Synthesis of a 2-aryl-3-aroyl indole salt (OXi8007) resembling combretastatin A-4 with application as a vascular disrupting agent

Abstract

The natural products colchicine and combretastatin A-4 are potent inhibitors of tubulin assembly, and they have inspired the design and synthesis of a large number of small-molecule, potential anticancer agents. The indole-based molecular scaffold is prominent among these SAR modifications, leading to a rapidly increasing number of agents. The water-soluble phosphate prodrug 33 (OXi8007) of 2-aryl-3-aroylindole-based phenol 8 (OXi8006) was prepared by chemical synthesis and found to be strongly cytotoxic against selected human cancer cell lines (GI₅₀ = 36 nM against DU-145 cells, for example). The free phenol, 8 (OXi8006), was a strong inhibitor (IC₅₀ = 1.1 μM) of tubulin assembly. The corresponding phosphate prodrug 33 (OXi8007) also demonstrated pronounced interference with tumor vasculature in a preliminary in vivo study utilizing a SCID mouse model bearing an orthotopic PC-3 (prostate) tumor as imaged by color Doppler ultrasound. The combination of these results provides evidence that the indole-based phosphate prodrug 33 (OXi8007) functions as a vascular disrupting agent that may prove useful for the treatment of cancer.

Figure 1

Representative small-molecule inhibitors of tubulin assembly.

Figure 2

Compilation of indole-based inhibitors of tubulin assembly (6: von Angerer, 1998 7–8: Pinney, 2001 9: Mahboobi and Beckers, 2001 10: Silvestri, 2004 11: Lee and Hsieh, 2004 12: Chang, 2007 13: Cachet, 2008 14–15: Chang, 2008 16: Romagnoli, 2008 17: Silvestri, 2011 18: Liou, 2011).

Figure 3

Dynamics of vascular disruption caused by 33 (OXi8007) in human prostate tumor PC-3 xenograft visualized by doppler ultrasound. Sequential transaxial images were acquired over a period of 80 min following injection of indole prodrug 33 (OXi8007) (350 mg/kg ip) in a SCID mouse. (A) Immediately post injection, (B) 20 min, (C) 40 min, (D) 60 min, (E) 80 min. Each image was acquired in color Doppler mode with extensive vasculature initially observed both within the tumor (outline in yellow) and in surrounding tissue. Within 20 min, the tumor vascular perfusion had decreased with further progressive decline, so that by 80 min no vascular flow was observed within the tumor. Flow was, however, still apparent in surrounding normal tissues (white arrow). White scale bar 2 mm. Heat scale bar representing flow ranging from ± 64.2 mm/s.

Scheme 1

Synthesis of indole analogues 8 and 30.

Scheme 2

Preparation of indole-based disodium phosphate prodrug salts 33 and 34.