Distribution

Paricalcitol is extensively bound to plasma proteins (≥ 99.8%). The mean apparent volume of distribution following a 0.24 mcg/kg dose of paricalcitol in healthy subjects was 34 L. The mean apparent volume of distribution following a 4 mcg dose of paricalcitol in CKD Stage 3 and a 3 mcg dose in CKD Stage 4 patients is between 44 and 46 L.

Metabolism

After oral administration of a 0.48 mcg/kg dose of 3H-paricalcitol, parent drug was extensively metabolized, with only about 2% of the dose eliminated unchanged in the feces, and no parent drug was found in the urine. Several metabolites were detected in both the urine and feces. Most of the systemic exposure was from the parent drug. Two minor metabolites, relative to paricalcitol, were detected in human plasma. One metabolite was identified as 24(R)-hydroxy paricalcitol, while the other metabolite was unidentified. The 24(R)-hydroxy paricalcitol is less active than paricalcitol in an in vivo rat model of PTH suppression.

In vitro data suggest that paricalcitol is metabolized by multiple hepatic and non-hepatic enzymes, including mitochondrial CYP24, as well as CYP3A4 and UGT1A4. The identified metabolites include the product of 24(R)-hydroxylation, 24,26- and 24,28-dihydroxylation and direct glucuronidation.

Elimination

Paricalcitol is eliminated primarily via hepatobiliary excretion; approximately 70% of the radiolabeled dose is recovered in the feces and 18% is recovered in the urine. While the mean elimination half-life of paricalcitol is 4 to 6 hours in healthy subjects, the mean elimination half-life of paricalcitol in CKD Stages 3, 4, and 5 (on HD and PD) patients ranged from 14 to 20 hours.
Table 3. Paricalcitol Capsule Pharmacokinetic Characteristics in CKD Stages 3, 4, and 5 Patients Pharmacokinetic Parameters CKD Stage 3
n = 15* CKD Stage 4
n = 14* CKD Stage 5 HD**
n = 14 CKD Stage 5 PD**
n = 8
Cmax (ng/mL) 0.11 ± 0.04 0.06 ± 0.01 0.575 ± 0.17 0.413 ± 0.06
AUC0-∞ (ng•h/mL) 2.42 ± 0.61 2.13 ± 0.73 11.67 ± 3.23 13.41 ± 5.48
CL/F (L/h) 1.77 ± 0.50 1.52 ± 0.36 1.82 ± 0.75 1.76 ± 0.77
V/F (L) 43.7 ± 14.4 46.4 ± 12.4 38 ± 16.4 48.7 ± 15.6
t1/2 16.8 ± 2.65 19.7 ± 7.2 13.9 ± 5.1 17.7 ± 9.6
* Four mcg paricalcitol capsules were given to CKD Stage 3 patients; three mcg paricalcitol capsules were given to CKD Stage 4 patients.
** CKD Stage 5 HD and PD patients received a 0.24 mcg/kg dose of paricalcitol as capsules.

Specific Populations

Geriatric

The pharmacokinetics of paricalcitol has not been investigated in geriatric patients greater than 65 years [see Use in Specific Populations (8.5)].

Pediatric

The pharmacokinetics of paricalcitol has not been investigated in patients less than 18 years of age.

Gender

The pharmacokinetics of paricalcitol following single doses over the 0.06 to 0.48 mcg/kg dose range was gender independent.

Hepatic Impairment

The disposition of paricalcitol (0.24 mcg/kg) was compared in patients with mild (n = 5) and moderate (n = 5) hepatic impairment (as indicated by the Child-Pugh method) and subjects with nor