Background: Delta-like canonical Notch ligand 3 (DLL3) is highly expressed on the cell surface of neuroendocrine carcinomas, which have few approved treatment options in the refractory, metastatic setting. HPN328 is DLL3-targeting T-cell engager. HPN328 has 3 binding domains including anti-DLL3 for target engagement, anti-albumin for half-life extension, and anti-CD3 for T cell engagement and activation. Methods: Pts with relapsed/refractory, metastatic SCLC, neuroendocrine prostate cancer (NEPC) and other NEC associated with DLL3 expression are eligible. Primary objectives are safety, maximum tolerated dose (MTD) and pharmacokinetics (PK). Secondary objectives are immunogenicity and efficacy. Overall response rate (ORR) is determined using modified RECIST v1.1 to include extracranial response assessment for pts treated with radiotherapy for brain metastases while on treatment. HPN328 is administered IV QW or Q2W with priming dose preceding target dose in higher dose cohorts. Results: As of January 5, 2024, 86 pts received HPN328 doses of 0.015-24 mg across 14 cohorts (SCLC [n = 54;63%]; other NEC [n = 32;37%]). The median (range) number of prior regimens was 3 (1-7); 83% previously received a PD-1/PD-L1 blocker. Treatment is ongoing in 24 of 46 (52%) pts in the dose optimization cohorts (1 mg priming dose with 12 or 24 mg target doses QW or Q2W). Treatment-related AEs in ≥ 10% of pts included CRS (59% [30% G1, 26% G2, 3% G3+]), dysgeusia (36%), fatigue (34%), diarrhea (19%), nausea (17%), vomiting (14%), decreased appetite and decreased neutrophil count (13% each), and weight decreased (11%). No G3-4 CRS was seen at target doses. Immune effector cell-associated neurotoxicity syndrome (ICANS) occurred in 9% of pts, all G1-2. The maximum tolerated priming dose was 1 mg; dose escalation of target dose continued up to 24 mg QW without reaching a maximum tolerated target dose. Among efficacy evaluable pts treated in dose optimization cohorts, the confirmed ORR in SCLC was 50% (12/24), with one complete response (CR). In NEC (other than NEPC) the confirmed ORR was 44% (4/9), with one CR. Four of eleven pts with NEPC had unconfirmed PRs in 1 mg priming dose cohorts, with 5 NEPC pts remaining on treatment > 20 weeks. HPN328 exhibited linear PK with dose-proportional increases in exposure and a median T_1/2 of 71 hrs. Transient increases in cytokines up to 24 hrs post-dose and T-cell activation were observed. Conclusions: HPN328 is well tolerated and clinically active in SCLC, NEC, and NEPC. Current dose optimization cohorts have completed enrollment and data continue to mature; selection of a recommended phase 2 dose will be made based upon complete mature data. Updated safety and efficacy results will be presented. Clinical trial information: NCT04471727.