Safe and Effective Combination of Donor-Derived, Allogeneic CD19/CD22-CAR T Cells with Myeloablative Graft-Engineered Allo-HCT for High-Risk B-ALL

Background: High-risk B-cell acute lymphoblastic leukemia (B-ALL) remains incurable for many patients despite treatment with autologous CAR T cells or allogeneic hematopoietic cell transplant (allo-HCT). We previously demonstrated that CAR T cells can be safely administered following allo-HCT without causing graft-versus-host disease (GVHD) in a preclinical murine model (Ghosh A, Smith M et al. Nat Med 2017). Hence, we hypothesized that adding allogeneic CD19/CD22-CAR T cells to a myeloablative allo-HCT containing donor T regulatory (Treg) cells (Orca-T) would augment graft versus leukemia without increasing acute GVHD or graft failure. We administered a CD19/CD22-CAR that was previously administered as an autologous CAR (NCT03233854) to patients with B-ALL where it demonstrated a high objective overall response but limited durable remission (Spiegel, J et. al. Nat Med 2021). Here, we report clinical outcomes of the initial dose escalation of donor-derived, allogeneic CD19/CD22-CAR T cells with Orca-T in our novel clinical trial design (NCT05507827).

Methods: This is a single-center, phase 1, dose escalation and expansion trial of allogeneic CD19/CD22-CAR T cells in adults with high-risk B-ALL. Myeloablative conditioning with cyclophosphamide and total body irradiation precede infusion of investigational Orca-T, which consists of infusions of HSPCs and Tregs on Day 0 and an infusion of T conventional (Tcon) cells on Day 2. Allogeneic CD19/CD22-CAR-T cells are also infused on Day 2. Single agent pharmacologic GVHD prophylaxis with tacrolimus starts on Day 3. Three dose levels (DL) of allogeneic CD19/CD22-CAR T cells are being studied: DL1 (1x10⁶), DL2 (2x10⁶) and DL3 (3x10⁶) CAR+ cells/kg. The primary objective is to evaluate safety and feasibility of administering allogeneic CD19/CD22-CAR T cells in combination with Orca-T. The primary endpoint is the incidence of engraftment without Grade III to IV acute GVHD at Day 42. Key secondary objectives include efficacy outcomes, CAR persistence, and immune reconstitution.

Results: To date, eight patients have been enrolled and treated: 3 on DL1, 3 on DL2, and 2 on DL3 of the dose escalation phase. The median age is 28 (range, 21-52); six (75%) patients are Hispanic. Regarding high-risk disease features, 2 patients had Ph-like ALL, 1 had TP53-mutated ALL, 7 were MRD+ post-induction, and 1 had relapsed disease. At enrollment, 3 (37%) were in an MRD- CR, 4 (50%) were in an MRD+ CR, and 1 (13%) had active disease. Seven (88%) received an HLA matched sibling donor allograft, while 1 (13%) patient received an allograft from a matched unrelated donor.

CAR T cells meeting the specified release criteria were successfully manufactured for all patients. All patients engrafted with median time to neutrophil and platelet recovery of 14 days (range, 12-20) and 15 days (range, 13-19), respectively. To date, no patients have experienced aGVHD, cGVHD, graft failure, or severe infectious complications. All patients experienced CRS (Grade 1, n= 7; Grade 2, n=1); there were no cases of ICANS.

At Day +42 post-treatment assessment, all patients demonstrated an MRD- CR as measured by flow cytometry and BCR::ABL1 PCR (Ph+ ALL), while 7/8 patients were MRD-negative by NGS (clonoSEQ). With a median follow-up of 190 days (range, 78-502, including 6 patients with >6 months of follow-up), all patients have an ongoing MRD- CR. At last assessment, whole blood and CD3 chimerism exceeds 95% in all patients.

All patients had good expansion of allogeneic CAR in the presence of immunosuppression with tacrolimus. Although the assessment of CAR T cell persistence is ongoing, 3/3 patients, all of whom were treated on DL1, have allogeneic CAR T cells detectable beyond one year. Six of the 8 patients treated will have at least one year of follow-up by the time of the meeting.

Conclusion: Here, we report very promising initial feasibility, safety and efficacy of the combination of allogeneic CD19/CD22-CAR T cells with Orca-T in patients with high-risk B-ALL. This paradigm shifting combination of allogeneic CAR T and allo-HCT resulted in 100% MRD- CR with full donor chimerism but without GVHD or severe CAR-mediated toxicity. These data, which demonstrate antigen-specific anti-tumor benefit of allogeneic CAR T cells in combination with GVHD prophylaxis mediated by Tregs and tacrolimus, have potential implications that could benefit patients with other hematologic diseases.