The treatment will begin on Cycle 1 Day 1. Each cycle lasts a specific duration, and the treatment will continue according to the schedule determined by the study protocol.

Two medications will be administered: trastuzumab deruxtecan (also called T-DXd) and pertuzumab. Both medications will be given through a vein, which is called intravenous infusion.

The specific dosage, frequency, and duration of each medication will be determined according to the study protocol and individual medical assessment.

Throughout the treatment, heart function will be monitored regularly. Before starting treatment, a test called LVEF (which measures how well the heart pumps blood) must show a result of at least 50%.

Physical examinations and blood tests will be performed at scheduled intervals to check blood cell counts, liver function, and kidney function.

Imaging scans will be conducted periodically to evaluate how the disease is responding to treatment. These assessments will follow RECIST version 1.1 criteria, which is a standardized method for measuring tumor response.

A smartphone with internet connection will be required throughout the study. The device must have Android 8.0 or newer, or iOS 15.0 or newer for one application, and Android 13.0 or newer, or iOS 17.0 or newer for another application.

Regular assessments will need to be completed using digital applications at specified times. These include surveys about symptoms and quality of life.

Oxygen level measurements using an oximeter (a device that clips onto a finger to measure oxygen in the blood) may be required at certain timepoints.

Educational content and self-management modules may be available through these digital tools.

Symptoms and side effects will be monitored throughout treatment using questionnaires called PRO-CTCAE (Patient-Reported Outcomes version of the Common Terminology Criteria for Adverse Events).

Quality of life will be assessed using standardized questionnaires: EORTC QLQ-C30 (a general cancer quality of life questionnaire) and EORTC QLQ-BR42 (a breast cancer-specific questionnaire).

These questionnaires will measure various aspects including physical functioning, role functioning, pain, nausea, vomiting, and overall health status.

If certain symptoms or alerts are triggered through the digital monitoring system, additional medical visits or interventions may be scheduled.

Medications to prevent or manage nausea and vomiting may be prescribed. A combination regimen of 3 anti-nausea medications may be used as a preventive measure.

If side effects occur, the dose of study medication may be reduced or temporarily stopped to allow recovery.

Additional supportive care medications may be prescribed as needed to manage any side effects that develop.

In some cases, if side effects are significant, hospitalization may be necessary.

If brain involvement is present and symptoms occur, corticosteroid medication (such as dexamethasone) may be used, but the dose should not exceed 3 mg per day or equivalent.

Treatment will continue as long as the disease does not progress and side effects remain manageable.

The actual amount of medication received and the duration of treatment will be recorded throughout the study.

If treatment needs to be interrupted due to side effects, it may be resumed once recovery occurs.

If the disease progresses or unacceptable side effects develop, treatment may be discontinued and alternative therapy may be considered.

After treatment ends, follow-up will continue to monitor disease status and overall health.

Information about any subsequent cancer treatments that are started after study treatment ends will be recorded.

Survival status and overall well-being will be tracked as part of the study follow-up.

The estimated duration of participation in the study, including follow-up, extends until May 2030.