Boosting Taxol Production

A newly optimized biosynthetic route in bacteria yields taxadiene, a precursor to the cancer drug paclitaxel (Taxol), at an amount that is 1,000-fold higher than previous efforts (Science 2010, 330, 70). Researchers have also for the first time engineered the oxidation of taxadiene to taxadien-5α-ol, the next step in the paclitaxel pathway.

Several subsequent genes in the paclitaxel biosynthesis pathway are unknown, so a full biosynthesis is not yet possible. Nevertheless, the increase in yield for taxadiene and its precursor chemicals is “quite impressive,” says Susan C. Roberts, a chemical engineering professor at the University of Massachussetts, Amherst, who was not involved in the new work. Roberts notes that the precursors are also intermediates in the biosynthesis of a variety of other natural products.

Paclitaxel was originally isolated from the bark of the Pacific yew tree, in amounts that necessitated sacrificing two to four trees per patient. The drug is currently produced by chemically modifying baccatin III, which is isolated from needles of the European yew tree.

Developing a biosynthetic pathway in bacteria or yeast should boost yields of the drug, as well as allow chemists to make derivatives for new drug candidates, say study authors Gregory N. Stephanopoulos and Blaine A. Pfeifer, chemical engineering professors at Massachusetts Institute of Technology and Tufts University, respectively.

The researchers divided the biosynthesis of taxadiene into two sections. One produces the building blocks isopentenyl pyrophosphate (IPP) and di­methyl­allyl pyrophosphate (DMAPP), and the other turns those chemicals into taxadiene. The group optimized each in Escherichia coli in a combinatorial fashion by varying parameters such as gene promoter sequences and the number of gene copies and eventually produced taxadiene at concentrations of 1 g/L.

They then engineered the next step in the biosynthesis, oxidation of taxadiene using a cytochrome P450 oxidase, to produce taxadien-5α-ol at 58 mg/L. “If one could make Taxol at 100 mg/L, then the global demand of Taxol of 1 ton per year would be possible to satisfy by a single 200-m³ fermentor, which is a rather small operation,” Stephanopoulos says.