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Abstract: PO1415

PF-06869206 Is a Selective Inhibitor of Phosphate Transport: Evidence from In Vitro and In Vivo Studies

Session Information

Category: Fluid, Electrolyte, and Acid-Base Disorders

  • 901 Fluid, Electrolyte, and Acid-Base Disorders: Basic

Authors

  • Thomas, Linto, USF Health Morsani College of Medicine, Tampa, Florida, United States
  • Xue, Jianxiang, USF Health Morsani College of Medicine, Tampa, Florida, United States
  • Dominguez Rieg, Jessica, USF Health Morsani College of Medicine, Tampa, Florida, United States
  • Rieg, Timo, USF Health Morsani College of Medicine, Tampa, Florida, United States
Background

The kidneys are key players in maintaining the body's phosphate (Pi) homeostasis, and patients with chronic kidney disease (CKD) develop hyperphosphatemia. Two renal transporters mediate the majority of Pi reabsorption, the Na+-phosphate cotransporters Npt2a and Npt2c, with Npt2a accounting for ~80% of Pi reabsorption. The aim of the current study was to determine the in vitro effects of a Npt2a-I (PF-06869206) in opossum kidney (OK) cells as well as its in vivo effects in Npt2a knockout (Npt2a-/-) mice.

Methods

To study the in vitro effects of Npt2a-I (0.1-100 µmol/L) on Pi uptake, 32P radiotracer was used in OK cells. In vivo, Npt2a-I dose-response (3-300 mg/kg, 1% bw by oral gavage) effects on urinary Pi excretion were assessed in metabolic cages for 3 hours in wild-type (WT) and Npt2a-/- mice. Effects on plasma Pi were studied before (baseline) and 2 hours after application of Npt2a-I (30 mg/kg, 1% bw by oral gavage).

Results

In vitro Npt2a inhibition caused a dose-dependent decrease in Pi uptake, showing a half-maximal inhibitory concentration of ~1.4 µmol/L. Kinetics of Npt2a uptake inhibition showed an increased Michaelis-Menten constant in response to Npt2a-I compared to vehicle (~2.4 fold); in contrast, Vmax was unaffected, indicating competitive inhibition. No differences were observed in live or dead cell populations after 24-hours treatment between vehicle or Npt2a-I. Parathyroid hormone (PTH) caused a dose-dependent decrease in Pi uptake, with 10 nmol/L being equivalent to the maximal inhibitory effect of Npt2a-I. No additional inhibitory effect was observed by co-incubation of PTH and Npt2a-I. In vivo, Npt2a inhibition caused a dose-dependent increase in urinary Pi excretion in WT mice with an EC50 of ~23 mg*kg-1. No effect on urinary Pi excretion was observed in Npt2a-/- mice. In WT mice, Npt2a-I caused a ~31% (P<0.05) decrease in plasma Pi levels; in contrast, plasma Pi levels were unchanged in Npt2a-/- mice.

Conclusion

Our studies show that PF-06869206 is a competitive inhibitor of Pi transport in OK cells and the effect on urinary Pi excretion and plasma Pi levels are mediated by Npt2a. Human studies are needed to identify if Npt2a inhibition is a useful treatment for hyperphosphatemia.

Funding

  • NIDDK Support