Background: HER2 (ERBB2) mutations have been reported in approximately 2-4% of patients (pts) with NSCLC, with exon 20 insertions being the most common. BAY 2927088 is an oral, reversible tyrosine kinase inhibitor that potently inhibits mutant HER2 and mutant EGFR in preclinical models. Encouraging objective responses were observed in pts with NSCLC harboring a HER2-activating mutation and treated with BAY 2927088 in the dose-escalation/backfill part of the Phase I/II SOHO-01 trial (NCT05099172). More recently the FDA has granted Breakthrough Therapy designation for BAY 2927088 for previously treated pts with advanced NSCLC and activating HER2 mutations. Here we report the safety, anti-tumor activity, and longitudinal circulating tumor DNA (ctDNA) data in a cohort of pts treated with BAY 2927088 from the expansion part of this trial. Methods: Pts with advanced NSCLC harboring a HER2-activating mutation and who experienced disease progression after at least 1 systemic therapy, but naïve to HER2-targeted therapy, were enrolled and received BAY 2927088 at 20 mg twice daily. Plasma samples were collected at baseline and several on-treatment time points for longitudinal ctDNA profiling using next-generation sequencing (NGS; Oncomine Precision Assay). Results: As of February 19, 2024, 34 pts were treated with BAY 2927088, with a median follow-up of 8 months. Median age was 62 years, 68% were female, 74% had never smoked, and 53% had received ≥2 lines of systemic anti-cancer therapy. Median duration of treatment with BAY 2927088 was 7.1 months (range 0.2-9.2). Treatment was ongoing in 17 pts (50%). Ten pts had a dose reduction, 8 had dose interruptions, and 3 discontinued study treatment due to a drug-related adverse event. The most common adverse events were diarrhea (85%; mainly grade 1-2) and rash (47%; grade 1-2). In 33 pts evaluable for efficacy, responses were observed in 23 (objective response rate 70%; 95% CI 51.3, 84.4) and 5 (15%) had stable disease for a disease control rate of 82% (95% CI 64.5, 93.0). Responses were rapid (median time to response 5.7 weeks) and durable (median duration of response not reached). Median progression-free survival was 8.1 months (95% CI 4.4, not evaluable). In a subset of 20 pts with successful paired (baseline, on-treatment) blood NGS and detectable HER2 ctDNA at baseline, 19/20 (95%) had a decrease in ctDNA and 1 pt with progressive disease had an increase in ctDNA; 15/20 (75%) had no detectable ctDNA after 6 weeks of treatment (including 3/4 pts with stable disease). Conclusions: BAY 2927088 led to rapid, substantial, and durable responses in pts with pretreated HER2-mutant NSCLC. The safety profile was consistent with previously reported data. These data support the further clinical development of BAY 2927088 in pts with HER2-mutant NSCLC. Clinical trial information: NCT05099172.