A Holistic Strategy for Characterizing the Safety of Metabolites through Drug Discovery and Development

Abstract

The subject of metabolites in safety testing has had much debate in the recent past and has shown itself to be a complex issue with no simple solutions to providing absolute assurance of drug safety. Much of the attention has focused on the ability to identify metabolites and then demonstrate that their risk has been adequately characterized, either through their exposure in toxicology species or, failing this, by direct safety testing. In this review, we summarize our forward operational strategy that combines the principles summarized in the FDA Guidance, together with discussions at scientific meetings and literature opinions. It is a balance between the primary goal of assuring patient safety with one of reasonable investment. A key principle in striking this balance is to build stepwise information on metabolites through the drug discovery and development continuum. This allows assessments to be made from early nonclinical studies onward as to whether or not metabolite safety is underwritten by exposure in toxicology species. This strategy does not require absolute quantitation of the metabolites in early clinical trials but relies upon comparison of relative exposures between animals and humans using the capabilities of modern analytical techniques. Through this strategy, human disproportionate metabolites can be identified to allow a decision regarding the need for absolute quantitation and direct safety testing of the metabolite. Definitive radiolabeled studies would be initiated following proof of pharmacology or efficacy in humans, and nonclinical safety coverage would be adequately assessed prior to large-scale clinical trials. In cases where metabolite safety is not supported through the parent compound toxicology program, approaches for the direct safety testing of metabolites with regard to general and reproductive toxicology, safety pharmacology, and genetic safety have been defined.

A Decade in the MIST: Learnings from Investigations of Drug Metabolites in Drug Development under the “Metabolites in Safety Testing” Regulatory Guidance

“…Typically, liver microsomes, liver S9 or hepatocytes are used for this purpose, but depending on the nature of the metabolites, other matrices such as recombinant enzymes or plasma could be used (Dalvie et al, 2009). …”

A Simple Liquid Chromatography-Tandem Mass Spectrometry Method to Determine Relative Plasma Exposures of Drug Metabolites across Species for Metabolite Safety Assessments

“…These have included the generation of a radiolabeled calibration standard of metabolite from a biological source (Zhang et al, 2007;Leclercq et al, 2009) or establishing a stock solution concentration of metabolite that has been isolated with 1 H NMR and using that solution for the construction of standard curves (Vishwanathan et al, 2009). It has been proposed that LC-MS/MS peak areas can be used as a relative concentration comparison between animals and humans to demonstrate the coverage of metabolite exposure in animals, and some bioanalytical issues have been discussed and remain to be resolved (Leclercq et al, 2009;Walker et al, 2009).…”

“…In this report, we describe the use of these mass spectrometric capabilities along with a previously described algorithm for MIST (Walker et al, 2009) to produce a resource-sparing strategy for the confident determination of the relative metabolite plasma exposures between the toxicological species and humans. This strategy is based on the notion that it is unnecessary to make actual determinations of plasma concentrations of metabolites in animals and humans, but that it is only necessary to demonstrate that animals have greater exposures relative to humans.…”

A Simple Liquid Chromatography-Tandem Mass Spectrometry Method to Determine Relative Plasma Exposures of Drug Metabolites across Species for Metabolite Safety Assessments (Metabolites in Safety Testing). II. Application to Unstable Metabolites

“…In several studies, there has been a focus on ensuring the safety not only of the drug itself but also of the circulating metabolites (Luffer-Atlas, 2008;Robison and Jacobs, 2009;Smith and Obach, 2009;Anderson et al, 2010;Frederick and Obach, 2010), which was triggered by a position offered by scientists from a pharmaceutical industry trade organization (Baillie et al, 2002). Many articles have been published, laying out various strategies for incorporating an assessment of human circulating metabolites (i.e., prediction and identification) (Leclercq et al, 2009;Nedderman, 2009;Walker et al, 2009;Nedderman and Wright, 2010) that can be used in determining whether further examination of safety attributes of metabolites is warranted. Although there has not been perfect agreement among all discussants on this topic, regulatory guidance documents have been issued by the International Conference on Harmonization as well as by the U.S. Food and Drug Administration in which guidelines defining the thresholds for when human circulating metabolites merit further consideration in risk assessment have been outlined.…”