#4394TACROLIMUS BUT NOT VOCLOSPORIN INHIBITS KIDNEY TUBULAR CALCIUM AND MAGNESIUM REABSORPTION IN RATS AT CLINICALLY THERAPEUTIC DOSES
Authors: Wei Kuang-Yu , van Heugten Martijn H , van Willigenburg Hester , Danser A H Jan , Rehaume Linda M , Cross Jennifer L , Viel John J , de Baaij Jeroen , Hoorn Ewout 
Published: June 2023

Abstract

Background and Aims
Clinically, the calcineurin inhibitor tacrolimus frequently causes hypercalciuria and hypomagnesemia by inhibiting kidney tubular calcium and magnesium reabsorption. Voclosporin, a novel calcineurin inhibitor approved in the USA and Europe for the treatment of adults with active lupus nephritis, has fewer side-effects including less hypomagnesemia. However, the differences between the kidney tubular effects of tacrolimus and voclosporin are unknown. To address this, we compared the effects of tacrolimus, voclosporin, and vehicle in rats.

Method
Tacrolimus (0.5 mg/kg) and voclosporin (0.5 mg/kg) were administrated by daily intraperitoneal injections in male Wistar rats and were compared against vehicle (Cremophor EL: 95% ethanol:saline (5:5:90, v:v:v)) for 28 days (n = 8–9/group). Dosages were determined based on pharmacokinetic studies in rats and aimed to achieve the area under the concentration-time curve observed in clinical studies. At day 18, blood and 24 h urine were collected to measure trough levels of the drugs and to analyze the fractional excretions of calcium and magnesium. At sacrifice, kidneys were harvested for immunoblotting of tubular proteins.

Results
Both tacrolimus and voclosporin reached clinically therapeutic doses with trough levels of 2.4 μg/L ± 0.6 μg/L and 25.8 μg/L ± 9.6 μg/L, respectively. Compared to vehicle, tacrolimus caused significantly higher fractional excretions of calcium (+348% ± 127%, P &lt; 0.001) and magnesium (+60% ± 38%, P &lt; 0.01) and also caused hypomagnesemia (plasma magnesium 0.65 mmol/L ± 0.04 mmol/L vs. 0.81 mmol/L ± 0.02 mmol/L, P &lt; 0.001). Compared to vehicle, voclosporin only caused a slight, but non-statistically significant, increase in fractional calcium excretion (+ 44% ± 38%, P = 0.08) and did not cause higher fractional magnesium excretion (+5% ± 30%, P = 0.96) or hypomagnesemia (plasma magnesium 0.8 mmol/L ± 0.04 mmol/L, P = 0.9). Compared to vehicle, tacrolimus caused an 11-fold decrease in the protein abundance of the cytosolic calcium-binding protein calbindin-D28K and a 2-fold decrease in the abundance of the sodium-chloride cotransporter (NCC). In contrast, voclosporin did not decrease the protein abundances of calbindin-D28K and NCC. No differences were observed between vehicle, tacrolimus, and voclosporin in the calcium channel TRPV5 and magnesium channel TRPM6 (Figure 1).

Conclusion
In contrast to the calcineurin inhibitor tacrolimus, voclosporin does not inhibit the kidney tubular reabsorption of calcium and magnesium and therefore does not cause hypercalciuria or hypomagnesemia. A possible explanation for this difference is that tacrolimus but not voclosporin affects tubular transport in the distal convoluted tubule, which was further supported by the selective inhibition of calbindin-D28K and NCC by tacrolimus. Our data show that tubulotoxicity of tacrolimus is not apparent with voclosporin treatment, at clinically relevant doses.