Myovant, Pfizer’s Endometriosis Pain Treatment Gets FDA Greenlight

The FDA has approved Myovant Sciences and Pfizer’s Myfembree, a once-daily pill for managing moderate to severe pain associated with endometriosis, a debilitating condition where tissue resembling the lining of the uterus is found outside the uterine cavity. Endometriosis causes disruptive symptoms like painful periods, heavy menstrual bleeding and pain in the lower back, abdomen and pelvis. 

“Endometriosis is a painful, chronic disease with limited therapies to manage symptoms,” said Juan Camilo Arjona Ferreira, MD, Chief Medical Officer of Myovant. The new indication for Myfembree will advance the company’s mission to redefine care for women by helping address a disease with high unmet need, he said. 

Myfembree was first approved in May 2021 for the management of heavy menstrual bleeding associated with uterine fibroids, a condition where noncancerous tumors appear in the uterus. 

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Myfembree Reduces Pain With Lower Risk Of Bone Loss

Myfembree is a mix of relugolix (40 mg), estradiol (1 mg) and norethindrone acetate (0.5 mg), with a treatment duration of up to 24 months. Relugolix, a gonadotropin-releasing hormone (GnRH receptor) antagonist, helps reduce the amount of estrogen and other hormones that exacerbate endometriosis. Estradiol, an estrogen, reduces the risk of bone loss. Norethindrone acetate, a progestin, is necessary when women with a womb take estrogen. 

The US approval is based on one-year data from the Phase 3 Spirit program which includes two 24-week clinical studies (Spirit 1 and Spirit 2) in more than 1,200 women with endometriosis pain. 

In Spirit 1 and 2, 75% of women receiving Myfembree achieved clinically meaningful reduction in period cramps compared with 27% and 30% of women in the placebo group at Week 24, meeting co-primary endpoints. For non-menstrual pelvic pain, Myfembree meaningfully reduced pain in 59% and 66% of women, compared with 40% and 43% of women in the placebo groups.

Myfembree is not recommended for prolonged treatment over 2 years. Due to its inhibitory effects on estrogen, an important driver of bone growth, long-term use of Myfembree may lead to irreversible bone loss. An extension study on women who had completed Spirit 1 or 2 showed mean bone mineral density loss of less than 1%, but some patients (19.7%) had losses exceeding 3%.  

Side effects that come with treatment include headache, hot flashes, mood disorders, back pain, abnormal uterine bleeding and decreased libido among others. The combination treatment comes with a Boxed Warning on thromboembolic disorders and vascular events, and is contraindicated in women with such disorders, including women over 35 years old who smoke or have uncontrolled hypertension. It is also not recommended for women with hormone-sensitive malignancies such as breast cancer. 

Building On A $4.2 Billion Deal 

Pfizer paid $650 million upfront back in 2020 for rights to commercialize relugolix, one of the components of Myfembree, outside the US and Canada. This happened after relugolix got approved by the FDA as Orgovyx for the treatment of advanced prostate cancer. 

The deal was estimated to be worth $4.2 billion, and included $200 million in regulatory milestones and up to $2.5 billion in net sales for prostate cancer and women’s health indications. 

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Joy Lin

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