Ixazomib with or without Rituximab Following Immunochemotherapy and Autologous Stem Cell Transplant in Mantle Cell Lymphoma

Background: Maintenance rituximab administered post autologous stem cell transplant (ASCT) in mantle cell lymphoma (MCL) is associated with improved overall survival (OS). Maintenance approaches with the proteasome inhibitor, bortezomib, are associated with improved progression-free survival (PFS), although its use is limited by peripheral neuropathy (Kaplan et al, 2020). We conducted a phase 1 study to evaluate the safety of maintenance therapy with ixazomib, an oral proteasome inhibitor, alone and in combination with rituximab for patients with MCL completing ASCT in first remission. Methods: Adult patients with MCL completing an ASCT in first partial or complete remission and who remained progression-free were enrolled between days 70-180 post-ASCT. Initially, patients received ixazomib monotherapy in cohorts of three using a standard 3+3 design. Patients self-administered ixazomib on days 1, 8, and 15 of each 28-day cycle at one of 3 dose levels: 2.3mg, 3mg, or 4mg. Patients continued therapy until disease progression, intolerance, or completion of 10 cycles. While we were enrolling to the 4mg dose level, the LYMA trial reported an OS benefit associated with maintenance rituximab (Le Gouill et al, 2017), so the protocol was amended to explore the combination of ixazomib and rituximab. Combination therapy consisted of ixazomib 4mg (given at the same schedule) and rituximab 375 mg/m2 given on day 1 of cycles 1, 3, 5, 7, and 9. The primary objective of the study was to evaluate the safety of this treatment and determine the maximum tolerated dose (MTD) while secondary objectives included efficacy. Results Twelve patients enrolled in the trial and received at least one dose of study therapy. Seven patients received ixazomib monotherapy (3mg = 3 patients, 4mg= 4 patients), and 5 patients received combination with rituximab and 4mg ixazomib. In all patients, the median age was 58 years (range 42-73 years) and 75% of the patients were male. Five patients had a high risk MIPI score. All patients entered the study in complete remission. Pre-transplant induction was R-HyperCVAD (n=6), VR-CAP/R-DHAP (n=2), NORDIC regimen (n=2), R-CHOP/R-DHAP (n=1), and R-DHAX (n=1). Transplant conditioning regimens were BEAM (n=7) or Bu/Cy/VP-16 (n=5). In the monotherapy group, 6 patients completed all 10 cycles of therapy. One patient experienced a dose limiting toxicity (DLT) which consisted of grade 3 neutropenia following cycle 1. In addition to the patient who experienced a DLT, one patient discontinued therapy early due to persistent abdominal pain that was ultimately determined to be unrelated to study therapy after an extensive evaluation. Two patients required dose reductions due to grade 3 hematologic toxicities. Two patients in the combination therapy group completed 10 cycles of therapy. Two patients experienced DLTs which consisted of grade 4 neutropenia, and 1 patient opted to discontinue therapy early due to persistent grade 2 peripheral neuropathy. In all patients, common adverse events (AEs) regardless of attribution were abdominal pain (n=6), leukopenia (n=5), nausea (n=4), thrombocytopenia (n=4), and peripheral neuropathy (n=3). Grade 3/4 AEs were thrombocytopenia (n=1) and neutropenia (n=4). No grade 3/4 peripheral neuropathy was observed. With a median follow up of 29.6 months, no patients experienced disease progression and all are alive. Due to the myelosuppression-related DLT's encountered in the combination arm, the study was closed to further accrual. Conclusions Our findings suggest adding rituximab to ixazomib at the evaluated dose and schedule as post-ASCT maintenance is associated with myelosuppression, and we would suggest future studies evaluate an alternative dose/schedule to mitigate this risk. The lack of early progressions is encouraging, confirming the findings from CALGB50403 that proteasome inhibition may provide therapeutic benefit in this setting. Further evaluation of rituximab-containing post-induction combination therapies may identify a safer dosing strategy while improving long-term remission rates. Disclosures Allen: Bayer: Consultancy, Other; Curio Sciences: Honoraria; Clinical Care Options: Speakers Bureau; Research to Practice: Speakers Bureau; Imbrium: Consultancy, Other. Waller: Verastem Oncology, Inc: Consultancy, Research Funding. Flowers: Bayer: Consultancy; Denovo Biopharma: Consultancy; Genentech, Inc./F. Hoffmann-La Roche Ltd: Consultancy, Research Funding; AbbVie: Consultancy, Research Funding; BeiGene: Consultancy; Celgene: Consultancy, Research Funding; Gilead: Consultancy, Research Funding; Karyopharm: Consultancy; OptumRx: Consultancy; Pharmacyclics/Janssen: Consultancy; Spectrum: Consultancy; Acerta: Research Funding; Millennium/Takeda: Consultancy, Research Funding; TG Therapeutics: Research Funding; Burroughs Wellcome Fund: Research Funding; Eastern Cooperative Oncology Group: Research Funding; National Cancer Institute: Research Funding; V Foundation: Research Funding; Cancer Prevention and Research Institute of Texas: Research Funding; Kite: Research Funding; Leukemia and Lymphoma Society: Membership on an entity's Board of Directors or advisory committees. Cohen: Genentech, BMS, Novartis, LAM, BioInvent, LRF, ASH, Astra Zeneca, Seattle Genetics: Research Funding; Janssen, Adicet, Astra Zeneca, Genentech, Aptitude Health, Cellectar, Kite/Gilead, Loxo: Consultancy.