⁃ Part A and B participants are eligible to be included in the study only if all of the following criteria apply:

• Male or female at birth, between 1 to \<18 years of chronological age at the time of signing the Informed Consent Form (ICF).

• Have a medically confirmed diagnosis of classic CAH due to 21-hydroxylase deficiency (21-OHD) based on standard medically accepted criteria such as elevated 17-OHP level, confirmed CYP21A2 genetic testing, positive newborn screening with confirmatory second tier testing, or cosyntropin stimulation.

• Participants must have an elevated morning serum A4 level \>ULN during Screening obtained prior to morning glucocorticoid (GC) administration.

• Participants must be on a stable supraphysiologic GC replacement therapy for at least one month prior to Screening.

• Compliance, as judged per Investigator discretion, with GC replacement and mineralocorticoid replacement (if applicable) regimen documented during the Screening Period.

• Normal thyroid stimulating hormone (TSH) and thyroxine (T4) within 3 months of Screening per age-appropriate range.

⁃ Part C inclusion criteria require participants to complete treatment in either Part A or Part B and in the Investigator's opinion it would benefit the participant to continue in Part C, regardless of age.