INTOPLICINE (RP-60475) AND ITS DERIVATIVES, A NEW CLASS OF ANTITUMOR AGENTS INHIBITING BOTH TOPOISOMERASE-I AND TOPOISOMERASE-II ACTIVITIES

Intoplicine (RP 60475, NSC 645008) is an antitumor derivative in the 7 H-benzo[e]pyrido[4,3-b]indole series which is now being tested in clin ical trials. Intoplicine strongly binds DNA (K(A) = 2 x 10(5) M-1) and thereby increases the length of linear DNA. These properties are cons istent with DNA unwinding by intoplicine. Intoplicine was found to be a dual topoisomerase I and II inhibitor, with DNA sites of enzyme inhi bition being different for these two enzymes. In this study, 22 analog ues of intoplicine were evaluated for their effects on topoisomerase I - and II-mediated DNA cleavage reactions by using enzymes purified fro m calf thymus. Site-specific DNA cleavage mediated by topoisomerase I was observed with 7H-benzo[e]pyrido[4,3-b]indole derivatives but not w ith 11H-benzo[g]-pyrido[4,3-b]indole derivatives. Site-specific DNA cl eavage mediated by topoisomerase II occurred with derivatives having h ydroxyl groups at the 3-position on the 7H-benzo[e]pyrido[4,3-b]indole ring or at the 4-position on the 11H-benzo[g]pyrido[4,3-b]indole ring . Study of the relationships between the in vivo antitumor activity on P388 leukemia and the topoisomerase I- and/or II-mediated DNA cleavag e activity revealed that the most highly active antitumor compounds po ssessed both topoisomerase I- and II-inhibitory properties. Compounds selectively inhibiting either topoisomerase I or II were less active. These results suggest that dual topoisomerase I and II inhibition is c ritical for the antitumor activity of this new series of antitumor com pounds.