Aptevo Presents Preliminary Results of mipletamig Study

Aptevo Therapeutics presented preliminary results from its ongoing Phase 1b/2 RAINIER study evaluating mipletamig, a CD123 x CD3 bispecific molecule, in combination with azacitidine and venetoclax for newly diagnosed acute myeloid leukemia patients who are unfit for intensive chemotherapy. The data were presented on December 8, 2025, in a poster session at the American Society of Hematology Annual Meeting. Across dose-optimization Cohorts 1-3, mipletamig combined with AZA/VEN demonstrated high remission rates and a compelling safety/tolerability profile, reinforcing the potential of T-cell engagement in frontline AML when safety can be effectively managed. Aptevo's proprietary use of the CRIS-7-derived CD3 binding domain differentiates mipletamig in the category. Key Findings: 100% of treated patients remained free of cytokine release syndrome across cohorts to date; 93% overall response rate among evaluable patients; 87% achieved CR/CRi; 73% achieved CR; 60% of MRD evaluable CR/CRi patients achieved minimum residual disease negative status, a result that is typically associated with stronger, more durable responses;43% of ORR patients had a TP53 genetic mutation, a marker that is typically associated with poor prognosis in AML patients Median patient age was 75, reflecting a population that is underserved by intensive therapies. In the RAINIER trial to date, the triplet regimen was generally well tolerated. Infusion-related reactions and hematologic events were the most common adverse events, consistent with expectations for this patient population. Importantly, no CRS was seen, supporting the molecule's differentiated safety profile in combination therapy. The RAINIER study continues to enroll patients across additional dose levels.