Differential toxicity of cocaine and its isomers, (+)-cocaine and (-)-psi-cocaine, is associated with stereoselective hydrolysis by hepatic carboxylesterases in cultured rat hepatocytes.

Cocaine induces acute lethal cell injury in rat hepatocytes following N-oxidative metabolic activation by cytochrome P450-dependent and flavin-dependent monooxygenases. Beside this oxidative bioactivation pathway, hepatic carboxylesterases may cleave the carboxymethylester or the benzoylester linkage which leads to molecules found to be non-toxic in vivo. To elucidate the structural requirements of the cocaine molecule for its bioactivation and inactivation, the cytotoxic potential of the natural (-)-cocaine relative to two isomeric forms, (+)-cocaine* (the unnatural enantiomer) and (-)-psi-cocaine (the C2 epimer of the unnatural cocaine) were investigated. Primary short-term cultures of rat hepatocytes obtained from phenobarbital (PB)-pretreated rats were exposed to the drugs for up to 24 h. (-)-Cocaine produced marked time- and concentration-dependent release of lactate dehydrogenase (LDH) into the extracellular medium, whereas the other forms were not cytotoxic (0-1 mM). Furthermore, depletion of cellular glutathione (GSH) with diethylmaleate enhanced LDH release in (-)-cocaine-treated cells and caused marginal cytotoxicity in hepatocytes exposed to the other isomers. To investigate the mechanisms that could be responsible for these isomer-specific effects, the time-dependent metabolic degradation was determined both in cultured hepatocytes and in hepatic microsomes in the presence or absence of the serine carboxylesterase inhibitors, phenylmethylsulfonylfluoride (PMSF) or NaF. All three cocaine analogs were enzymatically degraded, but the rates of ester cleavage greatly varied among the stereoisomers. (-)-Cocaine was primarily N-oxidized via SKF-525A-sensitive pathways, whereas (+)-cocaine was predominantly hydrolyzed by PMSF-sensitive carboxylesterases. In contrast, (-)-psi-cocaine, which is very stable in the absence of cells at 37 degrees C and pH 7.4, was subject to extremely fast enzymatic ester cleavage. In conclusion, these results indicate that the isomer-specific differential cytotoxicity of (-)-cocaine, (+)-cocaine and (-)-psi-cocaine in hepatocytes may be related to stereoselective differences in the rates of hydrolytic inactivation by hepatic carboxylesterases and that the N-oxidative pathway, resulting in hepatocyte injury, may thus be relevant only for (-)-cocaine.

Structure/activity relationships of the genotoxic potencies of sixteen pyrrolizidine alkaloids assayed for the induction of somatic mutation and recombination in wing cells of Drosophila melanogaster.

Sixteen pyrrolizidine alkaloids (PAs) were examined for their genotoxic potency in the wing spot test of Drosophila melanogaster following oral application. This in vivo assay tests for the induction of somatic mutation and mitotic recombination in cells of the developing wing primordia. All PAs tested except the C9-monoester supinine were clearly genotoxic. Depending on their chemical structure, however, genotoxicity of the PAs varied widely in a range encompassing about three orders of magnitude. In general, macrocyclic diester-type PAs were the most and 7-hydroxy C9-monoester types the least genotoxic representatives studied, while open diesters were intermediate in this respect. Stereoisomeric PAs mostly showed similar, but sometimes also clearly unequal genotoxicity. An increasing number of hydroxy groups in the PA molecule seemed to reduce its genotoxic potency. With respect to the structure/activity relationships, there appears to be a good correlation between hepatotoxicity of PAs in experimental rodents and genotoxicity in the wing spot test of Drosophila. This suggests that PAs are bioactivated along similar pathways in the mammalian liver and in the somatic cells of Drosophila. The genotoxic potential of PAs in the Drosophila wing spot test and their carcinogenic potential in mammals also seem correlated, although the information in the literature on carcinogenicity of the non-macrocyclic PAs with moderate to low genotoxic potency is concededly limited. Comparisons with other genotoxicity tests suggest that the wing spot test is particularly suitable for genotoxins like PAs, on the one hand because of the versatile metabolic bioactivation system of Drosophila and on the other hand also because of its excellent sensitivity to the crosslinking agents among the genotoxins.