Lanthanum Carbonate (Fosrenol®) to Reduce Oxalate Excretion in Patients With Secondary Hyperoxaluria and Nephrolithiasis

Inclusion Criteria: * able to give written informed consenct * hyperoxaluria (defined as urinary oxalate \> 45 mg/24 hours), demonstrated on 24-hour urine collection within 18 months prior to baseline visit * history of nephrolithiasis eGFR \> 60 mL/min/1.73m² (CKD-EPI formula) Exclusion Criteria: * primary hyperoxaluria, diagnosed by genetic testing * known allergy to Lanthanum Carbonate * hypophosphatemia (defined as serum phosphorus \< 0.81 mmol/L) * severe known liver insufficiency of biliary obstruction * rectocolitis ulcerohaemorraghica, Crohn's disease, bowel obstruction, stomach/duodenal ulceration * glucose/galactose malabsorption * severe diarrhea or other gastrointestinal disorder, which might interfere with the ability to absorb oral medication * pregnancy or breast-feeding * female participant of childbearing potential unwilling to take efficient contraceptive measures for the duration of the study * female participant without negative serum or urine pregnancy test * psychological illness or condition, interfering with the patient's compliance or ability to understand the requirements of the study * currently participating in another clinical trial