Nectin-4 PET to Predict Response to Nectin-4-Targeted Therapy in Advanced Breast Cancer

This is a prospective, single-arm, exploratory clinical trial that plans to enroll 40 patients with locally advanced or metastatic breast cancer who are expected to participate in a therapeutic clinical trial of a Nectin-4-targeted agent at our center and receive at least two treatment cycles. All participants must meet the eligibility criteria of the therapeutic clinical trial in which they intend to participate and provide separate written informed consent for this study. The Nectin-4-targeted agents used in the two clinical trials currently underway at our center are provided to participants free of charge. All participants must undergo an \^18F-FDG PET/CT scan and a complimentary \^68Ga-FZ-NR-1 PET/CT scan (Nectin-4 PET) within 28 days before initiating treatment with a Nectin-4-targeted agent. After completing the baseline assessments, participants will begin treatment in accordance with the protocol of the therapeutic clinical trial of the Nectin-4-targeted agent in which they are enrolled and will be followed according to the requirements of the corresponding protocol until disease progression, unacceptable toxicity, or withdrawal from the study. Participants will be encouraged to undergo an additional complimentary Nectin-4 PET/CT scan after completion of the first treatment cycle and within 7 days before administration of the second treatment cycle to assess early changes in target expression during treatment. Peripheral blood samples will be collected at baseline, before the first dose of Cycle 3, and at disease progression. Participants will also be encouraged to undergo needle biopsy of a metastatic lesion at baseline and at disease progression to explore potential mechanisms of resistance and other relevant biomarkers. All PET/CT images will be independently reviewed by two physicians from the Department of Nuclear Medicine at our center who have extensive experience in diagnostic imaging.

Inclusion Criteria: \- 1. \*\*Age ≥18 years.\*\* 2\. \*\*Histopathologically confirmed unresectable locally advanced or metastatic breast cancer.\*\* 3\. The participant must be confirmed during screening to meet all inclusion and exclusion criteria for at least one of the following therapeutic clinical trials involving a Nectin-4-targeted agent being conducted at this center and must have signed the corresponding informed consent form: 1. \*\*Protocol BC-MUL-IIT-SHRA2102:\*\* \*A Study of the Efficacy and Safety of SHR-A2102 in Patients With HR-Positive/HER2-Negative Advanced Breast Cancer Following Failure of a CDK4/6 Inhibitor Combined With Endocrine Therapy\*; 2. \*\*Protocol SC-101-201:\*\* \*A Phase IIa Clinical Study Evaluating the Efficacy and Safety of SC-101 in Patients With Advanced Malignant Tumors Positive for NECTIN4 Gene Amplification.\* * Note:\*\* Summaries of the study designs and eligibility criteria for these two clinical trials are provided in Appendix 1. 4\. \*\*Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1.\*\* 5\. Radiographic or objective evidence of disease progression during or after the most recent systemic therapy administered before initiation of the study treatment, or intolerance to toxicity associated with prior treatment. 6\. \*\*Life expectancy of ≥12 weeks at screening.\*\* 7\. At least one lesion that has not previously received radiotherapy and is accurately measurable at baseline by CT or MRI, with a longest diameter of ≥10 mm. For lymph nodes, the short-axis diameter must be ≥15 mm. For participants with bone-only disease, osteolytic or mixed osteolytic bone lesions evaluable by CT, MRI, or X-ray are acceptable. 8\. \*\*Left ventricular ejection fraction ≥50% within 28 days before randomization.\*\* 9\. Adequate organ and bone marrow function within 14 days before randomization. For all parameters listed below, the most recently obtained test results must meet the following eligibility criteria: 1. Hemoglobin ≥9 g/dL; 2. Absolute neutrophil count ≥1,500/mm³; 3. Platelet count ≥100,000/mm³; 4. At baseline, total bilirubin (TBL) ≤1.5 × the upper limit of normal (ULN) in the absence of liver metastases, or \