Warfarin withdrawal. Pharmacokinetic-pharmacodynamic considerations

Peri-operative management of ophthalmic patients taking antithrombotic therapy

Increasing number of patients presenting for ophthalmic surgery are using oral anti-coagulant and anti-platelet therapy. The current practice of discontinuing these drugs preoperatively because of a presumed increased risk of bleeding may not be evidence-based and could pose a significant risk to the patient's health. To provide an evidence-based review on the peri-operative management of ophthalmic patients who are taking anti-thrombotic therapy. In addition, we briefly discuss the underlying conditions that necessitate the use of these drugs as well as management of the operative field in anti-coagulated patients. A semi-systematic review of literature was performed. The databases searched included MEDLINE, EMBASE, database of abstracts of reviews of effects (DARE), Cochrane controlled trial register and Cochrane systematic reviews. In addition, the bibliographies of the included papers were also scanned for evidence. The published data suggests that aspirin did not appear to increase the risk of serious postoperative bleeding in any type of ophthalmic surgery. Topical, sub-tenon, peri-bulbar and retrobulbar anaesthesia appear to be safe in patients on anti-thrombotic (warfarin and aspirin) therapy. Warfarin does not increase the risk of significant bleeding in most types of ophthalmic surgery when the INR was within the therapeutic range. Current evidence supports the continued use of aspirin and with some exceptions, warfarin in the peri-operative period. The risk of thrombosis-related complications on disruption of anticoagulation may be higher than the risk of significant bleeding by continuing its use for most types of ophthalmic surgery.

Antithrombotic management of atrial fibrillation patients presenting with acute coronary syndrome and/or undergoing coronary stenting: executive summary--a Consensus Document of the European Society of Cardiology Working Group on Thrombosis, endorsed by the European Heart Rhythm Association (EHRA) and the European Association of Percutaneous Cardiovascular Interventions (EAPCI)

There remains uncertainty over optimal antithrombotic management strategy for patients with atrial fibrillation (AF) presenting with an acute coronary syndrome and/or undergoing percutaneous coronary intervention/stenting. Clinicians need to balance the risk of stroke and thromboembolism against the risk of recurrent cardiac ischaemia and/or stent thrombosis and the risk of bleeding. The full consensus document comprehensively reviews the published evidence and presents a consensus statement on a 'best practice' antithrombotic therapy guideline for the management of antithrombotic therapy in such AF patients. This executive summary highlights the main recommendations from the consensus document.

Risk of stroke after surgery in patients with and without chronic atrial fibrillation

SUMMARY BACKGROUND

The extent to which chronic atrial fibrillation affects the risk of postoperative stroke is largely unknown.

OBJECTIVES

We sought to determine the 30-day rate of stroke among patients with and without chronic AF who underwent 10 different types of surgery.

PATIENTS/METHODS

The crude incidence of stroke was retrospectively determined using a population-based linked administrative database of hospitalized patients who underwent specified operations between 1 January 1996 and 30 November 2005. The risk of stroke in patients with AF was adjusted for age, race, sex, presence of diabetes, heart failure, hypertension and prior stroke.

RESULTS

The overall 30-day rate of stroke in 69 202 patients with chronic AF was 1.8% (95% CI, 1.7-1.9%) vs. 0.6% (CI, 0.58-0.62%) in 2 470 649 patients without AF. The risk-adjusted odds of a postoperative stroke in patients with chronic AF were 2.1 (CI, 2.0-2.3). The highest incremental difference in the crude rate of stroke was observed in patients undergoing neurologic or vascular surgery, with a difference of approximately 2%.

CONCLUSION

Patients with chronic AF had twice the risk of postoperative stroke. Randomized trials are needed to determine if aggressive perioperative anticoagulation can reduce the incidence of postoperative stroke in patients with AF.

Validity of current recommendation for peri-operative interruption of warfarin in Asians

Temporary interruption of anticoagulation therapy is usually recommended for anticoagulated patients undergoing invasive procedures to minimize their bleeding risks. We validated the current consensus recommendation on warfarin interruption which were based on Western population studies to determine if they could safely be applied to Asians. The international normalized ratios (INR) in twenty warfarinised patients with a stable INR of 2-3 were prospectively measured at days 0, 3 and 5 after stopping warfarin for procedures or at completion of treatment. The median INR at days 0, 3 and 5 were 2.30 (95% CI 2.16-2.43), 1.32 (95% CI 1.22-1.57) and 1.06 (95% CI 1.05-1.13) respectively (P < 0.001 for trend). All patients were below therapeutic INRs by day 3 with 14 patients (70%, 95% CI 49.92-90.08) achieving INR readings below 1.5. By day 5, all INRs were below 1.5 and only 2 patients (10%, 95% CI -3.15 to 23.15) had INRs above 1.2. There were no significant peri-procedure bleeding or thrombotic events in the 1 month following interruption of warfarin. Our results suggest that the current international recommendation of stopping warfarin for 5 days prior to procedure can safely be applied to Asians without compromising risk of bleeding or thrombosis.

Update on perioperative bridging in patients on chronic oral anticoagulation

Oral anticoagulation (OAC) with vitamin K antagonists is commonly used for long-term prevention or treatment of arterial or venous thromboembolism. In the USA alone, approximately 250,000 patients will require temporary interruption of OAC annually. Managing anticoagulation in those patients on chronic OAC who require invasive procedures continues to be a major clinical dilemma. This article summarizes the existing evidence in light of the recommendations of the American College of Chest Physicians. Management of anticoagulation in the perioperative period will continue to be an important clinical challenge and an evolving area of research. If new oral anticoagulants are successful in replacing warfarin, the entire perioperative anticoagulation scene will change.

Experimental model of warfarin-associated intracerebral hemorrhage

BACKGROUND AND PURPOSE

Future demographic changes predict an increase in the number of patients with atrial fibrillation. As long-term anticoagulation for the prevention of ischemic strokes becomes more prevalent, the burden of warfarin-associated intracerebral hemorrhage (W-ICH) is likely to grow. However, little is known about the clinical aspects and pathophysiologic mechanisms of W-ICH. This study describes the development of a mouse model of W-ICH in which hematoma growth and outcomes can be correlated with anticoagulation parameters.

METHODS

CD-1 mice were treated with warfarin (2 mg/kg per 24 hours) added to drinking water. ICH was induced by stereotactic injection of collagenase type VII (0.075 U) into the right striatum. Hemorrhagic blood volume was quantified by means of a photometric hemoglobin assay 2 and 24 hours after hemorrhage induction. Neurologic outcomes were assessed on a 5-point scale.

RESULTS

The international normalized ratio in nonanticoagulated mice was 0.8+/-0.1. After 24 (W-24) and 30 (W-30) hours of warfarin pretreatment, international normalized ratio values increased to 3.5+/-0.9 and 7.2+/-3.4, respectively. Compared with nonanticoagulated mice, mean hemorrhagic blood volume determined 24 hours after hemorrhage induction was found to be 2.5-fold larger in W-24 mice (P=0.019) and 3.1-fold larger in W-30 mice (P<0.001, n=10 per group). Mortality at 24 hours after hemorrhage induction was 0% in nonanticoagulated mice, 10% in W-24 mice, and 30% in W-30 mice. Hematoma enlargement between 2 and 24 hours after hemorrhage induction was -1.4% for nonanticoagulated mice, 22.9% for W-24 mice, and 62.2% for W-30 mice.

CONCLUSIONS

This study characterizes the first experimental model of W-ICH. It may be helpful in gaining further insights into the pathophysiology of W-ICH and may be used for testing the efficacy of treatment strategies, such as hemostatic therapy, in this severe subtype of stroke.

Hemorrhagic and thromboembolic complications after bariatric surgery in patients receiving chronic anticoagulation therapy

BACKGROUND

Perioperative management of bariatric surgical patients receiving chronic anticoagulation requires an understanding of potential hemorrhagic and thromboembolic risks. The aim of this study is to evaluate hemorrhagic and thromboembolic complications in morbidly obese patients who are on oral anticoagulation treatment and subsequently undergo laparoscopic bariatric surgery.

METHODS

The medical records of all laparoscopic Roux-en-Y gastric bypass (LRYGB) patients from June 2001 to March 2006 were retrospectively reviewed. In addition, data of patients who received chronic anticoagulation therapy with Coumadin and underwent laparoscopic Roux-en-Y gastric bypass was analyzed. Clinical parameters included length of hospitalization, hemorrhagic complications, thromboembolic complications, conversion rate, reoperation, and blood transfusion.

RESULTS

During the study period, 1,700 consecutive patients underwent bariatric surgery for the treatment of morbid obesity. Of these, 21 patients were treated with chronic oral anticoagulation; 3 of the 21 (14%) had hemorrhagic complications: one patient had intraluminal hemorrhage and two patients had intraabdominal hemorrhage. Two patients required blood transfusion, and one patient underwent surgical reintervention. None of the 21 laparoscopic operations were converted to open procedures. There were no postoperative mortalities, and there were no thromboembolic events in this series.

CONCLUSIONS

Laparoscopic bariatric surgery can be performed relatively safely in morbidly obese patients who are treated with chronic oral anticoagulation. Even in the presence of bleeding, patients can be successfully treated without the need for reoperation.

Prävention unerwünschter Arzneimittelwirkungen bei älteren Patienten

Adverse drug reactions are among the most common adverse events and a significant cause of preventable morbidity and mortality. As multimorbidity and polypharmacy are frequent in this population, the elderly are at special risk for adverse drug events, although the calendar age has not been proved as independent risk factor in this context. In particular falls and delirium are clinically significant and typical adverse drug events in the elderly. In this review mechanisms and factors which determine adverse drug re actions are described, and possible strategies for an effective prevention are given. This covers pharmacokinetic, pharmacogenetic and pharmacodynamic aspects as well as factors influencing individual adherence to drug therapy. A significant portion of adverse drug reaction may be prevented by a thorough indication and prudent monitoring of pharmacotherapy. Also adherence to pharmacotherapy may be improved by tailored and individual means referring to the patient's needs and expectancies. In the elderly functional limitations such as reduced cognitive abilities, reduced visual acuity and impaired dexterity determine an ineffective pharmacotherapy and medication errors. Hereby these functional limitations are significant predictors of adverse drug events in the context of self-management of pharmacotherapy. Testing of functional abilities as provided in the geriatric assessment is helpful to identify these factors. Among altered pharmacokinetic factors in the elderly, reduced renal function is most important to avoid overdosage. Although a precise measurement of renal function is not possible in a bed-side manner, an estimation of actual renal function utilizing estimation-formulas should always take place.

Anticoagulation for venous thromboembolism

Longer duration of treatment does not reduce risk of recurrence unless continued indefinitely

Association of warfarin dose with genes involved in its action and metabolism

We report an extensive study of variability in genes encoding proteins that are believed to be involved in the action and biotransformation of warfarin. Warfarin is a commonly prescribed anticoagulant that is difficult to use because of the wide interindividual variation in dose requirements, the narrow therapeutic range and the risk of serious bleeding. We genotyped 201 patients for polymorphisms in 29 genes in the warfarin interactive pathways and tested them for association with dose requirement. In our study, polymorphisms in or flanking the genes VKORC1, CYP2C9, CYP2C18, CYP2C19, PROC, APOE, EPHX1, CALU, GGCX and ORM1-ORM2 and haplotypes of VKORC1, CYP2C9, CYP2C8, CYP2C19, PROC, F7, GGCX, PROZ, F9, NR1I2 and ORM1-ORM2 were associated with dose (P < 0.05). VKORC1, CYP2C9, CYP2C18 and CYP2C19 were significant after experiment-wise correction for multiple testing (P < 0.000175), however, the association of CYP2C18 and CYP2C19 was fully explained by linkage disequilibrium with CYP2C9*2 and/or *3. PROC and APOE were both significantly associated with dose after correction within each gene. A multiple regression model with VKORC1, CYP2C9, PROC and the non-genetic predictors age, bodyweight, drug interactions and indication for treatment jointly accounted for 62% of variance in warfarin dose. Weaker associations observed for other genes could explain up to approximately 10% additional dose variance, but require testing and validation in an independent and larger data set. Translation of this knowledge into clinical guidelines for warfarin prescription will be likely to have a major impact on the safety and efficacy of warfarin.

National survey of anticoagulation policy in endoscopy

BACKGROUND

Anticoagulated patients who need to undergo endoscopy present unique challenges to the gastroenterologist. The continuation of anticoagulant therapy increases the risk of haemorrhagic complications of gastrointestinal endoscopy. Reversing the anticoagulation increases the risk of thromboembolism. In our experience in various endoscopy units, there are variable policies on the management of anticoagulated patients undergoing gastrointestinal endoscopy.

METHODS

To study the current practice, survey questionnaires were sent to 2320 doctors, working in 231 hospitals across the United Kingdom.

RESULTS

Responses were obtained from 219 hospitals (94.8%), but only from 434 doctors (18.7%). The results show 40.8% endoscopists continued the patients on warfarin when performing a planned upper gastrointestinal endoscopy, whereas 26% stopped it; 33.2% gave varying reports, that is, they used their own judgement according to the disease for which the anticoagulant was being given. For planned lower gastrointestinal endoscopy, 48.7% doctors preferred to stop warfarin; 53.3% of the endoscopists stated that they have a policy in place at their hospital for both upper and lower gastrointestinal endoscopy in anticoagulated patients; 5.5% had a policy for upper gastrointestinal endoscopy only and 6.2% for lower gastrointestinal endoscopy only. Thirty-five per cent doctors reported that they did not have any standard policy. We compared the responses from within a hospital to see whether the doctors were uniformly aware of an existing policy in their hospital. For upper gastrointestinal endoscopy, the responses were the same (either yes or no) by 51% of the doctors, whereas they were different by 49%. For lower gastrointestinal endoscopies, the same response was given by 49% of the doctors, whereas 51% gave different answers. The poor response rate from the doctors, however, makes firm interpretation of the data difficult.

CONCLUSIONS

A wide variation in practice is seen across the country. A robust national guideline to streamline the endoscopy practice in anticoagulated patients is needed.

Discontinuation of perioperative antiplatelet and anticoagulant therapy in stroke patients

Growing evidence suggests that perioperative withdrawal of ASA for secondary stroke prevention increases thromboembolic risk without the associated benefit of decreased bleeding complications. ASA maintenance is acceptable in many procedures, including invasive ones. Many procedures, in particular ophthalmologic, dermatologic, and dental surgeries, also are safe while continuing oral AC. Warfarin has been continued successfully even in some surgeries that have high bleeding risk. When the risk is too high, temporary bridging therapy with LWMH is safe in many populations. Although the exact thromboembolic risks associated with temporary cessation of AP and AC are unknown and likely low, morbidity and mortality associated with thromboembolism are high. Further studies investigating the risks and benefits of maintaining AP and AC during procedures, particularly invasive ones, are needed. Meanwhile, it is critical that physicians understand the risks and benefits of perioperative AP and AC and the variety of procedures in which these agents can be safely continued.

Association of warfarin dose with genes involved in its action and metabolism

We report an extensive study of variability in genes encoding proteins that are believed to be involved in the action and biotransformation of warfarin. Warfarin is a commonly prescribed anticoagulant that is difficult to use because of the wide interindividual variation in dose requirements, the narrow therapeutic range and the risk of serious bleeding. We genotyped 201 patients for polymorphisms in 29 genes in the warfarin interactive pathways and tested them for association with dose requirement. In our study, polymorphisms in or flanking the genes VKORC1, CYP2C9, CYP2C18, CYP2C19, PROC, APOE, EPHX1, CALU, GGCX and ORM1-ORM2 and haplotypes of VKORC1, CYP2C9, CYP2C8, CYP2C19, PROC, F7, GGCX, PROZ, F9, NR1I2 and ORM1-ORM2 were associated with dose (P < 0.05). VKORC1, CYP2C9, CYP2C18 and CYP2C19 were significant after experiment-wise correction for multiple testing (P < 0.000175), however, the association of CYP2C18 and CYP2C19 was fully explained by linkage disequilibrium with CYP2C9*2 and/or *3. PROC and APOE were both significantly associated with dose after correction within each gene. A multiple regression model with VKORC1, CYP2C9, PROC and the non-genetic predictors age, bodyweight, drug interactions and indication for treatment jointly accounted for 62% of variance in warfarin dose. Weaker associations observed for other genes could explain up to approximately 10% additional dose variance, but require testing and validation in an independent and larger data set. Translation of this knowledge into clinical guidelines for warfarin prescription will be likely to have a major impact on the safety and efficacy of warfarin.

Pharmacogenetics of warfarin: current status and future challenges

Warfarin is an anticoagulant that is difficult to use because of the wide variation in dose required to achieve a therapeutic effect, and the risk of serious bleeding. Warfarin acts by interfering with the recycling of vitamin K in the liver, which leads to reduced activation of several clotting factors. Thirty genes that may be involved in the biotransformation and mode of action of warfarin are discussed in this review. The most important genes affecting the pharmacokinetic and pharmacodynamic parameters of warfarin are CYP2C9 (cytochrome P(450) 2C9) and VKORC1 (vitamin K epoxide reductase complex subunit 1). These two genes, together with environmental factors, partly explain the interindividual variation in warfarin dose requirements. Large ongoing studies of genes involved in the actions of warfarin, together with prospective assessment of environmental factors, will undoubtedly increase the capacity to accurately predict warfarin dose. Implementation of pre-prescription genotyping and individualized warfarin therapy represents an opportunity to minimize the risk of haemorrhage without compromising effectiveness.

Synthesis and anticoagulant activities of substituted 2,4-diketochromans, biscoumarins, and chromanocoumarins

Different substituted 2,4-diketochromans, biscoumarins, and chromanocoumarins are the final products when 4-hydroxycoumarin and aromatic aldehydes containing hydroxyl group in o-, m,- or p-position condense in boiling ethanol. We synthesized 14 compounds. Three of them are described for the first time. The X-ray crystal structure analysis of 3-[6-oxo-(6H, 7H)-benzopyrano[4,3-b]benzopyran-7-yl]-4-hydroxy-2H-1-benzopyran-2-one 1 confirmed the structure of this compound. Acute toxicity studies of the compounds were performed on mice by oral and intraperitoneal administration. A comparative pharmacological study of the in vivo anticoagulant effect of the derivatives with respect to warfarin showed that the synthesized compounds have different anticoagulant activities. The most prospective compounds are 3-(3'-hydroxybenzylidene)-2,4-diketochroman 4 and 3,3'-(2-pyridylmethylene)-bis-4-hydroxy-2H-1-benzopyran-2-one 11 with low toxicity and dose-dependent anticoagulant activity in vivo.

Synthesis, structure, toxicological and pharmacological investigations of 4-hydroxycoumarin derivatives

Twenty 4-hydroxycoumarin derivatives were synthesized. Five of them are described for the first time. The X-ray crystal structure analysis of 3,3'-(2,3,4-trimethoxyphenylmethylene)bis-(4-hydroxy-2H-1-benzopyran-2-one) (7) and 3,3'-(3,5-dimethoxy-4-hydroxyphenylmethylene)bis-(4-hydroxy-2H-1-benzopyran-2-one) (9) confirmed the structure of these compounds. A comparative pharmacological study of the anticoagulant effect with respect to Warfarin showed that the synthesized compounds have different anticoagulant activities. The most prospective compound is 3,3'-(4-chlorophenylmethylene)bis-(4-hydroxy-2H-1-benzopyran-2-one) (12) with low toxicity, very good index of absorption and dose dependent anticoagulant activity.

Perioperative Management of Oral Anticoagulation

Based on an individual assessment of risk factors for arterial or venous thrombosis and the risk of postoperative bleeding, this article outlines the preoperative and postoperative approach to anticoagulant management. Preceding this is a brief description of the therapies most commonly used in the perioperative period. The prevention of arterial thromboembolism is considered separately from the prevention of venous thrombosis.

The management of patients who require temporary reversal of vitamin K antagonists for surgery: a practical guide for clinicians

The management of patients who require temporary interruption of vitamin K antagonists is a common clinical problem, affecting an estimated 400 000 patients per year in Europe and North America. Managing such patients is challenging because of the lack of randomized trials assessing different perioperative anticoagulation management strategies and inconsistent recommendations from consensus groups. Recent non-randomized trials have helped to estimate the risks for arterial thromboembolism and bleeding with bridging anticoagulation involving low-molecular-weight heparin. The objectives of this review are to describe bridging anticoagulation and how it may be used with a short-acting heparin, such as unfractionated heparin or low-molecular-weight heparin, to discuss preoperative patient management, focusing on risk stratification for thromboembolic events and interruption of vitamin K antagonist therapy, and to discuss postoperative patient management, focusing on surgery-related bleeding risk and the resumption of bridging anticoagulation and vitamin K antagonist therapy.

Therapy Insight: approaching endoscopy in anticoagulated patients

Over the past decade, the application of anticoagulant and antiplatelet agents for various cardiovascular and hematologic conditions has become more widespread. Optimal management of these agents during the periendoscopic period requires consideration, but limited prospective data mean that guidelines have largely relied on expert opinion. Elective procedures should be delayed in patients on temporary anticoagulation therapy (e.g. those with deep vein thrombosis). For procedures considered to have a low risk of bleeding (e.g. diagnostic endoscopy and colonoscopy without polypectomy) there is no need to discontinue or adjust anticoagulation. For procedures with a higher risk of bleeding (e.g. polypectomy and biliary sphincterotomy) an individual approach is required. This approach might include stopping oral anticoagulant therapy with or without the administration of unfractionated heparin or low-molecular-weight heparin for the preprocedure and postprocedure periods, during which the patient's international normalized ratio is in the subtherapeutic range.

Low–Molecular-Weight-Heparins as Periprocedural Anticoagulation for Patients on Long-Term Warfarin Therapy: A Standardized Bridging Therapy Protocol

BACKGROUND

Over 2 million patients in North America are on warfarin anticoagulation therapy for prevention of thromboembolism. Suspension of warfarin therapy is often required to prepare patients for invasive procedures or surgeries. To protect these patients against thromboembolism while they are off warfarin, shorter-acting parenteral agents such as low-molecular-weight heparins (LMWHs) are often used. We conducted a retrospective observational study of our anticoagulation clinic patients to assess the safety and efficacy of LMWHs using a standardized protocol for periprocedural anticoagulation therapy.

METHODS

We included 69 consecutive patients who required interruption of their long-term warfarin therapy between August 2001 and August 2002, and were deemed by the treating physician to be at high enough risk for perioperative thromboembolism to justify bridging anticoagulation. We used a standard bridging therapy protocol in our anticoagulation clinic. Sixty-six patients received enoxaparin and three patients received tinzaparin for a mean duration of 7.7 days postoperatively. Outcomes were assessed for 30 days post-procedure. Safety outcomes included major bleeding and minor bleeding. Efficacy outcomes included thromboembolic event or death.

RESULTS

There were two major bleeding events, one minor bleeding event, and no cases of thromboembolism. Twelve patients experienced some bruising around the injection site.

CONCLUSIONS

LMWH administration using our standard outpatient bridging protocol for perioperative anticoagulation appears to be relatively safe and efficacious, offering an alternative to inpatient administration of intravenous unfractionated heparin (UFH). Our study provides additional evidence to the limited published observational data regarding the safety and efficacy of LMWH as bridging therapy in the perioperative and periprocedural setting. Large, multicenter, randomized controlled trials are necessary to fully assess the safety and efficacy of LMWH for perioperative anticoagulation.We conducted a retrospective observational study of 69 consecutive anticoagulation clinic patients on warfarin between August 2001 and August 2002, who were undergoing a procedure or surgery. The study was done to assess the safety and efficacy of an outpatient LMWH bridging protocol. Sixty-six patients received enoxaparin and three patients received tinzaparin for a mean duration of 3 days preoperatively and 7.7 days postoperatively. Outcomes were assessed for 30 days post-procedure. Safety outcomes included major bleeding and minor bleeding. Efficacy outcomes included thromboembolic event or death. There were two major bleeding events, one minor bleeding event, and no cases of thromboembolism. Twelve patients experienced some bruising around the injection site.

No effect of encapsulation on the pharmacokinetics of warfarin

BACKGROUND

In double-blind comparator studies with the oral direct thrombin inhibitor (oral DTI) ximelagatran, warfarin (Coumadin) was administered in encapsulated form in order to maintain patient and investigator blinding. This open, randomized, two-way crossover study was conducted to determine whether the encapsulated warfarin tablets (Coumadin) used in the ximelagatran studies are bioequivalent to nonencapsulated, commercially available warfarin (Coumadin) tablets.

METHODS AND RESULTS

Eighteen healthy men received two 2.5 mg tablets of encapsulated warfarin and two 2.5 mg tablets of nonencapsulated warfarin as single oral doses, 14 days apart. The 90% confidence intervals for the mean treatment ratio (encapsulated tablet/nonencapsulated tablet) fell within the limits considered to reflect bioequivalence (0.80, 1.25) for total area under the plasma concentration-versus-time curve (AUC(infinity)), AUC to the last evaluable concentration (AUC(t)), and maximum plasma concentration (C(max)) for both R-warfarin (AUC(infinity) [0.93, 1.03], AUC(t) [0.95, 1.03], C(max) [0.90, 1.04]) and S-warfarin (AUC(infinity) [0.93, 1.03], AUC(t) [0.94, 1.03], C(max) [0.90, 1.06]).

CONCLUSIONS

The encapsulated form of warfarin (Coumadin) used in comparator studies with the oral DTI ximelagatran is bioequivalent to the nonencapsulated, commercially available form of warfarin (Coumadin). Thus, the results of ximelagatran clinical trials with encapsulated warfarin can be generalized to the commercially available form.

A feasibility study of continuing dose-reduced warfarin for invasive procedures in patients with high thromboembolic risk

BACKGROUND

The management of perioperative anticoagulation therapy for patients having a high risk of thromboembolism who are receiving long-term oral anticoagulant therapy is uncertain. The prevalent approach is to discontinue oral anticoagulation therapy and initiate heparin therapy. Another potential strategy is to continue oral anticoagulation therapy with a temporary adjustment of warfarin intensity to a preoperative international normalized ratio (INR) of 1.5 to 2.0. Such moderate-dose anticoagulation therapy with warfarin has been shown to be hemostatically safe yet effective in the prevention of thromboembolism after hip or knee replacement.

METHODS

Over an 11-year period (ie, 1993 to 2003), our hemostatic consultative service prospectively identified 100 consecutive patients for whom we continued warfarin therapy at adjusted doses during the perioperative period, targeting a goal for the INR of 1.5 to 2.0. Patients were assigned a score for venous thromboembolic risk as well as overall surgical risk using published instruments. Score assignment was based on what was deemed to be extremely high risk for thromboembolism in patients who were receiving long-term warfarin therapy. Although the patients were accrued prospectively, the final retrospective analysis was made after all patients were treated.

RESULTS

The most common indication (62%) for high-risk assignment was a thromboembolic event within the past 6 months. The second most prevalent reason was prior postoperative venous thromboembolism (VTE) [11%]. Indications for long-term anticoagulation therapy were recent VTE (62%), inherited thrombophilia (7%), antiphospholipid syndrome (13%), mechanical heart valves (18%), and prior cerebrovascular accident (4%). The prevalence of inherited thrombophilia probably has been grossly underestimated, as neither factor V Leiden mutation nor prothrombin 20210 mutation had been described during the bulk of the accrual time. Most surgical procedures (58%) were significantly invasive (Johns Hopkins category 3 to 5). The mean INR values were 2.1 on the day prior to surgery (SD, 0.9594; range, 1.2 to 6.5; n = 65), 1.8 on the day of surgery (SD, 0.4899; range, 1.2 to 4.9; n = 75), and 1.8 on the first postoperative day (SD, 0.4436; range, 1.1 to 3.3; n = 70). Two patients had major bleeding, and four patients had minor bleeding. One patient developed deep venous thrombosis. Several weeks after surgery, one patient with a prosthetic heart valve died from an embolic stroke, which was associated with a failure to increase his anticoagulation to therapeutic levels.

CONCLUSIONS

Moderate-intensity anticoagulant therapy with warfarin, targeting a goal INR of 1.5 to 2.0, appears to be a safe and feasible method for preventing thromboembolic complications in high-risk surgical patients who are receiving long-term oral anticoagulant therapy. This may be considered a reasonable method to afford thromboprophylaxis in highly selected patients who are occasionally seen in clinical practice. This observational study does not prove equality, let alone superiority, to other proposed methods of anticoagulation therapy.

The management of anticoagulants in the periendoscopic period for patients with atrial fibrillation: a decision analysis

PURPOSE

The management of patients who undergo endoscopy while being treated with warfarin is challenging. We used decision analysis to determine the preferred strategy to manage anticoagulants in the periendoscopic period.

METHODS

We designed a Markov model to estimate costs and quality-adjusted survival during a 10-year period in patients with nonvalvular atrial fibrillation undergoing screening colonoscopy. We compared six alternatives to the continue-warfarin strategy, which was to perform colonoscopy while the patient was taking full-dose warfarin. The hold-warfarin strategy was to stop warfarin 5 days before the colonoscopy. The repeat endoscopy strategy was to continue warfarin for a diagnostic colonoscopy, followed by a repeat procedure after cessation of warfarin if polypectomy was required. The dose-reduction strategy was to reduce the warfarin dose before colonoscopy. The low molecular weight heparin strategy was to administer subcutaneous low molecular weight heparin for 2 days before and 2 days after colonoscopy. The unfractionated heparin strategy was to administer intravenous unfractionated heparin for 2 days before and 2 days after the procedure. The vitamin K strategy was to hold warfarin for 4 days and to administer vitamin K if the international normalized ratio (INR) exceeded 2.0 the day before the procedure, or low molecular weight heparin if the INR was less than 1.5.

RESULTS

For screening colonoscopy, assuming that polyps would be removed in 35% of examinations, the hold-warfarin and dose-reduction arms were both cost-effective strategies. The hold-warfarin arm was most cost-effective if the likelihood of polypectomy exceeded 60%, or if there was a low risk of stroke despite atrial fibrillation. The continue-warfarin strategy was preferred if the probability of polypectomy was 1% or less.

CONCLUSION

Temporary warfarin cessation or halving the warfarin dose for several days before endoscopy was the preferred strategy for most patients. Periendoscopic heparin therapy was not cost-effective for patients with nonvalvular atrial fibrillation.

Avoidance of Bleeding During Surgery in Patients Receiving Anticoagulant and/or Antiplatelet Therapy

Perioperative management of chronically anticoagulated patients and/or patients treated with antiplatelet therapy is a complex medical problem. This review considers the pharmacokinetic and pharmacodynamic properties of commonly used antiplatelet and anticoagulant drugs with special emphasis on loss of effects after discontinuation and possible counteracting (or antidote) strategies. These drugs are aspirin (acetylsalicylic acid), ticlopidine/clopidogrel, abciximab, tirofiban and eptifibatide, heparin (unfractionated and low-molecular-weight), warfarin and direct thrombin inhibitors. Since the pharmacological mechanisms of some of these drugs are based on irreversible or slowly reversible effects, their pharmacokinetic profiles are not necessarily predictive for their pharmacodynamic profiles. A close and direct relationship between plasma concentrations and effects is seen only for the glycoprotein (GP) IIb/IIIa inhibitors tirofiban and eptifibatide with a fast off-rate for dissociation from the GPIIb/IIIa receptor, and for direct thrombin inhibitors (hirudin and argatroban). For other compounds, drug concentrations in plasma and pharmacodynamic effects are not closely correlated because of, for example, irreversible binding to their target (aspirin, clopidogrel and abciximab), inhibition of the generation of a subset of clotting factors with differing regeneration and degradation rates (coumarins) or sustained binding to the vascular wall (heparins). Surgery in patients on anticoagulant and/or antiplatelet therapy may be categorised as: (i) elective versus urgent; and (ii) cardiopulmonary bypass (CPB) versus non-CPB. Monotherapy with clopidogrel or aspirin need not be discontinued in elective non-CPB surgery, and temporary discontinuation of warfarin should be accompanied by preoperative intravenous heparin only in selected high-risk patients. Vitamin K as an antidote for warfarin should only be used subcutaneously and solely in urgent/emergency surgery. In elective surgery requiring CPB (coronary artery bypass grafting), it is recommended to discontinue aspirin 7 days preoperatively in patients with a low risk profile. Patients requiring urgent CPB surgery (e.g. after failure of a percutaneous coronary angioplasty with or without coronary stent deployment) are usually pretreated with several antiplatelet agents (e.g. aspirin and clopidogrel, together with a GPIIb/IIIa inhibitor) together with unfractionated or low-molecular-weight heparin. With judicious planning, urgent/emergency cardiac surgery can be safely performed on these patients. Delaying surgery (e.g. for 12 hours in patients treated with abciximab) should be considered if possible. Standard heparin doses should be given to achieve optimal anticoagulation for CPB. Prophylactic use of aprotinin (intra- and/or postoperatively), aminocaproic acid or tranexamic acid should be considered. Early (in the operating theatre prior to chest closure) and judicious use of replacement blood products (platelets) should be commenced when clinically indicated.

The use of vitamin K in patients on anticoagulant therapy: a practical guide

Anticoagulation with antivitamin K (AVK) is very effective for primary and secondary prevention of thromboembolic events. However, questions persist about the risks and management of over-anticoagulation. For reversal of excessive anticoagulation by warfarin, AVK withdrawal, oral or parenteral vitamin K administration, prothrombin complex or fresh frozen plasma may be used, depending on the excess of anticoagulation, the existence and site of active bleeding, patient characteristics and the indication for AVK. In over-anticoagulated patients, vitamin K aims at rapid lowering of the international normalized ratio (INR) into a safe range to reduce the risk of major bleeding and therefore improving patient outcome without exposing the patient to the risk of thromboembolism due to overcorrection, resistance to AVK, or an allergic reaction to the medication. The risk of bleeding increases dramatically when the INR exceeds 4.0-6.0, although the absolute risk of bleeding remains fairly low, <5.5 per 1000 per day. Patient characteristics, including advanced age, treated hypertension, history of stroke, and concomitant use of various drugs, affect the risk of bleeding. The absolute risk of thromboembolism associated with overcorrection appears to be in the same range as the risk of bleeding due to over-anticoagulation. The use of vitamin K in patients with warfarin over-anticoagulation lowers excessively elevated INR faster than withholding warfarin alone; however, it has not been clearly demonstrated that vitamin K treatment does, in fact, lower the risk of major hemorrhage. As vitamin K administration via the intravenous route may be complicated by anaphylactoid reactions, and via the subcutaneous route by cutaneous reactions, oral administration is preferred. A dose of 1-2.5mg of oral phytomenadione (vitamin K(1)), reduces the range of INR from 5.0-9.0 to 2.0-5.0 within 24-48 hours, and for an INR >10.0, a dose of 5mg may be more appropriate. Overcorrection of the INR or resistance to warfarin is unlikely if the above doses of vitamin K are used. Vitamin K is less effective for over-anticoagulation after treatment with acenocoumarol or phenprocoumon than after treatment with warfarin.

Comparative pharmacokinetics of coumarin anticoagulants L: Physiologic modeling of S-warfarin in rats and pharmacologic target-mediated warfarin disposition in man

The anticoagulant warfarin exemplifies a type of drug that exhibits high affinity to pharmacologic target sites of limited capacity, resulting in unusual concentration-dependent distribution and elimination properties. The time course of warfarin concentrations in the serum, liver, kidneys, muscle, and abdominal fat of male Sprague-Dawley rats was determined by high-performance liquid chromatography after IV injection of a 0.25 or 1.0 mg/kg dose. The rats were preclassified on the basis of their serum free fraction of warfarin; animals with free fraction values of approximately 0.004 and 0.01 and corresponding differences in elimination half-life were selected for study, yielding four experimental groups. Several rats of each group were sacrificed periodically over approximately 80-240 h for determination of drug concentrations. S-warfarin concentrations in serum declined apparently exponentially over at least one order of magnitude. During this time, concentrations in all other assayed tissues declined much more slowly. In another experiment, S-warfarin concentrations in serum and liver were followed for approximately 50 days after IV injection of a 1 mg/kg dose. This revealed a terminal, very slow elimination phase in serum nearly parallel to the decline in liver drug concentrations. Simultaneous physiologic modeling of all data (30 equations) using ADAPT II (Biomedical Simulations Resource, Los Angeles, CA), with intrinsic clearance, the dissociation constant of the warfarin-high affinity binding site complex, and two binding parameters for the (unassayed) remainder tissue compartment as parameters of unknown value, yielded very good fittings and parameter estimates with relatively small standard deviations. The unusual dose-dependent accumulation characteristics of this type of drug during continuous infusion are demonstrated by computer simulation of published results of warfarin infusions in rats. Utilization of a model premised on similar target-mediated drug disposition also allowed characterization of data from the literature for racemic warfarin pharmacokinetics in man.

Thrombotic disorders: diagnosis and treatment

Hematologists are increasingly involved in the diagnosis and management of patients with venous and arterial thromboembolic disorders. There have been major advances in recent years in our understanding of the central role of hypercoagulability in the pathogenesis of thrombosis. This has led to new approaches to the diagnosis of patients at risk for thrombosis and the development of more rational antithrombotic strategies. In Section I, Dr. Andrew Schafer reviews current concepts of acquired and inherited hypercoagulable states. It is now recognized that most, if not all, patients with venous thromboembolism have a genetic basis for the disorder ("thrombophilia"). The level of lifelong, baseline hypercoagulability in any individual may be determined by the type(s) and number of thrombophilia(s) that are inherited. Clinical episodes of thrombosis are precipitated by acquired thrombogenic triggers, which may be overt (e.g., pregnancy) or subclinical. In Section II, Dr. Mark Levine discusses the complex problem of thrombosis in patients with cancer. The goals of treating acute venous thromboembolism in cancer patients are to prevent recurrence, minimize the risk of anticoagulant-induced bleeding, and improve quality of life. New developments have improved treatment of venous thromboembolism in these patients, including outpatient therapy and secondary prevention with low-molecular-weight heparin. In Section III, Dr. Barbara Konkle reviews the diagnosis and management of thrombotic complications associated with pregnancy and hormonal therapy. Patient management is discussed based on data on thrombotic risks associated with hormonal treatment of infertility, pregnancy and the post-partum period in women with and without underlying thrombophilic risk factors. In Section IV, Dr. Clive Kearon discusses the management of anticoagulation before and after elective surgery. In the past, there has been no consensus on the perioperative management of anticoagulation for patients who require long-term warfarin therapy. This review considers the expected risks and benefits of different approaches to anticoagulation in patients who require warfarin because of atrial fibrillation, a mechanical heart valve, or a history of venous thromboembolism.

Interaction of ochratoxin A with human serum albumin. Binding sites localized by competitive interactions with the native protein and its recombinant fragments

Competitive interactions of ochratoxin A (OTA) and several other acidic compounds were utilized to gain insight into the localization of binding sites and the nature of binding interactions between anionic species and human serum albumin (HSA). Depolarization of OTA fluorescence in the presence of a competing anion was used to quantify ligand-protein interactions. The results obtained were rationalized in terms of OTA displacement from its major binding site. Based on their ability to displace OTA, two distinct groups of the anionic ligands were revealed. The first group contained structurally diverse compounds that shared a common binding site in subdomain IIA (Sudlow Site I). The second group consisted of three non-steroidal anti-inflammatory drugs, which showed much lower affinity to Site I than the OTA dianion. The major site for these drugs was located in domain III. Fluorescence spectroscopy measurements of OTA, warfarin (WAR) and naproxen (NAP) complexes with recombinant proteins corresponding to the domains of HSA (D1-D3) revealed binding to all domains but with different affinities. The binding constants for OTA and WAR decreased in the series D2z.Gt;D3>D1. In contrast, NAP showed the most favorable interaction with D3 and comparable affinities to the two remaining domains. The OTA binding constant for D2, 7.9 x 10(5) M(-1), was smaller than the largest constant for HSA by a factor of approximately 7. The binding constant for OTA with D3, 1.1 x 10(5) M(-1), was very close to that of the secondary binding site for HSA.

Perioperative anticoagulation management in patients who are receiving oral anticoagulant therapy: a practical guide for clinicians

The management of patients who require temporary interruption of oral anticoagulant therapy because of surgery or other invasive procedures is a clinically important topic because of the increasing prevalence of patients who are receiving oral anticoagulants and the availability of low-molecular-weight heparins (LMWHs), which allow out-of-hospital perioperative anticoagulation. The optimal management of such patients has been hampered by the lack of well-designed prospective studies investigating the efficacy and safety of different perioperative management strategies. The two main issues that need to be considered in perioperative anticoagulant management is the patient's risk of thromboembolic event when anticoagulant therapy is interrupted and the risk of bleeding that is associated with the surgery or procedure. An assessment of these factors will determine the perioperative management approach. The objectives of this review are to focus on practical issues relating to perioperative anticoagulation and the implementation of a perioperative anticoagulation management approach that can be used in everyday clinical practice.

Oral surgery in patients on anticoagulant therapy

OBJECTIVE

Surgery is the main oral healthcare hazard to the patient with a bleeding tendency, which is mostly caused by the use of anticoagulants. The traditional management entails the interruption of anticoagulant therapy for dental surgery to prevent hemorrhage. However, this practice may increase the risk of a potentially life-threatening thromboembolism. Because this issue is still controversial, it is the aim of this paper to review the evidence, to highlight the areas of major concern, and to suggest management regimens for patients on the 3 main types of anticoagulants: coumarins, heparins, and aspirin. MATERIALS REVIEWED: The pertinent literature and clinical protocols of hospital dentistry departments have been extensively reviewed and discussed.

RESULTS

Several evolving clinical practices in the last years have been detected: anticoagulant use is generally not discontinued; oral surgery is performed despite laboratory values showing significant bleeding tendency; new effective local methods are used to prevent bleeding; and patients at risk are referred to hospital-based clinics.

CONCLUSION

The management of oral surgery procedures on patients treated with anticoagulants should be influenced by several factors: extent and urgency of surgery, laboratory values, treating physician's recommendation, available facilities, dentist expertise, and patient's oral, medical, and general condition.

Substitution of generic warfarin for Coumadin in an HMO setting

BACKGROUND

Substitution of generic warfarin for Coumadin presents safety concerns due to warfarin's narrow therapeutic index and because a prior generic formulation was removed from the US market after it was associated with adverse events.

OBJECTIVE

To determine whether a health maintenance organization (HMO) can add generic warfarin to its formulary without adversely affecting warfarin management or increasing adverse events.

DESIGN

In a prospective, observational study, an HMO that formerly dispensed only Coumadin added a generic warfarin preparation (Barr Laboratories, Pomona, NY) to its formulary.

SETTING

An anticoagulation service (ACS) affiliated with an HMO that was based in St. Louis, MO.

PARTICIPANTS

The cohort consisted of 182 enrollees in the ACS as of May 1, 1999. At the start of the study, these participants were taking Coumadin; by October 31, 2000, all had switched to Barr warfarin.

MEASUREMENTS AND MAIN RESULTS

We collected data 8 months prior to and 10 months after the introduction of generic warfarin for the following endpoints: international normalized ratio (INR) control, frequency of INR monitoring, number of dose changes, and rate of thrombotic and hemorrhagic events. Statistical process control charts were used to differentiate between random variation in the endpoints and changes due to different warfarin formulations, and we used the Wilcoxon signed-rank test to look for a change in any endpoint after patients changed to generic warfarin. No significant differences were found in any endpoint.

CONCLUSIONS

Substitution of Barr warfarin for Coumadin did not significantly affect INR control, warfarin management, or adverse events. Our findings suggest that HMOs can safely substitute at least 1 generic formulation of warfarin without extra monitoring.

The management of anticoagulation before and after procedures

Patients maintained on warfarin for atrial fibrillation, mechanical heart valves, or deep venous thrombosis may occasionally need to stop their anticoagulation during invasive procedures. This article reviews the literature on bleeding risks of certain procedures, thrombosis risks of stopping anticoagulation, and heparin and warfarin pharmacokinetics. Recommendations regarding how to manage anticoagulated patients are discussed.

Extreme warfarin sensitivity in siblings associated with multiple cytochrome P450 polymorphisms

Warfarin use is complicated by an erratic dose response. Warfarin is metabolized by two distinct subfamilies of the cytochrome P450 (CYP) complex. We describe two siblings with extreme sensitivity to warfarin who share an unusual CYP genotype. These individuals illustrate both the importance of genetics in influencing the metabolism of warfarin as well as the potential utility of genetic testing as a guide to prescribing this medication.

Perioperative management and reversal of antithrombotic therapy

Patients of advanced age commonly undergo invasive procedures and surgery. With the number of elderly individuals being treated with long-term anticoagulant therapy growing annually, it is not uncommon that surgery is contemplated for older adults on long-term anticoagulant therapy. This article focuses on the management of elderly patients who are on long-term anticoagulant therapy, principally with warfarin, who must undergo invasive procedures. Although no consensus has been reached regarding the perioperative management of patients on long-term anticoagulation therapy, this discussion presents the current status and some recommendations for practice.

Therapeutic monitoring of warfarin: the appropriate response marker

Warfarin is a 4-hydroxycoumarin anticoagulant drug used for the prevention and management of thromboembolic and vascular diseases. It acts through the inhibition of the vitamin K-dependent transcarboxylation reactions that convert precursors of clotting factors into their active form. Appropriate use of warfarin requires patient monitoring and dosage adjustments, to ensure its safety and efficacy. The aim of this work was to clarify the relationship between traditional (prothrombin time, usually expressed as the international normalized ratio; INR) and alternative (clotting factors II and X) warfarin response markers to establish their usefulness for therapeutic drug monitoring. Seventy adult outpatients, aged between 31 and 86 years old, were involved in the study. All subjects received warfarin in a monotherapy regimen and had been on a stable dosing schedule for at least two weeks to assure a steady-state condition. A total of 81 prothrombin times (expressed as INR), and factor II and factor X activity were simultaneously determined. Eleven patients presented repeated measurements at different time periods under the same dosing regimen. The results obtained from regression and cluster analysis showed a close relationship between factors II and X (r = 0.73), a weak correlation between INR and both factor II (r = -0.35) and factor X (r = -0.36), and a very slight dependency between warfarin and the response markers used. In addition, it seems that independent of the selected response marker, in long-term warfarin therapy, reproducible responses can be obtained over time if a steady-state condition is achieved. The coefficients of variation for factors II and X were greater (35.44 and 37.93%, respectively) than INR (14.50%), indicating that INR is a more precise measure than either factor II or factor X. In conclusion, INR appears to be the most appropriate warfarin response marker for therapeutic drug monitoring due to its universality, objectivity as a direct physiological effect measurement, and the available information regarding appropriate endpoints. However, when INR values are not in accordance with patient response therapy, factor II and factor X should be considered as an alternative to optimize warfarin therapy.

Postoperative stroke after stopping warfarin for cutaneous surgery

BACKGROUND

Two patients undergoing cutaneous surgery had thromboembolic strokes within 1 week after surgery. Both patients had been taking warfarin for prevention of thromboembolism and warfarin was stopped 3-7 days prior to surgery.

OBJECTIVE

To examine the rationale and problems associated with preoperative warfarin discontinuation.

METHODS

Review of the medical literature.

RESULTS

When warfarin is stopped prior to surgery and restarted soon after surgery, the patient is at increased risk for thromboembolism. Although it is commonly believed that continuing warfarin during surgery is associated with an increased bleeding risk, for cutaneous surgery, this risk is extremely low and can be easily managed.

CONCLUSION

Warfarin should not be discontinued prior to cutaneous surgery because of the risk of thromboembolic stroke.

Open questions on bioequivalence: some problems and some solutions

This paper focuses on some specific situations where bioequivalence requires careful attention and tailored protocols in order to overcome intrinsic difficulties either marginally covered or fully neglected by operating guidelines. Some problems congregate with serious difficulties, namely high variability, very poorly absorbed drugs and endogenous substances with their own baseline. With endogenous substances, the dilemma faced is whether to subtract baseline from post-dose values in assessing bioequivalence. Either approach has intrinsic problems and is somewhat puzzling. In an attempt to resolve other existing problems, the most appropriate approach should be selected on a case-by-case basis, ensuring that the adopted procedure does not conflict with operating guidelines and scientific literature on the matter. Problematic cases include the management of trials with a predominant active metabolite, the absence of a reliable analytical bioassay, the availability of various strengths of the same drug on the market, a wide acceptability titre range, the management of studies on topical drugs that are devoid of systemic activity, the management of drugs that cannot be given for ethical reasons to healthy subjects or that may cause adverse events, especially when a steady state design is required. The parallel group study design appears to be more appropriate than the cross-over or the individual bioequivalence design in assessing drugs with a long half-life. Some pharmacokinetic and statistical analysis-related issues are also discussed such as the sequence/period interaction sometimes encountered in these trials, which, in the absence of the carry-over effect, does not bias the bioequivalence results and the need to process data with non-compartmental pharmacokinetic analysis.