A Pilot Study of Acalabrutinib with Bendamustine/Rituximab Followed By Cytarabine/Rituximab (R-ABC) for Untreated Mantle Cell Lymphoma

Introduction

The management of younger fit patients with mantle cell lymphoma (MCL) varies widely with no consensus on an optimal induction therapy. To date, the treatments with the longest progression-free survival incorporate a chemotherapy backbone that includes high dose cytarabine, followed by consolidation with an autologous stem-cell transplantation (ASCT) (Hermine et al. Lancet 2016, Eskelund et al. Br J Haematol 2016). Recent data showed that a regimen of bendamustine/rituximab followed by cytarabine/rituximab achieved high complete response rates with high minimal residual disease (MRD) negativity (Merryman RW et al. Blood Adv 2020). We hypothesized that adding the Bruton tyrosine kinase inhibitor acalabrutinib to the same chemotherapeutic backbone would be safe and increase complete response rates as well as minimal residual disease (MRD) negativity pre-transplant, and potentially improve clinical outcomes.

Methods

We conducted a single arm, single institution pilot study registered at clinicaltrials.gov (NCT03623373). Patients with untreated MCL, who were between ages 18-70 and were candidates for ASCT, were eligible. Patients received six 28-day cycles of treatment. Cycles 1-3 consisted of bendamustine 90 mg/m² on days 1 and 2, rituximab 375 mg/m² on day 1 and acalabrutinib 100mg BID on days 1 through 28. Cycles 4-6 consisted of rituximab 375 mg/m² on day 1, cytarabine 2 g/m² (1.5 g/m² if age>60) q12 hours on days 1 and 2, and acalabrutinib 100mg BID on days 1 through 7 and 22 through 28. Restaging PET/CT and response assessment based on the Lugano classification were obtained following cycles 3 and 6. After cycle 6 patients underwent leukapheresis and stem-cell collection as preparation for ASCT. Blood for MRD status was collected after cycles 2, 4 and 6 and will be evaluated using the ClonoSeq assay (Adaptive Biotechnologies). The primary objective was to determine the stem cell mobilization success rate. Secondary objectives included safety and tolerability, overall response rate (ORR), pre-transplant complete response rate (CR), and the MRD negativity rate during and after completion of therapy.

Results

The trial enrolled 14 patients from December 2018 to February 2020. One patient withdrew consent prior to start of treatment and another was found to have an undiagnosed adenocarcinoma shortly after starting MCL treatment. Both are excluded from the analysis. The median age was 57 years (range 52-66). 11 patients were males (92%), all patients had an ECOG performance status of 0-1. 11 patients (92%) presented with stage IV disease. The mean MCL International Prognostic Index (MIPI) score was 6.3 (25% high-risk, 42% intermediate-risk and 33% low-risk). Of the 12 patients who began treatment, 9 completed all 6 cycles. Three patients did not complete therapy due to: insurance issues (n = 1), and thrombocytopenia (n = 2) following cycle 5 and 4. The side effect profile showed expected hematologic toxicities with grade 3-4 cytopenias in all patients, mostly during cytarabine cycles. In total, 100% of patients developed grade 3-4 thrombocytopenia and 83% of patients developed grade 3-4 neutropenia. Three episodes of febrile neutropenia were observed. One patient had a grade 3 transaminase increase, and one patient had grade 3 diarrhea. No bleeding events or treatment related deaths occurred. The remainder of the side effects were low grade and the treatment was generally well tolerated. Of the 12 evaluable patients, 10 responded (ORR 83%) with 9 achieving CR (75%). One patient achieved PR prior to being removed from the study due to thrombocytopenia and then achieved CR off study. Two patients experienced PD during induction. With a median follow up of 9 months, no responding patients have relapsed. The median CD34+ stem cell collection was 3.84x10⁶ cells/kg (range 2.77 – 5.9). MRD results will be presented at the meeting.

Conclusions

This is the first study attempting to combine BTK inhibition with a high dose cytarabine containing regimen. The addition of acalabrutinib to a regimen of bendamustine/rituximab followed by cytarabine/rituximab appears to be safe. The R-ABC combination will be further tested in the recently activated intergroup trial EA4181.