Characterization of Extramedullary Disease in B-ALL and Response to CAR T-cell Therapy

Abstract

Chimeric antigen receptor (CAR) T-cells effectively eradicate medullary B-cell acute lymphoblastic leukemia (B-ALL) and can traffic to and clear central nervous system (CNS) involvement. CAR T-cell activity in non¬contral nervous system (CNS) extramedullary disease (EMD) has not been well-characterized. We systematically evaluated CAR T-cell kinetics, associated toxicities, and efficacy in B-ALL non-CNS EMD. We conducted a retrospective review of B-ALL patients with non-CNS EMD who were screened for/enrolled on one of three CAR trials at our institution (CD19, CD22, CD19/22). Non-CNS EMD was identified by histology or radiographic imaging at extramedullary sites excluding the cerebrospinal fluid and CNS parenchyma. Of approximately 180 patients with relapsed/refractory B-ALL screened across multiple early phase trials over an 8-year period, 38 (21.1%) presented with isolated non-CNS EMD (n=5) or combined medullary/non-CNS EMD (n=33) on FDG PET-CT imaging. A subset receiving CAR T-cells (18 infusions) obtained FDG PET-CT scans pre- and post-infusion to monitor response. At best response, 72.2% (13 of 18) of patients demonstrated a medullary MRD-negative complete remission and complete (CR, n=7) or partial (PR, n=6) non-CNS EMD response. Non-CNS EMD responses to CAR T-cells were delayed (n=3) and residual non-CNS EMD was substantial; rarely, discrepant responses (marrow without EMD response) were observed (n=2). Unique CAR-associated toxicities at non-CNS EMD sites were seen in select patients. CAR T-cells are active in B-ALL non-CNS EMD. Still, non-CNS EMD response to CAR T-cells may be delayed and sub-optimal, particularly with multifocal disease. Serial FDG PET-CT scans are necessary for identifying and monitoring non-CNS EMD.