Aptevo's Mipletamig Achieves 100% Remission With Favorable Safety In Phase 1b/2 AML Trial; Cohort 4 Now Open For Enrollment

No dose-limiting toxicities or cytokine release syndrome observed in RAINIER to date; mipletamig shows consistently favorable safety and tolerability

Trial progressing efficiently, Cohort 4 open for enrollment

SEATTLE, WA / ACCESS Newswire / September 16, 2025 / Aptevo Therapeutics ("Aptevo" or "the Company") (NASDAQ:APVO), a clinical-stage biotechnology company developing novel bispecific and trispecific immuno-oncology therapeutics, today announced a 100% remission rate in Cohort 3 of its Phase 1b/2 RAINIER trial evaluating mipletamig, the Company's first-in-class CD123 x CD3 bispecific antibody, in combination with venetoclax + azacitidine for newly diagnosed patients with acute myeloid leukemia (AML) unfit for intensive chemotherapy. Aptevo also reported that no dose-limiting toxicities or cytokine release syndrome have been observed in the RAINIER trial, or among any frontline patients treated with mipletamig to date.

Efficacy and Market Opportunity
The 100% remission rate (CR/CRi) achieved at the highest dose level of the RAINIER trial to date demonstrates mipletamig's potential to redefine the frontline treatment landscape for newly diagnosed AML patients who are unfit for intensive chemotherapy. Additionally, 40% of patients treated to date have achieved minimal residual disease (MRD)-negative status, a critical marker of remission that is strongly associated with improved overall outcomes in AML. Frontline AML represents a multibillion-dollar global market where current standard regimens achieve lower remission rates than those observed in RAINIER, leaving a substantial need for more effective options. By consistently delivering remissions across three increasing dose cohorts, and among frontline patients in prior trials, mipletamig positions Aptevo to compete for-and potentially expand-a high-value segment of the AML market, offering a therapy that could raise the bar for clinical outcomes and capture meaningful share of an underserved population.