Manganese inhibition of sarcoma induction by benzo[a]pyrene in rats.

The effects of Mn and Cr dusts upon carcinogenesis by polycyclic aromatic hydrocarbons were tested in male Fischer 344 rats. In Experiment I, rats were given i.m. injections of benzo[a]pyrene (BP, 1.2 mg), Mn dust (4.4 mg), and Cr dust (4.4 mg), alone, and in various combinations. By 100 weeks, sarcomas occurred at the injection site in 17/20 rats that received BP alone, versus 10/19 rats that received BP plus Mn dust (P < 0.05). The sarcoma incidences were 0/20 in 3 control groups that received the vehicle, Mn dust alone, or Cr dust alone, and 20/20 in rats that received BP plus Cr dust. In Experiment II, rats were given i.m. injections of BP (0.6 mg), 7,12-dimethylbenz[a]anthracene (DMBA, 0.6 mg), Mn dust (4.4 mg), and Cr dust (4.4 mg), alone, and in various combinations. Local sarcomas occurred in 17/20 rats that received BP alone, versus 5/17 rats that received BP plus Mn dust (P < 0.001). Sarcoma induction was not inhibited when BP was injected in the right thigh and Mn dust in the left thigh (sarcoma incidence = 16/20). The sarcoma incidences were 0/19 in a vehicle control group; 0/18 in 2 control groups that received Mn dust or Cr dust alone; 18/20 in rats that received BP plus Cr dust; 14/20 in rats that received DMBA alone; 17/19 in rats that received DMBA plus Mn dust; and 13/18 in rats that received DMBA plus Cr dust. These experiments show that Mn dust inhibits the carcinogenicity of BP when the Mn dust and BP are administered to rats by a single i.m. injection. Under identical conditions, Mn dust does not affect the carcinogenicity of DMBA; Cr dust does not affect the carcinogenicity of BP or DMBA.