Difficulty in completing enrollment within the planned time

This is an open-label, multicenter, dose-escalation and parallel-group expansion Phase II clinical trial to evaluate the efficacy, safety and tolerability of KN026 in combination with palbociclib and fulvestrant in women or male with HER2-positive metastatic breast cancer .The subjects will receive 20 mg/kg IV Q2W+ palbociclib 100/125 mg/day orally+/-Fulvestrant 500 mg IM until progressive disease, unacceptable toxicity or death.

KN026 is an anti-HER2 bispecific antibody that can simultaneously bind two non-overlapping epitopes of HER2, leading to a dual HER2 signal blockade.

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

diagnosing locally advanced unresectable or metastatic HER2-positive