Vol 7. Issue 31 / October 22, 2007
Inventing New Chemistry
By Mark Schrope
Whether in baseball or bioactivity, having a low success average is generally considered a bad thing. But for William Roush and his team at The Scripps Research Institute in Florida, a one percent success rate inspired a critical change in procedure, ultimately resulting in the discovery of a compound that may be useful in the treatment of Chagas' disease.
Chagas' disease, which claims some 14,000 lives per year, is a tropical illness caused by a parasite. The Roush lab's compound effectively kills this parasite and could one day be used to treat the most severe cases of Chagas'. The compound may also open new research paths to the development of a drug treatment for the tens of millions of disease carriers worldwide.
Roush, who is professor of chemistry, executive director of medicinal chemistry, and associate dean of Scripps Florida graduate studies, has a broad research program. In addition to synthesizing new compounds as potential treatments for specific diseases such as Chagas', his group also has a variety of projects aimed at developing new techniques for the synthesis of important natural products with potential to fight cancer and other disorders.
The Chagas' work is part of a long-term collaboration between Roush and researchers at the University of California, San Francisco (UCSF). The Roush lab's role was to synthesize a library of compounds targeted to exploit known weaknesses of the Chagas' parasite.
Aiming to block a cysteine protease critical to the parasite, the group began with a chemical starting point based on research by other groups, then designed, synthesized, and tested a range of modifications. These were intended to improve potency against the target enzymes, maximize characteristics important in drug treatments such as stability, and minimize potential side effects.
The group's screening protocol called for first testing a synthesized compound to see how well it inhibited the enzymes the parasite needs to survive. Compounds that showed good bioactivity against the enzymes were then tested for toxicity against the parasites themselves. Positive results in that second in vitro test would then lead to initial in vivo tests with mice afflicted with the Chagas' parasite. With such work, it's common to pursue chemical routes that ultimately prove unfruitful. "You have many hypotheses, most of which get dashed on the rocks after the compounds are made," says Roush, "Most turn out not to be useful."
Nonetheless, the team's success rate in the second bioassay had been hovering around one percent, inspiring Roush to call for a new plan in hopes of finding some better understanding of how the compounds were interacting with the parasites. So, he made the call to skip the first biochemical assay and begin testing the full library in the second assay involving the parasites themselves.
A Chemical Home Run
That's when the compound dubbed WRR483, the very first cysteine protease inhibitor the lab produced after joining Scripps Florida, began to shine. In that second assay, this compound showed remarkable potency in killing the parasite. Interestingly, WRR483 was actually designed as a negative control for use in a separate study of a related parasite, meaning it wasn't expected to work well against its target, much less against the Chagas' parasite.
"It's amazing WRR483 turned out to be a home run," says Roush, "It's that serendipity side of science that I find phenomenal."
With success in the second assay, the team then took the compound back to the first assay and found surprisingly low activity against the originally targeted enzymes. In other words, the promising compound would not have been identified had the team stuck to its original protocol.
James McKerrow is a microbiologist at UCSF who has collaborated with Roush for more than a decade. He says the WRR483's surprising efficacy against the parasites, and later positive results in mouse trials, illustrate the complexity of biology. One possible explanation for the results, he says, is that while the compound may be minimally toxic against the enzyme alone, some unique effect inside the parasite might concentrate and maximize the compound's effectiveness, thus killing the parasite.
"Once you get something into a biological system, so many other parameters can be operating," says McKerrow.
WRR483 is just one of several compounds from the Roush lab that the McKerrow group is studying for potential use against Chagas' disease as well as malaria, African sleeping sickness, and other topical maladies. The team has already secured support from the National Institutes of Health to take one of these compounds into the first round of human clinical trials. The researchers will make the final decision about which compound goes into human trials based on the results of more advanced animal studies now under way.
"Sometime in 2008 we'll open the envelope and have the winner," says McKerrow. Human trials could begin the following year.
Roush says that WRR483 is structurally complex, and, hence, potentially expensive to produce. However, it could still be used as a potent injectable drug for those suffering the worst cases of Chagas'.
The Roush group is already working to produce simpler compounds with equal or better properties that could one day fill the need for an economical drug that is more widely distributable. He says the success of WRR483 and related compounds could lead to a new understanding of ways to aid this quest.
"You have to take the leads that come to you and run with them," he says, "They could take you up a blind alley, but they could also open up totally different avenues for a research program to move into."
Roush's ultimate goal is to develop a single drug that could protect people in developing countries from the full range of tropical diseases that threaten them.
Interestingly, WRR483 also displays potent activity against Entamoeba histolytica, a water-born parasite that is classified as a Class B bioterrorism agent. This finding has led to a new collaboration with Professor Sharon Reed at the University of California, San Diego.
Though Roush says he thoroughly enjoys synthesizing new compounds targeted against diseases, such research is only one facet of his laboratory program. His group's other main focus is synthesizing compounds discovered in the natural world that might not be available from their natural sources in quantities sufficient for research or for eventual distribution as a drug treatment.
Such synthesis projects are typically focused on groups of compounds that share a particularly interesting chemical characteristic. One project focuses on compounds discovered in a marine sponge collected in New Caledonia from the chemical group of macrolide antibiotics that includes such pharmaceutical superstars as the antibiotic erythromycin.
The compounds in question, called superstolides, have proven effective in early tests against lung and a variety of other human cancer cell lines. Like most marine natural products, though, limited supply has also limited researchers' ability to study the compounds, highlighting the need for another production source such as laboratory synthesis.
One of Roush's goals has been to adopt the biochemical reaction sequences that nature uses to create such products and to use them in laboratory syntheses. With the superstolides, the team has successfully created compounds that mimic biosynthetic intermediates, but they were extremely reactive and unstable. "It was causing us to pull our hair out figuratively," says Roush. But the team at Scripps Florida was able to refine their techniques and use new methods that ultimately led to sufficient control of the compounds' reactivity and defined a route for their production.
Though a compound's biological profile is a factor, Roush says the motivation for the superstolide and related projects is broader. He is interested in the invention of new chemistry that accompanies a project like that because such fundamentals can ultimately enable production of a variety of new and needed compounds.
"If there are rules of the road, then there ought to be opportunities to exploit them," he says, "so you can use that information to improve other compounds."
For more information, see the Roush lab website.
Send comments to: mikaono[at]scripps.edu
"You have to take the leads that come to you and run with them," says Scripps Florida Professor William Roush.