Warfarin and vitamin K intake in the era of pharmacogenetics

Abstract

The considerable variability in the warfarin dose–response relationship between individuals, is explained mainly by genetic variation in its major metabolic (CYP2C9) and target (VKORC1) enzymes. Despite the predominance of pharmacogenetics, environmental factors also affect the pharmacokinetics and pharmacodynamics of warfarin, and are often overlooked. Among these factors, dietary and supplemental vitamin K consumption is a controllable contributor to within-, and between-patient variability of warfarin sensitivity. In this commentary we review the current role of vitamin K in warfarin anticoagulation therapy, with emphasis on the following:

1. The effect of dietary and supplemental vitamin K on warfarin anticoagulation, beyond the impact of genetic variability in CYP2C9 and VKORC1. We deal separately with the effects of vitamin K on warfarin dose requirements during the induction of therapy, as opposed to its effect on stability of anticoagulation control during maintenance therapy.

2. The role of vitamin K supplementation in warfarin treated patients with vitamin K deficiency as well as in patients with unstable warfarin anticoagulation, and

3. The role of therapeutic vitamin K in cases of warfarin over-anticoagulation.

Introduction

Vitamin K in its reduced form (vitamin K1 dihydroquinone, KH2) is the essential cofactor for post translational activation of the vitamin K dependent clotting factors, the procoagulants – factors II, VII, IX, X, and the anticoagulant proteins C and S. In the reaction, glutamic acid is converted to γ-carboxy-glutamic acid by γ-glutamyl-carboxylase, and vitamin K1 is converted to vitamin K epoxide which is rapidly reduced back to vitamin K quinone by the Vitamin K Epoxide Reductase Complex 1 (VKORC1) and then to vitamin K hydroquinone (KH2). VKORC1 is the molecular target inhibited by warfarin, which exerts its anticoagulant activity by interrupting the regeneration of KH2, the active (reduced) form of vitamin K, leading to decreased carboxylation of the vitamin K dependent clotting factors with loss of activity [1, 2].

Over the last two decades well known genetic polymorphisms in the S-warfarin (the main active warfarin isomer) metabolizing enzyme (cytochrome P450 CYP2C9) and in VKORC1, have emerged as major determinants of the high degree of between patient variability which characterizes the warfarin dose–response relationship. At therapeutic steady-state (stable anticoagulation) more than half of this variability is explainable, mostly attributable to CYP2C9 and VKORC1 genotype [3] and possibly, in part, to the newly described CYP4F2 V433M polymorphism [4, 5]. The remaining variance is only partially explained by clinical factors [5] as well as concurrent medications [6] interacting with warfarin. In cross-sectional studies at therapeutic steady-state, the impact of vitamin K intake (assessed either by plasma concentration or dietary intake) on predicted warfarin maintenance dose, after accounting for patient genotype, is seemingly minimal [7, 8]. Conversely, questions still remain open as to the effect of vitamin K on warfarin dose requirements at the initiation of therapy as well as under conditions of changing therapeutic stability. An additional point of current debate is that of the use of vitamin K in treating over-anticoagulation [9, 10]. Accordingly, we address the following issues which reflect on these questions:

1. Assessment of vitamin K status in healthy people and in warfarin treated patients.

2. The effects of dietary and supplemental vitamin K on warfarin dosing in patients commencing therapy and in those at stable or unstable therapeutic steady-state.

3. The role of therapeutic use of vitamin K in cases of excessive anticoagulation.

Assessment of vitamin K status

One of the major limitations in assessing the effect of vitamin K consumption on warfarin therapy stems from the inherent difficulties in determining vitamin K status or body stores [11]. The term ‘vitamin K’ refers to a family of compounds. The primary dietary source of vitamin K is phylloquinone (vitamin K1), the plant form of the vitamin, found in green leafy vegetables and certain vegetable oils [11, 12]. The menaquinones (vitamin K2) are synthesized by gut bacteria but play only a minor role in filling daily human requirements [11]. Body stores of vitamin K are mainly in the liver where turnover is rapid, resulting in depletion of hepatic reserves within a few days of restricted dietary intake [11, 13]. Increased hepatic vitamin K1 content, due to decreased hepatic oxidative metabolism of the vitamin, in patients carrying the newly described CYP4F2 V433M polymorphism [14] has been invoked to explain increased warfarin dose requirements.

Clinically evident vitamin K deficiency states are rare in adults [11] and no adverse effects associated with vitamin K consumption have been reported in adults other than those related to anticoagulation treatment. Data to support definition of meaningful daily requirements of vitamin K are lacking and therefore recommended dietary allowance (RDA) cannot be defined [15]. Adequate intake is set based on representative intake data from healthy individuals collected by the NHANES survey [16]. Various surrogate markers have been used to assess vitamin K status in humans. The prothrombin time is the only indicator associated with adverse clinical effects, but is usually insensitive to less than large changes in vitamin K intake, prothrombin time becoming prolonged only when prothrombin concentration drops below 50% of normal [17, 18].

Both phylloquinone and menaquinone plasma concentrations have been used to assess vitamin K status. Phylloquinone concentration reflects recent intake and has been shown to respond to changes in dietary intake within 24 h [19]. Other suggested markers for vitamin K status include plasma concentrations of undercarboxylated prothrombin (PIVKA II), percentage of under γ-carboxylated osteocalcin (%UCOC), and urinary γ-carboxyglutamyl (Gla) excretion. All these markers respond to changes in vitamin K intake, but their physiological impact is as yet unclear. Moreover, they do not provide a well-established basis for estimation of adequate vitamin K requirements [11, 20–22].

Vitamin K consumption in adults was assessed in a sub-population of the Framingham Offspring Study. Mean dietary phylloquinone intake was 115 µg day⁻¹ in men and 151 µg day⁻¹ in women. In a comprehensive review, intake reported for younger adults was 60–110 µg day⁻¹ and 80–210 µg day⁻¹ in those over age 55 years [11]. Current recommendations, based on data from NHANES III [16] are 90 µg day⁻¹ for women and 120 µg day⁻¹ for men. For research purposes, vitamin K plasma concentration, PIVKA II, %UCOC and urinary Gla excretion are used as surrogate markers, but no single marker can serve as a gold standard [11, 15].

Vitamin K and warfarin related genotypes

Polymorphisms in the CYP2C9 and VKORC1 genes are now well identified as the main contributors to the variability of warfarin pharmacokinetics and dynamics between patients [23, 24]. Mutations in the VKORC1 gene can lead to coding a protein that is either sensitive or resistant to warfarin inhibition [3]. The mechanism of the variable response to warfarin is not well characterized and the effect of vitamin K status among different VKORC1 genotype groups has been assessed in only a few studies. One pilot study in warfarin treated patients assessed the association between the VKORC1 -1639G > A (3673; rs 9923231) polymorphism and INR response, subsequent to vitamin K supplementation. Carriers of the GG (wild-type) genotype required significantly higher warfarin doses compared to GA or AA carriers to achieve the same target INR. After a week of vitamin K supplementation, the ratio of plasma vitamin K : vitamin K 2,3-epoxide was higher in GG genotype carriers compared with GA or AA patients. This represented decreased reducing capacity of vitamin K among GA and AA carriers and led to larger decreases in INR and higher doses of warfarin required to achieve therapeutic INR, among GG carriers. These results suggested that vitamin K supplementation leads to greater regeneration of vitamin K hydroquinone in patients with the GG genotype [25]. Another study found an association between sequence variations in the VKORC1 gene and plasma phylloquinone concentrations. VKORC11542G > C (6853; rs8050894) GG homozygotes had higher plasma phylloquinone concentrations compared with CG or CC carriers. VKORC13730G > A (9041; rs7294) AA or AG genotype carriers had significantly lower plasma phylloquinone concentrations compared with GG homozygotes [26]. Although still preliminary, these data suggest that patients on warfarin who take vitamin K supplements may manifest different clinically relevant anticoagulation responses, depending on VKORC1 genotype, GA and AA carriers representing approximately 60% of the Caucasian population [27].

Effect of changes in vitamin K intake on warfarin anticoagulation

The effect of gross changes in vitamin K intake on anticoagulation is a classic. Since the early years of warfarin use myriad case reports and case series have described decreased anticoagulant response due to sudden excessive vitamin K intake. The causes were usually vitamin K rich, vegetable-based, weight reducing diets and food supplements or multivitamins. The culprit amounts of vitamin K consumed ranged from 25 to 6000 µg day⁻¹, but other causes for therapeutic failures were not always excluded [28–31]. Excessive anticoagulation has also been described after unrecorded dietary modification or discontinuation of multivitamin use [31, 32].

Vitamin K status and initiation of warfarin treatment

The effect of vitamin K intake on warfarin anticoagulation was targeted more specifically in several observational and interventional studies, in patients commencing treatment. The association between vitamin K status and warfarin sensitivity was studied in orthopaedic patients commencing warfarin therapy. Vitamin K status was assessed by vitamin K1 and vitamin K2, 3 epoxide plasma concentrations and questionnaire-based, estimated dietary vitamin K intake. A higher than average intake was significantly associated with slower rise of undercarboxylated prothrombin, which was not however reflected in changes in INR. Higher baseline plasma vitamin K1 concentration was associated with slower rise in INR but not with slower rise of undercarboxylated prothrombin. The study did not address a possible correlation between vitamin K intake and time to achieve therapeutic INR [33]. In another study, patients consuming more than 250 µg day⁻¹ exhibited decreased sensitivity to warfarin, manifested by lower day 5 INR and higher steady state warfarin doses [34].

These findings notwithstanding, warfarin dosing at the initiation of therapy is still based mainly on trial and error. Current large scale prospective studies are evaluating the role of genotype guided initial dosing as a better strategy. It is however noteworthy that these, as well as the most comprehensive association study [27] (comprised of two parts: a cross sectional association model and prospective validation in an independent patient cohort) have not addressed vitamin K status. In this context it is important to note that many patients are still instructed, at the onset of therapy, to restrict and even avoid vitamin K consumption. This could be reflected in the rates of low dietary intake or of biochemical vitamin K depletion, demonstrated in warfarin treated patients in different countries [31], as contributing to clinically significant day to day within-patient variability in INR response, during long term treatment [35].

Vitamin K status during maintenance warfarin treatment

The relative effect of habitual vitamin K intake on stable therapeutic warfarin anticoagulation varies across studies. Earlier reports found vitamin K consumption to be an independent predictor of INR response [36, 37]. Conversely, in recent studies which include the effect of CYP2C9 and VKORC1 genotypes on warfarin dose at steady-state, the relative effect of vitamin K status has been shown to be negligible [7, 8]. In practice however, once empirically optimized warfarin maintenance doses are determined, genotypes should not be a major factor influencing fluctuating therapeutic stability. In this situation vitamin K status would be expected to play a more prominent role. Accordingly, a recent study aimed at long-term warfarin treated patients, demonstrated that an intervention based on structured vitamin-K dietary instructions could improve returning INR levels to within the therapeutic range, following detection of sub-therapeutic values [35].

Multivitamins and vitamin K

Several studies have addressed the effects of supplemental vitamin K on response to warfarin. Thus, single dose administration of a 250 µg vitamin K1 tablet to patients stabilized on warfarin did not result in prothrombin times outside the therapeutic range. However, after 1 week of daily administration of the vitamin, increased doses of warfarin were required in order to maintain the therapeutic range, and doses of K1 tablets at 100 µg day⁻¹ for 1 week caused increased coagulability, but still within the therapeutic range [38]. A vitamin K containing multivitamin tablet given in small doses (vitamin K 25 µg day⁻¹ for 4 weeks) to patients at stable anticoagulation, resulted in subtherapeutic INRs, requiring warfarin dose increments, in vitamin K depleted patients (defined by plasma vitamin K concentrations <1.5 µg l⁻¹, the 10^th percentile, in warfarin users in that clinic) but not in vitamin K repleted patients (with plasma concentrations >4.5 µg l⁻¹, the median). Correspondingly, plasma vitamin K concentrations doubled after supplementation in the depleted group and were unchanged in the controls [39]. It appears that even small changes in vitamin K intake can alter coagulation status, especially in the subgroup of vitamin K depleted patients, given a 12% rate of undetectable plasma vitamin K concentrations among unselected warfarin treated patients, in some clinics [31]. Indeed, many multivitamin formulations which contain vitamin K, deliver 25 µg day⁻¹ or more. These considerations are relevant irrespective of patient genotype, as they come into play at therapeutic steady-state, whether achieved empirically or by genotype-based dosing.

The role of vitamin K supplementation as a unique strategy to lead to more stable anticoagulation has been demonstrated in recent studies. Unstable anticoagulation was shown to be associated with lower mean intake of vitamin K [40]. Vitamin K supplementation given to unstable patients resulted in a small but significant increase in the time within the target INR range. Similarly, in patients with unstable control of anticoagulation, vitamin K supplementation (150 µg day⁻¹) has been shown to improve anticoagulation stability compared with placebo, perhaps due to a decrease in day-to-day variability in vitamin K intake [41]. High variability in vitamin K intake was also found to correlate with variable weekly INR [42], consistent with the results of two other small uncontrolled studies [43, 44]. This evidence indicates that supplemental vitamin K, even in very small daily doses, may affect previously stable warfarin anticoagulation, particularly in vitamin K depleted patients, in whom even the small increment may represent a large proportional increase in daily intake. Many patients use multivitamin formulations [45] (often containing small amounts of vitamin K). As these supplements are not perceived as concurrent medications their use is often not reported to the physician. Accordingly, patients on warfarin should be questioned not only about their dietary habits but also about consumption (especially initiation or discontinuation) of vitamin K containing multi-vitamins.

Vitamin K in reversal of warfarin induced over-anticoagulation

Supra-therapeutic INR is a common complication of warfarin treatment, with an attendant 1% rate of major haemorrhage, as seen in patients with INR levels between 5.0 and 9.0, in one study [46]. Patients with excessive INR levels, but free of bleeding are treated by reducing or omitting several warfarin doses. There have been conflicting reports on the effect of oral vitamin K in such patients. In one study, in patients with INR 4.5–10, a dose of only 1 mg led to a more rapid drop in INR and fewer bleeding episodes than placebo, over 30 days of follow-up. However, 16% of the vitamin K treated patients and none of the placebo group had INR < 1.8 on the day after treatment [47]. In contrast, in patients with no active bleeding, correction of supratherapeutic INR with low oral vitamin K (1.25 mg) in a controlled study, had no effect on the rate of subsequent bleeding [48]. In one non-randomized study, 75 episodes of INR > 10 were treated in an outpatient setting, 51 with 2 mg oral vitamin K and 24 treated with warfarin withdrawal alone. There were no major bleeds in the vitamin K treated patients vs. three clinically important bleeds in those who were not given vitamin K [49]. In the absence of bleeding, current guidelines recommend omitting one or two warfarin doses and considering 1–2.5 mg vitamin K orally when INR is 5–9, or 2.5–5 mg, when INR > 10 [10]. Whenever possible, oral administration should be preferred, as anaphylactoid reactons to vitamin K have been associated mainly with the intravenous route [50]. Subcutaneous administration lowers INR more slowly than oral vitamin K when given to asymptomatic warfarin treated over anti-coagulated patients [51].

Dietary guidelines

Health care professionals working in specialized anticoagulation clinics are usually well aware of the potential interaction between warfarin and diet. However not all patients attend specialized clinics and in primary care settings, knowledge of the intricacies of warfarin-nutrition interactions is sometimes lacking [52]. Even in review articles, instructions on diet are often general or vague [53].

Many patients on warfarin are vitamin K depleted as a result of routine instructions to restrict vitamin K intake in an attempt to achieve optimal therapeutic response. As described above, in vitamin K depleted patients, small changes in intake may be an important determinant of day to day variability in warfarin dose–response. Older recommendations for diets low in vitamin K as appropriate for warfarin treated patients [54] should now be considered outdated. Assessment of vitamin K consumption must include specific questions regarding the use of multivitamin and food additives. This is especially highlighted by the estimated 40% of the adult US population using some form of multivitamin supplements regularly [55]. In an ongoing survey of medication use in the US population, 40% of adults took vitamin products in the preceding week, 26% in the form of multivitamins [56]. Patients often do not consider vitamins as medications and do not mention using them or having discontinued use, during clinic visits.

The most important advice for patients on warfarin should be to maintain their usual dietary pattern and to report any planned changes in diet or in multivitamin usage. For those with a variable INR response not attributable to any of the usual known causes for instability, a trial of daily low dose oral vitamin K (100 to 200 µg) may be considered, with initially close monitoring of the INR and warfarin dose adjustment to counter unwanted lowering of the INR.

In summary vitamin K intake is an important and controllable factor within the complex array affecting warfarin anticoagulation, which includes physiologic and genetic factors, as well as disease states and concurrent medications. Although pharmacogenetics are at the forefront of warfarin dose-response research, once empirical or genotype-based dose titration is accomplished in the individual patient, variations in vitamin K intake still play an important role in maintaining therapeutic stability. This dictates ongoing patient education, highlighting the importance of maintaining steady intake. Moreover, future studies on warfarin dosing should continue to be designed to include CYP2C9 and VKORC1 genotype in dose-optimizing algorithms, at the initiation of anticoagulation. However, the investigation of instability of anticoagulation during maintenance dosing (either in the research setting or in the case of the at-risk individual patient) should place proper emphasis on assessment of changes in vitamin K status, as genotype is not a confirmed determinant of fluctuations in this ‘optimized’ steady state phase of therapy.

Competing interests

The Department of Clinical Pharmacology and Toxicology, Sheba Medical Center (R.L. and H.H.) received research and development grants from: 1) the Israel National Center for Health Services Research, 2) Maccabi Healthcare Services Ltd, both supporting a prospective, community-based study of the cost-effectiveness of genotype based warfarin treatment, and 3) the Israel Ministry of Industry and Commerce to develop, jointly with Pronto Diagnostics^R, a rapid warfarin genotyping technology, based on a patent submission in which H.H. holds partial rights. There are no other competing interests to declare.