C&EN: COVER STORY - COMBINATORIAL CHEMISTRY - 'A Pharmaceutical Company Would Never Do This'

STU BORMAN, C&EN WASHINGTON

At the University of Pittsburgh, researchers have been working on a combinatorial project that drug firms would consider too much of a gamble, according to the project's principal investigator, chemistry professor Peter Wipf. He described the project--the pursuit of analogs of the natural product curacin A as potential anticancer lead compounds--at the recent ACS ProSpectives conference on combinatorial chemistry.

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	ONLY ACADEMIC? Wipf and coworkers have created several generations of curacin A analogs in an effort to develop potential anticancer agents. Shown are the natural product and two sites (blue) the researchers modified when they synthesized a first-generation analog library, a second generation library they created by mixture synthesis, a lead compound from that library that showed good activity, and an analog derived from that lead that exhibited better activity than curacin A on a tubulin polymerization assay but was metabolically unstable. COURTESY OF PETER WIPF

Curacin A--first isolated by professor of natural products chemistry William H. Gerwick and coworkers at Oregon State University, Corvallis--is an antimitotic algal natural product that acts by inhibiting tubulin polymerization in cells. It has an impressive antiproliferative profile, Wipf said, but its lability (it has a readily oxidized thiazoline heterocycle) and its high lipophilicity have been strong deterrents to its use as a lead compound for drug discovery. In addition, "the target of that natural product is the colchicine site on tubulin, which is not a validated site--that is, there's no clinically used drug that acts at that site," Wipf said.

Nevertheless, Wipf, grad student Jonathan T. Reeves, and coworkers used combichem and rational design to identify analogs that exceed the potency and selectivity of curacin A. "A pharmaceutical company would never do this," Wipf said, "because it's way too risky. The question for us was, could we actually demonstrate the utility of doing something that companies would totally reject out of hand? We are trying to show there is value in a type of combinatorial project that companies do not consider at all valuable."

Wipf and coworkers began by synthesizing a first-generation library of curacin A analogs [J. Med. Chem., 45, 1901 (2002)]. "We tried three different approaches to make those--solution, solid support, and fluorous synthesis," Wipf said. They screened about 100 of the analogs for activity, and found that most were completely inactive. "Also, in the first-generation approach, we were not able to produce the quantities of materials we really wanted to because the syntheses were long," he said.

So the researchers decided to use an efficient mixture synthesis technique to make second-generation analogs. They used a fluorous scavenging approach to streamline purification of the mixtures and then used liquid chromatography-nuclear magnetic resonance spectroscopy "to ensure we had made what we claimed and to evaluate the purity," Wipf said. Overall, the mixture synthesis approach increased their productivity sixfold relative to the first-generation process.

One second-generation analog turned out to be an interesting lead compound. In fact, "it was one of the most potent synthetic curacin A analogs identified to date, and it also had a simpler structure, greater water solubility, and better chemical stability" than the natural product, Wipf said.

Starting with that lead compound, he said, "we played around with the central part in a combinatorial sense by changing motifs and making a number of derivatives." This resulted in a bunch of inactive compounds.

But additional rational changes in the scaffold eventually led them to a hot prospect. Its activity in a tubulin polymerization assay "was remarkable in that its IC₅₀ [the concentration at which it inhibited 50% of tubulin polymerization activity] was 170 nM," Wipf said. "This was clearly superior to that of curacin A, which is 520 nM. It was the most attractive analog of curacin A reported to date."

Mouse studies unfortunately showed that the compound was metabolically unstable. However, "we were able to get useful information on what the first metabolites are," Wipf said, "and we think we've identified the enzyme that causes the degradation. Now we're going to build that information back into a new library to address the issue of metabolic instability."

Once they solve that problem, perhaps a company might want to take over the project, Wipf said. But ironically, at that point "we would kind of lose interest in it. It would be completed for us. It's a very academic approach."