BACKGROUND: NC-6300 is a novel epirubicin (EPI) drug conjugated polymeric micelle developed using cutting-edge micellar nanoparticle technology. The nanoparticle epirubicin conjugates EPI to a polymer via a pH-sensitive linker which enables the selective EPI release into tumor. Tumor activity was observed in a monotherapy phase Ib trial, where two of two patients with angiosarcoma achieved a partial response. To further explore the activity of NC-6300 in angiosarcoma, an expansion cohort was undertaken. METHODS: Ten patients with angiosarcoma were enrolled in the expansion cohort. Patients were dosed using the recommended dose of 150 mg/m2 intravenously (IV) once every 3 weeks. The primary endpoint was progression-free survival. RESULTS: The most common adverse events (AEs) of any grade, regardless of the causal relationship with NC-6300, were neutropenia (90%), fatigue, and thrombocytopenia (60% each) and nausea (50%). The most common grades 3 and 4 AEs were neutropenia (80%), thrombocytopenia (40%), and anemia and leukopenia (20% each). The median progression-free survival (mPFS) for all subjects was 5.4 months. The mPFS was 3.8 months in subjects with prior anthracycline treatment and 8.2 months in subjects without prior anthracycline treatment. CONCLUSION: NC-6300 was well tolerated, showing promising activity in angiosarcoma patients without prior anthracycline treatment. NC-6300 warrants further investigation (ClinicalTrials.gov Identifier: NCT03168061).