Hepatic uptake of a modified low molecular weight heparin in rats

Protein source and choice of anticoagulant decisively affect nanoparticle protein corona and cellular uptake

Protein adsorption on nanoparticles has been a focus of the field of nanocarrier research in the past few years and more and more papers are dealing with increasingly detailed lists of proteins adsorbed to a plethora of nanocarriers. While there is an urgent need to understand the influence of this protein corona on nanocarriers' interactions with cells the strong impact of the protein source on corona formation and the consequence for interaction with different cell types are factors that are regularly neglected, but should be taken into account for a meaningful analysis. In this study, the importance of the choice of protein source used for in vitro protein corona analysis is concisely investigated. Major and decisive differences in cellular uptake of a polystyrene nanoparticle incubated in fetal bovine serum, human serum, human citrate and heparin plasma are reported. Furthermore, the protein compositions are determined for coronas formed in the respective incubation media. A strong influence of heparin, which is used as an anticoagulant for plasma generation, on cell interaction is demonstrated. While heparin enhances the uptake into macrophages, it prevents internalization into HeLa cells. Taken together we can give the recommendation that human plasma anticoagulated with citrate seems to give the most relevant results for in vitro studies of nanoparticle uptake.

Clearance of adenovirus by Kupffer cells is mediated by scavenger receptors, natural antibodies, and complement

Kupffer cells (KCs) rapidly remove intravenously injected adenovirus (Ad) vectors from the circulation. A better understanding of the mechanisms involved could suggest strategies to improve Ad gene delivery by suppressing or evading KC uptake. We recently showed that clearance of Ad type 5 vectors by KCs does not involve the interaction of Ad with the well-established Ad receptors, namely, integrins or the coxsackievirus and Ad receptor (J. S. Smith, Z. Xu, J. Tian, S. C. Stevenson, and A. P. Byrnes, Hum. Gene Ther. 19:547-554, 2008). In the current study, we systematically quantified the contributions of various receptors and plasma proteins to the clearance of Ad by KCs. We found that scavenger receptors are a predominant mechanism for the clearance of Ad by KCs. In addition, we found that Ad is opsonized by natural immunoglobulin M antibodies and complement and that these opsonins play a contributory role in the clearance of Ad by KCs. We also examined additional mechanisms that have been postulated to be involved in the clearance of Ad, including the binding of Ad to platelets and vitamin K-dependent coagulation factors, but we found that neither of these were required for the clearance of Ad by KCs.

Liver sinusoidal endothelial cells are the principal site for elimination of unfractionated heparin from the circulation

The mechanism of elimination of blood borne heparin was studied. To this end unfractionated heparin (UFH) was tagged with FITC, which served as both a visual marker and a site of labeling with (125)I-iodine. UFH labeled in this manner did not alter the anticoagulant activity or binding specificity of the glycosaminoglycan. Labeled heparin administered intravenously to rats (0.1 IU/kg) had a circulatory t(1/2) of 1.7 min, which was increased to 16 min upon coinjection with unlabeled UFH (100 IU/kg). At 15 min after injection, 71% of recovered radioactivity was found in liver. Liver cell separation revealed the following relative uptake capacity, expressed per cell: liver sinusoidal endothelial cell (LSEC)-parenchymal cell-Kupffer cell = 15:3.6:1. Fluorescence microscopy on liver sections showed accumulation of FITC-UFH only in cells lining the liver sinusoids. No fluorescence was detected in parenchymal cells or endothelial cells lining the central vein. Fluorescence microscopy of cultured LSECs following binding of FITC-UFH at 4 degrees C and chasing at 37 degrees C, showed accumulation of the probe in vesicles located at the periphery of the cells after 10 min, with transfer to large, evenly stained vesicles in the perinuclear region after 2 h. Immunogold electron microscopy of LSECs to probe the presence of FITC following injection of FITC-UFH along with BSA-gold to mark lysosomes demonstrated colocalization of the probe with the gold particles in the lysosomal compartment. Receptor-ligand competition experiments in primary cultures of LSECs indicated the presence of a specific heparin receptor, functionally distinct from the hyaluronan/scavenger receptor (Stabilin2). The results suggest a major role for LSECs in heparin elimination.

Semi-synthetic heparin derivatives: chemical modifications of heparin beyond chain length, sulfate substitution pattern and N-sulfo/N-acetyl groups

The glycosaminoglycan heparin is a polyanionic polysaccharide most recognized for its anticoagulant activity. Heparin binds to cationic regions in hundreds of prokaryotic and eukaryotic proteins, termed heparin-binding proteins. The endogenous ligand for many of these heparin-binding proteins is a structurally similar glycosaminoglycan, heparan sulfate (HS). Chemical and biosynthetic modifications of heparin and HS have been employed to discern specific sequences and charge-substitution patterns required for these polysaccharides to bind specific proteins, with the goal of understanding structural requirements for protein binding well enough to elucidate the function of the saccharide-protein interactions and/or to develop new or improved heparin-based pharmaceuticals. The most common modifications to heparin structure have been alteration of sulfate substitution patterns, carboxyl reduction, replacement N-sulfo groups with N-acetyl groups, and chain fragmentation. However, an accumulation of reports over the past 50 years describe semi-synthetic heparin derivatives obtained by incorporating aliphatic, aryl, and heteroaryl moieties into the heparin structure. A primary goal in many of these reports has been to identify heparin-derived structures as new or improved heparin-based therapeutics. Presented here is a perspective on the introduction of non-anionic structural motifs into heparin structure, with a focus on such modifications as a strategy to generate novel reduced-charge heparin-based bind-and-block antagonists of HS-protein interactions. The chemical methods employed to synthesize such derivatives, as well as other unique heparin conjugates, are reviewed.

Are novel scavenger-like receptors involved in the hepatic uptake of heparin?

Heparin is an anionic macromolecular drug. It has been widely used as an anticoagulant, and numerous efforts to clarify the mechanism of its disposition in the body have been made to help expand its clinical applications, using its newly found biological activities, as well as to further improve its use in anticoagulant therapy. It has now been shown that heparin is taken up extensively not only by Kupffer cells but also by parenchymal cells in the liver, the major distribution organ, and a receptor-mediated endocytotic mechanism, which is shared by heparin analogs and various anionic macromolecules, is responsible for heparin uptake in both types of cells. Although the characteristics of the receptors for heparin in both cells have lots of similarities to those of scavenger receptors, the receptors in parenchymal cells do not accept acetylated low density lipoprotein (Ac-LDL) as a ligand, which is the only striking difference between them and major scavenger receptors. Although the receptors in Kupffer cells, which accept Ac-LDL as a ligand, may belong to class A scavenger receptors, this remains to be established. We therefore conclude at present that it is likely that novel scavenger-like receptors for heparin (heparin receptors) or unidentified scavenger receptors are responsible for heparin uptake in the liver.

Bioequivalence of subcutaneous and intravenous body-weight-independent high-dose low-molecular-weight heparin Certoparin on anti-Xa, Heptest, and tissue factor pathway inhibitor activity in volunteers

The objective of the study was to demonstrate the bioequivalence of subcutaneously (s.c.) and intravenously (i.v.) administered fixed, high-dose low-molecular-weight heparin (LMWH) on anti-activated factor X activity (anti-FXa). Secondary objectives were the analysis of the pharmacodynamic effects on Heptest, thrombin inhibition, tissue factor pathway inhibitor (TFPI), and the urinary excretion of LMWH in the randomized cross-over study following i.v. and s.c. application of 8000 anti-FXa units LMWH Certoparin in 18 healthy subjects. The bioequivalence following s.c. administration was demonstrated from the antilog of the point estimator for the application differences (s.c. minus i.v.) by an area under the activity-time curve (0-24 h) of 101% (range, 93-110%). LMWH was bioequivalent also on Heptest and TFPI, and was 50% on thrombin inhibition. The urinary excretion of biologically active material was 4.1 and 3.6% following i.v. and s.c. administration, respectively. Differences in the pharmacodynamic parameters of the assays indicate specific biological actions of high and low molecular sacharide chains of the LMWH.

Heparin and chondroitin sulfate inhibit adenine nucleotide hydrolysis in liver and kidney membrane enriched fractions

The inhibition of adenine nucleotide hydrolysis by heparin and chondroitin sulfate (sulfated polysaccharides) was studied in membrane preparations from liver and kidney of adult rats. Hydrolysis was measured by the activity of NTPDase and 5'-nucleotidase. The inhibition of NTPDase by heparin was observed at three different pH values (6.0, 8.0 and 10.0). In liver, the maximal inhibition observed for ATP and ADP hydrolysis was about 80% at pH 8.0 and 70% at pH 6.0 and 10.0. Similarly to the effect observed in liver, heparin caused inhibition of ATP and ADP hydrolysis that reached a maximum of 70% in kidney (pH 8.0). Na(+), K(+) and Rb(+) changed the inhibitory potency of heparin, suggesting that its effects may be related to charge interaction. In addition to heparin, chondroitin sulfate also caused a dose-dependent inhibition in liver and kidney membranes. The maximal inhibition observed for ATP and ADP hydrolysis was about 60 and 50%, respectively. In addition, the hepatic and renal activity of 5'-nucleotidase was inhibited by heparin and chondroitin sulfate, except for kidney membranes where chondroitin sulfate did not alter AMP hydrolysis. On this basis, the findings indicate that glycosaminoglycans have a potential role as inhibitors of adenine nucleotide hydrolysis on the surface of liver and kidney cell membranes in vitro.

Characteristics of tissue distribution of various polysaccharides as drug carriers: influences of molecular weight and anionic charge on tumor targeting

Using the Walker 256 model for carcinosarcoma-bearing rats, we intravenously administered 5 polysaccharide carriers with various molecular weights (MWs) and electric charges and tested for their plasma and tissue distribution. Two carriers, carboxymethylated-D-manno-D-glucan (CMMG) and CMdextran (CMDex), showed higher plasma AUC than the other carriers tested, namely, CMchitin (CMCh), N-desulfated N-acetylated heparin (DSH), and hyaluronic acid (HA). This was consistently found to be true over the range of MWs tested. For CMDex, the maximum value of plasma AUC was obtained when the MW exceeded 150 kDa. As for the anionic charge, CMDex (110-180 kDa) with a degree of substitution (DS) of the CM groups ranging from 0.2 to 0.6, showed maximum plasma AUC values. Twenty-four hours after administration, the concentration of CMDex (180-250 kDa; DS: 0.6-1.2) in tumors was more than 3% of dose/g--approximately 10-fold higher than those observed with CMCh, DSH and HA. Doxorubicin (DXR) was bound to these carriers via a peptide spacer, GlyGlyPheGly (GGFG), to give carrier-GGFG-DXR conjugates (DXR content: 4.2-7.0 (w/w)%), and the antitumor effects of these conjugates were tested with Walker 256 carcinosarcoma-bearing rats by monitoring the tumor weights after a single intravenous injection. Compared with free DXR, CMDex-GGFG-DXR and CMMG-GGFG-DXR conjugates significantly suppressed tumor growth, while the CMCh-GGFG-DXR, DSH-GGFG-DXR, and HA-GGFG-DXR conjugates in a similar comparison showed weak tumor growth inhibition. These findings suggest that the antitumor effect of the carrier-DXR conjugates was related to the extent with which the carriers accumulated in the tumors.

The variable anticoagulant response to unfractionated heparin in vivo reflects binding to plasma proteins rather than clearance

The anticoagulant response to fixed doses of unfractionated heparin is variable in patients with acute illness, and some patients with venous thromboembolism require high doses of heparin to achieve a therapeutic anticoagulant response. To investigate the mechanism responsible for the variable anticoagulant response to heparin in acute illness, heparin clearance and nonspecific protein binding were compared in control and endotoxin-treated rabbits. The plasma half-life (t 1/2) of radiolabeled heparin increased in a dose-dependent fashion. At all doses of heparin studied, the t 1/2 of radiolabeled heparin was unaffected by experimental endotoxemia when compared with control animals. In contrast, the amount of heparin recovered was lower in the plasma of endotoxemic animals because of increased binding to plasma proteins. A chemically modified heparin with low affinity for antithrombin III was added ex vivo or in vivo to displace heparin bound nonspecifically to plasma proteins. The proportion of heparin bound to plasma proteins was significantly greater in the plasma of endotoxemic animals than in controls. These findings indicate that acute inflammation alters heparin recovery but does not affect heparin clearance. The variability of the anticoagulant response to heparin seen in patients with thromboembolism may, in part, be due to this effect of the underlying disease process.

N' alkylamine low molecular mass heparins (LMM-heparin-tyramine and LMM-heparin-tyramine-fitc) exhibit long lasting anticoagulant effects

The pharmacodynamic and pharmacokinetic properties of endpoint-attached N'alkylamine derivatives of low molecular mass heparin (LMMH), low molecular mass heparin (LMMH), low molecular mass heparin-tyramine (LMMH-tyr) and low molecular mass heparin-tyramine-fluorescein-5-isothiocyanate (LMMH-tyr-fitc) were investigated ex vivo. After intravenous bolus injection of LMMH, LMMH-tyr and LMMH-tyr-fitc (150 aXa U/kg) to Sprague-Dawley rats (n = 8), LMMH-tyr and LMMH-tyr-fitc displayed decreased clearances. The beta-half-life time of the antifactor Xa (aXa) of "endpoint-attached heparins" was significantly prolonged: LMMH-tyr (125 min), LMMH-tyr-fitc (141 min) compared to LMMH (69 min). The pharmacokinetics of LMMH-tyr-fitc were measured with reversed phase high performance liquid chromatography (RP-HPLC). It showed a decreased clearance and a prolonged half-life time (132 min). The selectively tagged LMMH-tyramine and LMMH-tyramine-fitc may be used to investigate the pharmacokinetics, plasma protein and cellular binding of low molecular mass heparins.

Hepatic overexpression of bovine scavenger receptor type I in transgenic mice prevents diet-induced hyperbetalipoproteinemia

Hepatic scavenger receptors (SR) may play a protective role by clearing modified lipoproteins before they target the artery wall. To gain insight into this hypothesized function, transgenic mice expressing hepatic bovine SR (TgSR) were created and studied when fed chow, and during diet-induced hyperlipidemia. SR overexpression resulted in extensive hepatic parenchymal cell uptake of fluorescently labeled acetylated human low density lipoprotein (DiI ac-hLDL) and a twofold increase in 125I-acetylated-LDL clearance. Food intake and cholesterol absorption was indistinguishable between control and TgSR mice. In chow-fed mice, lipoprotein cholesterol was similar in control and TgSR mice. However, on a 3-wk high fat/cholesterol (HFHC) diet, the rise in apoB containing lipoproteins was suppressed in TgSR+/- and TgSR+/+ mice. The rise in HDL was similar in control and TgSR+/- mice, but significantly elevated in the TgSR+/+ mice. Overall, on chow, the ratio of apo-B containing lipoprotein cholesterol to HDL cholesterol was similar for all groups (control = 0.33; TgSR+/- = 0.32; TgSR+/+ = 0.38). However, after 3 wk on the HFHC diet, this ratio was markedly higher in control (2.34 +/- 0.21) than in either TgSR+/- (1.00 +/- 0.24) or TgSR+/+ (1.00 +/- 0.19) mice. In TgSR+/- mice, hepatic cholesteryl esters were reduced by 59%, 7 alpha-hydroxylase mRNA levels were elevated twofold, and a significant increase in fecal bile acid flux was observed after the 3-wk HFHC diet. These results suggest SR may play a protective role in liver by preventing diet-induced increases in apoB containing lipoproteins.

Uptake of fractionated 3H-heparin by isolated rat Kupffer cells

PURPOSE AND METHODS

The uptake of fractionated 3H-heparin by isolated rat Kupffer cells was examined to determine the uptake mechanism.

RESULTS

The association of fractionated 3H-heparin was concentration-dependent with a dissociation constant of 3.4 nM and a maximum association capacity of 1.3 pmol/10(6) cells, suggesting the involvement of a specialized mechanism. Although 2,4-dinitrophenol inhibited neither the association nor internalization of fractionated 3H-heparin, lowering the temperature from 37 degrees C to 4 degrees C reduced the internalization of fractionated 3H-heparin by 70% without affecting the association.

CONCLUSIONS

It is suggested that the uptake mechanism may differ from receptor-mediated endocytosis of polypeptides and be mediated by scavenger receptors, because organic anions, and several ligands of scavenger receptors, as well as several heparin analogs, inhibit the binding of fractionated 3H-heparin to Kupffer cells, while phenylarsine oxide, which is known to inhibit the receptor-mediated or absorptive endocytosis of polypeptides, inhibits neither the association nor internalization of fractionated 3H-heparin.

The injection of heparin prolongs the plasma clearance of oxidized low density lipoprotein in the rat

There is evidence that oxidized-LDL plays an important role in atherogenesis. We now report on the in vivo interaction between unfractionated heparin and oxidized LDL in rats. The recovery rates of the native LDL particles ranged between 75% and 85% of the injected dose. Heparin did not interfere with the clearance rates of native LDL. After administration of radioactive labeled oxidized-LDL particles, 26% of the material was measured in circulation after 5 minutes, 8% after 20 minutes, and 3% after 60 minutes. After injection of heparin 2 minutes prior to oxidized-LDL tracer particles, 44% of the tracer was found in blood after 5 minutes, 23% after 20 minutes, and 9% after 60 minutes. Oxidized-LDL tracer particles disappeared from blood with an alpha half-life of 5 minutes and a beta half-life of 7.5 minutes. After receptor blocking with unfractionated heparin the alpha half-life of the oxidized-LDL tracer was prolonged to 17.5 minutes and the beta half-life to 27.5 minutes. These results indicate that heparin molecules of a comparatively small molecular weight competed the scavenger receptor mediated uptake of oxidized-LDL particles in vivo. Oxidized-LDL particles are known to mediate their pro-atherosclerotic activity in part by stimulating smooth muscle cell proliferation by a scavenger receptor-mediated pathway. It can be speculated, if heparins interfere with the uptake of oxidized-LDL, heparins might thus in part exert their known antiatherosclerotic properties.

Pharmacokinetics of tinzaparin (Logiparin)--a low molecular weight heparin--after single and repeated intravenous administration in rats

After a single and repeated i.v. injections of 1 mg/kg 3H-radiolabelled tinzaparin once daily to rats for 7, 14, and 21 days, drug-related radioactivity in plasma, tissues, urine and faeces was measured by use of liquid scintillation counting. The decay in plasma could be described by a three-compartment model with half-lives of the two distributive phases and the terminal elimination phase of 15 min, 90 min, and 37 hrs, respectively. The peak plasma concentration did not change during repeated dosing, as opposed to the trough concentration which increased 3 fold. The decay in tissues was significantly different from that in plasma, and showed less fluctuations. Drug-related radioactivity accumulated gradually with repeated dosing, reaching accumulation ratios between 5 and 9, when based on trough concentrations. Slow elimination was observed from tissues, and significant amounts were still present 14 days after discontinuation of the repeated dosing. In the liver, the concentrations were almost constant during a dosing interval. After a single injection, 86% and 4% of the administered radioactive dose were excreted in urine and faeces over 7 days, respectively, the majority being recovered during the first 24 hrs, demonstrating that the major route of elimination was by renal excretion. The molecular mass distribution of radioactivity in urine was similar but not identical to the injected test substance. It was shifted slightly towards lower molecular mass and had no anti-factor Xa activity, suggesting that the heparin was either inactivated, presumably by desulphation, or that the antithrombin binding portion of the drug was cleared through a different route.