Placitaxel, better known by the brand name Taxol, is a compound found in the bark of the Pacific yew tree, Taxus brevifolia, that has proved effective against a variety of cancers.
In the early 1990s, when the drug was just coming into use, the bark from over three trees was needed in order to extract just one gramme of Taxol.
These days, however, it is produced from plant cells grown in culture or by chemically processing a molecule found in the needles of the European yew tree.
Scientists are looking at another challenging option. Would it be possible to genetically engineer microbes to produce the drug? If so, production could be greatly increased and the cost of the medicine reduced.
Three decades back, scientists successfully engineered a bacterium to churn out insulin. This was achieved by introducing a gene for human insulin into the organism.
Instead of having to rely on insulin extracted and purified from animal tissue, large quantities of human insulin could be safely produced by simply growing the genetically altered bacterium.
To produce Taxol, however, a complicated chemical pathway has to be configured into a microbe. Many different genes are needed to make enzymes, each of which catalyse one step and must work like a factory assembly line to synthesise this complex molecule.
Recently in a paper published in the journal Science, a team led by Gregory Stephanopoulos of the Massachusetts Institute of Technology (MIT) reported efforts to produce precursors of Taxol using the bacterium Escherichia coli (E. coli).
It was not simply a matter of introducing new genes into the microbe. Getting the organism to produce large quantities of the precursor molecules required careful optimisation of the entire pipeline.
First, the output from an endogenous pathway in the bacterium had to be increased, explained Parayil Kumaran Ajikumar, a post-doc in Prof. Stephanopoulos' laboratory and first author of the paper. For that, production of enzymes from four of eight genes in the pathway needed to be boosted. In addition, the output of this pathway had to be balanced with a new pathway with two plant-derived genes that then synthesised taxadiene, a precursor of Taxol.
For reasons that are not clear, if the two pathways were not balanced, the cell triggered the synthesis of indole, he told this correspondent. When indole accumulated inside the bacterium, it inhibited the production of taxadiene.
By balancing the two pathways, the scientists were able to create a strain of E. coli that produced 1,000 times more taxadiene than any other engineered microbial strain.
Then, by coaxing a plant gene to work in the bacterium, taxadiene was turned into taxadiene-5-alpha-ol. Their engineered strain proved capable of producing 2,400 times more of the latter molecule than had been possible with genetically modified yeast. Many more steps are needed before the bacterium is able to synthesise Taxol. The most realistic scenario, however, would be to get E. coli to synthesise the intermediate baccatin III and then use existing chemical methods, which are very efficient, to produce Taxol, observed Prof. Stephanopoulos in an email.
But, as the scientists noted in their paper, making E. coli that can synthesise baccatin III will be “a daunting task.” Several more steps, including some that have not been identified, would have to be effectively engineered into the bacterium.
“By mimicking nature, we can now begin to produce these intermediates that the plant makes, so people can look at them and see if they have any therapeutic properties,” said Dr. Stephanopoulos in a press release issued by the MIT.
Moreover, they can synthesise variants of these intermediates that may have therapeutic properties for other diseases.