Tamoxifen is a potent and effective drug reducing the risk of dying from breast cancer in the adjuvant setting. Although more modern drugs have partly replaced tamoxifen, it is helpful in the neoadjuvant and metastatic settings as a single drug. Despite that, in the adjuvant setting, it is a valuable drug.

This study aims to validate and study the feasibility of serial assessments, including therapeutic drug monitoring of tamoxifen, 4-hydroxytamoxifen and Z-endoxifen by capillary blood sampling, combined with patient-reported symptom scores. This will provide preliminary data to allow us to develop a future multicentre randomised clinical trial of personalised dose monitoring and adjustment of adjuvant tamoxifen therapy to enhance the quality of life and breast cancer outcomes.

This repeated-measures, prospective, open-label, single-centre study is designed for women with stage 0-3 breast cancer receiving adjuvant tamoxifen 20 mg/day.

Inclusion criteria:

1. Female patients aged ≥ 18 years with hormone-positive stage 0-3 breast cancer.
2. Performance status Eastern Cooperative Oncology Group (ECOG) 0-2.
3. Ongoing daily adjuvant tamoxifen minimum of 2 months ± gonadotropin-releasing hormone (GnRH) analogues ± radiation therapy (RT) for stage 3 breast cancer.
4. Locally recurrent disease, previously treated with adjuvant tamoxifen.
5. Able to use software applications developed specifically for small, wireless computing devices, such as smartphones and tablets.
6. Have small, wireless computing devices, such as smartphones and tablets.

Exclusion Criteria:

1. Fulfilling any of the contraindications for tamoxifen.
2. Metastatic (stage IV) breast cancer.
3. Included in other clinical studies receiving not approved investigational medicinal drug.
4. Ongoing pregnancy or lactation.
5. Any psychological, familial, sociological, or geographical condition potentially hampering compliance with the study protocol and follow-up schedule.

No. Of Subjects: 40 female subjects.

Measured components: Tamoxifen, 4-hydroxytamoxifen and Z-endoxifen

Study design: Blood samples for measurement of tamoxifen, 4-hydroxytamoxifen and Z-endoxifen will be drawn capillary in total at 4-time points, at inclusion (baseline), and after 1, 2, and 3 weeks for each participant; and venously in total at 2-time points, at inclusion (baseline), and after 3 weeks for each participant.

At each time, participants will be asked to leave 2 vials of capillary blood (50ul x2) using the rhelise™ kit and 2 samples of conventional venous blood for blood and plasma (5 ml x 2).

Inclusion Criteria:

1. Female patients aged ≥ 18 years with hormone-positive stage 0-3 breast cancer.
2. Performance status ECOG 0-2.
3. Ongoing daily adjuvant tamoxifen minimum of 2 months ± GnRH analogues ± RT for stage 3 breast cancer.
4. Locally recurrent disease, previously treated with adjuvant tamoxifen.
5. Able to use software applications developed specifically for small, wireless computing devices, such as smartphones and tablets.
6. Have small, wireless computing devices, such as smartphones and tablets.

Exclusion Criteria:

1. Fulfilling any of the contraindications for tamoxifen.
2. Metastatic (stage IV) breast cancer.
3. Included in other clinical studies receiving not approved investigational medicinal drug.
4. Ongoing pregnancy or lactation.
5. Any psychological, familial, sociological, or geographical condition potentially hampering compliance with the study protocol and follow-up schedule.