Bexmarilimab plus azacitidine for high-risk myelodysplastic syndrome and relapsed or refractory acute myeloid leukaemia: results from the dose-escalation part of a multicentre, single-arm, phase 1/2 trial.

Background: Bexmarilimab blocks Clever-1 on macrophages to enhance antigen presentation and T cell activation. Because Clever-1 is expressed by myeloid leukaemia cells, bexmarilimab may combat leukaemia and influence the tumour microenvironment to augment the effectiveness of standard-of-care therapy in patients with myelodysplastic syndrome and acute myeloid leukaemia. The aim of this study was to determine the safety of bexmarilimab in combination with standard-of-care treatment in myelodysplastic syndrome and acute myeloid leukaemia and to identify the recommended dose for expansion of bexmarilimab in combination with standard of care.

Methods: The phase 1 dose-escalation part of this multicentre, single-arm, phase 1/2 study was done at six centres in Finland and the USA. Patients aged 18 years or older (Eastern Cooperative Oncology Group performance status of 2 or less) with myelodysplastic syndrome (2016 WHO) with a Revised International Prognostic Scoring System (IPPS-R) score of 3·5 or more for USA (3·0 for the European Union), chronic myelomonocytic leukaemia (2016 WHO) with 10-19% marrow blasts, myelodysplastic syndrome or chronic myelomonocytic leukaemia with no response to or disease progression during hypomethylating agent treatment, or relapsed or refractory acute myeloid leukaemia were treated with escalating doses of bexmarilimab (1·0 mg/kg, 3·0 mg/kg, and 6·0 mg/kg, intravenous, once weekly, 28-day cycle) in combination with azacitidine, administered as per label. Here we report the phase 1 part of the study, for which the primary outcome was safety (the incidence and frequency of dose limiting toxicities and the frequency and severity of adverse events) as well as the determination of the maximum tolerated dose and recommended expansion dose for the phase 2 part using a Bayesian optimal interval design. All patients receiving at least one dose of bexmarilimab were included in safety analyses, and those with a post-baseline activity assessment were included in activity analyses. This trial is registered with ClinicalTrials.gov (NCT05428969) and EudraCT (2021-002104-12) databases. Phase 2 of the study is ongoing in patients with myelodysplastic syndrome with no response to hypomethylating agent.

Findings: Between June 2, 2022, and Dec 7, 2023, 33 patients (14 with myelodysplastic syndrome, 19 with relapsed or refractory acute myeloid leukaemia) were enrolled in phase 1; no patients with chronic myelomonocytic leukaemia were identified. 19 (58%) patients were male and 14 (42%) were female, and 24 (73%) patients were non-Hispanic ethnicity, and eight (24%) were White. Median follow-up time for all patients was 6·2 months (IQR 3·5-10·7). The maximum tolerated dose was not reached, and the recommended expansion dose for phase 2 was established as 6·0 mg/kg in patients with myelodysplastic syndrome with no response to hypomethylating agents. There were no dose-limiting toxicities. The most common grade 3-4 treatment-emergent adverse events were febrile neutropenia (n=8 [24%]), anemia (n=7 [21%]), and thrombocytopenia (n=5 [15%]). Treatment-emergent deaths occurred as a result of sepsis (n=1 [3%]), neutropenic infection (n=1 [3%]), and haemophagocytic lymphohistiocytosis (n=1 [3%]). Four patients presented treatment-related serious adverse events, one patient in the 1·0 mg/kg group, two in the 3·0 mg/kg group and one in the 6·0 mg/kg group. These included rash (grade 3), capillary leak syndrome (grade 3), cryptogenic organising pneumonia (grade 3) and haemophagocytic lymphohistiocytosis (grade 5) which led to one death. The objective response rate was 45% (15 of 33; 95% CI 28-62) across all doses.

Interpretation: Bexmarilimab in combination with azacitidine has a manageable safety profile, consistent with azacitidine, and shows promising clinical activity in patients with high-risk myelodysplastic syndrome.

Funding: Faron Pharmaceuticals.