Selective metal cation activation of a DNA alkylating agent: Synthesis andevaluation of methyl 1,2,9,9a-tetrahydrocyclopropa[c]pyrido[3,2-e]indol-4-one-7-carboxylate (CPyI)

The synthesis of methyl 1,2,9,9a-tetrahydrocyclopropa[c]pyrido[3,2-e]indol- 4-one-7-carboxylate (CPyI) containing a one carbon expansion of the C ring pyrrole found in the duocarmycin SA alkylation subunit and its incorporatio n into analogues of the natural product are detailed. The unique 8-ketoquin oline structure of CPyI was expected to provide a tunable means to effect a ctivation via selective metal cation complexation. The synthesis of CPyI wa s based on a modified Skraup quinoline synthesis followed by a 5-exo-trig a ryl radical cyclization onto an unactivated alkene with subsequent TEMPO tr ap or 5-exo-trig aryl radical cyclization onto a vinyl chloride for synthes is of the immediate precursor. Closure of the activated cyclopropane, accom plished by an Ar-3' spirocyclization, provided the CPyI nucleus in 10 steps and excellent overall conversion (29%). The evaluation of the CPyI-based a gents revealed an intrinsic stability comparable to that of CC-1065 and duo carmycin A but that it is more reactive than duocarmycin SA and the CBI-bas ed agents (3-4x). A pH-rate profile of the addition of nucleophiles to CPyI demonstrated that an acid-catalyzed reaction is observed below pH 4 and th at an uncatalyzed reaction predominates above pH 4. The expected predictabl e activation of CPyI by metal cations toward nucleophilic addition was foun d to directly correspond to established stabilities of the metal complexes with the addition product (Cu2+ > Ni2+ > Zn2+ > Mn2+ > Mg2+) and provides t he opportunity to selectively activate the agents upon addition of the appr opriate Lewis acid. This tunable metal cation activation of CPyI constitute s the first example of a new approach to in situ activation of a DNA bindin g agent complementary to the well-recognized methods of reductive, oxidativ e, or photochemical activation. Resolution and synthesis of a full set of n atural product analogues and subsequent evaluation of their DNA alkylation properties revealed that the CPyI analogues retain identical DNA alkylation sequence selectivity and near-identical DNA alkylation efficiencies compar ed to the natural products. Consistent with past studies and even with the deep-seated structural change in the alkylation subunit, the agents were fo und to exhibit potent cytotoxic activity that directly correlates with thei r inherent reactivity.