I think it depends on some individual factors, particularly renal clearance of the drug. I found this information in the drug reference:
According to the manufacturer's tests, the drug reaches maximum serum levels between 15 and 44 hours after subcutaneous injection of a single dose. The mean elimination half-life is 22 to 60 hours (mean = 40 hours). So, I would expect that after 7 days (4.2 half-lifes) that the concentration would be about 5.5% (0.5^4.2) of the maximum levels.
From the published pharmokinetic studies on Pegintron:
Following a single subcutaneous dose of PegIntron, the mean absorption half-life (t ½ ka) was 4.6 hours. Maximal serum concentrations (Cmax) occur between 15 and 44 hours post dose, and are sustained for up to 48 to 72 hours. The Cmax and AUC measurements of PegIntron increase in a dose-related manner. After multiple dosing, there is an increase in bioavailability of PegIntron. Week 48mean trough concentrations (320 pg/mL; range: 0, 2960) are approximately 3-fold higher than Week 4 mean trough concentrations (94 pg/mL; range: 0, 416). The mean PegIntron elimination half-life is approximately 40 hours (range: 22-60 hours) in patients with HCV infection. The apparent clearance of PegIntron is estimated to be approximately 22mL/hr·kg. Renal elimination accounts for 30%of the clearance.
Pegylation of interferon alfa-2b produces a product (PegIntron) whose clearance is lower than that of nonpegylated interferon alfa-2b. When compared to INTRON A, PegIntron (1 mcg/kg) has approximately a 7-fold lower mean apparent clearance and a 5-fold greater mean half-life, permitting a reduced dosing frequency. At effective therapeutic doses, PegIntron has approximately 10-fold greater Cmax and 50-fold greater AUC than interferon alfa-2b.
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