Studies on the metabolic fate of recombinant human erythropoietin(SNB-5001). II: Plasma level profiles, distribution, metabolism, excretion, accumulation in rats after intravenous and subcutaneous injection of 125I-SNB-5001.

Abstract

Pharmacokinetics and accumulation of recombinant human erythropoietin (SNB-5001) were studied in rats after the intravenous(i.v.) and subcutaneous injection(s.c.) of 125I-SNB-5001. Plasma level declined biexponentially with a rapid phase (t1/2α : 0.4 ?? 0.9hr) and a slower phase (t1/2β :3.0 ?? 5.2hr) in male and female rats after a single i.v. (50 ?? 1250U/kg). Dosedependent Ccnax was observed at 6 ?? 9 hr after s.c. Plasma level after s.c. declined slower than after i.v. AUC0→∞ in rats after both injections increased in dose-dependent manner. Bioavailabilities after s.c. were 65 ?? 75%. Maximal levels of radioactivity in most tissues were observed at 10 min ?? 1hr after i.v. and at 6 ?? 12 hr after s.c. Higher radioactivity was found in the plasma and bone marrow following by kidney, liver and spleen. It was difficult to find the radioactivity in the brain and fat tissue. The decrease of tissue levels was proportional to that in the plasma. The levels in plasma and bone marrow at 24 hr after s.c. were two-fold higher than those after i.v. Urinary excretion of radioactivity was about 80% of injected dose in male and female rats until 96hr after i.v. Fecal excretions until 96hr accounted for 6% of administered dose, similar results were observed in rats after s.c. Biliary excretion was 7 % in male rats until 48 hr after i.v. A small amount of high molecular weight radioactive compound was observed in the urine and bile just after the injection, and other consisted of a low molecular weight radioactive compounds as revealed by HPLC analysis. During the repeated injection (50U/kg), the plasma levels at 1 hr after each i.v. and at 6 hr after each s.c. remained almost constant since the first day. There were no pronounced differences in pharmacokinetics between single and repeated injection (for 7 days) as measured the immunoreactive radioactivity. Plasma levels of total and TCA-precipitable radioactivity declined slower than those after the single injection. The tissue levels at 24hr after repeated injection increased than those after single injection but the ratios of tissue to plasma level after repeated injection scarcely changed. The excretion rates in urine and feces were nearly constant during the period of repeated injection, and 80% of injected radioactivity was recovered in urine and 8% in feces until 96hr after the last injection. Free 125I and high molecular weight metabolites were observed in plasma and tissues as determined by HPLC analysis. Free 125I gradually increased as the levels of 125I-SNB-5001 decreased. High molecular weight metabolites, such as M-3 in the plasma and M-1 in tissues, had a slight immunoreactivity. The immunoreactive metabolites were not detected in thyroid. These metabolites present in plasma, tissues and urine after repeated injection were almost same as those after a single injection. It was suggested that there was no specific tissue accumulating SNB-5001.