Results of a Phase II Study of PD-1 Inhibition in Advanced Myeloproliferative Neoplasms

Background: Myelofibrosis (MF) is a hematopoietic stem cell neoplasm characterized by abnormal JAK-STAT signaling, increased inflammation, and evolution to acute myeloid leukemia. Most patients harbor a phenotypic driver mutation- JAK2, CALR or MPL. In the last decade, two JAK2 inhibitors have been approved, however, outside of hematopoietic stem cell transplantation (HSCT), there are no medications that meaningfully modify MF disease biology. Thus, additional treatments are needed.

We previously demonstrated the presence of multiple immunosuppressive mechanisms in MPN patients, including expanded myeloid derived suppressor cell (MDSC) populations and elevated expression of immune checkpoint receptors, particularly PD1, on T cells from MPN patients compared to healthy donors (Cimen Bozkus, Cancer Discovery 2019). Therefore, we conducted a multi-center, open-label, phase 2, single-arm study of pembrolizumab in patients with primary, post-essential thrombocythemia or post-polycythemia vera myelofibrosis (MF) (NCT03065400).

Methods: Patients with intermediate-2/high DIPSS MF, ineligible for or previously failed ruxolitinib were eligible. Pembrolizumab was administered at the FDA-approved dose of 200 mg every 3 weeks for 6 cycles. The study was a Simon two-stage design, 9 patients enrolled in the first arm. The study was terminated after completion of this arm due to the lack of responses. Response assessment was conducted after 6 cycles utilizing the International Working Group (IWG-MRT) criteria. In addition, patients with accelerated or blast phase (MPN-AP/BP) could enroll as a separate, ten patient, exploratory cohort.

Results: 9 patients enrolled in the MF cohort and 1 in the BP cohort between 6/2017 and 3/2020. Baseline characteristics are summarized in Table 1. Median weeks on treatment were 14.7 (range 4-20). All 9 MF patients were evaluable for response as they received at least 1 dose of pembrolizumab. Of the 9 patients who were evaluable for response, all had SD, 4 (44%) patients discontinued therapy prior to the end of cycle 6. Reasons for discontinuation were adverse events (n=1), patient decision (n=1), physician decision (n=2).

Grade 3/4 AEs included anemia (n=3), thrombocytopenia (n=2), leukopenia (n=1), hyperglycemia (n=2), hyperuricemia (n=1), dyspnea (n=1), headache (n=1). No grade 3/4 immune related AEs occurred (Table 2).

The effects of pembrolizumab on the immune suppressive milieu observed in MPN were analyzed. PD-1 was detected on peripheral blood T cells by flow cytometry at baseline, but not post-treatment, likely due to receptor occupancy by pembrolizumab. In all patients analyzed, the levels of soluble PD1 in the plasma by Olink assay were significantly higher post-treatment. Other soluble factors associated with T cell activation such as class I–restricted T cell–associated molecule (CRTAM) and CD27 were also elevated after pembrolizumab administration. Furthermore, ARG1, a molecule that mediates T-cell suppression by MDSC, was significantly reduced in the plasma of treated patients. An increase in peripheral blood T cell frequencies was observed in a subset of patients after two cycles.

Discussion: Pembrolizumab did not demonstrate clinical activity in this phase 2 trial. The relevance of the preliminary correlative findings will be further confirmed by in situ gene profiling of immune cells and their microenvironment. The complete results will be available at the meeting. These results suggest that pembrolizumab may promote a phenotypical and soluble signature suggestive of a restored immune response. The fact that these molecular changes were not associated with clinical responses indicate that pembrolizumab alone may not be sufficient to reverse the multifactorial causes of immune tolerance in MPN.