Stability of cimetidine hydrochloride in admixtures after microwave thawing

The stability of cimetidine hydrochloride in admixtures subjected to freezing and subsequent microwave thawing was studied. Admixtures containing cimetidine 300 mg were prepared using 18 50-ml bags of 5% dextrose injection and 18 100-ml bags of 0.9% sodium chloride injection. A sample for analysis of pH and drug concentration was withdrawn from each bag, and all admixtures were frozen for 28 days. Nine admixtures of each solution were thawed at room temperature and the other nine in a microwave oven until a final solution temperature of 24 +/- 2 degrees C was reached. An additional five cimetidine admixtures in 0.9% sodium chloride injection were prepared and irradiated in the microwave oven until boiling occurred for five seconds. Samples of all admixture solutions were withdrawn within 30 minutes of thawing and again at 24 +/- 2 hours after thawing to determine pH and cimetidine concentration. Concentrations were measured by high-performance liquid chromatography. Admixtures pH after thawing did not vary substantially from baseline values. A significant difference in cimetidine concentration for each solution was observed between samples drawn at 0 and 24 hours after thawing; drug concentrations in 5% dextrose increased by 5.9% while those in 0.9% sodium chloride decreased by 1.5%. Thawing method had no significant effect on drug concentration in either admixture solution. In the solutions and drug concentrations tested, microwave thawing did not affect cimetidine stability. Increases in cimetidine concentrations in 5% dextrose solution may have been attributable to greater insensible water losses produced by heating of the small 50-ml bags.