Metabolic fate of zetidoline, a new neuroleptic agent, 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.

Zetidoline metabolism by rat liver microsomes. Formation of metabolites with potential neuroleptic activity

1-(3'-Chlorophenyl)-3-[2-(3,3-dimethyl-1-azetidinyl)ethyl] imidazolidin-2-one, zetidoline, a new neuroleptic agent, when incubated with rat liver microsomes was rapidly metabolized to six free (mets B, D, I, L, M and N) and two conjugated metabolites (mets E and F). Sites of the metabolic attack (oxidation) were primarily the aromatic moiety, then the imidazolidinone and the azetidine rings. The metabolites were purified and structures assigned by means of EI-MS, 1H-NMR and chemical synthesis (mets B, D, L and M). The main metabolites, zetidoline, some chemical analogues and a few known dopamine antagonists were tested as in vitro inhibitors of 3H-zetidoline and 3H-spiperone binding to rat striatal membranes, and as in vivo inducers of prolactin release in female rats (inhibition of the estrus cycle). Two zetidoline metabolites, namely 4'-hydroxy zetidoline (met. B) and 5-hydroxy zetidoline (met. L), were found to have both in vitro and in vivo activities comparable to those of the parent drug. Identification of these active hydroxylated metabolites appears important both in the search of new leads of neuroleptics and for designing pro-drugs derivatives with improved pharmacokinetic profiles.