Impact of CAR-T cells expansion and tumor burden on the outcome after inotuzumab ozogamicin as bridging therapy in pediatric and young adult patients with relapse/refractory acute lymphoblastic leukemia. Free

Introduction: Bridging therapy (BT) before treatment with tisagenlecleucel (tisa) in patients with relapsed/refractory B-cell acute lymphoblastic leukemia (R/R B-ALL) aims to control disease progression with minimal toxicity during CAR-T cell manufacturing. Standard BT strategies include conventional chemotherapy (CT) and immunotherapy with inotuzumab ozogamicin (InO). The use of InO remains controversial due to its potential negative impact on CAR-T cell expansion and persistence, although this relationship has not been clearly established.

Methods: A retrospective observational study was conducted on 24 pediatric and young adult patients with R/R B-ALL treated with tisa. Clinical characteristics and in vivo CAR-T cell expansion were compared based on the type of BT received. Multiparametric Flow cytometry (MFC) was used to identify and evaluate different populations of CAR-T cells and lymphocytes of immune system.

Results: Ten from 24 patients (41.7%) received InO and 14 (58.3%) received CT/steroids. All patients achieved complete remission (CR/CRi) with B-cell aplasia at day +28 after CAR-T infusion, although duration of aplasia was shorter in the InO group (median 3.8 vs. 6 months, p = 0.29). Event-free survival (EFS) and overall survival (OS) at 12 months were 65.8% and 75.6%, respectively, with no significant differences between groups. In vivo CAR-T cell expansion was significantly lower in the InO group (10.2% vs. 30%, p = 0.02), even among low-burden patients (11.34% vs. 50.78%, p = 0.008). Notably, InO was more effective at reducing tumor burden: 80% of high-burden patients converted to low burden pre-infusion vs. 28.6% in the CT group (p = 0.032), with 60% achieving measurable residual disease (MRD) negativity vs. 14.3% in the CT group. In fact, when stratifying by tumor burden, patients with low disease burden (<5% blasts in bone marrow) before infusion had significantly better EFS (74.3% vs. 25%, p = 0.005). Moreover, patients who converted from high to low burden pre-infusion had a significantly improved 12-month EFS (81.8%) compared to those who remained high-burden (25%, p = 0.019), similar to patients who maintained low burden throughout (100%). In addition, CAR-T characterization revealed a higher proportion of CD8+ stem cell memory (SCM) T cells in patients who had received InO, which correlated with improved prognosis. In fact, patients maintaining long-term response had a greater proportion of CD8+ naïve and SCM CAR-T cells on day +28. Furthermore, a lower percentage of CD8+ SCM T cells (CAR+ or CAR−) in both peak CAR-T expansion and residual immune system on days +28 and +90 post-infusion was associated with reduced EFS.

Conclusion: BT with InO results in better pre-infusion tumor burden reduction and higher conversion rates to low burden, positively impacting EFS despite lower CAR-T expansion. The increased proportion of SCM T cells in InO-treated patients may contribute to long-term benefit.