Preliminary Clinical Response Data from a Phase 1b Study of Mavorixafor in Combination with Ibrutinib in Patients with Waldenström's Macroglobulinemia with MYD88 and CXCR4 Mutations

Background: Waldenström's macroglobulinemia (WM) is a rare, indolent B-cell lymphoproliferative disorder characterized by expansion of clonal IgM-secreting cells (Advani P, et al. Hematol Oncol Stem Cell Ther. 2019;12:179-188). While ibrutinib is the only Bruton tyrosine kinase inhibitor (BTKi) currently approved by the US FDA and EMA for WM, other BTKi's are currently under investigation. More than 90% of patients with WM have somatic mutations in MYD88, and 30%-40% of these patients have activating mutations in CXCR4WHIM (Xu L, et al. Br J Haematol. 2016;172:735-744; Treon SP, et al. Blood. 2014;123:2791-2796). CXCR4WHIM in WM is associated with high serum IgM level, symptomatic hyperviscosity, earlier time to treatment, and inferior response to approved and investigational BTKi (Schmidt J, et al. Br J Haematol. 2015;169:795-803;Tam C, et al. Blood. 2020; 136:2038-2050). Inhibition of CXCR4 can sensitize CXCR4WHIM-expressing cells to ibrutinib (Cao Y, et al. Leukemia. 2015;29:169-176). Mavorixafor is an oral small-molecule antagonist of CXCR4. In vitro data have shown that mavorixafor inhibits CXCL12 binding and extracellular signal regulated kinase hyperactivation for CXCR4 mutations. Aims: To report on an early assessment of the safety and clinical response of mavorixafor in combination with ibrutinib after ≥6 months (6 cycles) of treatment in patients with MYD88 and CXCR4WHIM WM. Methods: This is an ongoing phase 1b, open-label, multicenter, single-arm study (NCT04274738) examining intrapatient dose escalation, safety, pharmacokinetics (PK), and pharmacodynamics of mavorixafor in combination with ibrutinib (target N=18). Eligibility includes age ≥18 years, clinicopathological WM diagnosis, consensus criteria indication for treatment, measurable disease, 0-3 prior therapies, and confirmed MYD88 and CXCR4WHIM mutations. Patients are initiated on mavorixafor 200 mg (low-dose) and ibrutinib 420 mg, both oral and once-daily. Mavorixafor escalation to 400 mg (mid-dose) occurs after 28 days if no dose-limiting toxicities (DLTs) are observed and to 600 mg (high-dose) after 400 mg is deemed safe (