By David N. Leff

Science Editor

In the medicine cabinets of clinical oncologists there are 31 approved anticancer drugs. Despite this diverse arsenal of tumor-killing compounds, cancer remains the No. 2 cause of death in Western industrialized countries - second only to cardiovascular disease.

An estimated 560,000 Americans die each year from some type of malignancy. These attack one or another organ system in the body in widely differing ways, but they have one thing in common: Cancer cells divide rapidly and out of control. The drugs of anticancer chemotherapy have another common denominator: They all cause horrendous side effects - from alopecia (hair loss) and nausea to immune system breakdown and sundry neurotoxicities. These two common denominators are not unconnected. Stay tuned.

Of those 31 chemotherapeutic drugs, five in particular, the "mitotic spindle poisons," throw chemical wrenches into mitosis - the machinery of tumor-cell division. Three of these five are vinca alkaloids (e.g., vincristine), derived from the periwinkle plant. And two are versions of Taxol, the potent cancer-stopping compound extracted from the Pacific yew tree.

Mitosis would be a natural choke point for stopping tumor growth in its tracks, but these spindle-poisoning drugs have limited efficacy and (seemingly) unlimited side effects.

In the laboratory of cell biologist Timothy Mitchison, at Harvard University in Cambridge, Mass., lab members started hunting for a chemical compound that would expressly block mitosis. However, rather than looking for a protein important in a disease, and then screening compound libraries against that single target - the way industry does - the academic scientists screened a library against cell division, the key to mitosis.

Here is how Thomas Mayer, a postdoctoral fellow in Mitchison's lab, described their three-stage screening strategy: "We were thinking about how we can identify small molecules that perturb proteins required in mitosis," he told BioWorld Today. "So first of all, the lab purchased a huge collection of 16,320 easily synthesizable, random compounds with diverse functionalities. This commercial library called Diverset E was supplied by Chembridge Corp., of San Diego, which imported them from Russia, where they were synthesized. We did the initial funnel-like screening for anti-mitotic compounds as broadly as possible," he recounted. "It's known that most of the anti-mitotic compounds target tubulins, the subunits of the mitotic spindle.

"The goal of our screen," Mayer recounted, "was to identify the molecules that target known or unknown proteins. One major reason why we wanted to have small molecules for specific proteins is that we could use them as tools to inhibit these proteins. And we already knew the function of microtubulin - it makes the mitotic spindle, and manages transport in the cells."

The team then proceeded to their second screen, Mayer continued, "to sort out and discard all microtubulin-targeting compounds. Then in the third - the cytolysis - screen, we looked for compounds that specifically target mitosis, and we ended up with five compounds out of the initial library of 16,320."

One Needle In 16,320 Haystack Strands

"Four of these were of unknown function, but the fifth targeted a motor protein that's physically required for the spindle architecture that allows the faithful segregation of the daughter chromosomes to opposite poles of the mitotic spindle," he said. Motor proteins are like freight cars that roll along the parallel "tracks" of microtubulin. He and his colleagues named their one-in-16,320 compound "monastrol."

Mayer is lead author of a report in today's Science, dated Oct. 29, 1999, and Mitchison it's senior author. Their paper is titled, "Small molecule inhibitor of mitotic spindle bipolarity identified in a phenotype-based screen."

As their article illustrates, that small molecule - monastrol - actually blows up the spindle. "In a normal bipolar spindle," Mayer explained, "we have two poles that divide up each parent-cell's chromosomes. But in the spindles of monastrol-treated cells, instead of two poles at opposite ends, there is only one pole in the center, and a microtubule array radiating out like a fireworks star burst, bordered by a ring of chromosomes.

"This defect," he went on, "causes the effect that the cells arrest in mitosis. They cannot proceed because the DNA is not nicely aligned down the middle. There is a cell-cycle checkpoint that monitors this alignment, and if this is incorrect the cell stops dividing. If it proceeded, it would get mis-segregation of the DNA."

Currently, Mayer said, the co-authors "have from the chemical perspective synthesized derivatives of monastrol to see if we could get compounds that are more potent."

Sidelining Chemotherapy's Side Effects

"And for things like cancer treatment," he observed, "so far the most commonly used anti-cancer drugs are taxols, which target microtubules. And microtubules are required not only in dividing cells, like tumor cells, but also in non-dividing cells. I think it's known that Taxol kills four tumor cells and at the same time one normal cell. That's the reason you have these huge side effects. In contrast," Mayer pointed out, "our drug, monastrol, inhibits a motor protein required in mitosis, not a microtubulin.

"At the moment," he continued, "our chemist is on the way to making a small library of monastrol derivatives, to look for compounds that are more active. And we are using these compounds to study the role of the motor protein in mitosis, because this inhibitor is reversible. We can apply it, and wash it out. So we can concisely control the time of adding the drug and removing the drug."

As for eventual administration of such anti-mitotic, anti-cancer agents, Mayer said, "It's too early to say. What we can say is that the compound is small enough so it can enter cells; it can cross the cell membrane. But about monastrol's oral bioavailability, who knows what happens to this compound when it gets into the stomach?