Evaluating tirzepatide and letrozole for reducing tumor growth in postmenopausal hormone‑receptor‑positive breast cancer patients

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What is this study about?

The condition being studied is Hormone Receptor positive postmenopausal breast cancer, a type of breast cancer that grows in response to hormones and occurs after menopause. The study looks at how this cancer behaves when patients receive different medicines before having surgery.

One medicine used is tirzepatide, which belongs to a class called GLP-1/GIP receptor agonist. It is given as a subcutaneous injection, meaning the drug is injected just under the skin. Another medicine is letrozole, a pill taken by mouth that blocks estrogen, a hormone that can help some breast cancers grow. The goal is to see whether these drugs, alone or together, can slow tumor growth, which is described as an anti‑proliferative effect, meaning they may reduce the speed at which cancer cells multiply. Researchers will also watch for any adverse events, or side effects, and will check blood for small pieces of tumor genetic material known as circulating tumor DNA to learn more about the disease.

The purpose of the trial is to compare the change in a cell‑growth marker called KI67 between the start of treatment and the time of surgery across the different treatment groups. Participants will receive the assigned medication(s) for a short period before surgery, undergo the planned operation, and then have follow‑up visits to monitor recovery and any side effects.

1 baseline assessment

after joining the study, you will undergo an initial evaluation that includes medical history, physical examination, and laboratory tests. a tumor biopsy will be taken to measure ki67 expression and to collect blood for circulating tumor dna analysis.

2 start tirzepatide injection

you will begin receiving tirzepatide, a glp‑1/gip‑receptor agonist, by subcutaneous injection. the initial dose is 0.36 mg (0.36 milligram) per injection. the injection will be administered according to the schedule provided by the study staff and will continue until the day of surgery.

3 start letrozole tablets

you will also take letrozole, an oral medication, in the form of a 2.5 mg film‑coated tablet. the tablet is taken by mouth as directed by the study staff and will be continued until the day of surgery.

4 regular monitoring visits

throughout the treatment period you will attend scheduled visits where study staff will check your adherence to tirzepatide injections and letrozole tablets, assess any side effects, and perform laboratory tests. blood samples will be collected to evaluate circulating tumor dna at each visit.

5 surgery

after the prescribed treatment period, you will undergo surgery to remove the breast tumor. a repeat measurement of ki67 expression will be performed on the surgical specimen.

6 post‑operative follow‑up

for up to three weeks after surgery you will be monitored for any complications, side effects, or laboratory abnormalities. additional assessments may be done to evaluate recovery and any changes in circulating tumor dna.

Who Can Join the Study?

Must be a female (woman).
Must be postmenopausal (no menstrual periods for at least 12 months, as defined by the doctor).
The breast tumor must be at least 1 cm in size.
Must have a biopsy that confirms early‑stage breast cancer that is grade II or III, hormone‑receptor positive (ER+) and HER2‑negative. ER+ means the cancer cells have receptors for the hormone estrogen, and HER2‑negative means the cells do not have excess HER2 protein, which can affect treatment. The grade describes how abnormal the cells look under a microscope, with higher numbers indicating faster‑growing tumors.
All invasive breast cancer subtypes are allowed, such as invasive ductal carcinoma or invasive lobular carcinoma.
Must have a body‑mass index (BMI) of 30 kg/m² or higher, or a BMI of 27 kg/m² or higher together with at least one of the following weight‑related health conditions: hypertension (high blood pressure), dyslipidemia (high cholesterol or triglycerides), obstructive sleep apnea (a breathing disorder during sleep), or cardiovascular disease such as blocked arteries or heart failure (class I‑III).
Must be able to read and understand Dutch, French, or English.
Must be willing to follow the study schedule and complete all required visits and procedures.
Must agree to have a new breast biopsy if the original tissue sample cannot be used for the trial.

Who Cannot Join the Study?

Having type 1 or type 2 diabetes (a condition where the body cannot control blood sugar), or a past episode of ketoacidosis (dangerously high acid in the blood), hyperosmolar state (very high blood sugar causing dehydration), or coma.
Undergoing or having had an endoscopic or device‑based treatment for obesity (such as a gastric balloon, gastric artery embolization, intragastric balloon, or duodenal‑jejunal liner) or having the device removed within the last 6 months.
Having severe kidney problems measured as an eGFR (estimated glomerular filtration rate) lower than 30 L/min/1.73 m².
Having a serious stomach‑emptying disorder, such as severe gastroparesis (the stomach does not empty properly) or a gastric outlet blockage, or regularly taking medicines that directly affect gut movement.
Having a past or current history of pancreatitis (inflammation of the pancreas), whether chronic or acute.
Being treated with insulin or other blood‑sugar‑lowering drugs (examples include metformin, sulfonylureas, or DPP‑4 inhibitors).
Participating in another clinical trial that uses an experimental drug or device before surgery (the “neoadjuvant” setting).
Having obesity caused by other hormonal disorders (for example, Cushing syndrome) or genetic forms of obesity (such as Melanocortin‑4 receptor deficiency or Prader‑Willi syndrome).
Having acute or chronic hepatitis or any other liver disease (except non‑alcoholic fatty liver disease) with liver test results above the limits: ALT more than 3 times the normal range, ALP more than 2 times normal, or total bilirubin more than 1.5 times normal (unless you have known Gilbert’s syndrome).
Using systemic hormonal replacement therapy (hormone pills, patches, or injections) within 2 months before screening.
Using a GLP‑1, GIP, or GC receptor agonist (a type of medication that affects appetite and blood sugar) within 2 months before screening.
Having blood test results that suggest diabetes: an HbA1c level of 6.5 % or higher (which reflects average blood sugar over 3 months) or a fasting glucose level of 126 mg/dL (7.0 mmol/L) or higher.
Using any weight‑loss medication within 2 months before screening (examples include liraglutide, orlistat, phentermine, semaglutide, and many others).
Having a history of breast cancer that does not fit the allowed exceptions (the study only accepts certain in‑situ cancers without systemic treatment).
Having another invasive cancer that is still progressing or required active treatment within the past 3 years, except for cured skin cancers (basal or squamous cell) or in‑situ cervical cancer.
Currently receiving or having taken systemic glucocorticoids (steroid pills) for more than 2 weeks within the last 3 months, or having an active autoimmune disease that is likely to need such steroids.
Having a history of drug or alcohol abuse, an eating disorder, or a psychiatric condition that the doctor believes could prevent you from following the study procedures.
Having a personal or family history of medullary thyroid carcinoma (a type of thyroid cancer) or multiple endocrine neoplasia type 2 (MEN‑2) (a genetic syndrome that increases cancer risk).
Gaining or losing more than 5 kg (about 11 lb) of body weight in the 3 months before screening.
Having had prior surgical treatment for obesity, other than cosmetic procedures such as liposuction or abdominoplasty (tummy tuck).

Where you can join this trial?

Verified and Recommended Sites

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Verified Sites

Site Name	City	Country	Status
Katholieke Universiteit te Leuven	Leuven	Belgium

Other Sites

No sites found in this category

Trial status

Country	Status	Recruitment Start
Belgium	Not yet recruiting	01.08.2026

Trial locations

Investigated Drugs:

Mounjaro (tirzepatide) is a medication given as a subcutaneous injection. It works by activating two hormone receptors called GLP‑1 and GIP, which help control blood sugar and can also affect tumor growth. In this trial the drug is being tested to see if it can shrink breast cancer tumors when used before surgery, either alone or together with other treatments.

Letrozole is an oral tablet that belongs to a group of medicines known as aromatase inhibitors. It lowers the amount of estrogen in the body, which can help slow the growth of certain breast cancers that need estrogen to grow. In the study, letrozole is used to compare its effect on tumor size with the injection therapy and with surgery alone.

Hormone receptor‑positive postmenopausal breast cancer – It is a type of breast cancer that grows in response to estrogen or progesterone and occurs after menopause. The tumor usually begins in the milk‑producing ducts or lobules and can expand within the breast tissue. Over time it may extend into nearby lymph nodes and, if unchecked, can spread to distant parts of the body. Hormone‑driven growth often leads to a gradual increase in tumor size. The disease can evolve from a localized lesion to an invasive form that penetrates surrounding tissues.

Trial ID:

2026-525820-76-00

Protocol code:

S71736

Trial Phase:

Therapeutic exploratory (Phase II)

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Evaluating tirzepatide and letrozole for reducing tumor growth in postmenopausal hormone‑receptor‑positive breast cancer patients

What is this study about?

Who Can Join the Study?

Who Cannot Join the Study?