This study was an open-label phase Ⅱ study to evaluate the safety and efficacy of SSGJ-705 Monotherapy and Combination Therapy in patients with advanced HER2-Expressing Solid Tumors.

This study includes 2 Parts: Part 1 (705 monotherapy for advanced gastric cancer and breast cancer that have failed standard treatment), and Part 2 (705 in combination with chemotherapy for previously untreated advanced gastric cancer and lung cancer).

Inclusion Criteria:

• Males and/or females over age 18.
• Histologically and/or cytologically documented local advanced or metastatic non-Small Cell Lung Cancer（NSCLC），Breast Cancer（BC）or Gastric/Gastroesophageal Junction Cancer （G/GEJC).
• Eastern Cooperative Oncology Group (ECOG) performance status of 0-1.
• Expected survival >3 months.
• Signed informed consent form.

Exclusion Criteria:

• Any remaining adverse events (AEs) > grade 1 from prior anti-tumor treatment as per Common Terminology Criteria for Adverse Events（CTCAE） v5. 0, with exception of hair loss, fatigue, and grade 2 peripheral neurotoxicity.
• Pregnant or nursing women or women/men who are ready to give birth.
• symptomatic central nervous system metastasis.
• Allergy to other antibody drugs or any excipients in the study drugs.
• Inadequate organ or bone marrow function.