EFFECT OF METABOLITES ON THE HUMAN PLASMA CONCENTRATION AS TOTAL ACTIVITY AFTER ORAL ADMINISTRATION OF ROKITAMYCIN: COMPARISON OF THREE DIFFERENT BIOASSAY METHODS

Three different bioassay methods for rokitamycin (TMS-19-Q) were compared to each otherin vitro and in vivo. Method I was a paper disc method using Micrococcus luteus ATCC 9341 as the test organism in nutrient agar (pH 8.0), method II was a paper disc method using Streptococcus pyogenes COOK in brain heart infusion agar (pH 7.4) and method III was an agar well method using M. luteus ATCC 9341 in the agar medium provided in the Minimum Requirements for Antibiotic Products of Japan (pH 6.5). Ratios of relative potency of TMS-19-Q and its metabolites calculated from standard curves in human plasma using these bioassay methods, were 1.00: 1. 13: 3. 34: 0. 96 (TMS-19-Q: 10-OHTMS-19-Q: LM A7: LM V) in method I, 1.00: 1.58: 0.40 (TMS-19-Q: LM A7: LM V) in method II and 1.00: 0. 51: 0.86: 0.114 (TMS-19-Q: 10-OH-TMS-19-Q: LM A7: LM V) in method III. The relative potency obtained from method I, generally used for macrolide antibiotics, greatly contradicted MIC values of TMS-19-Q and its metabolites against clinically isolated 190 strains of Staphylococcus aureus. On the other hand, the relative potency obtained from method III reflected closely MIC values of TMS-19-Q and its metabolites. Therefore, method III should be the most suitable bioassay method to measure the plasma concentration of TMS-19-Q. Plasma concentrations measured by these 3 different methods were compared to each otherin vivo, after oral administration of TMS-19-Q at a dose of 1,200 mg to each of fasted healthy volunteers. Values obtained from method I were about 4.3 times higher than those method III. The difference in plasma concentrations among these 3 bioassay methods was found due to differences in relative potencies of metabolites to TMS-19-Q.