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.