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