Tissue distribution of [14C]sucrose octasulfate following oral administration to rats

Sucrose octasulfate selectively accelerates thrombin inactivation by heparin cofactor II

Inactivation of thrombin (T) by the serpins heparin cofactor II (HCII) and antithrombin (AT) is accelerated by a heparin template between the serpin and thrombin exosite II. Unlike AT, HCII also uses an allosteric interaction of its NH(2)-terminal segment with exosite I. Sucrose octasulfate (SOS) accelerated thrombin inactivation by HCII but not AT by 2000-fold. SOS bound to two sites on thrombin, with dissociation constants (K(D)) of 10 +/- 4 microm and 400 +/- 300 microm that were not kinetically resolvable, as evidenced by single hyperbolic SOS concentration dependences of the inactivation rate (k(obs)). SOS bound HCII with K(D) 1.45 +/- 0.30 mm, and this binding was tightened in the T.SOS.HCII complex, characterized by K(complex) of approximately 0.20 microm. Inactivation data were incompatible with a model solely depending on HCII.SOS but fit an equilibrium linkage model employing T.SOS binding in the pathway to higher order complex formation. Hirudin-(54-65)(SO(3)(-)) caused a hyperbolic decrease of the inactivation rates, suggesting partial competitive binding of hirudin-(54-65)(SO(3)(-)) and HCII to exosite I. Meizothrombin(des-fragment 1), binding SOS with K(D) = 1600 +/- 300 microm, and thrombin were inactivated at comparable rates, and an exosite II aptamer had no effect on the inactivation, suggesting limited exosite II involvement. SOS accelerated inactivation of meizothrombin 1000-fold, reflecting the contribution of direct exosite I interaction with HCII. Thrombin generation in plasma was suppressed by SOS, both in HCII-dependent and -independent processes. The ex vivo HCII-dependent process may utilize the proposed model and suggests a potential for oversulfated disaccharides in controlling HCII-regulated thrombin generation.

Evidence for the absorption of heparin by rat stomach

Antithrombotic activity and heparin with endothelium are observed in rats when heparin is administered by the oral route. Peak endothelial concentrations at 6 min suggest rapid absorption. To identify the site of absorption, stomach and duodenum were isolated by tying the pyloric sphincter of male Wistar rats and heparin (unfractionated bovine lung, 60 mg/kg) was administered by stomach tube or injected into the duodenum. Heparin in plasma and aortic endothelium, collected within 15 min, was determined by densitometry following agarose gel electrophoresis with toluidine blue staining. Heparin was recovered in 5 of 10 endothelial (0.136+/-0.068 microg/cm(2)) and 6 of 10 plasma (0.06+/-0.02 microg/ml) samples when administered in the stomach and in 0 of 9 endothelial and 2 of 9 plasma (0.02+/-0.02 microg/ml) samples when injected into the duodenum. To further study heparin distribution, stomach layers were separated and analysed 15 min and 4h following heparin administration by stomach tube. Heparin was recovered in muscle and mucosal layers as well as washes indicating that heparin passes through stomach tissue. Heparin was also recovered from the portal vein, endothelium and lung. These results indicate that heparin is absorbed following oral administration and that the stomach is an important site of absorption.

Tissue distribution of the low molecular weight heparin, tinzaparin, following administration to rats by the oral route

Heparins are antithrombotic drugs given by intravenous and subcutaneous routes. However, we have observed that heparins have antithrombotic activity in a rat model when administered orally despite low plasma levels, with low molecular weight heparins (LMWHs) being effective at lower single doses than unfractionated heparins (UFH). Since LMWHs may have other pharmaceutical uses and little is known regarding the pharmacokinetics of oral LMWHs, our objectives were to determine the distribution of the LMWH tinzaparin (Logiparin) following oral dosing. To study distribution at different doses, 0.025-15 mg/kg tinzaparin was given by stomach tube to rats. Gut and non-gut tissues were sampled 4 h later. In a time course study, plasma and tissue samples were collected at eight time points within 24 h after oral administration (60 mg/kg, 4 rats/time interval). Accumulated urine and faeces were collected over 4 and 24 h using metabolic cages. Gut tissue and washes, faeces, urine and non-gut tissue were extracted and analysed for heparin by agarose gel electrophoresis with toluidine blue staining. Activated partial thromboplastin time (APTT) and anti-Xa activity, by Heptest and chromogenic assay, estimated plasma tinzaparin concentrations. Stomach and lung tinzaparin concentrations demonstrated a dose-effect. Peak concentrations in tissue and washes of stomach, duodenum, jejunum, ileum and colon were at 6-30, 15-30, 30 min, 2 and 4 h, respectively. Amounts found at peak times in combined tissue and washes accounted for 46% and 0.5% in stomach (15 min) and colon (4 h), respectively. Tinzaparin was recovered from liver, lung, endothelial samples, and urine at 24 h, but not in faeces. Non-significant increases were seen in APTT and the Heptest, however, anti-Xa activity was significantly greater than control at all times examined, peaking at 2 h. No bleeding was observed. Results are consistent with oral absorption of tinzaparin with wide tissue distribution, likely on endothelium with little in plasma, as previously observed for UFH. Oral administration of LMWHs should be further studied.