The most abundant heterocyclic amine in fried ground beef, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), induces colon carcinomas in the male F344 rat. The potential chemopreventive effects of two compounds, namely, the 'interceptor molecule' chlorophyllin (CHL) and a modulator of carcinogen activation, indole-3-carbinol (I3C), were examined in a PhIP colon carcinogenesis model. During weeks 3 and 4 of a 16-week study, F344 rats were given PhIP by oral gavage (50 mg/kg body weight, alternating days). Inhibitors were given either before and during PhIP exposure, after PhIP treatment, or continuously for 16 weeks. Treatment of rats with 0.1% CHL in the drinking water inhibited the formation of aberrant crypt foci (ACF) with > or = 4 crypts/focus, from 1.4 +/- 0.9 in controls to 0.7 +/- 0.3 following post-initiation CHL treatment, and to 0.3 +/- 0.5 in rats given CHL continuously for 16 weeks (mean +/- SD; P < 0.05). Potent inhibition of PhIP-induced ACF occurred following initiation, post-initiation and continuous exposure to 0.1% I3C in the diet. Using the initiation protocol, I3C completely inhibited the induction of the ACF with > or = 4 crypts/focus. In a separate experiment, rats were given 0.1% CHL in the drinking water or 0.1% I3C in the diet for 4 weeks. At the end of week 3, animals received 50 mg PhIP/kg body weight by single oral gavage and PhIP-DNA adducts were quantified in the colon and several other tissues by 32P-postlabeling analysis. In addition, the urine and feces were collected to study the effects of inhibitor treatment on PhIP metabolism and excretion. No significant protection against PhIP-DNA adduct formation was detected in the colon after CHL dosing, nor was a consistent pattern of CHL inhibition observed in several other tissues. In contrast, I3C shifted the time-course of adducts in all tissue; compared with controls, adducts were increased by I3C at 6 h but decreased at 24 h and 7 days following PhIP treatment. Analysis of urine metabolites revealed that I3C and CHL decreased the excretion of unmetabolized PhIP and 4'-hydroxy- << PhIP but increased the phase II detoxification products PhIP-4'-O-glucuronide and PhIP-4'- sulfate. In the feces, the elimination of unmetabolized PhIP was increased from 54.5% in controls to approximately 67% in CHL-treated rats and decreased to 28% in rats given I3C (P < 0.05). These results support a protective role for CHL and I3C against PhIP-induced colon carcinogenesis through mechanisms which alter the uptake or metabolism of the carcinogen, and by suppression in the post-initiation phase.