Design, Synthesis and Biological Evaluation of 2-Phenyl Indole Analogues of OXi8006 as Colchicine Site Inhibitors of Tubulin Polymerization and Vascular Disrupting Agents

Inhibitors of tubulin polymerization represent a promising therapeutic approach for the treatment of solid tumors. Molecules that bind to the colchicine site are of interest as they can function with a dual mechanism of action as both potent antiproliferative agents and tumor-selective vascular disrupting agents (VDAs). One such example is a 2-aryl-3-aroyl-indole molecule (OXi8006) from our laboratory that demonstrates potent inhibition of tubulin polymerization and strong antiproliferative activity (cytotoxicity) against a variety of human cancer cell lines. A water-soluble prodrug OXi8007, synthesized from OXi8006, demonstrates in vivo disruption of tumor-associated microvessels in several tumor types (mouse models). The molecular framework of OXi8006 inspired a series of fourteen new 2-aryl-3-aroyl-indole analogues that incorporated various functional group modifications on both the indole core and the aroyl ring. Electron withdrawing and donating groups at the mono-substituted 3' position and the di-substituted 3',5' positions were all accommodated while maintaining inhibition of tubulin polymerization (IC₅₀ < 5 μM), with several analogues demonstrating activity comparable to OXi8006 and the benchmark natural product combretastatin A-4 (CA4). Preliminary structure-activity relationship (SAR) studies were further enhanced by molecular docking to predict possible colchicine site interactions. Two analogues (KGP366 and KGP369) previously synthesized in our laboratory were re-synthesized using a somewhat modified route to increase synthetic efficiency and were subsequently converted to their corresponding water-soluble phosphate prodrug salts to evaluate their efficacy as VDAs. Administration of the prodrug salt (KGP415) of KGP369 caused significant reduction in bioluminescence signal from an orthotopic kidney tumor (RENCA-luc) in BALB/c mice, indicative of VDA activity. Collectively, these new functionalized indole-based analogues have extended SAR knowledge related to the colchicine binding site, and the most biologically active analogues hold promise for continued development as pre-clinical candidates for cancer therapy.