Endogenous Oxalate Production in Primary Hyperoxaluria Type 1 Patients

Significance Statement Primary hyperoxaluria type 1 (PH1) is a rare genetic disorder characterized by increased endogenous oxalate production (EOP). The metabolic pathways underlying oxalate synthesis have not been fully elucidated. Measurement of EOP can help evaluate PH1 drugs under development. By infusing stable isotopes of oxalate, glycolate, and glycine, we measured EOP and the contribution of glycolate to EOP and glycine production (to assess pyridoxine responsiveness) in patients with PH1 and in healthy volunteers. In this study, we provide a precise method to quantify oxalate kinetics that could serve as an additional tool to evaluate therapeutic efficacy and inform important clinical decisions (e.g., suitability for a kidney-alone transplant and prevent a liver transplant after pyridoxine or RNAi treatment). Background Primary hyperoxaluria type 1 (PH1) is an inborn error of glyoxylate metabolism, characterized by increased endogenous oxalate production. The metabolic pathways underlying oxalate synthesis have not been fully elucidated, and upcoming therapies require more reliable outcome parameters than the currently used plasma oxalate levels and urinary oxalate excretion rates. We therefore developed a stable isotope infusion protocol to assess endogenous oxalate synthesis rate and the contribution of glycolate to both oxalate and glycine synthesis in vivo. Methods Eight healthy volunteers and eight patients with PH1 (stratified by pyridoxine responsiveness) underwent a combined primed continuous infusion of intravenous [1-C-13]glycolate, [U-C-13(2)]oxalate, and, in a subgroup, [D-5]glycine. Isotopic enrichment of C-13-labeled oxalate and glycolate were measured using a new gas chromatography?tandem mass spectrometry (GC-MS/MS) method. Stable isotope dilution and incorporation calculations quantified rates of appearance and synthetic rates, respectively. Results Total daily oxalate rates of appearance (mean [SD]) were 2.71 (0.54), 1.46 (0.23), and 0.79 (0.15) mmol/d in patients who were pyridoxine unresponsive, patients who were pyridoxine responsive, and controls, respectively (P=0.002). Mean (SD) contribution of glycolate to oxalate production was 47.3% (12.8) in patients and 1.3% (0.7) in controls. Using the incorporation of [1-C-13]glycolate tracer in glycine revealed significant conversion of glycolate into glycine in pyridoxine responsive, but not in patients with PH1 who were pyridoxine unresponsive. Conclusions This stable isotope infusion protocol could evaluate efficacy of new therapies, investigate pyridoxine responsiveness, and serve as a tool to further explore glyoxylate metabolism in humans.