Lack of promoting effect of clonazepam on the development of N-nitrosodiethylamine-initiated hepatocellular tumors in mice is correlated with its inability to inhibit cell-to-cell communication in mouse hepatocytes.

The tumor-promoting ability of clonazepam (CZP), a widely used benzodiazepine anticonvulsant, was investigated in an in vivo mouse liver tumor promotion assay and an in vitro mouse hepatocyte intercellular communication assay. The development of preneoplastic hepatocellular foci of cellular alteration and hepatocellular neoplasms was studied in male B6C3F1 mice initiated, at 5 weeks of age, with a single i.p. injection of N-nitrosodiethylamine (NDEA; 90 mg/kg body weight) in tricaprylin, followed by administration of either phenobarbital (PB; 0.05%) or CZP (0.068% or 0.136%) in diet beginning 2 weeks after carcinogen injection and continuing to 60 weeks of age. Several mice from each group were killed after 9, 21, 33 or 53 weeks on test diet, and portions of liver and other organs were fixed in formalin and examined histologically. Unlike PB, CZP did not promote the development of preneoplastic hepatocellular foci or neoplasms (adenomas and carcinomas) in NDEA-initiated mice. Following limited (2 weeks) dietary exposure at 0.15%, CZP was a potent inducer of hepatic P450IIB1-mediated alkoxyresorufin O-dealkylase activities. In contrast, the degree of induction in hepatic tissue from mice fed 0.136% CZP for 53 weeks was markedly lower than that in mice fed 0.05% PB for 53 weeks. In the in vitro assay, diazepam, a strong tumor promoter in mouse liver, significantly inhibited mouse hepatocyte gap junctional intercellular communication, while CZP had no significant effect on this parameter. Thus, CZP, a drug structurally related to diazepam, is inactive as a liver tumor promoter in mice.