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4901 Abatacept-Prophylaxis Based Haploidentical Transplantation May Allow Sustained Engraftment and Offset Gvhd in Non-Malignant Disorders

Program: Oral and Poster Abstracts
Session: 721. Allogeneic Transplantation: Conditioning Regimens, Engraftment and Acute Toxicities: Poster III
Hematology Disease Topics & Pathways:
clinical trials, Research, Clinical Research, patient-reported outcomes
Monday, December 11, 2023, 6:00 PM-8:00 PM

Alexander I. Ngwube, MD1*, Niketa C. Shah, MD2, Ginny Schulz3*, Lakshmanan Krishnamurti, MD4 and Shalini Shenoy, MD, MBBS5

1Center for Cancer and Blood Disorders, Phoenix Children's Hospital, Phoenix, AZ
2Department of Pediatric Hematology Oncology and Stem Cell Transplant, Yale University, New Haven, CT
3St Louis Children's Hospital, St Louis, MO
4Section of Pediatric Hematology, Oncology and Bone Marrow Transplant, Yale School of Medicine, Atlanta, GA
5Washington University Medical Center, Saint Louis, MO

Introduction: Non-malignant disorders (NMD) derive no benefit from graft-versus-host disease (GVHD) after allogeneic hematopoietic transplantation (HCT). Transplantation designed to optimize engraftment and immune reconstitution while minimizing GVHD and organ toxicities is desirable. Post-transplant cyclophosphamide (PTCy) decreases severe chronic GVHD risk in radiation-containing regimens for NMD when combined with serotherapy (anti-thymocyte globulin) (PMID: 22955919; PMID: 30500440). Abatacept effectively offsets GVHD following unrelated donor (URD) HCT (PMID: 33449816; PMID: 32813873). Based on these reports, NCT03128996 was developed for haploidentical transplantation in NMD after radiation-free reduced intensity conditioning (RIC) combining PTCy + Abatacept for GVHD prophylaxis. Serotherapy does not contribute to GVHD prophylaxis in this RIC approach. Outcomes focused on safety and GVHD- free donor engraftment in children with NMD and no better donor options.

Method: Conditioning included alemtuzumab (day -22 to -19), fludarabine (day -8 to -4), melphalan (day -3) and thiotepa (day -4) as previously published (hydroxyurea prior to RIC was used in proliferative marrow disorders) (PMID: 32813873). Stem cell source was bone marrow (BM) or CD34-selected peripheral blood cells with a fixed CD3 cell dose of 1x10^6/kg recipient weight. GVHD prophylaxis included tacrolimus (day -1, with wean beginning 6-9 months post-HCT), PTCy on days +3 and +4, mycophenolate mofetil until day +28, and monthly abatacept doses until 13 months post-HCT. Survival, safety, donor chimerism, GVHD incidence, and incidence of post HCT infection were primary outcome measures.

Results: Eleven patients 6 to 24 years old (median 9 years) underwent HCT for adrenoleukodystrophy (2), sickle cell disease (SCD) (6), aplastic anemia (1), second transplant for severe combined immunodeficiency (1) and STAT 5B gain-of-function mutation (1). Eight were trial participants and 3 treated according to trial guidelines. All patients tolerated abatacept infusions and engrafted with neutrophil recovery (>500/cu mm) at a median of 17 days (range 15-24 days). As of 7/31/2023, median follow up is 411 days (range 83-1979 days). The mean myeloid and T-cell chimerisms at D100 (n=10) and 1 year (n=6) were 97.8%+/- 5.3 and 96.3% +/- 6.4 and 98.6% +/- 3.3 and 95% +/-8.7, respectively. One patient with SCD has low myeloid engraftment (24%) at 7 months post-HCT (lymphoid 91%); a stem cell boost is planned after immune suppression with fludarabine/Cytoxan/ATG (PMID: 35798858). Viral reactivation was noted in 4 patients, none developed CMV disease or EBV post-transplant lymphoproliferation. One patient developed grade 1 acute GVHD. Of 6 evaluable patients, one developed localized chronic skin GVHD. This patient remains on systemic immune suppression beyond 13 months (ruxolitinib). The overall and GVHD-free survival at 1-year post HCT in evaluable patients (N=6) is 100 % and 83%, respectively.

Conclusion: Promising preliminary results in this cohort support continuation of the trial with longer term follow-up. The effective suppression of GVHD without serious infectious complications in the first year post-HCT is encouraging. Virus reactivation was however noted early post-HCT and requires monitoring and prophylaxis. Incorporating abatacept into GVHD prophylaxis was successful in achieving desired outcomes; the phase 1 portion of the study is completed and the phase 2 trial and follow-up are ongoing.

Disclosures: No relevant conflicts of interest to declare.

*signifies non-member of ASH