HER2-PET as a Precision Imaging Tool for Treatment With HER2-ADC in HER2-expressing mBC

Inclusion Criteria: * Female patients age ≥18 years. * Metastatic or locally advanced breast cancer with disease progression after ≥ 1 line of chemotherapy in the palliative setting, or with disease relapse within six months after completion of (neo-) adjuvant chemotherapy. * The patient must be able and willing to provide written consent to participate in the study. * At least one metastatic lesion ≥ 10 mm is available for biopsy o Exception can be made when a recent biopsy is available (no more than 12 months old and without exposition to HER2-targeted therapy or local radiotherapy to the specific lesion). * At least one additional metastatic index lesion ≥ 10 mm for evaluation of treatment effect (according to RECIST v1.1) * WHO performance status ≤ 2. * Expected survival \> 12 weeks. * Contraceptives: Females of child-bearing potential must agree to use adequate contraception prior to study entry, for the duration of the study treatment phase and for six months after the last dose of \[68Ga\]Ga-ABY-025. Examples of contraceptive methods with a failure rate of \< 1% per year include bilateral tubal ligation, male sterilization, established, proper use of hormonal contraceptives that inhibit ovulation, hormone- releasing intrauterine devices (IUDs), and copper IUDs. Periodic abstinence (e.g., calendar, ovulation, symptothermal, or postovulation methods) and withdrawal are not acceptable methods of contraception. Women must refrain from donating eggs during this same period. Should a female become pregnant or suspect she is pregnant while participating in this study, she should inform her study physician immediately. If a female participant is of child-bearing potential (females are considered not of childbearing potential if they are at least one year postmenopausal and/or surgically sterile), she must have a documented negative serum Pregnancy testing prior to each administration of the IMP is obligatory. Exclusion Criteria: * Contra-indications for treatment for trastuzumab deruxtecan and inability to undergo this treatment as per local treatment routines. * A previously documented metastatic tumor biopsy that was HER2-positive (IHC 3+ and/or HER2 gene amplification). * Other manifest malignancies except for basal cell carcinoma of the skin. * Inadequate cardiac, renal, bone marrow or liver function * Patients with increased risk of complications from biopsies, i.e. increased risk of bleeding, defined as * prothrombin time test (INR value) \>1.4, platelet count \<70 (109/l), activated partial thromboplastin time (APTT) \>30s. * known bleeding disorders such as haemophilia, von Willebrand disease or platelet disorders. * any anticoagulants or antiplatelet treatment that cannot be temporarily paused