As of 28 Mar 2022, 86 pts received ≥1 dose of study treatment in cohorts A+B and 30 pts in cohort C. The ORR by week 12 in cohort C was 3.3% (95% CI, 0.1, 17.2) with DCR of 80.0%. Twenty-eight of 30 pts (93.0%) crossed over to received TUC + Tras, with cORR of 17.9% (95% CI, 6.1, 36.9).TUC monotherapy and TUC + Tras after crossover were well tolerated, consistent with the primary analysis. Disease stabilization was observed in most pts on TUC monotherapy; radiographic responses increased slightly after Tras addition. Monotherapy, crossover, and response data from the primary analysis show that concurrent initiation of dual HER2 blockade with TUC + Tras achieves optimal clinical benefit.