Biol. Pharm. Bull. 28(6) 1126—1128 (2005)

androsten-17a-yl)propionic acid g-lactone (SPN), is a competitive aldosterone antagonist, which is used as a potassiumsparing diuretic in premature infants to reduce lung congestion. The stability of SPN in solution was reported by Pramar and Gupta. In the study, SPN was most stable at pH 4.5, and the degradation rate increased with increasing and decreasing pH. The apparent first-order rate constant of SPN at 40 °C in pH 7.3 buffer solutions was 8.3 10 4 h . SPN degradation seemed to be a type of hydrolysis. We previously reported that the degradation of (Me)Arg–Lys–Pro–Trp–tertLeu–Leu–OEt (NT-1) and nikkomycin Z, which were hydrolyzed in the solution, were accelerated in rat plasma. We therefore theorized that SPN degradation might also be accelerated in rat plasma. However, the stability of SPN in rat plasma has not been reported, and the blood, serum, and plasma samples in the pharmacokinetic studies were treated without considering the stability of SPN in the samples. If SPN in the samples is unstable, the pharmacokinetic data reported are believed to be incorrect. We therefore began a study into SPN stability in rat plasma. In this report, SPN stability in rat plasma is first described, and then the pharmacokinetic SPN data is shown in rats after an intravenous administration of 20 mg/kg obtained with temperature strictly controlled for plasma and blood samples.