Blinatumomab Combined with Venetoclax Increasing the Abundance of CD8 T Cell and NK Cell and Improve the Outcome in Relapsed/Refractory B-Cell Acute Lymphoblastic Leukemia Patients

Background: Blinatumomab targets CD19/CD3, exhibiting a high clearance rate of over 80% for minimal residual disease (MRD) in B-cell acute lymphoblastic leukemia (B-ALL). However, as a single agent, it has a relatively low complete remission (CR) rate in relapsed/refractory (R/R) B-ALL. Venetoclax, an inhibitor of the BCL-2 protein, has demonstrated anti-leukemic activity for ALL in both in vitro and in vivo studies. Whether the combination of blinatumomab and venetoclax can improve the outcome in R/R B-ALL patients and change immune cell repertoire are unclear. This study aimed to explore the efficacy of venetoclax plus blinatumomab and the therapy-driven effect on the immune microenvironment in patients with R/R or MRD-positive B-ALL receiving the combination therapy compared to blinatumomab monotherapy. Methods: A multicenter retrospective study was conducted. Data from consecutive patients receiving venetoclax (400mg QD for 7 days prior to blinatumomab initiation) plus blinatumomab (9ug from day 1 to 7, 28ug from day 8 to 28) or blinatumomab monotherapy were collected from 10 hospitals between October 2021 and June 2024. Flow cytometry was used to assess MRD using bone marrow sample in all patients, and analyze immune cell subgroups using blood sample in R/R and MRD positive patients from Peking University People's Hospital at baseline and day 28 of blinatumomab infusion. Results: A total of 64 patients with R/R (n = 33), MRD-positive (n = 25), or MRD-negative B-ALL (n = 6) were enrolled in this study, including 19 in the combination group and 45 in the monotherapy group. Median age was 46 years (IQR, 17-74). In R/R patients, median percentages of bone marrow blasts at baseline were 60% (IQR, 25-86%) in the combination group and 42% (IQR, 19-85%, P = 0.57) in the monotherapy group, respectively. In the combination group, 10 (83%) of 12 R/R patients achieved an overall response rate (ORR, CR+PR); 7 (58%), MRD negativity. In the monotherapy group, 14 (67%) of 21 R/R patients achieved an ORR; 13 (62%), MRD negativity. The ORR (83% vs. 33%, P = 0.057) and MRD negativity rate (67% vs. 22%, P = 0.085) showed a high trend in the combination group compared with the monotherapy group in those with > 60% blasts in bone marrow at baseline. In MRD-positive patients, MRD negativity rates were achieved in 74% in both groups (P = 0.956). In the monotherapy group, all patients achieved MRD negativity. In multivariate analysis increasing bone marrow blasts (OR=0.95, P=0.001) was the sole adverse variable significantly-associated with achieving CR and MRD negativity. With a median follow-up of 16 (IQR, 11 to 25) months, 52 patients received 1 cycle of combination therapy or monotherapy; 9, 2 cycles; 1, 3 cycles, 2, 4 cycles. 30 patients underwent transplant, 8 relapse, 19 died of no response or relapse. At last follow-up, 11 patients continued on chemotherapy. The 2-year probabilities of DFS and survival were 55% and 64%. 21 patients from the combination group (n = 11) or the monotherapy group (n = 10) were analyzed immune cell subgroups at baseline and on Day 28 of blinatumomab infusion therapy. Patients in the combination group showed significantly decreased proportions of Treg in CD4+ T cells (median value from 8% to 5%, P = 0.046) and CD8+CD45RA+ in CD8+ T cells (from 39% to 31%, P = 0.075) and significantly increased CD8+CD45RO+ proportion in CD8+ T cells (from 50% to 60%, P = 0.028) on Day 28 compared to baseline. However, patients in the monotherapy group exhibited significantly increased total T cells in lymphocytes proportion (from 74% to 78%, P = 0.038) and significantly decreased NKT in lymphocytes proportion (from 20% to 15%, P = 0.015). The combination therapy resulted in higher post-/pre-treatment ratios for NKT cells (P = 0.071) and CD8+CD45RO+ cells (P = 0.029) and a lower ratio for CD8+CD45RA+ cells (P = 0.011) compared to monotherapy. Conclusions: The combination of blinatumomab and venetoclax demonstrated a high efficacy in patients with high leukemia burden and significantly altered the immune microenvironment, notably increasing the proportion of CD8+CD45RO+ cells and NKT cells and decreasing the proportion of Treg in CD4+ T cells and CD8+CD45RA+ in CD8+ T cells compared to blinatumomab monotherapy in R/R B-ALL patients. These preliminary findings warrant confirmation in larger-scale studies.