Background: AML is associated with poor survival rates in patients (pts) who are not eligible for intensive chemotherapy or allogeneic stem cell transplant (allo-SCT) due to advanced age, comorbidities, and/or disease risk factors. Non-intensive therapies, such as the hypomethylating agents (HMAs) azacitidine and decitabine, are frequently employed in this setting; however, response rates and survival remain suboptimal in this pt population. Vadastuximab talirine (33A) is a CD33-directed antibody conjugated to pyrrolobenzodiazepine (PBD) dimers. Upon binding, 33A is internalized and transported to the lysosomes where PBD is released via proteolytic cleavage of the linker, leading to DNA crosslinking and cell death. In preclinical studies, HMA priming followed by 33A exposure upregulated CD33 expression, increased DNA incorporation of the PBD dimer, and enhanced cytotoxicity. In a phase 1 study, 33A plus HMA was generally well tolerated without a significant pattern of off-target toxicity. Activity of the combination was markedly improved compared to historical data of HMA monotherapy, with a high MRD-negative remission rate and activity maintained in the highest risk subgroups (Fathi et al, ASH 2016). Methods: This phase 3, randomized, double-blind, placebo-controlled global study was designed to compare overall survival between pts treated with 33A plus HMA vs. pts treated with placebo plus HMA (NCT02785900). Secondary endpoints include composite complete remission rate (CR/CRi), event-free survival, and safety. Approximately 500 pts will be randomized in a 1:1 manner to one of the study arms. Investigators may select either HMA (azacitidine or decitabine). Eligible pts are adults with newly diagnosed, previously untreated, de novo or secondary AML, and have intermediate or adverse cytogenetic risk. Pts must not have AML associated with favorable risk karyotype or be a candidate for allo-SCT. Combination treatment may be repeated every 4 weeks until disease progression, leukemic recurrence, or unacceptable toxicity. Study enrollment began in June 2016.