Simultaneous Targeting of CD19 and CD22: Phase I Study of AUTO3, a Bicistronic Chimeric Antigen Receptor (CAR) T-Cell Therapy, in Pediatric Patients with Relapsed/Refractory B-Cell Acute Lymphoblastic Leukemia (r/r B-ALL): Amelia Study

Introduction CAR T-cell therapies directed against CD19 or CD22 antigens have shown significant activity in pediatric patients with r/r B-ALL. Whilst complete response (CR) rates of 70‒90% have been observed, relapse due to target antigen downregulation or loss is the major cause of treatment failure. This Phase I/II study evaluates the safety and efficacy of AUTO3, a CAR T-cell therapy designed to target CD19 and CD22 simultaneously in order to reduce the likelihood of relapse due to antigen loss.

Methods & Patients

We constructed a novel bicistronic retroviral vector encoding both an anti-CD19 CAR and an anti-CD22 CAR. Antigen binding domains were humanized and both CARs are in "2nd generation" format (incorporating an OX40 co-stimulatory domain for the CD19 CAR and a 41BB for the CD22 CAR). The performance of the CD22 CAR was optimized by incorporating a novel pentameric spacer. The cell product was manufactured on a semi-automated and closed process using CliniMACS® Prodigy (Miltenyi Biotec).

Patients (1‒24 years of age) with high risk relapse (IBFM criteria) or relapse post-allogeneic stem cell transplant (SCT), adequate performance score/organ function, and an absolute lymphocyte count ≥0.5 x 10⁹/L are eligible. Patients with CNS 3 disease, active graft versus host disease, and clinically significant infection or serious toxicity from prior CAR T-cell are excluded.

Patients receive lymphodepletion with 30 mg/m²/day fludarabine x 4 days and 500 mg/m²/day cyclophosphamide x 2 days prior to AUTO3 infusion. Three dose levels are being explored (1 x 10⁶, 3 x 10⁶, and 5 x 10⁶ transduced CAR⁺ T cells/kg) and CAR T cells are infused as a single (for <25% blasts) or split (for >25% blasts) dose based on leukemia burden. Bridging therapy is allowed during the manufacturing period. The primary endpoint of Phase I is the frequency of dose-limiting toxicities (DLTs) and key secondary endpoints include proportion of patients achieving a morphological/minimal residual disease (MRD) negative CR, disease-free survival, overall survival, as well as biomarker endpoints including AUTO3 levels and persistence in blood and bone marrow.

Results

As of the data cut-off date (July 16, 2018), 9 patients have been enrolled and 8 have received AUTO3. It was possible to generate a product in all patients and the median transduction efficiency was 16% (range 9‒34%). Median age was 7.5 years (range 4‒16 years) and 5 (63%) patients had prior SCT. One patient (13%) had prior anti-CD19 CAR-T cells and blinatumomab. The disease burden at Day ‒7 ranged from 0% to 90% leukemic blasts.

Eight patients had a minimum of 4 weeks' follow up and were evaluable for safety and efficacy analysis. Three patients received ≤1 × 10⁶ CAR T cells/kg as single dose, 1 patient received 2 × 10⁶/kg as split dose, and 4 received 3 × 10⁶ CAR cells/kg (3 single infusions, 1 split). No AUTO3 related deaths and no DLTs were observed. The most common grade (Gr) ≥3 adverse events were neutropenia (63%), febrile neutropenia (50%), pyrexia (25%), and anemia (25%). Five patients (63%) had Gr 1 cytokine release syndrome (CRS); no Gr 2 or higher CRS was seen. Five patients (63%) experienced neurotoxicity: 4 had Gr 1 and 1 patient (13%) had Gr 3 encephalopathy that was considered likely related to prior intrathecal methotrexate. No patients required ICU admission.

Six of 8 patients achieved MRD negative CR, giving an objective response rate of 75% (95% CI 34.9‒96.8%) at 1 month. In patients treated at doses <3 x 10⁶/kg, 3 responded but subsequently relapsed. Importantly, no loss of CD19 or CD22 was noted in patients that relapsed. All 4 patients treated at the higher dose of 3 × 10⁶ CAR T cells/kg had an MRD negative CR with ongoing remission and B-cell aplasia, with the longest follow up of 4 months. CAR T-cell expansion was enhanced in patients receiving 3 x 10⁶/kg (median 79,282

copies/µg DNA in blood at peak) compared to those receiving lower doses (median 10,243 copies/µg DNA).

Conclusion

This interim data analysis demonstrates for the first time the feasibility and safety of simultaneous targeting of CD19 and CD22 with AUTO3. Promising efficacy was demonstrated at a dose level of 3 × 10⁶ CAR T cells/kg, as 4/4 patients achieved MRD complete remission with no antigen negative escape at this early stage. The study continues to enrol and updated follow up and additional patient data at higher dose levels, as well as cellular kinetics and additional biomarker analysis, will be presented.