Oligosaccharide Blocks PAR1 (Proteinase-Activated Receptor 1)-PAR4-Mediated Platelet Activation by Binding to Thrombin Exosite II and Impairs Thrombosis.

BACKGROUND: Inappropriate activation and aggregation of platelets can lead to arterial thrombosis. Thrombin is the most potent platelet agonist that activates human platelets via two PARs (proteinase-activated receptors), PAR1 and PAR4. The aim is to study the activity and mechanism of an oligosaccharide HS-11 (the undecasaccharide, derived from sea cucumber Holothuria fuscopunctata) in inhibiting thrombin-mediated platelet activation and aggregation and to evaluate its antithrombotic activity. METHODS: Platelet activation was analyzed by detecting CD62P/P-selectin expression using flow cytometry. The HS-11-thrombin interaction and the binding site were studied by biolayer interferometry. Intracellular Ca2+ mobilization of platelets was measured by FLIPR Tetra System using Fluo-4 AM (Fluo-4 acetoxymethyl). Platelet aggregation, thrombus formation, and bleeding Assay were assessed. RESULTS: An oligosaccharide HS-11, depolymerized from fucosylated glycosaminoglycan from sea cucumber Holothuria fuscopunctata blocks the interaction of thrombin with PAR1 and PAR4 complex by directly binding to thrombin exosite II, and completely inhibits platelet signal transduction, including intracellular Ca2+ mobilization and protein phosphorylation. Furthermore, HS-11 potently inhibits thrombin-PARs-mediated platelet aggregation and reduces thrombus formation in a model of ex vivo thrombosis. CONCLUSIONS: The study firstly report that the fucosylated glycosaminoglycan oligosaccharide has antiplatelet activity by binding to thrombin exosite II, and demonstrates that thrombin exosite II plays an important role in the simultaneous activation of PAR1 and PAR4, which may be a potential antithrombotic target for effective treatment of arterial thrombosis.

Pharmacokinetics and Pharmacodynamics of a Depolymerized Glycosaminoglycan from Holothuria fuscopunctata, a Novel Anticoagulant Candidate, in Rats by Bioanalytical Methods.

dHG-5 (Mw 5.3 kD) is a depolymerized glycosaminoglycan from sea cucumber Holothuria fuscopunctata. As a selective inhibitor of intrinsic Xase (iXase), preclinical study showed it was a promising anticoagulant candidate without obvious bleeding risk. In this work, two bioanalytical methods based on the anti-iXase and activated partial thromboplastin time (APTT) prolongation activities were established and validated to determine dHG-5 concentrations in plasma and urine samples. After single subcutaneous administration of dHG-5 at 5, 9, and 16.2 mg/kg to rats, the time to peak concentration (Tmax) was at about 1 h, and the peak concentration (Cmax) was 2.70, 6.50, and 10.11 µg/mL, respectively. The plasma elimination half-life(T1/2ß) was also about 1 h and dHG-5 could be almost completely absorbed after s.c. administration. Additionally, the pharmacodynamics of dHG-5 was positively correlated with its pharmacokinetics, as determined by rat plasma APTT and anti-iXase method, respectively. dHG-5 was mainly excreted by urine as the unchanged parent drug and about 60% was excreted within 48 h. The results suggested that dHG-5 could be almost completely absorbed after subcutaneous injection and the pharmacokinetics of dHG-5 are predictable. Studying pharmacokinetics of dHG-5 could provide valuable information for future clinical studies.

The Toxicology of Native Fucosylated Glycosaminoglycans and the Safety of Their Depolymerized Products as Anticoagulants.

Fucosylated glycosaminoglycan (FG) from sea cucumber is a potent anticoagulant by inhibiting intrinsic coagulation tenase (iXase). However, high-molecular-weight FGs can activate platelets and plasma contact system, and induce hypotension in rats, which limits its application. Herein, we found that FG from T. ananas (TaFG) and FG from H. fuscopunctata (HfFG) at 4.0 mg/kg (i.v.) could cause significant cardiovascular and respiratory dysfunction in rats, even lethality, while their depolymerized products had no obvious side effects. After injection, native FG increased rat plasma kallikrein activity and levels of the vasoactive peptide bradykinin (BK), consistent with their contact activation activity, which was assumed to be the cause of hypotension in rats. However, the hemodynamic effects of native FG cannot be prevented by the BK receptor antagonist. Further study showed that native FG induced in vivo procoagulation, thrombocytopenia, and pulmonary embolism. Additionally, its lethal effect could be prevented by anticoagulant combined with antiplatelet drugs. In summary, the acute toxicity of native FG is mainly ascribed to pulmonary microvessel embolism due to platelet aggregation and contact activation-mediated coagulation, while depolymerized FG is a safe anticoagulant candidate by selectively targeting iXase.

Structure and pharmacokinetics/pharmacodynamics of the anticoagulant tetradecasaccharide oHG-14 as an intrinsic tenase inhibitor.

The intrinsic tenase complex (iXase) is an attractive antithrombotic target to treat or prevent pathological thrombosis with negligible bleeding risk. Fucosylated glycosaminoglycan (FG) is a promising anticoagulant by inhibiting iXase. A depolymerized FG (dHG-5) as an anticoagulant has been approved for clinical trials. Given that dHG-5 is a multi-component drug candidate consisting of a homologous series of oligosaccharides, it is difficult to predict a clear pharmacokinetics. Here, as a major oligosaccharide component, the tetradecasaccharide (oHG-14) was purified from dHG-5 and its structure was defined as L-Fuc3S4S-α(1,3)-L-Δ4,5GlcA-α(1,3)-{D-GalNAc4S6S-ß(1,4)-[L-Fuc3S4S-α(1,]3)-D-GlcA-ß(1,3)-}3-D-GalNAc4S6S-ß(1,4)-[L-Fuc3S4S-α(1,]3)-D-GlcA-ol. oHG-14 showed potent iXase inhibitory activity in vitro and antithrombotic effect in vivo comparable to dHG-5. After single subcutaneous administration of oHG-14 at 8, 14.4 and 32.4 mg/kg to rats, the absolute bioavailability was 71.6 %-80.9 % determined by the validated bioanalytical methods. The maximum concentration (Cmax) was 3.73, 8.07, and 11.95 µg/mL, respectively, and the time reaching Cmax (Tmax) was about 1 h. oHG-14 was mainly excreted by kidney as the parent compound with the elimination kinetics of first-order linear model. Anticoagulant activity of oHG-14 was positively correlated with its concentration in rat plasma. The pharmacokinetics/pharmacodynamics (PK/PD) of oHG-14 is similar to that of dHG-5. This study could provide supportive data for the clinical trial of dHG-5 and further development of pure oligosaccharide as an antithrombotic drug candidate.

Structural characterization of oligosaccharides from free radical depolymerized fucosylated glycosaminoglycan and suggested mechanism of depolymerization.

Free radical depolymerization is a common method in structural analysis of polysaccharides, the major challenge is the analysis of the cleavage site and characterization of newly formed ends in this reaction. Here, a fucosylated glycosaminoglycan from H. fuscopunctata (HfFG) was depolymerized by H2O2 and a series of oligosaccharides were purified and their structures were elucidated. For non-reducing ends of the trisaccharides were intact GalNAc4S6S, the cleavage site should mainly be the ß(1,3) linkages between GlcA and GalNAc in the backbone of FG. Meanwhile, the reducing ends of the disaccharides and trisaccharides were almost dicarboxylic acid derivatives of GlcA, possibly arising from oxidative breaking of the CC bond of GlcA at the reducing ends. In addition, glycosidic linkages in D-GalNAc-ß(1,4)-D-GlcA and L-FucS-α(1,3)-D-GlcA located at the reducing end could be cleaved, and the released GalNAc4S6S were oxidized to N-acetylgalactosaminic acid.

Structural characterization and anticoagulant analysis of the novel branched fucosylated glycosaminoglycan from sea cucumber Holothuria nobilis.

Glycosaminoglycan HnFG was extracted from sea cucumber Holothuria nobilis. Its chemical structure was characterized by analyzing the physicochemical properties, oligosaccharides from its mild acid hydrolysates and depolymerized products. The disaccharide d-GalNAc4S6S-α1,2-l-Fuc3S-ol found in its mild acid hydrolysates provided a clue for the presence of a unique disaccharide-branch in HnFG. Furthermore, it was confirmed by a series of oligosaccharides from the low-molecular weight HnFG prepared by ß-eliminative depolymerization. Combining with the analysis of its peroxide depolymerized products, the precise structure of HnFG was determined: A chondroitin sulfate E (CS-E)-like backbone branched with sulfated monofucoses (~67%) and disaccharides d-GalNAcS-α1,2-l-Fuc3S (~33%) at O-3 position of each GlcUA. This is the first report on the novel branches in glycosaminoglycan. Biologically, the native and depolymerized HnFG showed potent activities in prolonging the activated partial thrombin time (APTT) and inhibiting intrinsic coagulation Xase (iXase), whereas the oligosaccharides (degree of polymerization ≤6) had no obvious effects.