Outcomes from the Multicenter ACCRU-LY-1804/CARiBOU (Cytarabine, Acalabrutinib and Rituximab integrated with Bortezomib-based Outpatient therapy) Trial in 1st line Mantle Cell Lymphoma

Abstract Introduction: First-line therapy for mantle cell lymphoma (MCL) represents a critical opportunity for clinical benefit via sustained complete response. Multitargeted combinations, including optimized chemotherapy with novel agents targeted to MCL, may best address the biologic heterogeneity of this disease. Building on prior experience using modified VR-CAP and R-cytarabine, we enhanced this regimen by adding acalabrutinib while reducing cytarabine, aiming for improved efficacy and safety without impeding stem cell collection. To evaluate this regimen, we conducted a phase 2 multicenter trial at four centers in the Academic and Community Cancer Research United (ACCRU) network. Methods: CARiBOU (ACCRU-LY-1804) employs alternating VR-CAP and R-cytarabine with continuous acalabrutinib, in six 21-day cycles, for previously untreated MCL. Eligible patients (pts) were at least 18 years old with adequate organ function, measurable disease, no CNS involvement; and were ASCT-eligible per investigator. VR-CAP (cycles 1, 3, and 5) employed bortezomib 1.3 mg/m² subcutaneously days 1,8, and 15; rituximab 375 mg/m² day 1; cyclophosphamide 750 mg/m² day 1; doxorubicin 50 mg/m2 day 1; and prednisone 100 mg PO days 1-5. Cycles 2,4, and 6 used outpatient rituximab with cytarabine at 3 g/m² x 2 daily doses in the first 5 pts, after which the dose was reduced to 2 gm/m2 x 2 daily doses due to thrombocytopenia. Acalabrutinib 100 mg p.o. twice daily was given continuously but held for grade 4 thrombocytopenia or clinically significant toxicity. Growth factors and prophylactic antimicrobials were required. Responses were assessed after six cycles using FDG PET/CT, bone marrow aspirate/biopsy (BM Bx), and endoscopy when indicated (Lugano 2014). The primary endpoint was complete metabolic response (CMR) rate at end of treatment (EOT). Secondary endpoints included safety, feasibility of stem cell collection, proportion proceeding to ASCT, and progression-free (PFS) and overall survival (OS). Peripheral blood measurable residual disease (MRD) was evaluated using the ClonoSEQ assay (Adaptive Biotechnologies). A Simon two-stage design was used, targeting a CMR rate ≥ 70% versus a null rate of 50%, with a total sample size of 39 evaluable pts. Results: 41 pts were accrued from October 2021 through May 2025, 37 of whom completed all 6 cycles and are evaluable for response; two are still on therapy, and two discontinued therapy early for toxicity. Median age was 63 years (range 41-73), 22% were female, and 85% were White. MIPI risk groups were low (44%), intermediate (39%), and high (17%), and Ki-67 was ≥ 30% in 54% of pts. Three pts (7%) had known TP53 mutations by sequencing. At EOT, the CMR rate was 85.3% among all pts (35/41; 95% CI 67.9%-92.9%), or 94.5% among evaluable pts (35/37). BM Bx was performed in 31 pts at EOT, 30 of which (96.8%) were negative for MCL. Undetectable peripheral blood MRD using a 10^6 threshold (uMRD6) was achieved at EOT in 78.1% (25/32) of tested pts, and 93.8% (30/32) achieved uMRD5. Nine of 41 pts underwent ASCT; no patient failed stem cell collection. Maintenance therapy was not protocol-defined but included rituximab in 91.9% of pts and a BTK inhibitor in 21.6% of pts. With a median follow-up of 16.7 months, PFS and OS rates at 18 months were 76.7% (95% CI: 60.7%-96.9%) and 95.5% (95% CI: 87.1%-100%). Toxicities were primarily hematologic. Grade 3+ toxicities included thrombocytopenia (66.6%), neutropenia (28.2%), and anemia (25.6%). Grade 3+ febrile neutropenia was observed in 10.2% of pts. Peripheral neuropathy was seen in 20.5% of pts and all grade 1-2. Bleeding events included purpura in 2 pts (5.2%) and epistaxis in 1 patient (2.6%). One case of atrial fibrillation was observed. Eighteen pts required temporary dose hold of acalabrutinib for toxicity (mainly thrombocytopenia), and 2 had acalabrutinib dose reductions. Serious adverse events occurred in 6 pts, and no treatment-related deaths occurred. Conclusions: CARiBOU is an efficient, highly effective outpatient regimen for 1st line MCL, employing intermediate dose cytarabine with continuous acalabrutinib. With supportive care and platelet monitoring, this approach safely yielded high rates of CMR and uMRD6, supporting multitargeted induction therapy for appropriate pts with untreated MCL.