Efficacy and Safety of Humanized Chimeric Antigen Receptor (CAR)-Modified T Cells Targeting CD19 in Children with Relapsed/Refractory ALL

Background

Targeted immunotherapy with T cells expressing a chimeric antigen receptor (CAR) can produce dramatic anti-tumor responses. We previously demonstrated complete remissions (CRs), prolonged persistence, and sustained responses in children and adults with relapsed/refractory (r/r) acute lymphoblastic leukemia (ALL) treated with CD19-specific CAR-modified T cells (CTL019). However, a subset (~25%) of patients has limited persistence of CTL019, which can increase the risk of relapse. We hypothesized that repeat infusion may prolong persistence in some patients but would be ineffective in patients with immune-mediated rejection. As most CAR single chain variable fragment (scFv) domains, including that of CTL019, are of murine origin, anti-mouse reactivity is one potential cause of immunogenicity that may be overcome by fully human or humanized CAR design. We now report on retreatment with murine or humanized CD19-directed CAR T cells.

Aims

We sought to 1) prolong persistence by reinfusion of murine CTL019 in children with r/r CD19+ ALL and evidence of poor persistence and 2) determine the safety and efficacy of CD19-directed CAR T cells with a humanized scFv domain in children with B cell recovery or CD19+ relapse after prior CAR T cell therapy.

Method

T cells collected from patients were transduced with a lentiviral vector encoding a CAR composed of murine or humanized anti-CD19 scFv, CD3z, and 4-1BB domains, activated/expanded ex vivo with anti-CD3/anti-CD28 beads, cryopreserved, and then infused. The humanized scFv was developed by grafting the complementary determining regions of both the heavy and light chains onto human germline acceptor frameworks.

Results

Of 50 patients in CR at 1 month after CTL019 infusion (50/53 CR), 14 patients received a repeat infusion of murine CTL019 at 3 and/or 6 months after initial infusion and 11 are evaluable for response. Indications were B cell recovery (n=4), CD19+ minimal residual disease (MRD, n=2), or undetectable CTL019 in peripheral blood by flow cytometry (≤0.1% CTL019+ of CD3+, n=5). Minimal toxicities were observed and included fever and fatigue. CTL019 reinfusion induced B cell aplasia for a second time in 1 of 4 children treated for B cell recovery. Two of these patients later relapsed, 1 with CD19+ and 1 with CD19- disease. Of 2 patients with CD19+ MRD, 1 had no response and subsequently experienced a CD19+ relapse and the second became MRD-negative but had B cell recovery. All 5 children reinfused for poor CTL019 persistence demonstrated continued B cell aplasia 3-15 months after repeat infusion with detectable CTL019 in peripheral blood by flow cytometry (0.2-2.8% CTL019+ of CD3+) in 4 patients. Of this group, 4 patients remain in remission 12-21 months after initial infusion, and 1 experienced a CD19- relapse.

To overcome the potential for anti-murine immune-mediated rejection, patients previously treated with CAR-modified T cells were eligible for a phase 1 study of humanized CD19-directed CAR T cells (CTL119). Five children were infused for B cell recovery (n=3) or CD19+ relapse (n=2). Cytokine release syndrome (CRS), seen in 2 patients, was limited to fever, headache, and nausea and did not require vasopressor or respiratory support. Three patients showed no response and progressed (n=1) or relapsed with CD19+ (n=1) or CD19- (n=1) ALL. Responses were demonstrated in 2 patients: 1) MRD-negative CR in a patient treated for CD19+ relapse and 2) reduction in MRD and return of B cell aplasia in a patient treated for CD19+ MRD and B cell recovery, with subsequent progression to CD19+ relapse. Both responding patients were previously resistant to murine CTL019 reinfusion, suggesting a possible immune-mediated rejection mechanism. Ongoing studies are investigating anti-CAR responses.

Conclusion

Repeat infusion of murine CTL019 may prolong B cell aplasia in patients with early evidence of poor persistence and in a fraction of patients with B cell recovery. In the first demonstration of humanized anti-CD19 CAR efficacy, retreatment with CTL119 expressing a humanized scFv induced remission of ALL refractory to prior CD19-directed murine CAR T cell therapy, suggesting that immune-mediated rejection may be an important mechanism of resistance in the subset of patients with rapid CAR cell loss and loss of B cell aplasia. Further investigation into anti-CAR responses will be vital to improve durable remission rates in this highly refractory population.