Validated specific HPLC method for determination of zidovudine during stability studies.

The objective of the current study was to develop a validated stability-indicating assay method (SIAM) for zidovudine (3'-azido-3'-deoxythymidine) after subjecting it to forced decomposition under hydrolysis, oxidation, photolysis and thermal stress conditions. The drug decomposed under hydrolytic stress upon refluxing, and also on exposure to light. It was stable to oxidation and thermal stress. The same major decomposition product could be seen in all the decomposed solutions, which was identified as thymine through comparison with the standard. Separation of drug from major and minor degradation products was successfully achieved on a C-18 column utilising water-methanol in the ratio of 77:23. The detection wavelength was 265 nm. The method was validated and response was found to be linear in the drug concentration range of 25-500 microg ml(-1). The mean values (+/-R.S.D.) of slope and correlation coefficient were 21,859 (+/-0.213) and 0.9995 (+/-0.00578), respectively. The R.S.D. values for intra- and inter-day precision were <0.9 and <1.6%, respectively. The method was established to have sufficient intermediate precision as similar separation was achieved on another instrument handled by a different operator. The recovery of the drug from a mixture of degraded samples ranged between 100.6 and 100.9%. PDA peak purity test confirmed the specificity of the method. The method was also successful in analysis of drug in marketed tablets subjected to stability testing under accelerated conditions of temperature, humidity, and to thermal and photolytic stress.

Establishment of inherent stability of stavudine and development of a validated stability-indicating HPLC assay method.

The present study describes degradation of stavudine under different stress conditions (hydrolysis, oxidation, photolysis and thermal stress), and establishment of a stability-indicating reversed-phase HPLC assay method. The drug was found to hydrolyse in acidic, neutral and alkaline conditions and also under oxidative stress. The major degradation product formed under various conditions was thymine, as evidenced through comparison with the standard and spectral studies (NMR, IR and MS) on the isolated product. Separation of drug, thymine and another minor degradation product was successfully achieved on a C-18 column utilising water-methanol in the ratio of 90:10. The detection wavelength was 265 nm. The method was validated with respect to linearity, precision (including intermediate precision), accuracy and specificity. The response was linear in the drug concentration range of 25-500 microg ml(-1). The mean values (+/-R.S.D.) of slope and correlation coefficient were 24256 (+/-0.679) and 0.9994 (+/-0.0265), respectively. The R.S.D. values for intra- and inter-day precision studies were <0.210 and <1%, respectively. The recovery of the drug ranged between 99.7 and 101.5% from a mixture of degraded samples. The method even proved to be affective on application to a stressed marketed capsule formulation.

Behavior of decomposition of rifampicin in the presence of isoniazid in the pH range 1-3.

The extent of decomposition of rifampicin in the presence of isoniazid was determined in the pH range 1-3 at 37 degrees C in 50 min, the mean stomach residence time. With increase in pH, the degradation initially increased from pH 1 to 2 and then decreased, resulting in a bell-shaped pH-decomposition profile. This showed that rifampicin degraded in the presence of isoniazid to a higher extent at pH 2, the maximum pH in the fasting condition, under which antituberculosis fixed-dose combination (FDC) products are administered. At this pH and in 50 min, rifampicin decomposed by approximately 34%, while the fall of isoniazid was 10%. The extent of decomposition for the two drugs was also determined in marketed formulations, and the values ranged between 13-35% and 4-11%, respectively. The extents of decomposition at stomach residence times of 15 min and 3 h were 11.94% and 62.57%, respectively, for rifampicin and 4.78% and 11.12%, respectively, for isoniazid. The results show that quite an extensive loss of rifampicin and isoniazid can occur as a result of interaction between them in fasting pH conditions. This emphasizes that antituberculosis FDC formulations, which contain both drugs, should be designed in a manner that the interaction of the two drugs is prevented when the formulations are administered on an empty stomach.

Evaluation of the recently reported USP gradient HPLC method for analysis of anti-tuberculosis drugs for its ability to resolve degradation products of rifampicin.

The recently notified USP gradient HPLC method for quantitative determination of rifampicin, isoniazid and pyrazinamide in fixed dose combination (FDC) formulations was evaluated to determine its ability to resolve major degradation products of rifampicin, viz. 3-formylrifamycin SV, rifampicin N-oxide, 25-desacetyl rifampicin, rifampicin quinone, and the newly reported isonicotinyl hydrazone, an interaction product of 3-formylrifamycin and isoniazid. The first observation was that the requirements of theoretical plates listed in the given method were met for rifampicin, but not for isoniazid and pyrazinamide, even on columns of different makes. The resolving power of the method was also dependent upon make of the column. On two of the three columns of the three tested, it was able to resolve most degradation products, except rifampicin N-oxide and 25-desacetylrifampicin, which were overlapping. The method was modified and an overall satisfactory resolution for all components was obtained by changing the buffer: organic modifier ratio of solution B in the gradient from 45:55 to 55:45 and decreasing the flow rate from 1.5 to 1.0 ml/min, keeping all other conditions constant.

New findings on degradation of famotidine under basic conditions: identification of a hitherto unknown degradation product and the condition for obtaining the propionamide intermediate in pure form.

The degradation behavior of famotidine (1) was investigated in 25% ammonia solution and in 2 M NaOH. The hydrolysis of the drug in ammonia resulted in separation of [3-[[2-[(diaminomethylene)amino]-4-thiazolyl]methyl]thio]propionamide (3), an impurity listed in British Pharmacopoeia. The treatment with 2 M NaOH resulted in formation of [3-[[[2-[(diaminomethylene)amino]-4-thiazolyl]methyl]thio]propionyl]sulfamide (2) and 3. These products further decomposed to [3-[[2-[(diaminomethylene)amino]-4-thiazolyl]methyl]thio]propionic acid (4) and a heretofore unknown product, 5. The latter separated out in the reaction mixture as brown shiny crystals. Proton and carbon-13 nuclear magnetic resonance spectroscopy, mass spectrometry, and elemental analysis of the charcoal-treated product established the structure. The formation of 5 is postulated to involve abstraction of a proton from the alpha-carbon of intermediates 2 and 3 followed by elimination of the thiol moiety.