Now we hear that a promising experimental cancer drug was derived from a tree that Zulu warriors in South Africa used as a charm against enemies. Although the Zulus used the bark to treat sores and venereal disease, they did not use it for cancer.

The compound is called combrestatin. Although THESE RESULTS ARE PRELIMINARY, combrestatin does have a unique ability to damage blood vessels in tumors -- while apparently sparing normal tissue.

The compound is being developed by Oxigene, which reported in May that combrestatin had passed phase I trials in the United Kingdom. That means it had not caused intolerable side effects in a small group of cancer patients.

Does this bark have a bite?
The company also reported that combrestatin had restricted blood flow in the tumors. Without the oxygen that blood delivers, tumors die. Although this happened in animal trials of the compound, remember that Phase I trials are designed to prove safety, not effectiveness.

Still, Oxigene crowed about in the study. "We are very encouraged by [the compound's] safety profile and tumor blood flow shutdown data," said chairman and CEO Bjorn Nordenvall in a company press release.

Targeting blood flow has several advantages. First, unlike existing chemotherapies, it does not kill cancer cells directly, so the cells presumably cannot mutate and become immune to combrestatin. Second, attacking tumor blood vessels is a new weapon against solid tumors, which require a blood supply to grow above microscopic size. (The technique is not expected to work against blood cancers, which don't need new blood vessels). At least initially, the drug would complement, not replace, existing treatments like radiation and surgery.

The red pipeline
If this reminds you of our story on treating cancer by interfering with the formation of new blood vessels, you've a good memory. However, while the better-known (but also experimental) anti-angiogenesis factors stop all new blood vessels, combrestatin attacks existing vessels in tumors (it also seems to inhibit the formation of new vessels).

The discovery of combrestatin, which was isolated in 1987 by George Pettit of Arizona State University, is another reason to believe that preserving biological diversity is a medical necessity, not just an ethical or esthetic requirement.

Why has it taken so long -- 14 years and counting -- for the drug to get to the medicine chest? Because combrestatin, like every other drug candidate, must first pass three phases of clinical trials.

Phase I: Is it safe?

Phase II: What's the best dosage?

Phase III: Does it work better than existing treatments?

With two phases left to go, combrestatin will not hit the market for several years at best. For warriors against cancer -- Zulu and otherwise -- the chemical from the charmed tree could not come soon enough.

Dusty skies could be a real enemy of rain.