Pharmacokinetics of heparin and related polysaccharides

Heparin and SARS-CoV-2: Multiple Pathophysiological Links

Low molecular weight heparin, enoxaparin, has been one of most used drugs to fight the SARS-CoV-2 pandemic. Pharmacological properties of heparin recognize its specific ability, as with other oligosaccharides and glycosaminoglycan, to bind several types of viruses during their pass through the extracellular matrix of the respiratory tract, as well as its anticoagulant activity to prevent venous thromboembolism. Antithrombotic actions of enoxaparin have been testified both for inpatients with COVID-19 in regular ward and for inpatients in Intensive Care Units (ICUs). Prophylactic doses seem to be able to prevent venous thromboembolism (VTE) in inpatients in the regular ward, while intermediate or therapeutic doses have been frequently adopted for inpatients with COVID-19 in ICU. On the other hand, although we reported several useful actions of heparin for inpatients with COVID-19, an increased rate of bleeding has been recorded, and it may be related to several conditions such as underlying diseases with increased risks of bleeding, increased doses or prolonged administration of heparin, personal trend to bleed, and so on.

High-dose Intravenous Dalteparin Can be Monitored Effectively Using Standard Coagulation Times

The objective of this study was to examine the pharmacokinetics of intravenous dalteparin (Fragmin, Pharmacia-Upjohn, Peapack, NJ) and to assess the accuracy of standard coagulation-based monitoring techniques as an estimate of drug concentration with which to guide dosing. Knowledge of the kinetic behavior of low-molecular-weight heparins (LMWHs) and the possible utility of coagulation times for monitoring may aid in the development of safe and effective dosing algorithms for percutaneous coronary interventions. Twenty normal volunteers were treated at 2-week intervals with each of three intravenous dalteparin doses. Measurement of anti-IIa, anti-Xa, activated partial thromboplastin time (aPTT), activated clotting time (ACT), and low-range ACT was performed at baseline and at seven additional time points over 8 hours. The half-life of intravenous dalteparin is 77 minutes with slight dose-related variation. The aPTT, LR-ACT, and standard ACT are prolonged after dalteparin administration with the increase closely correlated to anti-Xa activity (aPTT, r = 0.85; LR-ACT, r = 0.79). Classification of anticoagulation intensity range using aPTT or LR-ACT in comparison to anti-Xa activity (0.5-0.99, 1.0-1.49, 1.5-2, >2) displays a level of agreement (kappa: aPTT = 0.69, LR-ACT = 0.59) that is comparable to values reported for coagulation time guidance of unfractionated heparin administration. Standard coagulation times are sensitive to the anticoagulant effect of dalteparin with a degree of correlation that suggests their utility for estimating drug concentration during high dose therapy. Trials establishing a relationship between monitoring and clinical efficacy, and the risk/reward of different treatment ranges alone or in combination with GPIIb/IIIa inhibitors and clopidogrel, are necessary.

Low-dose intravenous heparin infusion in patients with aneurysmal subarachnoid hemorrhage: a preliminary assessment

OBJECT

Aneurysmal subarachnoid hemorrhage (aSAH) predisposes to delayed neurological deficits, including stroke and cognitive and neuropsychological abnormalities. Heparin is a pleiotropic drug that antagonizes many of the pathophysiological mechanisms implicated in secondary brain injury after aSAH.

METHODS

The authors performed a retrospective analysis in 86 consecutive patients with Fisher Grade 3 aSAH due to rupture of a supratentorial aneurysm who presented within 36 hours and were treated by surgical clipping within 48 hours of their ictus. Forty-three patients were managed postoperatively with a low-dose intravenous heparin infusion (Maryland low-dose intravenous heparin infusion protocol: 8 U/kg/hr progressing over 36 hours to 10 U/kg/hr) beginning 12 hours after surgery and continuing until Day 14 after the ictus. Forty-three control patients received conventional subcutaneous heparin twice daily as deep vein thrombosis prophylaxis.

RESULTS

Patients in the 2 groups were balanced in terms of baseline characteristics. In the heparin group, activated partial thromboplastin times were normal to mildly elevated; no clinically significant hemorrhages or instances of heparin-induced thrombocytopenia or deep vein thrombosis were encountered. In the control group, the incidence of clinical vasospasm requiring rescue therapy (induced hypertension, selective intraarterial verapamil, and angioplasty) was 20 (47%) of 43 patients, and 9 (21%) of 43 patients experienced a delayed infarct on CT scanning. In the heparin group, the incidence of clinical vasospasm requiring rescue therapy was 9% (4 of 43, p = 0.0002), and no patient suffered a delayed infarct (p = 0.003).

CONCLUSIONS

In patients with Fisher Grade 3 aSAH whose aneurysm is secured, postprocedure use of a low-dose intravenous heparin infusion may be safe and beneficial.

Dose effect relationship of reviparin in chronic hemodialysis: a crossover study versus nadroparin

Low molecular weight heparins (LMWHs) are used for prevention of clotting in the dialysis circuit. The aim of this trial was to define the optimal dose of a new LMWH and to test the efficiency of a single dose at the start of the session. Fifteen patients were treated according to a double blind and crossover design during 4 blocks of 5 consecutive reviparin doses assigned randomly as 50, 60, 70, 85, and 100 IU anti-Xa/kg. Assessment was carried out on screening of fibrin rings or clots in the arterial and venous air traps and on visual detection of fiber in the dialyzer after rinsing. These clinical results were compared to plasmatic anti-Xa activity and thrombin-antithrombin (TAT) complex generation. A standard dose of 70 IU anti-Xa/kg of nadroparin was used as the control. After a bolus of 50 to 100 IU anti-Xa/kg, the occurrence of fibrin rings and clots in the air traps was dependent on three factors: dose of LMWH, time of the session, and patient status. A bolus of 85 IU anti-Xa/kg of reviparin was effective and safe for sessions of 4 h. For this dose, plasmatic anti-Xa activity was 0.96 +/- 0.28 IU/ml at Hour 2 and 0.82 +/- 0.22 IU/ml at Hour 4. TAT complexes are good markers of the activation of the coagulation. They did not increase during a 4 h session after a reviparin bolus of 100 IU/kg. For the same LMWH dose, the trial shows a great variability of the clinical effect and anti-Xa activities from one patient to another. A single dose of 85 IU anti-Xa/kg of reviparin can be used at the start of the dialysis session as a loading dose. We advise adapting the dose during the subsequent sessions according to the appearance of the blood circuit. The benefit of monitoring anti-Xa activity and TAT complexes could be tested in a further trial.

Heparin sensitive and resistant vascular smooth muscle cells: biology and role in restenosis

Vascular smooth muscle cells (VSMC)s are characterized by their acute growth inhibition by heparin and heparan sulfates; however, recently the isolation of VSMCs which display greatly diminished sensitivity to the antiproliferative action of heparin have been reported. These heparin resistant (HR) VSMCs have been derived through multiple passage of normal rat VSMCs in culture media containing high heparin doses, by transformation of VSMCs with oncogene-containing vectors, or have been isolated from vascular tissues of spontaneously hypertensive rats, healthy humans, or humans with restenosis where their presence is not limited to sites of injury. Initial characterizations of HR VSMCs are reviewed, and here we propose a definition of HR VSMCs. To date the mechanisms underlying heparin insensitivity remain elusive. Further study of HR VSMCs may expand our understanding of cell growth regulation by heparin, establish whether HR VSMCs contribute to the reported failure of heparin to combat restenosis in humans, and identify cellular mechanisms driving certain vascular proliferative diseases.

Indium-111 DTPA-heparin: radiolabeling, pharmacokinetics, and biodistribution following intravenous administration in rat and rabbit

Heparin was coupled to DTPA using the bicyclic anhydride and labeled with Indium-111. This resulted in a radiochemically pure preparation (greater than 95% activity in one peak) as determined by high pressure liquid radiochromatography and did not affect the anticoagulant properties of heparin. Biodistribution in the rat at 1, 20, and 60 minutes after intravenous injection showed rapid blood clearance with uptake in the liver followed by bone and kidney when expressed as percent injected total dose per organ and liver followed by kidney and spleen when expressed as percent injected total dose per gram. Blood elimination in the rabbit was 18.5 minutes which decreased to 7.5 minutes when followed by the injection of protamine. Radioactivity cleared from the liver and lungs as a single exponential with a half-time of 30 minutes, but there was very rapid increase of radioactivity in the lungs, peaking at 1-2 minutes, following the injection of protamine. Indium-111 DTPA-heparin may be used to study in vivo pharmacokinetics and biodistribution of heparin.

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.

Contrasting effects of the intermittent and continuous administration of heparin in experimental restenosis

BACKGROUND

Heparin inhibits proliferation of smooth muscle cells in culture and intimal hyperplasia in experimental animals but paradoxically exacerbates vascular injury in clinical trials. To determine whether the difference in the means by which heparin was administered explained the benefit in animals and aggravation in humans, we examined the vascular effects of a range of heparin treatments.

METHODS AND RESULTS

When laboratory rats were injected subcutaneously with heparin (55.5 IU, approximately 1.0 mg/kg) per clinical trial protocols, intimal hyperplasia after arterial injury was exacerbated rather than alleviated. The intima to media area ratio was increased 22.5% with every-other-day injections and was increased 16.8% with daily injections. When the daily dose of heparin was increased to 7.2 mg/kg or when injections were initiated a week before injury, intimal hyperplasia was made even worse (52.2% and 59.9% above control). Twice-daily heparin, 7 and 17 hours apart, had no demonstrable effect one way or the other, and it was not until the heparin was administered at 12-hour intervals that intimal hyperplasia and cell proliferation were lessened (44.6% decrease). The greatest reduction in intimal hyperplasia was obtained when the heparin was administered continuously. The continuous osmotic pump intravenous infusion of heparin inhibited 62.5% of the expected proliferation, and perivascular polymeric device release of heparin blocked the response by 74.2%. While subcutaneous injections transiently increased activated partial thromboplastin time, neither mode of continuous delivery altered coagulation.

CONCLUSIONS

We might reconsider the use of heparin in vascular diseases and not neglect this promising compound because of inappropriate extrapolation from the laboratory to clinical use.

Relationship between pharmacokinetics and pharmacodynamics of tinzaparin (logiparin), a low molecular weight heparin, in dogs

1. Pharmacodynamic models relating the plasma concentrations (C) of radioactive heparin material to anticoagulant effect (E) have been investigated after single i.v. and s.c. doses of 3H-tinzaparin (1 and 4 mg/kg), a radiolabelled low molecular weight heparin, to six dogs. 2. A counterclockwise hysteresis, characterizing the C versus E relationship, was observed in all animals after s.c., but not i.v., doses indicating a possible delay (lag-time) in the systemic availability of pharmacologically-active heparin material following extravascular administration. A constant (Ke) was introduced into the model to account for this hysteresis. 3. At high plasma concentrations of radioactivity (> 10 micrograms/ml), E was related to C by a sum of two sigmoid Emax models, whereas, at lower concentrations, this reduced to the well-known sigmoid Emax model. It was proposed that tinzaparin activates two 'receptors' having different affinities for the drug. The values of EC50 associated with the activation of a single 'receptor' and of a proposed additional 'receptor' were 3 and 13 micrograms/ml of heparin material, respectively. 4. Heparin material was predominantly eliminated by renal excretion and underwent widespread tissue distribution. After s.c. administration, input of heparin material into systemic plasma was complete within 12 h post-dose, and the absorption process was characterized by a bi-exponential function. 5. We conclude that sigmoid Emax models adequately describe the C versus E relationship after s.c. and i.v. doses of 3H-tinzaparin in dogs and that the interindividual variation of the pharmacodynamic parameters derived from this model was relatively small.

Heparin pharmacokinetics and pharmacodynamics

Heparin was discovered approximately 75 years ago and has been used extensively for the last 50 years to treat thromboembolic disorders. An endogenous glycosaminoglycan, heparin is found largely in the liver, lung and intestine. It is available for exogenous administration both as unfractionated and low molecular weight heparin. Unfractionated heparin is a heterogenous mixture of polysaccharide chains of varying length resulting in a range of molecular weights from 3000 to 30,000D while low molecular weight heparin ranges from 3000 to 6000D. Heparin produces its antithrombotic effect by binding to antithrombin III and this complex then binds to thrombin. In order to accomplish this a total of 18 to 22 monosaccharide units is necessary including a specific pentasaccharide binding site for antithrombin III. After either subcutaneous or intravenous injection heparin is distributed primarily within the intravascular space. A short distribution phase is seen which is thought to correspond to endothelial cell binding and internalisation. The disposition curve for unfractionated heparin has a unique concave-convex shape which is the result of combined saturable and nonsaturable elimination mechanisms. The nonsaturable elimination mechanism is renal and is the primary route of elimination for low molecular weight heparins. For this reason, the concave-convex pattern is not seen with low molecular weight preparations. Both forms of heparin are useful antithrombotic agents; however, the correlation between the antithrombotic effect and an in vitro laboratory test for either type still needs further clarification.