CHICAGO (TheStreet) -- The CTI Biopharma (CTIC - Get Report) drug pacritinib is mostly effective for a small percentage of myelofibrosis patients with dangerously low platelet counts, according to new data from a phase III study presented Saturday afternoon at the American Society of Clinical Oncology (ASCO) annual meeting.

Myelofibrosis patients with higher platelet counts appear to benefit more from treatment with Incyte's (INCY - Get Report) Jakafi, based on a cross-trial comparison. To date, the two drugs have not been compared directly against each other in a clinical trial.

CTI Biopharma announced top-line results from the phase III study known as PERSIST-1 last March. The pacritinib spleen response rate was 19.1%, compared with 4.7% for best alternative therapy, achieving the primary endpoint of the study with statistical significance.

Myelofibrosis is a disorder in which abnormal bone marrow stem cells produce scar tissue that replaces healthy marrow. Patients with myelofibrosis suffer from anemia and enlarged spleens. Patients with 35% or greater reduction in the size of their spleen after 24 weeks of treatment were considered responders in the pacritinib study.

Incyte secured approval for Jakafi, in part, on studies of myelofibrosis patients which demonstrated a spleen response rate of 32% after 24 weeks. But Jakafi causes platelet counts to fall, so some myelofibrosis patients can't take the drug or must reduce the dose.

Of the 327 myelofibrosis patients enrolled in CTI Biopharma's phase III study, 15% had baseline platelet counts of 50,000 units or less -- too low to be treated with Jakafi.

In these dangerously low-platelet patients, the pacritinib spleen response was 23%, compared with zero responses in patients treated with best alternative care, according to new data presented Saturday at the ASCO meeting.

Investigators also analyzed the phase III study based on the 32% of patients enrolled with baseline platelet counts of 100,000 or less. Outside of a clinical trial, these patients would be candidates for treatment with a lowered dose of Jakafi. Here, the pacritinib spleen response was 17% versus zero response for the best alternative therapy.

"Both pacritinib and ruxolitinib will have roles" in myelofibrosis, says Dr. Ruben Mesa of The Mayo Clinic in Scottsdale, Ariz., and the lead investigator in the pacritinib study. "Blood count of patients is likely to be an important decision-making factor for doctors" when choosing between the two drugs, he adds.

Mesa estimates that one-third of the approximately 20,000 myelofibrosis patients in the U.S. have some degree of lowered platelet counts, defined as 100,000 or less.

Unlike Jakafi, pacritinib also improved anemia in some patients. In a subset of enrolled patients requiring red blood cell transfusions, 26% become transfusion independent following treatment with pacritinib. None of the patients treated in the control arm became transfusion independent.

A second, ongoing phase III study, dubbed PERSIST-2, compares pacritinib against best alternative care in myelofibrosis patients with baseline platelet counts less than 100,000. Patients in the control arm will be allowed to be treated with Incyte's Jakafi. Results from this study are expected in the second half of the year. 

If the second study is also successful, CTI Biopharma and its partner Baxter have told investors that they will seek regulatory approval for pacritinib before the end of the year.

Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.