Eligible participants were healthy male and female subjects, 18–55 years old, weighing at least 60 kg (men) and at least 50 kg (women) with a BMI of 18.5–34.9 kg/m2. Female subjects of childbearing potential agreed to use two medically accepted forms of contraception from the screening period through the final study visit. Male subjects with female partners of childbearing potential agreed to use a highly effective, medically acceptable form of contraception from the screening period through the final study visit or at least 90 days after the last dose 
Reasons for exclusion from the study included acute or chronic illness at screening; history of exposure to a mAb or Fc domain-bearing recombinant protein within 5 years prior to screening and history of hypersensitivity reactions with exposure; exposure to a biologic within 90 days prior to screening or live vaccine within 14 days prior to screening; any current or prior use of anabolic steroids, systemic corticosteroids, systemic beta-adrenergic agonists; use of dietary or herbal supplements and vitamins, including protein supplements, growth hormones, or other substances designed to affect muscle growth . Subjects were also excluded if they participated in strenuous exercise within 14 days prior to study day −1, donated blood (more than 450 mL) or serum within 60 days, or had a history of substance abuse or dependency.
Study Design and Treatment
This study was conducted in two parts. Part A was the SAD phase while part B was the MAD phase. Eligibility screening for parts A and B was performed up to 28 days prior to day 0 dosing. Subjects were admitted to the phase 1 study unit the day prior to each infusion. No subjects enrolled into part A were later enrolled into part B, nor were part B subjects enrolled into part A.
Subject demographics, current and past medical status, history of surgery, allergies, and concomitant medications were recorded. Vital signs and a complete physical examination were performed by the investigator or a qualified designee within 120 min prior to each infusion.
Part A: Single Ascending Dose, Dose Escalation
The doses (Table 1; Fig. 1) for part A (SAD) were 1 mg/kg (cohort 1), 3 mg/kg (cohort 2), 10 mg/kg (cohort 3), 20 mg/kg (cohort 4), and 30 mg/kg (cohort 5). Eight subjects were enrolled in each cohort and randomized 3:1 to receive either active apitegromab (n = 6) or matching placebo (n = 2) administered as an IV infusion. In part A, an initial sentinel group of two subjects in each cohort (one active, one placebo) were infused to permit initial safety assessments. A minimum of 24 h elapsed after the start of infusion of the last sentinel subject before the remainder of the cohort was treated. Dosing of subjects 3–8 proceeded if no safety issues were identified by the Dose Escalation Committee (DEC) for the first two subjects in each cohort.
All subjects in part A had continuous cardiac monitoring for approximately 12 h after dosing and were observed as inpatients for 2 days (48 h) after dosing. Following discharge from the clinic, subjects returned to the phase 1 unit for follow-up visits on days 7, 14, 21, 28, 42, 56, 84, and 112 for safety monitoring and blood sampling for immunogenicity, PK, and PD assessments. Safety data from each cohort were reviewed by the DEC prior to escalation to the next higher dose.
Part B: Multiple Ascending Dose, Dose escalation
During part B (MAD) (Table 1; Fig. 1), three cohorts received apitegromab 10 mg/kg (cohort 6), 20 mg/kg (cohort 7), and 30 mg/kg (cohort 8). Eight subjects were enrolled in each cohort and were randomized to receive apitegromab (n = 6) or matching placebo (n = 2) once every 2 weeks for a total of three repeat doses on days 0, 14, and 28. Initiation of part B did not occur until the planned cumulative apitegromab dose for part B (three doses of 10 mg/kg) had been administered as a single dose to the full cohort in part A (30 mg/kg in advance of part B, three doses of 10 mg/kg); and the DEC had determined that it was safe to proceed on the basis of a review of safety data through day 7 for the 30-mg/kg SAD cohort; and available safety data from earlier SAD cohorts. All subjects in part B had continuous cardiac monitoring for approximately 12 h and were observed as inpatients for the first 48 h after each infusion. In addition to clinic admission for dosing on days 0, 14, and 28, subjects returned to the phase 1 unit for follow-up visits on days 7, 21, 35, 42, 56, 84, 112, and 140 for safety monitoring and blood sampling for immunogenicity, PK and PD assessments.
Pharmacokinetic and Pharmacodynamic Assessments
Initial blood samples for PK and PD assessments were obtained within 15 min prior to apitegromab administration. For part A (SAD) and part B (MAD), blood samples for PK and serum latent myostatin (PD) were collected prior to infusion, at the end of infusion, and 4, 8, 24, and 48 h post-infusion. Additional samples were collected in part A on days 7, 14, 21, 28, 42, 56, 84, and 112 and in part B on days 7, 21, 35, 42, 56, 84, 112, and 140.
PK samples were analyzed by a validated sensitive plate-based electrochemiluminescence (ECL) assay developed to quantitate apitegromab in human serum with a lower limit of quantification of 200 ng/mL. Immunogenicity was assessed in all subjects who received apitegromab. Latent myostatin concentrations were measured with a qualified plate-based ECL immunoassay to quantitate latent myostatin in serum samples. The assay selectively recognizes latent myostatin and does not detect promyostatin, mature myostatin, or other closely related members of the TGFβ superfamily. The assay has a quantitation range of 2.3–5000 ng/mL and is tolerant up to 2.5 mg/mL of apitegromab.
Single-dose PK parameters were estimated using noncompartmental analysis (part A), while the multiple dose PK parameters were estimated using noncompartmental and compartmental methods (part B). Phoenix® WinNonlin® (Version 8.3; Princeton, NJ) was used to estimate these parameters. Only noncompartmental analysis results are presented in this publication.
Anti-Drug Antibody Testing
The presence of anti-drug antibodies was evaluated using a validated plate-based ECL bridging immunoassay. Both screening and confirmatory portions of the assay have a sensitivity of 100 ng/mL.
Clinical chemistry, hematology, and urinalysis assessments were performed using standard methods. Blood samples for laboratory assessments were obtained at the phase 1 unit by the study investigator or a trained designated staff member.
Safety was monitored through reported adverse events (AEs), vital signs, physical examinations, clinical laboratory testing, and infusion site reactions. Subjects underwent continuous electrocardiogram (ECG) monitoring for approximately 12 h after each infusion. AE severity was graded using NCI CTCAE v5.0 .
The sample size was chosen to provide adequate numbers of subjects to characterize the safety and tolerability of apitegromab. Safety analyses included randomized subjects who received at least one dose of apitegromab. PK assessments included subjects with at least one quantifiable apitegromab serum concentration. Data were described and analyzed using statistical software (SAS® System, v9.4; SAS Institute Inc., Cary, NC). Descriptive statistics included the number of subjects (N), mean, standard deviation (SD), median, minimum, and maximum results.
This study was approved by a commercial institutional review board (IRB; Advarra®, Columbia, MD, #00024966), complied with the ethical principles of Good Clinical Practice as required by the International Conference on Harmonisation, the Declaration of Helsinki as amended, and US Investigational New Drug regulations (21 CFR 56). In addition, the IRB approved the written informed consent form, including the ability to publish data from the study, any consent form updates, subject recruitment procedures (e.g., advertisements), and any written information provided to the subjects prior to implementation. All subjects provided informed written consent to participate in the study prior to any study-related activities.