Synthesis and Evaluation of Stable Bidentate Transition Metal Complexes of 1-(Chloromethyl)-5-hydroxy-3-(5,6,7-trimethoxyindol-2-ylcarbonyl)-2,3-dihydro-1H-pyrrolo[3,2-f]quinoline (seco-6-azaCBI-TMI) as Hypoxia Selective Cytotoxins

A series of metal complexes were prepared as potential prodrugs of the extremely toxic DNA minor groove alkylator 1-(chloromethyl)-5-hydroxy-3-[(5,6,7-trimethoxyindol-2-yl)carbonyl]-2,3-dihydro-1H-pyrrolo[3,2-f]quinoline (seco-6-azaCBI-TMI) and close analogues. The pyrrolo[3,2-f]quinoline cytotoxins were prepared from 2-methoxy-4-nitroaniline in a nine-step synthesis involving a Skraup construction of a quinoline intermediate, its appropriate functionalization, and a final radical cyclization. The metal complexes were prepared from these and the labile metal complex synthons [Co(cyclen)(OTf)2]+, [Cr(acac)2(H2O)2]+, and [Co2(Me2dtc)5]+. The cobalt complexes were considerably more stable than the free effectors and showed significant attenuation of the cytotoxicity of the latter, with IC50 ratios (complex/effector) of 50- to 150-fold, and substantial hypoxic cell selectivity, with IC50 ratios (oxic/hypoxic cells) of 20- to 40-fold. The cobalt complexes were also efficiently activated by ionizing radiation, with G values for loss of the compound close to the theoretical value for one-electron reduction of 0.68 μmol/J. This work extends earlier observations that cobalt cyclen complexes are suitable for both the bioreductive and radiolytic release of potent pyrrolo[3,2-f]quinoline effectors.