Your medical team will confirm that your breast cancer is hormone receptor-positive (meaning the cancer cells have receptors for estrogen or progesterone) and HER2 IHC 0 (a specific type of protein test showing no or minimal HER2 protein on cancer cells).

Your cancer must be either locally advanced and cannot be removed by surgery, or has spread to other parts of the body (metastatic).

Your cancer must have progressed during or after hormone therapy, or you are not suitable for further hormone-based treatment.

Your ECOG performance status (a measure of how the disease affects your daily living abilities) must be 0 or 1, meaning you are fully active or restricted in physically strenuous activity but able to carry out light work.

Your life expectancy must be at least 12 weeks.

A sample of your tumor tissue will be provided for testing.

Your disease must be measurable or evaluable on imaging scans according to specific criteria called RECIST 1.1.

Blood tests will be performed within 7 days before starting treatment to ensure your bone marrow, liver, and kidneys are functioning adequately.

You must not be a candidate for another treatment called T-DXd, either because your tumor characteristics do not match, you have other health conditions that prevent its use, or the treatment is not available to you.

You will receive datopotamab deruxtecan (also called Dato-DXd), which is the study medication being tested.

The medication will be given as an intravenous infusion, meaning it will be administered directly into your vein through a drip.

The infusion will be prepared as a solution specifically for this purpose.

You will receive the study medication according to a regular schedule determined by your medical team.

During treatment, you will have regular imaging scans to monitor how your cancer is responding to the medication.

Your medical team will assess whether your disease is progressing, staying stable, or shrinking according to RECIST 1.1 criteria.

Blood tests and physical examinations will be performed regularly to monitor your overall health and organ function.

Your medical team will closely monitor you for side effects related to the study medication.

Specific attention will be paid to monitoring for oral mucositis or stomatitis (inflammation or sores in the mouth), ocular events (eye-related problems), and any severe side effects (Grade 3 or higher).

Any side effects will be recorded and graded according to a standard system called NCI CTCAE 5.0.

Your medical team will manage any side effects that occur during treatment.

Your response to treatment will be evaluated at regular intervals, including at 24 weeks after starting the medication.

The assessment will determine if your cancer has completely disappeared (complete response), partially shrunk (partial response), remained stable (stable disease), or progressed.

If your cancer responds to treatment, the duration of that response will be tracked from when the response is first confirmed until the disease progresses or other specified events occur.

You will continue receiving the study medication as long as your disease is not progressing and you are tolerating the treatment.

Treatment may continue until your disease progresses according to RECIST 1.1 criteria, unacceptable side effects occur, or you and your medical team decide to discontinue for other reasons.

Your medical team will continue to monitor your condition throughout the treatment period.

After completing or discontinuing the study medication, you will continue to be followed by your medical team.

Follow-up will continue to track your overall survival and long-term outcomes.

This follow-up period is expected to continue until the study concludes, which is estimated to be in August 2028.