Lansoprazole is a substituted benzimidazole which inhibits gastric acid secretion by inhibiting the hydrogen-potassium ATPase (proton pump) in the parietal cell. The finding of Leydig cell hyperplasia and Leydig cell tumors in 2-year oral studies in Sprague-Dawley rats but not in CD-1 mice prompted investigative studies to determine the mechanism for the Leydig cell changes. hCG challenge studies in Sprague-Dawley rats revealed decreased testosterone responsiveness in rats treated orally for 1 or 2 weeks with lansoprazole. After 4 weeks of daily oral treatment increases in serum LH and decreases in serum testosterone were detected within a few hours after dosing. In a study where 9-month-old male F344 rats were given testosterone supplementation via Silastic implants and then treated with lansoprazole for 6 months, a high incidence of Leydig cell tumors was seen in lansoprazole-treated, unsupplemented rats, whereas no Leydig cell tumors were seen in testosterone supplemented rats. This implied that reduction of the normal feedback inhibition at the level of the hypothalamus and/or pituitary due to reduced testosterone levels, thus giving rise to elevated levels of LH, was involved in the induction of Leydig cell tumors by lansoprazole. In vitro studies with Leydig cells from rats using various stimulators and precursors of testosterone biosynthesis demonstrated that the most sensitive site for inhibition of testosterone synthesis by lansoprazole is the transport of cholesterol to the cholesterol side chain cleavage enzyme. The IC50s for inhibition of LH or hCG-stimulated testosterone synthesis in Leydig cells from rats, mice, and monkeys were 11-12, 8, and 24.7 micrograms/ml, respectively. In vitro studies with metabolites of lansoprazole revealed that three metabolites were more potent inhibitors of testosterone synthesis than the parent drug, two of them being at least 10 times more potent. These metabolites are present in rats at substantial levels but are undetectable in humans. The lack of induction of Leydig cell tumors in mice, lower sensitivity of primate Leydig cells, and the absence of testosterone synthesis-inhibiting metabolites in man suggest that Leydig cell tumors found in rats represent a species-specific sensitivity and does not imply a risk for clinical use in man.