1,25-dihydroxyvitamin D synthesis after renal transplantation: the role of fibroblast growth factor 23 and cyclosporine.

1,25-dihydroxyvitamin D(3) (1,25D) is tightly regulated by circulating factors, containing fibroblast growth factor 23 (FGF23). However, this control is disturbed in chronic kidney disease. Renal transplantation (RTX) alters 1,25D homeostasis. To examine the clinical relevance of 1,25D in RTX, we drew blood samples from 27 renal transplant recipients (20 cyclosporine-based, seven non-cyclosporine-based) and examined serum concentrations of 25-hydroxyvitamin D(3) (25D), 1,25D, and FGF23. Our protocol for cyclosporine was as follows, an initial dose of 8 mg/kg two d before RTX, and subsequently adjusted on the basis of the pharmacokinetic profile. No baseline differences were observed between cyclosporine-based and non-cyclosporine-based regimens before RTX. All variables except 1,25D levels changed similarly between the two groups. In the cyclosporine-based regimen, 1,25D levels increased steeply on day 2 and re-increased from days 7 to 21. Post-transplant FGF23 levels sharply decreased until day 14. Interestingly, the cyclosporine-treated group revealed an unexpected tendency between circulating 1,25D and FGF23 on day 21. Multiple regression analyses indicated the cyclosporine pharmacokinetic profile as a significant predictor for 1,25D levels. Post-transplant 1,25D production is induced by a steep fall in serum FGF23 and prompt graft function on day 2; 1,25D levels thereafter may be stimulated by circulating abundant cyclosporine.