Izalontamab Brengitecan (Iza-Bren), a First-in-Class EGFR-HER3 Bispecific Antibody-Drug Conjugate in Extensive-Stage Small Cell Lung Cancer: Results From a Phase Ib Study

PURPOSE: Small cell lung cancer (SCLC) remains one of the most aggressive malignancies, with limited treatment options beyond frontline chemo-immunotherapy. Izalontamab brengitecan (iza-bren, BL-B01D1) is a first-in-class bispecific antibody-drug conjugate cotargeting epidermal growth factor receptor (EGFR) and human epidermal growth factor receptor 3 (HER3). We expanded the SCLC cohort to further evaluate the efficacy and safety of iza-bren in patients with extensive-stage SCLC (ES-SCLC). METHODS: This open-label, multicenter, dose-expansion phase Ib study (ClinicalTrials.gov identifier: NCT05194982) enrolled patients with ES-SCLC who had progressed on prior systemic therapies. Patients received iza-bren 2.5 mg/kg once daily on days 1 and 8 of each 3-week cycle. The primary end points were objective response rate (ORR) and safety/tolerability. Secondary end points included disease control rate (DCR), duration of response (DOR), progression-free survival (PFS), and overall survival (OS). Exploratory end point included the assessment of potential associations between EGFR/HER3 expression and clinical outcomes. RESULTS: As of December 5, 2024, 52 patients were enrolled. The ORR was 48.1% (95% CI, 34.0 to 62.4), with a median PFS of 4.1 months (95% CI, 3.0 to 5.5) and a median OS of 12.2 months (95% CI, 9.1 to 13.2). In patients who received iza-bren as second-line treatment (n = 22), the ORR was 72.7% (95% CI, 49.8 to 89.3), median PFS 6.2 months (95% CI, 3.7 to 8.2), and median OS 15.0 months (95% CI, 8.7 to not reached). The most common treatment-related adverse events were hematologic toxicities (neutropenia, thrombocytopenia, anemia, and leukopenia). Exploratory biomarker analysis suggested that positive HER3 expression may be associated with better treatment response. CONCLUSION: Iza-bren showed encouraging antitumor activity in relapsed ES-SCLC, particularly in the second-line setting. A phase III randomized controlled trial of iza-bren compared with topotecan (ClinicalTrials.gov identifier: NCT06500026) is ongoing.