Immunex Corp. said Thursday that it is starting Phase Iclinical trials in sarcoma patients of PIXY321, a geneticallyengineered protein that stimulates the growth of white bloodcells and platelets.

Immunex said it hopes the "second-generation" colonystimulating factor will help patients suffering fromchemotherapy or radiation therapy-induced blood disorders.

These treatments may induce neutropenia, a decrease in thewhite blood cell count that can lead to infections, andthrombocytopenia, which is characterized by decreasedplatelet levels. Each year, up to 250,000 U.S. cancer patientsare treated with regimens that may cause these conditions,the company said.

Sarcomas are highly malignant tumors in bone, muscle or othertissue that can be spread through the blood. Other potentialuses for PIXY321 include use as an adjunct therapy for treatingcertain infectious diseases, blood disorders and AIDS. Immunexplans to begin, possibly this year, trials of PIXY321 for otherindications.

PIXY321 combines in a single molecule analogs of twocytokines: granulocyte macrophage colony stimulating factorand interleukin-3. GM-CSF stimulates the growth of infection-fighting neutrophils and macrophages. IL-3 may promote thegrowth of megakaryocytes that form platelets, the elements ofblood responsible for clotting.

"We believe the potential for this product could be greater thanthe potential for the individual products," said Immunexspokesman Jason Rubin. "What we've seen in animal studies ismuch greater potency of the fused molecule than for theindividual molecules." This may permit lower dosages,reducing both side effects and costs.

Shares of Seattle-based Immunex (NASDAQ:IMNX) closed at$42.50, down $1.50, on Thursday. -- Karen Bernstein

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