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NEW YORK — Shares of Cell Therapeutics rose Monday after the Seattle company said it had reached an agreement with regulators about the design of a key trial of an experimental drug.

The agreement covers the design and goals of a late-stage trial of pacritinib as a treatment for myelofibrosis, a blood disorder. Cell Therapeutics said it will enroll 300 patients in the 24-week trial. The study will compare pacritinib to the best available treatment, including Jakafi, a myelofibrosis drug that was approved in 2011. The study will measure the number of patients who experience at least a 35 percent reduction in spleen volume and the percentage of patients whose symptoms are reduced by at least 50 percent.

The agreement makes it more likely the FDA will approve pacritinib if it meets its goals in the trial.

Myelofibrosis causes anemia, fatigue, pain and swelling of the spleen. It causes abnormal blood cells to build up in bone marrow, forming thick scar tissue that slows the production of healthy blood cells. To make up for the shortage, other organs including the liver and the spleen begin producing blood cells.

The Seattle company is already running one late-stage trial of pacritinib.

Shares of Cell Therapeutics advanced 23 cents, or 14.5 percent, to $1.82 in early afternoon trading in New York.

Cell Therapeutics Inc. has one approved drug, the lymphoma drug Pixuvri. Revenue from Pixuvri totaled $300,000 in the second quarter. The company is also studying a drug called Opaxio as a treatment for glioblastoma, the most common type of brain cancer.

Jakafi is marketed by Incyte and Novartis.