Background: Mutations resistant to anti-EGFR treatment (tx), beyond those in RAS, have been reported and include EGFR S492R. We report results for pts with EGFR S492R mutations in the phase 3 ASPECCT trial. Methods: Pts were randomized 1:1 to receive pmab or cmab. The primary endpoint was non-inferiority of overall survival (OS). Secondary endpoints included progression-free survival (PFS), objective response rate (ORR), and safety. EGFR S492R was evaluated by digital droplet PCR in plasma samples collected pre-tx and post-tx (safety follow-up 4 wks after the last dose). Results from the primary analysis were analyzed by EGFR S492 status. Results: Of 999 pts randomized and treated, post-tx samples were available for EGFR S492 assessment from 53% of pts (261/496) in the pmab arm and 57% of pts (285/503) in the cmab arm. EGFR S492R was detected in 1% of pts in the pmab arm and 16% of pts in the cmab arm in post-tx samples. EGFR S492R was not detected in pre-tx samples. Results are shown (table). Conclusions: In an exploratory analysis of pts with available samples from ASPECCT, 16% of pts in the cmab arm and 1% of pts in the pmab developed EGFR S492R mutations. Pts with EGFR S492R in the cmab arm had longer tx duration before progressive disease (PD) and appeared to have worse OS vs pts with wild-type S492 in the cmab arm. Clinical trial information: NCT01001377