Study of KN026 in Combination With Palbociclib and Fulvestrant in Patients With Advanced Breast Cancer

Inclusion Criteria: * Male or female subject \>= 18 years; * Histologically or cytologically confirmed, metastatic or locally advanced unresectable HER2-positive; * Adequate organ function assessed within 7 days prior to first trial treatment * ECOG score 0 or 1; * Left ventricular ejection fraction (LVEF) ≥ 50% at baseline; * Life expectancy \>3 months Exclusion Criteria: * Untreated active CNS metastasis or leptomeningeal metastasis; * Uncontrolled tumor-related pain; * Has received other anti-tumor treatment or an investigational drug within 28 days or within 5 times of half-life (whichever is shorter, and no less than 2 weeks) prior to the first trial treatment; * Major surgery for any reason within 28 days; * Curative radiation within 3 months of the first dose of trial treatment; * History of uncontrolled intercurrent illness; * Other medical conditions that at the discretion of investigator interfere with the requirements of the trial in terms of safety or efficacy evaluation, or treatment compliance