BACKGROUND: HER2-amplified metastatic colorectal cancer (mCRC) is a clinically relevant subgroup for HER2-targeted therapy. The TRIUMPH and MyPathway studies demonstrated the activity of pertuzumab plus trastuzumab (PER + TRA) in that population. Our integrated analysis assessed the effect of prior anti-epidermal growth factor receptor (EGFR) therapy on PER + TRA efficacy and explored the role of baseline circulating tumor DNA (ctDNA) profiling. MATERIALS AND METHODS: The analysis enrolled patients with HER2-amplified mCRC from TRIUMPH and MyPathway who had baseline ctDNA available. Anti-EGFR resistance alterations were defined as RAS, BRAF, or PIK3CA mutations; EGFR ectodomain mutation; or MET amplification. Clinical outcomes with PER + TRA were analyzed in patients who had tissue-based RAS wild-type tumors, stratified by prior anti-EGFR therapy and the presence of resistance alterations. RESULTS: Of 66 enrolled patients, 47 had received prior anti-EGFR therapy. Prior anti-EGFR therapy was associated with inferior overall survival (OS) [median: 10.9 versus 19.7 months; hazard ratio (HR): 2.01] and numerically shorter progression-free survival. ctDNA profiling identified anti-EGFR resistance alterations in 18 patients (27.3%), with an associated absence of objective response (0% versus 31.3%) and a significantly shorter OS (median: 7.6 versus 16.5 months; HR: 2.15). Outcomes were markedly poorer (including shorter OS and lower response rates) in patients with either prior anti-EGFR therapy or resistance alterations than in patients with neither factor. CONCLUSIONS: Prior anti-EGFR therapy and resistance alterations detected by baseline ctDNA profiling are associated with reduced efficacy of PER + TRA in HER2-amplified mCRC. Integrating treatment history with baseline ctDNA profiling might enable improved patient selection for dual HER2-targeted therapy.