A Spoonful of Sugar to Help the Prodrug Go Down: The Synthesis of Galactose Prodrugs as Novel Inhibitors of Tubulin Polymerization and Potential Cancer Therapeutics

Microtubules provide crucial structural support for endothelial cells lining the vascular network of both healthy tissue and tumors. While healthy tissue possesses highly ordered vasculature with uniformly oxygenated cells, tumor tissue is characterized by underdeveloped and disordered blood vessels, leading to hypoxic regions of tumor tissue. This weakness within tumor-associated vasculature provides potential for both selective targeting and efficient treatment using vascular disrupting agents (VDAs). The natural product combretastatin A-4 (CA4), derived from the African bush willow, combretum caffrum, functions as both an antiproliferative agent (cytotoxin) and a promising VDA capable of selective and irreversible damage to tumor vasculature. CA4, administered as its corresponding water-soluble phosphate prodrug salt (CA4P), functions by binding to the colchicine site on the tubulin heterodimer resulting in morphological changes in the endothelial cells, shutdown of the vascular network, and overall inner-tumor necrosis. The Pinney Research Group (Baylor University) designed and synthesized an indole-based molecule, OXi8006, that bears structural similarities to both colchicine and CA4 and functions as a potent inhibitor of tubulin polymerization. OXi8006 and its phosphate prodrug salt (OXi8007) demonstrate inhibition of tubulin polymerization (OXi8006; IC50 = 1.1 μM, OXi8007; IC50 = 4.2 μM) and consistent cytotoxicity against human cancer cell lines [GI50 values ranging from 3.5 to 38 nM against NCI-H460 (lung), DU-145 (prostate), and SK-OV-3 (ovarian)]. Cancer cells that have increased resistance to cell-cycle inhibitors are often characterized by cellular senescence, such that a healthy cell exhibits growth arrest while a cancerous cell takes on the resistant qualities of senescent cells. In a study by Wagner and associates, lysosomal-β-galactosidase (GLB1) was found to be the origin of senescence-associated β-galactose activity. Since GLB1 hydrolyzes β-galactose, the addition of a galactose onto select VDA drug candidates could provide a novel approach to selectively target cells with high resistance towards other treatment. To this end, we synthesized an OXi8006 analogue through a covalent linkage of a galactose sugar to the phenolic oxygen of OXi8006 as a proposed selective inhibitor of tubulin polymerization.