Venetoclax in Combination with Pediatric-Inspired Chemotherapy in Adults with Newly Diagnosed Acute Lymphoblastic Leukemia: Results of a Phase I Trial

Introduction: Young adults with acute lymphoblastic leukemia (ALL) have favorable outcomes compared with historical standards when treated with an asparaginase-containing pediatric regimen (Stock et al., Blood 2019). We have demonstrated the safety and efficacy of pediatric inspired chemotherapy (PIC) for adults up to age 60 (MSK protocol 12-266, Geyer et al., Haematologica 2021). However, cure rates for adults with ALL remain below 60%, prompting the need for novel agents. Venetoclax (ven) is a BCL2 inhibitor being tested in several clinical trials for ALL. Here, we present the first report of ven used in combination with a multiagent, asparaginase-containing chemotherapy regimen for the upfront treatment of ALL.

Methods: We conducted a Phase I, single-center, single-arm clinical trial (NCT05386576) to evaluate the safety and efficacy of ven in combination with PIC in adults with newly diagnosed ALL. Adults aged ≤60 years diagnosed with Philadelphia chromosome negative ALL were eligible for the study. The primary objective was to evaluate the safety of ven in combination with asparaginase-based PIC and identify the maximum tolerated dose and recommended phase II dose. The secondary objectives were to assess the rate of complete remission (CR) and measurable residual disease (MRD) negative CR at day 15 and 28 of Induction I, and end of Induction II. The regimen consisted of 6 treatment phases as previously published (12-266). Ven was administered at a final dose of 400mg (with a rapid dose escalation from 100mg over 3 days) during Induction I (days 5-7 dose escalation, days 7-28 full dose), Induction II (days 1-14 and 29-42), Intensification I & II (days 1-22), and Re-Induction I & II (days 1-14 and 29-42). Measurable residual disease (MRD) was assessed by multiparameter flow cytometry with 10^-4 sensitivity. We used historical data from 22 patients (pts) treated on our previous study, who received the same chemotherapy backbone without ven, as a comparator. Median follow up was calculated using the reverse Kaplan Meier indicator. Duration of treatment was compared using the Wilcoxon rank-sum test. Proportions of pts who achieved MRD-CR were compared with the chi squared test.

Results: At the time of analysis, 8 pts had enrolled in the study; all 8 pts completed induction I and 6 (75%) pts had completed induction II (Table 1). Median follow up was 171 days. The median age was 38.5 years (min 25, max 60). 50% of pts had B-cell ALL, and 50% T-cell (including 1 patient with early T-cell progenitor). One patient (12.5%) had CNS disease (CNS 3) at diagnosis. Four (50%) pts had intermediate risk cytogenetics by CALGB 19802 classification (Stock et al., Cancer 2013), 2 (25%) had unfavorable, and 1 (12.5%) favorable. Two (40% of B-ALL) had a Philadelphia-like phenotype.

During Induction I & II, 4 (50%) pts experienced febrile neutropenia, 6 (75%) grade 4 thrombocytopenia, and all 8 (100%) grade 4 neutropenia. No dose limiting toxicities, as defined per protocol, were observed. The average duration of Induction I on study was 46.6 days compared to 43.6 on 12-266 (p = 0.11). The average duration of Induction II on study for pts who had completed it at the time of analysis was 102 days compared to 74 days on 12-266 (p = 0.02), primarily due to cytopenias.

Three (37.5%) of pts achieved an MRD negative CR by the end of Induction I. Of the 6 pts who had completed Induction II by time of analysis, 5 (83%) had achieved an MRD- CR, compared to 81% on 12-266 (p = 0.90) and 1 patient had refractory extramedullary disease. We assessed kinetics and depth of response by plotting the average blast percentage in the bone marrow by flow cytometry across three different timepoints. Pts on the study had numerically lower blast percentages at day 15, end of Induction I and end of Induction II compared to 12-266 (Figure 1). At the time of this analysis, no patient who entered CR had experienced disease relapse.

Conclusions: Ven addition to a PIC backbone was overall safe with high MRD negative remission rates in this preliminary report. Compared to historical controls, pts treated with ven had a longer duration of Induction II primarily due to cytopenias, and we are amending the study to shorten ven duration during induction II. Pts treated with ven had a numerically faster reduction in bone marrow blasts compared to historical controls. Longer follow up in a larger patient population in this ongoing study will be required to assess long term toxicity and efficacy of this regimen.