Pharmacokinetic and pharmacodynamic interactions of echinacea and policosanol with warfarin in healthy subjects

Abstract

AIMS

This study investigated the pharmacokinetic and pharmacodynamic interactions of echinacea and policosanol with warfarin in healthy subjects.

METHODS

This was an open-label, randomized, three-treatment, cross-over, clinical trial in healthy male subjects (n= 12) of known CYP2C9 and VKORC1 genotype who received a single oral dose of warfarin alone or after 2 weeks of pre-treatment with each herbal medicine at recommended doses. Pharmacodynamic (INR, platelet activity) and pharmacokinetic (warfarin enantiomer concentrations) end points were evaluated.

RESULTS

The apparent clearance of (S)-warfarin (90% CI of ratio; 1.01, 1.18) was significantly higher during concomitant treatment with echinacea but this did not lead to a clinically significant change in INR (90% CI of AUC of INR; 0.91, 1.31). Policosanol did not significantly affect warfarin enantiomer pharmacokinetics or warfarin response. Neither echinacea nor policosanol had a significant effect on platelet aggregation after 2 weeks of pre-treatment with the respective herbal medicines.

CONCLUSION

Echinacea significantly reduced plasma concentrations of S-warfarin. However, neither echinacea nor policosanol significantly affected warfarin pharmacodynamics, platelet aggregation or baseline clotting status in healthy subjects.

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

• Echinacea and policosanol are commonly used herbal medicines which may be ingested by patients receiving warfarin. Echinacea has been implicated in interacting with drug metabolizing enzymes and policosanol has been shown to decrease platelet aggregation. The potential interaction of echinacea and policosanol with warfarin has not previously been investigated.

WHAT THIS STUDY ADDS

• Concomitant treatments with echinacea increased the apparent clearance of S-warfarin but did not have a clinically significant effect on warfarin pharmacodynamics in healthy subjects. Policosanol did not significantly affect warfarin pharmacokinetics or pharmacodynamics.

Introduction

Warfarin is widely used in cardiovascular medicine to prevent thromboembolic complications. Warfarin exhibits wide inter-subject variability in its response which has been partly attributed to genetic factors, age, hepatic function, diet and drug interactions [1, 2]. The rise in the use of herbal medicines in the community increases the likelihood that herbal and conventional medicines will be administered concomitantly despite the limited evidence to support the safe use of many of these combinations. Therefore, two commonly used herbal medicines, echinacea and policosanol were investigated for their potential to interact with warfarin.

Echinacea is a popular herbal medicine reported to be an immunostimulant [3]. Of the different species used medicinally, Echinacea purpurea and Echinacea angustifolia are more commonly used in the community [3]. The main constituents in preparations of these two species are caffeic acid derivatives and alkamides (alkylamides) [3]. Alkamides have been shown to achieve systemic concentrations after administration of echinacea tablets [4–6]. In vitro and in vivo evidence indicates that constituents of echinacea have the potential to interact with cytochrome P450 enzymes [7, 8]. Gorski et al. [9] investigated the in vivo effects of Echinacea purpurea administration on the activity of different CYP enzymes, including those enzymes involved in the metabolism of the enantiomers of warfarin. This study indicated that echinacea significantly decreased CYP1A2 activity, significantly increased hepatic CYP3A4 activity and there was a trend towards decreased CYP2C9 activity. The possible interaction between echinacea and warfarin is difficult to predict due to the lack of a controlled clinical study investigating this potential herb-drug interaction but is of particular relevance given the narrow therapeutic index of warfarin.

Policosanol is a complex mixture of alcohols which are mostly extracted from sugar cane wax (Saccharum officinarum) but may also be obtained from wheat, rice and beeswax [10]. Policosanol contains a mixture of long chain (20–36 carbons) higher aliphatic primary alcohols [10] consisting of approximately 66% octacosanol (C₂₈), 12% triacontanol (C₃₀), 7% hexacosanol (C₂₆) and lower proportions of other alcohols including docosanol (C₂₂) and tetracosanol (C₂₄) [10]. Policosanol has been investigated and promoted for its cholesterol lowering effects [11] and hence may be used concomitantly by patients with cardiovascular disease receiving warfarin. There are no clinical studies investigating the potential policosanol-warfarin interaction, although controlled studies have shown that policosanol can decrease platelet aggregation, suggesting there may be an aspirin-like interaction with warfarin [12].

Hepatic CYP2C9 is the enzyme responsible for (S)-warfarin metabolism and the impact of genetic variation in CYP2C9 genotype on warfarin effects is well documented [13]. Variation in the vitamin K epoxide reductase complex subunit 1 (VKORC1) gene has been found to contribute to clinically meaningful variations in warfarin response [14]. The impact of genetic variation on the significance of herb-drug interactions with warfarin remains unclear.

The aim of this study was to investigative the possible pharmacokinetic or pharmacodynamic interaction of echinacea and policosanol with warfarin in healthy male subjects of known CYP2C9 and VKORC1 genotype. This study also examined the steady-state concentration^-time profiles of echinacea alkamides in healthy subjects after 2 weeks of regular dosing with echinacea.

Materials and methods

Study design

This study was approved by the St Vincent's Hospital Human Research Ethics Committee, Darlinghurst, Australia and the Human Ethics Committee of the University of Sydney, Australia. Healthy male subjects (n= 12), aged between 18 and 34 years, who were non-smokers and not taking any medication including any herbal medicines or dietary supplements (for at least 2 weeks), were recruited into the trial. Subjects who gave written consent to participate in the study were selected after a full medical history, physical examination and clinical laboratory evaluation. Subjects with any medical condition that could alter warfarin effects, including any clotting disorders, hepatic dysfunction or platelet dysfunction were excluded from the study. Subjects were randomly allocated to three treatment groups to receive a single dose of warfarin (25 mg, Coumadin 5 × 5 mg tablets; Boots HealthCare Australia Pty Ltd, North Ryde, NSW, Australia) alone or after 2 weeks pre-treatment with multiple doses of either echinacea (1275 mg four times daily containing a mixture of 600 mg of E. angustifolia roots and 675 mg of E. purpurea root; standardized to contain 5.75 mg of total alkamides per tablet; MediHerb Premium Echinacea™ tablets, MediHerb Pty. Ltd, Warwick, QLD, Australia) or policosanol (10 mg tablet twice daily; Policosanol derived from sugar cane wax; Blackmores, Balgowlah, NSW, Australia). Dosing with herbal medicines continued throughout the 7 days after warfarin administration, i.e. until the last blood sample was drawn. Subjects were subsequently crossed over to the other treatments following a wash-out period of 2 weeks. To enhance adherence to clinical visits and the treatment protocol subjects were provided with a visit calendar and a diary to record medication intake. Herbal medicine ingestion was not directly supervised. Adherence to medicines, general well-being and signs of possible adverse events were also assessed using regular mobile phone calls/text messages and e-mail contact. The study protocol stipulated that any signs of bruising, bleeding or elevated INR (greater than 4.0) would result in immediate referral for medical attention. Blood samples were collected into sodium citrate (for INR measurement) and EDTA (for warfarin enantiomer measurement and genotyping) vacutainer tubes before (−48, −24, 0 h) and after (1, 2, 4, 8, 12, 24, 48, 72, 96, 120, 144 and 168 h) warfarin dosing. Plasma was harvested by centrifugation at 1500 g for 15 min. Platelet aggregation was assessed in whole blood samples (collected in sodium citrate tubes) taken before warfarin administration in all three treatment arms.

CYP2C9 and VKORC1 genotyping

The CYP2C9*1,*2 and *3 polymorphisms of CYP2C9 were detected by polymerase chain reaction-based restriction fragment length polymorphism (PCR-RFLP) analysis. The primers and restriction sites for the CYP2C9*2 and CYP2C9*3 alleles used in this study were as described by Xu et al. [15]. The VKORC11173 T > C polymorphism was detected using allelic discrimination real-time PCR as described by Li et al. [16] with minor modifications.

Plasma warfarin enantiomers concentrations

Plasma samples were analyzed using a validated, chiral, HPLC assay [17]. Precision of the assay (expressed as coefficient of variation) was less than 15% for both warfarin enantiomers. Inter- and intra-day accuracy of the assay was within 15% of the nominal warfarin enantiomer concentration. The extraction recovery of both (S)- and (R)-warfarin was within the range of 86–93%. The fraction unbound of (S)-warfarin and (R)-warfarin in plasma samples was investigated using a validated ultra- filtration method [17] based upon the assumption that warfarin fraction unbound is independent of concentration up to 25 µg ml⁻¹ of rac-warfarin as demonstrated by Banfield et al. [18]. In brief, rac-warfarin (15 µg) was added to plasma samples (1 ml) from each subject obtained by pooling samples collected between 1–8 h, 12–72 h and 96–168 h after the warfarin dose. Unbound warfarin enantiomers were separated by ultrafiltration (Centrifree™ YM-30, Millipore, Australia Pty Ltd, North Ryde, NSW, Australia) with centrifugation at 1500 g for 20 min. Ultrafiltrate was assayed using the validated chiral HPLC assay described above [17]. The unbound fraction was calculated as the ratio of ultrafiltrate to plasma warfarin enantiomer concentrations.

Platelet aggregation and International Normalised Ratio (INR) measurement

Platelet aggregation was measured within 3 h of blood collection using a whole blood aggregometer (Chrono-par; Chrono-log Corp., USA, Edward Keller Australia Pty Ltd, Hallam, VIC, Australia) [17] by addition of adesonine diphosphate (ADP, 10 µm), arachidonic acid (0.5 mm), or collagen (2 µg ml⁻¹). INR was measured using a BFT II analyzer™ (Dade Behring Diagnostics Pty Ltd, Lane Cove, NSW, Australia) as previously reported [17].

Determination of echinacea alkamides in tablets and in plasma

Echinacea alkamide content in the echinacea tablets used in this study and in the plasma of subjects after oral administration of multiple doses of echinacea tablets were determined using a previously reported and validated LCMS method [5]. Echinacea alkamide content in plasma was determined from blood samples collected over a dosing interval after 2 weeks of regular echinacea intake.

Data analysis

The pharmacodynamic response of warfarin was assessed by measuring the area under the curve of the International Normalised Ratio vs. time (AUC_INR) profile estimated using the linear trapezoidal rule. INR at baseline (INR₀) prior to warfarin administration and the maximum observed INR (INR_max) were also recorded.

The elimination rate constant (λ_z) was calculated from the slope of the terminal portion of the natural logarithmic concentration–time curve for (S)- and (R)-warfarin. Area under the concentration–time curve (AUC) to the last quantifiable concentration (AUC(0,t)) was determined using the trapezoidal rule and extrapolated to infinity (AUC(0,∞)) by adding C_t/λ_z, where C_t is the observed warfarin enantiomer concentration in the last quantifiable sample. The half-life for each warfarin enantiomer was calculated as ln 2/λ_z, apparent clearance (CL/F) was calculated as Dose/AUC(0,∞) and apparent volume of distribution (V/F) by dividing apparent clearance by λ_z. The maximum concentration (C_max) and the time it occurred (t_max) were determined by observation.

Statistical analysis

A power calculation indicated that 12 subjects in a crossover study design would provide an 80% chance of detecting a 20% difference in the AUC(0,∞) of (S)-warfarin at the P= 0.05 level of significance. This study design has been used successfully in our previous herb–drug interaction studies with warfarin [17]. Pharmacokinetic and pharmacodynamic parameters after treatment with warfarin alone (control) and in warfarin with herbal medicine treatment arm (intervention) were logarithmically transformed and reported as the geometric mean ratios (intervention to control) and 90% confidence intervals (CI) of the ratio. If the 90% CI of the ratios included the value of 1.0 then the difference between parameters was considered as being not significantly different. Residual mean square error (RMSE) for each logarithmically transformed parameter for each treatment was obtained using anova with nested parameters in treatment and sequence order. Statistical analyses were conducted using Stata® 5.0 (Stata Corp., TX, USA).

Results

The mean (and range) age, weight and height of the 12 subjects in the study were 24 years (20–35 years), 68 kg (57–84 kg) and 177 cm (171–184 cm), respectively. Six subjects were of European ancestry and six subjects were of Asian ancestry (four of whom were of South Asian origin). Nine subjects had the CYP2C9*1/*1 genotype and one subject each carried the CYP2C9*1/*2, CYP2C9*1/*3 and CYP2C9*2/*2 variant. Four subjects carried the VKORC1 wild-type (CC) and eight subjects carried variant VKORC1 alleles including six with heterozygous (CT) and two with homozygous (TT) alleles.

All subjects completed the study and no adverse events related to either of the herbal medicines or warfarin were reported. One subject received a dose of 20 mg (rather than 25 mg) in the warfarin only treatment arm. The pharmacokinetic data for this subject were included after dose normalization but the pharmacodynamic data (AUC_INR, INR_max) were excluded from the final analysis.

Pharmacokinetics of warfarin

Figures 1 and 2 present the concentration–time profiles of (S)- and (R)-warfarin, respectively, in the different treatments arms of this study. Co-administration of echinacea with warfarin led to a significant decreased in the AUC of (S)-warfarin (Table 1), although the magnitude of the mean change was small (8%). Moreover, for the majority of subjects (n= 10), the apparent total clearance of (S)-warfarin in the warfarin and echinacea treatment arm tended to be higher when compared with the warfarin only treatment arm. However, the pharmacokinetics of (R)-warfarin (Table 1) were not affected by co-administration with echinacea, nor did echinacea significantly affect the unbound plasma concentrations of either (S)- or (R)-warfarin (Table 1).

Figure 1.

Plasma concentration–time profile of (S)-warfarin after a single oral dose of warfarin either alone (solid squares) or after multiple doses of echinacea tablets (open squares) or policosanol tablets (open triangles). Values are presented as means ± SD

Figure 2.

Plasma concentration–time profile of (R)-warfarin after a single oral dose of warfarin alone (solid squares) or after multiple doses of echinacea tablets (open squares) or policosanol tablets (open triangles). Values are presented as mean ± SD

Table 1.

Warfarin pharmacokinetic parameters of (S)- and (R)-warfarin (n= 12)

	Mean (95% CI)			Geometric mean ratio (90% CI)
Parameters	Warfarin	Warfarin and policosanol	Warfarin and echinacea	Warfarin and policosanol to warfarin only	Warfarin and echinacea to warfarin only
(S)-warfarin
tmax (h)	1.7 (1.1, 2.2)	1.2 (1.0, 1.4)	1.9 (1.4, 2.4)	NA	NA
Cmax (µg ml−1)	1.3 (1.1, 1.6)	1.4 (1.2, 1.7)	1.3 (1.1, 1.43)	1.09 (0.97, 1.24)	0.97 (0.86, 1.10)
t1/2 (h)	38.6 (34.3, 43.0)	38.3 (34.7, 41.8)	36.5 (34.1, 38.8)	0.99 (0.88, 1.13)	0.95 (0.84, 1.08)
AUC(0,∞) (µg ml−1 h)	53.9 (42.9, 64.8)	51.9 (40.9, 62.8)	49.0 (40.0, 57.9)	0.96 (0.89, 1.04)	0.92 (0.85, 0.99)
CL/F (ml h−1)	267.3 (198.4, 336.3)	278.0 (206.5, 349.4)	289.7 (218.2, 361.1)	1.04 (0.97, 1.12)	1.09 (1.01, 1.18)
V/F (l kg−1)	0.21 (0.16, 0.27)	0.23 (0.16, 0.30)	0.23 (0.16, 0.30)	1.03 (0.97, 1.12)	1.09 (1.03, 1.18)
Fraction unbound (fu)	0.01 (0.01, 0.01)	0.01 (0.01, 0.02)	0.01 (0.01, 0.02)	1.08 (0.86, 1.30)	1.01 (0.68, 1.34)
(R)-warfarin
tmax (h)	2.0 (1.3, 2.6)	1.2 (1.0, 1.4)	2.0 (1.5, 2.4)	NA	NA
Cmax (µg ml−1)	1.3 (1.1, 1.5)	1.4 (1.2, 1.6)	1.2 (1.1, 1.4)	1.09 (0.98, 1.23)	0.98 (0.88, 1.10)
t1/2 (h)	50.6 (44.9, 56.2)	50.9 (44.4, 57.4)	49.2 (44.3, 54.2)	1.00 (0.90, 1.11)	0.98 (0.88, 1.10)
AUC(0,∞) (µg ml−1 h)	79.4 (65.4, 93.4)	77.8 (64.8, 90.8)	74.9 (62.5, 87.2)	0.98 (0.91, 1.06)	0.95 (0.88, 1.03)
CL/F (ml h−1)	178.3 (132.7, 223.4)	181.7 (134.0, 229.4)	186.4 (140.5, 232.2)	1.02 (0.94, 1.10)	1.10 (0.97, 1.14)
V/F (l kg−1)	0.19 (0.13, 0.25)	0.20 (0.13, 0.26)	0.20 (0.13, 0.26)	1.02 (0.92, 1.13)	1.03 (0.93, 1.14)
Fraction unbound (fu)	0.01 (0.01, 0.02)	0.01 (0.01, 0.02)	0.01 (0.01, 0.02)	1.06 (0.89, 1.23)	1.02 (0.78, 1.26)

Co-administration of policosanol did not affect the pharmacokinetics of either (S)- or (R)-warfarin (Figures 1 and 2; Table 1).

Pharmacodynamic end points

Co-administration of either echinacea or policosanol did not significantly affect warfarin pharmacodynamics (Table 2). This analysis included pharmacodynamic data for 11 of the 12 subjects as one subject received a lower dose of warfarin and was therefore excluded. Neither echinacea nor policosanol altered the platelet aggregation induced by the three different platelet agonists (ADP, arachidonic acid and collagen) (Table 3).

Table 2.

Warfarin pharmacodynamics during treatment with herbal medicines (n= 11)

	Mean (95 % CI)			Geometric mean ratio (90% CI)
Parameter	Warfarin	Warfarin and policosanol	Warfarin and echinacea	Warfarin and policosanol to warfarin only	Warfarin and echinacea to warfarin only
INR0	1.0 (1.0, 1.0)	1.0 (1.0, 1.1)	1.0 (1.0, 1.1)	1.04 (1.01, 1.07)	1.01 (0.98, 1.04)
INRmax	1.7 (1.5, 1.9)	1.9 (1.7, 2.1)	1.8 (1.6, 2.0)	1.08 (0.99, 1.18)	1.04 (0.95, 1.13)
AUCINR	52.7 (38.3, 67.2)	55.2 (41.2, 69.2)	55.2 (42.8, 67.6)	1.11 (0.92, 1.33)	1.09 (0.91, 1.31)

Table 3.

Platelet aggregation (Ohms) after 2 weeks of echinacea and policosanol pre-treatment (n= 12)

	Control	Policosanol treatment		Echinacea treatment
Agonist	Mean (95% CI)	Mean (95% CI)	Geometric mean ratio to control (90% CI)	Mean (95% CI)	Geometric mean ratio to control (90% CI)
ADP*	7.8 (5.5, 10.0)	6.8 (4.9, 8.8)	0.92 (0.64, 1.30)	7.5 (4.1, 10.8)	0.84 (0.59, 1.19)
Arachidonic acid	11.0 (9.6, 12.5)	11.2 (9.4, 12.9)	1.00 (0.82, 1.23)	12.1 (9.3, 15.0)	1.06 (0.86, 1.30)
Collagen	13.0 (11.5, 14.6)	14.9 (12.6, 17.2)	1.13 (0.93, 1.37)	15.8 (12.2, 19.5)	1.17 (0.97, 1.42)

^*Adenosine diphosphate.

CYP2C9 and VKORC1 genotype (S)-warfarin pharmacokinetics and interaction with herbal medicines

The apparent clearance of (S)-warfarin was CYP2C9 genotype dependent in the order of CYP2C9*1/*1 > CYP2C9*1/*2 > CYP2C9*1/*3 > CYP2C9*2/*2 in all the treatment arms (data not shown). This study found no evidence of any apparent effect of CYP2C9 or VKORC1 genotype on the interaction with neither policosanol or echinacea and warfarin.

Pharmacokinetics of echinacea alkamides

The echinacea herbal medicine product used in the study was found to contain the following major echinacea alkamides; 2,4-dienes, 0.88 mg/tablet; 2-ene, 0.66 mg/tablet and tetraene, 0.93 mg/tablet. Trough plasma concentrations of the major echinacea alkamide tetraene ranged from 1 to 23 ng ml⁻¹, and the C_max was found to range from 13 to 65 ng ml⁻¹. These data supported the quality of the echinacea tablets used in this investigation and confirm that subjects were taking the echinacea herbal medicine product.

Discussion

Warfarin exhibits stereo- and regio-selective metabolism [19], with the more potent (S)-warfarin being metabolized by CYP2C9 and CYP3A4 [19] and the less active (R)-enantiomer being metabolized by CYP3A4 and CYP1A2 [19]. This metabolic profile, together with the well-recognised narrow therapeutic index of warfarin, and the likelihood that herbal medicines will be ingested concomitantly with warfarin, provided the rationale for investigating the possible interaction between echinacea and warfarin, and policosanol and warfarin.

However, this study found that echinacea treatment only slightly increased the apparent total clearance of the more potent (S)-warfarin and did not affect the pharmacokinetics of the (R)-enantiomer. Interestingly, this effect on the pharmacokinetics of (S)-warfarin was not associated with a significant effect on warfarin pharmacodynamics. Policosanol had no effect on either the pharmacokinetics or pharmacodynamics of warfarin. Neither herbal medicine altered platelet aggregation.

Gorski et al. [9] have reported that, in vivo, echinacea inhibited hepatic CYP1A2 enzymes (27% decrease in caffeine clearance) and to a lesser extent, hepatic CYP2C9 (12% decrease in tolbutamide clearance). Echinacea has also been reported to induce hepatic CYP3A4 enzyme activity (a 42% increase in the systematic clearance of midazolam) [9]. CYP3A4 and CYP2C9 enzymes have been reported to metabolize one of the main alkamide constituents of echinacea in vitro[20]. However, the lack of effect of echinacea on the pharmacodynamics of warfarin is reported here despite using a herbal medicine product of known quality that was demonstrated to yield quantifiable concentrations of echinacea constituents in the plasma of all subjects.

The study by Gorski et al. [9] employed echinacea tablets containing Echinacea purpurea root that contained greater than 1% phenols (caftaric acid, chlorogenic acid, echinacoside and cichoric acid) and also alkamides (2-ene and 2,4-dienes). This study and that by Matthias et al. [5] reported that alkamides from echinacea are orally available. By contrast, Matthias et al. found that phenols were not detected in blood samples after oral administration of tablets formulated from either E. purpurea or E. angustifolia[5]. In this study we were also able to quantitate the alkamide constituents of echinacea (prepared from roots of E. angustifolia and E. purpurea) during multiple dosing. The echinacea product used in this study contained similar constituents to the echinacea formulation used by Gorski et al. [9] which in addition also contained constituents from E. angustifolia roots. Similarities between the E. purpurea root and E. angustifolia root in terms of alkylamide content were reported. Furthermore pharmacokinetic parameters of alkamides of combination product of E. purpurea and E. angustifolia root and of E. angustifolia root alone were reported to be similar [5, 6, 21] and hence the results from the study Gorski et al. [9] can be compared with the present study with respect to alkylamide pharmacokinetics.

Policosanol has been reported to be effective in lowering serum cholesterol concentrations in both healthy subjects and in patients [22, 23]. However, recent studies provide conflicting data [24]. The optimal cholesterol lowering dose of policosanol is reported to be 20 mg daily [25] and dose-dependent effects have been reported with respect to platelet aggregation [26]. In the present study the possibility of a policosanol-warfarin interaction was investigated at a policosanol dose of 20 mg day⁻¹. Previous studies have reported that policosanol up to this dose inhibited platelet aggregation induced by ADP and collagen by more than 20% [27]. Interestingly, the present study showed no significant alteration in platelet aggregation after 2 weeks of treatment with 20 mg daily policosanol. The reason for the discrepancy with previous studies remains unclear. The sample size of this study is comparable with the previous studies in terms of the subjects who received the intervention. One possibility is that the observed differences may be attributable to variations in the constituents in policosanol used in each study. Typically, policosanol is standardized for octacosanol content, which is expected to contribute to its cholesterol lowering effects, and it is possible that other constituents are responsible for platelet effects. Reiner et al. [28] reported no change in blood coagulation factors after 8 weeks pre-treatment with rice policosanol 10 mg daily in hypercholesterolaemic patients from a randomized, double-blind, placebo-controlled, crossover trial which is in agreement with the findings from the present study.

This study found no significant relationship between CYP2C9 or VKORC1 genotype and interactions with these herbal medicines, although genotype-dependent interactions have been previously reported [29, 30]. However, the relatively small sample size for each genotype in the present study limits further interpretation of these findings.

In conclusion, this study found no clinically significant pharmacokinetic or pharmacodynamic interaction between echinacea or policosanol and warfarin in healthy male subjects. While treatment with echinacea tended to increase the apparent clearance of (S)-warfarin, this did not alter the pharmacodynamic response to warfarin. The results of the present study cannot unequivocally exclude the possibility of a herb-drug interaction in patients receiving warfarin as this study did not include subjects with possible contributing factors (such as older age, concomitant medicines, co-existing medical conditions and organ dysfunction).

Competing interests

K.M.W and R.O.D have received research funds from Blackmores, B.D.R has acted as a paid consultant for various manufacturers of herbal medicine products and A.M. and R.P.L. are employees of MediHerb Research Laboratories.

The authors acknowledge the financial support of a Project grant from the Australian National Health and Medical Research Council and the clinical support from the staff at the St Vincent's Clinical Trial Centre (Darlinghurst, NSW, Australia). The authors also acknowledge Dr Heather James (Institute of Medical and Veterinary Science, Adelaide, Australia) for the VKORC1 genotyping. Policosanol tablets were a generous gift from Blackmores, Australia, and echinacea was purchased from MediHerb Pty.