Pharmacokinetics of Novel Erythropoiesis Stimulating Protein Compared with Epoetin Alfa in Dialysis Patients

Recombinant human erythropoietin (Epoetin) has been used for the treatment of renal and other anemias since it was first licensed nearly a decade ago (1,2). It is most commonly administered by subcutaneous or intravenous injection two or three times a week, and its efficacy, particularly for the anemia of renal failure, is undisputed.

Novel erythropoiesis stimulating protein (NESP) is a hyperglycosylated analogue of Epoetin that stimulates erythropoiesis by the same mechanism as the endogenous hormone. To create NESP, two extra N-linked carbohydrate addition sites were introduced into the primary sequence of Epoetin using site-directed mutagenesis. NESP has five N-linked carbohydrate chains, whereas both Epoetin and the endogenous hormone have three (3). Previous work (4) demonstrated that the sialic acid-containing carbohydrate of erythropoietin determines its serum half-life and in vivo activity. NESP was designed to test the hypothesis that creating a molecule with a higher sialic acid content than that occurring naturally would result in it having a longer half-life.

In animal models, NESP has been shown to have a longer terminal half-life and greater in vivo biologic activity than Epoetin, allowing it to be administered less frequently to obtain the same biologic response (3). The aim of this first human study was to compare the pharmacokinetics of intravenously administered NESP and Epoetin in dialysis patients, and to characterize the pharmacokinetics, including bioavailability, of subcutaneously administered NESP in the same group of patients.

Materials and Methods

Results

Ten of the 11 patients completed the first (intravenous) phase of the study. One patient was withdrawn after receiving the first dose of study drug due to a change in dialysis modality from CAPD to hemodialysis. In all 10 patients, NESP had a longer terminal half-life (t_1/2,z) than Epoetin. The mean t_1/2,z for NESP following intravenous administration was threefold longer than for Epoetin (25.3 versus 8.5 h), a difference of 16.8 h (95% confidence interval, 9.4 to 24.2 h; P = 0.0008) (Figure 1, Table 1). The area under the serum concentration—time curve (AUC) for NESP from 0 to 96 h was more than double that for Epoetin (291.0 ± 7.6 ng · h per ml versus 131.9 ± 8.3 ng · h per ml; mean ± SEM; P < 0.0005), and clearance, estimated using AUC from time 0 to infinity, was significantly lower for NESP compared with Epoetin (1.6 ± 0.3 ml/h per kg versus 4.0 ± 0.3 ml/h per kg; mean ± SEM; P < 0.0005). There was no significant difference in volume of distribution at steady state (V_d) between NESP and Epoetin (52.4 ± 2.0 ml/kg versus 48.7 ± 2.1 ml/kg; mean ± SEM). At 4 d (96 h) after the intravenous injection, mean levels of NESP were approximately 0.5 ng/ml above baseline, whereas those for Epoetin were only 0.02 ng/ml above baseline.

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Figure 1.

Comparison of intravenous pharmacokinetic profiles of NESP (n = 11) and Epoetin alfa (n = 10). Mean ± SD.

Six patients completed the second (subcutaneous) phase of the study. The mean t_1/2,z of NESP following subcutaneous administration was 48.8 h (range, 33.5 to 68.0), which was twofold longer than that for the intravenous route in these six patients (mean difference, 24.9 h; 95% confidence interval, 8.5 to 41.3 h; P = 0.01) (Figure 2, Table 2). The mean time to maximum serum concentration (T_max) was 54.1 h (range, 36 to 72) after subcutaneous injection, and the mean maximum serum concentration (C_max) was approximately 10% of that for the equivalent intravenous dose. At 7 d (168 h) after the subcutaneous injection, mean levels of NESP were approximately 0.3 ng/ml above baseline. Bioavailability was estimated for each patient from the ratio of AUC from time 0 to infinity (∞) following subcutaneous and intravenous administration. The mean bioavailability of NESP was 36.9% (SEM, 3.0) by the subcutaneous route.

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Figure 2.

Comparison of subcutaneous (n = 6) and intravenous (n = 6) pharmacokinetic profiles of NESP in patients that received both treatments. Mean ± SD.

There were no safety concerns during the study; no reported adverse events were considered to be related to Epoetin or NESP administration.

Discussion

This is the first report on administration of NESP to humans. The results of this single-dose pharmacokinetic study show that NESP has approximately a threefold longer terminal half-life than Epoetin following intravenous administration (mean 25.3 versus 8.5 h) and a two- to threefold decrease in clearance. The volume of distribution at steady state for both molecules was approximately equivalent to plasma volume. These results are consistent with the findings from animal studies with NESP (3), which also showed approximately a threefold longer terminal half-life following intravenous administration, a threefold decrease in clearance, and a similar volume of distribution when compared with Epoetin.

As has been found for Epoetin, serum levels of NESP increase slowly after subcutaneous administration, reaching a peak at a mean of 54 h. This time to peak concentration is longer than has been observed for Epoetin (usually about 16 to 24 h) (6, 7, 8, 9, 10, 11), and this is likely to be due to the increased molecular size of NESP (38,000 versus 30,400 daltons). The mean terminal half-life of NESP was 48.8 h following subcutaneous administration, which is nearly double that by the intravenous route (25.3 h). This is due to the fact that the subcutaneous half-life represents a balance between absorption from the injection site and elimination from the circulation. Previous clinical studies have shown extremely variable results for the terminal half-life of Epoetin following subcutaneous administration, but the data suggest that the terminal half-life of NESP is at least twofold longer than that of Epoetin by this route (6, 7, 8, 9, 10, 11). The bioavailability of subcutaneous NESP was 37%, which is at least as great as that reported for subcutaneous Epoetin (6, 7, 8, 9, 10, 11).

Although the underlying mechanism is not known, research has indicated that the sialic acid-containing carbohydrate moieties of erythropoietin have a significant effect on serum clearance, and that serum clearance is the primary determinant of in vivo biologic activity (4,12). Increasing the sialic acid-containing carbohydrate of erythropoietin decreases serum clearance, thereby increasing the terminal half-life and in vivo activity. In animal models, NESP has increased potency compared with Epoetin and can be administered less frequently to obtain the same increase in hematocrit. For example, in normal mice, NESP was nearly 20-fold more potent than Epoetin when each was administered once weekly (3). The data presented here confirm the longer terminal half-life of NESP in anemic dialysis patients. A single dose of subcutaneous NESP equivalent to 100 U/kg Epoetin was able to maintain mean serum concentrations at least threefold above baseline for up to 7 d, a level considered sufficient to stimulate erythropoiesis in anemic patients (13). It is likely that the longer half-life of NESP will confer a clinical advantage over Epoetin by allowing less frequent dosing in patients treated for anemia, possibly allowing dosing every 1 or 2 wk. Studies are under way to investigate the clinical efficacy of NESP for the treatment of anemia in dialysis patients.