Background: In CLEOPATRA (NCT00567190), adding P to H + D significantly improved progression-free and overall survival (PFS/OS) v Pla + H + D in patients (pts) with previously untreated HER2-positive LR/MBC. PUFFIN (NCT02896855) is a China bridging study; the objective being to assess consistency of efficacy with CLEOPATRA. Methods: Pts with previously untreated HER2-positive LR/MBC were randomized 1:1 to P + H + D or Pla + H + D, stratified by visceral v non-visceral disease and hormone receptor status. The primary endpoint was investigator-assessed PFS. Secondary endpoints included objective response rate (ORR in pts with measurable baseline disease), OS, and safety. The target sample size (240) was determined based on the consistency threshold for PFS, defined as hazard ratio (HR) < 0.81, which maintains ≥ 50% of the risk reduction determined in CLEOPATRA (HR 0.62). Results: Two hundred forty-three pts were randomized. Baseline/disease characteristics and prior therapies were generally balanced between arms. For PFS, the HR was 0.69 (95% CI 0.49, 0.99) in the ITT population. No cases of heart failure or symptomatic left ventricular ejection fraction decline were reported. Efficacy/safety are shown in the table. Conclusions: PUFFIN met its primary endpoint. Overall, efficacy data were consistent with CLEOPATRA (ITT population and Asian subgroup). Safety was also consistent and in line with the known P safety profile, with no new or unexpected signals reported. PUFFIN adds to the totality of data with P in previously untreated HER2-positive LR/MBC, and supports the favorable benefit–risk profile of P in Chinese pts. Clinical trial information: NCT02896855

(S)AE, (serious) adverse event.