Pharmacia ( PHA) said Friday it was halting testing on its experimental cancer drug, SU5416, after an interim analysis of a late-stage test showed the drug to be ineffective.

The drug was being tested in patients with advanced forms of colon cancer.

The setback at Pharmacia could have far-reaching effects, because iSU5416 was among the first of a new class of cancer drugs. SU5416 was supposed to work by blocking those growth factors in solid tumors that encourage the development of new blood vessels, thereby keeping tumors alive. Cut off the blood supply to a tumor and it dies.

Researchers call this process antiangiogenesis. More than 40 drug and biotech companies are experimenting with various drugs that use this approach to treat cancer. The fear: If Pharmacia's SU5416 showed no efficacy at such a late-stage of testing, does that cast a cloud over the entire antiangiogenesis field?

Friday, Pharmacia-sponsored scientists said no.

"Angiogenesis inhibitors remain a promising new way to attack cancer," said Lee Rosen, a doctor at UCLA's Jonsson Cancer Center and the lead investigator of SU5416 in colon cancer, in a statement. "It is critical to recognize that the results of these SU5416 clinical trials do not invalidate the VEGF-R target or the entire angiogenesis field. Instead, we will take what we have learned from SU5416 in order to develop the next generation of compounds and studies."

The VEGF-R target mentioned by Rosen refers to vascular endothelial growth factor, a protein that seems to play a big role in jump-starting the formation of new blood vessels for cancerous tumors.

Genentech ( DNA) is one of the other companies developing an antiangiogenesis drug. Last October, the biotech firm stopped a midstage test of Avastin in advanced kidney cancer because the drug met its clinical goals earlier than expected. In this case, it meant that tumors in the Avastin patients grew slower than tumors in placebo patients. But all of the patients were terminally ill, and Avastin did not prolong their lives. Further testing of the drug is underway.

But Genentech has run into trouble when it tested Avastin in patients with non-small cell lung cancer. Some of the patients suffered serious pulmonary bleeding episodes, resulting in four deaths.

Other companies with antiangiogeneis drugs under development include Celgene ( CELG), Entremed ( ENMD) and Medimmune ( MEDI).

And while all of the current attention at ImClone Systems ( IMCL) is focused on its colon cancer drug Erbitux, the embattled biotech firm also has two antiangiogenesis drugs in its pipeline.

In recent trading, Pharmacia was down 25 cents, or 0.7%, to $37.25.

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