[Determination of aspirin and nicotinic acid in plasma when used in combination]

An ultraviolet spectrophotometric method has been developed for indirect determination of aspirin (ASA) using the difference between total salicylic acid and free salicylic acid in rabbit plasma. Recoveries of salicylic acid were found to be above 98.4% and the method was linear from 8-160 micrograms with a correlation coefficient of 0.9999. The coefficient of variation was less than 3.85%. Using this method for ASA and Carlson's method for nicotinic acid (NA), the plasma concentrations of these two drugs were determined when they were used iv alone or in combination in rabbits. The results analysed by a micro-computer were fitted to a two compartment open model with first-order elimination. Among the pharmacokinetic parameters of NA, T1/2 (beta) and AUC0-infinity for the group (n = 6) treated with NA 50 mg/kg in combination with ASA 50 mg/kg were 2.3374 h and 361.63 micrograms.h/ml, respectively, both significantly different from those for the group (n = 6) treated with NA 50 mg/kg alone which were 0.9846 h and 157.71 micrograms.h/ml, respectively (p less than 0.01). For ASA, no significant differences were demonstrated. The mechanism and significance of the prolonged T1/2 (beta) of NA were discussed.