Heterogeneity of 68Ga-FAPI Uptake As Imaging Biomarker in T-DXd Treatment for Brain Metastasis of HER2 Positive Breast Cancer

Inclusion criteria

• Patients aged over 18 years old.
• ECOG 0-2.
• Patients have been diagnosed with unresectable, locally advanced or metastatic HER2-positive breast cancer, which were by means of immunohistochemical analysis (with 3+ indicating positive status), fluorescence in situ hybridization (with an amplification ratio ≥2.0 indicating positive status), or both.
• Brain metastasis was confirmed by MRI; measurable disease as defined by at least one intracranial cerebral metastatic lesion with diameter ≥ 1.0 cm not previously treated with radiation.
• It is allowed to use mannitol, bevacizumab, or corticosteroids before enrollment, but dose should be stable for at least one week.
• Plan to receive Trastuzumab deruxtecan (T-DXd).
• Adequate bone marrow, liver, kidney and cardiac function.
• All patients can provide an informed consent before enrolment and data collection.

Exclusion criteria

• Leptomeningeal involvement.
• Uncontrolled large amount of pleural effusion and ascites.
• Previous treatment of T-DXd.
• Adequate treatment washout period before enrollment, defined as: major surgery ≥4 weeks, radiation therapy ≥4 weeks, chemotherapy ≥4 weeks, small-molecule targeted agents, anticancer hormonal therapy, antibody-based treatment ≥3 weeks.
• Patients with previous interstitial lung disease, drug-induced interstitial lung disease, radiation pneumonitis require corticosteroids treatment; or any clinically active interstitial lung disease currently.
• Use of any investigational agent within 14 d before initiation of treatment.
• Concomitant other anticancer therapy, including cytotoxic, targeted agents, immunotherapy, antibody, retinoid or anticancer hormonal treatment.
• History of malignancy other than squamous cell carcinoma, basal cell carcinoma of the skin or carcinoma in situ of the cervix within the last 3 years, including contralateral breast cancer.
• Clinically significant cardiac disease.
• Patients with known hypersensitivity to trastuzumab or 68Ga-FAPI.
• a history of uncontrolled seizures, central nervous system disorders or psychiatric disability judged by the investigator to be clinically significant and adversely affecting compliance to study drugs.
• Patients with the history of immunodeficiency, including HIV, or other acquired or congenital immunodeficiency diseases, or a history of organ transplantation.
• Patients with HBsAg positive and HBV >=1000, HCV antibody positive, TP-Ab positive or HIV positive.
• Pregnant or lactating women. Women with childbearing potential must have a negative pregnancy test at screening; also excluded are women with childbearing potential, including women whose last menstrual period was <1 year before screening, unable or unwilling to use adequate contraception from study start to 1 year after the last dose of protocol therapy. Acceptable contraception methods included the application of an intrauterine device, barrier method or total abstinence.