TWS119 The synthesis and secretion of adrenal androgens is a
The synthesis and secretion of adrenal androgens is apparently unique to humans and nonhuman primates, and requires the expression of key mediators such as CYP17 . Endocrine control of CYP17 expression in rhesus monkeys is comparable to that in humans . An analysis of baboon cDNA for CYP17 showed 96% homology with human CYP17, with only 28 predominantly conservative amino TWS119 differences between the two . Similarly, chimpanzee and rhesus cDNA sequences have been shown to be near identical to human CYP17 . Thus a nonhuman primate model would appear suitable for analysis of 17,20-lyase inhibition in the development of orteronel. As such, here we report the effects of orteronel on steroid hormone production in monkey adrenal cells and human adrenal tumor cell lines, and serum dehydroepiandrosterone (DHEA), cortisol, and testosterone levels after oral dosing in both castrated and intact male cynomolgus monkeys. Our findings reveal that orteronel effectively inhibits the 17,20-lyase activity of CYP17A1 in vitro and in vivo and suggest that orteronel may be an effective therapeutic option for both androgen-dependent and CRPC.
Materials and methods
Discussion Orteronel is a novel, non-steroidal, naphthylmethylimidazole derivative expressly designed to specifically inhibit the 17,20-lyase activity of CYP17A1 . Continuing the evaluation and development of this compound, here we investigated its biochemical effects as well as its in vitro and in vivo pharmacological activity in the cynomolgus monkey model system and in human adrenal tumor cells. In enzyme assays, orteronel showed potent inhibition for both 17,20-lyase and 17-hydroxylase activities of monkey adrenal CYP17, an effect that was comparable to abiraterone. This similarity in enzyme inhibitory activity vs 17,20-lyase and 17-hydroxylase most likely reflects low selectivity of inhibition for DHEA production vs cortisol production specifically in this non-human primate system , thus it is difficult to say whether the clinical effects of cortisol inhibition would be significant. It is important to note that the IC50 ratio between 17,20-lyase and 17-hydroxylase for orteronel was 27:38nmol/L (approximately 1:1.4) in monkeys but 140:760nmol/L (approximately 1:5.4) in humans, thus it can be expected that orteronel will have a lower effect on 17-hydroxylase in humans relative to monkeys and relative to 17,20-lyase inhibition. Abiraterone on the other hand, had an IC50 ratio of 27:30nmol/L (approximately 1:1.1), suggesting less specificity and thus potentially a greater effect on 17-hydroxylase relative to 17,20-lyase inhibition. Furthermore, abiraterone potently inhibited cortisol synthesis with an IC50 of 23nmol/L. Based on current phase 1/2 clinical data, single-agent orteronel did not appear to result in clinically meaningful changes in cortisol  at a dose level effective for testosterone lowering (300mg twice daily), while single-agent abiraterone has been reported to be associated with a 2-fold decrease in serum cortisol levels , . Further investigation will be required to determine if differences in the inhibitory effects of orteronel for 17,20-lyase vs 17-hydroxylase activity are clinically significant. The antifungal agent ketoconazole, which also inhibits CYP17A1 and has been occasionally used for CRPC , showed much lower inhibition of these key target enzymatic activities. For 11-hydroxylase activity, which is necessary for cortisol and corticosterone production but not testosterone production, orteronel showed significantly lower inhibitory activity when compared to either abiraterone or ketoconazole, suggesting that utilizing orteronel to reduce serum androgens should have reduced effects on glucocorticoids. In in vitro studies conducted in monkey adrenal cells, production of DHEA and androstenedione was suppressed by orteronel with IC50 values similar to those determined in the cell-free enzyme assays conducted in this model and reported above. Aldosterone production was also suppressed by orteronel, while at higher concentrations, corticosterone and progesterone production was increased. The increase in progesterone production is thought to be accumulation of substrate by inhibition of 17-hydroxylation leading to the increase in corticosterone levels. These findings support the contention that orteronel has weaker inhibitory activity for 11-hydroxylase and aldosterone synthase than for 17,20-lyase in monkeys as 11-hydroxylase activity is necessary for corticosterone production. In contrast, ketoconazole suppressed production of DHEA, androstenedione, cortisol, and aldosterone with similar IC50 values suggesting that ketoconazole also effectively inhibits enzymes such as 11- or 21-hydroxylase and SCC enzyme. Taken together, these data support the notion that orteronel has more potent and more specific CYP17 inhibitory activity than ketoconazole.