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DRUG STABILITY STUDIES
Contents
1. Introduction
2. Chemical Kinetics
a.Zero order process
b.First order process
c.Second order process
d.Determination of order
3 Routes of degradation
a.Hydrolysis
b.Oxidation-Reduction
c.Photolysis
d.Racemization
1. Accelerated stability analysis
2. Addition of overages
3. Factors affecting the stability
4. Preformulation studies
5. Storage considerations
a. ICH and WHO guidelines
DRUG STABILITY STUDIES
INTRODUCTION
The term stability with respect to drug dosage form refers to chemical and physical integrity of the dosage form unit and its ability to offer protection against microbial contamination. The degradation occurs mainly because of the chemical reaction of the active ingredients or additives.
Stability can be defined as the capacity of drug product to remain within specifications established to ensure its identity, strength, quality, and purity. Precisely, it is the ability of the product to resist deterioration. The stability of the product is expressed as the expiry period or shelf life.
Application of the principles of chemical kinetics helps in determining the various factors affecting stability. The results of such kinetic studies aids in selection of appropriate formulation and predicting the shelf-life of the product. The shelf-life of the dosage form is the time lapse from the initial preparation and packaging to the last day of specified expiration date i.e., when the potency reaches 90% of the labelled value. It is defined as the time required for the concentration of the reactant to reduce to 90% of its initial concentration. Shelf life is represented as t 90 and has the units of time/concentration.
During drug development, several bulk lots are produced. The first lot used for stability studied may not be the representative of the whole bulk, but it acts as a baseline for the consequent lots to be analyzed. The application of certain physicochemical principles in the performance of stability studies has proved to be considerable advantage in the development of stable dosage forms.
Most of the drugs are susceptible to some form of decomposition. As a result, the physical properties, chemical composition may change on ageing. Aged drug may contain significantly less amount of the active medicament and hence may show poor bioavailability. So it is necessary to mention the shelf life on the label. A manufacturer is obliged to indicate the shelf life of a drug on the label unless it is greater than 3 years. No drug may be sold after 5 years.
Stability studies are important for the following reasons.1. This is an assurance given by the manufacturer that the patient would receive a uniform dose
throughout the shelf life.2. The drug control administration insists on manufacturers on conducting the stability studies,
identity, strength, purity and quality of the drug for an extended period of time in the conditions of normal storage.
3. Stability testing prevents the possibility of marketing an unstable product.
Both physical and chemical degradation of drug can result in unstable product.
Physical degradation may be due to-
1. Loss of volatile constituents-Example: Nitroglycerine tablets.2. Loss of water- Example:An effervescent substance loose water3. Absorption of water-Example: Calcium chloride-A deliquescent substance absorbs water.4. Crystal growth-Example:10% w/v Calcium gluconate injection gives precipitation and
undergo crystallization due to supersaturated solution.5. Colour change-Example: Aspirin tablets become pink and Ascorbic tablets turns yellowish
brown when exposed to air.
Chemical degradation may be due to the functional group present. Some of them are:
1. Hydrolysis-Example: Aspirin and Procaine2. Oxidation-Example: Ascorbic acid and vegetable oil.3. Isomerisation- Example: (-) Adrenaline is more active and (+) Adrenaline is less potent.
Trans vitamin A palmitate is more active.4. Absorption of carbondioxide-Example: Sodium hexobarbitone IV injection5. Decarboxylation-Example: Procaine gives a dark colored liquid due to loss of carbon dioxide
Purpose of stability studies
Stability studeisis done to understand how to design a product and its packaging such tha the product has appropriate physical .chemical and microbiological properties during a defined shelf life when stored and used.
CHEMICAL KINETICS
Most of the degradative reactions in pharmaceutical formulations take place at definite rates and chemical in nature. An effective and efficient study of these reactions requires the applications of chemical kinetic principles. Most of the rate limiting phenomena is describable by some equation systems. The reaction rate depends upon the concentration of reactants and this describes the order of reaction. The degradation of most of the pharmaceuticals follows zero order, first order and pseudo first order.
Reaction Kinetics is the study of rate of chemical change and the way in which this rate is influenced by the conditions of the concentrations of reactants, products and other chemical species which may be present and by factors such as solvent, pressure and temperature. In case of pharmaceuticals, such information permits a rational approach to the stabilization of drug products and prediction of shelf life and decisions regarding optimum storage conditions.
Zero order process
When the rate of disappearance of a reactant ‘A’ is constant and independent of its concentration is said to be zero order process.
Mathematically it can be expressed as;
-dA = K or K= A0-At Where K is the rate constantdt t At is the amount of A remaining
at time ‘t’ A0 is the initial concentration.
t90%=0.1 At K
Example:1.Photochemical reactions in which the rate determining factor is the light intensity rather
than concentrations of reactant are zero order reactions, like loss of colour in multisulfa product.
2.Pharmaceuticals like Riboflavin, Nifedipine are common drugs which are extremely light sensitive and follow zero order process.
First order process
When the rate of disappearance of reactant ‘A’ is proportional to the concentration of ‘A’ at time ‘t’ is said to be first order process.
Mathematically it can be expressed as;
-dA/dt = KA or In At/Ao = kt
log a = K t t ½ = 0.693 (a - x) 2.303 K
t90%= 0.105/K
Where, the rate constant K is in almost all cases a function of the temperature T. For most pharmaceutical products, as t is increased, the rate constant and therefore the rate of degradation increases.A0 is the initial concentration and at is the concentration at time t.
Example:
1. Decomposition of Hydrogen peroxide catalyzed by Potassium iodide.2. Absorption, distribution. Metabolism and excretion of the drug.
Second order process
When the rate of disappearance of reactants ‘A’ and ‘B’ is proportional to the concentration of each of ‘A’ and ‘B’at time ‘t’ is said to be second order process.
Mathematically it can be expressed as;
-dA/dt = dB/dt = K(AB) or
dx/dt = K(a-x) . (b-x)
K = 2.303 log b(a-x) t(a-b) a(b-x)
Where (a-x) and (b-x) are the concentrations of A and B remaining at time‘t’ and x is the concentration of drug decomposed at time t.
Example:
1. Hydrolysis of Chlorobutanol in presence of sodium hydroxide.2. Alkaline hydrolysis of esters such as methyl acetate.
DETERMINATIONS OF ORDER
The order of a reaction may be determined by several methods,
Subsituation method
The data accumulated in a kinetic study may be substituted in the integrated form of the equations that describe the various orders. When the equation is found in which the calculated ‘K’ value remains constant within the limits of experimental variations, the reaction is considered to be of that order.
Graphic Method
If a straight line results when concentration is plotted against t, the reaction is zero order. The reacton is first order, if log (a-x) versus t yields a straight line and it is second order if 1 / (a-x) versus t gives a straight line.When a plot of 1 / (a-x)2 against ‘t’ produces a straight line, with all reactants at the same initial concentrates the reaction is third order.
Half-life method:
Order Integrated rate Equation Half life equation
0 x = Kt t ½ = a 2k
1 log a = K t t ½ = 0.693 (a - x) 2.303 K
2 x = Kt t ½ = 1 a( a - x) aK
3 2ax - x2 = 2 Kt t ½ = 3 1
a 2 (a-x) 2 a2 K
The relationship between these results shows that in general the half-life of a reaction in which concentration of all reactants are identical is given by
t ½ 1 n – 1
a
n is the order of the reaction. Thus if two reactions are run at different initial concentrations, a1
and a2, the half lifes t ½ (1) and t ½ (2) are related as follows,
log t ½ (1) = (n – 1) log a2
t ½ (2) a1
= n = log t ½ (1) / t ½ (2) +1log (a2/ a1)
ROUTES OF DEGRADATION
The major routes of drug degradation are summerized in the following table.
OXIDATION HYDROLYSIS PHOTO CHEMICALDEGRADATION
Stabilization: Stabilization: Stabilization:
- Addition of anti - Adjustment of pH - Storage in Amber Oxidants - Use of non-aqeous coloured containers
Solvent - Use of opaque wrapper- Addition of reducing - Use of complexing - Use of opacifiers
Agents agent incorporation of UV Light absorbers.
- Adjustment of pH
- Providing inert Environment.
DEGRADATIVE PATHWAY
ROUTES OF DEGRADATION
RECEMIZATION
POLYMERIZATION
ACYLATIONDECARBOXYLATION
POLYMORPHISM SOLUATE - FORMATION
HYDROLYSIS
Many pharmaceuticals contain ester or amide functional groups, which undergo hydrolysis in solution. Examples of drugs that tend to degrade by hydrolytic cleavage of an ester or amide linkage are anesthetics, antibiotics, vitamins and barbiturates.
Many pharmaceuticals contain ester or amide functional groups, which undergo hydrolysis in solution. Examples of drugs that tend to degrade by hydrolytic cleavage of an ester or amide linkage are anesthetics, antibiotics, vitamins and barbiturates.
Ester hydrolysis:
The hydrolysis of an ester into a mixture of an acid and alcohol essentially involves the rupture of a covalent linkage between a carbon atom and an oxygen atom. Although some of this hydrolysis can be affected in pure water, in the majority of cases, the presence of a catalyst is needed to promote the reaction.
These catalysts are capable of supplying hydrogen or hydorxyl ion to the reaction mixture. (Polar compounds)
a. Hydrolysis effected by pure H2 O; o o || + - ||
R - C - OR + H + - O H R - C - OH + HOR (Acid) (Alcohol)
b. In case of acid catalysed reactions:
o o o || 1 + || 1 || 1 R - C - OR + H R - C - OR - - R – C – OH + R OH |
H
c. In case of alkaline catalysed reactions:
o o || 1 || 1 1 2R - C - OR + O H R - C - OR RCOOH + R OH |
OH
A number of reports in the literature deal with detailed kinetic studies of the hydrolysis of pharmaceutical ingredients containing an ester group in the molecule. Degradation of aspirin in various buffer solutions and treated the overall reaction as pseudo – first order.
Aspirin hydrolysis:
The pH of optimum stability is at 2.4. At a pH of 5 to 7 the degradation reaction was essentially pH – independent and at a pH above 10, the stability of aspirin was found to decrease rapidly with increase in pH.
Other pharmaceutical materials that have been reported to degrade through ester hydrolysis are procaine, atropine, and methyl P – amino benzoate.
A number of drugs degrade through ester hydrolysis. To enhance the stability of pharmaceuticals undergoing this type of degradation, the following factors are to be considered.
1. pH: If physiologically permissible, the solution of the drug should be formulated as close as possible to its pH of optimum stability.
2. Type of Solvent: Partial or full replacement of water with a solvent of lower dielectric constant generally causes a considerable decrease in the velocity of ester hydrolysis. Examples of these nonaqueous solvents are ethenol, glycols, glucose and mannital solutions and substituted amides.
3. Complexation: The hydrolytic rates may be influenced in two ways by complex formation, namely, by either stearic or polar effects.
Example: Caffine complexes with local anesthetics, such as Benzocaine, Procaine and Tetracaine cause a reduction of the velocity of their hydrolytic degradation. The complexed fraction of the ester undergoes essentialy no degradation.
The velocity of the base catalysed decomposition of riboflavin was decreased by the Presence of caffeine in solution. It was found that the vitamin in its complexed form with Caffeine possessed negligible reactivity towards alkaline hydrolysis.
4. Surfactants:Non ionic, catonic and anionic surfactans stabilize the drug against base catalysis. Example: A 5% Sodium lauryl sulfate Solution (anionic) caused an 18 – fold increase in the half – life of bernzocaine. When 2.46% cetyl trimethyl ammonium bromide in solution (Cationic) is used, a ten fold increase in the half – life of benzocaine is seen.
5. Modification of chemical structure: By increasing the length of or by branching the acyl or alkyl chain, the rate of hydrolysis of the ester usually decreases, owing to steric hindrance. However, if an electrophilic or nucleophilic group is introduced into the acyl or alkyl side
chain of aromatic esters, the rate of hydrolysis can be increased or decreased by the electronic effect of these groups.
For example, alkaline hydrolysis of aromatic esters is promoted by the presence of electrophilic groups on the benzene ring (halogen or NO2), which attract electrons away from the reacton site (ester groups). The hydrolysis is retarded, on the other hand, by nucleophilic groups (CH3, OCH3 and NH2 which cause electrons to move toward the point of reacton, The reverse effect would be found in the case of hydrogen ion catalysed hydrolysis of aromatic esters.
6. Salts and esters: Another technique that is sometimes employed to increase the stability of pharmaceuticals undergoing degradation through ester hydrolysis is to reduce their solubility by forming less soluble esters of the drug.
Amide hydrolysis
Pharmaceutical compounds containing an amide group can undergo hydrolysis resulting in the formation of an acid and an amine.
o H o || | || R - C - N - R 1 + H2O R - C - OH + H2N - R 1
Amide Acid Amine
Pharmaceuticals such as niacinamide, phenethicillin, barbiturates and chloamphenical degrade by amide hydrolysis.
The basic hydrolysis proceeded as follows:
o o o || slow | fast || R - C - NH R 1 + OH R - C - NH R 1 R - C - OH + R 1 NH
| FAST
OHo||
RC - O+ R 1 NH2
The rate determining step in the hydroxide ion – catalysed reaction is the nucleophilic attack by the hydroxide ion.
The acid hydrolysis was as follows:
O O O|| 1 fast || 1 slow |
RC - NHR + H3O RC – NH2R + H2O R - C - NH2R | O | H H
O O || 1 fast || 1RC – OH + R NH3 RC – OH2 + R NH2
fast
The mechanism for acid hydrolysis of amides requires that subsituent should exert only weak polar effects, but that when suitably situated, they should exert strong stearic effects.
The methods available for retarding deterioration through amide hydrolysis are similar to those presented under ester hydrolysis.
Ring alteration
A hydrolytic reaction can proceed as a result of ring cleavage with subsequent attack by hydrogen or hydroxyl ion. Examples of drugs that have been reported to undergo hydrolysis by this mechanism include hydrochlorothiazide, pilocarpine and reserpine.
Example: The hydrolysis of pilocarpine in aqueous solution has been reported to involve a cyclic equilibrium process, which is catalysed by hydrogen ion and hydroxyl ion.
The concentration of pilocarpate and pilocarpic acid are influenced by pH. Pilocarpine is relatively stable in solutions of acidic pH. As the pH increases, pilocarpine progressively becomes unstable.
OXIDATION REDUCTION
The oxidative decomposition of pharmaceutical compounds is responsible for the instability of a considerable number of pharmaceutical preparations. For example, steroids, vitamins, antibiotics and epirephrine undergo oxidative degration. These reactions are medicated either by free radicals or by molecular oxygen.
A substance is said to be oxidized if electrons are removed from it. Oxidation oftern involves the addition of oxygen or the removal of hydrogen.
Ex: Ferrous ion is oxidised to ferric ion.++ +++ -
Fe Fe + e
The most common form of oxidative decomposition is auto oxidation; it may be defined as the reaction of any material with molecular oxygen.
The auto oxidation of an organic substance RH by a free radical chain process can be simply described as follows’
Activation .Initiation: RH R . + (H) Light, heat
.Propagation: R . + O2 RO2
.RO2 + RH ROOH + R.
Hydroperoxide Decomposition:
ROOH RO. + . OH
Termination: RO2 . + X inactive products
RO2 + RO2 inactive products
Heavy metals, particularly those possessing two or more valency states, with a suitable oxidation-reduction potential between them (copper, iron, cobalt and nickel) generally catalyze
oxidative deteriorations. These metals reduce the length of the induction period (the time in which no measurable oxidation occurs) and increase the maximum rate of oxidation. They can affect the rates of chain initiation, propagation and termination as well as the rate of hydroperoxide decomposition.
Many oxidations are catalysed by hydrogen and hydroxyl ions. Example quinone to hydroquinone.Although the oxygen concentration is of importance in the auto oxidation process, its significance is usually not adequately considered. For the most part, oxidative degradations of pharmaceutical compounds follow first order or second order kinetic expressions.
The solutions not containing any chelating agent degraded more rapidly as the buffer concentration increased, while the buffered solutions containing chelating agent showed that the rate of degradation was independent of the concentration of the buffer.
Rancidity, which can affect nearly all oils and fats is a widely known term covering many typical off – flavors formed by the auto - oxidation of unsaturated fatty acids present in an oil or fat. These off – flavours have a more or less distinct odour and are due to the volatile compounds that are formed upon oxidation of the oils and fats. These volatile compounds are generally short chain monomers that are formed by cleavage of the non-volatile hydroperoxide primary oxidation product.
Determination of iodine numbers can be employed as an indication of whether oxidation takes place across the double bond.The stability of pharmaceutical compounds undergoing oxidative degradation can be increased by several approaches.
Oxygen Content
Since, in many cases, oxidative degradation of a drug takes place in aqueous solution, it is helpful to keep the oxygen content of these solutions at a minimum. Most oxidative degradations of pharmaceutical compounds are probably autooxidative in nature and involve chain reactions that require only a small amount of oxygen for initiating the reaction, so it is necessary to add agents such as antioxidants and chelating agents to obtain acceptable protection against oxidative degradations.
Anti Oxidants:
The effect of antioxidants is to break up the chains formed during the propagation process by providing a hydrogen atom or an electron. Water soluble antioxidants act by preferentially undergoing oxidation in place of the drug. Oil – soluble antioxidants serve as free radical acceptors and inhibit the free radical chain process.
Antioxidants commonly used for aqueous systems are:
Sodium sulfite Sodium dioxide Thioglycolic acidSodium metabisulfite Ascorbic acid Sodium thiosulfiteSodium bisulfite Thioglycerol Cysteine hydro chloride
Antioxidants commonly used for oil systems are:
Ascorbyl palmilate Butylated hydroxyHydroquinone toluenePropyl gallate - tocopherol
Lecithin.
The effectiveness of these antioxidants can depend on the concentration used, whether they are used singularly or in combination, the solution pH and the package integrity and non reactivity. The effectiveness of antioxidants can be enhanced through the use of synergists such as chelating agents.
Chelating Agents
Chelating agents tend to form complexes with the trace amounts of heavy metal ions inactivating their catalytic activity in the oxidation of medicaments. Examples of some chelating agents are ethylenediamine, tetra acetic acids derivatives and salts, dihydroxy ehtyl glycerine, citric acid and tartaric acid.
pH It is also desirable to buffer solutions containing ingredients that are readily oxidizable to
a pH in the acid range. This causes an increase of the oxidation potential of the system with a concurrent increase in stability when oxidations are catalysed by hydrogen or hydroxyl ion. The pH of optimum stability in the acid range, however, must be determined experimentally for each drug.
SolventsSolvents other than water may have a catalysing effect on oxidation reactions when used
in combination with water or alone. For example, aldehyde, ethers and ketones may influence free radical reactions significantly.
Ascorbic acid Oxidation
The scheme of oxidation of ascorbic acid by cupric ions is as follows.
Ascorbate ion in solution---------------- Semiquinone-------------- Dehydro ascorbic acid---- Slow oxidation Rapid oxidation Oxalic acid + Threonic acid ------ Ketogulanic acid
When solutions are free from traces of copper, ascorbic acid is not oxidized by molecular oxygen to a measurable extent, except in alkaline solution. When CO and KCN are added they form complexes with the metal ions and therefore oxidation is limited.
Ascorbic acid can exist as a singly charged and doubly charged ion. Oxygen ion react with divalent ions at about 105 times faster compared to its reaction with the the monovalent ion. In alkaline medium auto oxidation proceeds more rapidly.
PHOTOLYSIS
Degradative reactions, such as oxidation, reduction, ring arrangement or modification and polymerization can be brought about by exposure to light at particular wavelength. According to the equation E = 2.859 x 105 / Kcal per mole, the shorter the wavelength of light, the more energy is absorbed per mole.
In a large number of systems that are photolyzed, free radicals are products that undergo subsequent reactions. If the molecules absorbing the radiation take part themselves in the main reaction, these actions is said to be a photochemical one. Where the absorbing molecules do not themselves participate directly in the reaction, but pass on their energy, to other molecules that do, the absorbing substances is said to be a photo sensitizer.
Examples of pharmaceutical compounds that undergoes, the photo decomposition are chlorpromazine hydrochloride, alcoholic solutions of hydrocortisone, prednisolone and methyl prednisolone.
RACEMIZATION
A racemization is a reaction in which, an optically active substance loses its optical activity without changing its chemical composition. This reaction is important to the stability of pharmaceutical formulations, since the biologic effect of the dextro form can be considerably less than that of the levo form. For example, levo adrenaline is 15 to 20 times more active than dextro – adrenaline.
Racemization reactions, in general, undergo degradation in accordance with first – order kinetic principles. The racemization of a compound appears to depend on the functional group bound to the asymmetric carbon atom. Aromatic groups tend to accelerate the racemization process.
ACCELERATED STABILITY ANALYSIS
In the past it was the practice in many pharmaceutical manufacturing companies to evaluate the stability of pharmaceutical preparations by observing them for a year or more, corresponding to the normal time that they would remain in stock and in use. Such a method was time – consuming and uneconomical. Therefore accelerated stability studies were carried out. The objective of this study is to predict the shelf life of a product by accelerating the rate of decomposition, preferably by increasing the temperature.
The method of accelerated testing of pharmaceutical products is based on the principles of chemical kinetics. According to this technique the K values for the decomposition of a drug in solution at various elevated temperatures are obtained by plotting function of concentration against time, as seen in figure –1 and 2
70c
60cConcentration 50c
40 40c 30c
50 Log K 25c
70 60 20c
|
1/TTime in hours
Figure 1 accelerated breakdown of a Figure 2 – arrhenius plot for drug in aqueous solution at predicting drug stability elevated temperature. at room temperature.
The logarithms of the specific rates of decomposition are then plotted against the reciprocals of the absolute temperatures as shown in figure and the resulting line is extrapolated to room temperature. The K25 degree is used to obtain a measure of the stability of the drug under ordinary shelf conditions.
Another similar method in which the fractional life period is plotted against reciprocal temperatures and the time in days required for the drug to decompose to some fraction of its original potency at room temperature is obtained. The approach is illustrated in figures 3 and 4.
100—
300__ 90— 40 200__ 80— 150__
100__ 70— 50 80__ 60— 60 60__
50— Days t90 40__
70 log scale 40— 20__
10__ 30— 8__
90 6__ | | | | | | | | | | | | | | | | |
0 24 48 72 96 120 2.7 2.8 2.9 3.0 3.1 3.2 3.3
| | | | | | | | 90 80 70 60 50 40 30 25
Time (days) Temperature (Degree Celsius)
Figure: 3 Time in days required for drug potency Figure:4 A log plot of t90 (i.e., time to fall to 90% of original value. These timer, to 90% potency) on the vertical axis designated as t90, are then plotted on a log scale against reciprocal temperature (both
Kelvin and centigrade scales areShown) on the horizontal axis.
As observed in Figure 3 the log percent of drug remaining is plotted against time in days and the time for the potency to fall to 90 degree of the original value (i.e., t90 ) is read from the graph. In figure 4 the log time to 90% is then plotted against 1/T and the time at 25 degree celsius gives the shelf-life of the product in days. The decomposition data illustrated in figure.3 result in a t90 value of 199 days. Shelf-life and expiration dates are estimated in this way.
By either of these methods, the overage, that is, the excess quantity of drug that must be added to the preparation to maintain at least 100% of the labeled amount during the expected shelf of the drug, can be easily calculated and added to the preparation at the time of manufacture.
Limitations
1. Valid only when the breakdown depends on temperature.2. Valid only when the Ea is 10-30 K Cal / mole.3. Shelf life for one set of condition cannot be applied to other preparation of the same drug.4. Stability prediction are of little use if the degradation is due to diffusion, Microbial contamination, Photochemical reaction or excessive agitation5. They cannot be used if the product looses its physical integrity at higher temperature like co-agulation, denaturation, breaking of emulsion, melting of ointments etc. 6. Predictions will become erroneous when the order changes at elevated temperature.7. Predictions will become erroneous when the reaction changes its order during the period of study.
ADDITON OF OVERAGE
Addition of overages is done to attain the desired shelf life to keep the content of the active ingredient within limits compatible with therapeutic requirements, for a predetermined period of time, so that at least 100% of the labeled amount during the expected shelf life period is maintained. The International Pharmaceutical Federation has recommended that overages be limited to a maximum of 30% over the labeled potency of an ingredient.Figure-5
110
100% label claim 90
80
0 100 200 300 400 500 600 700 800Time in days
Even if the pharmaceutical dosage form were found to extremely stable, the packaged product would become unsightly if stored for excessive number of years. For these reasons, outdates no longer than 60 months are used.
FACTORS AFFECTING STABILITY
EXTRINSIC INTRINSIC BOUNDARYTemperature PH Containers compositionLight Complexation PorosityGases Microbial growth Dosage from interactionsMoisture
PREFORMULATION STUDIES
Effects of Temperature
Most of the factors affecting drug stability can be controlled by careful selection of adjuvant or container / closure system. But the effects of temperature are not in control as majority of the marketed drugs are stored at room temperature. The room temperature may vary from place to place and from season to season and hence the effects of temperature on drug stability deserves a special attention.
The quantitative relationship of the specific reaction rate and temperature is given by the Arrhenius equation:
In k = In A - Ea . T R
Where, R = Gas constant ( 1.987 cal / degree . mole) T = Absolute temperature A = Frequency factor, Ea = Activation energy
A plot of K vs 1/T gives arrhenius plot from which Ea and A can be calculated.
FIGURE : 6 ARRHENIUS PLOT
100 -- log A
50 ---
10 –
.
K hours-1
2.68 2.84 2.92 3.0 3.1 3..2 3..3 3.4
1/T x 103
Arrhenius plot showing the method for determining activation energy and temperature dependency of degradation
During early phases of drug development, the greatest amount of information is sought in the least amount of time. The Kinetic and Predictive studies are, therefore, generally carried out under accelerated conditions. The objectives of accelerated tests are’
1. Aids in rapid detection of deterioration.2. Aids in selection of the best formulation (Fig. 2)3. Shelf life prediction (Fig. 3)4. A rapid means of quality control.
FIGURE - 7 FIGURE - 8
Formulation 1 (least stable) X High Stress
Amount of Decomposition X
Obtained after the Formulation 2 time t, is used to
Predict value of Yafter time t2
Formulation 3 Y(Most stable) Low stress
Time Time t1 t2
There are number of situations when Arrhenius predictions become invalid or errneous (e.g emulsions, viscous dispersions, drugs very sensitive to oxygen or water vapour etc.,)
Realistically, FDA Stability Guidelines stress on requiring that stability studies be performed in intended marketed container under conditions of 37 - 40 degree Celsius and 75 - 85% RH for three months. This can simulate tropical conditions also. This gives a more realistic shelf-life, though a longer period is required. Shelf-life (t90 %) at ambient conditions can be determined from Arrhenius plot.
FDA states that a drug product which is stable for three months at 37 - 40 degree C and 75% RH can be given a tentative experiment period of two years, if it does not contain an overage.
Influence of pH on degradation
The magnitude of the rate of hydrolytic reactions catalyzed by hydrogen and hydroxyl ion can vary considerably with pH. Hydrogen ion catalysis predominates at the lower pH range
whereas hydroxyl ion catalysis operates at the higher pH range. For such a study, product samples are kept at pH 2-12 at one selected temperature between 55-900 C for two weeks. The data can be presented in the form of pH rate profile plots of log K or K against pH. The pH of maximum stability can be read from the plot.
To determine the influence of pH on the degradative reaction, the decomposition is measured at several hydrogen ion concentrations. The pH of optimum stability can be determined by plotting the logarithm of the rate constant versus pH.
Figure-9The point of inflection of such a plot represents
the pH of optimum stability. Knowledge of this point is extremely useful in the development of stable dosageform, provided the pH is within safe physiologic limits.
pp
K-Hours
1 2 3 4 5 6 7 8
pH
General acid-base catalysis of degradation:
Buffer salts are commonly used in the formulation of pharmaceutical liquids to regulate the pH of the solution. Although these salts tend to maintain the pH of the solution at a constant level, they can also catalyze the degradation.
Commonly buffer salts such as acetate, phosphate and borate have been found to have catalytic effects on the degradation rate of drugs in solution.
Influence of ionic strength on degradation:
The rate of reaction can be influenced by the strength of the solution in accordance with the following equation,
Log K = log Ko + 1.02 ZAZB u
Where,ZA and ZB are the charges carried by the reacting species in solution.
U- the ionic strengthK- the rate constant of degradation
pH OF MAXIMUM STABILITY
Ko- the rate constant of infinite dilution.
The ionic strength is defined as half the sum of the terms obtained by multiplying the concentration of each of the ionic species present in the solution by the square of its valence.
+When the drug is positively charged and is ^
undergoing hydrogen ion catalysis, an increase inionic strength caused by the addition of a salt, such as NaCl, cause an increase in the rate of degradation. 0
A decrease in the rate of degradation results Reaction
if the positively charged, drug is undergoing rates hydroxyl ion catalysis and the ionic strength is increased by addition of a salt. | >
u
Figure 10: Square root of ionic strength dependence of reaction rates on ionic strength.
The change in the pH of a drug solution during stability testing can be indicative of either degradation of the active ingredient on interaction of one or more of the constituents of the solution either the container-Plastic, glass or the rubber.
Photo Stability Studies
Exposure to sunlight can change the color of products, degrade packaging or even lead to chemical decomposition of active ingredient. However to study the detrimental effects of light on drug product, samples are placed in open petri dishes and packaged in both clear and amber containers. Controls are placed in light – resistant container such as, amber, glass, foil-wrapped or in a cardboard box. These are placed into a well-vented temperature monitored light cabinet of specified lumens, exposed for at least four weeks and analyzed.
Effect of Humidity
In order to obtain relevant information, it is preferable to employ a range of humidities. This is achieved by wing saturated salt solution. The test should be carried out on both final packaged product and the unpacked material to get information regarding formulation adjuvant, type of environment suitable for a drug and the type of package needed.
Effect of Oxygen
To study the effect of oxygen, the product samples are placed into containers and stored at 75 degree celcius for one week. Prior to sealing, the head space is evacuated and purged with an inert gas such as argon or nitrogen. Air head space samples are used as positive controls. Oxidation may be evident by potency loss and /or color change.
Autoclaving Studies
For parenterals, determination of stability to autoclaving is necessary. Solutions at an established pH range are exposed to autoclaving conditions of 121 degree celcius at 30 psig for 20, 30, 45 and 90 minutes. Assay data are recorded together with evaluation of change in color, pH and particulate matter.
Studies of Microbial Quality
The presence of microbes in the product poses a threat to stability causing degradation of drug resulting in dosage impotency or toxic products. Bacterial or mold growth is undesirable from therapeutic and an aesthetic point of view .
For evaluating microbiological stability, it is necessary to monitor the preservative content at intervals through out the projected expiration period. This can be done by microbial challenge tests and by chemical assays of the preservatives. Products that do not contain preservatives but require control of microbes are subjected to microbial limit tests.
Dosage form factors:
The objective of preformulation studies is to identify compatible, potentially useful pharmaceutical excipients, so that a stable formulation can be developed. Generally, a drug or a mixture of drugs along with excipients like preservatives, buffering agents, antioxidants and chelating agents are subjected to accelerated studies. This requires screening of a large number of excipients under several storage conditions, thus analytical, chemical and physical data are developed for the new drug substance. Incompatibility tests are also carried out.
Three types of dosage forms are there, if it is a
6. Solid state dosage form:Crystallinity, UV, I.R, TLC, HPTLC analysis should be carried out.
7. Semi-solid:X-ray crystallinity, particle size, shape and particle distribution should be found out.
8. Liquids:pH studies, cosolvant studies are to be carried out. If it is a suspension-particle size distribution, sedimentation, rate should be carried out.
Effect of package on stability
The package has been described as an economical method of providing convenience, identification, presentation and protection for a given product until such time that is consumed. Protection is the main emphasis for packaging pharmaceutical products and should act to protect the drug during product shelf-life.
The commonly used packaging materials include glass, metal, plastic and rubber .For solid dosage form , strip and blister packaging are used. Containers are made up of plastic or bottle, even silica bag is used to avoid the absorbency of moisture. Proper polymers should be selected. In glass maybe containers leaching or sorption happen for which proper test ought to be carried out.
For liquid dosage form,bottles are most commonly used. For air resistance, moisture resistance, amber colored bottles are used. Blue colored bottles are used for milk of magnesia. For parenterals ampoules and vials are used as containers. For this powdered glass tests, water attack test, alkalinity test, thermal shock test, internal pressure test, whole ampoule test are carried out. For plastic containers, test for plastic should be carried out. For rubber closures, test for rubber should be carried out..
For semi-solid dosage form – Collapsible tubes are used as a container which is made up of plastic, aluminum and rarely tin. Respective tests should be carried out.
STORAGE CONSIDERATIONS
The storage information can be found on the label of the immediate pack or subsequent package. The pharmacist should be highly concerned about maintaining the integrity of the product and necessity of storing it in the proper environment. In addition to this general storage restrictions should be given to certain individual drug applicable to particular drug product. When no specific compendial storage direction or limitation is provided, it is understood that the storage conditions include protection from moisture freezing and excessive heat. Expiration date is meaningless unless accompanied by labeled direction for storage under controlled condition. Data is provided in the following tables for world climatic conditions and calculated storage conditions for world wide stability testing.
Both ICH and WHO guidelines are framed on the concept of derived storage conditions of temperature and % relative humidity in the various climatic zones around the world.
Compendial storage temperature definitions
I.P. USP/NF
Cold Temperaure not exceeding 80 C, Same Usually between 20-80 C
Cool Temperature between 80- 250 C Temperaturebetween 80-150 C
Room temperature The temperature prevailing in a SameWorking area
Warm temperature between 300- 400 C Same
Excessive Heat Temperature above 400 C Same
World climate zones
Climatic Zone I Zone II Zone III Zone IVConditions (Temperare) (Sub tropical) (Hot Dry) (Hot Humid)
Mean Annual 20.50 C20.5 – 240 C 240 C 24 0CTemperature
Kinetic Mean 21 0C 250 C 300 C 300 CTemperature
Mean Annual 45% 60% 35% 70%Relative Humidity
ZoneI: UK, North Europe, Canada, Russia.Zone II: United states, Japan, Southeren EuropeZoneIII: Iran, Iraq, Sudan,Zone IV: Brazil, Iondonesia, Philippines.
Calculated storage conditions for World Wide Stability Testing
Climatic Zones Storage ConditionsTemperature RH
1. Temperature Climate 19 C 40 – 60%2. Mediterranean and Sub Tropical Climate 26 C 60 – 65%3. Hot and Dry Climate 31 C < 65%4. Hot and Humid Climate 31 C > 65%
A pharmacist developing a product exclusively for export market may utilize this data.
General Considerations of ICH and WHO Drug Stability Testing Guidelines
ICH WHO1. Concept ICH guidelines have been developed to
harmonize drug stability data required to be submitted to registration authorities in ICH countries.
WHO guidelines similarly are meant to harmonize drug stability requirements of registration authorities in WHO associated countries.
2. Agencies involved in development
Drug regulatory authorities and experts from the pharmaceutical industry.
WHO expert committee involving regulatory authorities, scientists, and the pharmaceutical industry.
3. Countries of application
17 Countries in three regions viz. USA, Japan and EC.
Global, meant to cover 170 WHO member states outside the ICH exercise.
4. Applicability New Chemical Entities and their finished products.
Pharmaceutical products containing well established drug substances in conventional dosage forms
5. Stage of development
Final draft endorsed by ICH steering committee on 27 October, 1993.
Appeared recently as Annexure 4 to the report of 34th Meeting of Expert Committee on Specifications for Pharmaceutical Preparations.
6. Date of implementation
1 January, 1998 Not available
7. Contents Preamble Objective Scope Drug Substance General Stress Testing Formal Studies Selection of batches Test Procedures Specifications Storage Conditions Testing Frequency Packaging Containers Evaluation Statements/Labelling Drug Product General Selection of Batches Specifications
General Definition of Terms Purpose of Stability Testing In the development Phase for the
registration dossier in the post-registration period
Intended market Design of Stability Studies Test Samples Test Conditions Accelerated Studies Real Time Studies Frequency of Testing and Evaluation of
Test Results Analytical Methods Stability Report Shelf-Life and Recommended Storage
Conditions References
Test Procedures Storage Conditions Testing Frequency Packaging Containers Evaluation Statements/Labelling Annexure 1. Glossary and Information
Appendix 1. Survey of Stability of Pharmaceutical Preparations included in the WHO model list of essential drugs
Appendix 2. Stability Testing - Summary Sheet
Technical Features of ICH and WHO Drug Stability Testing Guidelines
ICH WHO Comments1. Consideration of climatic zone
Zone II Zone II and IV ICH countries mostly fall in Zone II with very few in Zone I. WHO guidelines, being meant for global marketing, cover both Zones II and IV.
2. Test Conditions
- Accelerated
- Real time
400C 20C/75% RH5% for
6 months
250C 20C/60% RH5% for
12 months, assurance to be
provided for continuity of
the test upto the end of the
expected shelf-life
400C 20C/75% RH5% for 6 months for Zone IV countries.400C 20C/75% RH5% for 3 months for Zone II countries.
300C 20C/60% RH5% for IV countries.250C 20C/60% RH5% for zone II countries.Data for 6 months minimum shall be available at the time of registration
The ICH and WHO guidelines are built upon the concept of derived temperature (calculated from mean kinetic temperatures) in four different zones. The testing conditions suggested in ICH guidelines are based on derived conditions existing in Zone II countries while WHO guidelines are based upon derived storage conditions in both zone II and IV countries. The use of zone II conditions for zone I, and zone IV conditions for zone III means testing in adverse conditions which is reasonable as it provides an element of safety and also is rational since subsequent to harmonisation the products would move frequently from one country to the other.
3. Test samples/ selection of batches
Stability information on long-term and accelerated testing to be provided on three batches of the same formulation, with two of three batches essentially being of pilot scale.Information on three batches also desired for drug substance
For test samples containing fairly stable active ingredients -from two different production batches. For products containing easily degradable active ingredients - three batches to be sampled. Sampled batches out to be representative of the manufacturing process, pilot plant or full production scale.
In WHO guidelines, classification of batches is based on stable and sensitive active ingredients, which is not the case in ICH guidelines.
4. Testing frequency
Testing suggested to be carried out every three months during the first year, every six months during the second year and then annually for drug substance as well as the drug product. ICH guidelines suggests use of matrixing and bracketing designs, if application is justified
For accelerated studies, 0,1,2,3 and when appropriate, 6 months. For real time studies, 0,6,12 months and beyond that once a year
Testing frequencies vary in the two guidelines. The matrixing and bracketing designs allowed under ICH guidelines are meant to rationally reduce the testing frequency without sacrificing on the end results. This aspect is adopted from US FDA guidelines of 1987. Not covered by WHO guideline.
5. Packaging containers
Packaging containers same or shall simulate the actual proposed packaging. The guideline also recommends generation of data on the unprotected product under the accelerated conditions for the purpose to study the worse effects of storage on product properties
Studies to be done on final dosage form in its final container and packaging
Conceptually, requirements of the two guidelines are same with respect to testing of packaged samples. The ICH directive of testing of unpackaged drug substance need to be incorporated by other guidelines as the same gives a lot of information on inherent stability of the compound in the formulation environment
6. Interpretation of stability data
Required to be presented in a systematic form and summarised. Shelf-life estimates to be made through statistical methods such as regression analysis on transformed and untransformed data. To be
WHO guidelines permit assignment of a tentative shelf-life of 24 months provided the active ingredient is known to be stable, stability studies as suggested above have been performed without
This aspect or ICH guideline is also adopted from US FDA guidelines of 1987. WHO guidelines, however, are straight and suggest direct assignment of shelf-life of 24 months if formulation shows no
followed when significant degradation or change in the properties of the product occur, meaning that the change is outside the 95% confidence limits of the analytical method
significant changes, supporting data indicate that similar formulations have been assigned a shelf-life of 24 months or more and the manufacturer continues with real-time studies until the proposed shelf-life is covered.
changes under the testing conditions
7. Labelling Product to be labelled with storage temperature range based on national/regional requirements. Use of `ambient conditions’or room temperature as terms for storage conditions not accepted. However allows mention of specific storage conditions.
Requires the product to be PROMINENTLY labelled with the following storage conditions:-store under normal conditions-store between 2-80C (under refrigeration, no freezing)-store below 80C (under refrigeration)-store below -180C (in a deep freezer)The normal conditions have been defined as `storage in dry, well ventillated premises at temperatures of 15-250C or depending on climatic conditions,up to 300CExtraneous odours,contamination and intense light have to be excluded Where normal conditions are not met, WHO guidelines even recommend determination of storage conditions at level of country of designated use.
ICH guidelines suggest labelling based on national/regional requirements and don’t define the storage conditions. WHO guidelines are more specific and the labelling conditons for storage are defined.
