CYP2C8-Mediated Formation of a Human Disproportionate Metabolite of the Selective NaV1.7 Inhibitor DS-1971a, a Mixed Cytochrome P450 and Aldehyde Oxidase Substrate

Predicting human disproportionate metabolites is difficult, especially when drugs undergo species-specific metabolism mediated by cytochrome P450s (P450s) and/or non-P450 enzymes. This study assessed human metabolites of DS-1971a, a potent Nav1.7-selective blocker, by performing human mass balance studies and characterizing DS-1971a metabolites, in accordance with the Metabolites in Safety Testing guidance. In addition, we investigated the mechanism by which the major human disproportionate metabolite (M1) was formed. After oral administration of radiolabeled DS-1971a, the major metabolites in human plasma were P450-mediated monoxidized metabolites M1 and M2 with area under the curve ratios of 27% and 10% of total drug-related exposure, respectively; the minor metabolites were dioxidized metabolites produced by aldehyde oxidase and P450s. By comparing exposure levels of M1 and M2 between humans and safety assessment animals, M1 but not M2 was found to be a human disproportionate metabolite, requiring further characterization under the Metabolites in Safety Testing guidance. Incubation studies with human liver microsomes indicated that CYP2C8 was responsible for the formation of M1. Docking simulation indicated that, in the formation of M1 and M2, there would be hydrogen bonding and/or electrostatic interactions between the pyrimidine and sulfonamide moieties of DS-1971a and amino acid residues Ser100, Ile102, Ile106, Thr107, and Asn217 in CYP2C8, and that the cyclohexane ring of DS-1971a would be located near the heme iron of CYP2C8. These results clearly indicate that M1 is the predominant metabolite in humans and a human disproportionate metabolite due to species-specific differences in metabolism. SIGNIFICANCE STATEMENT: This report is the first to show a human disproportionate metabolite generated by CYP2C8-mediated primary metabolism. We clearly demonstrate that DS-1971a, a mixed aldehyde oxidase and cytochrome P450 substrate, was predominantly metabolized by CYP2C8 to form M1, a human disproportionate metabolite. Species differences in the formation of M1 highlight the regio- and stereoselective metabolism by CYP2C8, and the proposed interaction between DS-1971a and CYP2C8 provides new knowledge of CYP2C8-mediated metabolism of cyclohexane-containing substrates.

Combined use of filtered and edited 1 H NMR spectroscopy to detect 13 C-enriched compounds in complex mixtures

In conventional metabolism and pharmacokinetic studies, radioactive isotopes are used to identify and quantify the breakdown products of xenobiotics. However, the stable isotope (13) C provides a cheaper and less hazardous alternative. Metabolites of (13) C-enriched xenobiotics can be detected, quantified and identified by (13) C-filtered NMR spectroscopy. However, one obstacle to using (13) C is its 1.1% natural abundance that produces a background signal in (13) C-filtered NMR spectra of crude biological extracts. The signal makes it difficult to distinguish between (13) C-enriched xenobiotics resonances from endogenous metabolites unrelated to the xenobiotic. This study proposes that the (13) C background signal can be distinguished from resonances of (13) C-enriched xenobiotics by the absence of a (12) C component in the xenobiotic. This is detected by combined analysis of (13) C-filtered and -edited NMR spectra. The theory underlying the approach is described and the method is demonstrated by the detection of sub-microgram amounts of (13) C-enriched phenacetin in crude extracts of hepatocyte microsomes.

The early estimation of circulating drug metabolites in humans

INTRODUCTION

An evolution in bioanalytical methodologies to identify and quantify drug metabolites has led to a wealth of biotransformation information during preclinical and early clinical testing phases. However, this abundance of metabolism data has not clarified how to select the most important circulating human metabolites for safety assessment. Consequently, more stringent regulatory expectations for a comprehensive approach to human metabolism have led pharmaceutical sponsors to employ a variety of novel methods to estimate circulating drug metabolites in humans and animals.

AREAS COVERED

This review provides context for 'why' human circulating metabolites must be qualified for safety in animals. A detailed overview is also presented concerning 'where,' 'how' and 'when' to conduct these assessments during drug development.

EXPERT OPINION

A human metabolite qualification strategy is now a required element of the drug safety package submitted with a new drug application (NDA). The important question is whether or not this additional information, about metabolite safety, is making human drugs any safer. Currently, this is a debatable issue, especially because stand-alone metabolite testing is fraught with its own challenges. As drug development moves into the twenty-first century, there is a pressing need for more sophisticated methodologies to address human drug and metabolite safety.

Meeting the MIST regulations: human metabolism in Phase I using AMS and a tiered bioanalytical approach

The metabolites in safety testing and ICH-M3 guidance documents emphasize the importance of metabolites when considering safety aspects for new drugs. Both guidances state that relevant metabolites should have safety coverage in humans (although the guidelines have different definitions of relevant metabolites). Not having safety coverage for important metabolites in humans may cause significant delay in the overall pharmaceutical development program. This article discusses the regulatory background regarding safety and metabolites, as well as outlines an integrated strategy taken by one pharmaceutical company, Lundbeck A/S. Lundbeck uses metabolite exposure data from first-in-man studies, obtained using an accelerator MS approach followed by a two-tiered bioanalytical investigation. This enables early availability of key data on this aspect and, overall, represents a powerful risk mitigation strategy.