Selective metal cation activation of a DNA alkylating agent:: Synthesis and evaluation 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 incorporation into analogues of the natural product are detailed. The unique 8-ketoquinoline structure of CPyI was expected to provide a tunable means to effect activation via selective metal cation complexation. The synthesis of CPyI was based on a modified Skraup quinoline synthesis followed by a 5-exo-trig aryl radical cyclization onto an unactivated alkene with subsequent TEMPO trap or 5-exo-trig aryl radical cyclization onto a vinyl chloride for synthesis of the immediate precursor. Closure of the activated cyclopropane, accomplished by an Ar-3' spirocyclization, provided the CPyI nucleus in 10 steps and excellent overall conversion (29%). The evaluation of the CPyI-based agents revealed an intrinsic stability comparable to that of CC-1065 and duocarmycin A but that it is more reactive than duocarmycin SA and the CBI-based 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 that an uncatalyzed reaction predominates above pH 4. The expected predictable activation of CPyI by metal cations toward nucleophilic addition was found to directly correspond to established stabilities of the metal complexes with the addition product (Cu2+ > Ni2+ > Zn2+ > Mn2+ > Mg2+) and provides the opportunity to selectively activate the agents upon addition of the appropriate Lewis acid. This tunable metal cation activation of CPyI constitutes the first example of a new approach to in situ activation of a DNA binding agent complementary to the well-recognized methods of reductive, oxidative, or photochemical activation. Resolution and synthesis of a full set of natural 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 compared to the natural products. Consistent with past studies and even with the deep-seated structural change in the alkylation subunit, the agents were found to exhibit potent cytotoxic activity that directly correlates with their inherent reactivity.