Intestinal absorption and cell transforming potential of PhIP-M1, a bacterial metabolite of the heterocyclic aromatic amine 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)

• The transport of PhIP-M1, a metabolite of the colon carcinogen PhIP produced by gut bacteria, in eight different intestinal segments from male Fischer 344 rats was examined in Ussing chambers.
• A very recently developed HPLC method to quantify the concentration of PhIP-M1 in biological fluids as well as in intestinal tissue was applied.
• At the most, 10–20% of the PhIP-M1 amount added to the mucosal compartment of the Ussing chambers per segment were absorbed within 90 min.
• PhIP-M1 in a concentration of up to 100 μM did not induce the malignant transformation of BALB/c 3T3 cells.
• Even if one would assume that 100% of the daily amount of PhIP ingested by a human being is converted into PhIP-M1 in the colon, this concentration most probably would not lead to carcinogenicity in the colorectal mucosa.

Previous studies have shown that in the rat, the colon carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is only absorbed to a limited extent in the small intestines and that a major fraction of unmetabolised PhIP reaches the colon. Moreover, PhIP is extensively metabolised when incubated with human stool samples to a major derivative, 7-hydroxy-5-methyl-3-phenyl-6,7,8,9-tetrahydropyrido [3′,2′:4,5]imidazo[1,2-a]pyrimidin-5-ium chloride (PhIP-M1). In the present study, the uptake and transport of PhIP-M1 in Ussing chamber experiments, its cytotoxicity in the different segments of the Fischer 344 rat gut and its transforming potential in the BALB/c 3T3 cell transformation assay were analysed. At the most, 10–20% of the PhIP-M1 amount added to the mucosal compartment of the Ussing chambers per segment were absorbed within 90 min. Therefore, the amount of PhIP-M1 detected in the tissues as well as in the serosal compartment of the Ussing chambers was extremely low. Moreover, human-relevant concentrations of PhIP-M1 were not cytotoxic and did not induce the malignant transformation of BALB/c 3T3 cells. In conclusion, even if one would assume that 100% of the daily amount of PhIP ingested by a human being is converted into PhIP-M1 in the colon, this concentration most probably would not lead to cytotoxicity and/or carcinogenicity in the colorectal mucosa.