Substituted hydroxyphenanthrenes in opium pyrolysates implicated in oesophageal cancer in iran: Structures and in vitro metabolic activation of a novel class of mutagens

Previous epidemiological and laboratory studies have indicated an association between the ingestion of opium pyrolysates, dietary deficiencies and the high incidence of oesophageal cancer in subjects in north-east Iran. Pyrolysates of opium, and particularly of morphine, a major opium alkaloid, were both shown to contain similar highly mutagenie substances that were also clastogenic in mammalian cells and which transformed hamster embryo cells in culture. We now report the isolation and characterization of nine of the most abundant mutagenic compounds present in morphine pyrolysates, using h.p.l.c, GC-MS and n.m.r. spectroscopy. The hitherto unknown compounds, all containing a hydroxyphenanthrene moiety, were identified as: I, 3-methyl-3H-naphth [1,2-e]indol-10-ol; II, 1,2-dihydro-3-methyl-3H-naphth [1,2-e]indol-10-ol; III, 1-methyl-1H-naphth [2,1-g]indol-10-ol; IV, 2-methylphenanthro [3,4-d]-[1,3]oxazol-10-ol; V, 6-methylaminophenanthren-3-ol; VI, 2-methyl-3H-phenanthro [3,4-d]imidazol-10-ol;1,2-dimethyl-1H-phenanthro [3,4-d]imidazol-10-ol; VIII, 2,5-dimethyl-3H-phenanthro [3,4-d]imidazol-1O-ol; and IX, 2-ethyl-3H-phenanthro [3,4-d]imidazol-1O-ol; Structures for the heterocycic rings of compounds IV and VI to IX are tentative. Mutagenicity in Salmonella typhimurium TA98 in the presence of rat liver homogenates increased in the order listed and ranged over four orders of magnitude, IX being 1000 times more active than benzo[a]pyrene Compounds I and VII were converted by rat liver 9000 g supernatant into phenols and dihydrodiols, implicating arene oxides as ultimate mutagens. The formation and reaction of these arene oxides was shown by trapping experiments in vitro with ethanethiol and subsequent characterization of the ethyl sulfide reaction products. The order of biological activity of compounds I-IX, dependent on the structure of the heterocyclic ring, suggests that carhocations, resonance-stabilized as quinone methides, are their ultimate reactive metabolites. Our results lend additional support to the role of opium pyrolysates as an etiological factor in oesophageal cancer in north-east Iran.