Introduction
It is important to study the stability of pharmaceutical products and to know stability testing techniques for three main reasons:
- From the point of view of safety of the patient. It is important that the patient receives uniform dosing of the prescribed drug throughout its shelf-life and particularly throughout the duration of treatment. Again although most have been shown to be safe
to use from various kinetic studies, the decomposition products may not be so safe to use. So it’s the responsibility of the drug manufacturer to at least minimize, if not totally prevent the decomposition of the products. This is of particular significance with
parenteral products, since injections involve; a greater risk than other forms of drug administration. - Legal requirements must consider the acceptable identity, strength, purity and quality of the drug product as specified in the official manuals and good manufacturing practice
(GMP). - Quite apart from conforming with legal requirements, it makes economic sense for a manufacturer to prevent marketing of unsafe drugs. The sale of such unstable product is hardly the best of advertisement for a manufacturer and the withdrawal and subsequent reformulation of the drug may result in considerable financial loss.
Chemical Degradation of Pharmaceutical Products
Drug products differ considerably in their composition, so naturally they are subject to
different forms of chemical decomposition and in addition there may be several
simultaneous decomposition reactions occurring in a product. Some drugs are known to undergo isomerization, oxidation and hydrolysis resulting in their rapid deterioration and production of diverse decomposition products. It is needful then to establish reliable methods of reducing or even eliminating the causes of instability in such drug products.
Types of Chemical Degradation of Pharmaceutical Products (Drugs)
The major types of chemical degradation of drugs are:
1. Hydrolysis
This is the major cause of drug instability especially for aqueous drug solutions. Hydrolysis may be defined as the reaction of a compound with water and one may distinguish between ionic and molecular forms of hydrolysis. Ionic hydrolysis occurs when the salts of weak acid (e.g. potassium acetate, and bases e.g. codeine phosphate), interact with water to give alkaline and acidic solutions respectively. It is in an instantaneous equilibrium process. On
the other hand, molecular hydrolysis involves cleavage of the drug molecule and is a much slower, irreversible process. This latter form of hydrolysis is mainly responsible for the decomposition of pharmaceutical products. For example, esters, amides are subject to hydrolysis. Examples of Esters are the local anaesthetics arnethocaine and benzocaine and amides are the sulphonamides and nitriles. This form of hydrolytic reaction is frequently catalysed by H or OH ions. The rate of decomposition is critically dependent upon the pH of the system.
In many other cases, however, hydrolysis may be catalysed by acidic and basic species other than the hydrogen and hydroxyl ions.
The drug molecule undergoing hydrolysis may carry a charge or be uncharged. From the preamble it is evident that a drug may undergo several simultaneous catalytic reactions all of which are hydrolytic in nature. A good example is the decomposition of aspirin into salicylic acid and acetic acids. Over the pH range of 1 to 12 it has been predicted that six simultaneous reactions occur.
Although hydrolysis will occur principally with drugs in aqueous solution, and suspensions but solid dosage forms are also susceptible to hydrolytic decomposition. It has been shown
(Leeson and Mattocks, 1958) that moisture is rapidly absorbed on to the surface of aspirin particles causing solution of a portion of the drug in the water layer around the particles. As the aspirin in solution hydrolyses, more of the solid material dissolves and decomposition continues.
2. Oxidation
The decomposition of pharmaceutical preparations due to oxidation is nearly of equal occurrence as that due to hydrolysis. Morphine, adrenaline, fixed oils and fats, oil-soluble and water-soluble vitamins, volatile oils and phenols are some of the examples of drug products that exhibit oxidative decomposition.
Oxidation and reduction involve the loss and gain of electrons, respectively. Many oxidative reactions result from the presence of atmospheric oxygen but the required loss of electrons may sometimes occur even when oxygen is absent. For examples in reactions between oxidizing and reducing agents, however, the decomposition of medicinal compounds usually involves molecular oxygen. Such oxidations are usually termed auto-oxidations because they occur spontaneously under normal conditions and often involve free radicals.
Free radicals which contain one or more unpaired electrons, are particularly reactive and the products of their reactions are often free radicals themselves, hence chain reactions are initiated and proceed until the remaining free radicals are destroyed or rendered less active.
Thus auto-oxidation comprises three distinct steps:
- (i) Initiation
- ii) The propagation step
- (iii) Termination step
(i) Initiation
An organic compound (RH) is converted into an active free radical (R during this step, as a result of the influence of some factor such as heat, light, presence of trace metals or other free radicals.
(ii) The propagation step
This is the chain reaction during which the free radicals absorb a molecule of oxygen to form a peroxy radical (ROO ). The peroxy radical then abstracts hydrogen from another molecule of RH to form a hydroperoxide (ROOH) and a new free radical which then will absorb a molecule of oxygen and thus continue the reaction
The primary products of auto-oxidation are the odourless and tasteless hydroperoxides.
They however breakdown to yield aldehydes, ketones and short-chain fatty acids that are
responsible for the rancid odour of the oil or fat.
In theory, the propagation step can continue until either all of the organic compound or the oxygen has been consumed.
(iii) Termination step
However, in practice, a termination step intervenes since certain of the free radicals can combine to form inactive products which break the chain reaction
The above chain termination comprises:
- (a) Self termination which involves reaction between two free radicals and the production of inactive products (non-free radicals).
- (b) Chain Breaking Termination which involves reaction between free radicals and compounds that are known as chain inhibitors and results in the formation of stable and comparatively unreactive free radicals.
3. Isomerisation
Isomerisation means the conversion of an active drug into a less active, or inactive isomer having the same structural formula but differing in stereochemical configuration. One may distinguish between optical isomerization involving optically active compounds containing one or more asymmetric carbon atoms, and geometrical isomerization which refers to
changes in the relative spatial configuration of groupings around a double bond or bonds in a molecule.
Racemization in solution involves the conversion of an optically active form of a drug to a less pharmacologically active form or enantiomorphs.
4. Polymerization
This involves the combination of two or more identical molecules to form a much larger and more complex molecule. It can lead to degradation of pharmaceutical products.
Polymerization reaction may not be the initial cause of drug degradation but does occur as a further degradation process of primary decomposition products. However, polymerization is the prime cause of degradation of the antiseptic, formuladehyde. On standing, particularly in
the cold, the aldehyde forms paraformaldehyde which is precipitated out of solution as white deposit. To prevent this 10 to 15% of methyl alcohol is added to the aldehyde as a stabilizer. Adrenochrome, the primary oxidation product of adrenaline in acid solution undergoes further oxidation and is finally converted into black and brown polymeric pigments.
5. Decarboxylation
Decarboxylation is the removal of carbon dioxide (C02) from a compound.
Decarboxylation is commonly encountered when autoclaving parenteral solutions of sodium bicarbonate. In order to minimize the decomposition of sodium bicarbonate, carbon dioxide is passed into the solution for one minute and the containers are sealed so as to be gas-tight prior to autoclaving. Decarboxylation also occurs when autoclaving sodium aminosalicylate (a tuberculostatic agent).
6. Absorption of Carbon Dioxide
The absorption of carbon dioxide from the atmosphere by a pharmaceutical product is a more frequent occurrence than the loss of carbon dioxide by decarboxylation. Solutions of potassium hydroxide, calcium hydroxide, sodium hydroxide and lead subacetate become turbid due to the formation of insoluble carbonates.
Magnesium oxide and two volatile nasal
decongestants (amphetamine, propyihexedrine) also absorb carbon dioxide from the atmosphere.
More importantly is the effect that carbon dioxide has upon the stability of solutions of the sodium salts of bicarbonates. For example, the short-acting intravenous barbiturate sodium hexobarbitone, since it is the salt of a strong base and a weak acid, hydrolysis to give an alkaline solution in water. The solution can absorb carbon dioxide and this results in precipitation of hexabarbitone. This is a serious problem in the preparation of intravenous solutions of soluble barbiturates. They are therefore distributed as sterile powders in ampoules with instructions to dissolve immediately before use in water for injections free
from carbon dioxide.
Physical Factors Influencing Chemical Degradation of Pharmaceutical Products
1. Temperature
The rate of chemical degradation of most pharmaceutical products increase with
temperature. It is, therefore, important to consider this factor when formulating drug
products intended for use in tropical parts of the world where the ambient temperature for storage is relatively high (about 28 — 40°C). it also plays a role when a product has to be autoclaved or heat sterilized before use. Autoclaving aqueous injections, e.g. dextrose injection or sterilizing oil e.g. ethyl oleate, and powders e.g. suiphonamide, by dry heat methods can result in the decomposition of the parent drug, if thermolabile.
Thus, it is possible to reduce the rate of decomposition of thermolabile drugs by careful storage at low temperatures, by storing in a cool place. This is particularly required for biological products such as insulin, oxytocin and vasopressin injections, as well as for penicillin and its preparations, which may all be stored in a refrigerator.
There are, however, a few instances where low temperature storage might in fact accelerate decomposition. For example, there is a tendency for increased rate of polymerization of formaldehyde at temperatures below 15°C.
Again, although a reduction in temperature tends to reduce the rate of oxidation, it also
produces an increase in the solubility of oxygen in solution which tends to promote the rate of reaction decomposition but not to the extent predicted by kinetic reaction theories.
2. Moisture
The presence of moisture in the environment of a solid drug will often increase the rate of decomposition if it is susceptible to hydrolysis. This is particularly true of aspirin, the pencillins, and other antibodies such as streptomycin and tetracycline. Adsorbed moisture
may also in some cases also increase the rate of oxidation of a susceptible product since oxygen is dissolve in the water layer surrounding the drug particles. For example, ferrous sulphate crystals are more rapidly oxidized in moist air than in dry surroundings.
It is important that susceptible drugs should be stored in a dry cool environment. Their
manufacture should also be undertaken in an environment of controlled humidity. Also
excipients selected for formulation of susceptible drug should have low moisture content, to prevent the transfer of moisture to the parent drug.
3. Light
There is strong evidence in the literature that many pharmaceutical products undergo some forms of instability when exposed to strong light. In some instances the instability may be due to the heat accompanying the sun’s rays, yet light energy can initiate and accelerate
decomposition. It is important to distinguish between the effects of heat and light separately in order to determine whether cool storage or packaging that protects from light rays is appropriate.
The relationship between the amount of light absorbed and the amount of reaction taking place was proposed by Einstein. Law of Photochemical Equivalence proposed by Einstein which states that each molecule can only undergo chemical reaction as a direct result of absorption of light, if one quantum of radiant energy causing the reaction is taken up. The unit of radiant energy equivalent to one quantum is called the photon. The energy of the photon is directly proportional to the frequency of the absorbed radiation (inversely proportional to the wavelength). Hence there is more energy in a photon of short wavelength than in a photon of light of longer wavelength. Consequently photochemical destruction of pharmaceutical products is usually due to the absorption of sunlight of the visible blue and violet and ultraviolet wavelengths (500 — 300nm).
Many types of chemical reaction are induced by exposure to light of high energy. For example amyl nitrite and ethyl nitrite undergo hydrolysis, and many autoxidation process e.g. volatile oil oxidation are initiated. In some cases substances become coloured, e.g. morphine, codeine and quinine become yellowish brown while others fade. Photochemical reactions are rarely simple, since reaction products are subsequently involved in thermal reactions and its difficult to distinguish between the primary and secondary processes.
4. Radiation
Although ionizing radiation can be a useful technique for sterilizing certain drug products, especially those that are thermolabile, unfortunately radiation treatment can also produce deleterious changes in the products. This is because the procedure also causes ionization in the irradiated material.
Prediction of Shelf-Life
It is possible to predict the expiry date of a drug product based on the application of the
Arrhenius equation, which indicates the effect of temperature on the rate constant, k, of a chemical reaction.
The substitution of this value of-k into the appropriate order of reaction allows the amount of decomposition after a given time to be calculated. Thus this approach involves a knowledge of the order of the reaction and preliminary experiments are carried out to determine this order.
There are several difficulties and limitations involved in this aspect of accelerated stability tests but in spite of these difficulties, the application of accelerated stability to pharmaceutical products is useful in predicting fairly accurate shelf-lives.
Factors Influencing, and Methods of Reducing Chemical Degradation of Pharmaceutical Products
1. HYDROLYSIS
Methods of reducing hydrolysis include the following:
- Adjustment of pH
- Choice of Solvent
- Production of an Insoluble Drug Form
- Presence of Surface-active agents
- The Presence of Complexing Agent
(a) Adjustment of pH
It has been shown that hydrolysis is catalysed by H+ and/or OHand the rate of decomposition is critically dependent upon pH. A useful method for reducing hydrolysis of a drug exhibiting acid-base catalysis is to adjust the pH to the point corresponding to the minimum for the hydrolysis of the drug product. In many instances, pH affects the solubility and therapeutic activity of the drug as well as its stability.
(b) Choice of Solvent
Non-aqueous solvents, e.g. alcohol and propylene glycol, have often been used to replace a portion or all of the water in a solution in order to reduce hydrolytic decomposition of a drug. It is widely believed that replacement of water by a non-aqueous solvent
automatically enhances the stability of the product.
(c) Production of an Insoluble Drug Form
Hydrolysis only occurs with that portion of a drug which is in aqueous solution. Therefore, if the majority of the drug is present in a suspension with minimal amount in solution, there will be a reduction in the amount of hydrolysis. The solubility of a drug may sometimes be reduced by the adjustment of the pH of the aqueous vehicle thus favouring stability of the susceptible drug.
(d) Presence of Surface-active agents
Surface-active agents are widely used in pharmacy as solubilising and emulsifying agents.
Thus the presence of surface-active agents can often result in a significant improvement in stability, but this is by no means a general rule. There are some exceptions.
The rate of hydrolysis of emulsified systems of a water-insoluble compound has been found to be affected by the ability of the surface-active agent to solubilize the material. Since the bulk of the drug is present in oil globules and is inaccessible to hydrolytic attacks, the rate of decomposition is normally slow.
(e) The Presence of Complexing Agent
The concept of protecting a drug from hydrolysis by the addition of a chemical stabilizer was derived from the fact that adding a compound which would form a water-soluble complex with the drug might to some extent decrease the rate of decomposition.
The technique of reducing hydrolysis by addition of a complexing agent has been proved useful in the stabilization of procaine injection containing 3.42 per cent w/v of an
equimolecular compound of procaine and caffeine in a solution of sodium chloride solution.
2. Oxidation
Methods of reducing oxidation include the following:
- The presence of anti-oxidants
- The Presence of Reducing Agents
- Removal of Oxygen
- Other methods
(a) The presence of anti-oxidants
The decomposition of many readily oxidizable materials e.g. fats, may be reduced by adding a small quantity of a substance that will retard the autoxidation process. Such compounds are known as antioxidants. The term antioxidant embraces several classes of chemically unrelated compounds which may possess different mechanisms of action.
Primary anti-oxidants act by interfering with the propagation step of the autoxidation
process. It is evident that a primary antioxidant is used up by taking part in the chain process instead of the drug.
The classes of primary antioxidants more commonly used to retard autoxidation in
pharmaceutical preparations include tocopherols gallic acid, butylated hydroxyanisole (BHA) e.t.c.
(b) The Presence of Reducing Agents
In some instance, oxidation of pharmaceutical products may be retarded by the addition of a reducing agent. In contrast to antioxidants, reducing agents are effective against oxidaizing agents as well as atmospheric oxygen, and act by being oxidized in preference to the drug
they are protecting. The reducing agents popularly used in pharmacy are the potassium and sodium metabisulphites, bisulphate and sulphites.
(c) Removal of Oxygen
By limiting contact of the drug with the atmosphere, those oxidative decompositions dependent upon atmospheric O2 may be minimized. In some cases, it is only necessary to store the drug in well-filled air-tight containers e.g. codeine phosphate. With single-dose injections, it is often possible to reduce oxidation by displacing the air with an inert gas. N2
is frequently used, CO2 may also be employed. Carbon dioxide however is appreciably soluble in water at room temperature and this may result in pH change of the solution.
(d) Other methods involve
- (i) Adjustment of pH
- (ii) The presence of surface-active Agents
3. Photochemical Degradation
The photochemical degradation of a sensitive material can be reduced by protecting it from direct sunlight. This may be achieved by storing the product in a clear glass container, and then either placing it in the dark or packaging it in an opaque container. Alternatively, light -resistant containers may be used