1 research outputs found
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approved in 2006 for the treatment of obesity but was withdrawn in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. <i>In
vitro</i> 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 <i>in vitro</i> 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 [<sup>14</sup>C]-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 <i>N</i>-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 [<sup>14</sup>C]-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