Interaction of thrombopoietin with the platelet c‐mpl receptor in plasma: binding, internalization, stability and pharmacokinetics

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Abstract
Thrombopoietin (TPO) is the primary regulator of platelet production and acts through binding its receptor, c‐mpl, found on megakaryocyte progenitor cells, megakaryocytes and platelets. Circulating levels of TPO are regulated primarily by the clearance of TPO after it binds to c‐mpl receptors on circulating platelets. In this study the interaction of TPO with the platelet c‐mpl receptor has been analysed under physiological conditions using radiochemical and pharmacokinetic approaches. 125I‐rHuTPO was prepared using a novel method of gentle iodination that preserved its biological activity and used to demonstrate that platelets, but not endothelial cells, have a single class of binding sites (56 ± 17 binding sites/platelet) with high affinity (Kd = 163 ± 31 pM). Cross‐linking experiments confirmed that TPO, but not erythropoietin (EPO), specifically associated with the 95 kD platelet c‐mpl receptor. Upon addition of TPO to platelets, 80% of the TPO binding sites were internalized within an hour and were not recycled. TPO that was not bound by platelets was stable for up to 6 d in both platelet‐poor and platelet‐rich plasma. Using unlabelled recombinant human TPO (rHuTPO), standard pharmacokinetic analysis demonstrated that platelets have an average TPO clearance of 1.24 ± 0.38 ml/h/109 platelets and that TPO clearance was reduced by low temperature but not by a number of drugs or metabolic inhibitors. The maximal amount of TPO removed by platelets in vitro was identical to that predicted by the total number of TPO binding sites. These results provide a biochemical and pharmacokinetic basis for the clinical use of TPO and for understanding possible disorders of platelet production.