Post-Transplant Cyclophosphamide, Abatacept, and Short Course of Tacrolimus Combination (CAST) Is Safe and Seems Highly Effective in Preventing Graft-Versus-Host Disease Following Haploidentical Peripheral Blood Stem Cell Transplantation

The introduction of post-transplant cyclophosphamide (PTCy) has circumvented the need for T-cell depletion following haploidentical stem cell transplantation (SCT). By expanding the donor pool for patients from certain ethnic minorities, this has addressed to some degree an important health care disparity issue in SCT. However, a recent registry study showed increased incidence GvHD and inferior outcomes in patients receiving haploidentical SCT with PTCy, tacrolimus and mycophenolate mofetil for GvHD prevention as opposed to matched unrelated donor SCT with PTCy-based GvHD prevention. Seeking to improve the results of GvHD prevention in the setting of haploidentical SCT, we examined a combination of PTCy, abatacept and a short course of tacrolimus (CAST). Abatacept is a recombinant soluble fusion protein composed of the extracellular domain of cytotoxic T-lymphocyte associated antigen-4 (CTLA-4) fused to the Fc region of IgG1. Abatacept blocks CD28-CD80I86 axis and prevents T-cell co-stimulation. In early studies, abatacept has shown promising results when added to methotrexate and tacrolimus in matched and mismatched donor SCT. We initiated a phase Ib-II clinical trial for patients with hematological malignancies undergoing haploidentical SCT. Patients received G-CSF mobilized peripheral blood grafts from related haploidentical donors. GvHD prevention consisted of PTCy 50mg/kg IV on day +3 and +4 with forced hydration, abatacept 10mg/kg IV on day +5, +14 and +28 and tacrolimus. Tacrolimus was started on day +5 at 0.02mg/kg/day by continuous IV and adjusted thereafter to maintain a trough level of 5-12ng/mL. Tacrolimus taper was planned to begin on day +60 and complete by day +90 in the absence of GvHD. All patients received standard supportive care including levofloxacin until neutrophil engraftment, posaconazole until day +75, acyclovir for 1 year and, if CMV positive by serology, letermovir until day +100. Pneumocystis Jiroveci prophylaxis was started after neutrophil engraftment and continued until 6 months post-transplant. G-CSF was administered routinely until neutrophil engraftment.

Since September 2020, 19 patients were enrolled. Three patients are too early in their post-transplant course and were excluded from this analysis. Patients' characteristics are summarized in the table. All but 2 patients received cryopreserved products. Median times to ANC and platelet engraftment were 18.5 days (14-30) and 28.5 (16-61). All 16 patients achieved full whole blood donor chimerism by day +30. There was no secondary graft failure. With a median follow-up was 149.5 days (41-308) with 10 patients having >120 days and 8 >180 days of follow-up, 4 patients developed skin acute GvHD (all grade I). No patient developed grade II-IV acute GvHD. Two patients developed skin chronic GvHD (limited, both moderate). Both cases were diagnosed following COVID-19 vaccination. Fifteen patients completed tacrolimus taper by day +90. Two patients received systemic steroids, one for treatment of cGvHD. The remaining patients required no further immunosuppressive therapy beyond day +90. CMV activation rate was 25%. One patient had EBV reactivation and required preemptive therapy with 2 weekly rituximab doses. There were no cases of adenovirus, HHV-6 virus or BK virus reactivation. Four patients developed renal insufficiency (3 in the setting of acute sepsis and 1 with thrombotic microangiopathy, which resolved after tapering off tacrolimus. One patient with adult T-cell leukemia/lymphoma relapsed and died. All other patients are alive and well.

In summary, our preliminary results suggest that CAST with shortened course of tacrolimus is feasible and seems to offer very promising outcomes with low rates of acute GvHD. The study is accruing actively and the results of a larger cohort with longer follow-up will be presented at the meeting. If confirmed, by improving the outcomes of haploidentical SCT, this regimen may further address a health care disparity issue, offering almost every patient in need of allogeneic SCT an alternative donor option with equal outcomes.

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