Synthesis and Biological Evaluation of 2-Phenyl Indole Analogues as Potential Anti-Cancer Agents
Introduction
Inhibitors of tubulin polymerization represent a promising therapeutic approach for the treatment of solid tumors. Natural products such as colchicine, combretastatin A-1 (CA1), and combretastatin A-4 (CA4) have inspired numerous molecular scaffolds that target the colchicine site at the interface of the α and β-tubulin heterodimer. Although several of these small-molecule anti-cancer agents have advanced to human clinical trials, none have resulted in an approved drug for single-agent use in the treatment of cancer. The dual functionality of a subset of inhibitors of tubulin polymerization as antiproliferative agents and vascular disrupting agents (VDAs) has drawn interest and contributed to the discovery of a 2-aryl-3-aroyl-indole analogue (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, demonstrated in vivo disruption of tumor-associated microvessels.
Methods
The molecular framework of OXi8006 inspired a new series of 2-aryl-3-aroyl-indole analogues that incorporated various functional group substitutions on both the indole core and the aroyl ring. Further exploration of the substitution patterns at the 1,3-positions of the aroyl ring with halogens was evaluated, as well as modulation of the indole backbone with both electron donating and electron withdrawing substituents. Structure activity relationship studies were further enhanced by molecular docking to predict possible colchicine binding site interactions. KGP369, an OXi8006 analogue previously synthesized by the Pinney Group, was resynthesized using a somewhat modified route that utilized a microwave reactor to increase efficiency. KGP369 was subsequently converted to its corresponding water-soluble phosphate prodrug salt (KGP415) to evaluate its efficacy (in vivo) as a VDA.
Results
Several of these analogues demonstrated potent inhibition of tubulin polymerization comparable to that of OXi8006 and the benchmark natural product CA4. Notably KGP662 (IC50= 0.74 ± 0.1 µM, inhibition of tubulin polymerization) with a 3,4-dichloro substitution on the aroyl ring, and aniline based analog KGP463 (IC50= 0.83 ± 0.05 µM, inhibition of tubulin polymerization)) proved especially promising. Additionally, both molecules exhibited potent cytotoxicity (nM GI50 values) against the MCF-7 human cancer cell line. The prodrug salt of KGP369 (KGP415) demonstrated significant reduction in bioluminescence signal (>95% reduction at 100 mg/kg IP) when evaluated in vivo against an orthotopic kidney tumor (RENCA-luc) in BALB/c mice, indicative of VDA activity.
Conclusion
This structurally diverse library of indole analogues provides further insight regarding the important structural motifs that affect both inhibition of tubulin polymerization and corresponding cytotoxicity. Several molecules in this study demonstrated potent inhibition of tubulin polymerization and corresponding strong cytotoxicity against human cancer cell lines (MCF-7). Evaluation of KGP369 as its corresponding phosphate prodrug salt (KGP415) confirmed its efficacy as a vascular disrupting agent.