Trastuzumab Deruxtecan for the Treatment of HER2 Immunohistochemistry 0 Unresectable or Metastatic Breast Cancer

Inclusion Criteria

• Must be competent and able to comprehend, sign, and date an institutional review board (IRB) approved informed consent form (ICF) before performance of any study-specific procedures or tests
• Men or women ≥ 18 years old
• Pathologically documented breast cancer that: * Is unresectable or metastatic. * Has always been HER2-IHC 0 and never previously HER2-positive (IHC 3+ or in situ hybridization [ISH]+) or HER2-low (1+, or 2+ ISH-) on prior pathology testing according to American Society of Clinical Oncology College of American Pathologists (ASCO-CAP) guidelines * Is either hormone receptor (HR)-positive or HR-negative per ASCO-CAP guidelines * If HR-positive, has or has not been treated with a CDK4/6 inhibitor * Has been treated with 1 or 2 prior lines of systemic therapy (which includes either standard cytotoxic chemotherapy and/or antibody-drug conjugates [e.g sacituzumab-govitecan, datopotamab-deruxtecan, or others]) in the metastatic setting. If recurrence occurred within months of the last dose of (neo)adjuvant chemotherapy, (neo)adjuvant chemotherapy would count as 1 line of therapy. Endocrine therapies, immune checkpoint inhibition without systemic chemotherapy, and targeted-therapies (e.g. olaparib, or others) do not count as systemic cytotoxic chemotherapy lines and are unlimited prior to enrollment * Was never previously treated with anti-HER2 therapy
• Documented radiologic progression (during or after most recent treatment)
• Adequate archival tumor sample <3 years-old available for assessment of HER2 status by IHC and by HS-HER2 quantitative assay. If archival tissue is not available or inadequate for assessment (e.g. decalcified bone, cytology, or other), a newly obtained biopsy from a metastatic site (or breast tissue if locally advanced/unresectable disease as the only site of advanced disease) is required on enrollment
• Presence of at least 1 measurable lesion based on computed tomography (CT) or magnetic resonance imaging (MRI) per modified Response Evaluation Criteria in Solid Tumors (mRECIST) version 1.1
• Eastern Cooperative Oncology Group (ECOG) performance status (PS) ≤ 2
• Left ventricular ejection fraction (LVEF) ≥ 50% within 28 days prior to cycle 1 day 1 (C1D1)
• Platelet count ≥ 100,000/mm^3 (within 28 days prior to C1D1) (Platelet transfusion is not allowed within 1 week prior to screening assessment)
• Hemoglobin level ≥ 9.0 g/dL (within 28 days prior to C1D1) (red blood cell transfusion is not allowed within 1 week prior to screening assessment)
• Absolute neutrophil count ≥ 1500/mm^3 (within 28 days prior to C1D1) (granulocyte colony-stimulating factor administration is not allowed within 1 week prior to screening assessment)
• Creatinine clearance ≥ 30 mL/min, as calculated using the Cockcroft-Gault equation (within 28 days prior to C1D1)
• Aspartate aminotransferase (AST)/alanine aminotransferase (ALT) ≤ 3 x upper limit of normal (ULN) (within 28 days prior to C1D1) (< 5 x ULN in participants with liver metastases)
• Total bilirubin ≤ 1.5 x ULN if no liver metastases or < 3 x ULN in the presence of documented Gilbert's syndrome (unconjugated hyperbilirubinemia) or liver metastases at baseline (within 28 days prior to C1D1)
• International normalized ratio or prothrombin time and either partial thromboplastin or activated partial thromboplastin time ≤ 1.5 x ULN (within 28 days prior to C1D1)
• Adequate treatment washout period before C1D1: * Major surgery ≥ 4 weeks * Radiation Therapy including palliative stereotactic radiation therapy to chest ≥ 4 weeks * Palliative stereotactic radiation therapy to other anatomic areas including whole brain radiation ≥ 2 weeks * Anti-Cancer chemotherapy (Immunotherapy [non-antibody based therapy]), hormonal therapy, antibody-based therapy, or retinoid therapy ≥ 3 weeks * Targeted agents and small molecules ≥ 2 weeks or 5 half-lives, whichever is longer * Cell-free and concentrated ascites reinfusion therapy (CART), peritoneal shunt or drainage of pleural effusion, ascites or pericardial effusion ≥ 2 weeks prior to screening assessment
• 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 (beta-HCG) pregnancy test prior to each administration of TDXd. Women of childbearing potential are defined as those who are not surgically sterile (i.e. underwent bilateral salpingectomy, bilateral oophorectomy, or complete hysterectomy) or post-menopausal. Women will be considered post-menopausal if they have been amenorrheic 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, from the time of screening (those using hormonal methods must have been stable on their chosen form of contraception for 3 months prior to study entry) and must agree to continue using such precautions for 7 months after the last dose of T-DXd. 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 T-DXd. 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 * Non-hormonal methods: ** Total heterosexual abstinence (evaluate in relation to the duration of the clinical study and the preferred and usual lifestyle choice of the participant) ** Vasectomised sexual partner (provided that partner is the sole sexual partner of the trial participant and that the vasectomised partner has received medical assessment of the surgical success) ** Bilateral tubal occlusion ** Intrauterine device (provided coils are copper banded) * Hormonal methods: ** Combined (estrogen and progestogen containing) hormonal contraception associated with inhibition of ovulation *** Oral *** Intravaginal *** Transdermal ** Progestogen-only hormonal contraception associated with inhibition of ovulation *** Oral *** Injectable *** Implantable ** Intrauterine hormone-releasing system (IUS)
• Non-sterilized male patients who are sexually active with a female partner of childbearing potential must use a condom from screening to 4 months after the final dose of T-DXd. 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. In addition, male patients should refrain from fathering a child, or freezing or donating sperm from the time of enrollment, throughout the study and for 4 months after the last dose of TDXd. Preservation of sperm should be considered prior to enrollment in this study
• Female subjects must not donate, or retrieve for their own use, ova from the time of enrollment 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

Exclusion Criteria

• Uncontrolled or significant cardiovascular disease, including any of the following: * History of myocardial infarction within 6 months before enrollment * History of symptomatic congestive heart failure (New York Heart Association class II to IV) * Corrected QT interval (QTc) prolongation to > 470 ms (females) or > 450 ms (male) based on average of screening 12 lead electrocardiogram (ECG)
• Has a history of (noninfectious) interstitial lung disease (ILD)/pneumonitis that required steroids, has current ILD/pneumonitis, or where suspected ILD/pneumonitis cannot be ruled out by imaging at screening
• Has spinal cord compression or clinically active central nervous system metastases, defined as untreated and symptomatic, or requiring therapy with corticosteroids to control associated symptoms * Subjects with clinically inactive brain metastases may be included in the study * Subjects with treated brain metastases that are no longer symptomatic and who require no treatment with corticosteroids may be included in the study if they have recovered from the acute toxic effect of radiotherapy. A minimum of 2 weeks must have elapsed between the end of whole brain radiotherapy and study enrollment
• Has multiple primary malignancies within 3 years, except adequately resected non-melanoma skin cancer, curatively treated in situ disease, or contralateral breast cancer
• Has a history of severe hypersensitivity reactions to either the drug substances or inactive ingredients in the drug product
• Has a history of severe hypersensitivity reactions to other monoclonal antibodies
• Has an uncontrolled infection requiring ongoing IV antibiotics, antivirals, or antifungals
• Substance abuse or medical conditions such as clinically significant cardiac or pulmonary diseases or psychological conditions, that would, in the opinion of the investigator, increase the safety risk to the subject or interfere with the subject’s participation in the clinical study or evaluation of the clinical study results
• Social, familial, or geographical factors that would interfere with study participation or follow-up
• Has known history of human immunodeficiency virus (HIV) infection with detectable viral load or CD4 count < 200 cells per cubic millimeter or active hepatitis B (hepatitis B virus surface antigen [HBsAg] positive) or C (hepatitis B virus [HCV] positive ribonucleic acid [RNA]) infection
• Has unresolved toxicities from previous anticancer therapy, defined as toxicities (other than alopecia) not yet resolved to grade ≤ 1 or baseline. Subjects with chronic grade 2 toxicities may be eligible per the discretion of the sponsor-investigator (eg, grade 2 chemotherapy-induced neuropathy, fatigue, residual endocrinopathies from use of immunotherapy such as hypothyroidism/hyperthyroidism, type 1 diabetes, hyperglycemia, adrenal insufficiency, adrenalitis, skin hypopigmentation [vitiligo])
• Therapeutic radiation therapy or major surgery within 4 weeks before study treatment or palliative stereotactic radiation therapy (other than abdominal radiation) within 2 weeks before study treatment
• Systemic treatment with anticancer therapy, antibody-based therapy, retinoid therapy, or hormonal therapy within 3 weeks before study treatment; or treatment with nitrosoureas or mitomycin C within 6 weeks before study treatment; or treatment with small-molecule targeted agents within 2 weeks, or 5 half-lives before study treatment, whichever is longer
• Current treatment with strong cytochrome P450 (CYP3A4) and organic anion transporting polypeptide (OATP) inhibitors and any monoclonal antibody treatment (washout period of ≥ 3 elimination half-lives of the inhibitor/antibody is required)
• Participation in a therapeutic clinical study within 3 weeks before study treatment (for small-molecule targeted agents, this non-participation period is 2 weeks or 5 half-lives, whichever is longer), or current participation in other investigational procedures
• Is pregnant or breastfeeding, or planning to become pregnant
• Subject must not be study site personnel directly involved in the clinical trial, or an immediate family member of someone directly involved
• Receipt of live, attenuated vaccine (messenger [m]RNA and replication deficient adenoviral vaccines are not considered attenuated live vaccines) within 30 days prior to the first exposure to study intervention * Note: Participants, if enrolled, should not receive live vaccine during the study and up to 30 days after the last dose of study intervention
• Lung criteria: * Lung-specific intercurrent clinically significant illnesses including, but not limited to, any underlying pulmonary disorder (e.g. pulmonary emboli within three months of the study enrollment, severe asthma, severe chronic obstructive pulmonary disorder, restrictive lung disease, symptomatic, recurrent and uncontrolled pleural effusion etc.) * Any autoimmune, connective tissue or inflammatory disorders, including rheumatoid arthritis, Sjogren's, and sarcoidosis, where there is documented, or a suspicion of pulmonary involvement at the time of screening. Full details of the disorder should be recorded in the electronic case report form (eCRF) for participants who are included in the study * Prior pneumonectomy (complete)
• Otherwise considered inappropriate for the study by the investigator