12 CLINICAL PHARMACOLOGY
12.1 Mechanism of Action
Epoetin alfa products stimulate erythropoiesis by the same mechanism as endogenous erythropoietin.
Epoetin alfa products increase the reticulocyte count within 10 days of initiation, followed by increases in the RBC count, hemoglobin, and hematocrit, usually within 2 to 6 weeks. The rate of hemoglobin increase varies among patients and is dependent upon the dose of epoetin alfa products administered. For correction of anemia in hemodialysis patients, a greater biologic response is not observed at doses exceeding 300 Units/kg 3 times weekly.
In adult and pediatric patients with CKD, the elimination half-life (t1/2) of plasma erythropoietin after intravenous administration of epoetin alfa ranged from 4 to 13 hours. After subcutaneous administration, Cmax was achieved within 5 to 24 hours. The t1/2 in adult patients with serum creatinine greater than 3 mg/dL was similar between those not on dialysis and those maintained on dialysis. The pharmacokinetic data indicate no apparent difference in epoetin alfa t1/2 among adult patients above or below 65 years of age.
A pharmacokinetic study comparing 150 Units/kg subcutaneous 3 times weekly to 40,000 Units subcutaneous weekly dosing regimen was conducted for 4 weeks in healthy subjects (n = 12) and for 6 weeks in anemic cancer patients (n = 32) receiving cyclic chemotherapy. There was no accumulation of serum erythropoietin after the 2 dosing regimens during the study period. The 40,000 Units weekly regimen had a higher Cmax (3- to 7-fold), longer Tmax (2- to 3-fold), higher AUC0–168 h (2- to 3-fold) of erythropoietin and lower clearance (CL) (50%) than the 150 Units/kg 3 times weekly regimen. In anemic cancer patients, the average t1/2 was similar (40 hours with range of 16 to 67 hours) after both dosing regimens. After the 150 Units/kg 3 times weekly dosing, the values of Tmax and CL were similar (13.3 ± 12.4 vs. 14.2 ± 6.7 hours, and 20.2 ± 15.9 vs. 23.6 ± 9.5 mL/hr/kg) between week 1 when patients were receiving chemotherapy (n = 14) and week 3 when patients were not receiving chemotherapy (n = 4). Differences were observed after the 40,000 Units weekly dosing with longer Tmax (38 ± 18 hours) and lower CL (9.2 ± 4.7 mL/hr/kg) during week 1 when patients were receiving chemotherapy (n = 18) compared with those (22 ± 4.5 hours, 13.9 ± 7.6 mL/hr/kg, respectively) during week 3 when patients were not receiving chemotherapy (n = 7).
The pharmacokinetic profile of epoetin alfa in pediatric patients appeared similar to that of adults.
The pharmacokinetics of epoetin alfa products has not been studied in patients with HIV-infection.