A Phase 2 Study Evaluating the Addition of Ublituximab and Umbralisib (U2) to Ibrutinib in Patients with Chronic Lymphocytic Leukemia (CLL): A Minimal Residual Disease (MRD)-Driven, Time-Limited Approach

Background: While time-limited novel agent combinations have demonstrated high overall response rates and durable responses for patients with chronic lymphocytic leukemia (CLL), they also have high rates of adverse events and possibly overtreat many favorable risk patients. Meanwhile, patients receiving indefinite ibrutinib monotherapy are at risk for cumulative toxicity and acquired resistance with continuous exposure. To address these challenges, we utilized an “add-on” approach to combination therapy after a period of ibrutinib monotherapy exposure. We examined the addition of umbralisib (a selective PI3Kδ and casein kinase-1epsilon [CK1ε] inhibitor) and ublituximab (a novel anti-CD20 monoclonal antibody glycoengineered for enhanced antibody-dependent cellular cytotoxicity; U2) to ibrutinib in CLL patients with detectable minimal residual disease (MRD) after an initial period of treatment with ibrutinib monotherapy. With this strategy, we aimed to induce undetectable MRD (uMRD), minimize the risk of developing BTKi resistance mutations, stop all CLL-directed therapy, and achieve a durable treatment-free observation (TFO) period in CLL patients who would most benefit from combination therapy.

Methods: This is a phase II, multicenter, open label clinical trial (NCT04016805). Eligible patients were receiving ongoing ibrutinib, in any line of therapy, for a minimum duration of 6 months and had detectable residual CLL in the peripheral blood via MRD assay (flow cytometry with a cutoff of 10^-4 for uMRD). Umbralisib (administered daily at 800mg) and ublituximab (administered intravenously at 900 mg on Days 1/2 [split 150/750 mg], 8, and 15 of Cycle 1, Day 1 of Cycles 2–6, and on Day 1 every 3 cycles after Cycle 6) were added to ibrutinib, and patients were monitored serially for MRD starting on Cycle 3 Day 1. Once uMRD was achieved and confirmed 4 weeks later, patients entered a period of TFO. Patients who did not attain uMRD continued treatment for up to 24 cycles followed by TFO. The primary objective of the study was rate of uMRD, with a prespecified MRD conversion rate of 25% defined as promising. Secondary endpoints included safety and durability of clinical benefit after treatment discontinuation.

Results: In this fully accrued study, U2 was added to ibrutinib in 26 patients. Median age was 63 years (range, 48-81) and 77% were male. Disease characteristics included del(17p) in 8%, del(11q) in 12%, and unmutated IGHV in 31%. Median duration of ibrutinib treatment prior to addition of U2 was 22 months (range 7–66 months). The swimmer plot in Figure 1 depicts time on ibrutinib prior to enrolling in this study, the duration of treatment with triplet therapy, achievement of uMRD, and TFO.

MRD has been assessed in 24 patients; 71% (17/24) had uMRD in at least 1 assessment. Median time to first uMRD result was 5 months (range 2 – 12). A total of 16 patients (67%) entered TFO; 15 had 2 consecutive uMRD assessments and 1 completed 24 cycles with detectable MRD. TFO appears durable, with a median of 242 days off therapy (range 5-538 days) as of the data cutoff. 73% remain uMRD at last follow up. No patient has progressed or required re-treatment per iwCLL criteria.

U2 plus ibrutinib was well tolerated. All-causality grade 3/4 adverse events of special interest included ALT/AST increase (4%), diarrhea (4%), and hypertension (8%). Two patients discontinued all therapy due to rash; both were uMRD at the time of treatment discontinuation and remain uMRD. One patient died due to complications of COVID-19.

Conclusions: This is the first MRD-driven approach utilizing the combination of BTKi, PI3Ki, and anti-CD20 monoclonal antibody. This novel agent combination therapy was well tolerated and effective, with achievement of uMRD in 71% of evaluable patients. This “add-on” approach for patients on continuous ibrutinib resulted in deep remissions that allowed for a tailored, time-limited therapy and sustained treatment-free observation.