A Study to Learn How Well the Study Treatment Finerenone Works and How Safe it is in People With Long-term Decrease in the Kidneys' Ability to Work Properly (Chronic Kidney Disease) Together With Type 1 Diabetes

Researchers are looking for a better way to treat people with chronic kidney disease (CKD), a progressive decrease in the kidneys' ability to work properly, and type 1 diabetes.

In people with type 1 diabetes, the body does not make enough of a hormone called insulin, resulting in high blood sugar levels that can cause damage to the kidneys. CKD often occurs together with or as a consequence of type 1 diabetes.

The study treatment finerenone works by blocking certain proteins, called mineralocorticoid receptors. An increased stimulation of these proteins is thought to damage the kidneys and the heart. By lowering their stimulation, finerenone reduces the risk of kidney disease progressively getting worse. Finerenone is approved for doctors to prescribe to people with CKD and type 2 diabetes.

In this study, researchers want to learn if finerenone works better than placebo in reducing the participants' kidney disease from getting worse when given in addition to standard of care (SOC) treatment. A placebo looks like a treatment but does not have any medicine in it. SOC is a procedure or treatment that medical experts consider most appropriate for a condition or disease. To find out how well finerenone works, the level of a protein (albumin) in the urine will be measured.

Researchers also want to know how safe finerenone is. To do this, the researchers will collect the number of participants with:

* medical problems (also called treatment-emergent adverse events (TEAEs))
* serious TEAEs. An TEAE is considered 'serious' when it leads to death, puts the participant's life at risk, requires hospitalization, causes disability, causes a baby being born with medical problems, or is medically important
* higher than normal blood levels of potassium (hyperkalaemia). Depending on the treatment group, the participants will either take finerenone or placebo, Importantly, the participants will also continue to take their regular SOC medicines.

The participants will be in the study for up to 7.5 months and will take the study treatments for 6 months. During the study, they will visit the study site at least 6 times.

The study team will:

* collect blood and urine samples
* check the participants' vital signs such as blood pressure and heart rate
* do a physical examination including height and weight
* check the participants' heart health by using an electrocardiogram (ECG)
* do pregnancy tests in women of childbearing potential


Eligibility Criteria:
Inclusion Criteria:

* Participant must be ≥18 years of age (or the legal age of consent according to local legislation) at the time of signing the informed consent.
* Participants with Type 1 diabetes (T1D), i.e. T1D continuously treated with insulin, started within one year from diagnosis.

  * If the onset was after age 35, documentation of the presence of one or more of the following:

    * Circulating T1D-associated autoantibodies
    * Hospitalization for diabetic ketoacidosis
    * Plasma C-peptide below the limit of detection with standard assay (with concurrent blood glucose \>100 mg/dl).
* HbA1c at Screening \<10% (central assessment).

  * Note: One reassessment is allowed for HbA1c during the Screening period in case the first measurement is missing/unreadable/invalid.
* K+ ≤ 4.8 mmol/L at Screening (local assessment)
* Participants with a clinical diagnosis of CKD and fulfilling both the criteria (central assessment):

  * eGFR ≥25 and \<90 mL/min/1.73 m\^2 using CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) 2009 formula at the Screening visit
  * UACR ≥200 mg/g (22.6 mg/mmol) to \<5000 mg/g (565 mg/mmol) at the Screening visit (geometric mean of the 3 measurements)
* Participants on a stable (preferably without any change in the dosage for at least 4 weeks prior to the Screening visit) ACEI (Angiotensin-converting enzyme inhibitor) or ARB (Angiotensin receptor blocker) treatment.

Exclusion Criteria:

* Participant with T2D (Type 2 diabetes).
* Participant with mean BP (Blood pressure) higher than 160/100 mmHg or mean systolic BP lower than 90 mmHg at the Screening visit
* Symptomatic heart failure with reduced ejection fraction with class 1A indication for Mineralocorticoid receptor antagonists (MRAs).
* Participants with current or previous (within 8 weeks prior to the Screening visit) treatment with a SGLT-2/-1 (Sodium-Glucose co-transporter-2/-1) inhibitor or GLP1 (Glucagon-like peptide-1) receptor agonist.