Background: FS-1502 is an anti-HER2 antibody-drug conjugate that was well tolerated and showed good antitumor activity in patients (pts) with advanced HER2-expressing solid tumors in the dose-escalation stage of the current Phase 1 study. The results were presented at SABCS 2022. The recommend Phase 2 dose (RP2D) was 2.3 mg/kg IV once every 3 weeks. Here, we report the efficacy and safety of HER2-positive breast cancer (BC) pts treated with FS-1502 at the RP2D. Methods: This open-label Phase 1 trial was conducted at 9 hospitals in China. Heavily pretreated HER2-positive (IHC 3+ or 2+/FISH+) BC pts who had failed prior anti-HER2 therapy were recruited in the dose-expansion stage. All HER2-positive BC pts treated at the RP2D were pooled for analysis. Efficacy endpoints included objective response rate (ORR) and disease control rate (DCR) per RECIST v1.1. Safety endpoints included treatment-emergent adverse events (TEAEs) and the laboratory data. Results: As of cutoff date, December 24, 2022, the median study follow-up duration was 5.2 months. A total of 70 female HER2-positive BC pts, with a median age of 51.8 (range: 31-76) years, were enrolled and received at least one dose of FS-1502. The median prior therapy line was 3 (range: 1-16), and 91.4% (64/70) of pts had previously received at least 2 therapy lines. The most common metastasis organs were lung 55.7% (39/70), lymph node 54.3% (38/70) and bone 50% (35/70). 81.4% (57/70) of pts had at least 3 metastatic lesions. All pts had previously received anti-HER2 therapy, among which trastuzumab ± pertuzumab (n = 67, 96%) and pyrotinib (n = 52, 74%) were the most common. Among 68 efficacy-evaluable pts, the best ORR of 52.9% (36/68) (95% CI, 40.45-65.17) was achieved with 2 complete responses and 34 partial responses. DCR was 88.2% (60/68), median progression-free survival was 15.5 months (95% CI, 4.57,-), and median duration of response was 12.9 months (95% CI, 5.72,-). FS-1502 was well tolerated. No related interstitial lung disease occurred in any grade. Eye toxicity was mostly Grade 1 or 2 of dry eye and keratitis. Grade ≥ 3 study-related TEAEs were reported in 27 (38.6%) pts, in which the most common events (≥ 5%) were hypokalemia (18.6%), platelet count decreased (7.1%), and neutrophil count decreased (5.7%). One (1.4%) patient experienced adverse events leading to drug discontinuation (hemoptysis). Adverse events leading to death were reported in 2 (2.9%) pts (bacterial pneumonia and hemoptysis, respectively). Bacterial pneumonia was not related to FS-1502, while hemoptysis was possibly related to the drug. Conclusions: FS-1502 demonstrated very promising antitumor activity and was well tolerated in heavily pretreated HER2-positive BC, with mild eye toxicity and no related interstitial lung disease reported. A Phase 3 clinical trial is under planning to further confirm efficacy and safety of FS-1502 in pts with advanced HER2-positive BC. Clinical trial information: NCT03944499.