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Approximately 50% of acute myeloid leukemia (AML) patients do not respond to induction therapy (primary induction failure [PIF]) or relapse after <6 months (early relapse [ER]). We have recently shown an association between an immune-infiltrated tumor microenvironment (TME) and resistance to cytarabine-based chemotherapy but responsiveness to flotetuzumab, a bispecific DART antibody-based molecule to CD3ε and CD123. This paper reports the results of a multicenter, open-label, phase 1/2 study of flotetuzumab in 88 adults with relapsed/refractory AML: 42 in a dose-finding segment and 46 at the recommended phase 2 dose (RP2D) of 500 ng/kg per day. The most frequent adverse events were infusion-related reactions (IRRs)/cytokine release syndrome (CRS), largely grade 1-2. Stepwise dosing during week 1, pretreatment dexamethasone, prompt use of tocilizumab, and temporary dose reductions/interruptions successfully prevented severe IRR/CRS. Clinical benefit accrued to PIF/ER patients showing an immune-infiltrated TME. Among 30 PIF/ER patients treated at the RP2D, the complete remission (CR)/CR with partial hematological recovery (CRh) rate was 26.7%, with an overall response rate (CR/CRh/CR with incomplete hematological recovery) of 30.0%. In PIF/ER patients who achieved CR/CRh, median overall survival was 10.2 months (range, 1.87-27.27), with 6- and 12-month survival rates of 75% (95% confidence interval [CI], 0.450-1.05) and 50% (95% CI, 0.154-0.846). Bone marrow transcriptomic analysis showed that a parsimonious 10-gene signature predicted CRs to flotetuzumab (area under the receiver operating characteristic curve = 0.904 vs 0.672 for the European LeukemiaNet classifier). Flotetuzumab represents an innovative experimental approach associated with acceptable safety and encouraging evidence of activity in PIF/ER patients. This trial was registered at www.clinicaltrials.gov as #NCT02152956., (© 2021 by The American Society of Hematology.)

In immunotherapy dose-finding trials, the optimal dose is usually defined based on both toxicity and response because the relationship between toxicity and response is different than that seen with cytotoxic anti-neoplastic therapies. In immunotherapy trials, toxicity and response often require a longer follow-up time compared to trials with cytotoxic agents. The rapid enrollment design has been proposed for dose-finding trials to find the maximum-tolerated dose where the follow-up for toxicity is long and it is desirable to assign a patient to a dose of a new therapy as soon as the patient is enrolled. We extend the rapid enrollment design to immunotherapy trials to find the optimal dose. We further describe how to use the design in immunotherapy trials with ordered groups where efficacy and safety considerations dictate running dose-finding trials in each group separately as efficacy and toxicity at the same dose can vary across groups. The estimation of the optimal dose in each of the groups can be improved in many, but not all, cases by using the monotonicity of toxicity and response among groups.