A Prospective Randomized Clinical Trial Comparing a Calcineurin-Free Gvhd Prophylaxis Regimen with Post-Transplant Cyclophosphamide and Abatacept to Methotrexate and Tacrolimus

Background: Chronic Graft Versus Host Disease (GVHD) is one of the major causes of mortality and morbidity after an allogeneic stem cell transplantation. We hypothesized that we could induce post-transplant tolerance by using the combination of post-transplant cyclophosphamide (PTCy) and abatacept (CTLA4Ig) for GVHD prophylaxis. PTCy can eliminate most host-reactive donor T cells. Abatacept (CTLA4Ig) blocks the costimulatory signals given to naïve T cells during immune reconstitution, thus favoring an anergic phenotype and promoting tolerance towards recipient derived antigens. CTLA4Ig gives an activating signal to NK cells and has therefore the ability to preserve the graft-versus-tumor effect.

Methods: We conducted a prospective randomized clinical trial. Patients with hematologic malignancies in need of an allogeneic transplant and with an 8/8 matched related or unrelated donor were randomized 1:1 to tacrolimus and methotrexate for GVHD prophylaxis (SOC arm) or PTCy on days +3 and +4 followed by abatacept on an extended schedule: days +5, +14,+28, +56, +84, +112, +140 and +168 (Aba arm). Patients were stratified by conditioning regimen (myeloablative vs reduced intensity) and donor type (matched sibling vs matched unrelated donor). They received peripherally collected stem cells. The primary endpoint was moderate and severe chronic GVHD at 1 year and the secondary endpoints overall chronic GVHD, Grade II-IV and III-IV acute GVHD rate, overall (OS), disease-free (DFS), and GVHD-relapse-free-survival (GRFS) at on year, transplant related mortality and infection rate. The study stopped enrolling on the SOC arm when abatacept was approved for GVHD prophylaxis and the institutional SOC changed to include abatacept.

Results: 40 patients were treated on this study, 25 of which on the aba arm. There were two cases of secondary graft failure (one on each arm) that were excluded from further analysis other than OS. The trial met its primary end point: Kaplan-Meier estimates of moderate/severe cGVHD were 0% on the aba and 65.8% on the SOC arm (p < 0.0001). GRFS was 58.3% in the aba and 21.4% in the SOC arm (p=0.043). There were no treatment related deaths on the aba and two on the SOC arm. There was no statistically significant difference in the rates of OS (92% aba, 80% SOC, p=0.28), DFS (62.5% aba, 85.7% SOC, p=0.103) and infection. Grade III-IV acute GVHD rate was 4.1% on the aba and 20% on the SOC arm (p=0.132), whereas Grade II-IV 12.5% on aba and 35.7% on SOC (p=0.117). There were no cases of acute GVHD of the gut or liver on the aba arm. Overall chronic GVHD rate (including mild) was 9.1% on the aba and 74% on the SOC arm (p<0.0001).

Conclusions: The calcineurin inhibitor-free combination of PTCy and abatacept for GVHD prophylaxis is feasible, well tolerated and associated with a very low rate of chronic GVHD and a favorable GRFS rate.