Abstract 4594: EGFR PACC mutations occur more frequently as compound mutations with better responses to EGFR TKIs

Abstract EGFR mutations are key oncogenic drivers in NSCLC and can occur either as single or compound mutations. The frequency of classical (Ex19del and L858R) and non-classical PACC mutations in single and compound forms is not fully characterized, and the impact of single vs. compound EGFR mutations on drug sensitivity and clinical outcomes with EGFR TKIs remains unclear. We aimed to identify the effect of single and compound EGFR mutations to improve precision treatment and outcomes for EGFR-mutant NSCLC patients. We queried a circulating cfDNA sequencing database of de-identified NSCLC patients from Guardant Health and analyzed the incidences of EGFR mutations in single and compound forms. In this dataset, after excluding samples with T790M/C797S or Ex20ins, the most frequent classical mutations were Ex19del (6,670, 38.1%) and L858R (4,700, 26.9%), and the most frequent PACC mutations were G719X (712, 4.1%), S768X (335, 1.9%) and E709X (207, 1.2%). 84.22% of classical mutations occurred as single mutations, with Ex19del in 89.5% and L858R in 76.7% of samples, respectively. In contrast, 80.12% of PACC mutations occurred as compound mutations, with G719X in 73.2%, S768X in 86.9%, and E709X in 97.1% of samples. We engineered Ba/F3 cells to express single or compound PACC mutations and evaluated sensitivity to clinically available EGFR TKIs. Compound PACC mutants showed significantly increased sensitivity to 2nd-gen compared to 1st- and 3rd-gen TKIs. Cells with compound PACC mutations exhibited a tendency towards enhanced TKI sensitivity compared to cells with single mutations. Cells with single or compound G719X/E709X, single V769L and compound S768I/V769L were most sensitive to 2nd-gen TKIs. Cells with single S768I had the highest sensitivity to 3rd-gen TKIs. In a retrospective clinical data analysis we assessed treatment outcomes for patients with single or compound PACC mutations (n=1268). Patients with either form of PACC mutations showed the best response rates (RR) (single: 54.67%, compound: 70.34%) and longest mPFS (single: 9mo, compound: 15mo) with 2nd-gen TKIs. Compound PACC mutations were associated with better RRs and longer mPFS than single forms across all TKIs. Specifically, patients with G719X or E709X, regardless of mutation form, had the longest mPFS with 2nd-gen TKIs (single vs. compound: G719X:11.2 vs. 18.5mo, E709X: 10.9 vs. 8.9mo). Patients with single S768X showed the longest mPFS to 3rd-gen TKIs (13.25mo), differing from those with compound S768X, whose mPFS was the longest with 2nd-gen TKIs (8.9mo). Collectively, our analysis indicates that EGFR PACC mutations predominantly occur as compound mutations while classical mutations most frequently present as single mutations. Compound and single PACC mutations were associated with improved clinical responses to 2nd-gen EGFR TKIs. Citation Format: Ximeng Liu, Moses Stamboulian, Maxime Borgeaud, Ashwin Ravichandran, Caroline Weipert, Monique B. Nilsson, Erjia Zhu, Hong Chen, Courtney Lewis, Simon Heeke, Junqin He, John Lawson, Jianjun Zhang, Addeo Alfredo, John V. Heymach, Xiuning Le. EGFR PACC mutations occur more frequently as compound mutations with better responses to EGFR TKIs [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 4594.