A new clinical trial suggests that a low daily dose of endoxifen may help lower breast cancer risk by reducing breast density. Breast density is an important risk factor for breast cancer and can also make tumors harder to detect on mammograms. In this study, researchers found that even small doses of endoxifen led to meaningful reductions in breast density with manageable side effects.
Why Breast Density Matters in Breast Cancer Risk
Breast density refers to how much glandular and connective tissue appears on a mammogram compared with fatty tissue. Women with dense breasts face two main concerns:
- Higher risk of developing breast cancer
- Greater difficulty detecting cancer on a mammogram
Previous research has shown that when tamoxifen lowers breast density, women often have better outcomes and a reduced risk of breast cancer. Because of this, a drop in breast density is considered an important sign that a prevention treatment may be working.
What Is Endoxifen and How Is It Different From Tamoxifen?
Tamoxifen has long been used to treat and prevent certain types of breast cancer. It works by blocking estrogen receptors. Estrogen can help some breast cancers grow.
However, tamoxifen is a “pro-drug,” meaning:
- It must be converted in the liver into its active form
- The most powerful active form is called (Z)-endoxifen
- Not everyone converts tamoxifen efficiently
By giving endoxifen directly, doctors may avoid differences in how the body processes tamoxifen. This could lead to a more predictable effect with possibly fewer side effects.
How the Study Was Designed
The KARISMA Endoxifen trial was a randomized, double-blind, placebo-controlled phase 2 study. This means:
- Participants were randomly assigned to treatment groups
- Neither participants nor researchers knew who received the active drug
- One group received a placebo
The study included 240 healthy premenopausal women aged 40 to 55.
They were assigned to one of three groups:
- Placebo
- 1 mg of endoxifen daily
- 2 mg of endoxifen daily
Treatment lasted 6 months. Researchers measured breast density at the beginning and end of the study using digital mammography and automated imaging tools.
What Did the Study Find?
Both active doses significantly reduced breast density compared with placebo.
Results showed:
- 1 mg daily reduced breast density by about 19%
- 2 mg daily reduced breast density by about 26%
- Placebo showed almost no meaningful change
Even relatively low blood levels of endoxifen were enough to produce a reduction in breast density. Higher concentrations did not appear to provide much additional benefit beyond a certain level.
Importantly, the degree of density reduction was similar to what has been seen in earlier studies using standard 20 mg tamoxifen.
Side Effects and Safety
Most women reported at least one side effect during the study, but many were mild.
The most common side effects included:
- Irregular menstrual cycles
- Hot flashes
- Night sweats
Key safety findings:
- About twice as many women in the treatment groups reported side effects related to the drug compared with placebo
- The 2 mg group had more hot flashes and night sweats
- The 1 mg group did not show a significant increase in overall symptom severity compared with placebo
- Serious side effects were rare and were not believed to be caused by the study drug
More women stopped treatment in the 2 mg group than in the 1 mg group. The 1 mg dose appeared to offer a better balance between effectiveness and tolerability.
What This Could Mean for Breast Cancer Prevention
This study did not directly measure breast cancer cases. Instead, it used breast density as a marker for potential prevention benefit.
Past research has shown:
- Women whose breast density decreases while taking tamoxifen are more likely to experience reduced breast cancer risk
- Density reduction is linked to better long-term outcomes
Based on these findings, low-dose endoxifen—especially the 1 mg dose—may offer a promising option for breast cancer prevention with fewer side effects than traditional tamoxifen therapy.
However, larger and longer-term studies are needed to confirm whether endoxifen directly lowers breast cancer rates.
If future trials confirm these results, low-dose endoxifen could become an important new prevention strategy for women at increased risk of breast cancer.
Source:
Hall P, Hammarström M, Bergqvist J, Czene K, Eriksson M, Gabrielson M, et al. Endoxifen for mammographic density reduction—results from the KARISMA endoxifen trial. J Natl Cancer Inst. 2026;00(00):1-8. doi:10.1093/jnci/djag087. Accessed by: https://academic.oup.com/jnci/advance-article/doi/10.1093/jnci/djag087/8665489
