This study involves children and teenagers with classic congenital adrenal hyperplasia, which is a condition where the adrenal glands do not produce certain hormones properly. Because of this condition, the body makes too much of some hormones called androgens and not enough of a hormone called cortisol. People with this condition usually need to take replacement hormones called glucocorticoids, which are medicines that replace the missing cortisol. However, sometimes these medicines need to be given in higher doses than the body would normally make, which can cause unwanted effects. The study will test a medicine called atumelnant, which is also known by its code name CRN04894. This medicine comes in tablet form and is taken by mouth. It works by blocking certain receptors in the body to help control the production of androgens.

The purpose of this study is to see if atumelnant is safe and works well in children with congenital adrenal hyperplasia, and whether it can help reduce the amount of glucocorticoid medicine they need to take while keeping their hormone levels normal. The study is divided into three parts. In Part A, participants will receive atumelnant for eight weeks to check how safe it is and to measure changes in a hormone called androstenedione, which is one of the androgens that becomes too high in people with this condition. In Part B, which lasts twenty-eight weeks, the study will look at whether atumelnant allows doctors to lower the daily dose of glucocorticoid medicine while keeping androstenedione levels normal. Some participants may receive placebo. In Part C, which is a long-term extension, participants who completed Part A or Part B can continue treatment to see how atumelnant works over a longer period of time.

During the study, participants will have regular visits where doctors will check their hormone levels through blood tests, monitor their safety, and measure how much medicine is in their blood. The study will also look at changes in another hormone called 17-hydroxyprogesterone, which is also elevated in people with congenital adrenal hyperplasia. The doctors will carefully watch for any side effects and see how well participants tolerate the medicine. Throughout the study, participants will continue to take their regular replacement hormone medicines, and doctors will adjust the doses as needed based on how the participant responds to atumelnant.