Kidney Week

Abstract: PO1415

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

Session Information

• Fluid, Electrolyte, and Acid-Base Disorders: Basic
  October 22, 2020 | Location: On-Demand
  Abstract Time: 10:00 AM - 12:00 PM

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

Linto Thomas,

• Xue, Jianxiang, USF Health Morsani College of Medicine, Tampa, Florida, United States

Jianxiang Xue,

• Dominguez Rieg, Jessica, USF Health Morsani College of Medicine, Tampa, Florida, United States

Jessica Dominguez Rieg,

• Rieg, Timo, USF Health Morsani College of Medicine, Tampa, Florida, United States

Timo Rieg,

Background

The kidneys are key players in maintaining the body's phosphate (P_i) homeostasis, and patients with chronic kidney disease (CKD) develop hyperphosphatemia. Two renal transporters mediate the majority of P_i reabsorption, the Na⁺-phosphate cotransporters Npt2a and Npt2c, with Npt2a accounting for ~80% of P_i 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 P_i 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 P_i excretion were assessed in metabolic cages for 3 hours in wild-type (WT) and Npt2a^-/- mice. Effects on plasma P_i 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 P_i 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, V_max 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 P_i 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 P_i excretion in WT mice with an EC₅₀ of ~23 mg*kg^-1. No effect on urinary P_i excretion was observed in Npt2a^-/- mice. In WT mice, Npt2a-I caused a ~31% (P<0.05) decrease in plasma P_i levels; in contrast, plasma P_i levels were unchanged in Npt2a^-/- mice.

Conclusion

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

Funding

• NIDDK Support