Background: SCLC is an aggressive neuroendocrine tumor; response to initial chemotherapy and radiotherapy is often followed by recurrence, rapid progression, and resistance to current therapies. Delta-like ligand 3 (DLL3) is an inhibitory ligand of Notch receptors that is expressed in most SCLC tumors but minimally expressed in normal tissues. DLL3 may therefore be a promising target for T cell–redirecting immunotherapy. AMG 757 is a half-life extended BiTE antibody construct that is designed to transiently connect DLL3-positive cells to CD3-positive T cells and induce T cell–mediated cell lysis and concomitant T cell proliferation. AMG 757 induces potent killing of SCLC cell lines in vitro and inhibits tumor growth in the SHP-77 human SCLC xenograft model in vivo. AMG 757 was well tolerated in a preclinical multi-dose toxicology study in cynomolgus monkeys, with no evidence of tissue damage at weekly doses up to 4.5 mg/kg. Methods: NCT03319940 is an open-label, ascending, multiple dose, phase 1 study evaluating AMG 757. The study will initially enroll adult patients with relapsed/refractory SCLC who have progressed after at least 1 platinum-based chemotherapy regimen. Additional inclusion criteria include ECOG performance status 0–2, at least 2 measurable lesions per modified RECIST 1.1, no untreated or symptomatic brain metastases, and adequate organ function. Primary objectives are to evaluate safety and tolerability and to determine the maximum tolerated dose (MTD) or recommended phase 2 dose (RP2D). Secondary objectives are to characterize pharmacokinetics (PK) and evaluate preliminary antitumor activity. In the dose exploration phase, patients will be monitored for dose-limiting toxicities during the first 28 days. A Bayesian logistic regression model will be used to inform dose escalation/de-escalation decisions. The dose expansion phase will confirm the MTD or RP2D and collect further safety and efficacy data. AMG 757 will be administered as a short-term intravenous infusion once every 2 weeks. Alternative dosing schedules may be explored based on emerging PK and safety data. Clinical trial information: NCT03319940