Background: In patients (pts) with NSCLC harboring EGFR exon 20 insertions (ex20ins), treatment options are limited, with platinum-based chemotherapy with/without programmed death-ligand 1 inhibitors being the standard of care in first line, and recent accelerated approvals of mobocertinib and amivantamab in the USA for second-line treatment. These second-line therapies have overall response rates of 28% and 40%, respectively, limited CNS activity, and are associated with AEs such as severe gastrointestinal AEs and edema. BLU-451 is an investigational, CNS-penetrant, potent, selective, and wild-type sparing EGFR tyrosine kinase inhibitor that is under investigation in the ongoing phase 1/2 CONCERTO trial (NCT05241873), including in pts with ex20ins. Methods: In phase 1 dose escalation using a 3+3 design, BLU-451 monotherapy was administered orally QD or BID and given in continuous 21-day cycles. The primary objectives include maximum tolerated dose (MTD) and/or recommended phase 2 dose (RP2D) determinations and safety. Key secondary objectives include antitumor activity, PK, and pharmacodynamics. Pts with EGFR ex20ins–positive NSCLC after prior platinum-based chemotherapy were enrolled. Prior ex20ins–targeted therapy and asymptomatic CNS disease were permitted. Tumor response was assessed per RECIST v1.1. Serial blood samples for analysis of BLU-451 PK and circulating tumor DNA (ctDNA) were collected. Results: As of Jan 20, 2023, 28 pts have been enrolled and dosed in 5 cohorts (100 mg–400 mg QD and 200 mg BID), including 19 (68%) pts with exon 20 insertions. The median number of prior systemic lines of therapy was 2.5 (range, 1–10), and 19/28 (68%) pts had prior ex20ins-targeted agents. The most common AEs were diarrhea (21%), cough (18%), fatigue (14%), pruritis (14%) and rash (14%); all were Grade 1 or 2. There were no dose-limiting toxicities. Partial responses, including a disappearance of a CNS target lesion, were seen, and early evidence of on-target activity with ctDNA reductions was observed. Conclusions: As of the data cutoff, BLU-451 monotherapy was generally well tolerated, with early evidence of clinical activity in heavily pretreated pts with EGFR ex20ins–positive NSCLC. Early data at initial dose levels consisted of tumor reduction including responses, CNS activity, and ctDNA responses. The dose escalation is ongoing to determine the MTD and/or RP2D. Clinical trial information: NCT05241873.