Competition between approved and investigational drugs is undoubtedly good for patients. In myelofibrosis (MF), Incyte’s drug ruxolitinib has been the sole JAK inhibitor approved to date, primarily on the basis of the registrational trials COMFORT-I and II. Cell Therapeutics and their partner Baxter have also set sights on the MF space, having initiated PERSIST-1 and 2 to further the cause for pacritinib, their JAK inhibitor.
Incyte’s registration trials, COMFORT-I and II, enrolled patients with baseline platelet counts >100,000/uL, and had dose reduction strategies in their protocols for emergent exacerbation of thrombocytopenia. Subsequent to these trials, Incyte has initiated additional studies that have accrued results from ruxolitinib dosing strategies to accommodate patients with platelet counts below 100,000/uL. To wit, the prescribing information for ruxolitinib includes dosing strategies for patients with platelet counts between 50,000 - 100,000/uL. Nonetheless, ruxolitinib was approved on the strength of its treatment effect in patients with >100,000/uL platelet count. The development strategy for pacritinib is focused on this point, and pays special attention to MF patients with baseline platelet counts below 100,000/uL. The first large study for pacritinib is PERSIST-1, with topline data released in March and more details released at ASCO.
Table 1 provides a comparison of spleen responses for the various pacritinib and ruxolitinib trials to date. When possible, the data have been split up into platelet subgroups to facilitate comparisons. No such comparison is perfect, but absent head-to-head data, it’s a reasonable exercise.
Based on the results to date, some trends are visible. First and foremost, for patients with platelets >100,000/uL, ruxolitinib provides better spleen response. This population is, arguably, two thirds of the MF population. Conversely, for patients with platelet counts below 50,000/uL, ruxolitinib yields to pacritinib. The package insert directs caregivers to hold ruxolitinib administration if platelets fall below 50,000/uL, providing pacritinib with an open opportunity. As for the remaining patients with starting platelet counts between 50,000 and 100,000/uL, it is difficult to suggest superiority of one drug over the other. Cumulatively, 38 of 134 patients (~28%) treated with ruxolitinib have had responses as compared to ~11% for pacritinib. Nonetheless, these ruxolitinib data are from open label or expanded access trials, and would likely see a step down in a controlled, randomized setting.
Next up for pacritinib is PERSIST-2, a trial that is enrolling ~300 MF patients with platelet counts less than 100,000/uL. These patients are split into 3 arms: pacritinib 400 mg QD, pacritinib 200 mg BID, and a best available therapy arm that, unlike PERSIST-1, may include ruxolitinib. If the best available therapy arm primarily features ruxolitinib use, and the spleen responses remain similar to those outlined in Table 1, there is little chance of the pooled pacritinib arms besting ruxolitinib. To be more precise, the primary endpoint of PERSIST-2 takes into count both spleen responses and patient reported Total Symptoms Scores (TSS). In COMFORT-I and PERSIST-1, approximately 50% and 25% of ruxolitinib and pacritinib treated patients, respectively, noted a 50% or greater reduction in TSS. In effect, a head-to-head trial favors ruxolitinib for the majority of the MF population. Unless PERSIST-2 enrolls a large percentage of patients with starting platelets <50,000/uL, pacritinib is unlikely compare favourably if the best available therapy arm has extensive ruxolitinib use.
The following consideration for this setting is safety. Ruxolitinib has demonstrable impacts on both red blood cell and platelet counts, requiring dose reductions or transfusions. Pacritinib’s ability to be dosed for patients with platelets <50,000/uL likely confirms its advantage in eliciting less thrombocytopenia. At this time, it is difficult to compare anemia data across the trials, in part because the transfusion dependence criteria were divergent:
COMFORT-I: IWG criteria wherein “Baseline transfusion dependence was defined as the receipt of ≥2 units of RBC products in the 4 weeks prior to randomization, and on-study transfusion independence was defined as the absence of transfusions for any period of ≥8 weeks.”
- PERSIST-1: As per the press release, defined as “patients treated with pacritinib who were severely anemic and transfusion dependent – requiring at least six units of blood in the 90 days prior to study entry.”
By the above criteria, 41.2% vs 46.9% in the active and comparator arms in COMFORT-1 and 25% vs 0% in the active and comparator arms in PERSIST-1 became transfusion independent.
In all, the results from PERSIST-1 suggest that pacritinib is a less potent member of the JAK inhibitor class, signaled not only by its modest efficacy, but also by its more lenient adverse event profile. Going forward, pacritinib’s dataset in PERSIST-2 will be pivotal in determining its competitiveness in the MF space. If the best available therapy arm in PERSIST-2 is able to accommodate a high percentage of patients receiving ruxolitinib, the primary efficacy endpoint is unlikely to be met.