relationships predict slightly lower efficacy (at least two consecutive ≥ 30% reductions from baseline iPTH) with lower hypercalcemia rates (at least two consecutive serum calcium ≥ 10.5 mg/dL) for lower iPTH-based dosing regimens. Further lowering of hypercalcemia rates was predicted if the treatment with paricalcitol is initiated in patients with lower serum calcium levels at screening.
Based on these simulations, a dosing regimen of iPTH/80 with a screening serum calcium ≤ 9.5 mg/dL, approximately 76.5% (95% CI : 75.6% – 77.3%) of HD patients are predicted to achieve at least two consecutive ≥ 30% reductions from baseline iPTH over a duration of 12 weeks. The predicted incidence of hypercalcemia is 0.8% (95% CI : 0.7% – 1.0%). In PD patients, with this dosing regimen, approximately 83.3% (95% CI : 82.6% – 84.0%) of patients are predicted to achieve at least two consecutive ≥ 30% reductions from baseline iPTH. The predicted incidence of hypercalcemia is 12.4% (95% CI : 11.7% - 13.0%). (see CLINICAL STUDIES; CKD Stage 5 and DOSAGE AND ADMINISTRATION; CKD Stage 5)
Pharmacokinetics
Absorption
Paricalcitol is well absorbed. In healthy subjects, following oral administration of paricalcitol at 0.24 mcg/kg, the mean absolute bioavailability was approximately 72%; the mean maximum plasma concentration (Cmax), time to Cmax (Tmax), and area under the concentration time curve (AUC0-∞) were 0.630 ng/mL, 3 hours and 5.25 ng•h/mL, respectively. The mean absolute bioavailability of paricalcitol in CKD Stage 5 patients on hemodialysis (HD) or peritoneal dialysis (PD) was 79% or 86%, respectively. A food effect study in healthy subjects indicated that the Cmax and AUC0-∞ were unchanged when paricalcitol was administered with a high fat meal compared to fasting. Food delays Tmax about 2 hours. The AUC0-∞ of paricalcitol increased proportionally over the dose range of 0.06 to 0.48 mcg/kg in healthy subjects. Following multiple dosing, as once daily in CKD Stage 4 patients, the exposure (AUC) was slightly lower than that obtained after a single dose administration.
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 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 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; a |
