Phase 1 Study of SC291, a Hypoimmune, Allogeneic CD19-Directed CAR T Cell Therapy for Relapsed/Refractory B-Cell Malignancies (ARDENT) - Initial Clinical Data

ARDENT is a phase 1 study evaluating safety and tolerability of SC291, a hypoimmune (HIP), allogeneic, CD19-directed CAR T cell therapy in subjects with Non-Hodgkin's Lymphoma and Chronic Lymphocytic Leukemia (CLL). SC291 is derived from healthy donor CD4+ and CD8+ T cells that are genetically engineered to disrupt function of CD3 and HLA class I/II, overexpress CD47, and express a CD19-directed CAR. A primary obstacle for allogeneic CAR T cell approaches remains their immune recognition and rejection. Patient immune responses prevent durable allogeneic CAR T clinical responses despite the use of enhanced lymphodepletion (LD) regimens. We therefore developed hypoimmune CAR T cells that avoid immune recognition and have the potential to deliver clinical efficacy while using a standard LD chemotherapy regimen (as defined by approved autologous CAR T products). The HIP engineering approach involves the depletion of HLA class I/II to prevent adaptive immune rejection and the overexpression of CD47 to prevent innate immune rejection, which has previously demonstrated alloimmune protection in preclinical models ( Nat Biotechnol 2019;37(3):252-258; Proc Natl Acad Sci U S A 2021;118(28):e2022091118; J Exp Med. 2021;218(3):e20200839; Nat Biotechnol. 2023; doi: 10.1038/s41587-023-01784-x, and Nat Commun. 2023;14(1):2020). The initial patient, a 74-year-old male with CLL (unmutated IGHV, del(11q), BTK C481S mutation) and 3 prior lines of therapy (FCR, ibrutinib, venetoclax + rituximab), received a LD regimen of cyclophosphamide 500 mg/m 2 and fludarabine 24 mg/m 2 (daily for 3 days) followed by a starting dose of 60 million CAR+ SC291 cells (of which approximately 80% are fully HIP engineered cells). SC291 was well tolerated with no observed CRS or ICANS. The patient developed non-neutropenic fever after LD (prior to SC291 infusion). At the Day 28 visit post-SC291 infusion, the patient had a partial response (per iwCLL 2018 criteria), along with significant (99%) B-cell reduction and improvements in platelet levels. Analysis of cellular kinetics is ongoing. SC291 is a mixture of T cells, in which portions are partially or fully HIP-engineered. As such, we evaluated the initial patient's immune response against SC291 subpopulations using a panel of previously described in vitro assays. Blood samples from the patient were taken pre-treatment (Day -5) and at the Day 13 and Day 28 visits following SC291 treatment and were assessed for immune evasion. The patient's PBMCs were sorted into CD3+ T cells and CD3-CD56+ NK cells and incubated with the following subpopulations sorted from SC291 drug product (DP): (i) fully HIP engineered (CD19 HIP CAR T cells), (ii) HLA I/II negative T cells without CD47 overexpression (DKO), and (iii) HLA I/II positive CAR T cells overexpressing CD47 (WT CD19 CAR T cells). Patient serum from Day -5 and the Day 28 visit were used for donor-specific antibody analyses, complement-dependent cytotoxicity (CDC), and antibody-dependent cellular cytotoxicity (ADCC) against the DP subpopulations. At Day -5, T cell EliSpot assays showed no immune responses against any of the SC291 sorted subpopulations, but by Days 13 and 28, these assays produced high and increasing IFN-g spot frequencies against the WT CD19 CAR T cells. Similar results were shown with T cell cytotoxicity assays (Fig. 1B). In contrast, no T cell activation and no T cell killing was observed with DKO and CD19 HIP CAR T cells at any time point. As expected, patient NK cells vigorously killed DKO T cells at all time points but spared WT CD19 CAR T and CD19 HIP CAR T cells (Fig. 1B). No antibodies against any sorted subpopulation were detected at Day -5, but IgG antibody binding of the WT CD19 CAR T population was observed at Day 28. These antibodies did not bind to DKO or CD19 HIP CAR T cells, and there was no CDC or ADCC response against these subpopulations. Interestingly, with the LD regimen used, we observed a 95% and 99% reduction of CD19+ cells on Days 0 and 28, respectively, while T cells rebounded from an 88% reduction on day 0 to pretreatment levels on Day 28 (Fig. 1A). These data demonstrate the preliminary safety and tolerability of SC291 in the initial treated patient. Immune assays demonstrated that the CD19 HIP CAR T cell subpopulation effectively evades the host adaptive and innate immune responses and could overcome the allogeneic barrier in humans. Additional data from the ARDENT study will be presented at the time of the conference.