Recently, we developed a series of cluster mannosides that tion of mannose receptor-mediated t-PA clearance by M 6 L 5 as observed in the rat, for the human situation. (HEPATOLOGY were able to inhibit tissue-type plasminogen activator (t-PA) binding to the isolated mannose receptor. The mannoside 1997;26:1303-1310.) with the highest affinity was able to inhibit t-PA clearance by the liver in the rat. To test whether these mannosides would Tissue-type plasminogen activator (t-PA) is a serine protealso be efficient inhibitors in humans, we studied the expresase that activates fibrinolysis by converting plasminogen into sion of the mannose receptor in the human liver and deterplasmin, which cleaves fibrin into soluble degradation prodmined the efficacy of the mannosides to inhibit mannose reucts. 1,2 Because of its fibrin-selective action, t-PA is successceptor-mediated t-PA degradation by both rat and human fully used for thrombolytic therapy, e.g., after myocardial cells. Immunohistochemistry indicates that, like the rat, huinfarction. 3 Recombinant t-PA (70 kd) contains a single high man liver endothelial cells and human Kupffer cells do exmannose-type oligosaccharide and one or two complex-type press the mannose receptor. The mannosides do inhibit manoligosaccharides. 4 In both rat and humans, t-PA is rapidly nose receptor-mediated t-PA binding, association, and cleared mainly by the liver. 5,6 It has been shown that, in the degradation by isolated rat liver endothelial cells and t-PA rat, t-PA is cleared from plasma through both the mannose association and degradation by cultured human macrophages receptor and the a 2 -macroglobulin receptor/low-density liat similar concentrations. The cluster mannoside with six poprotein receptor-related protein (LRP). 5,7-10 mannose residues connected with a backbone of five lysine Because of its rapid clearance in humans, large doses of tgroups (M 6 L 5 ) was, like unlabeled t-PA, able to inhibit 125 I-PA must be administered to achieve efficient thrombolysis. t-PA degradation in the nmol/L range, while the mannoside Coadministration of inhibitors of t-PA clearance with t-PA M 5 L 4 inhibited 125 I-t-PA degradation in the mmol/L range. The would reduce the therapeutic dose of t-PA required for efficoncentrations of mannoside necessary to inhibit 125 I-t-PA cient thrombolysis. Furthermore, myocardial infarction padegradation in vitro were comparable with the concentrations tients can have a large variability of liver blood flow. 11 The necessary to inhibit mannose receptor-mediated 125 I-t-PA liver blood flow strongly influences the t-PA clearance. Reclearance in vivo. We conclude that there is no species differduced liver blood flow causes a decreased plasma clearance ence between rat and humans with respect to the distribution and increased plasma levels of endogenous or infused t-PA of the mannose receptor in the liver and the affinity of the in humans. 12,13 Thus, reduced liver blood flow may lead to cluster mannosides, establishing the relevance of the inhibit-PA overdosing and an increased bleeding risk in patients.
Coadministration of inhibitors of t-PA clearance would also reduce the influence of liver blood flow on the t-PA concen-Abbreviations: t-PA tissue-type plasminogen activator; LRP a2-macroglobulin retration, and thereby reduce the risk of t-PA overdosing. The ceptor/low-density lipoprotein receptor-related protein; M5L4,
inhibitors may in addition be used to inhibit the clearance [N-(p-(a-D-mannopyranosyloxy) anilino) thiocarbamyl] -L-lysyl]-N 6 -[N-(p-(a-D-manof endogenous t-PA to enhance the fibrinolytic activity in nopyranosyloxy) anilino) thiocarbamyl]-L-lysyl]-N 6 -[N-(p-(a-D-mannopyranosyloxy) blood and to prevent thrombosis. anilino) thiocarbamyl]-L-lysyl]-N 6 -[N-(p-(a-D-mannopyranosyl-oxy)-anilino) thiocarbamyl]-L-lysine; M6L5, N 2 -[N 2 -[N 2 -[N 2 -[N 2 , N 6 -Bis [N-(p-(a-D-mannopyranosyloxy) We have shown that the uptake and degradation of t-PA anilino) thiocarbamyl]-L-lysyl]-N 6 -[N-(p-(a-D-mannopyranosyloxy) anilino) thiocarby cultured human macrophages are mediated by both the bamyl]-L-lysyl]-N 6 -[N-(p-(a-D-mannopyranosyloxy) anilino) thiocarbamyl]-L-lysyl]mannose receptor and LRP; thus, these cells are a good in N 6 -[N-(p-(a-D-mannopyranosyloxy) anilino) thiocarbamyl]-L-lysyl]-N 6 -[N-(p-(a-Dvitro model for the plasma clearance of t-PA by the liver. 14 mannopyranosyl-oxy)-anilino) thiocarbamyl]-L-lysine; BSA, bovine serum albumin; GST-RAP, glutathione-S-transferase receptor-associated protein; PBS, phosphate-buf-
In the human liver, the LRP is present on hepatocytes and fered saline; IC50, half-maximal inhibitory concentration; CRD carbohydrate recogni-Kupffer cells. 15 The mannose receptor has been shown to be tion domain.