A Randomized, Open-Label, Dose-Optimization Phase II Clinical Trial to Evaluate the Efficacy and Safety of Single-Agent SMP-656 in Patients With HER2-Positive Locally Advanced or Metastatic Breast Cancer Who Progressed After Prior Treatment With HER2-Targeted Topoisomerase Inhibitor ADCs
The goal of this clinical trial is to learn if single-agent SMP-656 works to treat patients with HER2-positive locally advanced or metastatic breast cancer who have progressed after prior HER2-targeted topoisomerase inhibitor antibody-drug conjugate (ADC) treatment. It will also evaluate the safety of SMP-656 and identify the optimal dose for future trials. The main questions it aims to answer are: What is the objective tumor response rate (DOR, PFS, DCR, OS) of two different dose regimens of intravenous SMP-656? What are the side effects and safety risks of these two SMP-656 dose regimens? Which dose level achieves the best balance of anti-cancer activity and tolerability? This is a randomized, open-label, dose-optimization Phase II clinical trial. Participants will be randomly assigned 1:1 to receive one of two fixed doses of SMP-656 given intravenously once every 3 weeks. Participants will: Complete screening tests within 28 days before the first SMP-656 infusion to confirm eligibility Receive study treatment every 3 weeks until cancer progression, intolerable side effects, withdrawal, or other stopping criteria Have regular tumor imaging scans, physical exams, vital sign checks, and blood tests to monitor tumor response and safety Attend a safety follow-up visit 30 days after the last dose of SMP-656 Complete longer-term survival follow-up after the 30-day safety check Provide optional blood and tumor tissue samples for additional research studies
⁃ Voluntarily participate in this clinical trial, understand and comply with study procedures, and provide written informed consent voluntarily.
⁃ Female patients aged ≥ 18 years at the time of signing the informed consent form.
⁃ Patients with histologically or cytologically confirmed unresectable HER2-positive locally advanced or metastatic breast cancer, regardless of hormone receptor (HR) status.
⁃ HER2-positive status confirmed by testing at the study center or an accredited laboratory, where HER2 positivity is defined as IHC 3+, or IHC 2+ with positive fluorescence in situ hybridization (FISH+) results.
⁃ Patients with HER2-positive locally advanced or metastatic breast cancer who have received prior treatment with one TOP inhibitor ADC targeting HER2 (e.g., DS-8201 and other ADCs with topoisomerase inhibitor payloads), and have experienced disease progression.
‣ 1\) Endocrine monotherapy is excluded from the abovementioned standard therapy lines; 2) Disease recurrence occurring within 12 months following neoadjuvant or adjuvant chemotherapy will be regarded as progression after first-line standard therapy; 6. Have at least one measurable target lesion per the Response Evaluation Criteria in Solid Tumors Version 1.1 (RECIST 1.1) .
‣ 7\. ECOG performance status 0-1. 8. Expected survival ≥ 3 months. 9. Bone marrow, hepatic, renal and coagulation function shall be deemed adequate based on laboratory tests performed within 7 days prior to the first dose of the investigational product. Blood transfusion or growth factor supportive therapy is prohibited within 14 days before the first administration of the investigational product：
• Bone marrow function: absolute neutrophil count (ANC) ≥ 1.5 × 10⁹/L; platelet count (PLT) ≥ 80 × 10⁹/L; hemoglobin (Hb) ≥ 90 g/L;
• Hepatic function: aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤ 2.5 × upper limit of normal (ULN); total serum bilirubin (TBIL) ≤ 1.5 × ULN, with the following exceptions;
• For participants with confirmed liver metastases: AST and/or ALT ≤ 5 × ULN;
• For participants diagnosed with Gilbert's syndrome: TBIL ≤ 3 × ULN;
• Serum albumin ≥ 30 g/L;
• Renal function: Creatinine clearance (CrCL) ≥ 50 mL/min (calculated via the Cockcroft-Gault formula), OR serum creatinine ≤ 1.5 × ULN;
• Coagulation function: International Normalized Ratio (INR) ≤ 1.5 × ULN, and activated partial thromboplastin time (APTT) ≤ 1.5 × ULN.
• 10\. Women of childbearing potential (WOCBP) must agree to use highly effective contraception or maintain abstinence from the time of informed consent signature through 6 months after the last administration of the investigational product；For WOCBP, serum pregnancy testing performed within 7 days prior to the first dose of the investigational product must yield a negative result.