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Pharmacodynamic optimization of warfarin therapy

Doi SA

Clin. Pharmacol. Ther., 1994 May;55(5):597-601.
PMID: 8181204

Pharmacokinetic-pharmacodynamic information regarding warfarin is used to produce a predictive model based on the idea that pharmacodynamic variability is more important than pharmacokinetic variability in the overall dose-response variability to warfarin. A modification of the maximum effect model is tested on a group of patients initiating oral anticoagulation with warfarin. Results indicate that the model can account for at least half of the total variation in maintenance doses observed (sample coefficient of determination, 0.53) and offer the physician a framework for dose requirements at the onset of therapy. The basic prediction equation is as follows: Maintenance dose = (11/international normalized ratio)-1, with a coefficient of correlation of 0.73 (95% confidence limits, 0.46-0.88). Application of this model may improve on the traditional empiric approach to warfarin dose adjustment.

Matched MeSH terms: Warfarin/pharmacokinetics
The complexity of achieving anticoagulation control in the face of warfarin-phenytoin interaction: an Asian case report

Hassan Y, Awaisu A, Aziz NA, Ismail O

Pharm World Sci, 2005 Feb;27(1):16-9.
PMID: 15861930

Phenytoin has been reported to have major interactions with warfarin. Phenytoin induces warfarin's metabolism. However, there are many case reports which provide conflicting conclusions. Here, we report a case of a 65-year-old man with mechanical heart valve on chronic warfarin therapy who experienced persistent fluctuations of INR and bleeding secondary to probable warfarin-phenytoin interactions. The patient's anticoagulation clinic visits prior to hospitalization were thoroughly evaluated and we continued to follow-up the case for 3 months post-hospitalization. The reported interaction could be reasonably explained from the chronology of events and the pattern of INR fluctuations whenever phenytoin was either added or discontinued from his drug regimen.

Matched MeSH terms: Warfarin/pharmacokinetics*
Pharmacometabonomics Technique to Identify Warfarin Response Using Nuclear Magnetic Resonance Spectroscopy

Bawadikji AA, Teh CH, Kader MABSA, Sulaiman SAS, Ibrahim B

Curr Pharm Biotechnol, 2017;18(9):740-747.
PMID: 29110602 DOI: 10.2174/1389201018666171103141828

BACKGROUND: Warfarin, an anticoagulant medication, is prescribed regularly despite of its bleeding tendency for the prevention and/or treatment of various thromboembolic conditions, such as deep vein thrombosis, and complications associated with atrial fibrillation, and myocardial infarction, but because of its narrow therapeutic window, it has a lot of interactions with drugs and diet.
METHODS: Warfarin relies on regular monitoring of International Normalized Ratio which is a standardized test to measure prothrombin time and appropriate dose adjustment. Pharmacometabonomics is a novel scientific field which deals with identification and quantification of the metabolites present in the metabolome using spectroscopic techniques such as Nuclear Magnetic Resonance (NMR). Pharmacometabonomics helps to indicate perturbation in the levels of metabolites in the cells and tissues due to drug or ingestion of any substance. NMR is one of the most widely-used spectroscopic techniques in metabolomics because of its reproducibility and speed.
RESULTS: There are many factors that influence the metabolism of warfarin, making changes in drug dosage common, and clinical factors like drug-drug interactions, dietary interactions and age explain for the most part the variability in warfarin dosing. Some studies have showed that pharmacogenetic testing for warfarin dosing does not improve health outcomes, and around 26% of the variation in warfarin dose requirements remains unexplained yet.
CONCLUSION: Many recent pharmacometabonomics studies have been conducted to identify novel biomarkers of drug therapies such as paracetamol, aspirin and simvastatin. Thus, a technique such as NMR based pharmacometabonomics to find novel biomarkers in plasma and urine might be useful to predict warfarin outcome.

Matched MeSH terms: Warfarin/pharmacokinetics
Clinical relevance of VKORC1 (G-1639A and C1173T) and CYP2C9*3 among patients on warfarin

Teh LK, Langmia IM, Fazleen Haslinda MH, Ngow HA, Roziah MJ, Harun R, et al.

J Clin Pharm Ther, 2012 Apr;37(2):232-6.
PMID: 21507031 DOI: 10.1111/j.1365-2710.2011.01262.x

Testing for cytochrome P450-2C9 (CYP2C9) and vitamin K epoxide reductase complex subunit 1 (VKORC1) variant alleles is recommended by the FDA for dosing of warfarin. However, dose prediction models derived from data obtained in one population may not be applicable to another. We therefore studied the impact of genetic polymorphisms of CYP2C9 and VKORC1 on warfarin dose requirement in Malaysia.

Matched MeSH terms: Warfarin/pharmacokinetics
Interethnic variability of warfarin maintenance requirement is explained by VKORC1 genotype in an Asian population

Lee SC, Ng SS, Oldenburg J, Chong PY, Rost S, Guo JY, et al.

Clin. Pharmacol. Ther., 2006 Mar;79(3):197-205.
PMID: 16513444

Chinese and Malay subjects have been reported to require less maintenance warfarin than Indians that could not be accounted for by cytochrome P450 (CYP) 2C9 variants. Vitamin K epoxide reductase complex 1 (VKORC1) is the target enzyme of warfarin, and VKORC1 intronic variants and haplotypes have recently been shown to influence VKORC1 activity and warfarin requirements.

Matched MeSH terms: Warfarin/pharmacokinetics*
Comparing effectiveness of two anticoagulation management models in a Malaysian tertiary hospital

Thanimalai S, Shafie AA, Hassali MA, Sinnadurai J

Int J Clin Pharm, 2013 Oct;35(5):736-43.
PMID: 23715759 DOI: 10.1007/s11096-013-9796-6

BACKGROUNDS: Limited evidence is available regarding pharmacist managed anticoagulation clinic in the Southeast Asian region where there is marked difference in terms of care model, genetic composition and patient demographics.

OBJECTIVES: This study aimed at comparing the anticoagulation clinic managed by the pharmacist with physician advisory and the usual medical care provided in Kuala Lumpur Hospital (KLH) in terms of anticoagulation control and adverse outcomes.

SETTING: A 2,302 bedded government tertiary referral hospital in Malaysia.

METHODS: A 6-month retrospective cohort study of the effectiveness of two models of anticoagulation care, the pharmacist managed anticoagulation clinic which is known as warfarin medication therapy adherence clinic (WMTAC) and usual medical clinic (UMC) in KLH was conducted, where a random number generator was used to recruit patients. The UMC patients received standard medical care where they are managed by rotational medical officers in the physicians' clinic. As for the WMTAC with physician advisory, the pharmacist will counsel and review the patients internationalised normalization ratio at each clinic visit and also adjust the patients' warfarin dose accordingly. Patients are referred to physicians if immediate attention is required.

MAIN OUTCOME MEASURE: The main therapeutic outcome is time in therapeutic range (TTR) both actual and expanded TTR and thromboembolic and bleeding complications.

RESULTS: Each of the WMTAC and usual medical care recruited 92 patients, which totals to 184 patients. The patient demographics in terms of age, race and indication of treatment were comparable. At the end of the 6 months follow-up, patients in the WMTAC group had significantly higher actual-TTR (65.1 vs. 48.3 %; p < 0.05) compared to those in usual medical care group. Rates of admission were 6.5 versus 28.2 events per 100 person-years for the WMTAC and UMC groups, respectively. Though the bleeding incidences were not significantly different, it was reduced.

CONCLUSIONS: These findings will impact local warfarin patient management services and policies because there was no available evidence supporting the role of pharmacists in the management of warfarin patients prior to this study.

Matched MeSH terms: Warfarin/pharmacokinetics