Tumor promotion by an anticonvulsant agent, phenytoin, in mouse liver: correlation with CYP2B induction.

To investigate the liver tumor promoting effects of phenytoin (5,5-diphenylhydantoin; DPH), 5 week old male D2B6F1 mice were given a single i.p. dose of 90 mg N-nitrosodiethylamine (NDEA)/kg body wt in tricaprylin. Control groups received tricaprylin alone. After 2 weeks, the mice were given a diet containing 500, 250 or 125 p.p.m. DPH. Ten mice from each treatment group were killed at 30 weeks of age, at which time 10/10 mice given 500 p.p.m. DPH after NDEA initiation had developed multiple hepatocellular foci and adenomas. Such lesions were found only in 2/10 mice given NDEA alone. By 60 weeks, when the experiment was concluded, the incidences (and multiplicities, in units of tumors per tumor-bearing mouse) of hepatocellular adenomas were 60% (1.8 +/- 0.8), 100% (11.6 +/- 5.5), 77% (4.4 +/- 3.3) or 71% (2.6 +/- 1.3) in mice exposed to NDEA alone, or NDEA followed by 500, 250 or 125 p.p.m. DPH respectively. Hepatocellular carcinomas (87% incidence) and hepatoblastomas (33% incidence) were found only in mice given 500 p.p.m. DPH following NDEA initiation. Dose-dependent and profound increases in hepatic CYP2B-mediated benzyloxyresorufin O-dealkylase activity were detected in livers of B6C3F1 mice exposed for 14 days to dietary DPH (125, 250, 500 or 1000 p.p.m.). Similar increases in this activity were observed in D2B6F1 mice exposed to 500 and 250 p.p.m. for 30 or 60 weeks. Thus, increased hepatic CYP2B activity in mice exposed to DPH correlates with the tumor promoting effect of this compound.