Biochemical pharmacology and DNA-drug interactions by Cl-958, a new antitumor intercalator derived from a series of substituted 2H-[1]benzothiopyrano[4,3,2-cd]indazoles.

Cl-958, PD 121373, and PD 114595 belong to a new class of DNA complexers, substituted 2H-[1]benzothiopyrano[4,3,2-cd] indazoles, and are being further developed as antitumor drugs based on their curative properties against murine solid tumor models. The biochemical effects of these drugs on L1210 leukemia cells and their interaction with DNA were studied and compared to clinically used intercalators. The benzothiopyranoindazoles bound to DNA with a relatively high affinity, having intrinsic association constants of between 3 and 4 x 10(5) M-1. Based on viscosity measurements, the mode of DNA binding appears to be through intercalation. Unwinding angles were calculated to be approximately 18 degrees. The benzothiopyranoindazoles were potent inhibitors of nucleic acid synthesis, reducing both DNA and RNA synthesis to the same extent at similar concentrations. Like other known intercalators, these compounds produced DNA single- and double-strand breaks in a time- and concentration-dependent manner in L1210 cells. Between one and two DNA strand breaks were formed per protein-strand crosslink. Repair of these DNA lesions after the drug was removed from the cells was either very slow or did not occur at all for at least 2 hr. Finally, since the high incidence of cardiotoxicity associated with the administration of anthracyclines has been related to the formation of reactive oxygen species, the ability of the benzothiopyranoindazoles to augment superoxide dismutase-sensitive oxygen consumption was observed in a rat liver microsomal system. These compounds produced less than 5% of the activity in this assay that doxorubicin produced.