Background: Preliminary efficacy was observed with the combination of amivantamab, an EGFR-MET bispecific antibody, and lazertinib, a 3^rd-generation tyrosine kinase inhibitor, in both treatment-naïve and osimertinib (osi)-relapsed patients (pts) with EGFRm NSCLC (Cho Ann Oncol 2020;31:S813). We present updated results of the combination in osi-relapsed pts, including an analysis of potential biomarkers of response. Methods: Pts with EGFR exon 19 deletion or L858R mutation NSCLC, who had progressed on osi without intervening chemotherapy, were enrolled in the combination cohort of the ongoing CHRYSALIS study (NCT02609776). With pre-treatment tumor biopsies and ctDNA collected prospectively, pts received the combination dose of 1050/1400 mg amivantamab + 240 mg lazertinib to assess safety and efficacy in the osi-relapsed population. Response was assessed by investigator per RECIST v1.1. Osi-resistance mutations or amplifications in EGFR/MET identified by next-generation sequencing (NGS) in either ctDNA or tumor biopsy (biomarker-positive [pos]), were evaluated for enriching response. Immunohistochemistry (IHC) staining for EGFR and MET expression was also explored as a potential biomarker for response. Results: Of the 45 osi-relapsed pts, 36% (95% CI, 22–51) had a confirmed response (1 complete response and 15 partial responses [PR]). At a median follow-up of 8.2 mo (1.0–11.8), 20/45 pts (44%) remain on treatment. With 11/16 pts (69%) continuing in response (2.6–9.6+ mo), median duration of response has not been reached (NR). The median progression-free survival (mPFS) was 4.9 mo (95% CI, 3.7–8.3). In total, 44/45 pts were evaluable by ctDNA and 29/45 by tumor NGS. Genetic testing identified 17 biomarker-pos pts, of whom 8 (47%) responded. Of the remaining 28 pts, 8 (29%) responded. Among these 28 pts, 18 had unknown mechanisms of osi-resistance (8 PR) and 10 had non-EGFR/MET mechanisms of resistance identified (none responded). The mPFS (95% CI) for biomarker-pos and remaining pts was 6.7 mo (3.4–NR) and 4.1 mo (1.4–9.5), respectively. Adequate tissue was available for 20 pts to perform IHC testing for EGFR and MET–9/10 (90%) IHC high (combined EGFR+MET H score>400) pts responded to treatment, while 1/10 IHC low pts responded to treatment. Conclusions: Treatment with the combination of amivantamab and lazertinib yielded responses in 36% of chemotherapy-naïve pts who progressed on osi. Among these pts, genetic EGFR and MET-based biomarkers of resistance identified a subgroup of pts more likely to respond to amivantamab and lazertinib, although additional pts lacking identified resistance markers also responded. An IHC-based approach may identify pts most likely to benefit from the combination regimen, but further investigation is warranted. Clinical trial information: NCT02609776