Evaluation of the absorption, excretion and metabolism of [14C] etoperidone in man

What is the Objective of the Mass Balance Study? A Retrospective Analysis of Data in Animal and Human Excretion Studies Employing Radiolabeled Drugs

Mass balance excretion studies in laboratory animals and humans using radiolabeled compounds represent a standard part of the development process for new drugs. From these studies, the total fate of drug-related material is obtained: mass balance, routes of excretion, and, with additional analyses, metabolic pathways. However, rarely does the mass balance in radiolabeled excretion studies truly achieve 100% recovery. Many definitions of cutoff criteria for mass balance that identify acceptable versus unacceptable recovery have been presented as ad hoc statements without a strong rationale. To address this, a retrospective analysis was undertaken to explore the overall performance of mass balance studies in both laboratory animal species and humans using data for 27 proprietary compounds within Pfizer and extensive review of published studies. The review has examined variation in recovery and the question of whether low recovery was a cause for concern in terms of drug safety. Overall, mean recovery was greater in rats and dogs than in humans. When the circulating half-life of total radioactivity is greater than 50 h, the recovery tends to be lower. Excretion data from the literature were queried as to whether drugs linked with toxicities associated with sequestration in tissues or covalent binding exhibit low mass balance. This was not the case, unless the sequestration led to a long elimination half-life of drug-related material. In the vast majority of cases, sequestration or concentration of drug-related material in an organ or tissue was without deleterious effect and, in some cases, was related to the pharmacological mechanism of action. Overall, from these data, recovery of radiolabel would normally be equal to or greater than 90%, 85%, and 80% in rat, dog, and human, respectively. Since several technical limitations can underlie a lack of mass balance and since mass balance data are not sensitive indicators of the potential for toxicity arising via tissue sequestration, absolute recovery in humans should not be used as a major decision criteria as to whether a radiolabeled study has met its objectives. Instead, the study should be seen as an integral part of drug development answering four principal questions: 1) Is the proposed clearance mechanism sufficiently supported by the identities of the drug-related materials in excreta, so as to provide a complete understanding of clearance and potential contributors to interpatient variability and drug-drug interactions? 2) What are the drug-related entities present in circulation that are the active principals contributing to primary and secondary pharmacology? 3) Are there findings (low extraction recovery of radiolabel from plasma, metabolite structures indicative of chemically reactive intermediates) that suggest potential safety issues requiring further risk assessment? 4) Do questions 2 and 3 have appropriate preclinical support in terms of pharmacology, safety pharmacology, and toxicology? Only if one or more of these four questions remain unanswered should additional mass balance studies be considered.

In vitro identification of metabolic pathways and cytochrome P450 enzymes involved in the metabolism of etoperidone

1. In vitro studies have been carried out to investigate the metabolic pathways and identify the hepatic cytochrome P450 (CYP) enzymes involved in etoperidone (Et) metabolism. 2. Ten in vitro metabolites were profiled, quantified and tentatively identified after incubation with human hepatic S9 fractions. Et was metabolized via three metabolic pathways: (A) alkyl hydroxylation to form OH-ethyl-Et (M1); (B) phenyl hydroxylation to form OH-phenyl-Et (M2); and (C) N-dealkylation to form 1-m-chlorophenylpiperazine (mCPP, M8) and triazole propyl aldehyde (M6). Six additional metabolites were formed by further metabolism of M1, M2, M6 and M8. 3. Kinetic studies revealed that all metabolic pathways were monophasic, and the pathway leading to the formation of OH-ethyl-Et was the most efficient at eliminating the drug. On incubation with microsomes expressing individual recombinant CYPs, formation rates of M1-3 and M8 were 10-100-fold greater for CYP3A4 than that for other CYP forms. The formation of these metabolites was markedly inhibited by the CYP3A4-specific inhibitor ketoconazole, whereas other CYP-specific inhibitors did not show significant effects. In addition, the production of M1-3 and M8 was strongly correlated with CYP3A4-mediated testosterone 6beta-hydroxylase activities in 13 different human liver microsome samples. 4. Dealkylation of the major metabolite M1 to form mCPP (M8) was also investigated using microsomes containing recombinant CYP enzymes. The rate of conversion of M1 to mCPP by CYP3A4 was 503.0 +/- 3.1 pmole nmole(-1) min(-1). Metabolism of M1 to M8 by other CYP enzymes was insignificant. In addition, this metabolism in human liver microsomes was extensively inhibited by the CYP3A4 inhibitor ketoconazole, but not by other CYP-specific inhibitors. In addition, conversion of M1 to M8 was highly correlated with CYP3A4-mediated testosterone 6beta-hydroxylase activity. 5. The results strongly suggest that CYP3A4 is the predominant enzyme-metabolizing Et in humans.