Why stability studies ? Stability is an essential quality attribute for drug products. If there is any functionally relevant quality attribute of a drug product that changes with time, this evaluation checked by pharmaceutical scientists and regulators who quantify drug product stability and shelf life. The rate at which drug products degrade varies dramatically. e.g. radiopharmaceutical products Since the evaluation of the stability of drug product is highly specialized and esoteric nature. Drug stability concerns about drug product safety , efficacy, and quality, found it to appropriate. Stability studies is done through the regulatory agencies such as the FDA and the HPB (health protection branch ) Functional changes in dosage forms with time It may be related to changes in chemical or physical properties of drug and excipients, coating materials etc. or it may be related to complex interaction between components of dosage form. Chemical stability can be assessed similar to above for drugs and excipients. For assessments of physical changes to dosage form changes specific to each dosage form should be evaluated. Changes in Mechanical strength Storage in humid condition leads to moisture adsorption and so decreased mechanical strength of tablet in blister packages. The change in mechanical strength can be described as a function of moisture sensitivity, moisture permeability, and humidity condition which is used for prediction of storage period. Changes in drug dissolution from tablets and capsule Dissolution of a drug substance is very important characteristic for bioavailability and it changes on storage Changes in melting time of suppositories Changes in release rate from polymeric matrix dosage for , including microspheres Drug leakage from liposomes Aggregation in emulsion Discoloration Moisture adsorption Effect of changes on drug stability of drug products
Packaging play an important role in quality maintenance and the resistance of packaging material to moisture and light can significantly affect the stability of products Protection from light can be achieved using primary and secondary packaging made up of light resistant material. Incorporating oxygen adsorbents such iron powder can reduce the effect of oxygen.3
Relevant guidelines ICH Q1A(R2): Stability Testing of New Drug Substances and Products ICH Q1B: Photostability Testing of New Drug Substances and Products ICH Q1C: Stability Testing of New Dosage Forms ICH Q5C: Stability Testing of Biotechnological/ Biological Products ICH Q1D: Bracketing and Matrixing Designs ICH Q1E: Stability Data Evaluation ICH Q1F: Stability Data Package for Zones III and IV Potential adverse effects of instability in pharmaceutical products There is a various mechanisms by which drug products may degrade. Loss of active Increase in concentration of active Alteration in bioavailability Loss of content uniformity Decline of microbiological status Loss of content uniformity Decline of microbiological status Loss of pharmaceutical elegance and patient acceptability Formation of toxic degradation products Loss of package integrity Reduction of label quality Modification of any factor of functional relevance. 1 1. Loss of active loss of drug is main significance in the stability studies of many pharmaceutical products. However, it is certainly true that for many products loss of potency is of major importance. we regard any product that contains less than 90% of label claim of drug as being of unacceptable quality.
The potency of product stored at the appropriate temperature (250 C for products to be labelled “store at controlled room temperature”) 2. Increase in concentration of active For some products, loss of vehicle can result in an increase in the concentration of active drug . For e.g. lidocaine gels exhibit this behavior Perfusion bags sometimes allow solvent to escape and evaporate so that the product within the bag shows an increase in concentration. 3. Alteration in bioavailability Bioavailability of drug products is a subject of great importance to those concerned with drug product quality . If the rate or extent of absorption that characterizes a product changes on storage, then this of course, a stability problem. In particular, if any changes of dissolution test data with time it would effect bioavailability 4. Loss of content uniformity Suspensions are the drug delivery systems most likely to show a loss of content uniformity as function of time. For such systems, determination of ease of redispersion or sedimentation volume may therefore be included in a stability protocol 5. Decline of microbiological status Basically, there are some possible ways in which the microbiological status of a pharmaceutical product can change significantly with time. The microorganism present in the product at the time of manufacture may reproduce and thus increase the number of viable organisms. Thus a product that, when assayed for total bioburden at the time of manufacture, is within limits, then tested after 6 months storage, exceed the maximum permitted bioburden is maximum. 6. Loss of pharmaceutical elegance and patient acceptability It includes any aspect of the product that might suggest that the product is somehow substandard or variable. For example some drugs that contain amino functional groups, when made into direct compression tablets that contain spray-dried lactose, which results the some slight brown speckling on the surface of the tablet . This is due to the interaction of the drug with a minor component in the lactose Analysis of the tablets might reveal no loss potency or change in dissolution, but no reputable manufacture will market tablets because of its look. It is important point about drug products is that attributes such as appearance, taste, and smell should be reproducible and not so any significant batch to batch variation . 7. Formation of toxic degradation products
If a drug degrades to a molecular species that is toxic, there must be special attention given to the quantity of such a species found in the product during its shelf life. The classic example is epianhydrotetracycline from tetracycline. 8. Loss of package integrity Change in the package integrity during storage or distribution can be a stability problem that may require careful monitoring. For example, if a plastic screw cap loses back-off –torque, the possibility of chemical or microbiological hazard may be significantly increased. 9. Reduction of label quality The label of a drug product must be regarded as an essential element of the product. It provides information on identity, use, and safety. Thus if any aspect of the label deteriorates with time, this can be a serious stability problem. For example, if the plasticizer in a plastic bottle migrates into the label and causes the ink to run and thus effects legibility, this is major problem. 10. Modification of any factor of functional relevance If there is any change of any functionality relevant attribute of a drug product that adversely affects safety, efficacy, or patient acceptability. For example, when some transdermal patches were first introduce into the US, a problem of adhesion ageing is observed . Freshly prepared patches show excellent skin adhesion, while it stored in room temperature for weeks or months show loss of adhesion. Thus in use the patches had a tendency to fall off the patient’s skin 1. Reasons for stability testing Our concerns for patients welfare To protect the reputation of the producer Requirements of regulatory agencies To provide a database that may be of value in the formation of other products Modes of degradation Chemical Chemical degradation is like solvolysis and oxidation. Our knowledge of kinetics can be of material assistance in dealing with chemical degradation. Physical Physical degradation can be caused by a range of factors such as freezing, thawing, or shearing The physical methods that could be used in evaluation of tablet friability, suspension redispersibilty, or injection syringeability Biological In north America, Japan, and western Europe it is microbiological stability problems. However, in some parts of the rats, ants, and the non microbiological factors can be responsible for stability problems
Stability indicating method General tests Appearance Assays/potency Sterility /container integrity Moisture Degradation products Product specific tests Aggregation (proteins) Biological activity (proteins) Dosage form specific tests Dissolution/release rate (tablets/patches) Leachable/extractable (injections) Particle size and turbidity( injection) Preservatives The essential elements of stability program Commitment of the organization to quality Firm grasp of underlying scientific theory Up-to-date knowledge of all relevant polices of regulatory agencies and applicable pharmacopoeial standards Effective communication between R&D, production, QC/QA, complaints, and regulatory affairs Understanding of the limitations of the analytical methods used in the stability program Careful monitoring of the stability budget Managerial skills to coordinate and optimize the program.1 What is kinetics? kinetics is a rate of reaction which takes place in a particular compound. It may be change in parent compound either physical or chemical. Physical change include biotransformation. Chemical change include degradation Application of kinetics in stability To understand the mechanism of what kind of change. To estimate the degradation time. Some time half life or shelf life may be determine by the kinetic. For prediction of process mean by keeping the compound, what kind of change and how much time will be taken for that change will be estimated.3 Routes by which pharmaceuticals degrade
a. 1. 2. 3. 4. 5. 6.
Chemical degradative routes Solvolysis Oxidation Photolysis Dehydration Racemization Incompatibilities
b. Physical degradative routes 1. Vaporization 2. Aging 3. Adsorption 4. Physical instability in Heterogeneous Systems Rate of reaction Reaction can be of two type 1. Homogeneous this is uniform process and taking place in single phase. 2. Heterogeneous taking place in more than two phase. • Eg. decomposition of drug in suspension and enzyme catalyzed reaction. • Rate of reaction depended upon concentration of the reactant. • Chemical kinetics deals with the experimental determination of reaction rates from which rate laws and rate constants are derived. • Relatively simple rate laws exist for zero order reactions (for which reaction rates are independent of concentration), first order reactions, and second order reactions, and can be derived for others. • The activation energy for a reaction is experimentally determined through the Arrhenius equation and the Eyring equation. 1. 2. 3. 4.
The main factors that influence the reaction rate include: the physical state of the reactants, the concentrations of the reactants, the temperature at which the reaction occurs, and whether or not any catalysts are present in the reaction.
Factors affecting rate of reaction 1. Nature of the Reactants Depending upon what substances are reacting, the time varies. Acid reactions, the formation of salts, and ion exchange are fast reactions. When covalent bond formation takes place between the molecules and when large molecules are formed, the reactions tend to be very slow. 2. Physical State The physical state (solid, liquid, or gas) of a reactant is also an important factor of the rate of change.
Reaction can only occur at their area of contact, in the case of a liquid and a gas, at the surface of the liquid. Vigorous shaking and stirring may be needed to bring the reaction to completion. This means that the more finely divided a solid or liquid reactant, the greater its surface area per unit volume, and the more contact it makes with the other reactant, thus the faster the reaction. 3. Concentration As the concentration of the reactants increases, the frequency of the molecules colliding increases, striking each other more frequently by being in closer contact at any given point in time. By increasing the amount of one or more of the reactants you cause these collisions to happen more often, increasing the reaction rate. 4. Temperature Temperature usually has a major effect on the rate of a chemical reaction. Collision frequency is greater at higher temperatures, contributes very small proportion to the increase in rate of reaction. The important factor is reactant molecule should have reactive energy higher then activation energy ( E>Ea ) to react. 5. Catalysts A catalyst is a substance that accelerates the rate of a chemical reaction but remains chemically unchanged afterwards. The catalyst increases rate reaction by providing a different reaction mechanism to occur with a lower activation energy. Proteins that act as catalysts in biochemical reactions are called enzymes. 5
The manner in which the concentration of drug (or reactants) influences the rate of reaction or process is called as the order of reaction. If C is the conc. of drug A, the rate of decrease in C of drug A as it is changed to B can be expressed as a function of time t. dC/dt = -K Cn dC/dt = -K Cn _______(1) K= rate constant n= order of reaction If n = 0 than zero order reaction, If n =1 than first order reaction dC/dt = term Change in conc. (dC) with respect to time (dt) called as rate of reaction Zero order kinetics (constant rate processes) if n = 0 than, dC/dt = -K0 C0 = -K0 ____(2) K0 = zero order rate constant It is a reaction whose rate is independent of the concentration of drug undergoing reaction, so the rate can’t be increased further by increasing the concentration of reactants. Rearrangement of equation (2) yields: dC = -K0 dt Integration of above equation: C – C0 = -K0 t C = C0 - K0t ______(3) C = conc. of drug at time t C0 =conc. of drug at time t = 0 Eq. 3 states that the conc. of reactant decreases linearly with time. A plot of C vs. t yields straight line having slope –K0 and y-intercept C0
Zero order half life It is the time period req. for the conc. of drug to decrease by one-half. When t = t1/2 and C = C0 /2, then eq. 3 becomes C0/2 = C0 – K0 t1/2 Solving above eq. t1/2 = C0/2K0 = 0.5C0/K0 ______(4)
Eq. shows that t1/2 of a zero order process is not constant but proportional to the initial conc. of drug C0 and inversely proportional to the zero order rate constant K0. 4
First order kinetics (Linear kinetics) If n = 1 then eq. 1 becomes dC/dt = -K C _____(5) K = first order rate constant in time-1 Rate of reaction is directly proportional to the conc. of drug undergoing reaction, so greater the conc. , faster the reaction.
Rearrangement of eq. 5 yields dC/C = -K dt Integration of above eq. gives. ln C = ln C0 – Kt _____(6) This eq. can also be written as exponential form as: C = C0 e-Kt e = natural log base First order process is also known as monoexponential rate process and characterized by logarithmic or exponential kinetics. Since ln = 2.303 log, eq. 6 can be written as log C = log C0 – (Kt/2.303) _______(7) A semilogarithmic plot of eq. 7 gives a straight line with slope = -K/2.303 and yintercept = log C0
First order half life put the value of C = C0/2 at t = t1/2 in eq.7 and solving it t1/2 = 0.693/K _____(8) It indicate that half life of first order reaction is a constant and independent of initial drug conc. Hydrolytic reaction of many drugs follows second order kinetics but after excess amt. of water is added then conc. remain constant throughout the process. It called apparent first order kinetics. SECOND ORDER KINETICS
If a drug substance A react with a second substance B then: A+B=C Then the rate eq. is -d[A]/dt = k [A][B]
MIXED ORDER KINETICS (NONLINEAR KINETICS)
In some cases, the kinetics of a pharmacokinetic process change from predominantly first order to predominantly zero order with increasing dose or chronic medication. A mixture of both first order and zero order kinetics is called as mixed order kinetics. Also known as nonlinear kinetics or dose dependent kinetics. Nonlinearity observed in certain drugs like; vitamin C, naproxen, riboflavin. The kinetics of such capacity limited processes can be described by the Michaelis-Menten kinetics. Michaelis Menten Equation
Mixed order kinetics is best described by this equation:
-dC/dt = Vmax C/(Km + C) _____(9)
-dC/dt = rate of decline of drug concentration with time, Vmax = theoretical maximum rate of the process, Km = Michaelis constant. 4
Factors affecting stability of drug and dosage form These factors can be broadly classified into 3 types depending on their effect on different type of stability which are as follows: 1) Chemical factors 2) Physical factors 3) Biological factors 1) Chemical factors Various ways of chemical degradation includes hydrolysis dehydration isomerization & racemization decarboxylation & elimination oxidation photo degradation drug – excipients & drug – drug interactions such as a) Reaction of bisulfite, an oxidant b) Reaction of amines with reducing sugars3 A) THE ROLE OF MOLECULAR STRUCTURE
It has been noted earlier that molecular structure of drug substance determines its degradation mechanisms and that substituents around the reaction centre can strongly influence its reactivity. Example: drug sub. having an electron withdrawing group close to an ester bond will probably exhibit a higher propensity nucleophilic attack by hydroxide ion than will a similar ester without that functional group Steric factors can be significant for many chemical reactions.
B) TEMPERATURE It is one of the primary factors affecting drug stability. The rate constant/temperature relationship has traditionally been described by the Arhenius equation,
k = A exp (-Ea/RT)
where Ea = activation energy
A = frequency factor
Arhenius equation has traditionally been used to describe the temperature dependency for various chemical reaction by regarding A and Ea as independent of temperature.
Temperature is obviously an important parameter because most reactions proceed faster at elevated temperatures than at lower temperatures.
The terms Ea and ∆H are a measure of how sensitive the degradation rate of a drug is to temperature changes.
Quantitation of the temperature Dependency of degradation rate constants can be done by 3 ways: 1) Prediction of Degradation rate by Linear Regression Analysis of the Arhenius Equation 2) Prediction of degradation rate by Nonlinear Regression Analysis of the Arhenius Equation. 3) Nonisothermal Prediction of Degradation Rate C) pH AND pH RATE PROFILES Second most important parameter The effect of pH on degradation rate can be explained by the catalytic effects that hydronium or hydroxide ions can have on various chemical reactions. If critical path in a reaction involves a proton transfer or abstraction step, other acids and bases present in solution can affect the rate of reaction. For ionizable drugs, the fraction of drug present in any particular form will depend on the pH of the solution, So, if the reactivity of the drug depends on its form, its reactivity will be pHdependent. A reaction in which hydronium ion, hydroxide ion, and water catalysis are observed can be described by
Kobs = kH+ aH+ + KH2O + KoH- aOHWhere Kobs = sum of specific rate constants aH+ = activities of hydronium ion aOH- =activities of hydroxide ion This equation is for the case when drug is neutral in the pH range of study.i.e.where the ionization of drug does not have to take into account D) BUFFER
These buffer species, like H+ and OH-, participates in formation of break down of activated complexes of various reaction and determine their reaction rate. These catalytic species are referred to as general acid-base catalysts Studies with phosphate buffer indicates that it enhance the degradation of various drug substances such as carbenicillin etc. In addition to acting as proto donor or accepter, buffer species can also act as Lewis acid and base through nucleophilic or electrophilic mechanisms.
E) IONIC STRENGTH-PRIMARY SALT EFFECT
For drug degradation involving reactions with or between ionic species, the rate is affected by the presence of other ionic species such as salts of NaCl. Ionic strength affects the observed degradation rate constants, k, by its effect on the reactivity coefficients, f. Ionic strength µ is described by
µ = ½ ε Ci Zi2 Where Ci = conc. Of ionic species I Zi =its electric change
As ionic strength increases, the rate of reaction between ions of opposite charge decreases and the rate of reaction between ions of similar charges increases. So, studying the effect of ionic strength can help our understanding of the possible charges of the species involved in the degradation.
Drugs can be affected by the availability of oxygen. Some photo degradation reactions involve photo oxidative mechanisms that are dependent on conc. of oxygen. Oxygen participates as reactant and also alters the degradation rate Oxygen exists in various states such ground state triplate oxygen, etc. Singlate oxygen is highly oxidizing and capable of attacking olfenic bonds. Super oxide species is a mild reductant while hydrogen peroxide is fairly specific oxidant. G) LIGHT
The number and wavelength of incident photons affect the photo degradation rate of drugs. It is not easy to study the effect of light quantitatively as the wavelength dependence of degradation varies among drug substances and because light sources have different spectral distributions.
Photo degradation for drug strongly dependence on the spectral properties of the drug substances and the spectral distribution of the light source.
H) Crystalline state and polymorphism in solid drugs Drugs in the crystalline state have lower ground state free energy and exhibit higher ∆G and so, slow reactivity. Many drug substances exhibit polymorphism-each crystalline state has a different ground state free energy level and a different chemical reactivity. The stability of drugs in their amorphous form is generally lower than that of drugs
In their crystalline form due to higher free energy level of amorphous form. Decreased chemical stability of solid drugs brought about by mechanical stresses such as grinding is said to be due to change in crystalline state -eg: grinding of aspirin increased degradation rate in suspension form.3
I) MOISTURE AND HUMIDITY
Drug degradation in heterogeneous system such as solid and semisolid states is affected by moisture. Moisture plays important role in catalyzing chemical degradation: 1) Water participates in the drug degradation process itself as a reactant, leading to hydrolysis; hydration etc. Here degradation rate is directly affected by the concentration of water, hydronium ion, hydroxide ion. 2) Water absorbs onto the drug surface and forms a moisture-sorbed layer in which the drug is dissolved and degraded. Water adsorption may also change the physical state of the drugs, thereby affecting their reactivity. J) EXCIPIENTS
The role that excipients play in drug stability has been extensively reported-e.g.: accelerating the effect of talc on hydrolysis of thiamine hydrochloride, the accelerating effect of magnesium stearate on tablet containing amines and lactose etc. Additional informations include reports on compatibility and incompatibility of drugs. Excipients can affect drug stability via various mechanisms. The most obvious examples are those in which the excipients participate directly in degradation as reactants.
1) Effect of moisture present in excipients 2) The effect pH changes caused by excipients: 3) Effect of surfactants
K) MISCELLANEOUS FACTORS
Effect of γ irradiation: not a common variable employed for sterilization of pharmaceuticals e.g. decreased activity of insulin after γ irradiations Components of pharmaceuticals exist in various physical states like amorphous, hydrated and solvated form. The rate of conversion will depend on chemical potential corresponding to the free energy difference between 2 states.
The various physical changes that can occur in drugs and excipients are as follow: 1) Crystallization of amorphous drugs Attempts are made to formulate poorly soluble drugs into amorphous form as it has higher solubility that of crystalline state. But amorphous form change to crystalline state duo low free energy of it. So crystallization of amorphous drug may occur on long storage leading to change in release character of drugs and so in its effect. E.g. :amorphous nifedipine co precipitated with polyvinylpyrrolidone, undergoes partial crystallization under high humidity conditions resulted altered dissolution and solubility 2) Vapor phase transfers including sublimation Pharmaceuticals containing components that easily sublime may undergo changes in drug content due to sublimation of it. E.g. nitroglycerine , which is liquid with significant vapor pressure,sublinguel tablet exhibited significant changes in drug content during storage due to inter tablet migration through the vapor phase 3) Moisture Adsorption generally observed with solid pharmaceuticals It leads changes in physical properties such as appearance and dissolution rate. Moisture adsorption is governed by the physical properties of drug and excipients.
E.g. :adsorption moisture by aspirin crystals enhanced by addition of hydrophilic excipients3
references 1. Drug stability: principles and practices, 3rd edition, by Jens T. Carstensen and C. T. Rhodes 2. Modern pharmaceutics, 4th edition, by Gilbert S. Banker and Christopher T. Rhodes 3. Stability of drugs and dosage forms by Sumie Yoshioka and Valentino J. Stella; Springer Publication 4. The theory & practice of Industrial Pharmacy by Leon Lachman, 3rd edition 5. www.wikipedia.com