The primary objective is to assess the safety and tolerability of REGN2477 alone and combined with REGN1033.

The secondary objectives are to:

• Assess the effect of REGN2477 alone, REGN1033 alone, and REGN2477 + REGN1033 in combination on thigh muscle volume
• Assess the effects of REGN2477 alone, REGN1033 alone, and REGN2477 + REGN1033 in combination on total and regional body composition
• Evaluate the Pharmacokinetic (PK) profile of REGN2477 and REGN1033
• Assess immunogenicity of REGN2477 or REGN1033
• Assess REGN2477 or REGN1033 target engagement

Key Inclusion Criteria:

• Postmenopausal women age 45 to 70 years and age 35 to 60 years inclusive for men not intending to father children
• BMI between 18 to 32 kg/m2, inclusive
• Willing and able to maintain current diet, supplements and physical activity level throughout the study
• Provides signed informed consent

Key Exclusion Criteria:

• Significant illness or history of significant illness
• Contraindication to MRI
• History of human immunodeficiency virus (HIV); hepatitis B or hepatitis C virus (HCV)
• History of immobilization, major surgical procedure, fracture, or major trauma within 9 months prior to screening
• History of significant gynecological disorders or malignancies; history of breast malignancies (uterine fibroids or dysfunctional uterine bleeding is acceptable)
• Inconsistent vigorous physical activity (on fewer than 5 days per week), such as intermittent weight lifting
• History of hypersensitivity reactions to tetracycline antibiotics (includes doxycycline), vaccines, or biologics
• Use of agents that alter muscle mass that have not been at a stable dose for 3 months prior to screening (includes protein supplements), or use of any weight altering or anabolic steroid drugs (includes drugs for obesity, diuretics, testosterone)
• Participants treated with a biologic therapy or biologic immunotherapy in the previous 12 weeks prior to screening and during the study

Note: Other protocol Inclusion/Exclusion criteria apply.