Due to its potentially beneficial impact on human health, the polyphenol quercetin has come into the focus of medicinal interest. However, data on the bioavailability of quercetin after oral intake are scarce and contradictory. Previous investigations indicate that the disposition of quercetin may depend on the sugar moiety of the glycoside or the plant matrix. To determine the influence of the sugar moiety or matrix on the absorption of quercetin, two isolated quercetin glycosides and two plant extracts were administered to 12 healthy volunteers in a four-way crossover study. Each subject received an onion supplement or quercetin-4'-O-glucoside (both equivalent to 100 mg quercetin), as well as quercetin-3-O-rutinoside and buckwheat tea (both equivalent to 200 mg quercetin). Samples were analyzed by HPLC with a 12-channel coulometric array detector. In human plasma, only quercetin glucuronides, but no free quercetin, could be detected. There was no significant difference in the bioavailability and pharmacokinetic parameters between the onion supplement and quercetin-4'-O-glucoside. Peak plasma concentrations were 2.3 +/- 1.5 microg x mL(-1) and 2.1 +/- 1.6 microg x mL(-1) (mean +/- SD) and were reached after 0.7 +/- 0.2 hours and 0.7 +/- 0.3 hours, respectively. After administration of buckwheat tea and rutin, however, peak plasma levels were--despite the higher dose-only 0.6 +/- 0.7 microg x mL(-1) and 0.3 +/- 0.3 microg x mL(-1), respectively. Peak concentrations were reached 4.3 +/- 1.8 hours after administration of buckwheat tea and 7.0 +/- 2.9 hours after ingestion of rutin. The terminal elimination half-life was about 11 hours for all treatments. Thus, the disposition of quercetin in humans primarily depends on the sugar moiety. To a minor extent, the plant matrix influences both the rate and extent of absorption in the case of buckwheat tea administration compared with the isolated compound. The site of absorption seems to be different for quercetin-4'-O-glucoside and quercetin-3-O-rutinoside. The significance of specific carriers on the absorption of quercetin glycosides, as well as specific intestinal beta-glucosidases, needs to be further evaluated.

Herbal medicinal products containing natural volatiles are used in the treatment of gastrointestinal diseases, pain, colds and bronchitis. Many pharmacological studies report a wide variety of in vitro effects, with anti-inflammatory and antimicrobial activities investigated most frequently. In comparison, relatively few studies on the bioavailability and pharmacokinetics have been carried out. Thus, the relevance of the in vitro activity to the therapeutic effects found in individual studies or documented in textbooks of phytotherapy is still not established. Further studies with essential oils and their single compounds providing supporting evidence of efficacy and demonstrating systemic availability are necessary. Such data could also be important in the context of safety.Key words: Terpenes, volatiles, herbal medicinal products, bioavailability, pharmacokinetics. IntroductionEssential oils are mixtures of lipophilic, liquid, volatile, and often terpenoid compounds present in higher plants. More than 3000 compounds have been described so far (1). The clinical efficacy of volatiles is particularly well-established for chronic pulmonary obstruction and acute bronchitis. For these indications clinical trials have been carried out with products containing 1,8-cineole (2), (3), (4), standardised myrtol (1,8-cineole, a-pinene, limonene) (5) and thyme extract (6), (7). Further clinical studies have been carried out with peppermint oil for the treatment of irritable bowel syndrome (8), (9), non-ulcer dyspepsia (10), and tension-type headache (11). Numerous essential oils and their components have shown antimicrobial or antimycotic activity in in vitro studies (12), (13), (14), (15), (16).Various other in vitro activities of volatile oils or compounds have been reported. However, the clinical relevance of these activities depends on the systemic availability of these compounds in the respective target organs. Thus, investigation of absorption, distribution and metabolism is necessary to link in vitro with in vivo data. They may also be important in context of the safety of herbal medicinal products containing natural volatiles. However, pharmacokinetics of volatile natural compounds have not yet been investigated satisfactorily. For several monoterpenoid and phenylpropanoid compounds there is a large amount of experimental data, but crucially ± especially with respect to humans ± pharmacokinetic data are lacking.This article reviews the data currently available on the systemic fate of natural volatile terpenes and phenylpropanes. Unfortunately, methodological details, particularly concerning validation of the analytical methods used have not been published for most of the studies. Therefore the results of those studies discussed here have to be considered cautiously, since the assays used might not match the requirements for analytical validation. Absorption and Systemic Availability Dermal absorptionPreparations of eucalyptus oil or mountain pine oil containing a-and b-pinene, camphor, 3-carene, and limonen...

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