes indicate some kind of chemical or photochemical
decomposition of the active ingredients, dyes or other ingredients.
 Colour-fading of dyes is a fairly common type of instability.
 Indigo carmine dye tends to fade in the presence of reducing
substances (lactose and dextrose). Tartrazine tends to fade rapidly in
the presence of additives (surface active agents) or light.
 Colour development :- aspirin tablets become pink and ascorbic acid
tablets turn yellowish brown. Adrenaline on exposure to air becomes
red.
 Preventive measures :-
 protect the product from light and air.
 Avoid using reducing substances (dextrose etc.,) as additives.
 Include UV-absorbing substances such as 2,4-dihydrobenzophenone
in the formulation.

9. CHEMICAL DECOMPOSITION OF
DRUGS
-PREVENTIVE MEASURES

10. Hydrolysis :-
the general principles that govern hydrolysis reactions are,
 Drugs with ester and amide groups react with one molecule of water
and undergo hydrolysis. Ester groups break faster than amide groups.
 Drugs are either weak acids or bases, so they may be available as
ionic forms or neutral molecules. Hydrolysis reaction between ionic
species proceeds faster than with neutral molecules.
 Hydrolysis reactions are catalysed by H+ and (OH)- ions. Hydroxyl ions
catalyse hydrolysis by about 100 to 1000 times more actively than
hydrogen ions.
These principles help in rationalising the design of formulations from stability
point of view.

11. Examples of drugs decompose by hydrolytic pathway are :-
Esters Amides
Aspirin Chloramphenicol
Procaine Ampicillin
Atropine Cephalosporins
Barbituric acids

12. Drugs that contain ester groups undergo hydrolysis to give acids and
alcohols.
Ex :- procaine undergoes hydrolysis & give P-amino benzoic acid and
diethyl amino ethanol.
For the sterilization of procaine solution, it was suggested that
autoclaving at 1200C for shorter period of time is preferred to prolonged
heating at 1000C.

13. Amides undergo hydrolysis, though at a slower rate than esters.
Ex :- chloramphenicol decomposition is influenced by acids, bases,
phosphate ions, acidic and citrate buffers.

14. Protection against hydrolysis :-
 Hydrolysis reactions are known to occur in
presence of moisture, catalytic species H+ and
(OH-).
 pretective measures should aim at eleminating the
influence of these factors on the drug.

15. Buffers :-
 drugs may be stabilized by the use of buffers.
 The PH of the solution should be adjusted so that the drug will have
maximum stability and therapeutic activity.
 In general, optimal PH will be between 3.5 and 5, since in that range
the H+ and (OH-) catalysed hydrolysis are about equal.
Example :-
 Pilocarpine is highly active in alkaline PH and highly irritating to the
eye and also decomposes rapidly.
 Therefore, to prevent hydrolysis, acidic PH has to be selected.
 Select a buffer with low buffer capacity so that when administered in
the eye, the PH gradually rises and releases the free base for drug
action.
 For this reason, boric acid buffer of a pH 5.0 with low buffer capacity
is selected.

16. Complexation :-
 hydrolysis of benzocaine in aqueous solution can be inhibited by the
addition of caffeine which forms a complex.
 As a result of complexation, the attack of catalytic species on
benzocaine may be reduced.
 The ion-dipole interactions between (OH-) or H+ ions and drug
molecules will be reduced.
 now the rate of hydrolysis depends on the amount of free
uncomplexed benzocaine present in solution.
 As the amount of caffeine increases, more and more amount of
benzocaine will be complexed. This leads to decreased hydrolysis.
 Thus, the self life of the product can be prolonged.
 Other drugs which may be stabilized by complexation are procaine,
tetracaine etc.

17. Suppression of solubility :-
 when the solubility of a drug decreases, the concentration of drug in
solution phase will be decreased. Hence, the rate of hydrolysis is
reduced. As most of the drug is in the insoluble state, only a small
faction will be in solution form. Now, the rate depends on the
saturation solubility of the drug and follows zero order reaction.
i. Additives :- citrates, dextrose, sorbitol and gluconates, when
combined with drugs, the solubility of drugs will be suppressed,
probably because of decreased hydration of drug molecules.
ii. Salts :- the degradation of penicillin can be prevented by using
poorly soluble salt of procaine penicillin in the dosage form. This
preparation results in a suspension and follows zero order.
Ex :- benzathine penicillin G.
iii. Derivatives :- poorly water soluble derivatives such as esters (higher
fatty acids) of drugs can be used to reduce the tendency of
hydrolysis.
Ex :- erythromycin propionate,
erythromycin stearate,
chloramphenicol palmitate etc.

18. Removal of water :-
 as the presence of water is responsible for hydrolysis, it is
better to avoid its contact with the drug in the preparation.
 This is achieved by –
i. Storing the drug in dry form. When desired, reconstitute the
product.
Ex :- streptomycin dry powder for injection.
ii. Using water-immiscible vehicle for the dispersion of drug.
Ex :- aspirin in silicone fluid.

19. Oxidation :-
 Oxiditation involves the removal of electron from a molecule .
 The reaction between the compounds and molecular oxygen is called
autooxidation.
 In fats and oils, autooxidation of unsaturated fatty acids proceeds in
the presence of atmospheric oxygen, light and traces of heavy metals
or organic peroxides.
Ex :- The rate of oxidation of ascorbic acid is increased by a factor of
105, when copper ions are present in the concentration of 0.002 M.
similarly hydroperoxides contained in polyethylene glycol
suppository bases have been implicated in the oxidation of codeine to
codeine-N-oxide.

20. The general principles that govern an oxidation
reaction are :-
i. The presence of atmospheric oxygen (also air) promotes the rate of
oxidation.
ii. Since oxidation frequently involves free radicals, chain reactions
occur. Light provides the necessary energy to initiate the oxidation
process.
iii. The presence of trace metals also accelerate the rate of oxidation.
iv. Organic peroxides promote the chain initiation and propagate the
oxidation reaction.
v. Drugs are either weak acids or bases. Therefore, these may be
available as ionic forms or neutral molecules. Oxidation reaction
between ionic species proceeds faster than with neutral molecules
(to a large extent it is solubility related phenomenon).
vi. Oxidation reactions are catalysed by H+ and OH- ions. Hydroxyl ions
catalyse oxidation faster than hydrogen ions. Alkaline solutions are
known to react with atmospheric oxygen and forms oxides.

21. Drugs which decompose by oxidation pathways are :-
Arachil oil Vitamin A
Ethyl oleate Riboflavin
Clove oil Vitamin B12
Cinnamon oil Ascorbic acid
Promethazine Morphine
Epinephrine prednisolone

22. The autooxidation kinetics of ascorbic acid has been
extensively studied.
The overall reaction may be represented as :-

23. Influence of trace metals :-
the scheme of oxidation of ascorbic acid by cupric ion is as follows :
Ascorbate ion in solution slow oxidation, Cu2+ semiquinone
rapid oxidation, O2 dehydro ascorbic acid
 When solutions are free from traces of copper, ascorbic acid is not
oxidized by molecular oxygen to a measurable extent, except in
alkaline solutions.
 However, even traces of copper lead to the rapid oxidation of
ascorbic acid.
 When CO and KCN are added to the above reaction mixture, they
form complexes with metal ions, and therefore, oxidation of ascorbic
acid is inhibited.
 These reactions demonstrate the influence of cupric ion on the
oxidation of ascorbic acid.

24. Influence of air on oxidation :-
 The rate of decomposition decreases when higher
concentration of ascorbic acid is used.
 It is presumed that a part of the ascorbic acid reacts with
oxygen and thus depletes free oxygen.
 When air is bubbled through the reaction mixture, the rate of
oxidation is enhanced.
 When dissolved oxygen is maintained at saturation level, the
reaction rate remains constant.
 Therefore, oxygen is responsible for the autooxidation
reactions.

25. Influence of ionic species of drugs :-
 Ascorbic acid can exists as a singly charged or doubly
charged ion.
 In the absence of copper ions, oxygen is found to react with
divalent ions at about 105 times faster compared to its reaction
with monovalent ascorbate ion.
 When copper ions are added, oxidation of singly charged
ascorbate ion alone is found to be catalysed.

26. Influence of acidic and basic ion species :-
 The acid and base catalysed oxidation on ascorbic acid
proceeds as follows. Dehydroascorbic acid (degradation
product) further degrades to give ketogulonic acid, which
inturn gives threonic acid and oxalic acid.
Ascorbic acid H, (OH) dehydro ascorbic acid
ketogulonic acid Threonic acid + oxalic acid
 In general, autooxidation proceeds more readily in alkaline
medium than in acidic solution.
 Alkaline solutions are known to react with atmospheric oxygen and
form oxides.

27. Protection against oxidation :-
 Oxidation reactions are known to occur in presence
of oxygen, trace metals, H+ and (OH-) ions.
 Protective measures should aim at eliminating the
influence of these factors on the drug.

28. Antioxidants :-
 Tocopherols are the naturally occurring antioxidants.
 Other ex :-
 butylated hydroxyl anisole (BHA) ,
 butylated hydroxyl toluene (BHT),
 propyl gallate etc,
 These are widely used in foods, cosmetics and drugs.
 These agents are act by breaking the free radical chain reactions
at the step of chain propagation.
 Most of these compounds are oil-soluble antioxidants.
 Water soluble antioxidants act by preferentially undergoing
oxidation instead of the drug itself.
Ex :- ascorbic acid.
 Compounds having –SH groups consume molecular oxygen
present in solution.
Ex :- cysteine, acetylcysteine, thioglycolic acid etc.

29. Chelating agents :-
 Addition of a chelating agent to a product will be useful when traces
of heavy metals catalyse the oxidation.
 Substances such as EDTA (ethylenediamine tetraacetic acid) citric
acid and tartaric acid form complexes with heavy metals.
 Thus, metal ions are not available to catalyse the oxidation.
ex:- addition of EDTA to the buffer system prevents the degradation
of drugs such as prednisolone and ascorbic acid.
 Another variation is that boric acid forms a one to one chelate
directly with the drug, epinephrine.
 The chelated epinephrine is far less susceptible to sulfite attack
than free epinephrine.
 Thus oxidation of epinephrine is inhibited.

30. Vehicles :-
 usually water is used as a solvent for most products.
 The replacement of water by other solvents when used in
combination with water, they have catalyzing effect on
oxidation.
 Production of hydroperoxides through these solvents is
implicated in the degradation.
 Several physicochemical properties of solvents such as
internal pressure, solubility parameter, dielectric constant and
ionic strength are correlated for the rates of a reaction.

31. • Micellar solubilisation :-
.
 surfactants such as polysorbate 80 enhance the rate of oxidation
of ascorbic acid at low concentration, but protect above its
critical micelle concentration (CMC), presumably by entrapping
the drugs in the spherical micelles.
 Sometimes, spherical micelles offer a site for surface adsorption
of catalytic ions and enhance rate of reaction.
Buffers :-
 buffer system imparts stability when oxidation is catalysed by H+
or (OH-) ions.
 Choose a buffer with appropriate PH to maintain maximum stability
of the product.

32. Environmental control measures :-
 One or more of the preventive measures are employed to stabilize
the product from oxidation.
i. Prevent the exposure to light :- light is responsible for
oxidation. The preparation is protected from the light by
employing amber coloured bottles or using appropriate
packagingbmaterial (cardboard).
Ex :- morphine sulphate injection USP is protected from light by
using amber coloured ampoules.
ii. Oxygen free environment :- oxygen enhances oxidative
degradation. Therefore, air is replaced with inert gases such as
nitrogen or carbon di oxide. Similarly, use of oxygen free solvents
in manufacture is advisable.
iii. Low temperature :- since high temperature enhances the rate
of reaction, the product is stored in a cool place.

33. Miscellaneous reactions
preventive measures

34. Isomerism :-
 some drugs often have some structural formula, but possess different
stereochemical form into another leads to inactive or less active
drugs.
Optical isomerisation :-
 in solutions, the optically active form of a drug gets converted into its
enantiomorph.
 This process continues until the two forms are equal in concentration.
The optical activity at equilibrium will be zero, i.e., optically in active.
 In general, recemisation reactions undergo degradation in
accordance with first order kinetics.
 Ex :- (-)Adrenaline (+/-) Adrenaline
greater biological activity (+ and – is 50:50) less potent
Preventive measures :- the product is protected from light and heat.
Optimum PH has to be maintained for maximum activity.

35. .
Epimerization :- In this case, the compound has more than one
asymmetric carbon atoms. While one asymmetric atom remains static,
the other carbon rotates to give an epimer. At equilibrium, both
epimers are present, but need not to be in equal proportion. In other
words, the solution may still exhibit optical activity.
Ex :- ergometrine ergometrinine
in solution less active
Geometric isomerisation :- in this case, the compounds exists as trans
and cis isomers, based on their relative spatial configuration of
groups around a double bond(s). These changes may bring about a
corresponding changes in its biological activity.
Ex :-vitamin A palmitate 6-momo-cis derivative + 2,6-di-cis derivative
more acitve less active

36. .
Polymerization :- These types of reactions are not often the initial cause of
drug decomposition. Primary decomposition products may react further
and polymerise.
Ex :- dextrose injection autoclaving 5-hydroxymethyl furfural
polymerise straw coloured solution
Absorption of carbon dioxide :- solutions absorb carbon dioxide from the
atmosphere.
Ex :- sodium hexobarbitone iv injection salt hydrolysis solution basic PH
absorb CO2 acidic PH hexobarbitone precipitate
Preventive measures :- the product is stored in well-filled and well-closed
containers. Manufacturers supply the product as a dry sterile powder. The
instruction should be to dissolve the drug before use in carbon dioxide free
sterile water for injection.

37. .
Decarboxylation :- These type of reactions are normally observed when a
parentral solution contains sodium carbonate. During autoclaving, the
carboxylic acid groups will be kocked off.
Ex :- sodium p-aminosalicylic acid (PAS, Anti-TB drug),
procaine hydrochloride (local anesthetic)
Procain (clear solution) hydrolysis p-amino benzoic acid
-CO2 aniline liquid light drak coloured liquid
Preventive measures :- carbon dioxide gas is passed into the solution
for one minute. The container is sealed so as to be air-tigght prior to
autoclaving.