Metabolic, idiosyncratic toxicity of drugs: overview of the hepatic toxicity induced by the anxiolytic, panadiplon

Abstract

Preclinical drug safety evaluation studies, typically conducted in two or more animal species, reveal and define dose-dependent toxicities and undesirable effects related to pharmacological mechanism of action. Idiosyncratic toxic responses are often not detected during this phase in development due to their relative rarity in incidence and differences in species sensitivity. This paper reviews and discusses the metabolic idiosyncratic toxicity and species differences observed for the experimental non-benzodiazepine anxiolytic, panadiplon. This compound produced evidence of hepatic toxicity in Phase 1 clinical trial volunteers that was not predicted by rat, dog or monkey preclinical studies. However, subsequent studies in Dutch-belted rabbits revealed a hepatic toxic syndrome consistent with a Reye's Syndrome-like idiosyncratic response. Investigations into the mechanism of toxicity using rabbits and cultured hepatocytes from several species, including human, provided a sketch of the complex pathway required to produce hepatic injury. This pathway includes drug metabolism to a carboxylic acid metabolite (cyclopropane carboxylic acid), inhibition of mitochondrial fatty acid β-oxidation, and effects on intermediary metabolism including depletion of glycogen and disruption of glucose homeostasis. We also provide evidence suggesting that the carboxylic acid metabolite decreases the availability of liver CoA and carnitine secondary to the formation of unusual acyl derivatives. Hepatic toxicity could be ameliorated by administration of carnitine, and to a lesser extent by pantothenate. These hepatocellular pathway defects, though not directly resulting in cell death, rendered hepatocytes sensitive to secondary stress, which subsequently produced apoptosis and hepatocellular necrosis. Not all rabbits showed evidence of hepatic toxicity, suggesting that individual or species differences in any step along this pathway may account for idiosyncratic responses. These differences may be roughly applied to other metabolic idiosyncratic hepatotoxic responses and include variations in drug metabolism, effects on mitochondrial function, nutritional status, and health or underlying disease.

Introduction

Drug safety toxicity is conventionally determined by conducting animal toxicity studies in at least two species; human risk is then determined by extrapolation from animal study results and candidates with unacceptable risk are dropped from further development. The reliability of risk assessment, particularly when extrapolating between species, depends on the relatedness of the parameters measured to any potential mechanism of toxicity of the drug candidate, which is rarely determined, and the absence of idiosyncratic reactions. Determining the mechanism of toxicity for a xenobiotic requires a diversity of investigative techniques, sufficient time and a degree of serendipity. Practically, most mechanistic toxicology is conducted retrospectively, after a problem has been encountered, in an attempt to salvage a discovery program or clinical candidate. Idiosyncratic toxicities, functionally defined as those that can not be predicted based on dose, duration of exposure or mechanism of action, are particularly difficult to identify in preclinical animal studies. These responses are generally placed into one of two categories; those that are mediated by the immune system and those that are associated with altered drug disposition or action. Immune-mediated adverse drug reactions (reviewed in [1]) are thought to occur in response to drug-protein adducts that act as immunogens, driving antibody production or T-cell-mediated responses towards the drug in the target tissue. Generation of adducts requires the production of a reactive metabolite, and the metabolite-protein adduct may also precipitate an autoimmune response (reviewed in Ref. [2]). Non-immune idiosyncratic reactions can result from aberrant drug metabolism or clearance, leading to the accumulation of toxic metabolites and inhibition of critical cell processes (discussed in Refs. [3], [4]). In addition to immune responses and/or altered metabolism, there are clearly other factors that influence the initiation, severity and outcome of idiosyncratic toxicities. This paper describes the evidence supporting a mechanism of metabolic, non-immune mediated idiosyncratic toxicity for an experimental compound, and discusses the implications for similar toxicities produced by other xenobiotics.

The non-benzodiazepine anxiolytic, panadiplon (U-78875; 3-[5-cyclopropyl-1,2,4-oxadiazol-3yl]-5-[1-methylethyl]-imidazo {1,5-a}-quinoxalin-4[5H]-one), was withdrawn from development for treatment of Generalized Anxiety Disorder and Panic Disorder due to evidence of hepatic toxicity as indicated by elevations of serum transaminases in a few patients during Phase 1 multiple-dose clinical trials. Though not observed in rats, dogs or monkeys, we subsequently described a hepatic toxic syndrome in Dutch-belted rabbits [5]. The mechanism of toxicity was investigated by conducting in vivo studies in sensitive species (rabbit) compared to an insensitive species (rat), and in vitro studies with hepatocytes from sensitive species (rabbit and human) compared to insensitive species (rat and monkey). This toxicity was shown to result from mitochondrial inhibition by a carboxylic acid metabolite, cyclopropane carboxylic acid (CPCA; [6]). The hepatic toxicity produced by panadiplon is similar to that produced by a variety of xenobiotic carboxylic acids, including hypoglycin A, valproic acid and pentanoic acid [7], [8], [9], [10], [11], and has thus been ascribed to the group of Reye's Syndrome-like toxicities [12]. Discussed here are the various experimental findings for panadiplon and their implication in the overall progression of the toxicity. We also provide new data indicating a role for carnitine and coenzyme A in panadiplon-mediated hepatic toxicity.