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ABSORPTION, DISTRIBUTION, METABOLISM, AND EXCRETION

Pharmacokinetic/Pharmacodynamic Analysis of Paradoxal Regulation of Erythropoietin Production in Acute Anemia

Division of Pharmaceutics (N.H.A., V.P.-P.), The College of Pharmacy and Department of Pediatrics (J.A.W., R.L.S.), The College of Medicine, University of Iowa, Iowa City, Iowa

The regulatory mechanism responsible for a paradoxal, rapid drop in the erythropoietin (EPO) plasma level seen 2 to 4 days after acute, phlebotomy-induced anemia was investigated in seven adult sheep. To introduce acute anemia, each sheep underwent two phlebotomies where the hemoglobin (Hb) was reduced to 3 or 4 g/dl over 4 to 5 h. The phlebotomies were spaced 4 to 6 weeks apart in three animals, and 8 days apart in four other animals. EPO plasma levels, reticulocyte count, Hb, and p50 for oxygen-Hb dissociation were determined from frequent blood samplings throughout the study period. EPO's disposition pharmacokinetic (PK) and plasma clearance were determined from i.v. bolus injections of tracer amounts of a recombinant human EPO tracer. The controlled drop in Hb resulted in a rapid increase in plasma EPO to 836 ± 52 mU/ml (mean ± coefficient of variation percentage) that was followed by a paradoxical rapid drop 2 to 4 days after the phlebotomy while the animals were still very anemic (Hb = 4.3 ± 15 g/dl). The rapid drop in plasma EPO level could not be explained by the up-regulated clearance (clearance increased by a factor of less than 2.5) or by physiological adaptation (no change in p50, p > 0.05, second phlebotomy to Hb = 3g/dl inadequately stimulated the EPO production). The PK/pharmacodynamic (PD) analysis supports the hypothesis of a limited sustained high EPO production rate in acute anemia, which indicates an apparent deficiency in the regulation of EPO production in acute anemia. The hypothesis was supported by a PK/PD feedback inhibition model that showed good agreement with the data (r = 0.973 ± 1.57).

Address correspondence to: Dr. P. Veng-Pedersen, College of Pharmacy, University of Iowa, Iowa City, IA 52242. E-mail: veng{at}uiowa.edu

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