The purpose of this Phase 2, open-label, sequential dose cohort study is to evaluate the safety, efficacy, and pharmacokinetics (PK) of atumelnant (CRN04894) in participants with classic congenital adrenal hyperplasia (CAH) caused by 21-hydroxylase deficiency.

This Phase 2, open-label, sequential dose cohort study will evaluate the efficacy, safety, PK, and PD of atumelnant (CRN04894) when administered for 12 weeks in participants with CAH caused by 21-hydroxylase deficiency. Up to 42 participants will be enrolled in the study.

Inclusion Criteria:

1. Male or female participants ≥18 to 75 years of age at the time of signing the Informed Consent Form (ICF). Participants ≥16 years of age may be included in sites located in the United States
2. Classic 21-hydroxylase deficiency
3. On a stable regimen of glucocorticoid replacement (eg, hydrocortisone, prednisolone, prednisone, methylprednisolone)
4. Compliance with glucocorticoid replacement and mineralocorticoid replacement (if applicable) regimen during the Screening Period
5. Minimum total daily dose of ≥15 mg hydrocortisone (or equivalent). For Cohort 4, a mean daily dose of ≥11 mg/m²/day of hydrocortisone or hydrocortisone equivalents will be used for inclusion
6. If on estrogen therapy (any route), dose must be stable for at least 3 months prior to Screening

Exclusion Criteria:

1. Diagnosis of any other form of CAH other than classic 21-hydroxylase deficiency
2. Dexamethasone use within 30 days of Screening for Cohorts 1-3. In Cohort 4, dexamethasone is permitted
3. History of bilateral adrenalectomy, hypopituitarism, or other condition requiring chronic glucocorticoid therapy
4. Night shift workers or any other reason for abnormal sleep/wake cycles
5. Clinically significant unstable medical condition or chronic disease other than CAH
6. History of major surgery/surgical therapy for any cause within 4 weeks prior to Screening
7. Diabetes mellitus treated with insulin for less than 6 weeks prior to Screening, or with change in total daily insulin dose by >15% within 6 weeks prior to Screening
8. Poorly controlled diabetes mellitus defined as having a hemoglobin A1c (HbA1c) ≥8.5%(≥69 mmol/mL), or estimated HbA1c based on fructosamine if HbA1c is not evaluable (eg, due to hemoglobinopathies)
9. Participants with hypothyroidism who are not receiving adequate hormone replacement therapy based on thyroid hormone levels measured at the time of Screening
10. History of unstable angina or acute myocardial infarction within 12 weeks prior to Screening or other clinically significant cardiac disease at the time of Screening
11. History of cancer excluding cured/treated dermal squamous or basal cell carcinoma or cervical carcinoma in situ
12. Pregnant or lactating
13. Known history of illicit drug or alcohol abuse within the last year
14. Use of antiandrogen therapy in the past 3 months (eg, spironolactone, finasteride, cyproterone acetate, flutamide)
15. Use of testosterone, androgen-containing supplements, aromatase inhibitors, or growth hormone