Increase in International Normalized Ratio After Smoking Cessation in a Patient Receiving Warfarin

Subtherapeutic INR due to warfarin interaction with smokeless tobacco

We report an interesting case of warfarin resistance and subtherapeutic international normalized ratio (INR) in the setting of chronic smokeless tobacco use. A 33-year-old white male with a mechanical mitral valve failed to achieve therapeutic INR despite being on warfarin doses of up to 30 mg by mouth daily. The patient admits to chewing tobacco daily for the past 16 years. While evaluating potential causes of subtherapeutic INR, nonadherence, drug interactions, genetic polymorphisms, and dietary factors were considered. Subtherapeutic INR may be due to increased exposure to vitamin K from chewing tobacco. Tobacco is rich in vitamin K, and its chronic use may have caused the failure to attain a therapeutic INR. To our knowledge, there is only one other case of warfarin resistance resulting from smokeless tobacco described in published literature.

An evaluation of the potential drug interaction between warfarin and levothyroxine

BACKGROUND

Drug interaction references report that initiation of levothyroxine potentiates the effects of warfarin, and recommend more frequent International Normalized Ratio (INR) monitoring, but the mechanism is not well understood.

OBJECTIVE

To assess the impact of levothyroxine initiation on INR response.

PATIENTS/METHODS

A retrospective, self-controlled study was performed on patients aged ≥ 18 years receiving chronic warfarin therapy who were started on levothyroxine between 1 January 2006 and 30 June 2013, and who were followed for 90 days prior to and after levothyroxine initiation. The included patients had at least one elevated thyroid-stimulating hormone laboratory value in the pre-period, continuous warfarin therapy for 100 days prior to levothyroxine initiation, no purchases of medications known to interact with warfarin, no procedures requiring warfarin interruption, and no bleeding or thromboembolic event during the study period. The primary outcome was a comparison of the warfarin dose/INR ratio recorded before the initiation of levothyroxine with the ratio recorded during the post-period after two consecutive INRs with no warfarin dose change.

RESULTS

One hundred and two patients were included in the primary outcome. The mean warfarin dose/INR ratios in the pre-period and post-period were equivalent (P = 0.825). Although the mean warfarin dose was numerically lower in the post-period than in the pre-period, this difference did not reach statistical significance (P = 0.068).

CONCLUSION

No difference in the mean warfarin dose/INR ratio before and after initiation of levothyroxine was detected. The results suggest that there is not a clinically significant interaction between warfarin and levothyroxine, and so additional monitoring may not be necessary.

Therapeutic drug monitoring in a developing nation: a clinical guide

Therapeutic drug monitoring is aimed at using drug concentration measurements to manage a patient's medication requirement and optimise clinical outcome, particularly in respect of drugs with narrow therapeutic index. Typically, immunoassay methods of various techniques are employed with the advantage of rapid turnaround time and ease of operation. The chromatographic methods are specific and cost effective, though more demanding and require technical expertise. The most crucial aspect of any therapeutic drug monitoring service is the expert clinical interpretation of drug concentration measurements taking into consideration individual pharmacokinetic variability in drug disposition across different populations. The setting up of a therapeutic drug monitoring service requires enormous resources, both in terms of equipment and trained personnel. This poses considerable constraints in developing countries due to limited scarce resources, coupled with ignorance among health practitioners on the relevance of therapeutic drug monitoring in clinical practice. Consequently, the need for advocacy, training and encouragement of health practitioners on the usefulness of therapeutic drug monitoring in enhancing patient care and overall clinical outcome in a developing country such as Nigeria can never be over-emphasised.

CYP2C9, VKORC1, CYP4F2, ABCB1 and F5 variants: influence on quality of long-term anticoagulation

AIMS

The study aims to evaluate the impact of genetic, demographic and clinical data on various measures of outcome of anticoagulation quality in patients.

PATIENTS AND METHODS

The study consisted of 310 patients receiving long-term oral anticoagulation therapy in our hospital. Apart from demographic and clinical variables, 21 SNPs (in 7 genes) were analyzed and compared with the outcomes of anticoagulation therapy. Various outcomes that were measured are; supra therapeutic INRs (INR >3, >6), anticoagulation stabilization, time taken to stabilize and proportion of INRs within (2-3), above (>3) and below (<2) therapeutic range.

RESULTS

Supra therapeutic INRs were influenced by CYP2C9*2, *3, CYP4F2 rs2108622, VKORC1-1639G>A, 1173C>T, rs55894764 along with concomitant drugs, smoking, body weight and height. Persistently fluctuating INRs/absolute instability correlated with VKORC1-1639G>A, gender, height and body mass index. The time taken to stabilize was associated with CYP4F2 rs2108622, CYP2C9*14, smoking, clinical indication and concomitant drugs. The overall distribution of INR was influenced by variants in CYP4F2 rs2108622, CYP2C9*3, rs9332230, VKORC1 1173C>T, -1639G>A, rs55894764, ABCB1 rs2032582, rs1128503, rs1045642 and F5 rs6025, age, smoking and concomitant drugs.

CONCLUSIONS

Knowledge of factors influencing the quality of long term anticoagulation can help clinicians to customize therapy either by dose variation, therapy with alternate choice of drug, concurrent heparin therapy and/or frequent INR monitoring.

Patient Case

Introduction: The concurrent use of cigarettes while on warfarin therapy is a common occurrence. Smoking cessation among patients on chronic warfarin therapy is suspected to reduce drug clearance that may require dose adjustments. This type of interaction is particularly important when dealing with narrow therapeutic medications, as is the case with warfarin. Our case describes a series of supratherapeutic international normalized ratios (INRs) due to smoking cessation while on concomitant warfarin therapy. Patient Case: A 51-year-old male presented to the anticoagulation clinic for management of his warfarin therapy for an acute deep vein thrombosis. After 2 months of stable, therapeutic INR levels, the patient abruptly decreased his smoking from 1 pack/day to one-half pack/day and then subsequently stopped smoking completely. The patient’s smoking cessation resulted in a major modification of his required weekly warfarin dose to maintain a therapeutic INR (a 39% dose reduction). Conclusion: This case exemplifies how certain lifestyle factors, such as smoking, can alter the pharmacokinetics of patients on chronic warfarin therapy. This is the first case to demonstrate a greater than 30% reduction in the weekly warfarin dose following smoking cessation.

Oral anticoagulant therapy: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines

BACKGROUND

The objective of this article is to summarize the published literature concerning the pharmacokinetics and pharmacodynamics of oral anticoagulant drugs that are currently available for clinical use and other aspects related to their management.

METHODS

We carried out a standard review of published articles focusing on the laboratory and clinical characteristics of the vitamin K antagonists; the direct thrombin inhibitor, dabigatran etexilate; and the direct factor Xa inhibitor, rivaroxaban

RESULTS

The antithrombotic effect of each oral anticoagulant drug, the interactions, and the monitoring of anticoagulation intensity are described in detail and discussed without providing specific recommendations. Moreover, we describe and discuss the clinical applications and optimal dosages of oral anticoagulant therapies, practical issues related to their initiation and monitoring, adverse events such as bleeding and other potential side effects, and available strategies for reversal.

CONCLUSIONS

There is a large amount of evidence on laboratory and clinical characteristics of vitamin K antagonists. A growing body of evidence is becoming available on the first new oral anticoagulant drugs available for clinical use, dabigatran and rivaroxaban.

Assessing evidence of interaction between smoking and warfarin: a systematic review and meta-analysis

BACKGROUND

Chronic smoking, theoretically, can interfere with warfarin metabolism through enzyme-inducing effects of polycyclic aromatic hydrocarbons. However, clinical evidence of interactions between warfarin and smoking are inconclusive. This study aimed to systematically review all relevant clinical evidence of this interaction.

METHODS

We performed a systematic search using computerized databases, including PubMed, Embase, Cochrane Central Register of Controlled Trials, CINAHL, Allied and Complementary Medicine, PsycINFO, International Pharmaceutical Abstracts, and ClinicalTrials.gov from 1966 to December 2008. Keywords included "warfarin" with "smoking," "tobacco," "cigarette," and "polycyclic aromatic hydrocarbons." Original articles reporting interaction between warfarin and smoking were included. All articles were reviewed independently by two investigators for study design, population, outcomes, and quality of evidence.

RESULTS

Of the 1,240 studies retrieved, one experimental pharmacokinetic study and 12 cross-sectional studies were included. The pooled analyses of multivariate studies suggested that smoking was associated with a 12.13% (95% CI, 6.999-17.265; P < .001) increase in warfarin dosage requirement and an additional 2.26 mg (95% CI, 2.529-7.042; P = .355) per week compared with nonsmoking. Additional sensitivity analysis of four multivariate studies with adjustment for pharmacogenomic factors suggested that smoking was associated with a 13.21% (95% CI, 8.59%-17.83%; P < .001) increase in warfarin dosage requirement compared with nonsmokers. Results of an experimental pharmacokinetic study lend theoretical support to the findings.

CONCLUSIONS

Evidence suggests that smoking may potentially cause significant interaction with warfarin by increasing warfarin clearance, which leads to reduced warfarin effects. Close monitoring of warfarin therapy should be instituted when there is a change in smoking status of patients requiring warfarin therapy.

A review of smoking cessation: potentially risky effects on prescribed medications

AIMS AND OBJECTIVES

To identify prescription drugs that require dosage adjustment or monitoring in patients who quit smoking and to provide recommendations for dosage adjustment based on available evidence.

BACKGROUND

Health care providers are urged to facilitate smoking cessation for patients who smoke, but the effects of smoking cessation on the metabolism of some drugs is not routinely considered.

DESIGN

A comprehensive literature review.

METHODS

The review was conducted in 2008 using a computerised drug interaction program and multiple PubMed and CINAHL searches to identify prescription drugs with clinically significant pharmacokinetic or pharmacodynamic changes caused by smoking cessation.

RESULTS

Although much of the evidence is case report, dosage adjustments are clearly indicated for warfarin, olanzapine, clozapine and theophylline since they are metabolised by cytochrome P450 CYP1A2 and also have narrow therapeutic ratios. Careful monitoring is recommended for other CYP1A2 metabolised drugs, including those for hypertension and Alzheimer's disease. For many affected drugs, smoking cessation reverses smoking-induced CYP1A2 hepatic enzyme levels to normal, increasing plasma concentrations in patients whose dose was established while smoking. Because the effect on hepatic microsomal enzymes is not related to the nicotine component of tobacco, nicotine replacement will not alter the effect.

CONCLUSIONS

The effects of smoking cessation on drugs metabolised by CYP1A2 have been under-appreciated by health care providers. Smoking cessation may increase plasma levels of some drugs to potentially toxic levels. Further research is warranted to clarify this effect.

RELEVANCE TO CLINICAL PRACTICE

When patients stop smoking, providers should carefully review prescribed drug regimens and adjust or monitor drugs whose metabolism is affected by smoking cessation. This is particularly important for patients who abruptly stop smoking due to hospitalisation and for older patients who are likely to be taking multiple medications.

Probable interaction between warfarin and marijuana smoking

OBJECTIVE

To report a probable interaction between warfarin and marijuana smoking, resulting in increased international normalized ratio (INR) values and bleeding complications.

CASE SUMMARY

A 56-year-old white male had been receiving chronic warfarin therapy for 11 years after mechanical heart valve replacement. He was admitted to the hospital with a diagnosis of upper gastrointestinal bleeding. Upon admission, his INR value was supratherapeutic at 10.41, and his hemoglobin level was 6.6 g/dL. He received 4 units of fresh frozen plasma and one 10-mg dose of oral vitamin K; his INR was 1.8 the next day. He was discharged 7 days after admission. Fifteen days after hospital discharge, he was readmitted with a constant nosebleed and increased bruising. His INR value was 11.55. After treatment, he was discharged with an INR value of 1.14. The patient smoked marijuana more frequently throughout the period of these 2 hospitalizations due to his depression. He was counseled by the pharmacist on the potential interaction of warfarin and marijuana. The patient decided to stop smoking marijuana after the third counseling session. During the 9 months that he did not smoke marijuana, his INR values ranged from 1.08 to 4.40 with no significant bleeding complications.

DISCUSSION

Marijuana may increase warfarin anticoagulant effect by inhibiting its metabolism, and to a lesser extent, displacing warfarin from protein-binding sites. Other causes (eg, nonadherence) of the patient's increased INR were ruled out. Using the Horn Drug Interaction Probability Scale, our patient's warfarinmarijuana interaction appeared to be probable.

CONCLUSIONS

To our knowledge, there have been no other reported cases of warfarin-marijuana interaction. While more clinical reports would be useful to confirm this interaction, clinicians should be aware of its probability so as to manage patients appropriately.