Stability Of Drugs:Introduction
From Pharmpedia
Dear Students,
you all must have noticed in daily life, that all objects suffer from what we may call a ‘wear and tear’. All living beings go through a cycle of birth- growth-reproduction and death. All nonliving beings that are generated or grown or manufactured go through a life span in which they influence and are influenced by their environment. Metals corrode and wood is decomposed by the environment. The environment is spoiled by the presence of some chemicals such as benzene, carbon-di-oxide and methane. So what we may call as a material- environment interaction influences the longevity and purity of all items.
Food as we all know has to be well preserved if we want it to retain its quality. If raw materials such as rice and wheat grains
are not stored at the proper temperature and humidity they are spoiled by microbial contamination. Food is spoiled by three
varieties of decomposition; physical, chemical and microbiological. Have you noticed that rice grains left alone stay fine for
years together; rice made into flour is less stable (with time it develops a musty odor and off color) and rice processed into
food is stable only for a few hours. My point is “more processing- less stable”; “more contact with water – less stable”.
All this is true for drugs also. Pure drugs, solids, liquids, or gases are usually more stable
than their formulations. When they are formulated into medicines decomposition happens
faster because of the presence of excipients, and moisture and because of processing..
So when we say stability, we actually are talking about two
varieties of stability; that of the drug and that of the formulation. For example a capsule may become soft, an emulsion may
break, a suspension may cake and an ointment may bleed.
Even if solid pure powders of drugs are stored under ideal temperature and humidity conditions –even then there is some degradation. For example
consider aspirin. If you take a bottle of an old sample of pure aspirin and smell it you
can clearly feel the unmistakable odor of acetic acid. If you granulate aspirin and make it into tablets the rate of decomposition will be faster and if you formulate it into a suspension it will decompose totally
into acetic acid and salicylic acid in less than 25 days. This is because the breakdown is a chemical reaction involving the
collisions of the molecules, collisions having sufficient energy and the molecules having the proper orientation. This breakdown
is affected by various factors such as presence of oxygen, acidity, alkalinity, moisture and light. This breakdown is what we are
calling degradation. This is the reason why the
manufacturers of medicines are bound by rules to put an expiry date on the medicine. In fact if they can not put an expiry date
then they have to explain reasons for that.
So the researchers in charge of the formulation development do extensive studies to understand the mechanism of degradation and
the rate of degradation. Degradation happens because the molecules are hitting one another (like agitated football players in a
big field running madly without looking and hitting one another).But there is a method in this madness! And it is a
pharmaceutical technologist’s job to find the factors that enhance or reduce this madness (light? Heat? Air? Acidity?
Alkalinity?). And it is a pharmaceutical chemistry man’s job to find the mechanism of this madness- how is it triggered and how
is it happening? Oxidation? Hydrolysis? Racemisation? Photolysis?
So then dear students, I would like to bring your attention on to three different angles of stability, physical, chemical and
microbiological. It is imperative that the medicine that we are giving, is stable in its entirety from every angle throughout its
shelf life period. Often for a new formulation many batches go into the market and a lot of time elapses before all angles of the
stability of the product are understood.
The study of the changes that happen in a dosage form as it stands on the shelf of a drug store constitute the contents of the
chapter titled as “Drug Stability”. Physical stability implies that the formulation is totally unchanged throughout it shelf life and has not suffered any changes by way of
appearance, organoleptic properties, hardness, brittleness, particle size etc. The drug release nature (rate and mechanism)
should not be altered. Different formulations suffer from different physical challenges and are tested for different stability
criteria. Drug release is important from safety and efficacy angle and the physical stability of the formulation is significant
because of its effect on pharmaceutical elegance and on drug content uniformity and drug release rate. Chemical stability implies
the lack of any decomposition in the chemical moiety that is incorporated in the formulation as the drug. Chemicals present in
the formulation as preservatives or as other excipients may also decompose and their decomposition may influence the physical and
chemical stability of the drug. But to test for the chemical stability of a formulation we test for the drug content of the
formulation. When we say microbiological stability, we imply that the formulation has not suffered from any microbiological
attack and is meeting the standards with respect to lack of contamination/sterility which we have claimed when we kept the
product on the shelf.
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