By David N. Leff

Cancer chemotherapists who like Taxol are going to love epothilone.

For most of the 1990s, paclitaxel (generic name) has been approved for the treatment of ovarian cancer, and recently for breast cancer as well.

Epothilone is a natural product extracted from an unnatural soil bacterium, Sorangium cellulosum. Paclitaxel comes from the bark and needles of an evergreen tree, the Pacific yew, Taxus brevifolia.

The two products have similar bags of tricks for frustrating malignant tumor cells. The game they play is assembling tubulin, and stabilizing the microtubules in cells.

Microtubules are nanometer-size cylindrical elements of the cytoskeletons that give form and rigidity to plant and animal cells. During cell division (as in cancer), they latch on to the mitotic spindles that deal out the shared chromosomes to the two daughter cells.

"When Taxol or epothilone stabilize those microtubules," explained organic chemist K.C. Nicolaou, "they basically interrupt mitosis. They arrest the cell cycle, and therefore prevent the cell from replicating."

Despite their similar anticancer tactics, Nicolaou said, "At first glance, the molecular structures of epothilone and Taxol don't look anything alike. That of epothilone is rather simpler than that of Taxol, and that's where the excitement comes from, because we can now make epothilones in the laboratory in large quantities, then make analogues and screen these for drug discovery.

"The abbreviations of three words," Nicolaou explained, "describe the molecular structural features of the epothilones: 'Epo' stands for epoxide; 'thi' comes from thia, which means sulfur atoms; 'one' for a ketone."

Nicolaou chairs the department of chemistry at The Scripps Research Institute, in La Jolla, Calif. He is senior author of a paper in today's Nature, dated May 15, 1997. Its title: "Synthesis of epothilones A and B in solid and solution phase."

"The main thrust of our paper in Nature," he told BioWorld Today, "the most exciting part, is the synthesis of epothilone A in solid phase, which is amenable to combinatorial chemistry. This is the buzzword in the latest technology for drug discovery."

Nicolaou continued: "So we now have this rather interesting natural product that we can make in solid phase and apply combinatorial chemistry to it. Using this technique, we can make hundreds and hundreds of epothilone analogues, and immediately screen them in our laboratories for tubulin assembly."

At Scripps, Nicolaou has already set up biological assays jointly with biochemist and cell biologist Ernest Hamel, at the National Cancer Institute, in Frederick, Md. Hamel is a co-author of the Nature article.

"The difference between epothilones A and B," Hamel said, "is that a methyl group on B enhances its activity five- or 10-fold."

Patenting Bacterial Self-Defense Compound

He recounted that around 1991, fermentation chemist Gerhardt Höfle, at the Biotechnology Research Institute, in Braunschweig, Germany, extracted epothilone from the slime myxobacterium S. cellulosum, and two years later patented the natural product in Germany.

Meanwhile, Merck & Co. screened its library of some 8,000 natural products, and came up with only one showing Taxol-like activity. Learning of Höfle's patent, Merck dropped the subject.

"So, like Taxol itself," Hamel observed, "epothilone has very little patent protection in world markets." (Bristol-Myers Squibb Co., of New York, has trademarked Taxol.)

"S. cellulosum," Hamel pointed out, "doesn't cause disease in humans. Rather, its epothilone is ostensibly a defensive agent that kills fungi in its neighborhood, which also have microtubules. Höfle's rationalization," Hamel added, "is that the myxobacterium competes with fungi in the soil for whatever it grows on."

Taxol got its clinical start in the late 1970s. The NCI screened its collection of natural products, extracted the yew-tree compound, and injected it into mice that had been given leukemia cells. The extract caused their cure rate to increase.

By 1991, Hamel recalled, "In the initial human studies, a Phase I trial for toxicity, one patient with ovarian cancer had a complete response. That is, her tumor completely disappeared, and that's what initiated the extreme excitement about Taxol."

Records also show that in the Phase I Taxol study, another woman died suddenly for lack of pretreatment to prevent the lethal side effects of the drug.

Outlook For Epothilone Therapeutics

As for the epothilones' clinical prospects, Hamel observed, "It will still have to show in vivo activity in animals, then go through toxicology trials. Somebody has to make a commitment to develop them, which is compromised, I gather, at least from the pharmaceutical industry, by lack of patent protection."

Scripps' Nicolaou is more bullish: Whatever the patent status of the natural product, he pointed out, "Our synthetic materials, of course, are all new chemical entities. And we are taking patent positions on the new analogues we are synthesizing of epothilone."

A footnote to his paper in Nature cites grant support from "Novartis, Merck, DuPont-Merck, Schering-Plough, Hoffmann-La Roche and Amgen Inc., as well as the Skaggs Institute For Chemical Biology and the CaP CURE Foundation."

Scripps, Nicolaou said, has had "a lot of interest in these compounds from some pharmaceutical and biotechnology companies. "Obviously," he added, "we are negotiating with a major pharmaceutical company, but I cannot tell you its name. Preclinical in vivo trials will be in collaboration with our future partner."

In one unexpected way, epothilone showed anticancer superiority to paclitaxel.

"The main excitement about these epothilones compared to Taxol," Nicolaou concluded, "is that they have the ability to kill Taxol-resistant tumor cells where Taxol fails; in the lab, these epothilones are quite effective." *