A Single Centre Cohort Study of Monitoring B Cell Populations in the Bone Marrow and Peripheral Blood Post Tisagenlecleucel Therapy of Relapsed/Refractory Acute Lymphoblastic Leukaemia

Introduction: CD19-targeting CAR T cell therapy has transformed the landscape of relapsed/refractory (r/r) B cell acute lymphoblastic leukaemia (ALL). Outcome data from multiple study and real-world cohorts suggest early loss of B cell aplasia (i.e. within 6 months of infusion) is associated with a high relapse risk, but the optimal therapeutic strategy is not clear, with consideration given to e.g. consolidative hematopoietic stem cell transplant (HSCT) in order to mitigate relapse risk. CAR T cell persistence is inferred from ongoing B cell depletion assessed by enumeration of normal CD19-expressing B-cells or B-cell precursors in the peripheral blood (PB) and bone marrow (BM)respectively. However, internationally-agreed consensus definitions for B cell aplasia in the context of CAR T cell therapy are lacking. Whilst CAR T cells can be detected by flow cytometric and PCR-based assays, these are complex to validate robustly because of differing sensitivity and specificity, harmonisation efforts are underway. We systematically investigated recovery of healthy B cells in the BM and PB in patients undergoing tisagenlecleucel therapy for B cell ALL at our centre and mapped these events to MRD emergence, relapse and further therapy provision. Methods: Patients deemed eligible for this study were all those treated with tisagenlecleucel for r/r B-cell ALL at our centre and who entered an MRD negative complete remission with or without haematological recovery (CR/CRi). B cell aplasia was defined as having less than 0.01 CD19+ B cells x10^9/L in the peripheral blood and less than 1x10^-3 CD19+ BM B cell precursors enumerated against CD45+ BM WBC events after red cell lysis. B cell recovery in either PB or BM was defined by detection of greater than or equal to 0.01 CD19+ B cells x10^9/L in the peripheral blood or 1x10^-3 CD19+ BM B cell precursors respectively Patients were routinely monitored for B cell populations in PB monthly for the first 12 months. BM +/- CSF was assessed for disease/measurable residual disease (MRD) by flow cytometry and immunoreceptor gene rearrangements by qPCR as well as normal B cell regeneration at months 1, 3, 6, 12 post CAR T cell infusion. Patients were followed for recovery of B cells in the PB or BM until one of the following events: emergence of MRD, frank relapse or death in remission. Therapy (HSCT or maintenance chemotherapy) instigated for B cell recovery in patients remaining in an MRD negative CR/CRi was documented. Results: 46 patients meeting the eligibility criteria for this study were treated with tisagenlecleucel between January 2019 and May 2023. Patient characteristics are given in Table 1. Events for 28 patients with detection of normal B cells in the BM or PB at any time point post CAR T cell infusion are shown in the swimmer plot in Figure 1. Of 46 treated patients, 2 had not achieved B cell aplasia at 1 month post infusion. Of the remaining 44 patients, 26 subsequently (59%) had B cell recovery suggesting loss of CAR T cell persistence; 18 (41%) have B cell aplasia. Of 26 patients with B cell recovery, 16 (62%) recovered B cells in BM before PB with a median of 3 months (range 0-22 mo) between these. 8 (31%) patients had recovery of B cells in BM and PB contemporaneously and 1 (8%) had recovery detected in PB only at 22 mo post infusion Of 16 patients with first detection of BM B cells, 8 (50%) went on to have with HSCT or maintenance chemotherapy (for those with a contraindication to HSCT) whilst in an MRD neg remission, 1 had a repeat CART infusion, 2 had frank relapse, 1 developed secondary AML. 7 of the 9 treated patients are alive in MRD neg remission Of the 8 patients with recovery in both PB and BM compartments, 2 went on to have HSCT, 4 maintenance chemotherapy and 2 had second infusion. 4 remain alive and in MRD neg remission Of 46 patients achieving B cell aplasia, 1 patient had a frank CD19+ relapse at 648 days without prior detection of normal B cell recovery. 3 other patients without B cell recovery were noted to have emergence of MRD at 84, 93, and 206 days (CD19+, CD19- and CD19 unknown respectively) Conclusions: Following BM B cell populations can provide a window of opportunity to intervene before MRD emergence or relapse for patients with early B cell recovery suggesting loss of CAR T cell persistence.