Erythropoietin (Epo) is the primary hormone that stimulates erythroid proliferation and differentiation through its cell surface receptor (EpoR) on erythroid progenitor cells. Previous studies have suggested that the bone marrow plays an important role in Epo's elimination. The changes in the EpoR mRNA levels and Epo's clearance in the bone marrow of 11 newborn lambs were studied to elucidate the role of EpoR in Epo's clearance under anemic conditions. Epo mRNA levels were measured by realtime polymerase chain reaction, and relative expression of EpoR was calculated by using the comparative CT method. The glyceraldehyde-3-phosphate dehydrogenase housekeeping gene was chosen as a control gene for the calculations. All lambs showed significant increase in bone marrow EpoR mRNA levels after phlebotomy-induced anemia. Epo's clearance determined from simultaneous pharmacokinetic studies with 125 I-recombinant human Epo showed a significant increase after phlebotomy-induced anemia that was similar to the increase in EpoR. By day 28 after phlebotomy, EpoR mRNA levels and Epo clearance had returned toward baseline. These results indicate that the changes in Epo's clearance are not caused by body growth but result from significant changes in the pool of EpoR. A linear mixed-effect model was used to evaluate the quantitative relationship between EpoR and Epo's clearance. This analysis demonstrated a highly significant positive linear correlation between EpoR and Epo clearance. Together, these findings provide strong evidence that receptormediated Epo clearance is an important route for Epo's elimination.

Pre-clinical studies have demonstrated that bone marrow ablation has a profound effect in decreasing erythropoietin (EPO) elimination. The study's objective was to determine in humans if EPO pharmacokinetics (PKs) are perturbed following bone marrow ablation. EPO PK studies were performed in eight subjects, aged 4 to 61 years, undergoing fully myeloablative hematopoietic stem cell transplantation. Serial PK studies using intravenous injection of recombinant human EPO (92 ±2.0 U/kg) (mean±SEM) were carried out during four periods of altered marrow integrity: baseline pre-ablation, post-ablation pre-transplant, early post-transplant pre-engraftment, and late posttransplant full engraftment. Compared with baseline, post-ablation pre-transplant and early posttransplant EPO PKs demonstrated declines in clearance increases in terminal elimination half-life of 36 and 95%, respectively. Clearance and half-life returned to baseline following full engraftment. The association of EPO elimination with decreased bone marrow activity in patients undergoing transplantation conclusively establishes the bone marrow as a key determinant of EPO elimination in humans.Erythropoietin (EPO), a 34-kDa glycoprotein hormone, has a dominant action in the regulation of erythrocyte production. 1 EPO exerts its biological effect in stimulating the proliferation and differentiation of erythroid progenitors by binding to specific cell-surface receptors (EPO-Rs) that are in greatest abundance on erythroid progenitors located primarily in the bone marrow. 2 EPO-Rs are also located on virtually all non-hematopoietic tissues and have diverse additional biologic effects. 3 There is a paucity of information regarding which organ(s) and tissue(s) are important in EPO metabolism and elimination. Previous in vivo studies demonstrated that the kidney and liver exert no measurable effect on EPO in vivo elimination. [4][5][6] Erythropoietic tissues in rats, e.g., bone marrow and spleen, have also been shown to rapidly metabolize EPO with tissue uptake and clearance directly correlated with the number of erythroid colony-forming units. 7 These pre-clinical in vivo studies are supported by in vitro studies demonstrating that EPO is rapidly degraded by ligand-specific EPO-R erythroid progenitors, 8,9 and by clinical studies in anemic patients with hypoplastic marrows manifesting high serum EPO levels relative to their hemoglobin (Hb) levels. [10][11][12] Correspondence: JA Widness (john-widness@uiowa.edu). CONFLICT OF INTERESTThe authors declared no conflict of interest. In adults, EPO has demonstrated nonlinear pharmacokinetic (PK) behavior, with EPO clearance decreasing as the administered EPO dose increases. [12][13][14] The half-life of EPO ranges from 4 to 8 h in healthy adults given therapeutic doses of EPO. 13 In very-low-birth-weight premature infants, EPO also manifests nonlinear PK behavior, with clearances approximately threefold greater than adults. 15 Notably premature and term infants also manifest proportionally greater red marrow spac...