Neoplastic Transformation in Tissues of Rats Exposed to Monocrotaline or Dehydroretronecine

: Male Sprague-Dawley rats received sc injections biweekly of either the pyrrolizidine alkaloid monocrotaline or its metabolite dehydroretronecine for 1 year. The animals were then observed for an additional 12 months for the induction of neoplasms. Of 60 rats that received dehydroretronecine, 39 developed rhabdomyosarcomas at the injection site, and 5 of these neoplasms metastasized. In the 60 monocrotaline-treated rats, 31 widely dispersed tumors of various cell types were recorded. The reason suggested for the variation in tissue response was that the metabolite dehydroretronecine is a proximate carcinogen, whereas monocrotaline must first be metabolized before its carcinogenic potential is realized.