Failure to demonstrate chemoprevention by the monoterpene perillyl alcohol during early rat hepatocarcinogenesis: a cautionary note

The monoterpene perillyl alcohol (PA) is being considered as a useful chemopreventive and therapeutic agent against human cancers. However, no data are available on the effects of PA in the first stages of hepatocarcinogenesis. To study such effects, putatively initiated cells and preneoplastic foci in hepatocarcinogenesis were used as a model. Male Wistar rats were treated with a single dose of N-nitrosomorpholine (NNM). Between days 4 and 91 after NNM, subgroups of rats received either PA (1 g/kg body wt/day) or phenobarbital (PB) (50 mg/kg body wt/day) in the diet. Since PA treatment reduced food intake, one control group was fed ad libitum, while a second control was pair fed between days 4 and 91. In order to enhance any treatment effects, all groups, including the controls, were treated with the potent tumor promoter PB after day 91 until the end of the experiment at day 266. Rats were killed at multiple time points and putatively initiated cells and preneoplastic foci were identified by staining positively for placental glutathione S-transferase (G+). The following results were obtained. (i) A few days after NNM treatment single G+ cells emerged; a considerable portion of which developed into foci. (ii) Treatment with PB resulted in an increase in number and size of G+ foci. (iii) PA treatment failed to reduce the number of G+ cells; it somewhat lowered rates of apoptosis in G+ foci and clearly increased their average size. (iv) Eighty-seven days of PA revealed no protective effect on day 266, but, similar to PB treatment, increased the growth of foci. In conclusion, PA exerted no detectable chemopreventive effect in the early stages of rat hepatocarcinogenesis. It rather exerted a PB-like tumor promoting activity. These data argue against a recommendation of PA as a chemopreventive agent for healthy humans.