Sensitive Analytical Methods for Determination of Stability of Drugs in Pharmaceutical Dosage Forms

Mr.Vitthal V.Chopade

Stability of Drugs

The stability indicating method can be defined as Validated Quantitative analytical method that can detect the change with time in the chemical, physical or microbiological properties of the drug substance and drug product, and that are specific so that the content of active ingredient, degradation can be accurately measured without interference.

The ICH guideline indicates that stress testing is designed to determine the intrinsic stability of the molecule by establishing degradation pathway in order to identify the likely degradation products and to validate the stability indicating power of the analytical procedure used¹.

The International Conference on Harmonization (ICH) guidelines ‘stability testing of new drug substances and products’ Q1A requires that stress testing should be carried out to elucidate the substance. It suggests that the degradation product that are formed under the variety of condition should include the effect of temperature, humidity where appropriate oxidation, photolysis and susceptibility to hydrolysis across a wide range of pH value^2-6.

In the guideline, the study of effect of temperature is suggested to be done in 10C increment above the accelerated temperature (50C, 60C etc.) and that of humidity at a level of 75 % or greater. No details are however provided for the study of oxidation, photolysis and hydrolysis at different pH values. The ICH guidelines Q3B entitled ‘Impurities in New Drug Products emphasizes on providing documented evidence that analytical procedures are validated and suitable for the detection and quantitation of the degradation product. It is also required that analytical method should be validated to demonstrate that impurities unique to the new drug substance do not interfere with or are separated from specified and unspecified degradation products in the drug products.

Stress testing is a critical component of drug development. By generating key stress testing sample (i.e. partially degraded sample, stressed under various condition) predictive degradation information can be obtained early in the process and can be of significant value to a drug company in terms of time and money. In addition stress testing can help in the selection of more stable drug substance, salt forms and drug formulation^7-9.

figure:1. Flow Chart of Stability Studies.

Acid and alkali hydrolysis:

The hydrolytic degradation of a new drug in acidic and alkaline condition can be studied by refluxing the drug in 0.1 N HCl / 0.1 N NaOH. If reasonable degradation is seen, testing can be stopped at this point. However in case no degradation is seen under these conditions the drug should be refluxed in acid/alkali of higher strength & for longer duration of time. Alternatively if total degradation is seen after subjecting the drugs to initial condition, acid/alkali strength can be decreased with decrease in reaction temperature¹⁰.

Figure No 2: Flow chart of acid/alkali induced hydrolysis

Oxidation:

To test for oxidation, it is suggested to use hydrogen peroxide in the concentration range of 3 to 30 %. In some drugs extensive degradation is seen when exposed to 3% of hydrogen peroxide for very shorter time period at room temperature. In other cases exposure to high concentration of hydrogen peroxide, even under extreme condition does not cause any significant degradation. The behavior is on expected lines, as some drugs are in fact oxidisable, while there are others that are not. The latter are not expected to show any change even in the presence of high dose of oxidizing agent¹¹.

Figure No 3: Flow chart of oxidation

Photolytic degradation:

Sunlight: - The photolytic studies should cover the exposure of drug solution to sunlight. The drug solution should be exposed to sunlight for 4 days¹².   

UV light: The drug solution should be exposed to UV radiation, in UV chamber for 4 days to study the photolytic stability off drug.

Figure No 4: Flow chart of Photo stability

Neutral hydrolysis:

Stress testing under neutral condition can be started by refluxing the drug in water for 12 hours. Refluxing time should be increased or decreased as per the degradation obtained in 12 hours.

Dry heat:

Heating the drug powder at high temperature in oven can carry out stress testing for dry heat degradation. The heating time can be increased up to 12 hrs and above if there is no sufficient degradation seen in initial studies.

Wet heat:

Wet heat degradation can be studied by refluxing the drug solution for several hrs.

Handling the reaction samples for chromatographic studies:

Another practical aspect of stress testing that generates enquiries from practitioners is one that concern the best way to handle samples containing high concentrations of acid, alkali or oxidizing agent for HPLC¹³. One approach is to dilute the sample enough so that the concentration of reagent falls within the acceptable range. For HPLC the dilution can be performed in the mobile phase, where as for TLC a suitable solvent such as methanol or ethanol can be used. The second approach involves neutralization of acid and alkali solutions to tolerable pH. Dilution is often easier than neutralization. The problems with neutrlisations are that it is difficult to perform in a quantitative manner and moreover it generally leads to precipitation of the dissolved ingredients of the sample. That can be controlled by filtration of that sample by syringe filter.

References:

1. Ewing, G. W., Instrumental Methods of Chemical Analysis, Mc Graw Hill International Book Co, London, Ed. 4^th , 1981, 1-7.
2. Hager, R. N., Applications of UV-Visible Derivative Spectrophotometry, J. Anal. Chem, A. 1973, 13, 1131.
3. ICH Guidance on Analytical Method Validation, International Convention for the Pharmaceutical Industry, November 2005.
4. ICH, Q1A (R2) Stability testing of new drug substances and products, International Conference on Harmonization, 2003.
5. ICH, Q2A (R1) Validation of analytical procedure, International Conference on Harmonization, 1995.
6. ICH, Q2B Validation of analytical procedure: methodology, International Conference on Harmonization, 1997.
7. Pikering, W. F., Modern Analytical Chemistry, Macel Dekker Inc, New york, 1971, 265.
8. Sethi, P. D., Quantitative Analysis of Drugs in Pharmaceutical Formulations, Unique Publisher, 1997, 11.
9. Singh. S., Bakshi. M., Guidance on the Conduct of Stress Tests to Determine the Inherent Stability of Drugs, Pharmaceutical Technology, Asia Sep/Oct. 2000.
10. Skoog, D. A., West, D. M., Holler, F. J. Analytical Chemistry- An Introduction, Saunders College Publishing, Philadelphia, Ed. 7^th , 2002, 3-4, 5-11. 
11. Skoog, D. A., West, D. M., Principles of Instrumental Analysis, , Ed.2^nd, 1980, Saunderg Golden, Japan, 2-3.
12. Willard, H. H., Merrit, L. L., Dean, J. A., Instrumental Methods of Analysis, Ed. 6^th , CBS Publisher, 2000, 82-83.
13. Wilson and Wilson: A series of Comprehensive Analytical chemistry, 1986, 1-7, 653-656.

About Authors:

Mr.Vitthal V.Chopade
Lcturer in Pharmaceutics Modern College of Pharmacy, Yemuna Nagar Nigdi, Pune-44. He has done his M. Pharm in Quality Assurance from Nagpur University. He is a Life member of APTI. He has published and presented several research and review articles in national level and International level. E- Mail: vi_research@rediffmail.com Phone No. 9923277395

Miss. Suvarna J. Kshirsagar
Lcturer in Pharmaceutical Chemistry Modern College of Pharmacy, Yemuna Nagar Nigdi, Pune. E-mail: - suvarnajk_26d@yahoo.co.in

Prof.A.N.Tankar
Prof in Pharmacognosy Siddhant College of Pharmacy, Sudumbare, Pune. He has done his M. Pharm in Pharmacognosy. He is a Life member of APTI.