Humanized CD19 chimeric antigen receptor (CAR) T-cell therapy for high-risk and post-CAR relapse of B-cell acute lymphoblastic leukemia Free

Background CD19-directed chimeric antigen receptor T-cell (CART19) therapy has transformed the treatment landscape for relapsed/refractory (r/r) B-cell acute lymphoblastic leukemia (B-ALL). Yet relapse occurs in approximately 50% of recipients. These patients and subgroups of pediatric patients in first relapse, currently ineligible for commercial CART19, have very poor outcomes with current approaches, warranting investigation of alternative strategies.

We previously investigated a humanized CART19 (huCART19) in a Phase I trial with encouraging responses in both CAR-naïve and CAR-exposed cohorts (Myers JCO 2021). Here we report outcomes from a Phase II clinical trial (NCT03792633) of huCART19 for high risk r/r B-ALL, including early bone marrow (BM) relapse, a subgroup with historically poor outcomes despite intensive therapy.

Methods Patients aged 0-29 years (y) with CD19+ B-ALL were eligible in 2 cohorts: CAR-naïve patients with B-ALL that is refractory, in high risk first relapse, second or greater relapse, or relapsed after or ineligible for hematopoietic stem cell transplant (HSCT); CAR-exposed patients with poor response to prior cell therapy. Patients received huCART19 at a dose of 5x10⁶ CAR T cells/kg (maximum 2.5x10⁸) after lymphodepletion (LD) with fludarabine and cyclophosphamide. Response was assessed on day 28 by BM and cerebrospinal fluid morphology, minimal residual disease (MRD) by flow cytometry, and biologic response by B cell aplasia (BCA). The primary endpoint was event-free survival (EFS). Secondary endpoints included overall response rate (ORR), defined as rate of complete remission (CR) or CR with incomplete hematologic recovery (CRi) with BCA, and relapse-free survival (RFS). Data cutoff was July 1, 2025.

Results Of 106 patients enrolled, 100 were infused with huCART19 from 3/2019-8/2023. The median age at infusion was 12y (range 1-29), 44% were female, 20% Hispanic, 5% Asian, 7% Black, and 6% had Trisomy 21. Prior therapy included HSCT in 21%, blinatumomab in 21% and inotuzumab in 15%.

The CAR-naïve cohort (n=52) included 25 with first early BM relapse within 36m of diagnosis (12 <18m). On pre-infusion BM performed post-LD, 13/52 (25%) had >25% blasts, 25/52 (48%) <0.01%. Three patients were inevaluable for response: 1 deemed ineligible due to myeloid lineage switch on pre-infusion BM; 1 died on day 2; 1 lost to follow-up at day 28. By day 28, 46/49 (94%) were in CR/CRi, 45/46 MRD-negative, 1 MRD inevaluable. With a median follow up of 51m, EFS and RFS at 2y were 65% (95% CI 52-81%) and 70% (95% CI 57-86%), and at 4y, 57% (95% CI 42-76%) and 62% (95% CI 47-81%). Relapse occurred in 14/46 (30%), of which 7 (50%) were CD19-. Ten patients pursued alternative therapy in remission, 7 for loss of BCA and 3 for MRD recurrence (2 CD19+, 1 CD19-). Overall survival (OS) at 2y and 4y was 74% (95% CI 63-87%).

In a subgroup analysis of CAR-naïve patients treated for early BM relapse, ORR was 21/23 (91%), 2y EFS was 39% (95% CI 21-72%), RFS 45% (95% CI 25-81%), and OS 57% (95% CI 40-81%).

The CAR-exposed cohort (n=48) included 20 with post-CART19 relapse and 28 with early (<6m) loss of BCA without relapse. On pre-infusion BM, 5/48 (10%) had >25% blasts, 34/48 (71%) <0.01%. By day 28, 43/48 patients were in CR/CRi, 43/43 MRD-negative; 6/43 did not establish BCA resulting in an ORR of 37/48 (77%). With a median follow up of 43m, EFS and RFS at 2y were 53% (95% CI 39-71%) and 72% (95% CI 57-91%), and at 4y, 50% (95% CI 36-68%) and 68% (95% CI 52-88%). Relapse occurred in 9/37 (24%), of which 4 (44%) were CD19-. Eleven patients received alternative therapy in remission, 6 for loss of BCA, 4 for CD19+ MRD recurrence, and 1 for therapy-related myeloid neoplasm. OS at 2y and 4y was 77% (95% CI 66-90%).

CRS was reported in 47/52 (90%, 10 grade [Gr] 3, 5 Gr 4 on Penn scale) CAR-naïve patients and 38/48 (79%, 1 Gr 3, 2 Gr 4) CAR-exposed. There was 1 death prior to day 28, due to gastrointestinal hemorrhage on day 2 in the setting of progressive ALL and Gr 2 CRS. CAR neurotoxicity was reported in 14/52 (27%, 2 Gr 3, 2 Gr 4) CAR-naïve patients and 7/48 (15%, 1 Gr 3, 1 Gr 4) CAR-exposed, with 1 case of Gr 3 cerebral edema, fully recovered, and 1 case of ongoing myelopathy.

Conclusions HuCART19 produced durable remissions in high risk r/r B-ALL and demonstrated efficacy as salvage therapy for those with poor response to prior CAR therapy, comparing favorably to historical outcomes in this extremely high risk group.