The PPARα agonists fenofibrate and CP-778875 cause increased ß-oxidation, leading to oxidative injury in skeletal and cardiac muscle in the rat.

Weak peroxisome proliferator-activated receptor (PPAR) α agonists (fibrates) are used to treat dyslipidemia. This study compared the effects of the potent and selective PPARα agonist CP-778875 on peroxisomal ß-oxidation and cardiac and/or skeletal muscle injury with those of the weak PPARα agonist fenofibrate. We hypothesized that these muscle effects are mediated through the PPARα receptor, leading to increased ß-oxidation and consequent oxidative stress. CP-778875 (5 or 500 mg/kg) and fenofibrate (600 or 2,000→1,200 mg/kg, dose lowered because of intolerance) were administered to rats for six weeks. Standard end points, serum troponin I, heart and skeletal muscle ß-oxidation of palmitoyl-CoA, and acyl co-oxidase (AOX) mRNA were assessed. Both compounds dose-dependently increased the incidence and/or severity of cardiomyocyte degeneration and necrosis, heart weight, troponin I, and skeletal muscle degeneration. Mean heart ß-oxidation (3.4- to 5.1-fold control) and AOX mRNA (2.4- to 3.2-fold control) were increased with CP-778875 500 mg/kg and both doses of fenofibrate. ß-Oxidation of skeletal muscle was not affected by either compound; however, a significant increase in AOX mRNA (1.6- to 2.1-fold control) was observed with CP-778875 500 mg/kg and both doses of fenofibrate. Taken together, these findings were consistent with PPARα agonism and support the link between increased cardiac and skeletal muscle ß-oxidation and resultant muscle injury in the rat.

In vitro cytochrome P450 inhibition and induction.

The assessment of in vitro inhibition and induction of the cytochrome P450 enzymes of the liver is a critical part of the drug discovery and development process in order to ensure that two or more drugs can be safely coadministered without alterations in exposure. Early assessment of potential candidates using high throughput approaches provides key direction in choosing the most promising chemical series to pursue. In later stage development, the use of in vitro data to assess the potential for clinical interactions is now a practice readily accepted by regulatory authorities. Inhibition of drug metabolizing enzymes can occur via two principal mechanisms, reversible inhibition and time dependent inhibition (mechanism-based inactivation). Clinically, either of these mechanisms can lead to reduced clearance of a coadministered drug and potentially toxic levels may be reached. Inducers of a drug metabolizing enzyme can increase the clearance of other drugs, or itself, resulting in a decreased therapeutic effect; they can also increase the bioactivation of drugs that can produce reactive intermediates, leading to hepatotoxicity. A number of in vitro models composed of human-derived microsomes, recombinantly expressed human drug metabolizing enzymes, human-derived cell lines, as well as fresh and cryopreserved human hepatocytes, are increasingly in use to evaluate inhibition and induction. In this review, the authors' understanding of currently utilized enzyme inhibition and induction methodologies are presented and the authors provide recommendations regarding which assay types offer the greatest advantage during the drug development process.

The relationship among microsomal enzyme induction, liver weight, and histological change in cynomolgus monkey toxicology studies.

The purpose of this investigation was to examine the relationship among hepatic microsomal enzyme induction, liver weight, histological evidence of hepatic injury, and serum clinical chemistry markers of hepatic origin in the cynomolgus monkey. We report here the results from independent toxicology studies for 10 investigative drug candidates representing four therapeutic classes. Study conditions were selected to elicit target organ toxicity. We found that six of the 10 compounds altered cytochrome P450-associated activities in both male and female monkeys, two in females only, and one altered similar activities in males only. Frequently, significant treatment-related elevations in NADPH cytochrome c reductase and ethylmorphine N-demethylase were noted. When the results from all 10 studies were pooled, 14 cytochrome P450-associated activities were significantly increased and five were decreased in males compared to 15 significantly increased and three decreased in the females. Treatment-associated liver weight increases were noted in four studies. Except for hepatocellular hypertrophy in one study, no significant treatment-related microscopic changes in liver and no elevations of serum biomarkers commonly associated with liver toxicity were observed in any of the studies that demonstrated significant hepatic enzyme induction. Compared to parallel rat studies, one compound was an inducer only in monkeys and one was an inducer only in rats. Significant elevations of microsomal drug-metabolizing enzymes in the cynomolgus monkey liver are not accompanied by substantial hepatic changes except for hepatomegaly. These alterations in the hepatic drug-metabolizing enzyme system were benign based the absence of histopathological lesions and serum biomarkers of hepatobiliary toxicity.