A Study of the Safety, Tolerability, Pharmacokinetics, and Immunogenicity of BCX17725

This is a first-in-human, Phase 1/1b, 4-part study that includes the evaluation of safety, tolerability, pharmacokinetics (PK), and immunogenicity of BCX17725 when administered via single and multiple doses in healthy adult participants (Parts 1 and 2), and multiple doses in adult participants with Netherton syndrome (Part 3). In Part 4, the effectiveness, safety, and tolerability of BCX17725 when administered via multiple IV and/or SC doses through 12 weeks will be evaluated in adult and adolescent participants with Netherton syndrome.

Parts 1 and 2 are randomized, placebo-controlled, single-ascending-dose (SAD) and multiple-ascending-dose (MAD) study parts, respectively, in healthy participants. Part 3 will evaluate multiple dose administrations in participants with Netherton syndrome in an open-label design. Part 4 will evaluate multiple administrations of BCX17725 in participants with Netherton syndrome in an open-label study design over 12 weeks, with an 8-week post-treatment follow-up period.

Key Inclusion Criteria: * Male or non-pregnant, non-lactating female aged 18 to 55 years, inclusive (Parts 1 and 2), 18 to 65 years, inclusive (Part 3), or 12 to 65 years, inclusive (Part 4) * Confirmed diagnosis of Netherton syndrome (Parts 3 and 4) * IGA score of ≥ 3 (Parts 3 and 4) and IASI score of ≥ 16 (Part 4) * BMI between 18 and 30 kg/m\^2, inclusive (Parts 1 and 2) * Estimated glomerular filtration rate (eGFR) of ≥ 90 mL/min/1.73 m\^2 (Parts 1 and 2) or ≥ 60 mL/min/1.73 m\^2 (Part 3) * Agree to follow the protocol contraception requirements from screening until 90 days after the last dose of study drug * In the opinion of the investigator, expected to adequately comply with all required study procedures and restrictions for the duration of the study