title: Calcium balance during calcitriol and paricalcitol administration in healthy humans.
abstract: OBJECTIVE
  Secondary hyperparathyroidism in hemodialysis patients requires optimal correction of vitamin D deficiency with active vitamin D and analogues. It has been postulated that new vitamin D analogues, i.e. paricalcitol, efficiently suppress parathyroid hormone serum levels (PTH), but do not increase intestinal calcium absorption as much as calcitriol. The effects of calcitriol and paricalcitol on calcium balance can best be characterized under standardized conditions in healthy individuals with normal renal function, because the urinary calcium excretion at steady state corresponds to the net calcium absorption in the gut.
  
  
  METHODS
  In a randomized, double-blind, placebo-controlled, 3-way crossover Phase I study in 13 healthy individuals we investigated the changes compared to placebo in PTH and urinary calcium excretion during 6-day treatment periods with paricalcitol (1.5 microg/day) and calcitriol (0.5 microg/day).
  
  
  RESULTS
  24-hour urinary calcium excretion was stable during 6 days of placebo administration. Neither paricalcitol nor calcitriol significantly changed calcium excretion. Urinary creatinine, magnesium and phosphate excretion also remained unchanged over the study periods irrespective of the treatment. However, calcitriol was shown to be effective in reducing iPTH levels during 6 days of treatment (mean reduction 4.03+/-0.69 pmol/l), whereas paricalcitol had no effect.
  
  
  CONCLUSIONS
  Using a dosing ratio of 1:3 for calcitriol:paricalcitol, i.e. the same conversion factor used previously in studies on hemodialysis patients, only calcitriol was able to reduce iPTH levels in healthy individuals. Low-dose calcitriol reduced iPTH levels without raising calcium absorption and without including any hypercalcemia.
authors.affiliation: Department of Internal Medicine VI, Clinical Pharmacology and Pharmacoepidemiology, University of Heidelberg, Heidelberg, Germany.
authors.forename: V
authors.initials: V
authors.lastname: Hafner
authors.forename: C
authors.initials: C
authors.lastname: Rutsch
authors.forename: R
authors.initials: R
authors.lastname: Ding
authors.forename: T
authors.initials: T
authors.lastname: Heinrich
authors.forename: L
authors.initials: L
authors.lastname: Diedrichs
authors.forename: H
authors.initials: H
authors.lastname: Schmidt-Gayk
authors.forename: I
authors.initials: I
authors.lastname: Walter-Sack
authors.forename: J
authors.initials: J
authors.lastname: Bommer
authors.forename: G
authors.initials: G
authors.lastname: Mikus
mesh_terms: D000293:Adolescent; D000328:Adult; D002117:Calcitriol / Q000494:pharmacology*; D002118:Calcium / Q000652:urine*; D003404:Creatinine / Q000652:urine; D004311:Double-Blind Method; D004872:Ergocalciferols / Q000494:pharmacology*; D005260:Female; D006801:Humans; D008274:Magnesium / Q000652:urine; D008297:Male; D008875:Middle Aged; D010281:Parathyroid Hormone / Q000097:blood*; D010710:Phosphates / Q000652:urine; D014815:Vitamins / Q000494:pharmacology*
chemical_list: D004872:Ergocalciferols; D010281:Parathyroid Hormone; D010710:Phosphates; D014815:Vitamins; C084656:paricalcitol; D003404:Creatinine; D002117:Calcitriol; D008274:Magnesium; D002118:Calcium