An 18-month, Open-label, Single-arm Safety Extension Study of an age-and Bodyweight-adjusted Oral Finerenone Regimen, in Addition to an ACEI or ARB, for the Treatment of Children and Young Adults From 1 to 18 Years of Age With Chronic Kidney Disease and Proteinuria

Researchers are looking for a better way to treat children who have chronic kidney disease (CKD), which is long-term kidney disease, and proteinuria, a condition in which a person´s kidneys leak protein into the urine. The kidneys filter waste and fluid from the blood to form urine. In children with CKD, the kidney´s filters do not work as well as they should. This can lead to accumulation of waste and fluid in the body and proteinuria. CKD can lead to other medical problems, such as high blood pressure, also known as hypertension. Vice versa, hypertension and proteinuria can also contribute to worsening of CKD. Therefore, the treatment of CKD aims to control blood pressure and proteinuria. There are treatments available for doctors to prescribe to children with CKD and hypertension and/or proteinuria. These include "angiotensin-converting enzyme inhibitors" (ACEI) and "angiotensin receptor blockers" (ARB). Both ACEI and ARB can help improve kidney function by reducing the activity of the renin-angiotensin-aldosterone system (RAAS). The RAAS is a system that works with the kidneys to control blood pressure and the balance of fluid and electrolytes in the blood. In people with CKD, the RAAS is often too active, which can impair the ability of the kidneys to work properly and cause hypertension and proteinuria. However, ACEI or ARB treatment alone does not work for all patients with CKD as they only target the angiotensin part of the renin-angiotensin-aldosterone system. The study treatment, finerenone, is expected to help control RAAS overactivation together with an ACEI or ARB. So, the researchers in this study want to learn more about whether finerenone given in addition to either an ACEI or ARB can help their kidney function. The main purpose of this study is to learn how safe the treatment is when used of finerenone in addition to an ACEI or ARB in long-term. To see how safe the treatment is, the study team will collect information on medical problems which are also known as "treatment emergent adverse events" (TEAEs). And they will also collect levels of an electrolyte called potassium in the blood by taking blood samples, and measure blood pressure during the study. The secondary purpose of this study is to learn how well long-term use of finerenone can reduce the amount of protein in the participants' urine and benefit kidney function when taken with standard of care. To see how the treatment works, the study team will collect participants' urine samples to assess urinary albumin-to-creatinine ratio (UACR) and urinary protein-to-creatinine ratio (UPCR), which are important assessments for calculating the level of protein in the urine. Researchers will also collect blood samples to analyze serum creatinine and calculate estimated glomerular filtration rate (eGFR). A significant decline in eGFR indicates worsening kidney function. The study will include participants who had previously participated in FIONA study (NCT05196035). The participants will be aged from 1 year up to 18 years. The participants will be in the study for approximately 19 months. They will take study treatment for up to 18 months and will be follow up for 1 month. During this period, at least 12 visits are planned for patients who newly start finerenone, and at least 8 visits for patients who already received finerenone. In the visit, the study team will: * have their blood pressure, heart rate, temperature, height and weight measured * have blood and urine samples taken * have physical examinations * have their heart examined by an electrocardiogram and echocardiography (a sonogram of the heart) * answer questions about their medication and whether they have any adverse events, or have their parents or guardian's answer * answer questions about how they are feeling, or have their parents or guardian's answer * answer question about how they like the study medication, or have their parents or guardian's answer The doctors will keep track of any adverse events. An adverse event is any medical problem that a participant has during a study. Doctors keep track of all adverse events that happen in studies, even if they do not think the adverse events might be related to the study treatments. The doctors will check the participants' health about 30 days after the participants take their last treatment.