To our knowledge, these are the first genotype-guided loading and maintenance dose algorithms for PHE and ACE using large cohorts. The utility of these algorithms will be tested in randomized controlled trials.

In a prospective follow-up study of the effects of VKORC1 and CYP2C9 genotypes on the anticoagulation status of patients, we assessed the CYP2C9 and the VKORC1 C1173T genotypes of patients during the initial 6 months of phenprocoumon treatment. We used linear regression models and Cox proportional hazard models to determine the effects of the VKORC1 and CYP2C9 genotypes on phenprocoumon dose requirements, overanticoagulation, and time to achieve stability. Allele frequencies of interest within the cohort (N=281) were 40.8% VKORC1 T-1173, 12.8% CYP2C9*2, and 6.9% CYP2C9*3. In patients with the VKORC1 CC genotype, carriers of a CYP2C9 polymorphism needed dosages that were nearly 30% lower than those for CYP2C9*1/*1 patients (P<0.001). In patients with a VKORC1 polymorphism, differences between carriers of a CYP2C9 polymorphism and CYP2C9*1/*1 were far smaller and largely not statistically significant. A larger part of the variability in dose requirement was explained by the VKORC1 genotype than by the CYP2C9 genotype (28.7% and 7.2%, respectively). Carriers of a combination of a CYP2C9 polymorphism and a VKORC1 polymorphism had a strongly increased risk of severe overanticoagulation (hazard ratio (HR) 7.20, P=0.002). Only carriers of a CYP2C9*2 allele had a decreased chance to achieve stability compared to CYP2C9*1/*1 patients (HR 0.61, P=0.004). In conclusion, the VKORC1 genotype modifies the effect of the CYP2C9 genotype on phenprocoumon dose requirements. A combination of polymorphisms of both genotypes is associated with a strongly increased risk of overanticoagulation, whereas delayed stabilization is mainly associated with the CYP2C9 genotype.

SummaryThe risk of hemorrhage when using coumarin anticoagulants sharply increases when the International Normalised Ratio (INR) is ≥6.0. We performed a prospective cohort study with a nested case-control design among 17,056 outpatients of an anticoagulation clinic to determine the incidence of overanticoagulation and to study the association between overanticoagulation and characteristics of anticoagulant therapy and comorbidity. The incidence rate of an INR ≥6.0 was 7.8 per 10,000 treatment days in prevalent users on the starting date and 22.5 per 10,000 treatment days in incident users during the study period. 300 cases with an INR ≥6.0 were compared with 302 randomly selected matched controls with an INR within the target zone. Patients on acenocoumarol had an increased risk of an INR ≥6.0 compared to patients on phenprocoumon. Regarding comorbidity, impaired liver function, congestive heart failure, diarrhea and fever were risk factors for overanticoagulation. Increased monitoring of INR values if risk factors are present or avoidance of risk factors could prevent excess anticoagulation and potential bleeding complications.

The presence of at least 1 CYP2C9*2 or *3 allele in phenprocoumon users is associated with an increased risk of severe overanticoagulation. Similar to warfarin and acenocoumarol, phenprocoumon had a lower dosage requirement in carriers of CYP2C9*2 or *3 compared with that in CYP2C9 wild-type subjects.

BackgroundPoor anticoagulant stability in patients using vitamin K antagonists is a risk factor for both bleeding and thrombosis. In previous studies supplementation with low dose vitamin K1 was shown to improve the stability of anticoagulant control. We set up a study to confirm earlier reports and to determine the optimal daily dose of vitamin K1 in preparation of a large study with clinical endpoints. Design and MethodsFour hundred patients from two anticoagulation clinics starting with vitamin K antagonists, independently of a possible history of instable anticoagulation, were randomized to receive either placebo or 100, 150 or 200 μg of vitamin K1 together with their treatment with vitamin K antagonists. The treatment was administered for 6 to12 months. Anticoagulation stability, expressed as the percentage of time that the International Normalized Ratio was within the therapeutic range, was compared between the groups. ResultsAfter adjustment for age, sex, vitamin K antagonist used, anticoagulation clinic and interacting drugs as confounding factors the difference in percentage of time with the International Normalized Ratio within the therapeutic range between the placebo group and the vitamin K1 groups was 2.1% (95% CI: -3.2% -7.4%) for the group taking 100 μg, 2.7% (95% CI: -2.3% -7.6%) for the group taking 150 μg and 0.9% (95% CI: -4.5% -6.3%) for the group taking 200 μg vitamin K1 group, in favor of the vitamin K1 groups. The patients from both the 100 μg group and the 150 μg group had a 2-fold higher chance of reaching at least 85% of time with the International Normalized Ratio within the therapeutic range. There were no differences in thromboembolic or hemorrhagic complications between the groups. ConclusionsIn patients starting vitamin K antagonists, supplementation with low dose vitamin K1 resulted in an improvement of time that anticoagulation was within the therapeutic range. Differences between doses were, however, small and the improvement is unlikely to be of clinical relevance. For future studies we recommend selecting only patients with instable anticoagulant control. (This study was registered at . Haematologica 2011;96(4):583-589. doi:10.3324/haematol.2010 Vitamin K1 supplementation to improve the stability of anticoagulation therapy with vitamin K antagonists: a dose-finding study