CD28-Directed T Cell Costimulation Blockade with Abatacept to Prevent GvHD During High-Risk Unrelated HSCT: A First-in-Disease Feasibility Trial,
Authors: Kean Leslie S. , Langston Amelia , Qayad Muna , Khoury H. Jean , Tiwari Divya , Couture Cynthia , Renfroe Heather , Sen Sharon , Robertson Jennifer , Waller Edmund K , Horan John T. 
Published: October 2019

Abstract
Abstract 4078

Background:
Acute GvHD remains the major cause of complications and death following unrelated-donor HSCT. In a non-human primate model, we have previously shown that in vivo costimulatory blockade of donor T-cells could provide effective protection against GVHD. To begin to explore its clinical utility, we are conducting a trial (Clinical Trials.Org # NCT01012492) to determine the feasibility of combining abatacept (CTLA4-Ig) with cyclosporine and methotrexate as acute GVHD prophylaxis for patients undergoing unrelated marrow and peripheral blood stem cell transplants for hematologic malignancies.

Methods:
Patients older than 12 with advanced hematologic malignancies, conditioned with either TBI/Cytoxan, Busulfan/Cytoxan or Fludarabine/Melphalan are eligible. Abatacept is administered IV on days −1, +5, +14, and +28 at 10 mg/kg in addition to standard GvHD prophylaxis consisting of cyclosporine (day −2 to day 100), and methotrexate (15 mg/m2 on day +1 and 10 mg/m2 on days +3, 6 and 11). Patients are then followed for clinical outcomes and immunologic reconstitution through day +365.

Results:
9 patients (planned enrollment = 11 patients) have thus far been enrolled on the study of which 5 are evaluable for engraftment, toxicity and acute GvHD. The other four patients consist of 2 who are currently receiving abatacept, 1 who was discovered to have an ongoing viral infection at the start of the first abatacept infusion so was removed from the treatment regimen, and 1 who is awaiting transplant.
The median age for the 5 evaluable patients is 47 years (17–74 years). 3 patients had AML and 2 had ALL. Patients were conditioned with Bu/Cy (n=1), TBI/Cy (n=2) and Flu/Melphalan (n=2). 4 donor-recipient pairs were allele matched at 9 of 10 loci (A, B, C, DRB1 and DQB1), while 1 was fully matched. Four of the 5 patients are currently alive and in remission and 1 relapsed at day +98 (and died on day +121 with refractory AML). The four other patients are surviving without relapse with a follow-up of 155–313 days.
All 5 patients received the 4 scheduled abatacept doses. No infusional side effects were noted. All patients achieved neutrophil engraftment (median day +20 (11–47). 4 of 5 patients have achieved platelet engraftment (median day +27 (14–35). Donor engraftment (100% CD33 and 99–100% CD3 at Day +30) occurred in all cases. All patients have demonstrated rapid lymphocyte engraftment, with the mean ALC reconstituting to >500 cells/μL by day +21 post-transplant. At day +100, the mean CD3+ count was 673 +/− 251 cells/μL. Both CD8+ and CD4+ T cells reconstituted by day 100, with the mean CD8+ count = 384 +/− 148 cells/μL and the mean CD4+ count = 229 +/− 119 cells/μL. T cell reconstitution was accompanied by a shift away from naïve (Tn, CCR7+/CD45RA+) toward a CCR7-/CD45RA- effector memory (Tem)-predominant phenotype. Thus, the average proportion of CD4+ Tem cells in the recipient increased from 22 +/− 6% pre-transplant to 46 +/− 7% at day +100 with a concomitant loss of CD4+ Tn cells. Likewise, the proportion of CD8+ Tem also significantly increased, from an average of 15 +/− 4% pre-transplant to 32 +/− 7% at day +100, also with a reciprocal decrease in CD8+ Tn cells.
One patient developed steroid responsive grade 3 acute GVHD involving the skin and the liver, followed by steroid responsive liver chronic GvHD. This patient is currently weaning corticosteroids. Another patient developed steroid responsive late-onset (day +217) acute GVHD (liver and GI) during cyclosporine weaning, which was also steroid responsive, and is also currently weaning corticosteroids. No other systemic acute or chronic GvHD has occurred.
No unexpected complications or life-threatening infections were observed. 3 patients have experienced 5 episodes of CMV reactivation, all responsive to antiviral therapy. One patient developed polyclonal EBV-related PTLD (plasmacytic hyperplasia) in the absence of EBV viremia, which regressed without intervention. No other EBV-related disease has occurred.

Conclusions:
These preliminary data suggest that abatacept can be safely added to cyclosporine and methotrexate for GVHD prophylaxis in recipients of hematopoietic grafts from unrelated donors, with encouraging rates of acute GVHD. As such, they support the conduct of a larger, randomized phase 2 study.

Disclosures:
Off Label Use: Abatacept: It is an immunosuppressive agent that targets the CD28/B7 T cell costimulation pathway. It is approved for use in Rheumatoid arthritis.