The present study proposes an agar diffusion microbiological assay for quantitation of the antibiotic doripenem
in powder for injection. The method was developed according to official guidelines and its stability-indicating performance
was studied by stress testing in pre-defined conditions of heat and UV-C radiation. Based on the response obtained
from growth inhibition of a strain of Staphylococcus epidermidis ATCC 12228, the quantitation was carried out in the
range of 1.0 - 4.0 µg ml-1. The percentage of drug estimated was 99.42% (RSD=3.12%), with satisfactory intra-day and
inter-day precision. In the recovery test, the method was found to be reliable and accurate in quantitation of doripenem
(mean recovery = 100.30%). Through alkaline hydrolysis experiments and by comparison to HPLC, bioassay was found
to be specific. In kinetics degradation experiments, where the influence of temperature (45 ºC) and UV-C radiation were
evaluated, higher rate of degradation was observed for the first, with a drug remaining content of 55.74% after 12h of
storage. The reactions could be described by zero-order kinetics. The proposed bioassay was statistically compared to
HPLC and CE methods concerning quantitative purposes and the results indicated they are equivalent, which demonstrates
that the bioassay can be routinely used for determination of antibiotic potency, besides of being a complementary
tool in stability studies of drugs.
Keywords: Agar diffusion bioassay, degradation kinetics, doripenem, method validation, photostability, thermal stability.
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