Bioactivation of the Cannabinoid Receptor Antagonist Rimonabant to a Cytotoxic Iminium Ion Metabolite

The cannabinoid type 1 receptor (CB1r) antagonist rimonabant was approved in 2006 for the treatment of obesity but was withdrawn in 2008 due to serious drug-related psychiatric disorders. In vitro metabolism studies with rimonabant have revealed high levels of reactive metabolite formation, which re...

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Published in:	Chemical research in toxicology Vol. 26; no. 1; pp. 124 - 135
Main Authors:	Foster, Alison J, Prime, Lucy H, Gustafsson, Frida, Temesi, David G, Isin, Emre M, Midlöv, Johanna, Castagnoli, Neal, Kenna, J. Gerry
Format:	Journal Article
Language:	English
Published:	United States American Chemical Society 18-01-2013
Subjects:	Cannabinoid Receptor Antagonists - chemistry Cannabinoid Receptor Antagonists - metabolism Cannabinoid Receptor Antagonists - toxicity Carbon Radioisotopes - chemistry Cell Line, Transformed Cell Survival - drug effects Cytochrome P-450 Enzyme System - chemistry Cytochrome P-450 Enzyme System - metabolism Glutathione - metabolism Humans Imines - chemistry Ions - chemistry Metabolome - drug effects Piperidines - chemistry Piperidines - metabolism Piperidines - pharmacology Piperidines - toxicity Potassium Cyanide - chemistry Potassium Cyanide - pharmacology Protein Binding Proteins - chemistry Proteins - metabolism Pyrazoles - chemistry Pyrazoles - metabolism Pyrazoles - toxicity Ritonavir - chemistry Ritonavir - pharmacology
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Summary:	The cannabinoid type 1 receptor (CB1r) antagonist rimonabant was approved in 2006 for the treatment of obesity but was withdrawn in 2008 due to serious drug-related psychiatric disorders. In vitro metabolism studies with rimonabant have revealed high levels of reactive metabolite formation, which resulted in irreversible time-dependent P450 3A4 inhibition and in covalent binding to microsomal proteins. In the present study, an in vitro approach has been used to explore whether metabolic bioactivation of rimonabant might result in cell toxicity. A panel of SV40-T-antigen-immortalized human liver derived (THLE) cells that had been transfected with vectors encoding various human cytochrome P450 enzymes (THLE-1A2, 2C9, 2C19, 2D6, and 3A4) or with an empty vector (THLE-Null) were exposed to rimonabant. Cell toxicity and covalent binding to cellular proteins were evaluated, as was metabolite formation. Rimonabant exhibited markedly potentiated dose and time dependent cytotoxicity to THLE-3A4 cells, compared to that of all other THLE cell lines. This was accompanied by high levels of covalent binding of [14C]-rimonabant to THLE-3A4 cell proteins (1433 pmol drug equivalents/mg protein) and the formation of several metabolites that were not generated by THLE-Null cells. These included N-aminopiperidine (NAP) and an iminium ion species. However, no toxicity was observed when THLE cells were incubated with NAP. Glutathione depletion did not alter the observed potent cell cytotoxicity of rimonabant to THLE-3A4 cells. Preincubation of THLE-3A4 cells with the cytochrome P450 3A4 inhibitor ritonavir blocked the selective toxicity of rimonabant to these cells. In addition, ritonavir pretreatment blocked the metabolism of the compound in the cells and thereby significantly decreased the covalent binding of [14C]-rimonabant to THLE-3A4 cell proteins. We conclude that the potent toxicity of rimonabant in THLE-3A4 cells occurs by a mechanistic sequence, which is initiated by cytochrome P450 3A4 mediated formation of a highly cytotoxic reactive iminium ion metabolite that binds covalently to cellular proteins.
Bibliography:	ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23
ISSN:	0893-228X 1520-5010
DOI:	10.1021/tx300418w