The in vivo metabolism of recombinant human erythropoietin in the rat

We compared the in vivo plasma clearance and organ accumulation in anesthetized rats of ¹²⁵I-labeled, recombinant human erythropoietin and ¹²⁵I-labeled, desialated recombinant erythropoietin. The immediate volume of distribution of ¹²⁵I-labeled, recombinant erythropoietin approximated that of the plasma volume. Its plasma clearance was multiexponential, with an initial rapid distribution phase (t( 1/2 ) = 53 minutes) and a slower elimination phase (t( 1/2 ) = 180 minutes). Organ accumulation of labeled recombinant erythropoietin, as compared with ¹²⁵I-labeled human albumin, was negligible until 30 minutes after injection when small amounts appeared in the kidneys and bone marrow. Only 24% of the ¹²⁵I-labeled, desialated recombinant erythropoietin was recovered immediately after injection, and 96% of the hormone was cleared from the plasma with a t( 1/2 ) of 2.0 minutes. The bulk of the desialated hormone accumulated in the liver where it was rapidly catebolized and its breakdown products released back into the plasma. Significantly, in contrast to unmodified erythropoietin, there was also early accumulation of desialated hormone in the kidneys, marrow, and spleen. Desialated orosomucoid but not orosomucoid, yeast mannan, or dextran sulfate 500 inhibited the rapid plasma clearance and hepatic accumulation of desialated erythropoietin. Oxidation of the desialated hormone restored its plasma recovery and clearance to normal but rendered it biologically inactive, and accumulation in organs other than the kidney was negligible.