Pharmacology of Heparins and Fondaparinux
Unfractionated heparin (UFH) and low molecular weight heparin (LMWH) are the anticoagulants of choice in acute thrombosis due to their rapid onset of antithrombotic activity. Since heparins are dependent on the presence of AT(antithrombin) for clotting factor inhibition, they are considered indirect anticoagulants. Heparins have no fibrinolytic activity and will not lyse existing thrombi. Heparins contain an active pentasaccharide sequence that binds to(antithrombin) AT. Once heparin binds and activates AT, it can readily dissociate and bind to additional AT, providing a continuous anticoagulant effect. This binding produces a conformational change, accelerating AT binding and inactivation of coagulation factors XIIa, IXa, XIa, Xa and thrombin. The active pentasaccharide sequence responsible for catalyzing AT is found on one-third and one-fifth of the chains of UFH and LMWH, respectively. Fondaparinux is a synthetic analog of the naturally occurring pentasaccharide found in heparins.Fondaparinux selectively and irreversibly binds to AT. This results in neutralization of factor Xa, which ultimately inhibits thrombin formation and thrombus development.
:- Unfractionated Heparin
Pharmacodynamics and Monitoring
Intravenous (IV) infusion or subcutaneous injections are the available routes for UFH administration and IV is preferred. When given via subcutaneous injection for therapeutic anticoagulation, doses need to be large enough (>30,000 U/day) to overcome UFHs low bioavailability. UFH readily binds to plasma proteins, which contributes to its variable anticoagulant response after parenteral administration. Despite these limitations IV administration rapidly achieves therapeutic plasma concentrations that can be effectively monitored and adjusted based on infusion rates
UFH clearance from the systemic circulation is dose-related and occurs through two independent mechanisms The initial phase is the rapid and saturable binding to endothelial cells, macrophages, and local proteins where UFH is depolymerized. The second phase is a slower, non-saturable, renal-mediated clearance. At therapeutic doses, UFH is cleared primarily via depolymerization, with the higher molecular weight chains being cleared more rapidly than lower weight counterparts. As clearance becomes dependant on the kidney, increased or prolonged UFH dosing provides a disproportionate increase in both the intensity and the duration of the anticoagulant effect.
The anticoagulant response to UFH administration is monitored using the activated partial thromboplastin time (aPTT). The aPTT should be measured every 6 h with IV administration, and doses adjusted accordingly, until the patient has sustainable therapeutic levels. Once steady state is reached the frequency of monitoring can be extended.
To overcome variables delivering UFH, weight-based dosing nomograms are recommended for treatment of thromboembolic disease. Dosing nomograms have been associated with significantly higher initial UFH doses, shorter time to therapeutic activated aPTT, and no increase in bleeding events. UFH dosing nomograms will differ from hospital to hospital due to differences in thromboplastin agents and inter-laboratory standardizations in aPTT measurements [10].
Clinical Indications
Clinical indications for UFH include treatment of acute coronary syndromes (ACS), treatment or prevention of venous thromboembolism (VTE), bridge therapy for atrial fibrillation (AF), and cardioversion. UFH utilization has diminished with LMWH and fondaparinux availability and their superior pharmacokinetic profiles. UFH, with a short half-life and reversal capability, remains the best option in patients requiring higher UFH doses, in patients with underlying bleeding risk, or in those critically ill with organ dysfunction. Patients with fluctuating renal function or with a creatinine clearance less than 30 mL/min are not candidates for LMWH or fondaparinux due to the risk of accumulation and increased bleeding risk When used for thromboprophylaxis in medical patients, three times daily UFH dosing provides better efficacy in preventing VTE events compared to twice daily dosing but generates more major bleeding episodes.].Complications and Reversal of Effect:-
The major complications of UFH therapy include bleeding (major bleeding, 0–7 %; fatal bleeding, 0–3 %) and heparin-induced thrombocytopenia ( (HIT, 1–5 %). Patients receiving UFH for periods of more than 1 month are also at an increased risk for osteoporosis and development of vertebral fractures (approximately 2 % incidence). Hemorrhagic episodes are associated with the intensity of anticoagulation, route of administration (continuous infusions are associated with lower rates), and concomitant use of glycoprotein (gp) IIB/IIIA inhibitors, aspirin or fibrinolytic therapy. The relationship between supratherapeutic levels of UFH (elevated aPTT, heparin levels or anti-Xa levels) and major bleeding is not well established and has not been prospectively compared in clinical trials. Major bleeding can occur within therapeutic levels of anticoagulation. Patient-specific risk factors are the most important consideration when determining the bleeding risk, including: age, gender, renal failure, low body weight, and excessive alcohol consumption.
Anticoagulation management before and after surgery is a patient specific, risk versus benefit decision. It is based on the procedure and patient’s risk factors for bleeding and thrombosis. For patients requiring peri-operative anticoagulation in elective procedures or surgery, discontinuing therapeutic IV UFH doses 4 h prior to the procedure and measuring an aPTT is usually sufficient, as normal hemostasis is restored in this time frame in most cases. If the aPTT remains elevated, then hourly measurements are advised until the aPTT returns to baseline. Therapeutic UFH therapy can be restarted 12 h after major surgery, but should be delayed longer for evidence of continued bleeding. In patients receiving low-dose UFH subcutaneously, there is no contraindication to neuraxial techniques, as the risk for developing spinal hematoma appears to be minimal. In patients who are to receive intraoperative anticoagulation with UFH, the UFH infusion should be started at least 1 h after needle placement. Indwelling catheters should be removed 2–4 h after discontinuation of the UFH infusion and only after the patient’s coagulation status has been assessed.
Since UFH has a short half-life, reversal is not required in most bleed episodes. The treatment of clinically severe UFH-related bleeding includes anti-heparin therapy (protamine sulfate), transfusion therapy, and supportive care. Protamine dosing is dependent on timing of the last UFH dose. For immediate reversal (<30 min since the last UFH dose), 1 mg of protamine is administered for every 100 U of UFH and a follow up aPTT can evaluate the reversal response. When UFH is given as a continuous IV infusion, only UFH delivered during the preceding 2–2.5 h should be included in the calculation to determine the protamine dose. If the UFH dose is unknown, protamine 50 mg can be administered slowly over 10 min followed by serial measurements of aPTT. Severe adverse reactions to protamine, such as hypotension and bradycardia, are common. Reaction severity may be reduced by slowing the administration over 1–3 min (maximum administration rate is 5 mg/min). Allergic responses to protamine are more common in patients who have been previously exposed to the drug for UFH neutralization, or treated with protamine-containing insulin (neutral protamine Hagedorn insulin), have undergone vasectomy, or have hypersensitivity to fish. Patients at risk of developing anti-protamine antibodies can be pretreated with corticosteroid and anti-histamine medications
Low Molecular Weight Heparins:-
Pharmacodynamics and Monitoring
LMWHs have increased bioavailability after subcutaneous injection, renal clearance that is dose-independent, and a longer half-life (17–21 h) when compared to UFH. LMWHs are administered in fixed doses for thromboprophylaxis, or in total body weight adjusted doses for therapeutic anticoagulation
Clinical Indications:-
For medically ill and post-operative patients requiring parenteral VTE(Venous thromboembolism ) prophylaxis, LMWHs have become a suitable replacement for UFH. LMWHs require fewer injections and produce fewer adverse events. In hospitalized medical patients receiving thromboprophylaxis, LMWH was associated with a lower risk of deep vein thrombosis (DVT), fewer injection site hematomas, and no differences in bleeding when compared with UFH. LMWHs have largely replaced IV UFH in patients with acute VTE who are able to continue therapy, unmonitored in the ambulatory setting . In ACS, patients with ST-segment elevation myocardial infarction treated with fibrinolysis and LMWH had a lower incidence of death or non-fatal recurrent myocardial infarction but a higher rate of major bleeding than those treated with fibrinolysis and UFH. Similarly, in unstable angina/non-ST-segment elevation myocardial infarction, LMWH therapy reduced the incidence of death, myocardial infarction, or urgent revascularization when compared to UFH.
Complications and Reversal of Effect
Hemorrhage is the major complication of LMWH, with some data supporting decreased rates of bleeding compared to UFH. Rates of fatal bleeding are reported in 0–0.8 % and major bleeding in 0–3 % of patients. In the surgical setting, peri-procedural thromboembolic risk assessment, bleeding risk assessment, and physician preference will play a role in determining whether LMWH prophylactic dosing is continued or withheld. For patients receiving therapeutic LMWH dosing, discontinuation should be considered 12–24 h prior to procedure, or longer in patients with renal dysfunction. Therapeutic doses of LMWH should not be restarted for 24 h after a major procedure or after neuraxial anesthesia.
In the setting of overdose or life-threatening hemorrhage, protamine is administered IV. Protamine does not fully reverse LMWH but can neutralize the AT effect. Because longer heparin chains bind to protamine, protamine completely reverses the anti-factor IIa activity of LMWH but only reverses 60 % of the anti-factor Xa activity. If immediate reversal is warranted within 8 h of LMWH administration, a protamine dose of 1 mg neutralizes 100 U anti-Xa or 1 mg of LMWH. If bleeding continues, a second dose of 0.5 mg of protamine per 100 U anti-Xa may be administered. Smaller protamine doses are required if the LMWH administration interval is beyond 8 h.
HIT(heparin-induced thrombocytopenia) and HIT with thrombosis (heparin-induced thrombocytopenia witj thrombosis) (HITT) are immune-mediated disorders that result from antibodies being formed against the heparin–platelet factor IV complex. The incidence of HITT in critically ill patients ranges from 1 to 5 % and is associated with the development of thrombocytopenia and life-threatening thrombosis in approximately 30–50 % of HIT positive patients. This immune-mediated response typically occurs in patients exposed to UFH or LMWH for 5–7 days, or sooner if the patient was previously exposed. A 50 % decrease in platelet count occurring 4–10 days after the initiation of UFH or LMWH therapy or formation of a new thrombus while anticoagulated may be indicative of HIT. Platelet counts should be measured prior to the initiation of UFH or LMWH and monitored every other day for the first 4–10 days of therapy. The incidence of HIT is approximately one-tenth lower with LMWH than with UFH. In the setting of a HIT allergy or if positive HIT antibodies have been detected, LMWH cannot be used due to cross reactivity between glycosaminoglycans. Direct thrombin inhibitors (DTIs) are the treatment of choice for patients with HIT or HITT.
Osteoporosis reportedly occurs less frequently in patients treated with LMWH as compared to UFH, and it typically is associated with long-term therapy.
Fondaparinux
Pharmacodynamics and Monitoring
After subcutaneous administration, fondaparinux is rapidly and completely absorbed, exhibiting a half-life of 17–21 h in patients with normal renal function. Fondaparinux is excreted primarily unchanged in the urine with clearance reduced in patients with renal impairment. Similar to LMWH, with predictable pharmacokinetics, monitoring anti-Xa levels is not recommended during fondaparinux administration.
Clinical Indication
Fondaparinux has been proven to be at least as safe and effective as treatment of DVT and pulmonary embolism (PE) as LMWH and UFH, respectively. Fondaparinux has been studied extensively for thromboprophylaxis in medically ill and surgical patients In three trials fondaparinux showed superior efficacy in reducing VTE(Venous thromboembolism ) in patients. undergoing knee arthroplasty, hip arthroplasty, and hip fracture surgery. In a combined analysis, the overall incidence of major bleeding was statistically higher with fondaparinux (2.7 %) compared with LMWH (1.7 %). However, the incidence of clinically relevant bleeding, as defined as bleeding leading to death, reoperation, or occurring in a critical organ, did not differ between the agents. The differences in efficacy and safety outcomes could be related to dosing as well as the timing of peri-operative drug administration. The administration of fondaparinux given less than 6 h after surgery has been associated with an increased frequency of major bleeding. Holding therapy for at least 6 h post-procedure may be recommended in patients at risk of bleeding. Fondaparinux may be a potential option for thromboprophylaxis in the setting of an HIT allergy but no conclusive data is available. While fondaparinux has been studied in ACS(Acute Coronary Syndrom), it has not received Food and Drug Administration (FDA) approval..
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