Abstract
The contribution of the lymphatics to the absorption and systemic availability of recombinant human epoetin alfa (rHuEPO) following s.c. injection was examined using a cannulated sheep model. Parallel studies were conducted in sheep where a single bolus dose was administered either by i.v. (10, 100, or 1000 IU/kg) or s.c. (400 IU/kg) injection. The first s.c. group served as a control for the calculation of absolute bioavailability. In the second group, the efferent popliteal lymphatic duct was cannulated and peripheral lymph draining the injection site was continuously collected. In the third group, the thoracic duct was cannulated to allow collection of central lymph just prior to entry into the systemic circulation. Blood was periodically sampled from all animals, and concentrations in serum and lymph were determined by enzyme-linked immunosorbent assay. The cumulative amount of rHuEPO recovered in peripheral and central lymph was 83.9 ± 6.6% and 75.3 ± 3.9% of the administered dose, respectively, indicating almost complete absorption from the s.c. injection site and minimal clearance during transit through the lymphatic system. After i.v. administration, the systemic clearance of rHuEPO decreased with increasing dose, reflecting capacity-limited elimination kinetics. A pharmacokinetic model was developed to simultaneously fit experimental data for all treatment groups and estimate bioavailability. The direct measurement of >75% of the dose in peripheral and central lymph independently verifies the calculated bioavailability of 87% and demonstrates the major role of the lymphatic route in the overall s.c. bioavailability of rHuEPO after s.c. administration with this animal model.
Footnotes
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This work was supported by Amgen Inc.
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Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
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doi:10.1124/jpet.104.078790.
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ABBREVIATIONS: EPO, erythropoietin; rHuEPO, recombinant human epoetin alfa; CL, clearance; MRT, mean residence time; Vss, volume of distribution at steady state; QC, quality control; AUC(0-∞), area under the serum concentration-time curve extrapolated to infinity; tlag, lag time; kloss, degradation or loss from the s.c. injection site; Fabs, total fraction of the dose absorbed.
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↵1 Current Address: Department of Clinical Pharmacology, Sandwich Laboratories, Pfizer Ltd., Sandwich, Kent, United Kingdom.
- Received October 4, 2004.
- Accepted December 2, 2004.
- The American Society for Pharmacology and Experimental Therapeutics
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