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A Genetically Modified Yeast Turns Sugar Into Painkillers

Stanford scientists have engineered a strain of yeast that can produce opiates on its own

Yeast, a multipurpose microbe. (Stephanie Galanie)

Since ancient times, winemakers, brewers and bakers have harnessed the fermentation properties of yeast to leaven bread and produce alcoholic libations. But now, a team of scientists at Stanford University has genetically modified this renaissance microbe for a unique purpose: pumping out opiate painkillers.

While you are unlikely to find any opiates at your local microbrewery anytime soon, the results show great promise in speeding up the manufacturing process for these drugs, as well as opening doors for the discovery of new medicines.

“Many of our medicines are being shifted to production by biotechnology,” says study author Christina Smolke, an associate professor of bioengineering at Stanford. “Plants do really sophisticated biochemistry to make these compounds, but the challenge is that they don't necessarily do it efficiently.”

Historically, all opiate painkillers have been derived from the opium poppy, which is legally grown in locations such as Australia, Europe and India and shipped to manufacturing centers. The plant compounds are then isolated, refined and converted into prescription drugs in a process that can take a year or more from farm to pharmacy.

Like any other crop-dependent product, opiates can be subject to pest infestation, drought, changes in climate and other variables capable of limiting the manufacture of familiar drugs such as morphine, codeine, oxycodone and hydrocodone (more commonly known by the brand name Vicodin). Given these restrictions, the researchers wanted to compress the entire farming and manufacturing process into a single bioreactor that could produce painkillers in a matter of days.

“We wanted to show that you could take a process that is traditionally distributed across both biological and chemical synthesis and integrate it entirely within a synthesis route in yeast,” says Smolke.

Encouraging precedents existed for creating plant-based medicines using synthetic biology. In 2006, the anti-malarial drug artemisinin, derived from the sweet wormwood tree, was successfully produced from genetically altered yeast cells. This biosynthesis process expanded rapidly—yeast-made artemisinin currently accounts for approximately one-third of the world’s supply. And earlier this year, a team at UC Berkeley engineered brewer's yeast to make one of the building blocks of morphine.

To coax their yeast down the biochemical pathway for opiates, the Stanford researchers first had to break down and genetically re-create each enzyme-enabled step in the synthesis chain that converts tyrosine, an amino acid the yeast makes from sugar, to thebaine, a precursor for many common opioid painkillers. The scientists could then insert the genes necessary to convert thebaine to hydrocodone. But after all this biochemical construction work, the team ran into a technical hurdle—they were unable to create a sufficient quantity of opioid product. They discovered that the yeast was misreading the directions for making the protein necessary to reach a key stage in the production line.

“We then had to rewrite the instructions for how yeast should make the protein so that it more closely modeled how the plant was doing it,” says Smolke. By the end of the process, the researchers had reconstructed the yeast cells with 23 new genes from a variety of organisms, including several plant species, rats and bacteria. Even now, though, the overall process is too inefficient, requiring more than 4,400 gallons of yeast to produce a single dose of hydrocodone.

“By our estimates, we would need to improve the efficiency of the process by 100,000 times to be ready for commercial production,” says Smolke, whose team reports the results this week in Science. “But we believe this is feasible and have already begun that work.”

The authors point out several benefits that would result from optimizing their process. First, it would significantly reduce the manufacturing costs for opiates, creating opportunities to reach the estimated 5.5 billion people who have limited access to pain medications. And because this is an entirely self-contained process, it can take place anywhere—removing the dependency on geography and climate while enabling greater containment and quality control. The integrated yeast synthesis also frees up land for other types of farming—growing sugarcane to feed the yeast takes up far less land area than that required for poppy farming.

But perhaps the biggest benefit of this technology comes from its flexibility to explore new medicinal compounds that are more effective and have fewer side effects.

“People are working on all types of very interesting alternatives to conventional opiates,” says Kenneth Oye, an associate professor of political science and engineering systems at the Massachusetts Institute of Technology. “The big advantage of moving from traditional production techniques to these pathways for synthesis in yeast is that the pathways are far more easily modified, allowing for easier synthesis of new compounds.”

Still, making it easier to produce opiates carries important safety and abuse considerations.

“I don’t think that the strain developed by Christina Smolke’s lab poses a big threat to public health and safety as is,” says Oye. Indeed, Smolke recently tested the viability of their strain under home-brew conditions and found that it didn't produce opiates. “But if someone were to develop a strain of yeast with a pathway that went from glucose to heroin with high efficiency, then you have a problem. Such a strain might have the potential for home-brew opiates.”

Oye also points out that if such a strain of yeast were developed, control over distribution would be extremely difficult. “This is something that could be reproduced fairly easily and would be difficult to contain or recall,” he says. He argues that early dialogue is essential to ensure safe technical and policy precautions, such as engineering yeast strains to be dependent on nutrients that are hard to obtain, inserting markers that could help with detection, and increasing lab security. “In many instances, your options for mitigating potential risk are limited once the work is complete,” says Oye.

Smolke agrees, adding: “I believe there needs to be an open deliberative process to discuss the real concerns and how to develop strategies to mitigate these risks. It is not just dependent on the technology but also policy makers, law enforcement and the medical community. And if this research catalyzes a discussion around that, I think that is really important.”


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