A Phase II Study of SSGJ-612 Plus a HER2-Targeted ADC in HR-Positive, HER2-Low or HER2-Ultralow Expression Metastatic Breast Cancer

Prior treatment

• Patients who received investigational drug therapy in another clinical trial within 30 days prior to first study drug administration, or who are concurrently enrolled in another clinical trial. Note: Exclusions include participation in observational (non-interventional) clinical trials, or being in the follow-up phase of an interventional clinical trial.

Other illness

• Uncontrolled or severe cardiovascular disease.
• History of interstitial lung disease (ILD)/pneumonia requiring corticosteroid therapy (non-infectious), or current ILD/pneumonia, or suspected ILD/pneumonia not excluded by screening imaging.
• Presence of pulmonary-specific, clinically significant comorbid conditions.
• Presence of drug abuse, or other diseases (e.g., psychiatric disorders) that, in the investigator's judgment, may interfere with patient participation in the trial or affect the assessment of trial outcomes.

Other

• Presence of spinal cord compression, or clinically active central nervous system metastases.
• Female patients who are pregnant, breastfeeding, or planning pregnancy.

• Voluntary participation in this study, signed informed consent form, good treatment compliance, and willingness to cooperate to complete all trial procedures.
• Expected survival duration ≥ 3 months.
• Histopathologically confirmed breast cancer meeting all of the following criteria simultaneously:
  • Metastatic tumor;
  • Tumor samples tested by central laboratory confirmed as HER2-low or HER2 ultra-low expression;
  • No prior test results reporting HER2 positivity;
  • Tumor sample testing confirms hormone receptor-positive (HR+) status.
• Patients must satisfy either of the following conditions:
  • Disease progression within 6 months after initiation of first-line endocrine therapy combined with cyclin-dependent kinase 4/6 (CDK4/6) inhibitors for metastatic disease;
  • Received at least 2 lines of prior endocrine therapy for metastatic disease, with disease progression post-treatment.
• Participants in Part 1: Breast cancer patients who have received at least one line of chemotherapy for metastatic disease; Participants in Part 2: Breast cancer patients who have not received any chemotherapy for metastatic disease.
• At least one repeatedly and accurately measurable tumor lesion as target lesion assessed per RECIST v1.1 criteria.