A Bioreductive Approach to Taxoids Using Recombinant Yeast

The introduction of enzymes as catalysts into synthetic organic chemistry made possible many new highly enantioselective transformations. The employment of microorganisms provides access to enzymes that are difficult to use in an isolated form. In addition, such organisms are self-propagating sources of the desired biocatalyst and all cofactors necessary for the reaction. Therefore, the use of such "biocatalytic reagents" has spread among synthetic organic chemists over the past decade. Since fermentations are generally carried out in aqueous buffers the use of organic solvents and organometallic catalysts in the course of a synthesis is minimized, hence this methodology is of increasing interest for industrial applications. Baker`s yeast (Saccharomyces cerevisiae) has been the first and most popular whole-cell biocatalyst, particularly for asymmetric reductions of carbonyl compounds. Reductions catalyzed by this organism tolerate a large variety of ketones and side-reactions are rarely observed. Based on preliminary results an efficient enantioselective route to novel taxoids is proposed utilizing yeast mediated reductions of keto-lactams as the key synthetic step. Screening of a yeast genome library of recombinant overexpression strains will enable identification of the enzymes involved. Subsequent optimization of the biotransformation will provide access to novel taxoids. Paclitaxel (Taxol) was originally extracted in small quantities from the bark of the Pacific yew tree, and was quickly regarded as a most promising anti cancer agent. Taxol promotes microtuble assembly in vitro at concentrations attainable clinically during prolonged infusions. This upsets the normal dynamic equilibrium between soluble dimeric tubules and the microtubule polymers. Taxol is used in clinical therapy against ovarian, breast, and non-small cell lung cancer and preclinical screens have revealed significant activity against resistant murine melanoma. To overcome some of the major disadvantages of paclitaxel such as low water solubility and in order to increase the biological activity especially against resistant cancer cell lines a series of novel "second generation" taxoids will be prepared utilizing environmentally benign biotransformations by whole-cells. The effects of combining sterically bulky substitutents with polar groups at the C-13 side chain of baccatin III will be studied in detail. The conformational behavior of these novel compounds will provide new insight into the interaction with the receptor region and the rational design of highly active cytotoxines will be supported.