Immunomodulatory Effects of the Calcitriol Analog Paricalcitol

Paricalcitol (19-nor-1,25-dihydroxyvitamin D2) is a synthetic vitamin D receptor (VDR) agonist used for the treatment of secondary hyperparathyroidism mainly in chronic kidney disease (CKD), with a 10-times lower calcemic potential and 3-times less potent to the VDR than calcitriol. This narrative review examines the available preclinical and clinical evidence on the immunomodulatory actions of paricalcitol beyond mineral metabolism. Experimental studies show that paricalcitol can modulate immune-cell function, inflammatory signaling, oxidative stress and fibrosis through VDR-dependent pathways. Human evidence is derived mainly from patients with CKD and kidney transplantation, where reductions in inflammatory and oxidative stress markers have been reported; however, findings are not consistent across all studies. Evidence in other immune-mediated conditions, including IgA nephropathy, psoriasis, atopic dermatitis, autoimmune alopecia, and type 1 diabetes (T1D), remains limited and is based largely on case series and reports and non-RCTs. The PRECAL study (ISRCTN17354692) provides preliminary evidence that timely VDR activation may influence the course of T1D, requiring controlled confirmation. No dosing regimen has yet been established for immunomodulatory use, while the calcitriol replacement daily dose × 3/day (given the relevant potency) has been proven reasonably safe and effective in the PRECAL study. Predictive biomarkers of response are lacking, warranting further basic research.