This study is researching an experimental drug called mibavademab (also called "study drug"). The study is focused on patients with a condition called Functional Hypothalamic Amenorrhea (FHA). FHA is a condition where a woman stops menstruating because the brain is not sending the correct hormonal signals to the ovaries, which then are not able to cycle and ovulate properly.

The aim of the study is to see how well the study drug helps the body make the hormones necessary for ovulation and reproduction, and to assess its safety.

The study is looking at several other research questions, including:

• Whether the drug helps bone health
• What side effects may happen from taking the study drug
• How much study drug is in the blood at different times
• Whether the body makes antibodies against the study drug (which could make the drug less effective or could lead to side effects)

Key Inclusion Criteria:

1. Diagnosis of FHA after exclusion of anatomic or organic causes of amenorrhea
2. Has had no menstrual bleeding episode within 3 months (or longer) prior to day 1, as described in the protocol
3. Has a Body Mass Index (BMI) ≥18.5 and <25 kg/m^2 at screening, or BMI ≥25 to <30 kg/m^2 at screening AND percentage of body fat <20% from rigorous exercise as determined by the investigator through Standard of Care (SoC) methods (eg, skinfold thickness, DXA, bioimpedance) within 6 months of screening, as described in the protocol

Key Exclusion Criteria:

1. Has presence of primary amenorrhea (failure to reach menarche, first menstrual cycle)
2. Has blood estradiol ≥50 pg/mL or blood progesterone ≥1 ng/mL at screening
3. Has uterine (eg, absence of uterus, prior endometrial ablation, endometriosis, outflow tract disorders) or ovarian (eg, absence of an ovary, presence of polycystic ovaries) conditions that impact assessment of menses or of the HPO axis
4. Has conditions other than FHA that may cause amenorrhea or menstrual cycle disturbances (eg, polycystic ovarian syndrome, hyperprolactinemia, primary ovarian insufficiency, untreated primary hypothyroidism, primary hyperthyroidism)
5. Polycystic ovarian morphology with an ovarian volume >10 cc on TransVaginal UltraSound (TVUS) [or TransAbdominal Pelvic Ultrasound (TAPU) if applicable] at baseline
6. Presence of follicle ≥17 mm, evidence of ruptured follicle, and/or evidence of corpus luteum on TVUS (or TAPU if applicable) at baseline

NOTE: Other protocol-defined Inclusion/Exclusion criteria apply

All Individual Patient Data (IPD) that underlie publicly available results will be considered for sharing.

When Regeneron has:

• received marketing authorization from major health authorities (e.g., FDA, European Medicines Agency (EMA), Pharmaceuticals and Medical Devices Agency (PMDA), etc.) for the product and indication or has globally discontinued development of the product for all indications on or after April 2020 and has no plans for future development
• made the study results publicly available (e.g., scientific publication, scientific conference, clinical trial registry)
• the legal authority to share the data, and
• ensured the ability to protect participant privacy

Qualified researchers can submit a proposal for access to individual patient or aggregate level data from a Regeneron-sponsored clinical trial through Vivli. Regeneron's Independent Research Request Evaluation Criteria can be found at: https://www.regeneron.com/sites/default/files/Regeneron-External-Data-Sharing-Policy-and-Independent-Research-Request-Evaluation-Criteria.pdf