The importance of carcinogen dose in chemoprevention studies: quantitative interrelationships between, dibenzo[a,l]pyrene dose, chlorophyllin dose, target organ DNA adduct biomarkers and final tumor outcome

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Chlorophyllin (CHL) is a potent antimutagen in vitro, an effective anti-carcinogen in several animal models, and significantly reduced urinary biomarkers of aflatoxin B1 (AFB1) exposure in a human population. Here we report an expanded analysis of CHL chemoprevention using the potent environmental hydrocarbon dibenzo[a,l]pyrene (DBP). A dose–dose matrix design employed over 12 000 rainbow trout to evaluate the interrelationships among dietary carcinogen dose, anti-carcinogen dose, carcinogen–DNA adduct levels at exposure and eventual tumor outcome in two target organs. Included was an evaluation of the pharmaceutical CHL preparation (Derifil), used previously in a study of individuals chronically exposed to AFB1. CHL was pre-, co- and post-fed at doses of 0–6000 p.p.m. and co-fed with DBP at doses of 0–371.5 p.p.m. for 4 weeks. This protocol generated a total of 21 dose–dose treatment groups, each evaluated with three or more replicates of 100 animals. The DBP-only treatment produced dose-responsive increases in liver and stomach DBP–DNA adducts, whereas increasing CHL co-treatment doses produced successive inhibition in liver (49–83%) and stomach (47–75%) adduct levels at each DBP dose examined. The remaining 8711 trout were necropsied, 10 months later. DBP treatment alone produced a logit incidence versus log [DBP] dose–response curve in stomach that was linear; CHL co-treatment provided dose-dependent tumor inhibition which ranged from 30 to 68% and was predictable from the adduct response. The Derifil CHL preparation was also found to effectively reduce DNA adduction and final tumor incidence in stomach (as well as liver), with a potency compatible with its total chlorin content. Liver tumor incidence in the DBP-only groups appeared to plateau near 60%. At DBP doses of ≤80 p.p.m., increasing CHL doses generally reduced tumor incidence and multiplicity consistent with early DNA adducts as biomarkers. At 225 p.p.m. DBP, however, very high CHL doses were required to reduce tumor incidence below the 60% plateau. Apparent tumor multiplicity in liver was neither linear nor monotonic with DBP dose, but peaked at 80 p.p.m. DBP and declined at 225 p.p.m., where it was increased by all but one CHL dose. Consequently, the effects of a given CHL dose and the predictivity of DNA adducts as biomarkers were highly dependent on carcinogen dose. These results underscore the critical importance of establishing carcinogen-end point dose–response relationships in chemoprevention studies, and the potential otherwise for misleading interpretations in chemoprevention studies carried out solely at high-carcinogen dose. (Supported by USPHS grants ES03850, ES00210, CA34732, ES07060, ES06052 and ES03819.)

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