CC-1065, a novel antibiotic produced by Streptomyces zelensis, was active against several experimental tumors in vivo and a broad spectrum of human tumor cells in vitro. This report describes its biological and biochemical effects of L1210 leukemia cells. CC-1065 is one of the most cytotoxic agents known. The concentrations required for a 50 and 90% inhibition of cell growth are 0.02 and 0.05 ng/ml, respectively. It is about 400 times more cytotoxic than was Adriamycin. The action of CC-1065 is

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... rapid and is dose and time dependent. CC-1065 inhibits DNA synthesis much more than it inhibits RNA and protein synthesis. The concentrations required for a 50% inhibition of DNA synthesis and RNA synthesis are 4 to 6 and 45 to 60 ng/ml, respectively. Although the drug inhibition of DNA synthesis cannot completely account for its cytotoxic effects on L1210 cells, these results, along with those generated by other investigators, suggest that the inhibition of DNA synthesis represents a major mode of action of CC-1065. CC-1065 inhibited both thymidine kinase and DNA polymerase alpha activities, but the effect on highly purified DNA polymerase alpha was more pronounced. At 1 microgram/ml, CC-1065 inhibited more than 70% of the enzyme activity. In order to elucidate the mechanism of inhibition of DNA polymerase alpha, the interaction between CC-1065 and DNA was investigated. The studies with thermal melting of DNA and difference circular dichroism measurement indicate that CC-1065 is one of the strongest DNA-binding agents. It induced an increase in thermal melting temperature of calf thymus DNA by at least 31 degrees. The circular dichroism studies also reveal that CC-1065 binds only to double-stranded DNA but not to heat-denatured DNA or yeast RNA. These observations were supported by those obtained with two other experimental approaches. CC-1065 also appeared to interact with proteins, but the interaction was weak and reversible.