REVIEW pubs.acs.org/crt
Constituents in Kava Extracts Potentially Involved in Hepatotoxicity: A Review Line R. Olsen,*,† Mark P. Grillo,‡ and Christian Skonberg† † ‡
Department of Pharmaceutics and Analytical Chemistry, Faculty of Pharmaceutical Sciences, University of Copenhagen, Denmark Department of Pharmacokinetics and Drug Metabolism, Amgen Inc., South San Francisco, California ABSTRACT: Aqueous kava root preparations have been consumed in the South Pacific as an apparently safe ceremonial and cultural drink for centuries. However, several reports of hepatotoxicity have been linked to the consumption of kava extracts in Western countries, where mainly ethanolic or acetonic extracts are used. The mechanism of toxicity has not been established, although several theories have been put forward. The composition of the major constituents, the kava lactones, varies according to preparation method and species of kava plant, and thus, the toxicity of the individual lactones has been tested in order to establish whether a single lactone or a certain composition of lactones may be responsible for the increased prevalence of kava-induced hepatotoxicity in Western countries. However, no such conclusion has been made on the basis of current data. Inhibition or induction of the major metabolizing enzymes, which might result in drug interactions, has also gained attention, but ambiguous results have been reported. On the basis of the chemical structures of kava constituents, the formation of reactive metabolites has also been suggested as an explanation of toxicity. Furthermore, skin rash is a side effect in kava consumers, which may be indicative of the formation of reactive metabolites and covalent binding to skin proteins leading to immune-mediated responses. Reactive metabolites of kava lactones have been identified in vitro as glutathione (GSH) conjugates and in vivo as mercapturates excreted in urine. Addition of GSH to kava extracts has been shown to reduce cytotoxicity in vitro, which suggests the presence of inherently reactive constituents. Only a few studies have investigated the toxicity of the minor constituents present in kava extract, such as pipermethystine and the flavokavains, where some have been shown to display higher in vitro cytotoxicity than the lactones. To date, there remains no indisputable reason for the increased prevalence of kava-induced hepatotoxicity in Western countries.
’ CONTENTS 1. 2. 3. 4. 5. 6. 7.
Introduction Constituents of Kava Preparation Technique Composition of Lactones in Extraction Solvents In Vivo Toxicity Testing Lactone Cytotoxicity Alteration of Cytochrome P450 Activity 7.1. In Vitro Inhibitory Effect on P450 Activity by Crude Kava Extract 7.2. In Vitro Inhibitory Effect on P450 Enzymes by Individual Kava Lactones 7.3. In Vitro Inhibition of Transporters 7.4. In Vivo Effect on P450 Enzymes by Crude Kava Extract 8. P450 Isoenzyme Polymorphisms 9. Cytotoxicity of Minor Kava Constituents 10. Role of GSH Depletion by Kava 11. Reactive Metabolites r 2011 American Chemical Society
12. Chemically Reactive Kava Constituents 13. Further Studies 14. Conclusions Author Information References
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1. INTRODUCTION Aqueous extracts from the root of the kava plant (Piper methysticum, Forst. f.) have been used as an apparently safe ceremonial and social drink in the South Pacific for centuries.13 However, following the introduction of kava preparations to markets in Western countries, including Europe and the USA, there have been several reports of suspected kava-induced liver toxicity, and products containing kava extracts were therefore banned for a period of time in a number of European countries, the USA, and Canada.47 The legal status of kava-containing product is today unclear in a number of countries. The cause of
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Received: November 29, 2010 Published: April 20, 2011 992
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hepatotoxicity has not yet been established, although several investigative attempts have been made to determine the identity of the potentially hepatotoxic constituents present in kava extracts. Several examples exist in the literature, reporting on clinical findings and outcomes of the reported cases where ingestion of kava was a probable or possible reason for toxicity. These cases have been reviewed more thoroughly elsewhere.810 Most of the cases involve elevated liver enzymes (AST, ALT, and γ-GT), elevated bilirubin levels, hepatitis, and jaundice along with hepatocellular necrosis. There seems to be no apparent relation to age, but according to some reports, the majority of the reviewed cases are female.8,10 Furthermore, according to the WHO report from 2007, there was generally a delay of several months (average 111 days) or days from first dose to the onset of liver problems.8 In the reported cases, kava extracts from ethanol, acetone, or water, along with synthetic kava products, had been ingested, but in about half the cases, the type of extract was not identified. The WHO report was inconclusive concerning the mechanism(s) involved in kava hepatotoxicity but mentioned both immunologic and metabolic idiosyncratic reactions as possibilities. Instead, the various risk factors involved were defined, which include the use of organic extracts, excessive alcohol consumption, comedication, pre-existing liver disease, kava overdose, and genetic polymorphisms of P450 enzymes.8 The consumption of alcohol as a confounding factor in kava toxicity has not been proven clinically but should probably be regarded as a preventive measure by the WHO, on the basis of some of the toxicity case reports.8 One in vivo study in mice did find increased toxicity when kava extract and alcohol were coadminstered,11 but the doses used have been criticized for being outside the therapeutically relevant range.12,13 Furthermore, Clough et al. did not find any confounding effect of alcohol use on liver function tests in users of aqueous kava extracts.14 The WHO risk assessment has been criticized due to poor data quality and the causality assessment method applied.15,16 This critique has led to further evaluation of the case reports, using other causality assessment methods resulting in different conclusions in the individual case reports, suggesting that hepatotoxicity involving kava may not be a question of extraction method but rather a matter of overdose or poor quality of raw material.15,17 The original warning from the US FDA about kava was based on a number of case stories of hepatotoxicity involving nonaqueous preparations or dietary supplements.18 This, and other case stories, may have led to the notion that traditional aqueous preparations were safer than Western commercial preparations. However, a number of studies following the use among traditional kava users (i.e., in the South Pacific and among Australian Aboriginals) have shown that dermatological reactions (urticaria and dermopathy) and elevated liver enzymes are also prevalent, if not common among these users.9,19,20 These reactions are generally reported to be reversible upon the cessation of kava consumption. However, one 2009 publication reported a tourist presenting with overt liver toxicity after participation in a traditional kava ceremony.21 Clinical trials, assessing the efficacy of kava, have not revealed any hepatotoxic side effects among the participants. A metaanalysis of 7 randomized clinical trials reported gastrointestinal side effects, restlessness, drowsiness, tremor, headache, and tiredness as the most frequent adverse effects described.22 Also, two postmarketing surveillance studies monitoring 4049 and 3029 adults taking commercial kava extract (150 and 240 mg/day, respectively) revealed adverse events in 1.5% and 2.3% of the cases,
Figure 1. Major constituents of kava.
Figure 2. Structures of alkaloids in different species of Piper methysticum.
respectively. These were mainly gastrointestinal complaints or allergic reactions and ceased as kava treatment was discontinued.23 This review will cover the current published research concerning the chemical kava constituents likely to be involved in kava toxicity, in order to shed some light on the possible mechanisms of toxicity.
2. CONSTITUENTS OF KAVA The pharmacologically active compounds in kava are the kava lactones.2 They are responsible for the soothing and intoxicating effect of kava. To date, 18 kava lactones have been identified from organic extracts of kava root. The six compounds depicted in Figure 1 are the major lactones present in kava root and account for 320% dry weight and approximately 96% of the constituents in organic extracts.2,5,24 Other important constituents have been isolated from various parts of the plant. Dragull et al.25 examined the aerial parts of the plant and characterized the relative amount of three alkaloids in the stems and leaves of 11 different kava species. These alkaloids included pipermethystine, 3R,4R-epoxy-5β-pipermethystine, and awaine (Figure 2). 993
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4. COMPOSITION OF LACTONES IN EXTRACTION SOLVENTS A limited number of studies have focused on analyzing the composition of lactones obtained when using varied extraction solvents. Cote et al.35 examined the lactone composition in aqueous and organic extracts of kava root and compared it to the composition in commercially available kava caplets. Three different organic solvents were used (acetone, ethanol, and methanol), and all three resulted in a higher yield of lactones than in the corresponding aqueous extract. Furthermore, the composition of lactones differed significantly as concentrations of yangonin and desmethoxyyangonin were very low in the aqueous extract compared to that in the organic extracts and the commercially available caplets. The concentrations of kavain and dihydrokavain were much higher in the commercial product compared to that in both the organic and aqueous extracts, whereas the composition of the remaining lactones in the commercial product was relatively similar to that in the organic root extract. Loew and Franz36 examined aqueous, acetone, toluene, and ethanol kava extracts using thin layer chromatography. The extracts were produced from powdered kava rhizomes, and it was demonstrated that the qualitative outcome was comparable, although it was clearly demonstrated that polar extraction solvents were less efficient in extracting kava lactones.
Figure 3. Structures of flavokavains A, B, and C.
Pipermethystine and awaine were found in all the tested species in various amounts, but only one variant of the 11 examined species was shown to contain 3R,4R-epoxy-5β-pipermethystine, namely, Piper methysticum var. Isa. This was an interesting observation since the cultivar Isa is rarely used for traditional drinking purposes because it causes prolonged nausea.26 However, it is a popular species in the herbal industry due to its resistance toward kava dieback, a viral disease most important for kava yield reduction in the South Pacific.27 It is not surprising that the composition of the constituents has been determined to vary in accordance with species of the plant.2,26 The differences in composition have proven to cause varied intoxicating effects in the consumers of traditionally prepared kava extracts due to differences in absorption rate, half-life, and potency of the different lactones.26,28 For example, the cultivar called Tudei is so named because of the duration of the effect, which allegedly lasts two days due to the high content of dihydromethysticin.2,26 Minor kava constituents (
