Stability of polymyxin B sulfate diluted in 0.9% sodium chloride injection and stored at 4 or 25 °C

Purpose. The stability of polymyxin B sulfate in infusion bags containing 0.9% sodium chloride injection stored at 4 and 25 degrees C was studied. Methods. Seven manufacturing batches of polymyxin B from different sources were tested. The products were reconstituted in sterile water for injection, diluted in infusion bags containing 0.9% sodium chloride injection, and stored at room temperature (25 degrees C) or under refrigeration (4 degrees C). Samples were withdrawn at the same time on days 0, 1, 2, 3, 5, and 7. A modified microbiological assay was used to determine the concentrations, as indicated by zones of inhibition, of polymyxin B. Bordetella bronchiseptica served as the reference organism. Stability was defined as retention of >90% of the initial concentration. The decomposition kinetics of polymyxin B in 0.9% sodium chloride injection were evaluated by plotting the polymyxin B concentration remaining versus time. Results. On average, the samples retained over 90% of their initial concentration for up to two days at both storage temperatures. All samples retained over 90% of their initial concentration at 24 hours. The decomposition kinetics of polymyxin B in infusion bags containing 0.9% sodium chloride injection exhibited pseudo-first-order kinetics, with rate constants of 0.024-0.075 day(-1) at 25 degrees C and 0.022-0.043 day(-1) at 4 degrees C (p > 0.05). Conclusion. Polymyxin B was stable for at least one day when stored at 4 or 25 degrees C in infusion bags containing 0.9% sodium chloride injection. Stability did not differ significantly between the two storage temperatures.