⁃ Patients will be eligible for study participation if they comply with the following criteria:

• Written informed consent for all study procedures according to local regulatory requirements prior to beginning specific protocol procedures.

• Females or males, ≥ 18 years and ≤ 75 years of age.

• Invasive breast carcinoma as revealed by local pathology that is either:

∙ HER2-positive defined as an immunohistochemistry (IHC) score of 3+ and/or positive by in situ hybridization (ISH) in Her2 2+ tumors (as defined in 2018 American Society of Clinical Oncology - College of American Pathologists \[ASCO-CAP\] guidelines)

‣ HER2-low defined as an immunohistochemistry (IHC) score of 1+ or an IHC score of 2+ with a mandatory negative in situ hybridization (ISH), as defined in 2018 American Society of Clinical Oncology - College of American Pathologists \[ASCO-CAP\] guidelines.

• Complete resection of the tumor with resection margins free of invasive carcinoma (R0).

• Participation in the SURVIVE study and evidence of molecular relapse (as assessed based on a positive ctDNA result obtained in the SURVIVE-study)

• No evidence of metastatic relapse as revealed by a CT-scan (Abdomen/Chest) and a SPECT bone scan that must be performed within 8 weeks before randomization (M0).

• Completion of surgery, (neo-)adjuvant chemotherapy (if applicable) and radiation therapy (if applicable, whichever occurred last) at least 6 months before randomization.

• Adjuvant/Postneoadjuvant treatment with Trastuzumab, Pertuzumab, T-DM1, Capecitabine, Pembrolizumab, and Olaparib must be discontinued upon randomization into Arm A (treatment with trastuzumab deruxtecan). The washout periods (see Table 2) must be complied with. Endocrine therapy (i.e. Tamoxifen, Letrozol, Anastrozol, Fulvestrant or Exemestane) can be administered simultaneously to treatment with trastuzumab deruxtecan.

• Known HR status, per local laboratory assessment, as defined by ASCO-CAP guidelines (≥1%): HR-positive status defined by either positive estrogen receptor (ER) and/or positive progesterone receptor (PR) status. HR-negative status defined by both known negative ER and known negative PR

⁃ Left ventricular ejection fraction (LVEF) ≥50% within 28 days prior to randomization

⁃ Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 at Screening

⁃ Adequate organ and bone marrow function within 28 days before randomization as described in table 1. Organ and bone marrow function criteria must also be met when laboratory tests are repeated within 3 days before Cycle 1 Day 1. Transfusion (red blood cell or platelet) or G-CSF administration is not allowed within 2 weeks prior to the day on which marrow function is assessed.

⁃ Adequate treatment washout period before treatment with trastuzumab deruxtecan (in case of randomization into cohort A), defined in table 2.

⁃ Female subjects: Evidence of post-menopausal status or negative serum pregnancy test for females of childbearing potential who are sexually active with a non-sterilized male partner. For women of childbearing potential, a negative result for serum pregnancy test (test must have a sensitivity of at least 25 mIU/mL) must be available at the screening visit and urine beta-human chorionic gonadotropin (β-HCG) pregnancy test prior to each administration of Investigational Medicinal Product (IMP).

• Women of childbearing potential are defined as those who are not surgically sterile (underwent bilateral salpingectomy, bilateral oophorectomy, or complete hysterectomy) or post-menopausal. Women will be considered post-menopausal if they have been amenorrhoeic for 12 months without an alternative medical cause.

∙ Female patients of childbearing potential who are sexually active with a non-sterilized male partner must use at least one highly effective method of contraception (see 5.5.1.) from the time of screening and must agree to continue using such precautions for 7 months after the last dose of IMP. Not all methods of contraception are highly effective. Female patients must refrain from breastfeeding while on study and for 7 months after the last dose of IMP. Complete heterosexual abstinence for the duration of the study and drug washout period is an acceptable contraceptive method if it is line with the patient's usual lifestyle (consideration must be made to the duration of the clinical trial); however, periodic or occasional abstinence, the rhythm method, and the withdrawal method are not acceptable.

∙ Female subjects must not donate, or retrieve for their own use, ova from the time of randomization and throughout the study treatment period, and for at least 7 months after the final study drug administration. They should refrain from breastfeeding throughout this time. Preservation of ova may be considered prior to enrollment in this study.

⁃ Male subjects: Non-sterilized male patients who are sexually active with a female partner of childbearing potential must use a condom with spermicide from screening to 4 months after the final dose of IMP. Complete heterosexual abstinence for the duration of the study and drug washout period is an acceptable contraceptive method if it is in line with the patient's usual lifestyle (consideration must be made to the duration of the clinical trial); however, periodic or occasional abstinence, the rhythm method, and the withdrawal method are not acceptable. It is strongly recommended for the female partners of a male patient to also use at least one highly effective method of contraception throughout this period, as described in section 5.5.1. In addition, male patients should refrain from fathering a child, or freezing or donating sperm from the time of randomization/enrolment, throughout the study and for 4 months after the last dose of IMP. Preservation of sperm should be considered prior to enrolment in this study.