Bayer Starts New Phase III Study with Mirena® for the Treatment of Nonatypical Endometrial Hyperplasia

Key Terms

phase iii medical

A Phase III trial is the late-stage clinical study that tests whether a medical treatment works and is safe in a large group of patients, often comparing it to standard care. Think of it as a final dress rehearsal or full-scale road test before regulators decide on approval; positive or negative results strongly influence a drug maker’s chance to sell the treatment, future revenue, and investment risk.

levonorgestrel-releasing intrauterine system medical

A levonorgestrel-releasing intrauterine system is a small, T-shaped medical device placed inside the uterus that steadily releases a low dose of the hormone levonorgestrel to prevent pregnancy for several years. Investors follow it because it represents a regulated, long-acting medical product with predictable sales, regulatory approval pathways, and reimbursement issues—similar to a subscription product in healthcare—so approvals, safety data, patent status, or pricing changes can materially affect a company’s revenue.

endometrial hyperplasia medical

Endometrial hyperplasia is an abnormal thickening of the lining of the uterus caused by excess growth of its cells, often linked to hormone imbalance. It matters to investors because it can be a precursor to cancer or require medical treatment, which drives demand for diagnostics, drugs, and procedures; changes in study results, approvals, or guidelines can directly affect companies developing related tests or therapies, similar to how a rising crack in a building affects repair markets.

oral progestin medical

A man-made version of the natural hormone progesterone taken by mouth to mimic its effects in the body; it is used for birth control, hormone replacement and treating certain menstrual or uterine conditions. Investors care because oral progestins drive drug sales, clinical trial and regulatory risk, competitive positioning and patient adherence—like a more convenient tool in a toolbox, they can change how widely a therapy is used and how profitable it becomes.

uterine cancer medical

Uterine cancer is a disease where cells in the uterus, most often the lining of the womb, grow out of control and form a tumor; think of it like weeds overtaking a garden bed. Investors track uterine cancer because changes in how often it occurs, how it’s diagnosed, and what treatments are approved directly affect demand for drugs, medical devices, diagnostic tests, hospital services and insurers, which in turn influence revenues and risk for healthcare-related companies.

• Study further expands clinical development program of Bayer’s long-acting reversible intrauterine system Mirena® (levonorgestrel-releasing intrauterine system) 52 mg.
• Approximately 121 out of every 100,000 women develop nonatypical endometrial hyperplasia (NAEH), a benign form of endometrial thickening, each year.¹ Currently, there are no approved medical treatment options for NAEH in the U.S.
• Mirena is globally available in more than 120 countries, and approved in the U.S. as a progestin-containing intrauterine system (IUS) indicated for prevention of pregnancy for up to 8 years and is also indicated for the treatment of heavy menstrual bleeding for women who choose to use intrauterine contraception as their method of contraception for up to 5 years.² Mirena is not FDA approved for the treatment of NAEH.

WHIPPANY, N.J.--(BUSINESS WIRE)-- Bayer announced today the start of its Phase III study SUNFLOWER, an investigational study evaluating the efficacy and safety of 52mg levonorgestrel-releasing intrauterine system Mirena® for the treatment of nonatypical endometrial hyperplasia (NAEH) in women.

NAEH is an abnormal thickening of the lining of the uterus, known as the endometrium, due to an imbalance between estrogen and progesterone levels. This condition is classified as nonatypical as the cells in the endometrium do not show signs of cancer. It can be asymptomatic, present with heavy or abnormal bleeding, or bleeding after menopause. If untreated, this condition may lead to uterine cancer. Approximately 121 out of every 100,000 women develop NAEH each year.¹ Currently, there are no approved medical treatment options for this condition, presenting a high unmet medical need.

“Previous clinical studies indicate the therapeutic potential of intrauterine levonorgestrel and its ability to restore the endometrium to its normal state,” said George L. Maxwell, MD Principal Investigator of the SUNFLOWER study. “The SUNFLOWER study is an important step in investigating this potential benefit which could inform future therapeutic approaches for this patient population. Mirena is effective in pregnancy prevention as well as management of heavy menstrual bleeding in women who choose an IUD for birth control. This study aims to shed light on a potential additional therapeutic benefit of this long-established contraceptive.”

SUNFLOWER is a Phase III investigational clinical study that will evaluate the treatment of NAEH with 52mg levonorgestrel-releasing intrauterine system versus an oral progestin (OP). The study is anticipated to enroll 207 patients at about 90 centers in three countries with completion after around 18 months.

About Mirena
INDICATIONS FOR MIRENA
Mirena® (levonorgestrel-releasing intrauterine system) is a hormone-releasing IUD that prevents pregnancy for up to 8 years. Mirena also treats heavy periods for up to 5 years in women who choose intrauterine contraception.

IMPORTANT SAFETY INFORMATION

• If you have a pelvic or genital infection, get infections easily, or have certain cancers, don't use Mirena. Less than 1% of users get a serious pelvic infection called pelvic inflammatory disease (PID).
• If you have persistent pelvic or stomach pain, or excessive bleeding after placement, tell your healthcare provider (HCP). If Mirena comes out, call your HCP and avoid intercourse or use non-hormonal back-up birth control (such as condoms or spermicide). Mirena may go into or through the wall of the uterus and cause other problems.
• Pregnancy while using Mirena is uncommon but can be life threatening and may result in loss of pregnancy or fertility.
• Ovarian cysts may occur but usually disappear.
• Bleeding and spotting may increase in the first 3 to 6 months and remain irregular. Periods over time usually become shorter, lighter, or may stop.

Mirena does not protect against HIV or STIs.

Only you and your HCP can decide if Mirena is right for you. Mirena is available by prescription only.

For important risk and use information about Mirena, please see Full Prescribing Information.

About Bayer
Bayer is a global enterprise with core competencies in the life science fields of health care and nutrition. In line with its mission, “Health for all, Hunger for none,” the company’s products and services are designed to help people and the planet thrive by supporting efforts to master the major challenges presented by a growing and aging global population. Bayer is committed to driving sustainable development and generating a positive impact with its businesses. At the same time, the Group aims to increase its earning power and create value through innovation and growth. The Bayer brand stands for trust, reliability and quality throughout the world. In fiscal 2024, the Group employed around 93,000 people and had sales of 46.6 billion euros. R&D expenses amounted to 6.2 billion euros. For more information, go to www.bayer.com.

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Forward-Looking Statements
This release may contain forward-looking statements based on current assumptions and forecasts made by Bayer management. Various known and unknown risks, uncertainties and other factors could lead to material differences between the actual future results, financial situation, development or performance of the company and the estimates given here. These factors include those discussed in Bayer’s public reports which are available on the Bayer website at www.bayer.com. The company assumes no liability whatsoever to update these forward-looking statements or to conform them to future events or developments.

References
¹ Yuk J. The incidence rates of endometrial hyperplasia and endometrial cancer: a four-year population-based study. NIH: https://pmc.ncbi.nlm.nih.gov/articles/PMC5012286/#:~:text=The%20EH%20incidence%20is%20133,generated%20between%202009%20and%202012. Accessed November 2025
² Mirena Prescribing Information: Mirena_PI.pdf. Accessed November 2025

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Media Contact:

Jackie Zima-Evans

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Source: Bayer