Patiromer for Treatment of Hyperkalaemia in Children Under 12 Years of Age

Inclusion Criteria: * Paediatric participants (\<12 years of age) with hyperkalaemia at screening. * Participant's age should not reach 12 years during the 28 days of the pharmacodynamic/dose-ranging period. * Participant is able to receive regular external feeding and medication, including via tubes, i.e., percutaneous endoscopic gastrostomy (PEG) or entero-gastric feeding tube. * At screening/baseline, the results from 2 separate and consecutive potassium assessments using the same measurement method (whole blood, plasma, or serum) need to be above the age-appropriate upper limit of normal (ULN). * If taking any renin-angiotensin aldosterone system inhibitors (RAASi), beta blockers, fludrocortisone, or diuretic medications, must be on a stable dose for at least 14 days prior to screening. * Parent(s) or legally authorised representative(s) or another appropriate person delegated by the legally authorised representatives must be available to help the study-site personnel ensure follow-up; accompany the participant to the study site on each assessment day; accurately and reliably dispense investigational product as directed. * Females of childbearing potential must be non-lactating, must have a negative pregnancy test at screening, and must have used an effective, acceptable form of contraception (e.g., abstinence) for at least 1 month before patiromer administration. Females of childbearing potential must agree to continue using contraception throughout the study and for 1 month after the last dose of patiromer. * If undergoing peritoneal dialysis, participants must be on a stable treatment plan for a minimum of 4 weeks prior to screening, or at least 8 weeks prior to screening if newly initiated on peritoneal dialysis. Exclusion Criteria: * Preterm birth infants with \<37 weeks of gestation cannot be included in Cohort 3. * Participants who due to their general condition, e.g., anaemia or low body weight, are not suitable to have blood volume withdrawn. * Any of the following renal conditions: maintenance haemodialysis, renal artery stenosis, and acute kidney injury (defined by 2012 Kidney Disease Improving Global Outcomes) or a history of acute renal insufficiency in the past 3 months. Note: Chronic kidney disease (CKD) is not excluded. * A history of or current diagnosis of a severe gastrointestinal (GI) diagnosis or surgery that could affect GI transit of the drug (delayed gastric emptying), such as a severe swallowing disorder, severe gastroesophageal reflux, uncorrected pyloric stenosis, intussusception, any other intestinal obstruction (e.g., Hirschsprung disease, chronic intestinal pseudo-obstruction, clinically significant postsurgical abdominal adhesions) or any gut-shortening surgical procedure prior to screening. Pre-gastric above-mentioned pathologies may be disregarded in case of existence of a PEG or entero-gastric feeding tube, as the PEG or entero-gastric feeding tube will serve for nutrition and investigational product administration. * Active cancer, currently on cancer treatment, or history of cancer in the past 2 years (except for non-melanoma skin cancer). * Scheduled for kidney transplant procedure during the first 28 days after Day 1. * History of sudden infant death in a sibling (only for participants \<2 years of age at screening). * Use of the following medications if doses have not been stable for at least 14 days prior to screening or if doses are anticipated to change during the 4-week pharmacodynamic/ * Dose-ranging period: digoxin, bronchodilators, theophylline, heparins (including low molecular heparins), tacrolimus, mycophenolate mofetil, cyclosporine, trimethoprim, or cotrimoxazole. * Use of any investigational product for an unapproved indication within 30 days prior to screening or within 5 half-lives, whichever is longer. * Known hypersensitivity to patiromer or its components. * If the child is being breastfed: 1. There is suspicion of current alcohol or substance misuse/abuse in breastfeeding mother. 2. The breastfeeding mother is taking potassium supplements * Other protocol defined Inclusion/Exclusion criteria may apply.