Nitro-chloromethylbenzindolines: hypoxia-activated prodrugs of potent adenine N3 DNA minor groove alkylators.

Hypoxia represents an important therapeutic target in tumors because of the resistance of hypoxic cells to radiotherapy and chemotherapy and because it is more severe in many tumors than in normal tissues. Here, we describe a class of prodrugs, nitro-chloromethylindolines, which undergo hypoxia-selective activation by endogenous nitroreductases in tumor cells to form the corresponding amino compounds. The latter are chemically related to the cyclopropylindoline antitumor antibiotics and they share the same properties of sequence-selective DNA minor groove alkylation and high cytotoxic potency. Of three alkylating subunits investigated, the chloromethylbenzindoline (CBI) structure provided the most favorable prodrug properties: aerobic cytotoxic potency of the amines was approximately 90- to 3,000-fold higher than the corresponding nitro compounds, and the nitro compounds showed air/anoxia potency differentials of up to 300-fold. Selective alkylation of adenine N3 in calf thymus DNA by an amino-CBI was shown by characterization of the thermal depurination product; the same adduct was shown in hypoxic RIF-1 cells exposed to the corresponding nitro-CBI prodrug under hypoxic (but not oxic) conditions. The amino metabolite generated from a nitro-CBI by cells expressing Escherichia coli nfsB nitroreductase in multicellular layer cultures was shown to elicit bystander killing of surrounding cells. Nitro-CBI prodrugs were >500-fold less toxic to mice than amino-CBIs by i.p. administration and provided selective killing of hypoxic cells in RIF-1 tumors (although only at maximally tolerated doses). Nitro-CBIs are novel lead hypoxia-activated prodrugs that represent the first examples of hypoxia-selective generation of potent DNA minor groove alkylating agents.