Comparison of in vivo and in vitro rat hepatic toxicity of coumarin and methyl analogues, and application of quantitative morphometry to toxicity in vivo

Combined Risk Assessment of Food-derived Coumarin with in Silico Approaches

Hepatotoxicity associated with food-derived coumarin occurs occasionally in humans. We have, herein, assessed the data of existing clinical and nonclinical studies as well as those of in silico models for humans in order to shed more light on this association. The average intakes of food-derived coumarin are estimated to be 1-3 mg/day, while a ten-times higher level is expected in the worst-case scenarios. These levels are close to or above the tolerable daily intake suggested by a chronic study in dogs. The human internal exposure levels were estimated by a physiologically-based pharmacokinetic model with the use of virtual doses of coumarin in the amounts expected to derive from foods. Our results suggest that: (i) coumarin can be cleared rapidly via 7-hydroxylation in humans, and (ii) the plasma levels of coumarin and of its metabolite, o-hydroxyphenylacetic acid associated with hepatotoxicity, are considerably lower than those yielding hepatotoxicity in rats. Pharmacokinetic data suggest a low or negligible concern regarding a coumarin-induced hepatotoxicity in humans exposed to an average intake from foods. Detoxification of coumarin through the 7-hydroxylation, however, might vary among individuals due to genetic polymorphisms in CYP2A6 enzyme. In addition, the CYP1A2- and CYP2E1-mediated activation of coumarin can fluctuate as a result of induction caused by environmental factors. Furthermore, the daily consumption of food-contained coumarin was implicated in the potential risk of hepatotoxicity by the drug-induced liver injury score model developed by the US Food and Drug Administration. These results support the idea of the existence of human subpopulations that are highly sensitive to coumarin; therefore, a more precise risk assessment is needed. The present study also highlights the usefulness of in silico approaches of pharmacokinetics with the liver injury score model as battery components of a risk assessment.

In vitro platelet antiaggregatory properties of 4-methylcoumarins

Platelets play a crucial role in physiological haemostasis. However, in coronary arteries damaged by atherosclerosis, enhanced platelet aggregation, with subsequent thrombus formation, is a precipitating factor in acute myocardial infarction. Current therapeutic approaches are able to reduce approximately one quarter of cardiovascular events, but they are associated with an increased risk of bleeding and in some resistant patients are not efficient. Some coumarins possess antiplatelet activity and, due to their additional antioxidant effects, may be promising drugs for use in combination with the present therapeutic agents. The aim of this study was to analyse a series of simple 4-methylcoumarins for their antiplatelet activity. Human plasma platelet suspensions were treated with different aggregation inducers [arachidonic acid (AA), collagen and ADP] in the presence of the 4-methylcoumarins. Complementary experiments were performed to explain the mechanism of action. 5,7-Dihydroxy-4-methylcoumarins, in particular those containing a lipophilic side chain at C-3, reached the activity of acetylsalicylic acid on AA-induced aggregation. Other tested coumarins were less active. Some of the tested compounds mildly inhibited either collagen- or ADP-induced aggregation. 5,7-Dihydroxy-4-methylcoumarins did not interfere with the function of thromboxane synthase, but were competitive antagonists of thromboxane A(2) receptors and inhibited cyclooxygenase-1 as well. 5,7-Dihydroxy-4-methylcoumarins appear to be promising candidates for the extension of the current spectrum of antiplatelet drugs.

Synthesis and antifungal activity of derivatives of 2- and 3-benzofurancarboxylic acids

We found that amiodarone has potent antifungal activity against a broad range of fungi, potentially defining a new class of antimycotics. Investigations into its molecular mechanisms showed amiodarone mobilized intracellular Ca2+, which is thought to be an important antifungal characteristic of its fungicidal activity. Amiodarone is a synthetic drug based on the benzofuran ring system, which is contained in numerous compounds that are both synthetic and isolated from natural sources with antifungal activity. To define the structural components responsible for antifungal activity, we synthesized a series of benzofuran derivatives and tested them for the inhibition of growth of two pathogenic fungi, Cryptococcus neoformans and Aspergillus fumigatus, to find new compounds with antifungal activity. We found several derivatives that inhibited fungal growth, two of which had significant antifungal activity. We were surprised to find that calcium fluxes in cells treated with these derivatives did not correlate directly with their antifungal effects; however, the derivatives did augment the amiodarone-elicited calcium flux into the cytoplasm. We conclude that antifungal activity of these new compounds includes changes in cytoplasmic calcium concentration. Analyses of these benzofuran derivatives suggest that certain structural features are important for antifungal activity. Antifungal activity drastically increased on converting methyl 7-acetyl-6-hydroxy-3-methyl-2-benzofurancarboxylate (2b) into its dibromo derivative, methyl 7-acetyl-5-bromo-6-hydroxy-3-bromomethyl-2-benzofurancarboxylate (4).

Aquatic toxicity assessment of the additive 6-methylcoumarine using four experimental systems

The toxicity assessment of chemicals is one of the main issues in the current policies in order to protect the health of the environment and human beings. Food and cosmetic additives have been extensively studied in relation to their toxicity to humans, but data about their ecotoxicological effects are scarce. The aim of this study was to evaluate the toxic effects of the additive 6-methylcoumarine in the aquatic milieu using a test battery comprising experimental model systems from different trophic levels. The inhibition of bioluminiscence was studied in the bacteria Vibrio fischeri (decomposer), the inhibition of growth was evaluated in the alga Chlorella vulgaris (producer) and immobilization was studied in the cladoceran Daphnia magna (first consumer). Finally, several end points were evaluated in the RTG-2 salmonid fish cell line, including neutral red uptake, protein content, methylthiazol tetrazolium salt metabolization, glucose-6-phosphate dehydrogenase activity, lactate dehydrogenase activity and leakage, and morphology. The sensitivity of the test systems employed was as follows: V. fischeri > D. magna > C. vulgaris > RTG-2 cell line. The results show that 6-methylcoumarine is not expected to produce acute toxic effects on the aquatic biota. However, chronic and synergistic effects with other chemicals cannot be excluded and should be further investigated.

Selectivity of 7-alkoxycoumarins as probe substrates for rat hepatic cytochrome P450 forms is influenced by the substitution pattern on the coumarin nucleus

1. The O-dealkylation of 7-alkoxycoumarins is widely used as an assay to characterize cytochrome P450 (CYP) activity. These substrates can also undergo oxidative attack at additional sites on the coumarin nucleus, which may influence their apparent selectivity for particular CYP forms. 2. Accordingly, the effect of blockade of these additional sites was investigated on the selectivity towards rat hepatic CYP forms, with emphasis on the CYP1A and 2B forms. 3. Blockade of the 3-/4- and 6-positions resulted in substrates for which the CYP1A1/2 selectivity of the unsubstituted 7-alkoxycoumarins was altered to a CYP2B selectivity; this was achieved with little overall change in the molecular dimensions of the substrate. Limited analysis of other inducible CYP forms indicated at most only small effects of structure modification on activity. 4. The findings suggest that the sensitivity of probe substrates for CYP forms may be limited by the occurrence of competing side reactions of the substrate, and that better probes may be derived by blocking the sites of these side reactions.

Evaluation of the genotoxic potential of the isocoumarin paepalantine in in vivo and in vitro mammalian systems

Paepalantine is an isocoumarin isolated from Paepalanthus vellozioides which showed antimicrobial activity in in vitro experiments. In the present study, paepalantine was tested for possible clastogenic and cytotoxic action. Cultures from different individuals were treated with paepalantine at concentrations of 20, 40 and 80 microg/ml. The effect of isocoumarin was also tested in an in vivo assay using Wistar rat bone marrow cells. Paepalantine was administered intraperitoneally at concentrations of 6.25, 12.5 and 25 mg/kg body weight. Under these conditions paepalantine did not have a clastogenic effect, but was significantly cytotoxic in the in vitro and in vivo mammalian cell systems tested in the present work.

Coumarin metabolism, toxicity and carcinogenicity: relevance for human risk assessment

The metabolism, toxicity and results of tests for carcinogenicity have been reviewed with respect to the safety for humans of coumarin present in foodstuffs and from fragrance use in cosmetic products. Coumarin is a natural product which exhibits marked species differences in both metabolism and toxicity. The majority of tests for mutagenic and genotoxic potential suggest that coumarin is not a genotoxic agent. The target organs for toxicity and carcinogenicity in the rat and mouse are primarily the liver and lung. Moreover, the dose-response relationships for coumarin-induced toxicity and carcinogenicity are non-linear, with tumour formation only being observed at high doses which are associated with hepatic and pulmonary toxicity. Other species, including the Syrian hamster, are seemingly resistant to coumarin-induced toxicity. There are marked differences in coumarin metabolism between susceptible rodent species and other species including humans. It appears that the 7-hydroxylation pathway of coumarin metabolism, the major pathway in most human subjects but only a minor pathway in the rat and mouse, is a detoxification pathway. In contrast, the major route of coumarin metabolism in the rat and mouse is by a 3,4-epoxidation pathway resulting in the formation of toxic metabolites. The maximum daily human exposure to coumarin from dietary sources for a 60-kg consumer has been estimated to be 0.02 mg/kg/day. From fragrance use in cosmetic products, coumarin exposure has been estimated to be 0.04 mg/kg/day. The total daily human exposure from dietary sources together with fragrance use in cosmetic products is thus 0.06 mg/kg/day. No adverse effects of coumarin have been reported in susceptible species in response to doses which are more than 100 times the maximum human daily intake. The mechanism of coumarin-induced tumour formation in rodents is associated with metabolism-mediated, toxicity and it is concluded that exposure to coumarin from food and/or cosmetic products poses no health risk to humans.

Depression of glutathione content, elevation of CYP2E1-dependent activation, and the principal determinant of the fasting-mediated enhancement of 1,3-dichloro-2-propanol hepatotoxicity in the rat

The influence of fasting (18 hours) on the hepatotoxicity of 1,3-dichloro-2-propanol (1,3-DCP) and on various hepatic parameters has been assessed in the rat. Fasting produced an enhancement of the hepatotoxicity which was associated with alterations in a variety of hepatic parameters when measured relative to protein content, most notably glutathione (GSH) levels (decrease) and CYP2E1-mediated enzyme activity (increase), two parameters previously identified as being important determinants to the toxicity. Fasting also decreased the liver weight normalized to body weight. When this was taken into account, total liver CYP2E1-mediated enzyme activity was not significantly altered whereas the total liver GSH level was markedly reduced following fasting. These results imply that the reduction in hepatic GSH is the principal determinant of the enhanced susceptibility to 1,3-DCP hepatotoxicity following fasting.

Low-dose diethyldithiocarbamate attenuates the hepatotoxicity of 1,3-dichloro-2-propanol and selectively inhibits CYP2E1 activity in the rat

The effect of low doses of diethyldithiocarbamate (DEDC) on hepatic cytochrome P450-dependent enzyme activity and 1,3-dichloro-2-propanol (DCP) hepatotoxicity in the rat have been investigated. DEDC at a dose of 5 mg/kg selectively inhibited enzyme markers for CYP2E1 activity, and provided substantial protection against DCP hepatotoxicity. At a higher dose (25 mg/kg), DEDC also inhibited an enzyme marker for CYP1A2 activity and provided complete protection against DCP hepatotoxicity. It is concluded: (a) that DEDC at a dose of 5 mg/kg is a selective CYP2E1 inhibitor in the rat in vivo; and (b) that DCP hepatotoxicity is mediated principally by CYP2E1, with a possible contribution from CYP1A2.

7-Ethoxy-3,4-dimethylcoumarin: a substrate for a cytochrome P450-mediated mono-oxygenase activity that is highly induced by phenobarbitone and beta-naphthoflavone

The O-dealkylation of 7-ethoxy-3,4-dimethylcoumarin in rat liver microsomes was catalysed in a typical cytochrome P450-mediated reaction as judged by cofactor requirement and inhibition criteria, and displayed monophasic Michaelis-Menten kinetics. When measured at low substrate concentration, this activity was highly inducible by treatment with phenobarbitone or beta-naphthoflavone (44- and 78-fold induction, respectively). These data indicate the potential usefulness of this activity as a probe for P4501A1- and P4502B-mediated activities. The O-dealkylation of 7-methoxy- and 7-propoxy-3,4-dimethylcoumarin were much less inducible.

Lobar variation of carbon tetrachloride hepatotoxicity in the rat

The lobar variation in carbon tetrachloride-induced liver damage in the rat has been assessed by use of a systematic random sampling protocol and quantitative morphometry. A random inter-animal lobar variation in severity of damage was apparent, in contrast with the results obtained previously in which sampling bias, small animal numbers, and lack of fully quantitative measurement were apparent. The route of administration (oral vs. i.p.) did not influence these findings.