Targeting tumor-associated hypoxia with bioreductively activatable prodrug conjugates derived from dihydronaphthalene, benzosuberene, and indole-based inhibitors of tubulin polymerization

A strategy for targeting tumor-associated hypoxia utilizes reductase enzyme-mediated cleavage to convert biologically inert prodrugs to their corresponding biologically active parent therapeutic agents selectively in areas of pronounced hypoxia. Small-molecule inhibitors of tubulin polymerization represent unique therapeutic agents for this approach, with the most promising functioning as both antiproliferative agents (cytotoxins) and as vascular disrupting agents (VDAs). VDAs selectively and effectively disrupt tumor-associated microvessels, which are typically fragile and chaotic in nature. VDA treatment may augment existing tumor-associated hypoxia, thus enhancing the efficacy of hypoxia-selective prodrugs. Structure activity relationship-guided studies in our laboratories led to the discovery of promising lead molecules (OXi6196, KGP05, KGP18, and OXi8006) that bind to the colchicine site on the tubulin heterodimer. A series of bioreductively activatable prodrug conjugates (BAPCs) based on these molecules was synthesized utilizing ether-linked heteroaromatic hypoxia-selective triggers bearing a nitro group. Biological evaluation against the A549 human lung carcinoma cell line (under normoxic versus anoxic conditions) revealed several BAPCs with positive hypoxia cytotoxicity ratios. Preliminary <i>in vivo</i> evaluation of a representative BAPC (<b>KGP291</b>) demonstrated vascular shutdown in nude mice bearing orthotopic 4T1 breast tumors studied by bioluminescence imaging.