TAMOXIFEN for Breast cancer

Inclusion criteria

• {"criterion_text":"- Patients with treatment naïve, ≥cT1c ER+, Her2- primary breast cancer (tumor still in situ) with a Ki-67 ≥10%"}
• {"criterion_text":"- Age older than 18 year"}
• {"criterion_text":"- An indication for postoperatively endocrine treatment"}
• {"criterion_text":"- WHO performance status 0-2"}
• {"criterion_text":"- The primary breast tumor should be amendable for multiple biopsies"}
• {"criterion_text":"- Specific for tamoxifen: Indication for adjuvant tamoxifen treatment"}

Exclusion criteria

• {"criterion_text":"- Women who are pregnant or breast feeding"}
• {"criterion_text":"- Patients in whom neoadjuvant chemotherapy is indicated and delay is not desirable"}
• {"criterion_text":"- Patients with known alcoholism, drug addiction and/or psychiatric or physiological condition which in the opinion of the investigator would impair treatment compliance"}
• {"criterion_text":"- Evidence of any other disease, neurological or metabolic dysfunction, physical examination finding or laboratory finding giving reasonable suspicion of a disease or condition that contraindicates the use of endocrine treatment or puts the patient at high risk for treatment related complications."}
• {"criterion_text":"- Specific for tamoxifen: Endometrial cancer (diagnosis < 3 years ago)"}
• {"criterion_text":"- Specific for tamoxifen: •\tUse of strong CYP2D6-inhibitors without the possibility to stop this medication"}
• {"criterion_text":"- Specific for tamoxifen: •\tConcomitant use of over-the-counter-drugs, herbal or homeopathic remedies which (may) interfere with tamoxifen and not willing to stop such as (but not limited to ) St. John’s Wort, curcuma/piperine"}
• {"criterion_text":"- Specific for tamoxifen: Known poor metabolizer of CYP2D6"}

Primary endpoints

• {"endpoint_text":"- Endoxifen levels after three months of continuous use of 20mg (standard dose) of tamoxifen","definition_or_measurement_approach":"Measure endoxifen plasma/tissue levels after three months of continuous 20 mg tamoxifen"}
• {"endpoint_text":"- Tamoxifen resistance defined as Ki67 >10% after three weeks of tamoxifen use according to an adequate dosing schedule","definition_or_measurement_approach":"Tamoxifen resistance operationalised as Ki67 >10% measured after three weeks of tamoxifen treatment"}
• {"endpoint_text":"- Intratumoral endoxifen levels after three weeks and Ki67 response defined as difference between Ki67 percentage for and Ki67 percentage after three weeks of tamoxifen according to an adequate dosing schedule","definition_or_measurement_approach":"Measure intratumoral endoxifen levels at three weeks and calculate Ki67 response as the difference between pre-treatment Ki67 percentage and Ki67 percentage after three weeks"}

Secondary endpoints

• {"endpoint_text":"- Ki67 percentages on tumor material","definition_or_measurement_approach":"Ki67 percentage measured on tumor biopsy/resection material"}
• {"endpoint_text":"- Levels on estradiol and progesterone in blood after three weeks and three months of tamoxifen treatment","definition_or_measurement_approach":"Measure estradiol and progesterone blood levels at three weeks and three months of tamoxifen treatment"}
• {"endpoint_text":"- Ki67 percentages on tumor material measured by the pathologist and measured by an AI algorithm","definition_or_measurement_approach":"Compare Ki67 percentage as assessed by pathologist vs an AI algorithm on tumor material"}
• {"endpoint_text":"- Change in scores of the FACT-ES questionnaire","definition_or_measurement_approach":"Assess change in patient-reported FACT-ES scores to evaluate side-effect burden"}

Netherlands

Earliest CTIS Part Ii Submission Date

24-09-2025

Latest Decision Or Authorization Date

21-10-2025

Processing Time Days

27

Number Of Sites

1

Number Of Participants

175

Primary sponsor

Full Name

Erasmus Universitair Medisch Centrum Rotterdam (Erasmus MC)

Organisation Type

Hospital/Clinic/Other health care facility

Country Of Registered Address

Netherlands