Approaches for minimizing metabolic activation of new drug candidates in drug discovery

Metabolic activation and cytotoxicity of 4-Methylquinoline mediated by CYP3A4 and sulfotransferases in rats

4-Methylquinoline (4-MQ) is a quinoline derivative widely present in groundwater and soil and has been reported to be genotoxic. The mechanisms of the toxic action remain unknown. This study aimed to elucidate the metabolic activation of 4-MQ and to determine the possible role of reactive metabolites in 4-MQ-induced liver injury in rats. In the present study, a hydroxylation metabolite (M1), a GSH conjugate (M2) and an NAC conjugate (M3) derived from 4-MQ were detected in vitro and in vivo. The structures of the two conjugates were verified by chemical synthesis, mass spectrometry, and nuclear magnetic resonance. CYP3A4 was found to dominate the hydroxylation of 4-MQ. Sulfotransferases also participated in the metabolic activation of 4-MQ. Pretreatment of primary hepatocytes with ketoconazole (KTC) or 2,6-dichloro-4-nitrophenol (DCNP) not only reduced the production of GSH conjugate M2 but also decreased the susceptibility of hepatocytes to the cytotoxicity of 4-MQ. Urinary NAC conjugate M3 was found in rats given 4-MQ, and M3 may be a potential biomarker for 4-MQ exposure.

Case Study 11: Considerations for Enzyme Mapping Experiments-Interaction Between the Aldehyde Oxidase Inhibitor Hydralazine and Glutathione

Often it may be convenient and efficient to address multiple research questions with a single experiment. In many instances, however, the best approach is to design the experiment to address one question at a time. The design of enzyme mapping experiments is discussed in this chapter, focusing on considerations pertinent to the study of aldehyde oxidase (AO) vs. cytochrome P450 metabolism. Specifically, a case is presented in which reduced glutathione (GSH) was included in an experiment with human liver S9 fraction to trap reactive metabolites generated from cytochrome P450-mediated metabolism of lapatinib and its O-dealkylated metabolite, M1 (question 1). The AO inhibitor hydralazine was included in this experiment to investigate the involvement of AO-mediated metabolism of M1 (question 2). The presence of GSH was found to interfere with the inhibitory activity of hydralazine. Consideration of the time-dependent nature of hydralazine inhibitory activity toward AO when designing this experiment could have predicted the potential for GSH to interfere with hydralazine. This case underscores the importance of clearly identifying the research question, tailoring the experimental protocol to answer that question, and then meticulously considering how the experimental conditions could influence the results, particularly if attempting to address multiple questions with a single experiment.

In-Vitro Approaches to Predict and Study T-Cell Mediated Hypersensitivity to Drugs

Mitigating the risk of drug hypersensitivity reactions is an important facet of a given pharmaceutical, with poor performance in this area of safety often leading to warnings, restrictions and withdrawals. In the last 50 years, efforts to diagnose, manage, and circumvent these obscure, iatrogenic diseases have resulted in the development of assays at all stages of a drugs lifespan. Indeed, this begins with intelligent lead compound selection/design to minimize the existence of deleterious chemical reactivity through exclusion of ominous structural moieties. Preclinical studies then investigate how compounds interact with biological systems, with emphasis placed on modeling immunological/toxicological liabilities. During clinical use, competent and accurate diagnoses are sought to effectively manage patients with such ailments, and pharmacovigilance datasets can be used for stratification of patient populations in order to optimise safety profiles. Herein, an overview of some of the in-vitro approaches to predict intrinsic immunogenicity of drugs and diagnose culprit drugs in allergic patients after exposure is detailed, with current perspectives and opportunities provided.

Deleterious effects of reactive metabolites

A number of drugs have been withdrawn from the market or severely restricted in their use because of unexpected toxicities that become apparent only after the launch of new drug entities. Circumstantial evidence suggests that, in most cases, reactive metabolites are responsible for these unexpected toxicities. In this review, a general overview of the types of reactive metabolites and the consequences of their formation are presented. The current approaches to evaluate bioactivation potential of new compounds with particular emphasis on the advantages and limitation of these procedures will be discussed. Reasonable reasons for the excellent safety record of certain drugs susceptible to bioactivation will also be explored and should provide valuable guidance in the use of reactive-metabolite assessments when nominating drug candidates for development. This will, in turn, help us to design and bring safer drugs to the market.

Reversible time-dependent inhibition of cytochrome P450 enzymes by duloxetine and inertness of its thiophene ring towards bioactivation

Duloxetine is a selective serotonin-norepinephrine reuptake inhibitor (SNRI) approved to treat major depressive disorder and diabetic peripheral neuropathic pain. It is known to cause hepatotoxicity, in some cases leading to death. It has been reported that duloxetine causes time-dependent inhibition (TDI) of CYP1A2, CYP2B6, CYP2C19 and CYP3A4/5; but the nature of these TDI (whether reversible or irreversible) is not known. Irreversible TDI can cause clinically significant drug-drug interactions and also immune-mediated hepatotoxicity. Structurally, duloxetine possesses several toxicophores, i.e. the naphthyl and thiophene rings. It has been reported that the naphthyl ring undergoes epoxidation and was subsequently adducted to glutathione, but bioactivation related to the thiophene ring has not been completely elucidated. In this paper, the potential of duloxetine in causing irreversible TDI and generating reactive metabolites was investigated. Human liver microsomal assays demonstrated that duloxetine did not cause irreversible TDI of CYP1A2, CYP2B6, CYP2D6, CYP2C19 and CYP3A4/5. Subsequently, reactive metabolite trapping assays using soft nucleophiles (glutathione and glutathione ethyl ester) revealed a previously reported adduct at the naphthyl ring of duloxetine but not at the thiophene ring. Trapping assays utilizing a hard nucleophile (semicarbazide) did not demonstrate adducts with the thiophene ring, indicating an absence of thiophene ring opening. The hepatotoxicity of duloxetine is possibly not related to the irreversible TDI of CYP450 or the bioactivation of its thiophene moiety, but might be due to the epoxidation of its naphthyl ring.

Risk assessment and mitigation strategies for reactive metabolites in drug discovery and development

Drug toxicity is a leading cause of attrition of candidate drugs during drug development as well as of withdrawal of drugs post-licensing due to adverse drug reactions in man. These adverse drug reactions cause a broad range of clinically severe conditions including both highly reproducible and dose dependent toxicities as well as relatively infrequent and idiosyncratic adverse events. The underlying risk factors can be split into two groups: (1) drug-related and (2) patient-related. The drug-related risk factors include metabolic factors that determine the propensity of a molecule to form toxic reactive metabolites (RMs), and the RM and non-RM mediated mechanisms which cause cell and tissue injury. Patient related risk factors may vary markedly between individuals, and encompass genetic and non-genetic processes, e.g. environmental, that influence the disposition of drugs and their metabolites, the nature of the adverse responses elicited and the resulting biological consequences. We describe a new strategy, which builds upon the strategies used currently within numerous pharmaceutical companies to avoid and minimize RM formation during drug discovery, and that is intended to reduce the likelihood that candidate drugs will cause toxicity in the human population. The new strategy addresses drug-related safety hazards, but not patient-related risk factors. A common target organ of toxicity is the liver and to decrease the likelihood that candidate drugs will cause liver toxicity (both non-idiosyncratic and idiosyncratic), we propose use of an in vitro Hepatic Liability Panel alongside in vitro methods for the detection of RMs. This will enable design and selection of compounds in discovery that have reduced propensity to cause liver toxicity. In vitro Hepatic Liability is assessed using toxicity assays that quantify: CYP 450 dependent and CYP 450 independent cell toxicity; mitochondrial impairment; and inhibition of the Bile Salt Export Pump. Prior to progression into development, a Hepatotoxicity Hazard Matrix combines data from the Hepatic Liability Panel with the Estimated RM Body Burden. The latter is defined as the level of covalent binding of radiolabelled drug to human hepatocyte proteins in vitro adjusted for the predicted human dose. We exemplify the potential value of this approach by consideration of the thiazolidinedione class of drugs.

Role of biotransformation in drug-induced toxicity: influence of intra- and inter-species differences in drug metabolism

It is now widely appreciated that drug metabolites, in addition to the parent drugs themselves, can mediate the serious adverse effects exhibited by some new therapeutic agents, and as a result, there has been heightened interest in the field of drug metabolism from researchers in academia, the pharmaceutical industry, and regulatory agencies. Much progress has been made in recent years in understanding mechanisms of toxicities caused by drug metabolites, and in understanding the numerous factors that influence individual exposure to products of drug biotransformation. This review addresses some of these factors, including the role of drug-drug interactions, reactive metabolite formation, individual susceptibility, and species differences in drug disposition caused by genetic polymorphisms in drug-metabolizing enzymes. Examples are provided of adverse reactions that are linked to drug metabolism, and the mechanisms underlying variability in toxic response are discussed. Finally, some future directions for research in this field are highlighted in the context of the discovery and development of new therapeutic agents.