Preformulation method for parenteral preservative efficacy evaluation

A method is described for rapidly and reliably evaluating parenteral preservative efficacy. Solutions containing antimicrobial preservatives were challenged with microorganisms, sampled from 0.5 to 6 h following introduction of the challenge, cultured, and counted for surviving microbial cells. Data were analyzed by computer according to two models: linear and quadratic. Decimal reduction times (D values) were calculated for each microbial challenge in each preservative solution. A D value of less than or equal to 2 h for bacteria predicts that the preservative system will pass the British Pharmacopoeia (BP) preservative efficacy test, a more rigorous test than the USP test. Fourteen preservative systems were tested in both neutral isotonic saline solutions and neutral regular insulin solutions. D values and correlation coefficients for both models were calculated. The ranking of preservative effectiveness in neutral saline solutions closely correlated with the results found using neutral regular insulin solutions. The most effective preservative systems were found to be 0.3% m-cresol and various combinations of m-cresol and phenol. The advantages and limitations of this method are discussed.