Formulation of peginterferon alfa-2a and peginterferon alfa-2b

Pegylation is the process of covalent attachment of polyethylene glycol (PEG) polymer chains to another molecule, normally a drug or therapeutic protein. In general, pegylation results in improved pharmacokinetic and pharmacodynamic properties, increased drug stability, overall half-life and changes in tissue distribution pattern and elimination pathways. Molecular weight, configuration (linear or branched), and means of attachment of the PEG moiety are the primary contributors to the physicochemical properties of the PEG-peptide conjugate. Attachment of PEG at multiple sites can lead to steric interference between the compound and its receptor, which reduces the biologic activity of the conjugate. Pegylated interferon alfa-2a consists of a 40-kDa branched PEG moiety covalently attached to an interferon alfa-2a molecule. Pegylated interferon alfa-2b consists of a 12-kDa linear PEG molecule covalently attached to an interferon alfa-2b molecule. The branched PEG formulation has many advantages compared with the linear one. Although the 2 PEG molecules are of the same molecular mass, the branched PEG molecule acts as if it were much larger than the corresponding linear molecule. In addition, the branched-chain PEG conjugates have greater thermal and pH stability, are more resistant to proteolytic degradation, and are less antigenic because of the shielding of the attached polypeptide from the immune system. Key differences in the formulation of peginterferon alfa-2a and peginterferon alfa-2b are summarized in Table 1 .

Pharmacokinetics of peginterferon

The difference in pegylation between the 2 peginterferons has a significant effect on their pharmacokinetic properties. Peginterferon alfa-2b has a relatively rapid absorption rate, with a half-life of 4.6 hours and a volume of distribution of 0.99 L/kg, which are not significantly different from those of standard interferon alfa-2b (2.3 hours and approximately 1.4 L/kg, respectively). Maximum serum concentrations are achieved between 15 and 44 hours postdosing and are maintained for 48 to 72 hours, with a peak to trough ratio of greater than 10 after multiple doses. However, mean apparent clearance for pegylated interferon alfa-2b (22 mL/h/kg) is approximately one-tenth of that of nonpegylated interferon alfa-2b (231 mL/h/kg). Thus, the observed increase in the clinical efficacy of peginterferon alfa-2b is principally the result of reduced renal clearance rather than of the effects of pegylation on the rate of absorption or distribution.

Peginterferon alfa-2a is absorbed more slowly than peginterferon alfa-2b and has a restricted volume of distribution (confined largely to the vasculature and liver). In healthy volunteers, a single dose of 180 μg of peginterferon alfa-2a produced a mean maximum serum concentration of 14.2 μg/L in a mean time of 78 hours. After administration of multiple doses of peginterferon alfa-2a (180 μg weekly) to patients with chronic hepatitis C, the mean maximum serum concentration was 25.6 μg/L in a mean time of 45 hours. Peginterferon alfa-2a has a lower peak to trough ratio (1.5:2) than peginterferon alfa-2b, indicating less fluctuation in the serum concentration of the drug during the 1-week dosing interval. Peginterferon alfa-2a is cleared by both the kidney and the liver. Because of its relatively large size, peginterferon alfa-2a has more than a 100-fold reduction in renal clearance than interferon alfa-2b. The pharmacokinetics of the drug is unaffected by renal failure, and hence, no dose modifications are necessary in the setting of renal impairment, as opposed to peginterferon alfa-2b.

Head-to-head comparison of the 2 peginterferons indicated that peginterferon alfa-2b has a shorter half-life in serum than peginterferon alfa-2a. A significant proportion of patients who received peginterferon alfa-2b had levels of drug less than the limits of detection by the end of day 7. Low serum concentrations toward the end of the dosing schedule may be associated with viral rebound. These observations suggest that a shorter dosing interval is necessary for peginterferon alfa-2b. Indeed, one study has reported improved viral kinetics with a twice-weekly regimen, but SVR rates were not reported and a formal study has not been conducted.

Pharmacokinetics of peginterferon

The difference in pegylation between the 2 peginterferons has a significant effect on their pharmacokinetic properties. Peginterferon alfa-2b has a relatively rapid absorption rate, with a half-life of 4.6 hours and a volume of distribution of 0.99 L/kg, which are not significantly different from those of standard interferon alfa-2b (2.3 hours and approximately 1.4 L/kg, respectively). Maximum serum concentrations are achieved between 15 and 44 hours postdosing and are maintained for 48 to 72 hours, with a peak to trough ratio of greater than 10 after multiple doses. However, mean apparent clearance for pegylated interferon alfa-2b (22 mL/h/kg) is approximately one-tenth of that of nonpegylated interferon alfa-2b (231 mL/h/kg). Thus, the observed increase in the clinical efficacy of peginterferon alfa-2b is principally the result of reduced renal clearance rather than of the effects of pegylation on the rate of absorption or distribution.

Peginterferon alfa-2a is absorbed more slowly than peginterferon alfa-2b and has a restricted volume of distribution (confined largely to the vasculature and liver). In healthy volunteers, a single dose of 180 μg of peginterferon alfa-2a produced a mean maximum serum concentration of 14.2 μg/L in a mean time of 78 hours. After administration of multiple doses of peginterferon alfa-2a (180 μg weekly) to patients with chronic hepatitis C, the mean maximum serum concentration was 25.6 μg/L in a mean time of 45 hours. Peginterferon alfa-2a has a lower peak to trough ratio (1.5:2) than peginterferon alfa-2b, indicating less fluctuation in the serum concentration of the drug during the 1-week dosing interval. Peginterferon alfa-2a is cleared by both the kidney and the liver. Because of its relatively large size, peginterferon alfa-2a has more than a 100-fold reduction in renal clearance than interferon alfa-2b. The pharmacokinetics of the drug is unaffected by renal failure, and hence, no dose modifications are necessary in the setting of renal impairment, as opposed to peginterferon alfa-2b.

Head-to-head comparison of the 2 peginterferons indicated that peginterferon alfa-2b has a shorter half-life in serum than peginterferon alfa-2a. A significant proportion of patients who received peginterferon alfa-2b had levels of drug less than the limits of detection by the end of day 7. Low serum concentrations toward the end of the dosing schedule may be associated with viral rebound. These observations suggest that a shorter dosing interval is necessary for peginterferon alfa-2b. Indeed, one study has reported improved viral kinetics with a twice-weekly regimen, but SVR rates were not reported and a formal study has not been conducted.

Pharmacokinetics of ribavirin

Bioavailability of ribavirin was assessed in 6 healthy volunteers by administering an intravenous dose of ¹³ C3-ribavirin, 150 mg, followed 1 hour later by an oral dose of ribavirin, 400 mg. Intravenous and oral ribavirin produced mean maximum plasma concentrations of 4187 and 638 ng/mL, respectively. The mean bioavailability was 51.8% ± 21.8%, and the mean γ-phase half-life was 37.0 ± 14.2 hours. The mean renal clearance, metabolic clearance, and volume of distribution of the central compartment were 6.94 L/h, 18.1 L/h, and 17.8 L, respectively. There is no evidence that peginterferons alter the pharmacokinetics of ribavirin. After the administration of ribavirin, 600, 800, and 1000 to 1200 mg/d, in combination with peginterferon alfa-2b, the mean peak plasma ribavirin concentrations at week 1 were 741, 799, and 1101 ng/mL, respectively. The mean peak plasma ribavirin concentrations at week 4 were 1770, 2297, and 2750 ng/mL, respectively, for patients treated with 600, 800, and 1000 to 1200 mg of ribavirin daily in combination with peginterferon alfa-2b ribavirin. The mean time to the mean peak plasma levels occurred between 1 and 2 hours after dosing at weeks 1 and 4.

Pharmacodynamics of peginterferon

Interferon has both direct and indirect antiviral actions against HCV, but the mechanism of eradication of HCV infection by interferon-alfa is not certain. Interferon mediates its antiviral effect by inducing interferon-stimulated genes (ISGs), which encode for several effector proteins with antiviral effects, such as protein kinase, 2′,5′ oligoadenylate synthetase (2′,5′ OAS), adenosine deaminase, and protein GTPase MX, leading to the inhibition of messenger RNA (mRNA) translation, RNA degradation and editing, and production of nitric oxide. Interferon-alfa also acts indirectly via upregulation of major histocompatibility complex class I genes in antigen-presenting cells, promoting the cytotoxic T-cell clearance of virally infected cells.

Peginterferon alfa-2b is considered a prodrug of interferon alfa-2b. The urethane bond linking the 12-kDa PEG chain to interferon in peginterferon alfa-2b is unstable and susceptible to hydrolysis such that after injection, native interferon alfa-2b is released and circulates in the body. In contrast, the amide bond linking the PEG chain to interferon alfa-2a is not subject to hydrolysis. Overall, the in vitro antiviral activity of peginterferon alfa-2a is approximately 7% of that of interferon alfa-2a, whereas the antiviral activity of peginterferon alfa-2b is approximately 28% of that of interferon alfa-2b. This difference may be, in part, a result of the reduced affinity of peginterferon alfa-2a for its receptor in vitro because of the more stable covalent linkage of the PEG moiety.

The biologic activity of interferon-alfa is estimated by measuring the serum levels of ISGs or known endogenous proteins induced by interferon, such as 2′,5′ OAS, neopterin, and β ₂ -microglobulin. Serum levels of 2′,5′ OAS increased rapidly following single doses of standard interferon alfa-2a and peginterferon alfa-2a and peaked 24 or 48 hours after the administration. 2′,5′ OAS levels declined rapidly in recipients of standard interferon alfa-2a but remained near peak levels for 1 week in recipients of pegylated interferon alfa-2a. Similarly, the administration of peginterferon alfa-2b was associated with dose-related increases in serum neopterin levels and non–dose-related increases in serum 2′,5′ OAS levels in patients with chronic hepatitis C.

Pharmacodynamic studies comparing the 2 peginterferon preparations have yielded conflicting results. In a small study, 22 patients were randomized to receive either peginterferon alfa-2a (180 μg weekly) or peginterferon alfa-2b (1.0 μg weekly). The enzymatic activity of 2′,5′ OAS and the levels of neopterin and β ₂ -microglobulin were measured over a 7-day period. 2′,5′ OAS activity and the serum concentrations of neopterin and β ₂ -microglobulin did not differ significantly between the 2 patient groups at any time point, and there was no significant correlation between the serum area under the concentration-time curve of either peginterferon and that of 2′,5′ OAS, neopterin, and β ₂ -microglobulin. In contrast, in another study, 36 patients were randomized to receive either peginterferon alfa-2b (1.5 μg/kg/wk) or peginterferon alfa-2a (180 μg/wk) for 4 weeks and then in combination with ribavirin (13 mg/kg/d) for an additional 4 weeks. The pharmacokinetic profile of both peginterferons, mRNA expression of a selected group of ISG transcripts, and serum HCV RNA levels were assessed. Patients who received peginterferon alfa-2b had a significantly greater upregulation of interferon-alfa response genes than those who received peginterferon alfa-2a. Correspondingly, patients treated with peginterferon alfa-2b also had a significantly greater log ₁₀ maximum and log ₁₀ time-weighted average decrease in serum HCV RNA levels. The differences in the test to measures interferon’s effectiveness enzymatic assays compared with mRNA levels, and the study design account for the differences in the results. The pharmacodynamics of peginterferon alfa-2a is not affected by race.