A Study to Test the Effect of Different Doses of BI 685509 on Kidney Function in People With Diabetic Kidney Disease

Inclusion criteria: 1. Signed and dated written informed consent in accordance with International Council of Harmonisation-Good Clinical Practice (ICH-GCP) and local legislation prior to admission to the trial. 2. Male or female patients aged ≥ 18 years at time of consent. 3. eGFR (Chronic Kidney Disease Epidemiology Collaboration \[CKD-EPI\] formula) ≥ 20 and \< 90 mL/min/1.73 m2 at Visit 1 by central laboratory analysis. eGFR must remain ≥ 20 mL/min/1.73 m2 after Visit 1 up to the start of Visit 3, measured by central or any local laboratory analysis. 4. Urine Albumin Creatinine Ratio (UACR) ≥ 200 and \< 3,500 mg/g in spot urine (midstream urine sample) by central laboratory analysis at Visit 1. 5. Treatment with the highest tolerated dose of either Angiotensin Converting Enzyme inhibitor (ACEi) or Angiotensin Receptor Blocker (ARB) (but not both together), and stable dose for ≥ 4 weeks before Visit 1 with no planned change of the therapy during the trial. 6. If the patient is taking any of the following medications they should be on a stable dose at least 4 weeks prior to visit 1 until start of treatment, with no planned change of the therapy during the trial: anti-hypertensives, Non-steroidal anti-inflammatory drug(s) (NSAIDs), endothelin receptor antagonists, systemic steroids or Sodium-Glucose co-Transporter-2 (SGLT2) inhibitors. 7. Patients with stable type 1 or type 2 diabetes mellitus, diagnosed before informed consent. Treatment (including SGLT2 inhibitor and/or Glucagon-Like Peptide 1 (GLP1) receptor agonist) should have been unchanged or changes deemed minor (according to investigator's judgement) within 4 weeks before Visit 1 and until start of trial treatment. 8. Glycated Haemoglobin (HbA1c) \< 10.0% at Visit 1 measured by the central laboratory. Further inclusion criteria apply. Exclusion criteria: 1. Treatment with Renin Angiotensin Aldosterone System (RAAS) interventions (apart from either ACEi or ARB), phosphodiesterase 5 inhibitors, non-specific phosphodiesterase inhibitors (such as dipyridamole and theophylline), NO donors including nitrates, sGC-stimulators/activators (other than trial treatment) or any other restricted medication (including OATP1B1/3 inhibitors, UGT inhibitors/inducers) as provided in the Investigator Site File (ISF) within 4 weeks prior to visit 1 and throughout screening and baseline run-in. Patients who must or wish to continue the intake of restricted medications or any drug considered likely to interfere with the safe conduct of the trial are also excluded. 2. Any clinically relevant laboratory value from screening until start of trial treatment, which in the investigator's judgement puts the patient at additional risk. 3. Biopsy or otherwise confirmed non-diabetic chronic kidney disease, or non-diabetic chronic kidney disease in the opinion of investigator, e.g., Autosomal Dominant Polycystic Kidney Disease (ADPKD), uncontrolled lupus nephritis. The presence of a hypertensive etiology does not need to be excluded unless it is evident this is the only cause for the Chronic Kidney Disease (CKD). 4. Any immunosuppression therapy or immunotherapy in the last 3 months prior to visit 1 and throughout screening and baseline run-in (except prednisolone ≤10 mg or equivalent). 5. Acute kidney injury (AKI) according to the Kidney Disease: Improving Global Outcomes (KDIGO) in the 30 days prior to Visit 1 until the start of trial treatment. 6. Planned start of chronic renal replacement therapy during the trial or end stage renal disease before start of trial treatment. 7. Known history of moderate or severe symptomatic orthostatic dysregulation as judged by the investigator before start of trial treatment. 8. The patient has an active infection with Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) (or is known to have a positive test) from screening until randomisation. Further exclusion criteria apply.