Aptevo Therapeutics Reports Promising Results for Mipletamig Triplet in TP53-Mutated AML

SEATTLE, September 3, 2026, Aptevo Therapeutics Inc., a clinical-stage biotechnology company, has announced encouraging results from its mipletamig triplet treatment in patients with TP53-mutated acute myeloid leukemia (AML). The trial, which involved 14 evaluable patients, reported a 93% clinical benefit rate, with 79% achieving complete remission or complete remission with incomplete hematologic recovery (CR/CRi). This outcome nearly doubles the 41% composite remission rate observed in patients with similar characteristics treated with venetoclax and azacitidine, according to published data.

Dirk Huebner, M.D., Chief Medical Officer of Aptevo Therapeutics, commented, "TP53-mutated AML remains one of the most challenging subtypes to treat. Seeing this level of clinical benefit in a population that historically has not responded well to treatment is particularly promising."

The study, conducted as part of the RAINIER trial, evaluated the mipletamig triplet in combination with venetoclax and azacitidine. Of the 14 patients, 13 experienced clinical benefit, including nine complete remissions. The remaining two patients also showed clinical benefit, though the specific outcomes were not detailed.

These results are significant because they demonstrate the potential of mipletamig in addressing a patient population that has historically faced limited treatment options. Huebner added, "The robust responses observed with the mipletamig triplet highlight its potential to address a significant unmet medical need in this challenging AML subtype."

The RAINIER study is currently in its first phase, a dose optimization trial, which is expected to be completed by year-end. Regulatory interactions are planned for the first half of 2027 to determine the next steps in the development process.

Mipletamig, Aptevo's lead proprietary drug candidate, is designed to redirect the patient's immune system to target CD123, which is overexpressed on leukemic stem cells and AML blasts. Unlike some competitors, Aptevo's approach aims to reduce the likelihood and severity of cytokine release syndrome.

Aptevo Therapeutics Inc. (Nasdaq: APVO) is focused on developing novel bispecific and trispecific immunotherapies for cancer treatment. The company's lead candidate, mipletamig, is currently being evaluated in RAINIER, a two-part Phase 1b/2 trial for the treatment of frontline AML in combination with standard-of-care venetoclax and azacitidine. Mipletamig has received orphan drug designation for AML under the Orphan Drug Act.

The company is also advancing its solid tumor research, focusing on well-established targets and developing novel immunomodulatory platform backbones. Preclinical molecules are expected to enter investigational new drug (IND)-enabling studies in 2027.

Aptevo's mission is to improve treatment outcomes and transform the lives of cancer patients. The company's eight pipeline candidates are created using proprietary ADAPTIR™ and ADAPTIR-FLEX™ platforms, designed to generate differentiated multispecific therapeutics.