66P Early safety, pharmacokinetics, and translational profiling of HCB101 in combination with standard-of-care regimens: Results from the phase Ib/IIa HCB101-201 study

were centrally analyzed.Outcomes included safety, progression-free survival (PFS), overall survival (OS), treatment sequencing, and descriptive genomic profiling.Results: A total of 141 patients treated with avelumab maintenance between July 2022 and October 2025 were identified.Median age was 69 years; 64% had ECOG PS 0 and 19% had visceral metastases.Prior chemotherapy was cisplatin-based in 73%.After a median follow-up of 20 months, 58% experienced disease progression and 36 patients remained on treatment.Observed response rate was 14%.Immune-related adverse events occurred in 21%, 6%, and 2% for grade 2, 3, and 4 toxicities, with treatment discontinuation in 7%.Following progression, 41 patients (50%) received further therapy, most commonly enfortumab vedotin (EV; n=22).Median PFS and OS from avelumab initiation were 9 months (95% CI 7-14) and 28 months (95% CI 22-31).Median OS from first-line chemotherapy was 33 months overall and 37 months in patients who received second-line EV.Actionable genomic alterations were detected in 16 of 57 profiled patients (28%), most commonly FGFR2/3 alterations and ERBB2 amplification.Targeted therapy was administered in 4 patients (25% of those with actionable findings). Conclusions:In routine clinical practice, avelumab maintenance provides durable benefit with manageable toxicity in aUC.Effective post-progression treatment sequencing, including EV, is feasible and associated with prolonged survival.Genomic profiling identifies actionable alterations in a subset of patients and may inform laterline treatment decisions.