THE DISCODERMOLIDE STORY

When Dr. Shirley Pomponi first collected the Caribbean sponge Discodermia dissoluta in March of 1987 she knew one thing; it was a sponge she had never seen in her previous three years of collecting marine specimens with Harbor Branch Oceanographic Institution. Now, the sponge has shown great promise by yielding a chemical compound currently being tested in human clinical trials for treatment of pancreatic cancer.

Pomponi is a research professor at Harbor Branch Oceanographic, which is a research institute of Florida Atlantic University. She recalls when she and her colleague and dive partner John Reed were ascending from a scuba dive near the Bahamas after collecting specimens for their potential use in fighting human diseases. She noticed the orange-brown sponge at a depth of around 110 feet, an anomaly for what turned out to be a deepwater organism. She signaled for Reed to wait as she scooped it up, placed it in a plastic bag and continued her climb to the surface.

"It's really rare in the Bahamas at that depth, and in fact once we found the extract from that was active John and I went back there to try and collect more of it," says Pomponi. "We were doing three to five dives a day for about four days, all along the area where we dove before, working between 100 and 120 feet trying to find the sponge, and we never found it."

The nearly 20 years since the initial discovery have included field and laboratory work as well as collaboration with a pharmaceutical company in order to transform the natural compound, discodermolide, into an anti-cancer drug.

Discodermolide is among the marine biomedical compounds currently available for licensing through the Florida Atlantic University Technology Transfer office.

Potentially disease-fighting compounds like discodermolide are built into a sponge's natural defenses and compounds such as this form the basis for marine biomedical research. Sponges and many other marine inhabitants are sessile, remaining anchored to the seafloor throughought their lives. Unable to run from predators, they must nevertheless defend themselves while also competing for resources such as food and space along crowded reefs. To this end, these organisms have evolved an array of bioactive secondary metabolites that aid in defense, reproduction and communication. Fortunately for us, many of these compounds are also showing promise as drugs incombating many human maladies. Realizing this potential, researchers at Harbor Branch have collected more than 30,000 ocean samples over the last 20 years To date, discodermolide is the only one to reach human testing with a pharmaceutical company although several others show promise.

From the Sea to the Pharmacy

There is considerable difficulty involved in the development of a marine-derived drug, from exploration and collection through scuba and submersible dives, to identifying and characterizing target natural products and verifying their source, to balancing sufficient collection of the sea organism for research while still ensuring viable natural populations. This makes "drugs from the sea" research even more daunting than many other pharmaceutical endeavors. However, as with Discodermia, perseverance in this area of research can yield novel marine natural products with amazing potential for fighting drug-resistant and other hard-to-treat diseases such as cancer and AIDS.

After collection, technicians at Harbor Branch ground up the sponge in ethyl alcohol to extract chemical components. They then applied many analytical tools, such as n magnetic resonance (NMR), high performance liquid chromatography (HPLC) and X-ray crystallography, to identify the purified constituents.



Dr. Sarath Gunasekera, a DBMR natural product chemist, spent long hours determining the complex structure of discodermolide, belonging to a class of compounds called polyketides. The compound was then put through a battery of screens to determine whether it exhibited activity toward certain cultured cell lines and thereby would be useful against various cancers, fungal and bacterial infections, viruses or as an immune system suppressor. Initially, Harbor Branch scientists pursued discodermolide as an immune-suppressing agent which could be used to treat organ transplant patients. However, the drug proved to be too toxic and this avenue was abandoned.

"The high dosages that one would need for it to be effective as an immune-suppressive drug were too toxic. It was killing the mice," says Pomponi. Instead, realizing that discodermolide might function in a similar way to the breast cancer drug Taxol® (originally derived from the bark of the Pacific yew tree) Pomponi and her team explored the compound for its anti-cancer activity. As it turns out, discodermolide actually appears to be more potent than Taxol, and it also works against many drug-resistant cancers. ņIt's different enough that discodermolide works where Taxol doesn't," says Pomponi. In addition, unlike the hydrophobic Taxol, discodermolide is water-soluble, and therefore, easier to dissolve in liquids for better patient delivery. To make Taxol into a drug that could be injected into patients, manufacturers had to combine it with another chemical, which can cause side effects.

Despite their distinct chemical structures, both drugs work by targeting certain aspects of mitosis, the fundamental process by which virtually all cells in the body replicate by dividing to form a pair of daughter cells. The drugs bind at the molecular level to protein components called tubulin that make up the microtubules. These are rod-like structures which assemble and disassemble throughout a cell's life and help to pull daughter cells apart during division. Normally, microtubules accumulate at the beginning of the cell doubling process and break down in the end. However, discodermolide prevents microtubule breakdown, causing the cancer cells to become so over-crowded with microtubule filaments that they can't divide and spread. Eventually the cancer cells initiate a process called apoptosis, or "cell suicide". Researchers think that discodermolide and Taxol attach to slightly different areas on the microtubules, explaining their differing abilities to attack certain tumor types.

Pipeline Partners

The fact that the new compound functions in a similar way to Taxol piqued the interest of pharmaceutical and academic establishments interested in pursuing further research and development. However, discodermolide was competing with easier-to-produce laboratory drugs, so Pomponi had to prove to the drug companies the compound's stellar potential. "It has to be pretty darn hot usually to compete with some of the things that they're working with, and especially if it's competing with a synthetic compound, something that they can easily make on their own," says Pomponi.

After talking to a few drug corporations, Harbor Branch licensed the compound to the Swiss company Novartis AG in 1998. "I was the one who negotiated the license agreement with Novartis, which was something I had never done before. It's not something you learn in graduate school."

But licensing meant the division researchers had to find enough of the elusive sponge for another round of testing by Novartis. The challenge was to do this without negatively impacting the natural population, typically one of the more formidiable barriers to developing marine-derived drugs. Not only does Discodermia occur sparsely in the deeper waters (usually between 450 and 750 feet) of the Caribbean, but the paucity of the compound itself within the sponge, means supply is limited. "It's always a tug for us, because on the one hand, we are very mindful of the fact that you can't just take everything that's on the bottom. . . You can't wipe out the population," says Pomponi. She adds, "On the other hand, we're trying to develop this drug. So we're just trying to balance that and trying to figure out ways of extracting more of the compound from the sponge and really tailoring experiments so we would have to use less material."

To circumvent supply problems, the drug company has labored to develop a method of chemical synthesis, and although very expensive and laborious, is currently the most feasible alternative to ocean collection. Making discodermolide in the lab was no small feat, according to Dr. Stuart J. Mickel, head of the chemical development division at Novartis in Basel, Switzerland. "This is the most complex molecule Novartis process research has ever had to deal with," said Mickel in 2003. The final synthesis route included 36 steps and was a combination of two other methods found in the scientific literature. From July of 2000 through March of 2002, his team of scientists struggled to complete each step of the formidable process to transform commercially-available starting materials into the final product, while meeting strict criteria such as a high yield and the ability to reproduce the whole series of steps. The final product was a fluffy white powder, and a total of 25 kg was synthesized. This was estimated to be enough to treat approximately a half a million patients, although Mickel said the company hasn't determined exact dosage yet.

After his long work in the laboratory, Mickel still remembered the source of this precious chemical structure. "Novartis is very ecologically minded and we saved three tons of sponge," he said, referring to the amount of sponge it would take to produce this much discodermolide.

Synthetic work in In Mickel's lab has continued, and in 2004 Novartis reported the first industrial-scale total synthesis of discodermolide.

Pomponi says that Harbor Branch scientists continue to examine other production methods such as culturing the sponges in a laboratory setting (which has never been successful with a deepwater sponge) as well as looking for an associated microbe that may be the true producer of the molecule.

In addition, DBMR researchers Ross Longley (now of Taxolog, Inc.) and Gunasekera, along with researchers from Novartis and the University of Rochester, are investigating alternative discodermolide-like compounds. This current research, funded by the National Institutes of Health(NIH), focuses on the preparation and evaluation of natural and synthetic compounds that have the same function as discodermolide but a different structure. These analogs may lead to a better understanding of the mechanism by which the chemical works and an improved synthesis of the compound. Ultimately, the scientists hope to find a compound that is more potent and easier to make in the lab than discodermolide.

Discodermolide has been in advanced human trials to determine safety and efficacy since 2003. As discodermolide winds its way through the clinical pipeline and gains momentum as an anti-cancer agent other marine natural products have shown promise. For instance, topsentin, a compound derived from the deepwater sponge Spongosorites ruetzleri, is ready for licensing as an anti-inflammatory agent. Pomponi says this drug should be the next Harbor Branch addition to the marine pharmacopoeia, especially because the compound has already been synthesized in the lab.

While marine-derived drugs could revolutionize biomedical research, Pomponi believes the movement will also bring public awareness to the value of the world's oceans. "The fact of the matter is that because there are so many compounds, like a dozen or so compounds in clinical trials, it has already raised the awareness of the general public to the importance of marine organisms and the oceans for positive things for human health."