A Single- and Multiple-Ascending Dose Study of SRK-439 in Healthy Adults

Key Inclusion Criteria: 1. Male or female participants, ≥18 to ≤55 years of age, at the time of informed consent 2. BMI of ≥18 kg/m2 to ≤35 kg/m2 (inclusive) at screening Exclusion Criteria: 1. Previous treatment with SRK-439 or apitegromab, or with other anti-myostatin therapies, including ACTR antagonists. 2. History of immunosuppressive, chemotherapeutic, or radiation treatment within 12 months prior to screening. 3. Current or prior use of anabolic steroids, growth hormones, or other substances with known effects on muscle. 4. Use of therapies with potentially significant muscle effects (eg, insulin-like growth factor, systemic beta-agonist, neurotoxins, or muscle relaxants or muscle-enhancing supplements) in any form within 3 months prior to screening or during the study. 5. Any history of exposure to a mAb or Fc domain-bearing recombinant protein within 5 years prior to screening with any prior history of hypersensitivity reactions from exposure 6. Treatment with other investigational drugs or devices in a clinical study or exposure to a biologic therapeutic within 3 months or 5 half-lives, whichever is longer, prior to screening.