Protein Design Labs ( PDLI) said Friday that a late-stage test of its experimental cancer drug Zamyl produced mixed results.

Zamyl is being developed to treat acute myeloid leukemia. In a late-stage test of 191 patients, 43% of patients taking Zamyl plus traditional chemotherapy drugs successfully reduced the number of cancer cells in their blood. This is compared with just 26% of patients taking chemotherapy drugs alone.

PDL called these results a success, but on an equally important measure, Zamyl failed. The company said patients didn't reach this "complete response" target within 70 days of starting treatment. This time target was the primary goal of the late-stage test.

In recent trading on Friday, the company's stock was lower by $1.71, or 4.2%, to $38.86.

PDL is best known as a drug technology company that licenses its know-how to other biotech firms. But with products such as Zamyl, the company is trying to use its technology to develop its own drugs .

The mixed Zamyl results do not come as much of a surprise. Wall Street analysts had been downplaying expectations for the drug in recent weeks. The company uwrapped the drug's results at the annual meeting of the American Society of Hematology, which starts today and runs through Dec. 11. On Monday, PDL will release partial results from testing on another of its young drugs, Remitogen, for the treatment of non-Hodgkin's lymphoma.

PDL is testing Zamyl on patients who failed other treatments for acute myeloid leukemia, or who had relapsed within one year. In the trial, a complete response was defined as a patient with less than 5% of cancerous cells in his body and who no longer required blood transfusions.

The 43% overall response rate for patients taking Zamyl and chemotherapy was measured using a set of criteria also used to test a competing and approved leukemia drug, Mylotarg, which is sold by American Home Products ( AHP).

But under a more stringent definition of "complete response," Zamyl didn't perform as well: It produced a complete response in 30% of patients, compared with 21% of patients using chemo drugs alone, the company said.

PDL executives believe the trial, despite its mixed results, proves that Zamyl can be an effective and safe leukemia drug.

"We are delighted by the positive results we have seen in this trial, and the potential benefits Zamyl might provide to myeloid leukemia patients," said company CEO Larry Korn, in a statement. "If the complete analysis of the data, which PDL will be conducting over the next two or three months, confirms this preliminary review, PDL plans to hold discussions with the FDA and European regulatory authorities. We would then plan to file for marketing clearance for Zamyl in the United States and Europe."

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