Background: ctDNA is an important tool to diagnose and monitor mutations in pts with non–small cell lung cancer (NSCLC). We evaluated epidermal growth factor receptor gene (EGFR) ex20ins mutations in tumor vs plasma samples, assessed changes in EGFR ex20ins variant allele frequency (VAF) with mobocertinib treatment and correlation with response, and identified potential emerging variants of acquired resistance. Methods: Tumor tissue samples were collected at baseline (BL) from pts with EGFR ex20ins+ advanced NSCLC receiving mobocertinib 160 mg QD in a phase 1/2 study (NCT02716116); plasma samples were collected at BL, after 2 treatment cycles (Cycle 3, Day 1 [C3D1]) and at disease progression/end of treatment (DP/EOT). ctDNA samples were analyzed by next-generation sequencing for EGFR ex20ins to determine concordance rate detection between tissue and plasma ctDNA at BL. Changes in VAF for EGFR ex20ins from BL were analyzed at C3D1 and DP/EOT by confirmed response (RECIST v1.1) to mobocertinib per independent review committee. Emerging variants at DP/EOT were evaluated by elimination of germline variants seen in healthy populations (gnomAD databases and 1000 Genomes) and nonharmful variants predicted by PolyPhen and SIFT tools or annotated as benign in ClinVar database. Results: BL EGFR ex20ins mutations were detected by ctDNA sequencing in 29 of 38 pts (76%) with tissue-confirmed EGFR ex20ins+ NSCLC. VAFs for EGFR ex20ins significantly decreased at C3D1 in mobocertinib-treated pts with confirmed partial response (PR; P=0.0057) or stable disease (SD; P=0.0016), but not in pts with progressive disease (PD; P=0.14) (Table). ctDNA at EOT/DP analysis identified numerous genetic variants; EGFR, TP53, and DNMT3A were the most common genes with emerging variants. Twelve emerging missense mutations were identified in EGFR in 9 pts, including mutations located in the exon 20 loop following the C-helix (6), T790M (5), and D379E (1). Conclusions: Concordance between tissue and plasma ctDNA for EGFR ex20ins mutations at BL was 76%. EGFR ex20ins VAF decreased significantly after 2 treatment cycles in mobocertinib-treated patients with PR and SD. Plasma ctDNA longitudinal monitoring may be useful to assess mutation status and disease progression in pts with NSCLC treated with mobocertinib. Clinical trial information: NCT02716116.

IQR, interquartile range; PD, progressive disease. ^aP≤0.05 vs BL (Wilcoxon rank sum).