Lack of effect of coumarin on unscheduled DNA synthesis in precision-cut human liver slices

Coumarin-Induced Hepatotoxicity: A Narrative Review

Coumarin is an effective treatment for primary lymphoedema, as well as lymphoedema related to breast cancer radiotherapy or surgery. However, its clinical use is limited in several countries due to the possible occurrence of hepatotoxicity, mainly in the form of mild to moderate transaminase elevation. It is worth noting that only a few cases of severe hepatotoxicity have been described in the literature, with no reported cases of liver failure. Data available on coumarin absorption, distribution, metabolism, and excretion have been reviewed, focusing on hepatotoxicity studies carried out in vitro and in vivo. Finally, safety and tolerability data from clinical trials have been thoroughly discussed. Based on these data, coumarin-induced hepatotoxicity is restricted to a small subset of patients, probably due to the activation in these individuals of alternative metabolic pathways involving specific CYP450s isoforms. The aim of this work is to stimulate research to clearly identify patients at risk of developing hepatotoxicity following coumarin treatment. Early identification of this subset of patients could open the possibility of more safely exploiting the therapeutical properties of coumarin, allowing patients suffering from lymphoedema to benefit from the anti-oedematous activity of the treatment.

Cancer Hazard Identification Integrating Human Variability: The Case of Coumarin

Coumarin is a naturally occurring sweet-smelling benzopyrone that may be extracted from plants or synthesized for commercial uses. Its uses include as a flavoring agent, fragrance enhancer, and odor-masking additive. We reviewed and evaluated the scientific evidence on the carcinogenicity of coumarin, integrating information from carcinogenicity studies in animals with mechanistic and other relevant data, including data from toxicogenomic, genotoxicity, and metabolism studies, and studies of human variability of a key enzyme, CYP2A6. Increases in tumors were observed in multiple studies in rats and mice in multiple tissues. Our functional pathway analysis identified several common cancer-related biological processes/pathways affected by coumarin in rat liver following in vivo exposure and in human primary hepatocytes exposed in vitro. When coumarin 7-hydroxylation by CYP2A6 is compromised, this can lead to a shift in metabolism to the 3,4-epoxidation pathway and increased generation of electrophilic metabolites. Mechanistic data align with 3 key characteristics of carcinogens, namely formation of electrophilic metabolites, genotoxicity, and induction of oxidative stress. Considerations of metabolism, human variability in CYP2A6 activity, and coumarin hepatotoxicity in susceptible individuals provide additional support for carcinogenicity concern. Our analysis illustrates the importance of integrating information on human variability in the cancer hazard identification process.

The effect of feeding piglets with the diet containing green tea extracts or coumarin on in vitro metabolism of aflatoxin B1 by their tissues

To clarify whether enzymes involved in aflatoxin B1 (AFB1) metabolism in pigs respond to antioxidant agents, the effect of feeding piglets with diets containing green tea extracts (Sunphenon) and coumarin on in vitro AFB1 metabolism by their liver and intestinal tissues was studied. The results showed that coumarin reduced AFB1-DNA adduct formation by both liver and intestinal microsomes, while Sunphenon did not have any effects. Both coumarin and Sunphenon enhanced the glutathione S-transferase (GST) activity to conjugate AFB1 to glutathione GSH in the intestine, although no effects were noted in the liver. Changes of the expression of mRNA of GSTA2 and GSTO1 were not in parallel with the observed changes of GST activity, suggesting that other GST subtypes are involved in the GST activity toward AFB1. As for lipophilic-free AFB1 metabolites, coumarin reduced the liver microsomal conversion of AFB1 to aflatoxin M1 (AFM1) and aflatoxin Q1 (AFQ1), but Sunphenon exerted no effects. Both coumarin and Sunphenon enhanced the conversion of AFB1 to aflatoxicol in the liver. All the results suggest that feeding with a diet containing coumarin affects AFB1 metabolism to enhance AFB1 detoxification through the suppression of P450 enzyme activity in the liver and the enhancement of GST activity in the intestine. Feeding with a diet containing Sunphenon enhances AFB1 detoxification, but the effects are noted mainly in the intestine.

Lack of effect of furfural on unscheduled DNA synthesis in the in vivo rat and mouse hepatocyte DNA repair assays and in precision-cut human liver slices

The ability of furfural to induce unscheduled DNA synthesis (UDS) in hepatocytes of male and female B6C3F(1) mice and male F344 rats after in vivo administration and in vitro in precision-cut human liver slices has been studied. Preliminary toxicity studies established the maximum tolerated dose (MTD) of furfural to be 320 and 50 mg/kg in the mouse and rat, respectively. Furfural was dosed by gavage at levels of 0 (control), 50, 175 and 320 mg/kg to male and female mice and 0, 5, 16.7 and 50 mg/kg to male rats. Hepatocytes were isolated by liver perfusion either 2-4 h or 12-16 h after treatment, cultured in medium containing [3H]thymidine for 4 h and assessed for UDS by grain counting of autoradiographs. Furfural treatment did not produce any statistically significant increase or any dose-related effects on UDS in mouse and rat hepatocytes either 2-4 h or 12-16 h after dosing. In contrast, UDS was markedly induced in mice and rats 2-4 h after treatment with 20 mg/kg dimethylnitrosamine and 12-16 h after treatment of mice and rats with 200 mg/kg o-aminoazotoluene and 50 mg/kg 2-acetylaminofluorene (2-AAF), respectively. Precision-cut human liver slices from four donors were cultured for 24 h in medium containing [3H]thymidine and 0-10 mM furfural. Small increases in the net grain count (i.e. nuclear grain count less mean cytoplasmic grain count) observed with 2-10 mM furfural were not due to any increase in the nuclear grain count. Rather, it was the result of concentration-dependent decreases in the mean cytoplasmic grain counts and to a lesser extent in nuclear grain counts, due to furfural-induced cytotoxicity. In contrast, marked increases in UDS (both net grain and nuclear grain counts) were observed in human liver slices treated with 0.02 and 0.05 mM 2-AAF, 0.002 and 0.02 mM aflatoxin B(1) and 0.005 and 0.05 mM 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine. This study demonstrates that furfural does not induce UDS in the hepatocytes of male and female B6C3F(1) mice and male F344 rats after oral treatment at doses up to the MTDs. Moreover, human liver slice studies suggest that furfural is also not a genotoxic agent in human liver.

Lack of effect of coumarin on the formation of micronuclei in an in vivo mouse micronucleus assay

Coumarin was tested for its potential to cause genotoxic effects in mouse bone marrow cells using an in vivo micronucleus assay. Male and female Swiss mice were administered a single oral dose of coumarin at 50, 100 or 200 mg/kg by gavage in corn oil vehicle. Control animals received only the vehicle. Groups of male mice were also administered mitomycin C at 0.75 mg/kg and served as positive controls. At 24 h after treatment, mice from all dose levels, and at 48 h after treatment, mice from the high dose level only were sacrificed. Bone marrow cells were collected and assayed for the presence of micronuclei. Coumarin did not cause any increase in the incidence of micronucleated polychromatic erythrocytes in male or female mice at any of the dose levels, the positive control mitomycin C produced a significant increase. There was no evidence of coumarin or mitomycin C treatment related cytotoxicity to bone marrow cells. The results of this study demonstrate that coumarin is negative in the mouse in vivo micronucleus assay.

Lack of effect of coumarin on unscheduled DNA synthesis in the in vivo rat hepatocyte DNA repair assay

The ability of coumarin to induce UDS in male Sprague-Dawley CD rat hepatocytes in vivo was assessed using the unscheduled DNA synthesis (UDS) assay. From a preliminary toxicity study the oral maximum tolerated dose (MTD) of coumarin was determined to be 320 mg/kg body weight. For the UDS studies, rats were treated with 0 (corn oil control), 32 (one-tenth the MTD), 107 (one-third the MTD) and 320 (MTD) mg/kg coumarin via oral gavage. Rats were also treated with 20mg/kg body weight dimethylnitrosamine (DMN) or 50mg/kg body weight 2-acetylaminofluorene (2-AAF) as positive controls for the 2-4 hr and 12-16 hr expression of UDS, respectively. Hepatocytes were isolated by liver perfusion either 2-4 hr or 12-16 hr after treatment and cultured in medium containing [methyl-(3)H]thymidine for 4 hr and assessed for UDS by grain counting of autoradiographs. Coumarin treatment at doses of 32-320 mg/kg body weight had no statistically significant or dose-related effect on UDS in rat hepatocytes either 2-4 hr or 12-16 hr after dosing. In contrast, both DMN 2-4 hr after dosing and 2-AAF 12-16 hr after dosing produced significant increases in UDS assessed as the net nuclear grain count. Both genotoxins also increased the percentage of hepatocyte nuclei with greater than 5 net grains. Treatment with coumarin, DMN and 2-AAF had no statistically significant effect on the proportion of rat hepatocytes undergoing replicative DNA synthesis. In summary, this study demonstrates that coumarin does not induce UDS in hepatocytes of male Sprague-Dawley CD rats after oral administration at doses up to the MTD of 320 mg/kg. The responsiveness of the animals used in this study to genotoxic agents was demonstrated by the clear induction of DNA repair after treatment with DMN and 2-AAF.

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.