Development of synthetic methodology suitable for the radiosynthesis of combretastatin A-1 (CA1) and its corresponding prodrug CA1P.

Synthetic methodology has been established suitable for the preparation of combretastatin A-1 (CA1) and its corresponding phosphate prodrug salt (CA1P) in high specific activity radiolabeled form. Judicious selection of appropriate phenolic protecting groups to distinguish positions on the A-ring from the B-ring of the stilbenoid was paramount for the success of this project. Methylation of the C-4' phenolic moiety by removal of the tert-butyldimethylsilyl protecting group in the presence of methyl iodide was accomplished in excellent yield without significant Z to E isomerization. This step (carried out with (12)C-methyl iodide as proof of concept in this study) represents the process in which a (14)C radioisotope could be incorporated in an actual radiosynthesis. CA1 is a natural product isolated from the African bush willow tree (Combretum caffrum) that has important medicinal value due, in part, to its ability to inhibit tubulin assembly. As a prodrug, CA1P (OXi4503) is in human clinical trials as a vascular disrupting agent.