Nancy L. Shapiro

Genetic and Clinical Predictors of Warfarin Dose Requirements in African Americans

The objective of this study was to determine whether, in African‐American patients, additional vitamin K oxidoreductase complex subunit 1 (VKORC1), cytochrome P450 2C9 (CYP2C9), CYP4F2, or apolipoprotein E (APOE) polymorphisms contribute to variability in the warfarin maintenance dose beyond what is attributable to the CYP2C9*2 and *3 alleles and the VKORC1 −1639G>A genotype. In a cohort of 226 African‐American patients, weekly warfarin dose requirements were lower in those with the CYP2C9*8 allele (34 (30–47) mg; P = 0.023) and the CYP2C9 *2, *3, *5, *6, or *11 allele (33(28–40 mg); P < 0.001) as compared with those with the CYP2C9*1/*1 genotype (43 (35–56) mg). The combination of CYP2C9 alleles, VKORC1 −1639G>A genotype, and clinical variables explained 36% of the interpatient variability in warfarin dose requirements. By comparison, a model without the CYP2C9*5, *6, *8, and *11 alleles explained 30% of the variability in dose. No other VKORC1, CYP4F2, or APOE polymorphism contributed to the variance. The inclusion of additional CYP2C9 variants may improve the predictive ability of warfarin dosing algorithms for African Americans.

Warfarin and its interactions with foods, herbs and other dietary supplements

Despite its complex pharmacokinetic and pharmacodynamic profile, warfarin is still one of the most widely used oral anticoagulant agents. Attaining optimal anticoagulation with this agent is clinically challenging in view of its many food and drug interactions. Inappropriate anticoagulation control can expose patients to an increased risk of bleeding or thromboembolic complications, due to over and underanticoagulation, respectively. Fluctuations in dietary vitamin K intake can have a significant effect on the degree of anticoagulation in patients treated with warfarin. In addition, the explosion in use of various dietary supplements and herbal products can lead to undesired outcomes on anticoagulant levels. The aim of this review is to discuss the scope and the potential clinical impact of the most commonly reported food, dietary supplement and herbal interactions with warfarin therapy. Practical steps for patients and providers to minimise these interactions are highlighted.

Factors influencing warfarin dose requirements in African–Americans

INTRODUCTION African-Americans are under-represented in studies assessing contributors to warfarin response. Our primary objective was to determine whether the genes for cytochrome P450 (CYP) 2C9, nicotinamide adenine dinucleotide phosphate, reduced, quinone oxidoreductase (NQO1) and vitamin K epoxide reductase complex subunit 1 (VKORC1) are associated with warfarin dose requirements in African-Americans. PATIENTS AND METHODS The following factors were assessed: demographics; clinical data; the CYP2C9 Arg144Cys (*2), Ile358Leu (*3) and Asp360Glu (*5); NQO1 Pro187Ser (*1/*2); and VKORC1 G6853C genotypes were analyzed in 115 African-Americans on stable warfarin doses. RESULTS Allele frequencies were 0.05 for the CYP2C9 *2, *3 or *5 alleles; 0.20 for NQO1 *2; and 0.25 for VKORC1 6853C. Possession of a CYP2C9*2, *3 or *5 allele was associated with a 38% lower warfarin dose compared with the *1/*1 genotype (30 +/- 13 vs 48 +/- 18 mg/week; p = 0.003). Neither the NQO1 *1/*2 nor VKORC1 G6853C genotype was associated with warfarin dose requirements in the population as a whole or in CYP2C9*1 allele homozygotes. Multiple regression analysis revealed that CYP2C9 genotype (p = 0.015), age (p < 0.001) and body surface area (p < 0.001) were jointly associated with warfarin dose requirements, and together explained 33% of the variability in warfarin dose requirements among African-Americans. DISCUSSION Our data suggest that CYP2C9 genotype, age and body size are important determinants of warfarin dose requirements in African-Americans. Our data further suggest that the VKORC1 G6853C polymorphism alone may not be useful for predicting warfarin dose requirements in this racial group.

Decreased Warfarin Clearance Associated With the CYP2C9 R150H (*8) Polymorphism

The cytochrome P450 (CYP) 2C9 R150H (*8) allele occurs commonly in African Americans and is associated with lower warfarin dose requirements. We conducted a pharmacokinetic study to examine whether the CYP2C9*8 allele impacts warfarin clearance in African‐American patients. We also conducted an in vitro kinetic study of S‐warfarin 7‐hydroxylation using complementary DNA (cDNA)‐expressed CYP2C9 enzymes. We observed a 30% reduction in the unbound oral clearance of S‐warfarin and a 25% lower R‐ to S‐warfarin plasma concentration ratio in patients with the CYP2C9*8 allele (n = 12) as compared to CYP2C9*1 homozygotes (n = 26). Consistent with these findings, the in vitro intrinsic clearance of S‐warfarin was 30% lower with the cDNA‐expressed R150H protein as compared to the wild‐type protein. These data show that the R150H variant protein expressed by the CYP2C9*8 allele is associated with lower S‐warfarin clearance. This finding provides clinical and experimental evidence to explain the lower warfarin dose requirements in patients with the CYP2C9*8 allele.

Ezetimibe: a selective cholesterol absorption inhibitor.

Ezetimibe is the first agent of a novel class of selective cholesterol absorption inhibitors recently approved by the Food and Drug Administration for treatment in the United States. Ezetimibe inhibits the absorption of biliary and dietary cholesterol from the small intestine without affecting the absorption of fat‐soluble vitamins, triglycerides, or bile acids. Ezetimibe localizes at the brush border of the small intestine and decreases cholesterol uptake into the enterocytes. Preclinical studies demonstrated lipid‐lowering properties of ezetimibe as monotherapy and showed a synergistic effect in combination with 3‐hydroxy‐3‐methylglutaryl coenzyme A reductase inhibitors (statins). The efficacy and safety of ezetimibe 10 mg/day have been established in phase III clinical trials. In these trials, ezetimibe was investigated as monotherapy, as an add‐on to ongoing statin therapy, and as combination therapy with statins in patients with primary hyper‐cholesterolemia. In addition, ezetimibe has been evaluated in patients with homozygous and heterozygous familial hypercholesterolemia and in those with sitosterolemia. When given as monotherapy or in combination with statins or fenofibrate, ezetimibe reduces low‐density lipoprotein cholesterol (LDL) by 15–20% while increasing high‐density lipoprotein cholesterol by 2.5–5%. Unlike other intestinally acting lipid‐lowering agents, ezetimibe does not adversely affect triglyceride levels and, due to its minimal systemic absorption, drug interactions are few. Ezetimibes side‐effect profile resembles that of placebo when given as monotherapy or in combination with statins. In clinical practice, ezetimibe has a role as monotherapy for patients who require modest LDL reductions or cannot tolerate other lipid‐lowering agents. In combination therapy with a statin, ezetimibe is used in patients who cannot tolerate high statin doses or in those who need additional LDL reductions despite maximum statin doses.

Interaction Between Warfarin and Cranberry Juice

Warfarin is extensively used for anticoagulation to a target international normalized ratio of 2.0–3.0 for most indications or 2.5–3.5 for high‐risk indications; however, many drugs and dietary supplements induce fluctuations in the international normalized ratio. Such fluctuations may lead to therapeutic failure or bleeding complications. Cranberry juice is increasingly used for the prevention and adjunctive treatment of urinary tract infections. The United Kingdoms Committee on Safety of Medicines has alerted clinicians to a potential interaction between warfarin and cranberry juice and has advised that patients avoid their concurrent use. Review and analysis of the literature revealed that ingestion of large volumes of cranberry juice destabilize warfarin therapy. Small amounts of juice are not expected to cause such an interaction. Clinicians should be aware of this potential interaction and monitor and counsel patients accordingly.

New anticoagulant agents: direct thrombin inhibitors.

Decades of research have been devoted to developing effective, safe, and convenient anticoagulant agents. In recent years, much emphasis has been placed on the development of direct thrombin inhibitors (DTIs) that offer benefits over agents like heparin and warfarin including the inhibition of both circulating and clot-bound thrombin; a more predictable anticoagulant response, because they do not bind to plasma proteins and are not neutralized by platelet factor 4; lack of required cofactors, such as antithrombin or heparin cofactor II; inhibiting thrombin-induced platelet aggregation; and absence of induction of immune-mediated thrombocytopenia. Various injectable DTIs are currently available and used for many indications. In addition, research is now focusing on oral DTIs that seem promising and offer various advantages, such as oral administration, predictable pharmacokinetics and pharmacodynamics, a broad therapeutic window, no routine monitoring, no significant drug interactions, and fixed-dose administration.

Pharmacogenomics of Warfarin dose requirements in Hispanics

While Hispanics are the largest and most rapidly growing minority population in the United States, they are underrepresented in pharmacogenomic studies with warfarin. We sought to determine the combination of clinical and genetic influences of warfarin dose requirements in Hispanics. In addition, we tested the performance of published warfarin dosing algorithms derived from largely non-Hispanic cohorts in an inner-city U.S. Hispanic population. Genetic samples and clinical data were obtained from 50 Hispanics on a stable dose of warfarin. The contribution of cytochrome P450 2C9 (CYP2C9) and vitamin K epoxide reductase complex-1 (VKORC1) genotypes and clinical factors to warfarin dose requirements was determined. The correlation between the predicted dose using published algorithms and therapeutic dose was also assessed. Compared to the VKORC1-1639 GG genotype, warfarin dose requirements were 30% and 62% lower with the GA and AA genotypes, respectively (p=0.001). The combination of the VKORC1-1639G>A and CYP2C9 genotypes and clinical factors explained 56% of the inter-patient variability in warfarin dose. Warfarin dose predicted using algorithms derived from mostly non-Hispanic cohorts was significantly correlated with the therapeutic dose in our Hispanic cohort (r(2)=0.43 to 0.49; p<0.001); the predicted dose was within 1.0 mg/day of the therapeutic dose for 40% to 50% of patients. Our data suggest that factors influencing warfarin dose requirements in Hispanic Caucasians are similar to those previously described in European Caucasians and that dosing algorithms derived from non-Hispanic Caucasian cohorts are applicable to Hispanics living in the U.S.

Association of the GGCX (CAA)16/17 repeat polymorphism with higher warfarin dose requirements in African Americans

Objective Little is known about genetic contributors to higher than usual warfarin dose requirements, particularly for African Americans. This study tested the hypothesis that the &ggr;-glutamyl carboxylase (GGCX) genotype contributes to warfarin dose requirements greater than 7.5 mg/day in an African American population. Methods A total of 338 African Americans on a stable dose of warfarin were enrolled. The GGCX rs10654848 (CAA)n, rs12714145 (G>A), and rs699664 (p.R325Q); VKORC1 c.-1639G>A and rs61162043; and CYP2C9*2, *3, *5, *8, *11, and rs7089580 genotypes were tested for their association with dose requirements greater than 7.5 mg/day alone and in the context of other variables known to influence dose variability. Results The GGCX rs10654848 (CAA)16 or 17 repeat occurred at a frequency of 2.6% in African Americans and was overrepresented among patients requiring greater than 7.5 mg/day versus those who required lower doses (12 vs. 3%, P=0.003; odds ratio 4.0, 95% confidence interval, 1.5–10.5). The GGCX rs10654848 genotype remained associated with high dose requirements on regression analysis including age, body size, and VKORC1 genotype. On linear regression, the GGCX rs10654848 genotype explained 2% of the overall variability in warfarin dose in African Americans. An examination of the GGCX rs10654848 genotype in warfarin-treated Caucasians revealed a (CAA)16 repeat frequency of only 0.27% (P=0.008 compared with African Americans). Conclusion These data support the GGCX rs10654848 genotype as a predictor of higher than usual warfarin doses in African Americans, who have a 10-fold higher frequency of the (CAA)16/17 repeat compared with Caucasians.

Bleeding Incidence with Concomitant Use of Antidepressants and Warfarin

Introduction: Bleeding is the major complication associated with warfarin therapy. Some antidepressants are also associated with increased bleeding risk. Warfarin and antidepressants are used frequently in combination, but it is unclear whether concomitant use increases the risk of bleeding beyond that with warfarin alone. The primary goal of this study was to determine whether the use of warfarin and an antidepressant increases the risk for bleeding outcomes compared with the use of warfarin alone. The secondary goal was to characterize the risk of bleeding in warfarin-treated patients taking one specific class of antidepressant, selective serotonin reuptake inhibitors (SSRIs). Materials and Methods: This was a retrospective, single-center, study of warfarin-treated patients prescribed (n = 46) and not prescribed (n = 54) an antidepressant. Medical records over 6 months were reviewed for international normalized ratio values, medical history, bleeding type and incidence, and hospitalization due to bleeding. Patients were included in the antidepressant group if they were taking concomitant warfarin and antidepressant therapy consistently for a period of 6 months and in the control group if they were not taking an antidepressant with warfarin. Results: The use of any antidepressant with warfarin was not associated with the incidence of any bleeding or major bleeding during the 6-month period. However, the use of an SSRI with warfarin was associated with an increase in any bleeding event (odds ratio 2.6, 95% confidence interval, 1.01-6.4 P = 0.04). The use of an SSRI remained a significant predictor of bleeding after accounting for other factors associated with bleeding risk. Conclusions: Based on these data, it is important to clarify the interaction between warfarin and SSRIs in regard to bleeding risk given the high frequency of their concomitant use.