Dose-dependent milk transfer and tissue distribution of the food mutagen PhIP in rats and their suckling pups.

The transfer of the neonatal carcinogen and food mutagen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) into milk of lactating Sprague-Dawley rats and the uptake in 10 day old suckling pups were investigated. PhIP was quantified by HPLC. In dams given a single i.v. injection of PhIP (0.5 mg/kg body wt) and milked 1 h later, 0.04-0.16% of the dose per ml milk was excreted. In dams dosed i.v. with PhIP (0, 0.05, 0.5 and 5.0 mg/kg body wt) and killed 4 h later a linear dose-dependent milk excretion and uptake of PhIP in the tissues were observed (y = 76.1 chi + 1.0; r = 0.94; P < 0.001). In addition, a linear correlation was found between PhIP levels in dam's liver and milk (y = 0.61 chi + 37.4; r = 0.97; P < 0.001). Following administration of PhIP at doses of 0.05 and 0.5 mg/kg body wt, the highest level of PhIP was observed in the milk samples. Following injection of 5.0 mg PhIP/kg body wt, the highest level of PhIP was observed in the mammary gland and liver. The milk/plasma ratio was 9.3 +/- 6.8 at the highest dose at 4 h. High levels of unmetabolized PhIP were also detected in liver and blood of pups allowed to suck the PhIP-dosed dams for 3 h. Autoradiography of pups injected i.p. with [2-14C]PhIP (4.8 mg/kg body wt) showed that only a low level of radioactivity was irreversibly bound in the liver, and high levels of radioactivity were found in stomach milk, intestinal contents and in the urine, 1 h and 4 h following injection. In addition, radioactivity was present in the skin, liver and kidney. Since milk is the major dietary source for nursing infants, the milk transfer of PhIP is of great concern.

Transfer of the food mutagen PhIP to foetuses and newborn mice following maternal exposure.

The food mutagen PhIP (2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine) is a liver carcinogen in neonatal mice. In this study the transfer of PhIP to foetuses and neonates following a single ip dose (4.7-5.2 mg/kg body weight) to pregnant or lactating C57Bl/6 mice was studied by tissue extraction and HPLC analysis. A transplacental transfer of unchanged [3H]PhIP to foetuses was demonstrated; the highest foetal levels were observed at late gestation. Autoradiography of mice injected iv with [2-14C]PhIP (1.4 mg/kg) during late gestation showed a high and selective localization of radioactivity in the pigmented parts of the foetal eye and a moderate level of radioactivity in the foetal liver, gastro-intestinal contents, urine and in the uterine fluid. HPLC analysis of stomach contents and tissues of newborn mice exposed for 4 hr to lactating dams dosed ip with [3H]PhIP (5.2 mg/kg) showed the presence of unchanged PhIP suggesting excretion of PhIP in the milk of the dams. The results in this study raise concern that exposure to PhIP during pregnancy and nursing may result in a transfer of this food mutagen to foetuses and infants.

Binding of the food mutagen PhIP in pigmented tissues of mice.

The distribution of the 14C-labelled food mutagen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) in the tissues of C57B1/6 and NMRI mice was studied. The results showed a high and selective binding of radioactivity in the pigment epithelium of the eye and in the fur following a single dose (0.3-4 mg/kg) of [14C]PhIP in the pigmented C57B1/6 mice whereas no such localization of radioactivity was present in the albino NMRI mice. A low but selective covalent binding of radioactivity was observed in the liver, inner cortex of the kidney and in the tracheal mucosa of [14C]PhIP-injected mice. PhIP was firmly bound to synthetic melanin pigment in vitro; only 3% was released by extraction with a phosphate buffer (pH 7) whereas 72% was released by extraction by methanol:conc. NH3 (15:1). Three hours to 7 days following a single injection of [14C]PhIP in C57B1/6 mice the radioactivity in the eye was 3- to 6-fold higher than that in the liver or kidney. Almost 60% of the radioactive material present in the pigmented epithelium of the eye 3 and 24 h following injection could be extracted by basic methanol and identified as unchanged PhIP. The residual radioactivity in the pigmented epithelium of the eyes may represent a covalent binding of [14C]PhIP metabolites to cellular constituents or to a basic methanol-resistant binding of [14C]PhIP to melanin. The results indicate that pigmented tissues may be potential target tissues for the toxic effects of PhIP and suggest that the use of hair for biological monitoring of PhIP should be examined.