Gloria R. Grice

Use of Pharmacogenetic and Clinical Factors to Predict the Therapeutic Dose of Warfarin

Initiation of warfarin therapy using trial‐and‐error dosing is problematic. Our goal was to develop and validate a pharmacogenetic algorithm. In the derivation cohort of 1,015 participants, the independent predictors of therapeutic dose were: VKORC1 polymorphism −1639/3673 G>A (−28% per allele), body surface area (BSA) (+11% per 0.25 m2), CYP2C9*3 (−33% per allele), CYP2C9*2 (−19% per allele), age (−7% per decade), target international normalized ratio (INR) (+11% per 0.5 unit increase), amiodarone use (−22%), smoker status (+10%), race (−9%), and current thrombosis (+7%). This pharmacogenetic equation explained 53–54% of the variability in the warfarin dose in the derivation and validation (N= 292) cohorts. For comparison, a clinical equation explained only 17–22% of the dose variability (P < 0.001). In the validation cohort, we prospectively used the pharmacogenetic‐dosing algorithm in patients initiating warfarin therapy, two of whom had a major hemorrhage. To facilitate use of these pharmacogenetic and clinical algorithms, we developed a nonprofit website, http://www.WarfarinDosing.org.

Integration of genetic, clinical, and INR data to refine warfarin dosing

Well‐characterized genes that affect warfarin metabolism (cytochrome P450 (CYP) 2C9) and sensitivity (vitamin K epoxide reductase complex 1 (VKORC1)) explain one‐third of the variability in therapeutic dose before the international normalized ratio (INR) is measured. To determine genotypic relevance after INR becomes available, we derived clinical and pharmacogenetic refinement algorithms on the basis of INR values (on day 4 or 5 of therapy), clinical factors, and genotype. After adjusting for INR, CYP2C9 and VKORC1 genotypes remained significant predictors (P < 0.001) of warfarin dose. The clinical algorithm had an R2 of 48% (median absolute error (MAE): 7.0 mg/week) and the pharmacogenetic algorithm had an R2 of 63% (MAE: 5.5 mg/week) in the derivation set (N = 969). In independent validation sets, the R2 was 26–43% with the clinical algorithm and 42–58% when genotype was added (P = 0.002). After several days of therapy, a pharmacogenetic algorithm estimates the therapeutic warfarin dose more accurately than one using clinical factors and INR response alone.

Laboratory and Clinical Outcomes of Pharmacogenetic vs. Clinical Protocols for Warfarin Initiation in Orthopedic Patients

Summary.  Background: Warfarin is commonly prescribed for prophylaxis and treatment of thromboembolism after orthopedic surgery. During warfarin initiation, out‐of‐range International Normalized Ratio (INR) values and adverse events are common. Methods: In orthopedic patients beginning warfarin therapy, we developed and prospectively validated pharmacogenetic and clinical dose refinement algorithms to revise the estimated therapeutic dose after 4 days of therapy. Results: The pharmacogenetic algorithm used the cytochrome P450 (CYP) 2C9 genotype, smoking status, peri‐operative blood loss, liver disease, INR values and dose history to predict the therapeutic dose. The R2 was 82% in a derivation cohort (n = 86) and 70% when used prospectively (n = 146). The R2 of the clinical algorithm that used INR values and dose history to predict the therapeutic dose was 57% in a derivation cohort (n = 178) and 48% in a prospective validation cohort (n = 146). In 1 month of prospective follow‐up, the percent time spent in the therapeutic range was 7% higher (95% CI: 2.7–11.7) in the pharmacogenetic cohort. The risk of a laboratory or clinical adverse event was also significantly reduced in the pharmacogenetic cohort (Hazard Ratio 0.54; 95% CI: 0.30–0.97). Conclusions: Warfarin dose adjustments that incorporate genotype and clinical variables available after four warfarin doses are accurate. In this non‐randomized, prospective study, pharmacogenetic dose refinements were associated with more time spent in the therapeutic range and fewer laboratory or clinical adverse events. To facilitate gene‐guided warfarin dosing we created a non‐profit website, http://www.WarfarinDosing.org.

A Polymorphism in the VKORC1 Regulator Calumenin Predicts Higher Warfarin Dose Requirements in African Americans

Warfarin demonstrates a wide interindividual variability in response that is mediated partly by variants in cytochrome P450 2C9 (CYP2C9) and vitamin K 2,3‐epoxide reductase complex subunit 1 (VKORC1). It is not known whether variants in calumenin (CALU) (vitamin K reductase regulator) have an influence on warfarin dose requirements. We resequenced CALU regions in a discovery cohort of dose outliers: patients with high (>90th percentile, n = 55) or low (<10th percentile, n = 53) warfarin dose requirements (after accounting for known genetic and nongenetic variables). One CALU variant, rs339097, was associated with high doses (P = 0.01). We validated this variant as a predictor of higher warfarin doses in two replication cohorts: (i) 496 patients of mixed ethnicity and (ii) 194 African‐American patients. The G allele of rs339097 (the allele frequency was 0.14 in African Americans and 0.002 in Caucasians) was associated with the requirement for a 14.5% (SD ± 7%) higher therapeutic dose (P = 0.03) in the first replication cohort and a higher‐than‐predicted dose in the second replication cohort (allele frequency 0.14, one‐sided P = 0.03). CALU rs339097 A>G is associated with higher warfarin dose requirements, independent of known genetic and nongenetic predictors of warfarin dose in African Americans.

Gamma-glutamyl carboxylase and its influence on warfarin dose

Via generation of vitamin K-dependent proteins, gamma-glutamyl carboxylase (GGCX) plays a critical role in the vitamin K cycle. Single nucleotide polymorphisms (SNPs) in GGCX, therefore, may affect dosing of the vitamin K antagonist, warfarin. In a multi-centered, cross-sectional study of 985 patients prescribed warfarin therapy, we genotyped for two GGCX SNPs (rs11676382 and rs12714145) and quantified their relationship to therapeutic dose. GGCX rs11676382 was a significant (p=0.03) predictor of residual dosing error and was associated with a 6.1% reduction in warfarin dose (95% CI: 0.6%-11.4%) per G allele. The prevalence was 14.1% in our predominantly (78%) Caucasian cohort, but the overall contribution to dosing accuracy was modest (partial R2 = 0.2%). GGCX rs12714145 was not a significant predictor of therapeutic dose (p = 0.26). GGCX rs11676382 is a statistically significant predictor of warfarin dose, but the clinical relevance is modest. Given the potentially low marginal cost of adding this SNP to existing genotyping platforms, we have modified our non-profit website (www.WarfarinDosing.org) to accommodate knowledge of this variant.

Pharmacogenomics of warfarin: Uncovering a piece of the warfarin mystery

PURPOSE The literature on the pharmacogenomics of warfarin and the use of genetic testing to optimize initial and maintenance warfarin dosing is reviewed. SUMMARY Warfarin tablets contain a racemic mixture of R- and S-isomers. The S-isomer is responsible for about 70% of warfarins anticoagulant effect. Cytochrome P-450 isoenzyme 2C9 (CYP2C9) metabolizes S-warfarin into two inactive metabolites. Genetic variations to the gene encoding CYP2C9 (CYP2C9 ) are known to affect warfarin clearance. Single nucleotide polymorphisms (SNPs) have been identified that clearly influence warfarin metabolism and sensitivity, including SNP variants of CYP2C9 and SNPs in vitamin K epoxide reductase complex subunit 1 (VKORC1), which influence an individuals sensitivity to a given dose. Retrospective studies have evaluated potential factors influencing warfarin metabolism, maintenance dosing, and variability. Several dosing models used to predict warfarin dosing (initial or refinement) have been retrospectively evaluated in diverse patient populations. There are several arguments to support incorporating its use in current clinical practice; however, many expert clinicians in anticoagulation have expressed concern that the push for genotyping patients for CYP2C9 and VKORC1 is premature and not based on good, prospective evidence. Large, randomized controlled trials, in multiple patient populations, comparing clinical dosing to genetic-guided dosing are needed to fully determine the benefits of pharmacogenetic warfarin dosing. CONCLUSION The increased understanding of pharmacogenomics may improve patient safety during initial dosing of warfarin. At this time, it is unknown if genotype-based dosing will become the standard of care for patients receiving the drug.

Optimal Initial Dose Adjustment of Warfarin in Orthopedic Patients

Background: Warfarin sodium is commonly prescribed for the prophylaxis and treatment of venous thromboembolism. Dosing algorithms have not been widely adopted because they require a fixed initial warfarin dose (eg, 5 mg) and are not tailored to other factors that may affect the international normalized ratio (INR). Objective: To develop an algorithm that could predict a therapeutic warfarin dose based on drug interactions. INR response after the initial warfarin doses, and other clinical factors. Methods: We used stepwise regression to quantify the relationship between these factors in patients beginning prophylactic warfarin therapy immediately prior to joint replacement. In the derivation cohort (n = 271), we separately modeled the therapeutic dose after 2 and 3 initial doses. We prospectively validated these 2 models in an independent cohort (n = 105). Results: About half of the therapeutic dose variability was predictable after 3 days of therapy: R2 was 53% in the derivation cohort and 42% in the validation cohort. INR response after 3 warfarin doses (INR3) inversely correlated with therapeutic dose (p < 0.001). Intraoperative blood loss transiently, but significantly, elevated the postoperative INR values. Other significant (p < 0.03) predictors were the first and second warfarin doses (+7% and +6%, respectively, per 1 mg), and statin use (–15.0%). The model derived after 2 warfarin doses explained 32% of the variability in therapeutic dose. Conclusions: We developed and validated algorithms that estimate therapeutic warfarin doses based on clinical factors and INR response available after 2–3 days of warfarin therapy. The algorithms are implemented online at www.WarfarinDosing.org

Dosing anticoagulant therapy with coumarin drugs: is genotyping clinically useful? Yes

*St Louis College of Pharmacy, St Louis, MO; and Department of Medicine, Washington University Medical School, St Louis, MO, USATo cite this article: Thacker SM, Grice GR, Milligan PE, Gage BF. Dosing anticoagulant therapy with coumarin drugs: is genotyping clinically useful?Yes. J Thromb Haemost 2008; 6: 1445–9.See also Mannucci PM, Spreafico M, Peyvandi F. Dosing anticoagulant therapy with coumarin drugs: is genotyping clinically useful? No. This issue,pp 1450–2.

Comparison of Patient Simulation Methods Used in a Physical Assessment Course

Objective. To determine whether there is a difference in student pharmacists’ learning or satisfaction when standardized patients or manikins are used to teach physical assessment. Design. Third-year student pharmacists were randomized to learn physical assessment (cardiac and pulmonary examinations) using either a standardized patient or a manikin. Assessment. Performance scores on the final examination and satisfaction with the learning method were compared between groups. Eighty and 74 student pharmacists completed the cardiac and pulmonary examinations, respectively. There was no difference in performance scores between student pharmacists who were trained using manikins vs standardized patients (93.8% vs. 93.5%, p=0.81). Student pharmacists who were trained using manikins indicated that they would have probably learned to perform cardiac and pulmonary examinations better had they been taught using standardized patients (p<0.001) and that they were less satisfied with their method of learning (p=0.04). Conclusions. Training using standardized patients and manikins are equally effective methods of learning physical assessment, but student pharmacists preferred using standardized patients.

Assessment of Students’ Critical-Thinking and Problem-Solving Abilities Across a 6-Year Doctor of Pharmacy Program

Objective. To determine the feasibility of using a validated set of assessment rubrics to assess students’ critical-thinking and problem-solving abilities across a doctor of pharmacy (PharmD) curriculum. Methods. Trained faculty assessors used validated rubrics to assess student work samples for critical-thinking and problem-solving abilities. Assessment scores were collected and analyzed to determine student achievement of these 2 ability outcomes across the curriculum. Feasibility of the process was evaluated in terms of time and resources used. Results. One hundred sixty-one samples were assessed for critical thinking, and 159 samples were assessed for problem-solving. Rubric scoring allowed assessors to evaluate four 5- to 7-page work samples per hour. The analysis indicated that overall critical-thinking scores improved over the curriculum. Although low yield for problem-solving samples precluded meaningful data analysis, it was informative for identifying potentially needed curricular improvements. Conclusions. Use of assessment rubrics for program ability outcomes was deemed authentic and feasible. Problem-solving was identified as a curricular area that may need improving. This assessment method has great potential to inform continuous quality improvement of a PharmD program.