Pacritinib's Clinical Hold for Myelofibrosis Is Lifted

The US Food and Drug Administration’s (FDA) clinical hold on trials for pacritinib were lifted, according to CTI BioPharma, the developer of the JAK2/FLT3 inhibitor.

In February 2016, the FDA placed a full clinical hold on pacritinib studies following reports of patient deaths related to intracranial hemorrhage, cardiac failure and cardiac arrest in the phase 3 PERSIST-2 myelofibrosis trial. To rectify the hold, CTI BioPharma provided the FDA with final clinical study reports from the PERSIST-1 and 2 myelofibrosis trials and initiated the new PAC203 trial.

The PAC203 study is examining the safety and efficacy of pacritinib in patients with primary myelofibrosis who previously received Jakafi (ruxolitinib). Patients will receive pacritinib at either 100 mg once daily, 100 mg twice daily, or 200 mg twice daily. The targeted enrollment is 105 patients for the trial, which CTI BioPharma will likely launch in spring 2017.

“We are pleased to resolve the full clinical hold through working diligently with the FDA to provide a comprehensive response to their requests,” said Richard Love, interim president and CEO of CTI BioPharma. “We look forward to discussing with the FDA the future development of pacritinib. We believe pacritinib can ultimately address the unmet need of patients with myelofibrosis who are ineligible to receive or are not benefitting from the approved JAK1/JAK2 inhibitor, ruxolitinib, as these patients have limited treatment options.”

In the open-label PERSIST-2 trial, patients with myelofibrosis and platelet counts unfrt 100,000/μL were randomized in a 2 to 1 ratio to pacritinib or best available therapy, which could include Jakafi, which became the first drug specifically approved for patients with myelofibrosis in 2011. The trial had already reached its accrual goal at the time of the clinical hold.

Prior to the hold, pacritinib had demonstrated promising data in the PERSIST-1 trial, which was presented at the 2015 ASCO Annual Meeting. In this study, the tyrosine kinase inhibitor demonstrated improvements in spleen volume and symptom control versus best available therapy for patients with myelofibrosis.

Findings from the PERSIST-1 trial had been submitted to the FDA; however, following the clinical hold, CTI BioPharma announced that it had withdrawn its new drug application. In August 2014, pacritinib was granted a fast track designation for the treatment of intermediate and high-risk myelofibrosis.

In the PERSIST-1 trial, 327 patients were randomized two to one to receive 400 mg pacritinib daily versus physician’s choice of best available therapy, excluding Jakafi. The most frequently used treatments in the control arm were hydroxycarbamide (55.7 percent) and a watch-and-wait strategy (25.5 percent).

Patients with primary myelofibrosis, post-polycythemia vera myelofibrosis, or post-essential thrombocythema-myelofibrosis were eligible for the trial. There was no required platelet level for enrollment, with 32 percent of patients having platelet levels under 100,000/μL and 16 percent having platelet counts under 50,000/μL. The primary endpoint of the study was spleen volume reduction greater than 35 percent.

The median duration of treatment was 16.2 months with pacritinib compared with 5.9 months in the control arm. Overall, 79 percent of those in the control arm crossed over to receive pacritinib. After 24 weeks of follow-up, the spleen volume was reduced by at least 35 percent for 19.1 percent of patients treated with pacritinib versus 4.7 percent of those in the best available therapy arm.

In those with platelets less than 100,000/μL, spleen volume reduction was seen in 16.7 percent of patients treated with pacritinib versus 0 percent with best available therapy. In those with platelet counts under 50,000/μL, 22.9 percent of patients treated with pacritinib experienced spleen volume reduction versus none in the control arm.

Additionally, more patients treated with pacritinib experienced a at least 50 percent reduction in total symptom score at 24 weeks using the Myeloproliferative Neoplasm Symptom Assessment. Overall, 24.5 percent of patients in the pacritinib arm experienced a symptom score reduction versus 9.9 percent in the control group.

Among patients who were dependent upon red blood cell transfusions, 25 percent in the pacritinib arm became independent during the trial compared with none of those on best available therapy.

The most common adverse events (AEs) for pacritinib were diarrhea, nausea and vomiting. There was a low incidence of grade 3 AEs. In the pacritinib arm, three patients discontinued therapy and 13 required dose interruptions for diarrhea. There were no grade 4 gastrointestinal events reported.

The most common all-grade AEs for pacritinib versus best available therapy, respectively, were diarrhea (53.2 percent vs 12.3 percent), nausea (26.8 percent vs 6.6 percent), anemia (22.3 percent vs 19.8 percent), thrombocytopenia (16.8 percent vs 13.2 percent) and vomiting (15.9 percent vs 5.7 percent).

In the United States, CTI BioPharma and Baxalta are jointly developing pacritinib, following a license agreement in November 2013. Outside of the United States, Baxter has exclusive rights for pacritinib. At this point, the future remains unclear for the medication.