Gentamicin, Erythromycin,Biological Assay,Doxycycline Tetracycline

This study was carried out to determine the stability of antibiotics after they were exposed to drastic conditions of high temperature and high humidity by the use of microbial methods of analysis. Sudan is classified as zone IV country (hot and humid) based on the classification of the WHO of climatic zones all over the world, and the drugs are exposed to very hard conditions during transport and storage, so drugs must be highly stable in order to withstand these conditions. Accelerated stability studies are used to estimate the stability of the drug formulation when used under normal conditions during its real shelf life. The study was done using four antibiotics, doxycycline, tetracycline, gentamicin, and erythromycin in different dosage forms, tablets, capsules, injections, and oral suspensions. The samples were stored in stability cabinet at temperature of 50°C and relative humidity of 75% for six months. Samples were removed every month for the first three months and at the sixth month. These samples were assayed by the microbial methods and examined for changes occurred in the their physical appearance. Other tests like dissolution and disintegration of tablets and capsules were also done. The microbial assay was done by two methods. Agar diffusion method was used for the assay of doxycycline tablets, gentamicin injections, and erythromycin suspensions, while tetracycline capsules were assayed by the use of turbidimetric method. In agar diffusion method the melted agar media were inoculated with suitable test organisms then poured into sterile petri dishes and the antibiotic was applied in stainless cylinders put on the surface of the solidified media, then the petri dishes were incubated for 18 hours and the diameters of zones of inhibition were measured. For turbidimetric method, liquid media was inoculated with suitable test organism and added to the antibiotics in sterile test tubes, then incubated for 2-4 hours and the turbidity caused by growth of the microorganism was read in the form of transmittance by the use of spectrophotometer. In both methods the assay was based on comparison of the response (zone diameter or transmittance) of the samples with the response of standard of known potency. Serial dilutions of both samples and standards were used, the response was measured, and the content was calculated by statistical methods. Media, test organisms, serial dilutions, and other test conditions were obtained from the different Pharmacopoeias. The results of the assays in every month were obtained and statistically analysed by comparing the content of active ingredient at the zero time, (before storage) and its content in each following month. The statistical analysis showed that no statically significant changes had occurred in the content of active ingredient in all samples, but on observation of the results, it was found that the contents of doxycyline and tetracycline were clearly decreased while gentamicin and 14 erythromycin were not greatly affected. Also the colours of doxycyline, tetracycline and the erythromycin suspension were changed. The study concluded that doxycycline and tetracycline are not stable under the conditions of accelerated stability testing, while gentamicin and erythromycin are stable. The study gave some recommendations with regard to packaging materials and some excipients used in the formulations of the drugs