Epidemiologic evidence indicates that exposure to heterocyclic amines in the diet is an important risk factor for the development of colon cancer. Well-done cooked meats contain significant levels of heterocyclic amines, which have been shown to cause cancer in laboratory animals. To better understand the mechanisms of heterocyclic amine bioactivation in humans, the most mass abundant heterocyclic amine, 2-amino-1-methyl-6-phenylimidazo [4,5-b]pyridine (PhIP), was used to assess the relationship between PhIP metabolism and DNA adduct formation. Ten human volunteers where administered a dietary relevant dose of [ 14 C]PhIP 48 to 72 hours before surgery to remove colon tumors. Urine was collected for 24 hours after dosing for metabolite analysis, and DNA was extracted from colon tissue and analyzed by accelerator mass spectrometry for DNA adducts. All 10 subjects were phenotyped for cytochrome P4501A2 (CYP1A2), N-acetyltransferase 2, and sulfotransferase 1A1 enzyme activity. Twelve PhIP metabolites were detected in the urine samples. The most abundant metabolite in all volunteers was N-hydroxy-PhIP-N 2 -glucuronide. Metabolite levels varied significantly between the volunteers. Interindividual differences in colon DNA adducts levels were observed between each individual. The data showed that individuals with a rapid CYP1A2 phenotype and high levels of urinary N-hydroxy-PhIP-N 2 -glucuronide had the lowest level of colon PhIP-DNA adducts. This suggests that glucuronidation plays a significant role in detoxifying N-hydroxy-PhIP. The levels of urinary N-hydroxy-PhIP-N 2 -glucuronide were negatively correlated to colon DNA adduct levels. Although it is difficult to make definite conclusions from a small data set, the results from this pilot study have encouraged further investigations using a much larger study group. (Cancer Res 2006; 66(21): 10541-7)

The metabolism of 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) was investigated in five human volunteers given a dietary equivalent of 14C-labeled MeIQx. The amount of the dose excreted in urine ranged from 20.2% to 58.6%, with unmetabolized MeIQx accounting for 0.7-2.8% of the dose. Five principal metabolites were detected in urine, and four of the derivatives were characterized by on-line UV spectroscopy and by HPLC-MS following immunoaffinity chromatography. Two metabolites were identified as the phase II conjugates N2-(3,8-dimethylimidazo[4,5-f]quinoxalin-2-yl)sulfamic acid (MeIQx-N2-SO3(-)) and N2-(beta-1-glucosiduronyl)-2-amino-3,8-dimethylimidazo[4,5-f ]quinoxaline (MeIQx-N2-Gl). Two other metabolites were the cytochrome P450-mediated (P450) oxidation products 2-amino-8-(hydroxymethyl)-3-methylimidazo[4,5-f]quinoxaline (8-CH2OH-MeIQx), and N2-(beta-1-glucosiduronyl)-N-hydroxy-2-amino-3,8-dimethylimidaz o[4,5-f]quinoxaline (NOH-MeIQx-N2-Gl). The latter product is a conjugate of the genotoxic metabolite 2-(hydroxyamino)-3,8-dimethylimidazo-[4,5-f]quinoxaline (NHOH-MeIQx). A large interindividual variation was observed in the metabolism and disposition of MeIQx; these four metabolites and unchanged MeIQx combined accounted for 6.3-26.7% of the total dose. The remaining principal metabolite found in all subjects accounted for 7.6-28% of the dose. It has not been previously identified in rodents or nonhuman primates, and its structure remains unknown. P450-mediated ring oxidation of MeIQx at the C-5 position, a major pathway of detoxication in rodents, was not detected in humans. Both 8-CH2OH-MeIQx formation and NHOH-MeIQx formation are catalyzed by P450 1A2 and may be useful biomarkers of P450 1A2 activity in humans. The levels of NHOH-MeIQx-N2-Gl found in human urine ranged from 1.4% to 10.0% of the dose, which is significantly higher than that formed in rodents and nonhuman primates undergoing cancer bioassays. Thus, bioactivation of MeIQx by P450-mediated N-oxidation is extensive in humans.

Chlorophyll (Chla) and chlorophyllin (CHL) were shown previously to reduce carcinogen bioavailability, biomarker damage, and tumorigenicity in trout and rats. These findings were partially extended to humans, where CHL reduced excretion of aflatoxin B1 (AFB1)-DNA repair products in Chinese unavoidably exposed to dietary AFB1. However, neither AFB1 pharmacokinetics nor Chla effects were examined. We conducted an unblinded crossover study to establish AFB1 pharmacokinetic parameters among four human volunteers, and to explore possible effects of CHL or Chla cotreatment in three of those volunteers. For protocol 1, fasted subjects received an Institutional Review Board–approved dose of 14C-AFB1 (30 ng, 5 nCi) by capsule with 100 mL water, followed by normal eating and drinking after 2 hours. Blood and cumulative urine samples were collected over 72 hours, and 14C- AFB1 equivalents were determined by accelerator mass spectrometry. Protocols 2 and 3 were similar except capsules also contained 150 mg of purified Chla or CHL, respectively. Protocols were repeated thrice for each volunteer. The study revealed rapid human AFB1 uptake (plasma ka, 5.05 ± 1.10 h−1; Tmax, 1.0 hour) and urinary elimination (95% complete by 24 hours) kinetics. Chla and CHL treatment each significantly impeded AFB1 absorption and reduced Cmax and AUCs (plasma and urine) in one or more subjects. These initial results provide AFB1 pharmacokinetic parameters previously unavailable for humans, and suggest that Chla or CHL co-consumption may limit the bioavailability of ingested aflatoxin in humans, as they do in animal models.