Short-term tests with biochemical end points relevant to the prediction of carcinogenic potential of drugs

Effects of the tumor promoter 12-O-tetradecanoylphorbol-13-acetate on phenytoin-induced embryopathy in mice

The anticonvulsant drug phenytoin may be cooxidized during prostaglandin biosynthesis to a reactive free radical intermediate capable of exerting embryopathic effects. Since 12-O-tetradecanoylphorbol-13-acetate (TPA), a potent tumor promoter, activates the enzymatic release of arachidonic acid, thereby initiating prostaglandin synthesis, the potential teratological synergism between these two compounds was investigated. Pregnant CD-1 mice were given a fixed dose of phenytoin, 65 mg/kg intraperitoneally (ip), on Gestational Days 12 and 13, followed 2 hr later by varying doses of TPA, 0.2 to 2000 micrograms/kg ip. The dams were killed on Day 19 and fetuses were examined for anomalies. TPA post-treatment with doses of 2 to 200 micrograms/kg produced a TPA dose-related increase in maternal lethality of up to 100%, compared with no lethality in dams given TPA or phenytoin alone. Subsequent studies therefore were restricted to TPA doses between 0.2 and 20 micrograms/kg. TPA 20 micrograms/kg caused a threefold increase (p = 0.11) in the incidence of phenytoin-induced cleft palates, which likely was an underestimate due to the extremely high (97%) incidence of fetal resorptions. With lower TPA doses of 0.2 to 20 micrograms/kg, the incidence of fetal resorptions in animals treated with both phenytoin and TPA increased in a TPA dose-related manner, to over twofold (p less than 0.05), compared with phenytoin controls. Postpartum fetal lethality was enhanced similarly by the combination treatment, reaching a maximum of 100% (p less than 0.05). Lower doses of TPA also significantly enhanced the fetal weight loss in phenytoin-treated mice, while in contrast, TPA alone significantly increased fetal body weight compared with vehicle-treated controls. In animals treated with only TPA, the incidence of embryopathy generally was either comparable to controls or significantly less than that in phenytoin controls. These data indicate that TPA can potentiate phenytoin-induced embryopathy, possibly through a mechanism involving bioactivation by prostaglandin synthetase.

Aziridine biotransformation by microsomes and lethality to hepatocytes isolated from rat

To clarify the relationship of aziridine biotransformation to their cytotoxic activities, the metabolism of optical isomers of typical cytotoxic and non-cytotoxic aziridines was studied in isolated hepatocytes, rat liver microsomes, mitochondria and L-1210 mouse leukemia cells. Cytotoxic 1-methyl-2-beta-naphthylaziridine (NAZ) gave nitrosomethane as one of the bioactivation products in isolated hepatocytes and simultaneously induced a marked decrease in cellular ATP followed by cell lethality. NAZ itself did not directly affect the respiratory function of mitochondria in isolated hepatocytes or in buffer solution, however, it inhibited the mitochondrial activity in the presence of microsomes in the buffer solution. Nitroso-t-butane or nitrosomethane dimer, used as a substitute for extremely labile nitrosomethane, strongly inhibited the respiration of mitochondria. On the other hand, optical isomers of 2-aziridinecarboxylic acid (AZC) which did not give nitrosomethane in isolated hepatocytes or microsomes also did not show cytotoxicity. Thus, the cytotoxicity of NAZ seems to be induced by bioactivation via cellular oxidases with the nitrosomethane generated being a major toxic component. This may occur with most of the cytotoxic aziridine derivatives.