Preservative efficacy testing of refrigerated pharmaceuticals: choice of challenging isolate and storage temperature

The antimicrobial effectiveness test assesses the performance of the preservatives and/or antimicrobial substances added to multiuse sterile or nonsterile dosage forms to protect these products from microbiological growth. In this study, insulin products underwent the European Pharmacopoeia and the United States Pharmacopeia antimicrobial preservative effectiveness tests at both room and refrigerated temperatures. Notably, a psychrotolerant strain, <i>Serratia marcescens</i>, originally isolated from a refrigerated pharmaceutical product, was included in the antimicrobial effectiveness testing. When evaluated against the compendial requirements, both neutral protamine Hagedorn and regular insulin stored at room temperature successfully met the European Pharmacopoeia-A criteria and the United States Pharmacopeia standards. However, although both formulations stored under refrigeration conformed to the United States Pharmacopeia criteria, they failed to satisfy the European Pharmacopoeia-A criteria at contact times of <7 days. The reductions in <i>Serratia marcescens</i> counts were the same at both incubation temperatures. This study indicated that for sterile multidose vials, performing the antimicrobial effectiveness test at both room and refrigerated temperatures might give a more accurate indication of antimicrobial preservative efficacy.