JAK2/mTOR Inhibition Fails to Prevent Acute GVHD Despite Reduced Th1/Th17 Cells: Final Phase 2 Trial Results

Hematology Immunology Medicine Life Sciences
JAK2/mTOR inhibition acute GVHD Th1/Th17 cells allogeneic hematopoietic cell transplantation clinical trial immunosuppression cell differentiation

Inhibition of JAK2/mTOR Fails to Effectively Prevent Acute Graft-versus-Host Disease (GVHD): Limitations of Targeting Th1/Th17 Cells

In recent years, the role of systemic immunosuppressive therapies in preventing acute graft-versus-host disease (aGVHD) following allogeneic hematopoietic cell transplantation (alloHCT) has come under scrutiny. Th1 (T-helper cell Type 1) and Th17 (T-helper cell Type 17) play critical roles in the pathogenesis of aGVHD, and the JAK2 (Janus kinase 2) and mTOR (mammalian target of rapamycin) signaling pathways are essential for the differentiation and function of these T cells. However, despite substantial clinical trial activity showing that JAK2 inhibition might regulate these inflammatory pathways, the efficacy and underlying mechanisms remain controversial.

Of note, the phase II clinical trial published in Blood investigates the combination of the JAK2 inhibitor Pacritinib with the mTOR inhibitors Sirolimus and Tacrolimus (referred to as Pac/Sir/Tac), aiming to evaluate its biological effects on reducing Th1/Th17 cell differentiation and its efficacy in preventing acute GVHD. Led by researchers at the Moffitt Cancer Center in Florida and the University of Minnesota, including Joseph Pidala and Shernan G. Holtan, this study provides new data to expand the GVHD treatment toolbox.

Background and Study Objectives

The team initiated this study to address the suboptimal outcomes of prior therapies targeting JAK2 signaling. GVHD is characterized by donor T cells attacking host antigens, such as those in the liver, gastrointestinal tract, and skin, leading to severe immune responses. Cytokines such as IL-6, IL-12, and IL-23 and their associated signaling pathways are pivotal in these processes. Previous studies and clinical trials have shown that treatments targeting the IL-6 receptor (e.g., Tocilizumab) can mitigate Th1 and Th17 responses in models but have limited efficacy in human clinical trials.

The key objectives of this study included: 1. Investigating whether Pac/Sir/Tac effectively reduces phosphorylated Stat3 (pStat3) expression in CD4+ T cells on post-transplant day 21 (primary endpoint). 2. Assessing whether it can reduce the incidence of acute GVHD compared to historical control groups without such targeted therapies (secondary endpoint). 3. Examining how JAK2/STAT3 inhibition modulates Th cell subsets (Th1, Th17, Treg).

Methods and Design

This study adopted a single-arm phase II design involving 28 patients who underwent HLA-matched allogeneic hematopoietic stem cell transplantation. The researchers focused on the effects of selective JAK2 inhibition (Pacritinib) combined with immunosuppressive agents (Sirolimus and Tacrolimus).

Study Design Overview:

  • Patient Inclusion Criteria: Patients included those with hematologic malignancies, such as acute myeloid leukemia (AML) or myelodysplastic syndrome (MDS), and had to meet strict organ function criteria. All underwent fully HLA-matched hematopoietic stem cell transplantation.

  • Treatment and Dosing Regimen: Pacritinib was administered at the recommended dose (100 mg twice daily) from the day of transplant to Day 70, followed by gradual tapering and termination by Day 100. The doses of Sirolimus and Tacrolimus were adjusted based on therapeutic drug monitoring.

  • Endpoints and Statistical Methods:

    • Primary Endpoint: Proportion of pStat3+ CD4+ T cells on Day 21 post-transplant (aim: ≤35%).
    • Secondary Endpoint: GVHD grading and incidence. Statistical methods included one-sample t-tests, with Kaplan-Meier estimates for GVHD, infections, relapse rates, and overall survival.

Results

  1. Primary Endpoint Findings: Pacritinib significantly reduced the proportion of pStat3+ CD4+ T cells on Day 21 (mean reduced to 9.62%), meeting the primary biological target (≤35%). This suppression was consistent across patients and unaffected by short-term treatment interruptions (e.g., due to diarrhea from chemotherapy).

  2. Regulation of Th Cell Subsets: Pac/Sir/Tac effectively reduced Th1 and Th17 cell numbers on Day 21, while increasing the Treg-to-Th1/Th17 ratio. However, the increase in absolute Treg numbers was modest, consistent with earlier phase I trial results.

  3. Acute GVHD Incidence: Despite robust biological inhibition, Pac/Sir/Tac did not significantly improve clinical outcomes in aGVHD prevention. The 100-day cumulative incidence of grade II-IV aGVHD (46%) was comparable to historical control with Sir/Tac alone (43%). The incidence of severe grade III-IV aGVHD was 18%.

  4. Long-Term Outcomes:

    • Chronic GVHD incidence reached 45% at 24 months.
    • The relapse rate was low at 11%.
    • Overall survival at 24 months was 71%, with no significant increase in infectious complications observed during the trial.

  5. Notable Observations: The study highlighted that Pac/Sir/Tac had minimal influence on Aurora kinase A activity, a key pathway implicated in some donor T cell immune escape mechanisms.

Significance and Limitations

Key Findings and Theoretical Advances

  • This study adds significant validation to the biological effects of JAK2 inhibition combined with mTOR pathway regulation in the context of GVHD treatment, while also highlighting its limitations in achieving sufficient GVHD prevention.
  • It suggests the need for combined therapeutic approaches, such as incorporating Aurora kinase A inhibitors (e.g., Vic-1911) or broader JAK1/2 inhibitors (e.g., Ruxolitinib).

Clinical and Experimental Limitations

  • Due to suboptimal efficacy for aGVHD prevention, patient enrollment was halted early, potentially limiting external generalizability.
  • Adverse events such as diarrhea, unrelated to Pacritinib pharmacodynamics, led to treatment interruptions, complicating adherence and long-term evaluation.

Future Directions

Although this study did not achieve a breakthrough in aGVHD prevention, it underscores the biological efficacy of JAK2/mTOR modulation and its limitations. These findings suggest that future therapeutic strategies should integrate broader immune regulation (e.g., targeting Aurora kinase A alongside Th cell differentiation), aiming for a more comprehensive and effective approach to GVHD prevention and improved long-term transplant survival outcomes.