Determination of genotoxic potencies of pyrrolizidine alkaloids in HepaRG cells using the γH2AX assay

Comprehensive review of pyrrolizidine alkaloids in bee products: Occurrence, extraction, and analytical methods

Pyrrolizidine alkaloids (PAs) and their N-oxides (PANOs) are hepatotoxic secondary metabolites present in certain plant genera, raising health concerns due to their inevitable occurrence in bee products like honey, pollen, royal jelly, and propolis. The European Commission has set a 500 μg kg-1 limit for PAs/PANOs in pollen-based supplements to ensure safety, emphasizing the need for sensitive analytical methods. This review, based on studies published between 2019 and 2024, identifies 51 compounds in bee products, including 32 PAs and 19 PANOs, with lycopsamine, senecionine, echimidine, intermedine, and retrorsine being the most studied. Solvent extraction, often combined with SPE or QuEChERS, is the most used preparation method, while liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) is preferred for detection. Few studies assessed the risk of PAs consumption. These findings support regulatory monitoring of bee product safety and highlight the need for refining detection methods and establishing standardized limits and monitoring for PAs/PANOs.

Quantifying the effect of human interindividual kinetic differences on the relative potency value for riddelliine N-oxide at low dose levels by a new approach methodology

Pyrrolizidine alkaloid N-oxides (PA-N-oxides) are predominant in plants and herbal foods, and are converted to pyrrolizidine alkaloids (PAs) upon consumption, leading to toxicity. The effect of interindividual kinetic differences on the relative potency values of PA-N-oxides compared to their PAs (REPPANO to PA) was studied, with riddelliine N-oxide (RIDO) and riddelliine (RID) as model compounds. In vitro kinetic data measured in incubations with 30 fecal and 25 liver S9 donor samples showed high variation across individuals, where the interindividual variability was captured with Bayesian multilevel regression. The distributions of influential PBK model parameters were used as input for physiologically based kinetic (PBK) modeling combined with Monte Carlo (MC) simulations to calculate the probability distribution of REPRIDO to RID values. At low dose levels, interindividual differences were shown to be a factor that influences the REPRIDO to RID value while neither dose nor endpoint used plays a role. The distribution of the REPRIDO to RID value ranged from 0.71 to 0.97 (95th percentile) with a mean value of 0.87. The approach described enables determination of interindividual REPPANO to PA values at low dose levels, which are not accessible in in vivo experiments quantifying the REPPANO to PAvalue.

Repair of Retrorsine-Induced DNA Damage in Rat Livers: Insights Gained from Transcriptomic and Proteomic Studies

Pyrrolizidine alkaloids (PAs) are common phytotoxins that are found worldwide. Upon hepatic metabolic activation, the reactive PA metabolites covalently bind to DNAs and form DNA adducts, causing mutagenicity and tumorigenicity in the liver. However, the molecular basis of the formation and removal of PA-derived DNA adducts remains largely unexplored. In the present study, Sprague Dawley (SD) rats were exposed to retrorsine (RTS), a representative PA, at a human-relevant dose of 3.3 mg/kg/day for 28 days. The rats were divided into three groups: control, RTS-28 (sacrificed after continuous RTS exposure), and RTS-161 (sacrificed at 133 days post-RTS-exposure). The multi-omics analyses demonstrated the involvement of homologous recombination (HR) and non-homologous end joining (NHEJ) repair pathways as a response to PA-induced DNA damage. Additionally, the characteristic guanine adducts induced by RTS exposure were in accordance with the higher expression of XPA and XPC, indicating that nucleotide excision repair (NER) and base excision repair (BER) also contributed to repairing RTS-induced DNA damage. Furthermore, we also showed that DNA damage persisted after PA exposure, and mutagenically related repair errors might occur due to the prolonged genotoxic effects. The present study lays the foundation for bridging PA-derived DNA adducts, DNA damage, DNA repair, and the follow-up mutagenesis and carcinogenesis associated with PA exposure.

Modeling HepaRG metabolome responses to pyrrolizidine alkaloid exposure for insight into points of departure and modes of action

The new challenges in toxicology demand novel and innovative in vitro approaches for deriving points of departure (PODs) and determining the mode of action (MOA) of chemicals. Therefore, the aim of this original study was to couple in vitro studies with untargeted metabolomics to model the concentration-response of extra- and intracellular metabolome data on human HepaRG cells treated for 48 h with three pyrrolizidine alkaloids (PAs): heliotrine, retrorsine and lasiocarpine. Modeling revealed that the three PAs induced various monotonic and, importantly, biphasic curves of metabolite content. Based on unannotated metabolites, the endometabolome was more sensitive than the exometabolome in terms of metabolomic effects, and benchmark concentrations (BMCs) confirmed that lasiocarpine was the most hepatotoxic PA. Regarding its MOA, impairment of lipid metabolism was highlighted at a very low BMC (first quartile, 0.003 µM). Moreover, results confirmed that lasiocarpine targets bile acids, as well as amino acid and steroid metabolisms. Analysis of the endometabolome, based on coupling concentration-response and PODs, gave encouraging results for ranking toxins according to their hepatotoxic effects. Therefore, this novel approach is a promising tool for next-generation risk assessment, readily applicable to a broad range of compounds and toxic endpoints.

Utilization of a novel human hepatocyte-endothelial cell coculture model to determine differential toxicities of pyrrolizidine alkaloid food contaminants

Pyrrolizidine alkaloids (PA) are comprised of a family of hundreds of metabolites, produced by plants as a mechanism to protect against herbivory. Upon ingestion and metabolism, dehydropyrrolizidine alkaloids are formed, which are known to generate DNA adducts and subsequently double-strand DNA breaks. Within the liver, the most sensitive cell type to PA exposure is the sinusoidal endothelial cell, as evidenced by the generation of veno-occlusive disease in the human population. PAs are a common crop contaminant and have been regulated by some agencies, using the precautionary principle; each equally potent and genotoxic. Therefore, as a proof of principle we have established a human in vitro coculture model system, utilizing the metabolically active HepaRG hepatocyte and the SK-Hep-1 endothelial cell, to determine differential potencies of different PAs commonly found in crops and food products, notably cell death, targeting of endothelial cells, and genotoxicity comparing the micronucleus assay versus γH2AX assay. Our results demonstrate differential potencies of the PAs used, which encompass three esterification states (monoester, cyclic diester, and open-chain diester). The results suggest that a more nuanced approach to the regulation of PAs may be more appropriate in the regulatory decision-making process.

Application of HepaRG cells for genotoxicity assessment: a review

There has been growing interest in the use of human-derived metabolically competent cells for genotoxicity testing. The HepaRG cell line is considered one of the most promising cell models because it is TP53-proficient and retains many characteristics of primary human hepatocytes. In recent years, HepaRG cells, cultured in both a traditional two-dimensional (2D) format and as more advanced in-vivo-like 3D spheroids, have been employed in assays that measure different types of genetic toxicity endpoints, including DNA damage, mutations, and chromosomal damage. This review summarizes published studies that have used HepaRG cells for genotoxicity assessment, including cell model evaluation studies and risk assessment for various compounds. Both 2D and 3D HepaRG models can be adapted to several high-throughput genotoxicity assays, generating a large number of data points that facilitate quantitative benchmark concentration modeling. With further validation, HepaRG cells could serve as a unique, human-based new alternative methodology for in vitro genotoxicity testing.

Toxins in Botanical Drugs and Plant-derived Food and Feed - from Science to Regulation: A Workshop Review

In September 2022, the 3rd International Workshop on pyrrolizidine alkaloids (PAs) and related phytotoxins was held on-line, entitled 'Toxins in botanical drugs and plant-derived food and feed - from science to regulation'. The workshop focused on new findings about the occurrence, exposure, toxicity, and risk assessment of PAs. In addition, new scientific results related to the risk assessment of alkenylbenzenes, a distinct class of herbal constituents, were presented. The presence of PAs and alkenylbenzenes in plant-derived food, feed, and herbal medicines has raised health concerns with respect to their acute and chronic toxicity but mainly related to the genotoxic and carcinogenic properties of several congeners. The compounds are natural constituents of a variety of plant families and species widely used in medicinal, food, and feed products. Their individual occurrence, levels, and toxic properties, together with the broad range of congeners present in nature, represent a striking challenge to modern toxicology. This review tries to provide an overview of the current knowledge on these compounds and indicates needs and perspectives for future research.

In vitro-in silico study on the influence of dose, fraction bioactivated and endpoint used on the relative potency value of pyrrolizidine alkaloid N-oxides compared to parent pyrrolizidine alkaloids

Pyrrolizidine alkaloids (PAs) and their N-oxides (PA-N-oxides) are phytotoxins found in food, feed and the environment. Yet, limited data exist from which the relative potency of a PA-N-oxide relative to its corresponding PA (REPPANO to PA) can be defined. This study aims to investigate the influence of dose, fraction bioactivated and endpoint on the REPPANO to PA of a series of pyrrolizidine N-oxides using in vitro-in silico data and physiologically based kinetic (PBK) modeling. The first endpoint used to calculate the REPPANO to PA was the ratio of the area under the concentration-time curve of PA resulting from an oral dose of PA-N-oxide divided by that from an equimolar dose of PA (Method 1). The second endpoint was the ratio of the amount of pyrrole-protein adducts formed under these conditions (Method 2). REPPANO to PA values appeared to decrease with increasing dose, with the decrease for Method 2 already starting at lower dose level than for Method 1. At dose levels as low as estimated daily human intakes, REPPANO to PA values amounted to 0.92, 0.81, 0.78, and 0.68 for retrorsine N-oxide, seneciphylline N-oxide, riddelliine N-oxide and senecivernine N-oxide, respectively, and became independent of the dose or fraction bioactivated, because no GSH depletion, saturation of PA clearance or PA-N-oxide reduction occurs. Overall, the results demonstrate the strength of using PBK modeling in defining REPPANO to PA values, thereby substantiating the use of the same approach for other PA-N-oxides for which in vivo data are lacking.

Levels, Toxic Effects, and Risk Assessment of Pyrrolizidine Alkaloids in Foods: A Review

Pyrrolizidine alkaloids (PAs) are naturally occurring secondary metabolites of plants. To date, more than 660 types of PAs have been identified from an estimated 6000 plants, and approximately 120 of these PAs are hepatotoxic. As a result of PAs being found in spices, herbal teas, honey, and milk, PAs are considered contaminants in foods, posing a potential risk to human health. Here, we summarize the chemical structure, toxic effects, levels, and regulation of PAs in different countries to provide a better understanding of their toxicity and risk assessment. With recent research on the risk assessment of PAs, this review also discusses the challenges facing this field, aiming to provide a scientific basis for PA toxicity research and safety assessment.

Pyrrolizidine Alkaloids in Foods, Herbal Drugs, and Food Supplements: Chemistry, Metabolism, Toxicological Significance, Analytical Methods, Occurrence, and Challenges for Future

Consumers are increasingly seeking natural alternatives to chemical compounds, including the use of dried aromatic plants as seasonings instead of salt. However, the presence of pyrrolizidine alkaloids (PAs) in food supplements and dried plants has become a concern because of their link to liver diseases and their classification as carcinogenic by the International Agency for Research on Cancer (IARC). Despite European Union (EU) Regulation (EU) 2023/915, non-compliance issues persist, as indicated by alerts on the Rapid Alert System for Food and Feed (RASFF) portal. Analyzing PAs poses a challenge because of their diverse chemical structures and low concentrations in these products, necessitating highly sensitive analytical methods. Despite these challenges, ongoing advancements in analytical techniques coupled with effective sampling and extraction strategies offer the potential to enhance safety measures. These developments aim to minimize consumer exposure to PAs and safeguard their health while addressing the growing demand for natural alternatives in the marketplace.

OCT1-dependent uptake of structurally diverse pyrrolizidine alkaloids in human liver cells is crucial for their genotoxic and cytotoxic effects

Pyrrolizidine alkaloids (PAs) are important plant hepatotoxins, which occur as contaminants in plant-based foods, feeds and phytomedicines. Numerous studies demonstrated that the genotoxicity and cytotoxicity of PAs depend on their chemical structure, allowing for potency ranking and grouping. Organic cation transporter-1 (OCT1) was previously shown to be involved in the cellular uptake of the cyclic PA diesters monocrotaline, retrorsine and senescionine. However, little is known about the structure-dependent transport of PAs. Therefore, we investigated the impact of OCT1 on the uptake and toxicity of three structurally diverse PAs (heliotrine, lasiocarpine and riddelliine) differing in their degree and type of esterification in metabolically competent human liver cell models and hamster fibroblasts. Human HepG2-CYP3A4 liver cells were exposed to the respective PA in the presence or absence of the OCT1-inhibitors D-THP and quinidine, revealing a strongly attenuated cytotoxicity upon OCT1 inhibition. The same experiments were repeated in V79-CYP3A4 hamster fibroblasts, confirming that OCT1 inhibition prevents the cytotoxic effects of all tested PAs. Interestingly, OCT1 protein levels were much lower in V79-CYP3A4 than in HepG2-CYP3A4 cells, which correlated with their lower susceptibility to PA-induced cytotoxicity. The cytoprotective effect of OCT1 inhibiton was also demonstrated in primary human hepatocytes following PA exposure. Our experiments further showed that the genotoxic effects triggered by the three PAs are blocked by OCT1 inhibition as evidenced by strongly reduced γH2AX and p53 levels. Consistently, inhibition of OCT1-mediated uptake suppressed the activation of the DNA damage response (DDR) as revealed by decreased phosphorylation of checkpoint kinases upon PA treatment. In conclusion, we demonstrated that PAs, independent of their degree of esterification, are substrates for OCT1-mediated uptake into human liver cells. We further provided evidence that OCT1 inhibition prevents PA-triggered genotoxicity, DDR activation and subsequent cytotoxicity. These findings highlight the crucial role of OCT1 together with CYP3A4-dependent metabolic activation for PA toxicity.

Occurrence and associated health risks of pyrrolizidine alkaloids in supplements marketed in Ghana for improved sexual performance

Pyrrolizidine alkaloids (PAs) are noted for their hepatotoxic, genotoxic, and carcinogenic effects in animals and humans following metabolic activation in the liver. In this study, herbal supplements sold in Ghana for sexual improvement were analysed for the presence of 64 PAs using LC-MS/MS analysis. Up to 17 different PAs were identified in 19 out of the 37 samples analysed. The sum of PAs in samples ranged from 5 to 3204 μg kg-1. Since the PA content in the herbal medicinal preparations was generally lower than in honey samples, their presence was mainly attributed to cross-contamination. The observed levels would result in estimated daily intakes from 0.01 to 12 μg per day or 0.0002 to 0.2 μg kg-1 bw day-1 for a person weighing 70 kg. The margins of exposure ranged from 1200 to 1,400,000 with eight samples showing values below 10,000, thus indicating a health concern.

Comparative Pharmacokinetic Study of Two Pyrrolizidine Alkaloids Lasiocarpine and Heliotrine in Rats

Lasiocarpine (LAS) and heliotrine (HEL) are two different ester types of toxic pyrrolizidine alkaloids (PAs): open-chain diester and monoester. However, the pharmacokinetics of these two types of PAs in rats have not been reported. In the present study, two LC-MS/MS methods for determining LAS and HEL were established and validated. The methods exhibited good linearity, accuracy, and precision and were then applied to a comparative pharmacokinetic study. After intravenous administration to male rats at 1 mg/kg, the AUC_0-t values of LAS and HEL were 336 ± 26 ng/mL × h and 170 ± 5 ng/mL × h. After oral administration at 10 mg/kg, the AUC_0-t of LAS was much lower than that of HEL (18.2 ± 3.8 ng/mL × h vs. 396 ± 18 ng/mL × h), while the C_max of LAS was lower than that of HEL (51.7 ± 22.5 ng/mL × h vs. 320 ± 26 ng/mL × h). The absolute oral bioavailability of LAS was 0.5%, which was significantly lower than that of HEL (23.3%). The results revealed that the pharmacokinetic behaviors of LAS differed from that of HEL.

Potency ranking of pyrrolizidine alkaloids in metabolically competent human liver cancer cells and primary human hepatocytes using a genotoxicity test battery

Pyrrolizidine alkaloids (PAs) occur as contaminants in plant-based foods and herbal medicines. Following metabolic activation by cytochrome P450 (CYP) enzymes, PAs induce DNA damage, hepatotoxicity and can cause liver cancer in rodents. There is ample evidence that the chemical structure of PAs determines their toxicity. However, more quantitative genotoxicity data are required, particularly in primary human hepatocytes (PHH). Here, the genotoxicity of eleven structurally different PAs was investigated in human HepG2 liver cells with CYP3A4 overexpression and PHH using an in vitro test battery. Furthermore, the data were subject to benchmark dose (BMD) modeling to derive the genotoxic potency of individual PAs. The cytotoxicity was initially determined in HepG2-CYP3A4 cells, revealing a clear structure-toxicity relationship for the PAs. Importantly, experiments in PHH confirmed the structure-dependent toxicity and cytotoxic potency ranking of the tested PAs. The genotoxicity markers γH2AX and p53 as well as the alkaline Comet assay consistently demonstrated a structure-dependent genotoxicity of PAs in HepG2-CYP3A4 cells, correlating well with their cytotoxic potency. BMD modeling yielded BMD values in the range of 0.1-10 µM for most cyclic and open diesters, followed by the monoesters. While retrorsine showed the highest genotoxic potency, monocrotaline and lycopsamine displayed the lowest genotoxicity. Finally, experiments in PHH corroborated the genotoxic potency ranking, and revealed genotoxic effects even in the absence of detectable cytotoxicity. In conclusion, our findings strongly support the concept of grouping PAs into potency classes and help to pave the way for a broader acceptance of relative potency factors in risk assessment.

Physiologically based kinetic modeling of senecionine N-oxide in rats as a new approach methodology to define the effects of dose and endpoint used on relative potency values of pyrrolizidine alkaloid N-oxides

Over 1,000 pyrrolizidine alkaloids (PAs) and their N-oxides (PA-N-oxides) occur in 3% of all flowering plants. PA-N-oxides are toxic when reduced to their parent PAs, which are bioactivated into pyrrole intermediates that generate protein- and DNA-adducts resulting in liver toxicity and carcinogenicity. Literature data for senecionine N-oxide in rats indicate that the relative potency (REP) value of this PA-N-oxide compared to its parent PA senecionine varies with the endpoint used. The first endpoint was the ratio between the area under the concentration-time curve (AUC) for senecionine upon dosing senecionine N-oxide or an equimolar dose of senecionine, while the second endpoint was the ratio between the amount for pyrrole-protein adducts formed under these conditions. This study aimed to investigate the mode of action underlying this endpoint dependent REP value for senecionine N-oxide with physiologically based kinetic (PBK) modeling. Results obtained reveal that limitation of 7-GS-DHP adduct formation due to GSH depletion, resulting in increased pyrrole-protein adduct formation, occurs more likely upon high dose oral PA administration than upon an equimolar dose of PA-N-oxide. At high dose levels, this results in a lower REP value when based on pyrrole-protein adduct levels than when based on PA concentrations. At low dose levels, the difference no longer exists. Altogether, the results of the study show how the REP value for senecionine N-oxide depends on dose and endpoint used, and that PBK modeling provides a way to characterize REP values for PA-N-oxides at realistic low dietary exposure levels, thus reducing the need for animal experiments.

Risk assessment of short-term intake of pyrrolizidine alkaloids in food: derivation of an acute reference dose

Pyrrolizidine alkaloids (PA) are phytochemicals that are known to act as human hepatotoxins and are also considered to be genotoxic carcinogens. Several plant-derived foods are frequently contaminated with PA, like teas and herbal infusions, spices and herbs or certain food supplements. With respect to the chronic toxicity of PA, the carcinogenic potential of PA is generally regarded as the critical toxicological effect. The risk assessment of the short-term toxicity of PA, however, is internationally less consistent. The characteristic pathological syndrome of acute PA toxicity is hepatic veno-occlusive disease. High PA exposure levels may lead to liver failure and even death as documented by several case reports. In the present report, we suggest a risk assessment approach for the derivation of an acute reference dose (ARfD) for PA of 1 µg/kg body weight per day based on a sub-acute animal toxicity study in rats after oral PA administration. The derived ARfD value is further supported by several case reports describing acute human poisoning following accidental PA intake. The here derived ARfD value may be used for PA risk assessment in cases where the short-term toxicity of PA is of interest in addition to the assessment of the long-term risks.

Physiologically-Based Kinetic Modeling Predicts Similar In Vivo Relative Potency of Senecionine N-Oxide for Rat and Human at Realistic Low Exposure Levels

SCOPE

This study aims to determine if previously developed physiologically-based kinetic (PBK) model in rat can be modified for senecionine (SEN) and its N-oxide (SENO), and be used to investigate potential species differences between rat and human in relative potency (REP) of the N-oxide relative to the parent pyrrolizidine alkaloid (PA).

METHODS AND RESULTS

In vitro derived kinetic parameters including the apparent maximum velocities (V_max ) and Michaelis-Menten constants (K_m ) for SENO reduction and SEN clearance are used to define the PBK models. The rat model is validated with published animal data, and the toxicokinetic profiles of SEN from either orally-administered SENO or SEN are simulated. REP values of SENO relative to SEN amount to 0.84 and 0.89 in rat and human, respectively.

CONCLUSION

The REP value can be dose- and species-dependent, with the values for rat and human being comparable at low realistic exposure scenarios. In summary, PBK modeling serves as a valuable New Approach Methodology (NAM) tool for predicting REP values of PA-N-oxides and may actually result in more accurate REP values for human risk assessment than what would be defined using in vivo animal experiments.

Genotoxicity of pyrrolizidine alkaloids in metabolically inactive human cervical cancer HeLa cells co-cultured with human hepatoma HepG2 cells

Pyrrolizidine alkaloids (PAs) are secondary plant metabolites, which can be found as contaminant in various foods and herbal products. Several PAs can cause hepatotoxicity and liver cancer via damaging hepatic sinusoidal endothelial cells (HSECs) after hepatic metabolization. HSECs themselves do not express the required metabolic enzymes for activation of PAs. Here we applied a co-culture model to mimic the in vivo hepatic environment and to study PA-induced effects on not metabolically active neighbour cells. In this co-culture model, bioactivation of PA was enabled by metabolically capable human hepatoma cells HepG2, which excrete the toxic and mutagenic pyrrole metabolites. The human cervical epithelial HeLa cells tagged with H2B-GFP were utilized as non-metabolically active neighbours because they can be identified easily based on their green fluorescence in the co-culture. The PAs europine, riddelliine and lasiocarpine induced micronuclei in HepG2 cells, and in HeLa H2B-GFP cells co-cultured with HepG2 cells, but not in HeLa H2B-GFP cells cultured alone. Metabolic inhibition of cytochrome P450 enzymes with ketoconazole abrogated micronucleus formation. The efflux transporter inhibitors verapamil and benzbromarone reduced micronucleus formation in the co-culture model. Furthermore, mitotic disturbances as an additional genotoxic mechanism of action were observed in HepG2 cells and in HeLa H2B-GFP cells co-cultured with HepG2 cells, but not in HeLa H2B-GFP cells cultured alone. Overall, we were able to show that PAs were activated by HepG2 cells and the metabolites induced genomic damage in co-cultured HeLa cells.

Seasonal Variability in Pyrrolizidine Alkaloids in Jacobaea alpina from the Trentino-Alto Adige Region (Northern Italy)

In recent years, increased attention has been given to plants containing toxic pyrrolizidine alkaloids (PAs). Jacobaea alpina (syn. Senecio cordatus) is a tall forb growing on mountain pastures and meadows containing such alkaloids and therefore, the plant is considered as a noxious weed in these environments. The repartition of toxic macrocyclic PAs in the plant and their evolution during the vegetation period has been studied in two populations. Eight PAs were found where senciphylline and senecionine accounted in most samples for more than 85 % of total alkaloids. Leaves in April and stems in May started with high PA concentrations (19-22 mg/g dry matter), then alkaloid levels declined. This decrease was more rapid in stems than in leaves. Depending on the population, fully developed inflorescences in June and July PA contents were higher or lower than in the respective leaves. Later, also in the inflorescences PA concentration decreased. Combined with growth data total alkaloid content in the whole plant as mg/plant was highest in midsummer and declined afterwards. Finally, new emerging leaves in September had high PA levels, which declined markedly towards the end of the season in November. In sum, over a large period PA concentration appeared to be high enough to present a health risk for grazing animals.

Structure-Dependent Toxicokinetics of Selected Pyrrolizidine Alkaloids In Vitro

Phytochemicals like pyrrolizidine alkaloids (PAs) can affect the health of humans and animals. PAs can occur for example in tea, honey or herbs. Some PAs are known to be cytotoxic, genotoxic, and carcinogenic. Upon intake of high amounts, hepatotoxic and pneumotoxic effects were observed in humans. This study aims to elucidate different toxicokinetic parameters like the uptake of PAs and their metabolism with in vitro models. We examined the transport rates of differently structured PAs (monoester, open-chained diester, cyclic diester) over a model of the intestinal barrier. After passing the intestinal barrier, PAs reach the liver, where they are metabolized into partially instable electrophilic metabolites interacting with nucleophilic centers. We investigated this process by the usage of human liver, intestinal, and lung microsomal preparations for incubation with different PAs. These results are completed with the detection of apoptosis as indicator for bioactivation of the PAs. Our results show a structure-dependent passage of PAs over the intestinal barrier. PAs are structure-dependently metabolized by liver microsomes and, to a smaller extent, by lung microsomes. The detection of apoptosis of A549 cells treated with lasiocarpine and monocrotaline following bioactivation by human liver or lung microsomes underlines this result. Conclusively, our results help to shape the picture of PA toxicokinetics which could further improve the knowledge of molecular processes leading to observed effects of PAs in vivo.

Bioassay-directed analysis-based identification of relevant pyrrolizidine alkaloids

Pyrrolizidine alkaloids (PAs) are produced by various plant species and have been detected as contaminants in food and feed. Monitoring programmes should include PAs that are present in relevant matrices and that exhibit a high toxic potential. The aim of the present study was to use a bioassay-directed analysis approach to identify relevant PAs not yet included in monitoring programmes. To that end, extracts of Heliotropium europaeum and H. popovii were prepared and analysed with LC–MS/MS for the presence of 35 PAs included in monitoring programmes, as well as for genotoxic activity in the HepaRG/γH2AX assay. Europine, heliotrine and lasiocarpine were found to be the most abundant PAs. The extracts showed a higher γH2AX activity than related artificial mixtures of quantified known PAs, which might point to the presence of unknown toxic PAs. The H. europaeum extract was fractionated and γH2AX activities of individual fractions were determined. Fractions were further analysed applying LC–Orbitrap-MS analysis and Compound Discoverer software, identifying various candidate PAs responsible for the non-explained genotoxic activity. Altogether, the results obtained show that bioassay-directed analysis allows identification of candidate PAs that can be included in monitoring programmes.

The chemical structure impairs the intensity of genotoxic effects promoted by 1,2-unsaturated pyrrolizidine alkaloids in vitro

1,2-unsaturated pyrrolizidine alkaloids (PAs) represent a large group of secondary plant metabolites exhibiting hepatotoxic, genotoxic, and carcinogenic properties upon bioactivation. To examine how the degree of esterification affects the genotoxic profile of PA we investigated cytotoxicity, histone H2AX phosphorylation, DNA strand break induction, cell cycle perturbation, micronuclei formation, and aneugenic effects in different cell models. Analysis of cytotoxicity and phosphorylation of histone H2AX was structure- and concentration-dependent: diester-type PAs (except monocrotaline) showed more pronounced effects than monoester-type PAs. Cell cycle analysis identified that diester-type PAs induced a S-phase arrest and a decrease in the occurrence of cells in the G1-phase. The same structure-dependency was observed by flow-cytometric analysis of PA-induced micronuclei in CYP3A4-overexpressing V79 cells. Analysis of centromeres induced by lasiocarpine in the micronuclei by fluorescence in situ hybridization indicated an aneugenic effect in V79_h3A4 cells. Comet assays revealed no significant induction of DNA strand breaks for all investigated PAs. Overall, diester-type PAs induced more pronounced effects than monoester-type PAs. Furthermore, our results indicate aneugenic effects upon exposure towards lasiocarpine in vitro. These data improve our understanding how structural features of PA influence the genotoxic profile. Especially, the monoester-type PAs seem to induce less severe effects than other PAs.

The Role of Kinetics as Key Determinant in Toxicity of Pyrrolizidine Alkaloids and Their N-Oxides

Pyrrolizidine alkaloids (PAs) are a large group of plant constituents of which especially the 1,2- unsaturated PAs raise a concern because of their liver toxicity and potential genotoxic carcinogenicity. This toxicity of PAs depends on their kinetics. Differences in absorption, distribution, metabolism, and excretion (ADME) characteristics of PAs may substantially alter the relative toxicity of PAs. As a result, kinetics will also affect relative potency (REP) values. The present review summarizes the current state-of-the art on PA kinetics and resulting consequences for toxicity and illustrates how physiologically-based kinetic (PBK) modelling can be applied to take kinetics into account when defining the relative differences in toxicity between PAs in the in vivo situation. We conclude that toxicokinetics play an important role in the overall toxicity of pyrrolizidine alkaloids. and that kinetics should therefore be considered when defining REP values for combined risk assessment. New approach methodologies (NAMs) can be of use to quantify these kinetic differences between PAs and their N-oxides, thus contributing to the 3Rs (Replacement, Reduction and Refinement) in animal studies.

Novel Insights into Pyrrolizidine Alkaloid Toxicity and Implications for Risk Assessment: Occurrence, Genotoxicity, Toxicokinetics, Risk Assessment-A Workshop Report

This paper reports on the major contributions and results of the 2nd International Workshop of Pyrrolizidine Alkaloids held in September 2020 in Kaiserslautern, Germany. Pyrrolizidine alkaloids are among the most relevant plant toxins contaminating food, feed, and medicinal products of plant origin. Hundreds of PA congeners with widespread occurrence are known, and thousands of plants are assumed to contain PAs. Due to certain PAs' pronounced liver toxicity and carcinogenicity, their occurrence in food, feed, and phytomedicines has raised serious human health concerns. This is particularly true for herbal teas, certain food supplements, honey, and certain phytomedicinal drugs. Due to the limited availability of animal data, broader use of in vitro data appears warranted to improve the risk assessment of a large number of relevant, 1,2-unsaturated PAs. This is true, for example, for the derivation of both toxicokinetic and toxicodynamic data. These efforts aim to understand better the modes of action, uptake, metabolism, elimination, toxicity, and genotoxicity of PAs to enable a detailed dose-response analysis and ultimately quantify differing toxic potencies between relevant PAs. Accordingly, risk-limiting measures comprising production, marketing, and regulation of food, feed, and medicinal products are discussed.

Regulatory Perspectives of Pyrrolizidine Alkaloid Contamination in Herbal Medicinal Products

The toxicity of plants containing certain pyrrolizidine alkaloids has long been recognized in grazing animals and humans. Genotoxicity and carcinogenicity data from in vitro and in vivo (animal) studies were published over the last few decades for some of the 1,2-unsaturated pyrrolizidine alkaloids, leading to regulatory action on herbal medicinal products with pyrrolizidine alkaloid-containing plants more than 30 years ago. In recent years, it has become evident that in addition to herbal medicinal products containing pyrrolizidine alkaloid-containing plants, these products may also contain pyrrolizidine alkaloids without actually including pyrrolizidine alkaloid-containing plants. This is explained by contamination by accessory herbs (weeds). The national competent authorities of the European member states and the European Medicines Agency, in this case, the Committee on Herbal Medicinal Products, reacted to these findings by setting limits for all herbal medicinal products. This review article will briefly discuss the data leading to the establishment of thresholds and the regulatory developments and consequences, as well as the current discussions and research in this area.

Organic Cation Transporter I and Na+ /taurocholate Co-Transporting Polypeptide are Involved in Retrorsine- and Senecionine-Induced Hepatotoxicity in HepaRG cells

SCOPE

1,2-unsaturated pyrrolizidine alkaloids (PAs) are secondary plant metabolites that are found in many plant species throughout the world. They are of concern for risk assessment as consumption of contaminated foodstuff can cause severe liver damage. Of late, transporter-mediated uptake and transport has advanced as a vital determinant of PA toxicity. In this study, the authors investigate a transporter-mediated uptake of PAs and its implications in PA toxicity.

METHODS AND RESULTS

We show that transporter expression levels are significantly affected by treatment with the PAs senecionine (Sc) and retrorsine (Re) in the human hepatoma cell line HepaRG. Furthermore, the specific contribution to PA uptake of the two transporters Na⁺ /taurocholate co-transporting polypeptide (SLC10A1) and organic cation transporter I (SLC22A1), both belonging to the heterogeneous solute carrier super family, is investigated by means of a siRNA-mediated knockdown approach. Knockdown of both uptake transporters result in reduced uptake of Re and Sc in a time-dependent manner and attenuated PA-mediated cytotoxic effects in HepaRG cells.

CONCLUSION

Our results confirm previous findings of active transport mechanisms of PAs into hepatocytes and highlight the importance of toxicokinetic studies for the risk assessment of PAs.

Physiologically based kinetic modelling predicts the in vivo relative potency of riddelliine N-oxide compared to riddelliine in rat to be dose dependent

Pyrrolizidine alkaloids (PAs) are toxic plant constituents occurring often in their N-oxide form. This raises the question on the relative potency (REP) values of PA-N-oxides compared to the corresponding parent PAs. The present study aims to quantify the in vivo REP value of riddelliine N-oxide compared to riddelliine using physiologically based kinetic (PBK) modelling, taking into account that the toxicity of riddelliine N-oxide depends on its conversion to riddelliine by intestinal microbiota and in the liver. The models predicted a lower C_max and higher T_max for the blood concentration of riddelliine upon oral administration of riddelliine N-oxide compared to the C_max and T_max predicted for an equimolar oral dose of riddelliine. Comparison of the area under the riddelliine concentration–time curve (AUC_RID) obtained upon dosing either the N-oxide or riddelliine itself revealed a ratio of 0.67, which reflects the in vivo REP for riddelliine N-oxide compared to riddelliine, and appeared to closely match the REP value derived from available in vivo data. The models also predicted that the REP value will decrease with increasing dose level, because of saturation of riddelliine N-oxide reduction by the intestinal microbiota and of riddelliine clearance by the liver. It is concluded that PBK modeling provides a way to define in vivo REP values of PA-N-oxides as compared to their parent PAs, without a need for animal experiments.

A Comparison of Nine Machine Learning Mutagenicity Models and Their Application for Predicting Pyrrolizidine Alkaloids

Random forest, support vector machine, logistic regression, neural networks and k-nearest neighbor (lazar) algorithms, were applied to a new Salmonella mutagenicity dataset with 8,290 unique chemical structures utilizing MolPrint2D and Chemistry Development Kit (CDK) descriptors. Crossvalidation accuracies of all investigated models ranged from 80 to 85% which is comparable with the interlaboratory variability of the Salmonella mutagenicity assay. Pyrrolizidine alkaloid predictions showed a clear distinction between chemical groups, where otonecines had the highest proportion of positive mutagenicity predictions and monoesters the lowest.

Metabolism-mediated cytotoxicity and genotoxicity of pyrrolizidine alkaloids

Pyrrolizidine alkaloids (PAs) and PA N-oxides are common phytotoxins produced by over 6000 plant species. Humans are frequently exposed to PAs via ingestion of PA-containing herbal products or PA-contaminated foods. PAs require metabolic activation to form pyrrole-protein adducts and pyrrole-DNA adducts which lead to cytotoxicity and genotoxicity. Individual PAs differ in their metabolic activation patterns, which may cause significant difference in toxic potency of different PAs. This review discusses the current knowledge and recent advances of metabolic pathways of different PAs, especially the metabolic activation and metabolism-mediated cytotoxicity and genotoxicity, and the risk evaluation methods of PA exposure. In addition, this review provides perspectives of precision toxicity assessment strategies and biomarker development for the risk control and translational investigations of human intoxication by PAs.

The Food Contaminants Pyrrolizidine Alkaloids Disturb Bile Acid Homeostasis Structure-Dependently in the Human Hepatoma Cell Line HepaRG

Pyrrolizidine alkaloids (PAs) are a group of secondary plant metabolites being contained in various plant species. The consumption of contaminated food can lead to acute intoxications in humans and exert severe hepatotoxicity. The development of jaundice and elevated bile acid concentrations in blood have been reported in acute human PA intoxication, indicating a connection between PA exposure and the induction of cholestasis. Additionally, it is considered that differences in toxicity of individual PAs is based on their individual chemical structures. Therefore, we aimed to elucidate the structure-dependent disturbance of bile acid homeostasis by PAs in the human hepatoma cell line HepaRG. A set of 14 different PAs, including representatives of all major structural characteristics, namely, the four different necine bases retronecine, heliotridine, otonecine and platynecine and different grades of esterification, was analyzed in regard to the expression of genes involved in bile acid synthesis, metabolism and transport. Additionally, intra- and extracellular bile acid levels were analyzed after PA treatment. In summary, our data show significant structure-dependent effects of PAs on bile acid homeostasis. Especially PAs of diester type caused the strongest dysregulation of expression of genes associated with cholestasis and led to a strong decrease of intra- and extracellular bile acid concentrations.

Comparison of gene expression and biotransformation activity of HepaRG cells under static and dynamic culture conditions

Flow conditions have been shown to be important in improving longevity and functionality of primary hepatocytes, but the impact of flow on HepaRG cells is largely unknown. We studied the expression of genes encoding CYP enzymes and transporter proteins and CYP1 and CYP3A4 activity during 8 weeks of culture in HepaRG cells cultured under static conditions (conventional 24-/96-well plate culture with common bicarbonate/CO2 buffering) and under flow conditions in an organ-on-chip (OOC) device. Since the OOC-device is a closed system, bicarbonate/CO2 buffering was not possible, requiring application of another buffering agent, such as HEPES. In order to disentangle the effects of HEPES from the effects of flow, we also applied HEPES-supplemented medium in static cultures and studied gene expression and CYP activity. We found that cells cultured under flow conditions in the OOC-device, as well as cells cultured under static conditions with HEPES-supplemented medium, showed more stable gene expression levels. Furthermore, only cells cultured in the OOC-device showed relatively high baseline CYP1 activity, and their gene expression levels of selected CYPs and transporters were most similar to gene expression levels in human primary hepatocytes. However, there was a decrease in baseline CYP3A4 activity under flow conditions compared to HepaRG cells cultured under static conditions. Altogether, the present study shows that HepaRG cells cultured in the OOC-device were more stable than in static cultures, being a promising in vitro model to study hepatoxicity of chemicals upon chronic exposure.

Active Transport of Hepatotoxic Pyrrolizidine Alkaloids in HepaRG Cells

1,2-unsaturated pyrrolizidine alkaloids (PAs) are secondary plant metabolites occurring as food contaminants that can cause severe liver damage upon metabolic activation in hepatocytes. However, it is yet unknown how these contaminants enter the cells. The role of hepatic transporters is only at the beginning of being recognized as a key determinant of PA toxicity. Therefore, this study concentrated on assessing the general mode of action of PA transport in the human hepatoma cell line HepaRG using seven structurally different PAs. Furthermore, several hepatic uptake and efflux transporters were targeted with pharmacological inhibitors to identify their role in the uptake of the PAs retrorsine and senecionine and in the disposition of their N-oxides (PANO). For this purpose, PA and PANO content was measured in the supernatant using LC-MS/MS. Also, PA-mediated cytotoxicity was analyzed after transport inhibition. It was found that PAs are taken up into HepaRG cells in a predominantly active and structure-dependent manner. This pattern correlates with other experimental endpoints such as cytotoxicity. Pharmacological inhibition of the influx transporters Na⁺/taurocholate co-transporting polypeptide (SLC10A1) and organic cation transporter 1 (SLC22A1) led to a reduced uptake of retrorsine and senecionine into HepaRG cells, emphasizing the relevance of these transporters for PA toxicokinetics.

Analysis of pyrrolizidine alkaloids in Eupatorium fortunei Turcz. and their in vitro neurotoxicity

This study was to analyze the pyrrolizidine alkaloids (PAs) in Eupatorium fortunei herbs and its derived finished products with a view to evaluating their effects on the proliferation and oligodendrogenesis of neural progenitor cells (NPCs). Using a LC-MS/MS method with 32 PAs reference standards, 8 PAs including intermedine, intermedine N-oxide, lycopsamine, lycopsamine N-oxide, retronecine, seneciphylline and senkirkine and 7-acetylintermedine N-oxide were identified with intermedine N-oxide and lycopsamine N-oxide being most abundant. The total PA amounts were found to vary from 0.18 to 61.81 μg/g in 30 batches of herbs and from 0.86 to 36.96 μg/g in 4 commercial finished products, respectively. Risk assessments indicated that the short-term intake seemed unlikely lead to acute toxic effects but the chronic use warranted cautions. Using NPCs derived from mouse induced pluripotent stem cells as an in vitro testing model, intermedine, intermedine N-oxide and lycopsamine N-oxide appeared to decrease cell viability at 30 μM whereas intermedine N-oxide inhibited oligodendrogenesis of NPCs at 10 μM. The present results suggested that the PAs in the majority of E. fortunei herbs and the derived products not only resulted in their exposure far exceeding the acceptable intake limit (i. e. 1.0 μg PA per day for adults) in herbal medicinal products recommended by the European Medicines Agency but also induced neurotoxicity to NPCs in vitro.

Determination of Toxic Pyrrolizidine Alkaloids in Traditional Chinese Herbal Medicines by UPLC-MS/MS and Accompanying Risk Assessment for Human Health

Pyrrolizidine alkaloids (PAs) are a class of natural toxins with hepatotoxicity, genotoxicity and carcinogenicity. They are endogenous and adulterated toxic components widely found in food and herbal products. In this study, a sensitive and efficient ultra-high performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) method was used to detect the PAs in 386 kinds of Chinese herbal medicines recorded in the Chinese Pharmacopoeia (2020). The estimated daily intake (EDI) of 0.007 μg/kg body weight (bw)/day was adopted as the safety baseline. The margin of exposure (MOE) approach was applied to evaluate the chronic exposure risk for the genotoxic and carcinogenic potential of PAs. Results showed that PAs was detected in 271 out of 386 samples with a content of 0.1–25,567.4 μg/kg, and there were 20 samples with EDI values above the baseline, 0.007 μg/kg bw/day. Beyond that, the MOE values for 10 out of 271 positive samples were below 10,000. Considering the actual situation, Haber’s rule was used to assume two weeks exposure every year during lifetime, and still the MOE values for four out of 271 positive samples were under 10,000, indicating these products may have potential health risk. The developed method was successfully applied to detect the PAs-containing Chinese herbal medicines. This study provides convincing data that can support risk management actions in China and a meaningful reference for the rational and safe use of Chinese herbal medicines.

Metabolic Pattern of Hepatotoxic Pyrrolizidine Alkaloids in Liver Cells

Contamination with 1,2-unsaturated pyrrolizidine alkaloids (PAs) is a serious problem for certain phytomedicines, foods, and animal feeds. Several of these PAs are genotoxic and carcinogenic, primarily in the liver, upon cytochrome P450 (CYP)-catalyzed activation into reactive (pyrrolic and pyrrole-like) metabolites. Here we investigated the metabolism of selected PAs (echimidine, europine, lasiocarpine, lycopsamine, retrorsine, and senecionine) in rat hepatocytes in primary culture and in human CYP3A4-transfected HepG2 cells. The open-chained diesters echimidine and lasiocarpine and the cyclic diester senecionine were extensively metabolized in rat hepatocytes into a broad spectrum of products released into the medium. A large portion of unidentified, possibly irreversibly bound, products remained in the cells while detectable amounts of reactive and other metabolites were found in the incubation media. In HepG2-CYP3A4 cells, lasiocarpine was more extensively metabolized than echimidine and senecionine which also gave rise to the release of pyrrolic metabolites. In human cells, no pyrrolic metabolites were detected in retrorsine or lycopsamine incubations, while no such metabolites were detected from europine in both cell types. Other types of metabolic changes comprised modifications such as side chain demethylation or oxygenation reactions like the formation of N-oxides. The latter, considered as a detoxification step, was a major pathway with cyclic diesters, was less distinctive for echimidine and lycopsamine and almost negligible for lasiocarpine and europine. Our data are in agreement with previously published cyto- and genotoxicity findings and suggests that the metabolic pattern may contribute substantially to the specific toxic potency of a certain congener. In addition, marked differences were found for certain congeners between rat hepatocytes and transfected human HepG2 cells, whereby a high level of bioactivation was found for lasiocarpine, whereas a very low level of bioactivation was observed for monoesters, in particular in human cells.

Comparative Risk Assessment of Three Native Heliotropium Species in Israel

Pyrrolizidine alkaloids (PAs) are genotoxic carcinogenic phytotoxins mostly prevalent in the Boraginaceae, Asteraceae and Fabaceae families. Heliotropium species (Boraginaceae) are PA-producing weeds, widely distributed in the Mediterranean region, that have been implicated with lethal intoxications in livestock and humans. In Israel, H. europaeum, H. rotundifolium and H. suaveolens are the most prevalent species. The toxicity of PA-producing plants depends on the PA concentration and composition. PAs occur in plants as mixtures of dozens of various PA congeners. Hence, the risk arising from simultaneous exposure to different congeners has to be evaluated. The comparative risk evaluation of the three Heliotropium species was based on recently proposed interim relative potency (iREP) factors, which take into account certain structural features as well as in vitro and in vivo toxicity data obtained for several PAs of different classes. The aim of the present study was to determine the PA profile of the major organ parts of H. europaeum, H. rotundifolium and H. suaveolens in order to assess the plants' relative toxic potential by utilizing the iREP concept. In total, 31 different PAs were found, among which 20 PAs were described for the first time for H. rotundifolium and H. suaveolens. The most prominent PAs were heliotrine-N-oxide, europine-N-oxide and lasiocarpine-N-oxide. Europine-N-oxide displayed significant differences among the three species. The PA levels ranged between 0.5 and 5% of the dry weight. The flowers of the three species were rich in PAs, while the PA content in the root and flowers of H. europaeum was higher than that of the other species. H. europaeum was found to pose a higher risk to mammals than H. rotundifolium, whereas no differences were found between H. europaeum and H. suaveolens as well as H. suaveolens and H. rotundifolium.

Pyrrolizidine Alkaloids Induce Cell Death in Human HepaRG Cells in a Structure-Dependent Manner

Pyrrolizidine alkaloids (PAs) are a group of secondary metabolites produced in various plant species as a defense mechanism against herbivores. PAs consist of a necine base, which is esterified with one or two necine acids. Humans are exposed to PAs by consumption of contaminated food. PA intoxication in humans causes acute and chronic hepatotoxicity. It is considered that enzymatic PA toxification in hepatocytes is structure-dependent. In this study, we aimed to elucidate the induction of PA-induced cell death associated with apoptosis activation. Therefore, 22 structurally different PAs were analyzed concerning the disturbance of cell viability in the metabolically competent human hepatoma cell line HepaRG. The chosen PAs represent the main necine base structures and the different esterification types. Open-chained and cyclic heliotridine- and retronecine-type diesters induced strong cytotoxic effects, while treatment of HepaRG with monoesters did not affect cell viability. For more detailed investigation of apoptosis induction, comprising caspase activation and gene expression analysis, 14 PA representatives were selected. The proapoptotic effects were in line with the potency observed in cell viability studies. In vitro data point towards a strong structure–activity relationship whose effectiveness needs to be investigated in vivo and can then be the basis for a structure-associated risk assessment.

Genotoxicity of selected pyrrolizidine alkaloids in human hepatoma cell lines HepG2 and Huh6

INTRODUCTION

Pyrrolizidine alkaloids (PAs) are found in many plant species as secondary metabolites which affect humans via contaminated food sources, herbal medicines and dietary supplements. Hundreds of compounds belonging to PAs have been identified. PAs undergo hepatic metabolism, after which they can induce hepatotoxicity and carcinogenicity. Many aspects of their mechanism of carcinogenicity are still unclear and it is important for human risk assessment to investigate this class of compounds further.

MATERIAL AND METHODS

Human hepatoma cells HepG2 were used to investigate the genotoxicity of different chemical structural classes of PAs, namely europine, lycopsamine, retrorsine, riddelliine, seneciphylline, echimidine and lasiocarpine, in the cytokinesis-block micronucleus (CBMN) assay. The different ester type PAs europine, seneciphylline, and lasiocarpine were also tested in human hepatoma Huh6 cells. Six different PAs were investigated in a crosslink comet assay in HepG2 cells.

RESULTS

The maximal increase of micronucleus formation was for all PAs in the range of 1.64-2.0 fold. The lowest concentrations at which significant induction of micronuclei were found were 3.2 μM for lasiocarpine and riddelliine, 32 μM for retrorsine and echimidine, and 100 μM for seneciphylline, europine and lycopsamine. Significant induction of micronuclei by lasiocarpine, seneciphylline, and europine were achieved in Huh6 cells at similar concentrations. Reduced tail formation after hydrogen peroxide treatment was found in the crosslink comet assay for all diester type PAs, while an equimolar concentration of the monoesters europine and lycopsamine did not significantly reduce DNA migration.

CONCLUSION

The widely available human hepatoma cell lines HepG2 and Huh6 were suitable for the assessment of PA-induced genotoxicity. Selected PAs confirmed previously published potency rankings in the micronucleus assay. In HepG2 cells, the crosslinking activity was related to the ester type, which is a first report of PA mediated effects in the comet assay.

Comprehensive investigation and risk study on pyrrolizidine alkaloid contamination in Chinese retail honey

Pyrrolizidine alkaloids (PAs) are common phytotoxins. We performed the first comprehensive investigation on PA contamination in Chinese honeys. LC-MS analysis revealed that 58% of 255 honey samples purchased from 17 regions across Mainland China and Taiwan contained PAs with total content ranging over 0.2-281.1 μg/kg. Monocrotaline (from Crotalaria spp), a PA never found in honey in other regions, together with echimidine (Echium plantagineum) and lycopsamine (from Senecio spp.), were three predominant PAs in PA-contaminated Chinese honeys. Further, PAs present in honeys were found to have geographically distinct pattern, indicating possible control of such contamination in future honey production. Moreover, we proposed a new risk estimation approach, which considered both content and toxic potency of individual PAs in honeys, and found that 12% of the PA-contaminated Chinese honeys tested might pose potential health risk. This study revealed a high prevalence and potential health risk of PA contamination in Chinese honeys.

Evaluation of pyrrolizidine alkaloid-induced genotoxicity using metabolically competent TK6 cell lines

Pyrrolizidine alkaloid (PA)-containing plants are among the most common poisonous plants affecting humans, livestock, and wildlife worldwide. A large number of PAs are known to induce genetic damage after metabolic activation. In the present study, using a battery of fourteen newly developed TK6 cell lines, each expressing a single human cytochrome P450 (CYP1A1, 1A2, 1B1, 2A6, 2B6, 2C8, 2C18, 2C9, 2C19, 2D6, 2E1, 3A4, 3A5, and 3A7), we identified specific CYPs responsible for bioactivating three PAs - lasiocarpine, riddelliine, and senkirkine. Among the fourteen cell lines, cells expressing CYP3A4 showed significant increases in PA-induced cytotoxicity, evidenced by decreased ATP production and cell viability, and increased caspase 3/7 activities. LC-MS/MS analysis revealed the formation of 1-hydroxymethyl-7-hydroxy-6,7-dihydropyrrolizine (DHP), the main reactive metabolite of PAs, in CYP3A4-expressing TK6 cells. DHP was also detected in CYP3A5- and 3A7-expressing cells after PA exposure, but to a much lesser extent. Subsequently, using a high-throughput micronucleus assay, we demonstrated that PAs induced concentration-dependent increases in micronuclei and G2/M phase cell cycle arrest in three CYP3A variant-expressing TK6 cell lines. Using Western blotting, we observed that PA-induced apoptosis, cell cycle changes, and DNA damage were primarily mediated by CYP3A4. Benchmark dose (BMD) modeling demonstrated that lasiocarpine, of the three PAs, was the most potent inducer of micronuclei, with a BMD₁₀₀ of 0.036 μM. These results indicate that our TK6 cell system holds promise for genotoxicity screening of compounds requiring metabolic activation, identifying specific CYPs involved in bioactivation, and discriminating the genotoxic compounds that have different chemical structures.

Structure-dependent genotoxic potencies of selected pyrrolizidine alkaloids in metabolically competent HepG2 cells

1,2-unsaturated pyrrolizidine alkaloids (PAs) are natural plant constituents comprising more than 600 different structures. A major source of human exposure is thought to be cross-contamination of food, feed and phytomedicines with PA plants. In humans, laboratory and farm animals, certain PAs exert pronounced liver toxicity and can induce malignant liver tumors in rodents. Here, we investigated the cytotoxicity and genotoxicity of eleven PAs belonging to different structural classes. Although all PAs were negative in the fluctuation Ames test in Salmonella, they were cytotoxic and induced micronuclei in human HepG2 hepatoblastoma cells over-expressing human cytochrome P450 3A4. Lasiocarpine and cyclic diesters except monocrotaline were the most potent congeners both in cytotoxicity and micronucleus assays with concentrations below 3 μM inducing a doubling in micronuclei counts. Other open di-esters and all monoesters exhibited weaker or much weaker geno- and cytotoxicity. The findings were in agreement with recently suggested interim Relative Potency (iREP) factors with the exceptions of europine and monocrotaline. A more detailed micronuclei analysis at low concentrations of lasiocarpine, retrorsine or senecionine indicated that pronounced hypolinearity of the concentration–response curves was evident for retrorsine and senecionine but not for lasiocarpine. Our findings show that the genotoxic and cytotoxic potencies of PAs in a human hepatic cell line vary in a structure-dependent manner. Both the low potency of monoesters and the shape of prototype concentration–response relationships warrant a substance- and structure-specific approach in the risk assessment of PAs.

In vitro biotransformation of pyrrolizidine alkaloids in different species: part II-identification and quantitative assessment of the metabolite profile of six structurally different pyrrolizidine alkaloids

Pyrrolizidine alkaloids (PA) exert their toxic effects only after bioactivation. Although their toxicity has already been studied and metabolic pathways including important metabolites were described, the quantification of the latter revealed a large unknown portion of the metabolized PA. In this study, the qualitative and quantitative metabolite profiles of structurally different PAs in rat and human liver microsomes were investigated. Between five metabolites for europine and up to 48 metabolites for lasiocarpine were detected. Proposals for the chemical structure of each metabolite were derived based on fragmentation patterns using high-resolution mass spectrometry. The metabolite profiles of the diester PAs showed a relatively good agreement between both species. The metabolic reactions were summarized into three groups: dehydrogenation, oxygenation, and shortening of necic acid(s). While dehydrogenation of the necine base is considered as bioactivation, both other routes are considered as detoxification steps. The most abundant changes found for open chained diesters were dealkylations, while the major metabolic pathway for cyclic diesters was oxygenation especially at the nitrogen atom. In addition, all diester PAs formed several dehydrogenation products, via the insertion of a second double bond in the necine base, including the formation of glutathione conjugates. In rat liver microsomes, all investigated PAs formed dehydropyrrolizidine metabolites with the highest amount formed by lasiocarpine, whereas in human liver microsomes, these metabolites could only be detected for diesters. Our findings demonstrate that an extensive analysis of PA metabolism can provide the basis for a better understanding of PA toxicity and support future risk assessment.

Monocrotaline-induced liver toxicity in rat predicted by a combined in vitro physiologically based kinetic modeling approach

The aim of the present study was to use an in vitro–in silico approach to predict the in vivo acute liver toxicity of monocrotaline and to characterize the influence of its metabolism on its relative toxic potency compared to lasiocarpine and riddelliine. In the absence of data on acute liver toxicity of monocrotaline upon oral exposure, the predicted dose–response curve for acute liver toxicity in rats and the resulting benchmark dose lower and upper confidence limits for 10% effect (BMDL₁₀ and BMDU₁₀) were compared to data obtained in studies with intraperitoneal or subcutaneous dosing regimens. This indicated the predicted BMDL₁₀ value to be in line with the no-observed-adverse-effect levels (NOAELs) derived from availabe in vivo studies. The predicted BMDL₁₀–BMDU₁₀ of 1.1–4.9 mg/kg bw/day also matched the oral dose range of 1–3 mg PA/kg bw/day at which adverse effects in human are reported. A comparison to the oral toxicity of the related pyrrolizidine alkaloids (PAs) lasiocarpine and riddelliine revealed that, although in the rat hepatocytes monocrotaline was less toxic than lasiocarpine and riddelliine, due to its relatively inefficient clearance, its in vivo acute liver toxicity was predicted to be comparable. It is concluded that the combined in vitro-PBK modeling approach can provide insight in monocrotaline-induced acute liver toxicity in rats, thereby filling existing gaps in the database on PA toxicity. Furthermore, the results reveal that the kinetic and metabolic properties of PAs can vary substantially and should be taken into account when considering differences in relative potency between different PAs.

Mode of action-based risk assessment of genotoxic carcinogens

The risk assessment of chemical carcinogens is one major task in toxicology. Even though exposure has been mitigated effectively during the last decades, low levels of carcinogenic substances in food and at the workplace are still present and often not completely avoidable. The distinction between genotoxic and non-genotoxic carcinogens has traditionally been regarded as particularly relevant for risk assessment, with the assumption of the existence of no-effect concentrations (threshold levels) in case of the latter group. In contrast, genotoxic carcinogens, their metabolic precursors and DNA reactive metabolites are considered to represent risk factors at all concentrations since even one or a few DNA lesions may in principle result in mutations and, thus, increase tumour risk. Within the current document, an updated risk evaluation for genotoxic carcinogens is proposed, based on mechanistic knowledge regarding the substance (group) under investigation, and taking into account recent improvements in analytical techniques used to quantify DNA lesions and mutations as well as "omics" approaches. Furthermore, wherever possible and appropriate, special attention is given to the integration of background levels of the same or comparable DNA lesions. Within part A, fundamental considerations highlight the terms hazard and risk with respect to DNA reactivity of genotoxic agents, as compared to non-genotoxic agents. Also, current methodologies used in genetic toxicology as well as in dosimetry of exposure are described. Special focus is given on the elucidation of modes of action (MOA) and on the relation between DNA damage and cancer risk. Part B addresses specific examples of genotoxic carcinogens, including those humans are exposed to exogenously and endogenously, such as formaldehyde, acetaldehyde and the corresponding alcohols as well as some alkylating agents, ethylene oxide, and acrylamide, but also examples resulting from exogenous sources like aflatoxin B1, allylalkoxybenzenes, 2-amino-3,8-dimethylimidazo[4,5-f] quinoxaline (MeIQx), benzo[a]pyrene and pyrrolizidine alkaloids. Additionally, special attention is given to some carcinogenic metal compounds, which are considered indirect genotoxins, by accelerating mutagenicity via interactions with the cellular response to DNA damage even at low exposure conditions. Part C finally encompasses conclusions and perspectives, suggesting a refined strategy for the assessment of the carcinogenic risk associated with an exposure to genotoxic compounds and addressing research needs.

Hepatotoxic pyrrolizidine alkaloids induce DNA damage response in rat liver in a 28-day feeding study

Pyrrolizidine alkaloids (PA) are secondary plant metabolites that occur as food and feed contaminants. Acute and subacute PA poisoning can lead to severe liver damage in humans and animals, comprising liver pain, hepatomegaly and the development of ascites due to occlusion of the hepatic sinusoids (veno-occlusive disease). Chronic exposure to low levels of PA can induce liver cirrhosis and liver cancer. However, it is not well understood which transcriptional changes are induced by PA and whether all hepatotoxic PA, regardless of their structure, induce similar responses. Therefore, a 28-day subacute rat feeding study was performed with six structurally different PA heliotrine, echimidine, lasiocarpine, senecionine, senkirkine, and platyphylline, administered at not acutely toxic doses from 0.1 to 3.3 mg/kg body weight. This dose range is relevant for humans, since consumption of contaminated tea may result in doses of ~ 8 µg/kg in adults and cases of PA ingestion by contaminated food was reported for infants with doses up to 3 mg/kg body weight. ALT and AST were not increased in all treatment groups. Whole-genome microarray analyses revealed pronounced effects on gene expression in the high-dose treatment groups resulting in a set of 36 commonly regulated genes. However, platyphylline, the only 1,2-saturated and, therefore, presumably non-hepatotoxic PA, did not induce significant expression changes. Biological functions identified to be affected by high-dose treatments (3.3 mg/kg body weight) comprise cell-cycle regulation associated with DNA damage response. These functions were found to be affected by all analyzed 1,2-unsaturated PA.

In conclusion, 1,2-unsaturated hepatotoxic PA induced cell cycle regulation processes associated with DNA damage response. Similar effects were observed for all hepatotoxic PA. Effects were observed in a dose range inducing no histopathological alterations and no increase in liver enzymes. Therefore, transcriptomics studies identified changes in expression of genes known to be involved in response to genotoxic compounds at PA doses relevant to humans under worst case exposure scenarios.

Benchmark dose analyses of γH2AX and pH3 endpoints for quantitative comparison of in vitro genotoxicity potential of lipophilic phycotoxins

The phycotoxins, okadaic acid (OA) and dinophysistoxins 1 and 2 (DTX-1 and -2), are protein phosphatase PP2A and PP1 inhibitors involved in diarrhetic shellfish poisoning (DSP) in humans. Data on the in vivo acute toxicity of the OA-group toxins show some differences and the European Food Safety Authority (EFSA) has determined toxicity equivalent factors (TEFs) of one for the reference toxin, OA, as well as for DTX-1 and 0.6 for DTX-2. However, recent in vitro studies indicated that DTX-1 seems to be more toxic than OA. As OA was described as apoptotic and aneugenic compound, we analyzed the DNA damage responses induced by the 3 toxins through γH2AX and pH3 biomarkers on proliferative HepaRG cells using High Content Analysis. We quantitatively examined the responses for γH2AX and pH3 by benchmark dose analyzing (BMD) using PROAST software. We found that the three toxins increased both γH2AX- and pH3-positive cells populations in a concentration-dependent manner. The 3 toxins induced mitotic arrest, characteristic of aneugenic compounds, as well as DNA strand-breaks concomitantly to cytotoxicity. BMD analysis showed that DTX-1 is the most potent inducer of DNA damage, followed by OA and DTX-2. The quantitative genotoxic data provided in this study are additional findings for reconsidering the estimated TEFs of this group of phycotoxins.

Detection of pyrrolizidine alkaloids in jamu available on the Indonesian market and accompanying safety assessment for human consumption

The occurrence and accompanying risks of pyrrolizidine alkaloids (PAs) in Indonesian jamu were evaluated. PAs were detected in 34 out of 35 jamu containing PA-producing botanicals, in the range of 12.3-235,376 μg/kg. A total PA level of 5.9-3,421 μg/kg was found in 17 out of 23 jamu made of non-PA-producing botanicals pointing to contamination with PA-producing plants. Short-time consumption of jamu is unlikely to result in acute toxic effects, although one sample would exceed an intake of 10 μg PA/kg bw/day which may cause hepatic veno-occlusive disease (HVOD) in humans. The risk assessment for the genotoxic and carcinogenic potential of PAs revealed Margin of Exposure (MOE) values below 10,000 for 27 out of all samples analysed (46.6%), indicating a priority for risk management when assuming daily lifelong consumption. Assuming consumption for two weeks every year during a lifetime, and using Haber's rule, 13 out of 35 jamu samples containing PA-producing botanicals (37%) still pose a priority, while the jamu consisting of non-PA-producing botanicals would be of low priority (MOE>10,000). This study provides data that can support risk management actions in Indonesia to minimize the potential health risk for jamu consumers due to the occurrence of toxic PAs in these products.

First study on the pyrrolizidine alkaloids of Pardoglossum cheirifolium (L.) E.Barbier & Mathez.: GC-MS analysis of their volatile components in the whole plant

In this work, the first study on the pyrrolizidine alkaloids content of Pardoglossum cheirifolium (L.) E.Barbier & Mathez. was reported. The studied exemplar was collected in Algeria and the analysis was carried out by means of GC-MS on the crude base extract deriving from the whole plant. Nine compounds were tentatively identified and they all represent new phytochemicals at the species level. Their presence confirms the botanical classification of the plant as a member of the Boraginaceae family. In addition, it points out some interesting chemosystematic relations within the family.