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 (CA4) 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 a 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 (VDA) that may prove useful for the treatment of cancer.

The vascular disrupting activity of OXi8006 in endothelial cells and its phosphate prodrug OXi8007 in breast tumor xenografts

“…Our previous studies demonstrated that OXi8006 was a potent inhibitor of tubulin polymerization (IC 50 = 1.1 µM) and competed with radiolabeled colchicine at the colchicine binding site of tubulin [17]. We also found OXi8006 to be cytotoxic against three evaluated human cancer cell lines, NCI-H460, DU-145 and SK-OV-3, with an average GI 50 of 25.7 nM [17].…”

“…Our previous studies demonstrated that OXi8006 was a potent inhibitor of tubulin polymerization (IC 50 = 1.1 µM) and competed with radiolabeled colchicine at the colchicine binding site of tubulin [17]. We also found OXi8006 to be cytotoxic against three evaluated human cancer cell lines, NCI-H460, DU-145 and SK-OV-3, with an average GI 50 of 25.7 nM [17]. The core indole molecular structure is now widely represented as a key component of a variety of inhibitors of tubulin assembly [18], but OXi8006 remains one of the most active compounds in this group with respect to tubulin binding and cytotoxicity.…”

Blocking Blood Flow to Solid Tumors by Destabilizing Tubulin: An Approach to Targeting Tumor Growth

“…As recently discussed, 34 the reason for this apparent inconsistency is not completely clear, but there are several factors that may contribute to explain this experimental fact. One factor could be that the stoichiometry (compound versus tubulin concentration) required in the tubulin (cell-free) assay to achieve an IC 50 value might be completely different from the stoichiometry that exists when inhibiting tubulin in cell culture.…”

“…In addition, there may be factors directly linked to the limitations and sensitivity of the protein assay. 34 2 36 Combretastatin A-4 derivatives have also been reviewed by Shan et al, 37 Marrelli et al, 38 and very recently by Patil et al 39 A review focusing on cis-restricted isomers has been published by Rajak et al 40 For those particularly interested in the synthetic pathways applied to obtain these compounds, relevant information can be obtained in the review article of Kaur et al 41 Other authors have performed a classification of the compounds based on structural and docking studies, as the exhaustive review of Alvarez et al 42 An overview of the role played by computational approaches in the context of the colchicine-binding domain has also been recently reported by Massarotti et al 43 In our survey through the different families of compounds that behave as VDAs, we have centered our attention on those compounds that are active in the nM range in cell culture and/ or for which animal experiments have been conducted as proof of concept of their VDA capacity. However, no comparison between data coming from different laboratories has been performed.…”

Mechanistic considerations in the synthesis of 2-aryl-indole analogues under Bischler–Mohlau conditions

“…The synthetic route to 13 C-labeled bromoacetophenone intermediate 16 followed a similar sequence as the non-labeled bromoacetophenone intermediate from our previous studies with the indole based vascular disrupting agent (VDA) OXi8006 6–7,33 along with related work by von Angerer and co-workers, 34 and simply replaces the traditional methylation step reagents to install the 13 C carbon atom at the alpha position (Scheme 6). …”