Metabolic activation of retronecine and retronecine N-oxide – formation of DHP-derived DNA adducts

We have previously reported that metabolism of a series of pyrrolizidine alkaloids in vitro and in vivo generated a set of (+/−)6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP)-derived DNA adducts. It has also been shown that the levels of the DHP-derived DNA adduct formation correlated closely with the tumorigenic potencies of the mice fed with different doses of riddelliine. Retronecine is the necine base and the structurally smallest chemical of the retronecine-type pyrrolizidine alkaloids. Although it has been reported that microsomal metabolism of retronecine generated DHP as a metabolite, it was yet not known whether metabolism of retronecine in vivo could generate DHP-derived DNA adducts and if formed, whether or not the levels of DNA adducts were comparable with those formed from the other tumorigenic retronecine-type pyrrolizidine alkaloids, such as riddelliine, retrorsine, and monocrotaline. In this investigation, the in-vitro and in-vivo metabolic activation of retronecine was studied. Rat liver microsomal metabolism of retronecine in the presence of calf thymus DNA resulted in the formation of a set of DHP–DNA adducts. The metabolism of retronecine N-oxide under similar conditions also formed the similar set of DHP–DNA adducts. The level of DNA adducts from retronecine was enhanced when metabolism by liver microsomes from phenobarbital (PB)-induced rats were used. The DHP–DNA adducts were also found in the liver DNA of female F344 rats treated with retronecine or retronecine N-oxide. The highest level of the total DHP–DNA adducts was found in liver DNA from the rats treated with dehydroretronecine (DHR). The order of the levels of DNA adducts in the liver DNA samples from rats treated with various pyrrolizidine alkaloids was: DHR > riddelliine > riddelliine N-oxide >> retronecine > retronecine N-oxide. The results indicate that 1) retronecine can be metabolized to form DHP by rat liver microsomal enzymes and interacts with DNA to produce DHP–DNA adducts and 2) retronecine N-oxide undergoes the biotransformation to the parent compound, retronecine. The results from this and our previous findings strongly suggest that formation of DHP–DNA adducts may be a potential biomarker for pyrrolizidine alkaloid carcinogenesis.