An Open-label, Single-arm Study of Prophylaxis for Datopotamab Deruxtecan (Dato-DXd) -Related Stomatitis in Eligible Patients With Metastatic or Inoperable Locally Recurrent Breast Cancer or Locally Advanced or Metastatic Epidermal Growth Factor Receptor-Mutated Non-small Cell Lung Cancer.

Inclusion criteria

• Patients are eligible to be included in the study only if all the following criteria apply: Disease Characteristics
• Patients with any of the following disease characteristics are eligible (specific population subject to FDA Dato-DXd label): • Patients with unresectable or metastatic HR+/HER2- (IHC 0, IHC 1+, or IHC 2+/ISH-) breast cancer who have received prior ET and chemotherapy for unresectable or metastatic disease. OR • Adult patients with unresectable or metastatic TNBC who are not candidates for PD-1/PD-L1 inhibitor therapy. Participants will be enrolled upon FDA approval on use of Dato-DXd for this indication. OR • Patients with locally advanced or metastatic EGFRm NSCLC who have received prior EGFR-directed therapy and platinum-based chemotherapy. Age
• Patient must be ≥ 18 years, at the time of signing the informed consent. Type of Patient and Disease Characteristics-
• Has documentation that Dato-DXd will be prescribed for the labelled indication and has not received any dose of Dato-DXd prior to enrollment.
• Is willing to comply with use of prophylactic dexamethasone mouthwash at the start of first infusion and throughout Dato-DXd administration.
• ECOG performance status 0 or 1.
• Has adequate bone marrow function (hemoglobin ≥ 9 g/dL; red blood cell/plasma transfusion is not allowed within 1 week prior to screening assessment).
• All women of childbearing potential must have a negative serum pregnancy test result at screening. Informed Consent
• Capable of giving signed informed consent as described in the protocol which includes compliance with the requirements and restrictions listed in the ICF and the protocol.

Exclusion criteria

• Patients are excluded from the study if any of the following criteria apply:
• Has received Dato-DXd prior to enrollment. Medical Conditions
• As judged by the investigator, any evidence of cardiac, pulmonary, vascular, and other renal conditions which in the investigator's opinion makes it undesirable for the patient to participate in the study.
• Has any-grade active and uncontrolled stomatitis or mouth ulcers at baseline (participants with prior medical history of stomatitis or mouth ulcers are eligible).
• Use of steroid-containing mouthwash is contraindicated in, including but not limited to:
• Active oral infections (eg, viral, bacterial, or fungal), where the use of a steroid-containing mouthwash may impair local immune response and delay healing
• Known hypersensitivity or allergy to corticosteroids or any excipients contained in the mouthwash formulation
• Uncontrolled infection requiring IV antibiotics, antivirals, or antifungals; suspected infections (eg, prodromal symptoms); or inability to rule out infections (participants with localized fungal infections of skin or nails are eligible).
• Has clinically significant corneal disease.
• Has a history of severe hypersensitivity reactions to either the drug or inactive ingredients of Dato-DXd (including, but not limited to, polysorbate 80).
• Has a history of severe hypersensitivity reactions to other monoclonal antibodies.
• Is pregnant, or breastfeeding, or planning to become pregnant. Other Exclusions
• Involvement in the planning and/or conduct of the study (applies to both AstraZeneca staff and/or staff at the study site).
• Judgment by the investigator that the patient should not participate in the study if the patient is unlikely to comply with study procedures, restrictions, and requirements.
• Previous enrollment in the present study.