The rational design of potential chemotherapeutic agents: synthesis of bryostatin analogues.
The bryostatins are a unique family of cancer chemotherapeutic candidates isolated from marine bryozoa. While their molecular mode of action is not known, these macrolactones exhibit high affinities for protein kinase C (PKC) isozymes, compete for the phorbol ester binding site on PKC, and stimulate kinase activity in vitro and in vivo. Unlike the phorbol esters, they do not act as tumor promoters. Despite promising biological properties, the supply of these compounds is limited by the difficulty of their isolation from natural sources and their synthetic complexity. A new class of bryostatin analogues which retain the putative recognition domain of the bryostatins but are simplified through deletions and modifications in the C1-C14 spacer domain have been designed using computer models. A convergent synthesis has been realized for the production, in gram quantities, of these recognition and spacer domains whose coupling allows for the generation of a range of analogues. The final closure process involves a novel macrotransacetalization reaction which proceeds with complete stereoselectivity. The solution structures of two synthetic analogues were determined by NMR spectroscopy and found to be very similar to the previously reported structures of bryostatins 1 and 10. In addition, these structures appear to indicate that the stereochemistry of the C3 hydroxyl group plays a significant role in the conformation of the macrolactone. All analogues bound strongly to a mixture of PKC isozymes, and several exhibited significant levels of in vitro growth inhibitory activity against human cancer cell lines. Taken together, this work provides important steps toward the development and understanding of simplified, synthetically accessible analogues of the bryostatins as potential chemotherapeutic agents.