Metabolic Switching of BILR 355 in the Presence of Ritonavir. I. Identifying an Unexpected Disproportionate Human Metabolite

Abstract

18 metabolites was observed. Their structures were proposed on the basis of high-performance liquid chromatography-tandem mass spectrometry technologies, and 10 metabolites were confirmed by comparison with synthetic standards. We were surprised to find that a disproportionate human metabolite, BILR 516, was uncovered during this metabolite profiling study and pharmacokinetic analysis of BILR 516 showed that it had a longer half-life and higher exposure than the parent compound at steady state. Of interest, BILR 516 was not detected in human plasma when BILR 355 was administered alone. Therefore, whereas RTV boosted the exposure of BILR 355, it resulted in a significant metabolic switching of BILR 355. Overall, this article demonstrates an unusual example of metabolic switching and raises concern about the consequence of metabolic switching during drug development.

Metabolic Switching of BILR 355 in the Presence of Ritonavir. II. Uncovering Novel Contributions by Gut Bacteria and Aldehyde Oxidase

“…The level of BILR 355 initially increased with the incubation time (0.15 M BILR 355 was formed by 4 min when the substrate concentration was 1 M, and 0.9 M BILR 355 was formed at the same time when the substrate concentration was 10 M) and then started to decrease. This observation is consistent with the fact that BILR 355 is also a good substrate of CYP3A (Li et al, 2012) and would be further metabolized by CYP3A as the incubation proceeded. The amount of each P450 enzyme used in the incubations was adjusted on the basis of the relative level of each P450 isoform in an average set of human liver microsomes (Shimada et al, 1994).…”

“…However, an unexpected consequence of this boosting strategy was that a metabolite, BILR 516 ( Fig. 1), which was not detected previously in humans given BILR 355 alone, emerged as a disproportionate human metabolite (DHM) with plasma levels exceeding those of the parent at steady-state (Li et al, 2012).…”

“…Previous studies with BILR 355 had shown that it was extensively metabolized by CYP3A (Li et al, 2012). Concomitant administration of RTV with BILR 355 to inhibit CYP3A and boost the exposure of BILR 355 had the desired effect (Huang et al, 2008).…”

“…In vitro metabolism studies suggested that cytochrome P450 (P450) 3A4 was playing a significant role in limiting systemic exposure to BILR 355 (Li et al, 2012). Ritonavir (RTV) is a potent inhibitor of CYP3A that has been used as a boosting agent to increase the exposure of a number of drugs, such as atazanavir and darunavir (Hull and Montaner, 2011).…”

“…Enzyme kinetics (K m and V max ) of the depletion of BILR 402 and the formation of BILR 355 by HLMs were measured and the clearance (CL int, in vitro and CL h ) of both processes was calculated using the same procedure as described in Li et al (2012).…”