Design, synthesis, and biological evaluation of benzosuberene and chalcone-based tubulin binding agents inspired by the natural product combretastatin A-4 and prodrugs targeting tumor-associated hypoxia.


Cancer therapeutic strategies have moved, increasingly, toward methods that are designed to exploit inherent characteristics of tumor cells and the tumor microenvironment. One strategy is to take advantage of the primitive nature of tumor vasculature, which, in comparison to healthy vasculature, is chaotic, unevenly distributed, and high permeability. Tumors require a robust vascular network to grow and ultimately metathesize into other areas of the body making this an attractive target to cut the tumor off from oxygen and nutrients. A class of small molecules that are capable of affecting tumor vasculature are referred to as vascular disrupting agents (VDAs). Several VDAs bind at the colchicine binding site on the β-tubulin subunit of the tubulin heterodimer, causing inhibition of tubulin polymerization. Inhibition of tubulin polymerization interferes with the dynamics of the tubulin-microtubule protein system causing rapid morphological changes in the endothelial cells lining tumor-associated vasculature leading to collapse, blood flow loss, and eventually necrosis. The natural products, combretastatin A-4 (CA4) and combretastatin A-1 (CA1) are potent VDAs that have inspired significant structure activity relationship (SAR) campaigns to design analogues that impart enhanced efficacy of vascular shutdown and cytotoxicity leading to improved therapeutics for the potential treatment of cancer. The Pinney Research Group has developed a diverse library of analogues inspired by these natural products (CA4 and CA1) that have been shown to be potent cytotoxic agents against human cancer lines, strong inhibitors of tubulin polymerization, and effective VDAs. One lead compound, KGP18, is a benzosuberene-based analogue that has inspired further SAR-driven molecular design and synthesis. The fused and pendant aryl ring motifs associated with this benzosuberene system are conformationally restricted thus mimicking the Z-geometry associated with the aryl rings of the natural products CA4 and CA1. A series of new benzosuberene analogues, designed and prepared by chemical synthesis, were evaluated biologically [in collaborative studies with Dr. Mary Lynn Trawick (Baylor University), Dr. Ralph P. Mason (University of Texas Southwestern Medical Center), and Dr. Ernest Hamel (National Cancer Institute)] for their cytotoxicity against human cancer cell lines, inhibition of tubulin polymerization, and ability to function as VDAs (preliminary in vivo studies). Additionally, a series of α-conformationally restricted chalcones and related analogues were designed, synthesized, and evaluated biologically in a similar fashion. Another strategy employed to specifically target tumors for therapeutic intervention exploits characteristic tumor-associated hypoxia. Bioreductively activatable prodrug conjugates (BAPCs) are intended to be biologically inert but are designed to selectively release a highly cytotoxic parent compound through enzyme-mediated cleavage in regions of pronounced tumor-associated hypoxia. A series of phenstatin BAPCs were synthesized and evaluated under both normoxic and hypoxic conditions to determine their hypoxia cytotoxicity ratio (HCR) and for preliminary evaluation as potential therapeutic agents for the treatment of cancer.

Cancer. Tumor-associated hypoxia. Combretastatin A-4. Combretastatin A-1.