Phase II Study of Acalabrutinib and High-Frequency Low-Dose Subcutaneous Rituximab in Patients with Previously Untreated Chronic Lymphocytic Leukemia/Small Lymphocytic Lymphoma (CLL/SLL)

Background: Continuous Bruton's tyrosine kinase (BTK) inhibition represents an effective and easily administered oral therapy for patients with CLL; however, it is not curative, can have serious side effects, and is expensive. Novel combinations may provide deep remissions allowing fixed duration therapy. The second generation BTK inhibitor acalabrutinib (ACALA) has demonstrated an improved safety profile compared to ibrutinib. Importantly, unlike ibrutinib, ACALA does not inhibit anti-CD20 monoclonal antibody dependent cellular phagocytosis (VanDerMeid et al, Cancer Immuno Res 2018). Using standard doses, rituximab (RTX) rapidly exhausts the finite innate immune system cytotoxic capacity (Pinney, et al Blood 2020) and also causes loss of cell membrane CD20 from CLL cells by trogocytosis. Previous studies have shown that high frequency low dose (HFLD) IV RTX (20mg/m ² three times per week) was effective and limited loss of CD20 (Zent, et al Am J Hematol, 2014). Subcutaneous (SQ) RTX is FDA approved in CLL, has similar efficacy and pharmacokinetics, and can be self-administered. This phase II study tested the efficacy and tolerability of the combination of ACALA and HFLD RTX as initial treatment for patients with treatment-naïve CLL.

Methods: Eligible patients were treated with 50mg RTX on day 1 and 3 of each week for six 28-day cycles. The first dose was administered IV over 2 hours. If tolerated, subsequent doses were SQ and could be self-administered at home by trained patients. ACALA 100mg BID therapy was initiated on cycle 1 day 8 for a minimum of 12 cycles. Treatment response was assessed during cycles 12 and 24. Patients achieving an iwCLL complete response (CR) with undetectable minimal residual disease (uMRD) by 6-color flow cytometry (£ 1:10 ^-4)at either time point could stop therapy. The primary objective was to determine the rate of iwCLL CR with a secondary endpoint of rate of uMRD.

Results: 37 patients have been treated with a median follow-up of 14 months. Baseline demographics were male/female (22/15) and median age 67 years (range 40-78). High-risk genetic features included TP53 mutation (21.6%), del17p (13.5%), del 11q (16.2%), unmutated IGHV (62.2%), NOTCH1 mutation (21.6%) and SF3B1 mutation (10.8%).

Grade 3/4 AEs occurring in ≥5% of patients were infections (13.5%), neutropenia (8.1%) and anemia (8.1%). No patients discontinued therapy due to AEs and there were no deaths on treatment. The most common (≥20%) AEs (all grades and all causality) were infusion-related reactions (62.1%), infections (56.8%) (upper respiratory infections in 29.7% of patients, urinary tract infections in 18.9%, COVID-19 pneumonia in 8.1%), fatigue (51.3%), anemia (51.3%), headache (43.2%), rash or other skin changes (32.4%), thrombocytopenia (29.7%), bruising (27.0%), and diarrhea (21.6%). Injection site reactions (8.1%) from SQ RTX were grade 1. Three patients contracted COVID-19 while on study during times of high community transmission prior to the availability of vaccines. Two required hospitalization, one contracted the virus following cycle 1 requiring a delay in RTX, and all patients remained on ACALA while COVID-19 positive.

27 patients have completed 12 cycles and been evaluated for response. All patients responded with 1 MRD+ CR, 20 partial responses (PR), and 6 PR with sustained lymphocytosis. 10 of these patients have completed 24 cycles and had a sustained PR. One patient with del17p and TP53 mutation had progressive disease after 25 cycles of therapy. All other patients remain on treatment per protocol.

Conclusion: HFLD RTX and ACALA is a tolerable, effective and convenient therapy that could be the basis for regimens incorporating other novel agents. It is notable that three patients have contracted COVID-19 during the trial; however, none required intubation, and all remained on ACALA during their infection. This at-home combination markedly decreased patient infection risk during the COVID-19 pandemic. This regimen has the potential to enable RTX to be administered at facilities with limited medical IV infusion capacity which could be very useful in rural and economically disadvantaged areas. While all patients have responded to therapy, no patients to date have achieved an uMRD CR, suggesting that additional agents are required to allow for time-limited treatment.