JAK2/mTOR inhibition fails to prevent acute GVHD despite reduced Th1/Th17 cells: final phase 2 trial results

Our phase 1 graft-versus-host disease (GVHD) prevention trial of JAK2 inhibitor, pacritinib (PAC; recommended phase 2 dose: 100 mg orally twice a day on day 0 to +70) plus sirolimus and tacrolimus (SIR/TAC) demonstrated the regimen was safe and free of pan-JAK myelosuppression after allogeneic hematopoietic cell transplantation (alloHCT). PAC inhibits interleukin 6 (IL-6) receptor activity and pathogenic T helper cell 1 (Th1)/Th17 differentiation in preclinical models and the phase 1 trial. Herein, we report on our completed phase 2 trial of PAC/SIR/TAC after 8/8 human leukocyte antigen matched alloHCT. This single-arm phase 2 trial (NCT02891603) was powered to determine if PAC/SIR/TAC suppressed percentage phosphorylated STAT3 (pSTAT3)+ CD4+ T cells at day +21 (primary end point: percentage pSTAT3+ CD4+ T cells ≤ 35%) and estimated grade II to IV acute GVHD by day +100. The impact of PAC/SIR/TAC on T-cell subsets, CD28 (pS6 and pH3ser10), and IL-2 receptor (pSTAT5) signal transduction was also evaluated. Eligible patients (n = 28) received alloHCT for hematologic malignancies or myeloproliferative neoplasms. Reduced or myeloablative intensity conditioning was permitted. PAC/SIR/TAC met the primary end point, reducing percentage pSTAT3+ CD4+ T cells to 9.62% at day +21. Th1/Th17 cells were decreased at day +21, increasing the ratio of regulatory T cells to Th1 and Th17 cells with PAC/SIR/TAC at recommended phase 2 dose PAC compared with dose level 1 PAC. The cumulative incidence of grade II to IV acute GVHD by day +100 with PAC/SIR/TAC was similar to historic SIR/TAC values (46% vs 43%). Although PAC/SIR/TAC suppressed pSTAT3 and Th1/Th17 cells, the regimen did not improve acute GVHD prevention.