Making Medicine: Cooperating with Nature

Pieces of white stem and red flower from a thorny shrub native to the American Southwest lie atop scholarly books in the office of Professor Paul A. Grieco, holder of the Earl Blough Professorship in Chemistry. Despite the spare beauty that the plants give to the office, the shrubs are not decorative. They are nature's source of quassinoids, highly toxic compounds that have potential application in medicine, and they are intimately bound with Grieco's work of the past fifteen years. A report in the mid-1970s first sparked Grieco's interest when one study suggested that bruceantin, a quassinoid, appeared to be effective against leukemia. Researchers were eager to test this compound against tumor cells, but only minute quantities of the compound could be isolated from shrub matter. The efficacy of the drug could not be tested unless sufficient amounts were obtained. Total synthesis of a quassinoid was a daunting task and had never been tried, but Grieco wanted to make the attempt. Grieco had earlier done key work in lactone synthesis, something that turned out to be a pivotal development in the synthesis of natural products. With that background to build on, he now undertook to synthesize highly complex quassinoids.

Having begun his work on quassinoid synthesis at the University of Pittsburgh, Grieco brought his research to Indiana University when he joined the faculty in 1980. By 1981 his laboratory at IU had become the first in the world to synthesize a quassinoid, the parent compound called quassin. Once accomplished, that strategy opened the door to synthesis of more complex structures because all quassinoids in nature are derived from quassin. Grieco and his team embarked on a long and painstaking effort to synthesize bruceantin and other useful quassinoids from quassin. In the meantime, bruceantin extracted from plant matter was being used by the National Cancer Institute for in vitro testing against P-388 lymphocytic leukemia.

Along the way to synthesis of bruceantin and the closely related quassinoid glaucarubolone, Grieco tried out a new and revolutionary solvent, and, against all odds, the solvent worked. What is more astonishing is that this solvent--water--was so abundant and cheap. "If a graduate student had put that in a paper back in the late seventies, I would have failed the student. There was no way water should have worked. We went ahead and tried it expecting it wouldn't work," recalls Grieco, shaking his head. "But it worked."

The use of water utilized an important principle about chemical reactions. Many compounds exhibit hydrophobia, a tendency of molecules to avoid exposure to water by bunching together. It is this bunching that instigates chemical reaction. "It's a good thing we tried water as a solvent," says Grieco. "Without it, we never would have achieved synthesis of the more complex quassinoids."

Grieco has since worked with other unconventional solvents to coax organic reactions. He mixed lithium perchlorate--a salt--with diethyl ether and found that the mixture causes perhaps even greater bunching of molecules to avoid exposure to the medium. But water remains an attractive favorite as a new solvent: it is cheap, gentle, and environmentally harmless.

Meanwhile, bruceantin turned out to be unsatisfactory in fighting leukemia cells, and in 1986 the National Cancer Institute stopped its testing of the drug, which had reached phase I clinical trials. But Grieco's team kept working on quassinoids and, aided by the use of unconventional solvents, made remarkable breakthroughs in the synthesis of a number of them. By 1991 Grieco's laboratory had succeeded in the total synthesis of three additional quassinoids: chaparrinone, glaucarubolone, and glaucarubinone. Grieco's leadership in this area is underlined by the fact that since then two other laboratories in the world have succeeded in only partial synthesis of quassinoids.

At this period Dr. Frederick Valeriote, an oncologist at Wayne State University in Detroit, was looking for compounds that might be effective against solid tumors of the type seen in colon and pancreatic cancer. Grieco offered to supply Valeriote with some likely candidates from among the compounds he had synthesized. One of these, glaucarubolone, showed itself to be effective against solid tumor cells. The Wayne State research team proceeded from in vitro cell studies to in vivo studies with mice, and again the results were encouraging.

Grieco and his team, faced with ever-increasing demand for large quantities, realized there was a practical problem with production of glaucarubolone. The complicated procedures involved in total synthesis--more than forty steps, each step involving minute changes in the molecular structure until the target compound was reached--meant the cost of producing even small quantities of glaucarubolone would be exorbitant. Grieco looked for a way to reduce the cost and found it in a cooperative venture with nature.

The plant genus Castela of the family Simaroubaceae has many shrub-like species that thrive in warm, dry climates and are rich in quassinoids. By collecting the plants from nature, then grinding the bark, roots, or stems and extracting through the use of solvents, quassinoids can be isolated from the plants. This extraction process yields a number of similar compounds in rather small amounts. Total synthesis of these compounds would likewise be prohibitively expensive. Why not, thought Grieco, save steps by utilizing nature? He decided to try a combination: isolate quassinoids from the natural plant matter, then use chemical techniques to transform them into the target compound.

Grieco now oversees a laboratory production line, as technicians grind down and extract quassinoids from plant material harvested from Castela emoryi, which is collected in Arizona. Two or three compounds are isolated. Extraction of the natural compounds in these plants gives the team chemicals already many stages closer to the medically useful compounds they want to produce. Grieco's collaborator at Wayne State has taken compounds produced by Grieco's laboratory and tested their effects on solid tumors. The Wayne State team found that one of the compounds-- its name not yet released due to patent procedures underway--proved to be three times more effective against cancer cells in vivo than glaucarubolone. It has become the single target drug of all the extraction and synthesis work.

Now Grieco and his team take all the compounds extracted from plants and then go to work carrying out a series of four or five transformations to reach that target compound so desired by the Wayne State testing team. Soon will come toxicity studies on animals and phase I clinical trials in humans, as medical experts at various sites around the country seek to apply the compound to the treatment of colon and pancreatic cancer tumors. Preliminary studies indicate that the drug is toxic to tumor cells yet does not harm healthy tissue in the body. It seeks out its prey and destroys the cancerous cells with remarkable efficiency.

Where does this finally lead? "I hope to see it used someday to fight colon and pancreatic cancer. I want to see it on the market, in use. That's my dream," says Grieco. "It's exciting. This is what it's all about."

Faced with the need to keep medical researchers supplied, Grieco runs one of the largest laboratory teams at IU. His team understands the importance of turning out enough compound for all the tests underway. But that is not the only work for Grieco. He is chair of the chemistry department and has served in that capacity since 1988. These two positions would tax the energies of an ordinary person--demanding every moment of one's time at the chemistry building. But he has extraordinary energy. Grieco goes home at night to a farm north of Bloomington that he operates with his sixteen-year-old son. "We keep seventy-five head of cattle, and we put up our own hay." His calloused hands are proof he really does work at the farm in his time away from the campus. "I've got a lot of nervous energy," he admits. "I have to use it somehow, or I go crazy."

--William Rozycki