Stability Studies

DRUG STABILITY

STABILITY:

Stability of a pharmaceutical product or a drug is defined as “extent to

which a product retains, within specified limits, and throughout its period

of storage and use (i-e its shelf life) the same properties and

characteristics that is possessed at the time of its manufacture ~ USP.

IMPORTANCE OF STABILITY:

Chemical and physical degradation of drug substances may change

their PHARMACOLOGICAL effects, resulting in altered efficacy

therapeutic as well as toxicological consequences.

Because pharmaceuticals are used therapeutically based on their

efficacy and safety, and being stable

For maintenance of quality until the time of usage or until

their expiration date stability is important.

The quality should be maintained under the various conditions that

pharmaceuticals encounter, during production, storage in

warehouses, transportation, and storage in hospital and community

pharmacies, as well as in the home.

KINETIC PRINCIPLES:

kinetics in pharmaceuticals, IS defined as the study of physical and

chemical reactions in drugs and dosage forms, FACTORS influencing

these chemical reactions are, ACCELERATED stability testing and

prediction of shelf life of formulations.

RATES AND ORDER OF REACTION:

To find whether how many constituents of the drug are active and which

primarily cause DEGRADATION AND factors like these, we need to

know the order of reaction

TYPES OF ORDER OF REACTION:

1. .zero order reaction

2. . First order reaction

3. .second order reaction

4. .pseudo first order reaction

5. .pseudo zero order reaction

ZERO ORDER REACTION:

If a rate of reaction is independent of concentration of reacting specie

the reaction is zero order reaction.

ZERO ORDER REACTION (continued).

The rate of zero order reaction is given by,

-dA/dt =k

dA is the change in concentration with respect of change in time. “-“ sign

indicates that concentration is decreasing⌠∫

This rate equation can be integrated between initial concentration A®

(original) the At after time “t”.

∫ dA = -k ∫ dt

At -A⁰ = -kt

At = A⁰ - kt

The above equation can also be written as,

K =A⁰ -At/t or

T= A⁰ -At/k

Half life of zero order reaction:

At = ½ A⁰ or

t 1/2= ½ A⁰/K

The time required for the drug to decompose by 10% (i.e. to 90% of

its original concentration). Thus,

At = 0.9A⁰

ZERO ORDER REACTION (continued).

Substituting above equation we get,

T0.9 = A⁰ -0.9A⁰ /k

When a graph is plotted of concentration against time it gives a straight

line

FIRST ORDER REACTION:

A first-order reaction is a reaction that proceeds at a rate that depends

linearly only on one reactant concentration.

The differential equation describing first order kinetics is:

A→ Product

Rate is the reaction rate (in unites of molar/time) and k is the reaction

rate coefficient (in units of 1/time).

The rate of reaction of first order reaction is given by

-dc/dt = kc

Dc/c = -kdt

Integrating between limits of concentration C⁰ at time t=0 and

concentration c at time t=t

∫ dc/c = -k∫ dt

Or converting to common log we get,

Log c =log C⁰ -kt /2.303 or

K= 2.303/t log c⁰/c or

K=2.303/t log a/(a-x)

FIRST ORDER REACTION (continued):

Where,

‘a’ is the initial concentration equal to C⁰

‘x’ is decrease in concentration in time “t”

‘a-x)’ is concentration remaining after time ‘t’ and equal to c in above

equation.

Here the unit for “k” is sec-1.

HALF LIFE OF FIRST ORDER REACTION:

t 1/2 = 2.303/k log C⁰/1/2 C⁰

= 2.303/k log2

=0.693 /k

SHELF LIFE OF FIRST ORDER REACTION:

T0.9 = 2.303/k log C⁰/0.9 C⁰

=2.303/k × 0.0457

=0.1052/k.

Where dc represents the concentration and dt represents time interval k

is the constant.

FIRST ORDER REACTION (continued):

These equations can be shown by plotting graph of concentration

against time which is shown in fig (a) and if graph is plotted of log c

against t, a straight line is obtained with a slope equal to –k/2.303

which is shown in figure (b) rate constant can be obtained from the

slope of the line.

SECOND ORDER REACTION:

A reaction is said to be second order when experimentally determined rate of reaction is proportional to first power of two reactants

A + B = Product

If a and b represents initial concentrations of A and B and x is the amount of A and B reacting in time t, the reaction rate dx/dt is given by,

Dx/dt = (a-x) (b-x)

Where (a-x) and (b-x) are conc. Of A and B.

If initial concentration of two components is equal then equation becomes

Dx/dt = k (a-x) 2

On integrating x=0 at t=0 and x=dx at t=t we get,

∫ dx/(a – x ) = k∫ dt

Kt =1/at × x / (a-x)

And if the initial concentration is not equal then,

SECOND ORDER REACTION (Continued):

Kt = 2.303/ (a-b) × log b (a-x) / a (b-x)

x slope = k

a(a-x)

Time

Plot of x/a (a-x) versus time for a second order reaction giving a

diagonal straight line.

NOTE:

For instance we have to find the value of "K" or "expiry date of a drug" or "in how

much time the drug will be reduced to half”,we can find all these by substituting

the experimentally found values in a certain order of reaction's equation. That

particular order of reaction is found by graphical method mostly i.e. seeing the

shape of the curve for example straight line is for zero order reaction.

To illustrate the above lines let us take an example.

Consider that the initial concentration of a drug is (c° = 94 unit/ml) and

point/value at which drug was degraded or not suitable for use was ( c= 45

unit/ml ) where k is 2.06 × 10 -5 hr -1, reaction following first order. Find expiry

date?

Here we can put the value in the following equation of first order,

T = 2.303/k log c°/c

T= 2.303/2.06× 10 -5 log 94/45

T= 4 years approx.

PSEUDO FIRST ORDER REACTION:

In a second order reaction ,if concentration of one component is too

large it virtually remains constant and rate of change in concentration

follows first order. Hydrolysis reaction are good example.

PSEUDO ZERO ORDER REACTION:

A compound decomposing in a solution exhibits first order reaction ,but

if more of it is present in insoluble form. The concentration of the

compound in a solution, thus remains essentially remains constant till

any of insoluble drug remains constant. Such a reaction is pseudo order

reaction. For example in ASPIRIN SUSPENSION it remains zero order

until aspirin is in solid form as it gets dissolved the reaction becomes

first order.

METHODS FOR FINDING ORDER OF REACTION:

1. Substitution Method

2. Graphical Method

3. Half Life Method

4. Ostwald Isolation Method

Substitution Method:

In this method experimentally obtained data is substituted in relevant

rate equation. The equation that yields a fairly constant value of k

indicates the order of reaction. Table below shows the rate and half life

equation for different order reactions.

GRAPHICAL METHOD:

In this method the data obtained from experiment is plotted to the

relevant form, to determine the particular order of reaction. For example

if a straight line comes it is a zero order reaction.

Half Life Method:

In this method half life's are determined graphically by plotting a versus t

at two different initial concentrations a1 and a2 .The values of half life

and initial concentration are then substituted in the equation below and

order of reaction is obtained directly.

Ostwald Isolation Method

This method is introduced by Ostwald and is used to determine the

order of a reaction with respect to one reactant at a time. The total order

of the reaction can be calculated by adding all the orders of there action

for individual reactant.

log t1/2(1)/t1/2(2)

log (a2/a1)n =

Ostwald Isolation Method (Continued):

The principle behind this method is that if the concentration of all but one reactant is taken in excess, then during the course of reaction, the concentration of those reactants taken in excess will remain constantan the variation in the rate will correspond to the concentration of that reactant whose concentration is small. DEGRADATION FACTORS:

1. Temperature: high temperature accelerate oxidation, reduction and hydrolysis reaction which lead to drug degradation

2. pH:

• Acidic and alkaline pH influence the rate of decomposition of most drugs.

• Many drugs are stable between pH 4 and 8.

• Weekly acidic and basic drugs show good solubility when they are ionized and they also decompose faster when they are ionized. So if the pH of a drug solution has to be adjusted to improve solubility and the resultant

Ostwald Isolation Method (Continued):

pH leads to instability then a way out of this tricky problem is to

introduce a water miscible solvent into the product.

• Reactions catalyzed by pH are monitored by measuring degradation

rates against pH, keeping temperature, ionic strength and solvent

concentration constant. Some buffers such as acetate, citrate, lactate,

phosphate and ascorbate buffers are utilized to prevent drastic change

in ph.

• Sometimes pH can have a very serious effect on decomposition. As

little as 1 pH unit change in pH can cause a change of ten fold in rate

constant. So when we are formulating a drug into a solution we should

carefully prepare a pH – decomposition profile and then formulate the

solution at a pH which is acceptable physiologically and stability-wise

also.

3. Moisture:

a. Water catalyzes chemical reactions as oxidation, hydrolysis and

reduction reaction

b. Water promotes microbial growth

4. Light: affects drug stability through its energy or thermal effect which

lead to oxidation.

5. Pharmaceutical dosage forms: solid dosage forms are more stable than liquid dosage forms for presence of water.

6. Concentration: rate of drug degradation is constant for the solutions of the same drug with different concentration. So, ratio of degraded part to total amount of drug in diluted solution is bigger than of concentrated solution.

7. UV light: It has a high energy level .it is the cause of many degradation reactions, exposure to UV illumination may cause oxidation (photo-oxidation) and scission (photolysis) of covalent bonds. Yellow-green glass gives the best protection against the UV.

STABILITY TESTING:

First we need to know about the climatic zones .

Zone 1: temperate

Zone 2: Sub tropical with high positive humidity

Zone 3:Hot ,Dry

Zone 4: Hot ,humid

It is categorized in three types majorly,

Accelerated stability testing:

Studies designed to increase the rate of chemical degradation or

physical change of an active substance or drug product by using

exaggerated storage conditions as part of the formal, definitive storage

program.

o Temperature & humidity specifications: In this study ,a storage

condition of 40®c ±2®c and relative humidity of 75±5% for all the four

zones for drug substances to be stored at 25 or 30®c.

o The studies are carried out for 6 months .

o For drug substances recommended to be stored in a refrigerator the

study should be carried out at 25±2®c and 60±5%.

o In general the accelerated storage conditions must be at least 15%

above the expected actual temperature and appropriate relative

humidity.

o Input of heat required to cause product failure is determined. This is

done to subject the product to a condition that accelerates

degradation. This information is then projected to predict shelf life or

used to compare the relative stability of alternative formulations .

o Accelerated stability testing (continued):

This usually provides an early indication of the product shelf life and thus shortening the

development schedule. In addition to temperature, stress conditions applied during accelerated

stability testing are moisture, light, agitation, gravity, pH and package accelerated stability

testing, a product is stressed at several high temperatures .

In accelerated stability testing, a product is stressed at several high (warmer than ambient)

temperatures and the amount of heat input required to cause product failure is determined.

This is done to subject the product to a condition that accelerates degradation. This information

is then projected to predict shelf life or used to compare the relative stability of alternative

formulations. This usually provides an early indication of the product shelf life and thus

shortening the development schedule. In addition to temperature, stress conditions applied

during accelerated stability testing are moisture, light, agitation, gravity, pH are also applied.

o Accelerated stability testing (continued):

This usually provides an early indication of the product shelf life and thus shortening the

development schedule. In addition to temperature, stress conditions applied during accelerated

stability testing are moisture, light, agitation, gravity, pH and package accelerated stability

testing, a product is stressed at several high temperatures .

In accelerated stability testing, a product is stressed at several high (warmer than ambient)

temperatures and the amount of heat input required to cause product failure is determined.

This is done to subject the product to a condition that accelerates degradation. This information

is then projected to predict shelf life or used to compare the relative stability of alternative

formulations. This usually provides an early indication of the product shelf life and thus

shortening the development schedule. In addition to temperature, stress conditions applied

during accelerated stability testing are moisture, light, agitation, gravity, pH are also applied.

LONG TERM STABILITY TESTING:

According to latest guidelines Expiry date is determined from Long-

Term Stability Test Data Accelerated data considered only supportive.

For long-term studies, frequency of testing should be sufficient to

establish the stability profile of the API.

For APIs with a proposed re-test period or shelf-life of at least 12

months, the frequency of testing at the long-term storage condition

should normally be every three months over the first year, every six

months over the second year and annually thereafter throughout the

proposed re-test period or shelf-life.

TEMPERATURE AND HUMIDITY SPECIFICATIONS: In zone 1 and 2

temperature range is 25±2ºc and relative humidity is 60±5%, and in

zone 3 and 4 temperature conditions are 30±2ºc and relative humidity

is 65±5%.

Long term stability testing is normally performed for longer

duration of the test period in order to allow significant

product degradation under recommended storage conditions

.During the testing, data is collected at an appropriate

frequency such that a trend analysis is able to distinguish

instability from day-to-day ambiguity. The reliability of data

interpretation can be increased by including a single batch of

reference material for which stability characteristics have

already been established. Stability of the reference material

also includes the stability of reagents as well as consistency of

the performance of the instrument to be used throughout the

period of stability testing. However, system performance and

control for drift and discontinuity resulting from changes in

both reagents and instrumentation must be monitored

LONG TERM STABILITY TESTING (continued):

Long term testing in substances which are to be stored in refrigerator are

recommended at 5±3ºc and for those who are to be stored in freezer are

recommended at -20±5ºc.

INTERMEDIATE STABILITY TESTING:

30±2ºc /60 ±5%.

Cyclic temperature stress testing:

This is not a routine testing method for marketed products. In this method,

cyclic temperature stress tests are designed on knowledge of the product so

as to mimic likely conditions in market place storage.

THEORATIC CONSIDERATIONS:

Stability study requirements and expiration dating are covered in GMP the

USP and FDA.

The main considerations are as follows,

Product labeling of official articles should provided recommended storage

conditions and expiry date assigned to specific formulation.

According to FDA guidelines, now registration of pharmaceuticals

worldwide is appreciated.