Synthesis and evaluation of CBI-based agents: Enhanced functional analogs of the potent antitumor antibiotics (+)-CC-1065 and the duocarmycins

(+)-CC-1065 and the duocarmycins are potent antitumor natural products which exert their antitumor effects through sequence selective DNA alkylation. A 5-exo-trig aryl free radical-alkene cyclization approach to the practical synthesis of the simplified DNA alkylation subunit of (+)-CC-1065 and the duocarmycins, 1,2,9,9a-tetrahydrocyclopropa (c) benz (e) indol-4-one (CBI), has been developed. Coupling of CBI to a series of synthetic DNA binding subunits with deep-seated structural modifications has resulted in the preparation of (+)- and ent-($-$)-CBI-TMI (38, IC$\sb{50}$ = 30 pM), (+)-seco-CBI-indole-NMe$\sb3\sp+$ (41, 42 and 43, IC$\sb{50}$ = 8-17 nM), (+)-CBI-CDPBI (57) and (+)-CBI-CDPBO (58), and (+)-CBI-indole$\sb2$ bioisosteres (93-97, IC$\sb{50}$ = 5-15 pM). Detailed examination of the structure-activity relationship of these agents along with four simple N$\sp2$ substituted CBI-analogs (29-32) has led to the establishment of a fundamental linear relationship between the agents chemical stability and their biological potency. That is, for agents which possess sufficient reactivity to alkylate duplex DNA, the chemically more stable agents constitute the biologically more potent agents. DNA alkylation studies have shown that, unlike (+)-CC-1065, (+)-duocarmycin A and SA reversibly alkylate DNA. The synthetic analogs possess essentially the same sequence selectivity, and their alkylation efficiency follows the common trend observed in the SAR studies. That is, the more stable agents exhibit greater DNA alkylation efficiency. The natural enantiomers of the agents related to the duocarmycins alkylate duplex DNA at an AT-rich 3.5 base-pair site with a binding orientation extending from the adenine N3 alkylation site in the 3$\sp\prime$ to 5$\sp\prime$ direction. The AT-rich 3.5 base-pair site for the unnatural enantiomers extends in the reverse 5$\sp\prime$ to 3$\sp\prime$ direction and is offset from the adenine N3 alkylation site. Detailed models of the enantiomeric alkylation of duplex DNA by CBI-TMI are presented. In an attempt to separate the desired biological potency and the undesired delayed toxicity observed for (+)-CC-1065, five new bioisosteres of (+)-CBI-indole$\sb2$ (93-97) have been prepared. Their in vivo antitumor activities and clinical potential are currently being evaluated for the treatment of cancer