Over several decades, warfarin has been used as the drug of choice for prevention and treatment of thromboembolic disorders. The impact of dietary vitamin K intake on anticoagulation response to warfarin is well established. However, the effect of dietary vitamin K on the activity of the novel oral anticoagulants is less well known. Since the activation of both factor X and thrombin is inextricably linked to the vitamin K cycle in the coagulation cascade, there is the possibility that the pharmacological activity of agents that inhibit these proteins could be influenced by alterations in vitamin K availability.
To examine the influence of dietary vitamin K intake on the pharmacological activity of rivaroxaban in man.
The pharmacological activity of rivaroxaban was evaluated ex-vivo. Thirty one medically stable elderly inpatients (12 males) with a poor dietary intake in terms of calories and nutrients including vitamin K (12 males) and 28 healthy subjects (7 males) with adequate diets were enrolled into the study. An overnight fasted venous blood sample (20 ml) was taken from each subject. The plasma was stored at -80 degree C for the later measurement of the effects of rivaroxaban on clotting times and clotting factor activity. Each subject completed a validated dietary questionnaire for quantification of vitamin K content of foods eaten in the past week. The plasma samples from each subject were incubated with a range of rivaroxaban concentrations (100 - 500 ng/ml) in order to simulate the therapeutic drug plasma concentrations. Normal prothrombin time (PT), modified prothrombin time (mPT), and vitamin K dependent clotting factors activity were measured according to established in-house methods.
The mean±SD age of elderly patients and healthy subjects was 87±6 and 36±10 years, respectively. Rivaroxaban produced a greater prolongation of both PT (P=0.01) and m-PT (P=0.02) in the plasma of elderly subjects than the younger healthy subjects over the drug concentration range studied. The mean difference in PT between the two groups ranged from 1.8s to 5.0s at 100 ng/ml and 500 ng/ml plasma rivaroxaban concentrations respectively. The mean difference in m-PT between the two groups ranged from 10.0s to 34.0s at 100 ng/ml and 500 ng/ml plasma rivaroxaban concentrations respectively. There was also a greater inhibition of factor Xa (P=0.005) activity by rivaroxaban in the elderly subjects compared to the healthy subjects, whereas factor IXa (P=0.03) activity was significantly inhibited in the healthy subjects compared to the elderly subjects. However, there were no significant differences between the two groups in factors II and VII activity and fibrinogen concentration. The analysis of dietary questionnaires for vitamin K intake and plasma for fasting vitamin K levels is in progress and the results will be presented.
We have previously demonstrated in a proof-of-concept study in rats that vitamin K deficiency significantly enhanced the pharmacological activity of the prototype DTI, ximelagatran [1]. The present study findings suggest that the anticoagulation response to rivaroxaban may be influenced by poor dietary vitamin K intake. Assessment of whether dietary vitamin K influences anticoagulation response as well as any influence of age, co morbidities and drug intake in patients taking rivaroxaban is warranted.
[1] Kamali F, Wood P, Ward A. Vitamin K deficiency amplifies anticoagulation response to ximelagatran: possible implications for direct thrombin inhibitors and their clinical safety. Annals of Hematology 2009; 88:141-149.
No relevant conflicts of interest to declare.
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