This study is a multicenter, double-cohort clinical research aimed at systematically evaluating the value of jointly detecting the HER2 expression of CTCs/CTECs and the serum HER2 ECD level in predicting the efficacy of T-DXd treatment for patients with HER2 low expression (IHC 1+ or IHC 2+/ISH-) and HER2 IHC 0 advanced breast cancer.

The study plans to include 100 patients and divide them into cohort A (HER2 low expression, 70 cases) and cohort B (HER2 IHC 0, 30 cases) based on the tissue HER2 status. This study adopts a non-random design, and patients will be directly included in the corresponding cohort according to their baseline tissue HER2 status. All enrolled patients will receive standard T-DXd treatment after signing the informed consent form until disease progression, occurrence of intolerable toxic reactions, or study termination. The primary endpoint of the study is to explore the correlation between the HER2 expression status of CTCs/CTECs and ORR. The secondary endpoints include the correlation between the baseline HER2 expression status of CTCs/CTECs and the DCR, PFS, and OS of the entire population; the correlation between the baseline serum HER2 ECD level and the ORR, DCR, PFS, and OS of the entire population; and the ORR and PFS of each cohort. Patients will be followed up from the first administration and samples of peripheral blood will be collected for biomarker dynamic monitoring before treatment, at 2 cycles of treatment, and at the time of disease progression (the number of tests may be increased or decreased according to the patient's condition) until disease progression, death, loss to follow-up, or study closure.

Inclusion Criteria:

1. Patients with a pathological record of breast cancer;
2. Group A: Diagnosed with low expression of HER2 (IHC 1+ or IHC 2+ and ISH-); Group B: Diagnosed with HER2 0 expression (IHC 0);
3. No history of other malignant tumors;
4. Signed the informed consent form at the age of 18 or above;
5. Possess relatively complete clinical case characteristic data.

Exclusion Criteria:

• 1. Male breast cancer patients; 2. Pregnant or breastfeeding; 3. Patients known to have an allergic reaction to the active substances or any excipients of T-DXd; 4. Patients judged by the investigators to be unsuitable to participate in this study.

female

This study focused on breast cancer patients who had received treatment with anti-HER2-ADC drugs.