12 CLINICAL PHARMACOLOGY
12.1 Mechanism of Action
Paricalcitol is a synthetic, biologically active vitamin D2 analog. Preclinical and in vitro studies have demonstrated that paricalcitol's biological actions are mediated through binding of the vitamin D receptor (VDR), which results in the selective activation of vitamin D responsive pathways. Vitamin D and paricalcitol have been shown to reduce parathyroid hormone levels by inhibiting PTH synthesis and secretion.
Within two hours after administering paricalcitol intravenous doses ranging from 0.04 to 0.24 mcg/kg, concentrations of paricalcitol decreased rapidly; thereafter, concentrations of paricalcitol declined log-linearly. No accumulation of paricalcitol was observed with three times a week dosing.
Paricalcitol is extensively bound to plasma proteins (≥ 99.8%). In healthy subjects, the steady state volume of distribution is approximately 23.8 L. The mean apparent volume of distribution following a 0.24 mcg/kg dose of paricalcitol in CKD patients requiring hemodialysis (HD) and peritoneal dialysis (PD) is between 31 and 35 L.
After intravenous 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 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 (present at low levels in plasma), as well as 24,26- and 24,28-dihydroxylation and direct glucuronidation.
Paricalcitol is excreted primarily by hepatobiliary excretion. Approximately 63% of a radioactive dose was recovered in the feces and 19% was recovered in the urine in healthy subjects. In healthy subjects, the mean elimination half-life of paricalcitol is about five to seven hours over the studied dose range of 0.04 to 0.16 mcg/kg. The pharmacokinetics of paricalcitol has been studied in CKD patients requiring hemodialysis (HD) and peritoneal dialysis (PD). The mean elimination half-life of paricalcitol after administration of 0.24 mcg/kg paricalcitol intravenous bolus dose in CKD HD and PD patients is 13.9 and 15.4 hours, respectively (Table 5).
|CKD - HD|
n = 14
|CKD - PD|
n = 8
|Cmax (ng/mL)||1.680 ± 0.511||1.832 ± 0.315|
|AUC0–∞ (ng∙h/mL)||14.51 ± 4.12||16.01 ± 5.98|
|β (1/h)||0.050 ± 0.023||0.045 ± 0.026|
|t1/2 (h)*||13.9 ± 7.3||15.4 ± 10.5|
|CL (L/h)||1.49 ± 0.60||1.54 ± 0.95|
|Vdβ (L)||30.8 ± 7.5||34.9 ± 9.5|
The pharmacokinetics of paricalcitol has not been investigated in geriatric and pediatric patients.
Patients with Hepatic Impairment
The disposition of intravenous 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 normal hepatic function (n=10). The pharmacokinetics of unbound paricalcitol were similar across the range of hepatic function evaluated in this study. The influence of severe hepatic impairment on the pharmacokinetics of paricalcitol has not been evaluated.
Patients with Renal Impairment
The pharmacokinetics of paricalcitol have been studied in CKD patients requiring hemodialysis (HD) and peritoneal dialysis (PD). Hemodialysis procedure has essentially no effect on paricalcitol elimination. However, compared to healthy subjects, CKD patients on dialysis showed a decreased CL and increased half-life.
Drug Interaction Studies
An in vitro study indicates that paricalcitol is not an inhibitor of CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, or CYP3A at concentrations up to 50 nM (21 ng/mL) (approximately 20-fold greater than that obtained after highest tested dose). In fresh primary cultured hepatocytes, the induction observed at paricalcitol concentrations up to 50 nM was less than two-fold for CYP2B6, CYP2C9 or CYP3A, where the positive controls rendered a six- to nineteen-fold induction. Hence, paricalcitol is not expected to inhibit or induce the clearance of drugs metabolized by these enzymes.
Drug interactions with Paricalcitol Injection have not been studied. The following studies have been performed with oral paricalcitol capsules.
The pharmacokinetic interaction between paricalcitol capsule (16 mcg) and omeprazole (40 mg; oral), a strong inhibitor of CYP2C19, was investigated in a single dose, crossover study in healthy subjects. The pharmacokinetics of paricalcitol were unaffected when omeprazole was administrated approximately 2 hours prior to the paricalcitol dose.
Strong CYP3A Inhibitors
The effect of multiple doses of ketoconazole, a strong inhibitor of CYP3A, administered as 200 mg BID for 5 days on the pharmacokinetics of paricalcitol capsule has been studied in healthy subjects. The Cmax of paricalcitol was minimally affected, but AUC0–∞ approximately doubled in the presence of ketoconazole. The mean half-life of paricalcitol was 17.0 hours in the presence of ketoconazole as compared to 9.8 hours, when paricalcitol was administered alone [see Drug Interactions (7)].