Pharmacokinetic Optimisation of the Treatment of Deep Vein Thrombosis
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The current treatment for deep vein thrombosis is a 5- to 10-day course of heparin followed by 3 to 6 months of oral anticoagulants. Both heparin and oral anticoagulants present a high inter- and intra-individual variability and require individualisation and monitoring of their dosage. The pharmacokinetic properties of heparin have been difficult to assess through the radiolabelling procedures typically used for many other drugs. This is partially a result of the heterogeneous nature of heparin. Thus, the pharmacokinetics of heparin are expressed in terms of its pharmacodynamic activity. Improved coagulation test methodology coupled with the incorporation of patient factors such as bodyweight, height, baseline coagulation status, pretreatment heparin sensitivity and heparin concentrations, can be used to improve the accuracy of heparin dosage determination. Computer-based systems are now available to assist clinicians in quantitating dosage requirements, estimating bleeding risks, and storing patient dose-response relationships for future therapy monitoring. Low molecular weight heparin products might improve our ability to control anticoagulant therapy because drug concentration, as well as the effect on the clotting system, will be more predictable in patients receiving these products. In addition, low molecular weight heparins produce a more consistent, predictable anticoagulant response, and clinicians have a new pharmacological tool which may readily lend itself to patient-controlled, home-based anticoagulant pharmacotherapy. Where pharmacokinetics and pharmacodynamics could contribute to the optimisation of warfarin treatment is in the initiation of treatment, the estimation of the dosage required, the methods for drug monitoring, the assessment of unusual responses and the avoidance of drug interactions. Traditional pharmaco kinetic methods have limited applicability to the optimisation of warfarin therapy because there is no direct relationship between drug concentration and therapeutic effect. However, a variety of simple or sophisticated computer-assisted methods have been developed to help clinicians in individualising and monitoring warfarin treatment. New therapeutic approaches, such as direct thrombin inhibitors and thrombolytic agents, could overcome some limitations of the standard heparin plus oral anticoagulation therapy.