Cyclic Diester Research Articles

Enantiomorphic Site-Assisted Chain End Control Stereospecific Alternating Copolymerization of Chiral Cyclic Diesters.

Stereospecific alternating copolymerization of different chiral cyclic esters is one feasible approach to enrich the structural diversity of copolyesters and tailor their properties. However, dramatically different reactivities of different cyclic esters let a perfectly stereospecific alternating polymerization of these cyclic esters be a challenge, thus the catalyst is required to balance their reactivities. Herein, a remarkable enantiomorphic site effect on chain end control was discovered and successfully utilized to balance the reactivities of highly reactive S, S-lactide (S, S-LA) and low reactive R, R-ethylglycolide (R, R-EG)/R, R-propylglycolide (R, R-PG) during their heterospecific alternating copolymerization. The enantiomorphic site of R, R-SalenAl complex can increase the relative reactivity of R, R-EG/R, R-PG and suppress that of S, S-LA, then a perfectly alternating sequence of the copolymer of S, S-LA and R, R-EG/R, R-PG can be achieved (Palt=0.96/0.91); inversely, using S, S-SalenAl complex, the significant enantiomorphic site effect enlarges the reactivity difference of two monomers, the alternating level was just 0.70/0.68 even to 0.61. Poly(S, S-LA-alt-R, R-EG) with a high alternating regularity exhibits lower glass transition temperatures and a dramatically higher elongation at break (ϵB=449±51 % (Palt=0.96) vs ϵB=6±1% (Palt=0.70)).

Enantiomorphic Site‐Assisted Chain End Control Stereospecific Alternating Copolymerization of Chiral Cyclic Diesters

AbstractStereospecific alternating copolymerization of different chiral cyclic esters is one feasible approach to enrich the structural diversity of copolyesters and tailor their properties. However, dramatically different reactivities of different cyclic esters let a perfectly stereospecific alternating polymerization of these cyclic esters be a challenge, thus the catalyst is required to balance their reactivities. Herein, a remarkable enantiomorphic site effect on chain end control was discovered and successfully utilized to balance the reactivities of highly reactive S, S‐lactide (S, S‐LA) and low reactive R, R‐ethylglycolide (R, R‐EG)/R, R‐propylglycolide (R, R‐PG) during their heterospecific alternating copolymerization. The enantiomorphic site of R, R‐SalenAl complex can increase the relative reactivity of R, R‐EG/R, R‐PG and suppress that of S, S‐LA, then a perfectly alternating sequence of the copolymer of S, S‐LA and R, R‐EG/R, R‐PG can be achieved (Palt=0.96/0.91); inversely, using S, S‐SalenAl complex, the significant enantiomorphic site effect enlarges the reactivity difference of two monomers, the alternating level was just 0.70/0.68 even to 0.61. Poly(S, S‐LA‐alt‐R, R‐EG) with a high alternating regularity exhibits lower glass transition temperatures and a dramatically higher elongation at break (ϵB=449±51 % (Palt=0.96) vs ϵB=6±1% (Palt=0.70)).

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.

Safety profile of ZnO active packaging PBAT based biomaterial for food packaging. First tier evaluation

Materials produced with polybutylene adipate terephthalate (PBAT) and starch are raising great interest for packaging and food contact applications, including as support for active antimicrobial agents such as zinc oxide nanoparticles (ZnO). Studies focusing on the safety of these materials as evaluated with the current reference rules for food contact materials, are lacking. A commercially available PBAT/starch-based material was incorporated with ZnO and the overall and specific migration of the films were studied at different conditions of simulants and temperature. The overall migration (OM) limit is exceeded due to the release of starch as confirmed by infrared spectroscopy. The impact of temperature on the OM was higher for the ethanol 10%. The incorporation of ZnO particles reduced the OM, for both temperatures tested for ethanol 10%. The impact of incorporating ZnO in the migration into acetic acid was relevant at 20 °C only. Beside starch, most relevant migrants were PBAT oligomers made of butanediol and two different kinds of diacids, terephthalic acid or adipic acid. The cyclic diester of 1,4-butanediol and adipic acid, butenyl butanediol adipate in either cyclic and linear form and oligomers terephthalic acid, but-3-enyl hexadecyl ester were detected in an untargeted screening with GC-MS. A second TPA oligomer was not fully identified. The specific migration was in several cases, depending on the simulant and on the temperature, higher than 50 μgkg−1 (semi-quantification), the threshold value for requiring additional toxicity test (genotoxicity tests applied to oligomers below 1000 Da). This indicates the need for more detailed studies with more precise quantification to verify the need for toxicity tests.

Efficient Access to Sequence-Controlled Poly(α-hydroxy acids) with Ax(BC)yDz-Type and Ax(BC)yAz-Type Triblock Structures via Self-Switchable Ring-Opening Polymerization of Monomer Mixtures.

Synthesizing block-sequence-controlled poly(α-hydroxy acids) of three or four α-hydroxy acids remains challenging in one step. In this study, a strategy was employed using three monomers of O-carboxyanhydrides (OCAs) consisting of one α-hydroxy acid (A), asymmetric cyclic diester (B and C, two different α-hydroxy acids of B and C), and symmetric cyclic diester (one α-hydroxy acid of D) with remarkably different activities toward a stereoselective, regioselective, and chemoselective initiator of a zirconium complex. Then, via a self-switchable approach, these monomers can be copolymerized in a well-controlled block sequence of Ax(BC)yDz and Ax(BC)yAz without an external stimulus. Moreover, upon addition of more monomer mixtures during the copolymerization process, more complicated sequence-controlled poly(α-hydroxy acids) can be achieved with up to 15 blocks.

Thermally Stable Biodegradable Polyesters with Camphor and Tartaric Acid Coupled Cyclic Diester Monomers for Controlled Hydrolytic Degradations

The development of biodegradable engineering plastics is challenging, because increases in biodegradability mostly degrade other material properties, such as mechanical and thermal properties of polymers. Here, we use bioderivatives of diester monomers, dimethyl (1S,4R)-1,7,7-trimethylspiro[bicyclo[2.2.1]heptane-2,2′-[1,3]dioxolane]-4′,5′-dicarboxylate (camphor dimethyl dl-tartrate, Ct diester), for the synthesis of phthalate-based copolyesters to enhance degradability without a notable loss of thermal stability. Ct diester synthesized by acetalization of tartaric acid and camphor contains a bridged type of rigid bicycling ring with high thermal stability and controlled degradation via hydrolysis of acetal groups within the spiro-ring structure. Various alkyl-length diols of ethylene, butylene, and hexylene are copolymerized with mixtures of dimethyl phthalate (DMT) and Ct diester in a 9:1 molar ratio. The glass transition temperature and degradation temperature of the copolyesters are comparable to those of DMT-based homopolyesters. Due to decreases in crystallinity, copolyesters exhibit slight decreases in melting temperature of 10 to 20 °C, which can be advantageous for reducing processing temperatures. Notably, the copolyesters demonstrate enhanced hydrolytic degradation of 9.3 to 26.2% in pH 2 environment for 18 days, as measured by the degree of molecular weight reductions. These values are between 1.7 and 3.5 times higher than the degradation rates of their control samples of DMT-based homopolyesters under the same degradation conditions.

Crystalline aliphatic polyesters from eight‐membered cyclic (di)esters

AbstractRing‐opening polymerization (ROP) of lactones or cyclic (di)esters is a powerful method to produce well‐defined, high‐molecular‐weight (bio)degradable aliphatic polyesters. While the ROP of lactones of various ring sizes has been extensively studied, the ROP of the simplest eight‐membered lactone, 7‐heptanolactone (7‐HL), has not been reported using metal‐based catalysts. Accordingly, this contribution reports the ROP of 7‐HL via metal‐catalyzed coordinative‐insertion polymerization to the corresponding high‐molecular‐weight polyester, poly(7‐hydroxyheptanoate) (P7HHp). The resulting P7HHp is a semi‐crystalline material, with a Tm of 68 °C, which is ~10 °C higher than poly(ε‐caprolactone) derived from the seven‐membered lactone. Mechanical testing showed that P7HHp is a hard and tough plastic, with elongation at break >670%. P7HHp‐based polyesters with higher Tm values have been achieved through stereoselective copolymerization of 7‐HL with an eight‐membered cyclic diester, racemic dimethyl diolide (rac‐8DLMe), known to lead to high Tm poly(3‐hydroxyburtyrate) (P3HB). Notably, catalyst's strong kinetic preference for polymerizing rac‐8DLMe over 7‐HL in the 1/1 comonomer mixture rendered the formation of di‐block copolymer P3HB‐b‐P7HHp, showing two crystalline domains with Tm1 ~ 65 °C and Tm2 ~ 160 °C. Semi‐crystalline random copolymers with Tm up to 164 °C have also been obtained by adjusting copolymerization conditions. Mechanical testing showed that P3HB‐b‐P7HHp can synergistically combine the high modulus of isotactic P3HB with the high ductility of P7HHp.

Mechanistic insights into the self-esterification of lactic acid under neutral and acidic conditions

Lactic acid (LA) is frequently used as the starting material for the production of biodegradable and/or bio-compatible polymers. We study several mechanisms which have been envisaged as important for the formation of lactide (LT, the cyclic diester of LA) in toluene solutions, both under neutral and acidic conditions. Quantum mechanical calculations of molecules and aggregates have been performed at the DFT level. We determine the free energies and molecular geometries of the reactants, products, the transition states, and intermediates. Among many proposed mechanisms, we confirm that, under neutral conditions, there is no promising reaction at the prevailing experimental conditions. This is to a large extent due to entropic effects. Under acidic conditions, on the other hand, both initial conversion of LA to dimers and the subsequent conversion to lactide will proceed, in particular if additional LA molecules are available (asynchronous concerted trimolecular reaction). However, to be selective towards LT, the systems should not contain an excess of LA, a condition that could be met in suitable porous media.

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.

Aryloxy ‘biometal’ complexes as efficient catalysts for the synthesis of poly(butylene adipate terephthalate)

A comparative study of catalytic activity of aryloxy complexes of Mg, Zn and Al in the synthesis of poly(butylene adipate terephthalate) demonstrated the preference of Al-based catalyst containing two bulky substituents in ortho-position of phenol ligand in terms of higher values of PBAT MW and lower yields of cyclic diester side product.

Bio-Degradable Polyesters with Rigid Cyclic Diester from Camphor and Tartaric Acid.

Despite their excellent, useful, and stable properties, thermoplastics are constantly subject to environmental risks because of their low degradability under thermal, chemical, and mechanical stresses. To overcome the aforementioned issues, we hereby introduce an eco-friendly camphor (Ct) cyclic diester. The Ct diester is designed as a monomer, including a ketal group from the Ct, and shows high thermal stability via a rigid spiro-ring and a bridged bicyclic structure. A series of polyester was synthesized using the Ct diester, including various types of diols and dimethyl terephthalate. PETxCty copolyesters showed appropriate thermal stability up to 414 °C and a high glass transition temperature. This thermal behavior led to amorphous regions as the Ct diester content increased. Regarding the proportion of the Ct diester in the polyester, it was sensitive to hydrolysis and contributed to the degradation of the polyester in acid buffer conditions.Graphical

Support for Regulatory Assessment of Percutaneous Absorption of Retronecine-type Pyrrolizidine Alkaloids through Human Skin.

1,2-unsaturated pyrrolizidine alkaloids are found naturally in Symphytum officinale, well known as comfrey, which has a longstanding use for the topical treatment of painful muscle and joint complaints. Pyrrolizidine alkaloids (PA) are a relevant concern for the safety assessment due to their liver genotoxicity profile, and close attention is paid during manufacturing to minimizing their levels. Current regulatory risk assessment approaches include setting limits that derive from toxicity data coming from the oral route of exposure. This study investigated to what extent pyrrolizidine alkaloids are bioavailable following topical exposure, assessing penetration of retronecine-type PAs in an in vitro human skin model. A single comfrey root formulation was spiked with 3 different congeners (a 7R-monoester, an open-chained 7R-diester, and a cyclic diester) and percutaneous absorption measured per OECD guidelines and good laboratory practices. The measured penetration for all 3 PAs was low and compared favourably with existing in vitro data. Although consideration of different regulatory guidance influences the determination of dermally absorbed dose, these data facilitate the understanding of absorption differences following topical exposure, which in turn can be taken into account in the risk assessment.

Isotactic-Alternating, Heterotactic-Alternating, and ABAA-Type Sequence-Controlled Copolyester Syntheses via Highly Stereoselective and Regioselective Ring-Opening Polymerization of Cyclic Diesters.

Synthesizing different types of sequence-controlled copolyesters can enrich the diversity of copolyesters and modify their properties more precisely, but it is still a challenge to synthesize a complicated sequence-controlled copolyester using different hydroxy acids in a living polymerization manner. In this work, a highly regioselective and stereoselective catalytic system was developed to synthesize biorenewable and biodegradable copolyesters of mandelic acid and lactic acid with isotactic-alternating, heterotactic-alternating, and ABAA-type precise and complicated sequences. Because of the regular incorporation of mandelic acid into polylactide, these sequence-controlled copolymers of mandelic acid and lactic acid show higher glass-transition temperatures than polylactide and a random copolymer. A stereocomplexation interaction between two opposite enantiomeric isotactic polymer chains was also discovered in the isotactic-alternating copolymer.

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.

Chirality-Directed Regioselectivity: An Approach for the Synthesis of Alternating Poly(Lactic-co-Glycolic Acid).

We report the synthesis of alternating poly(lactic-co-glycolic acid) via a regioselective ring-opening polymerization of (S)-methyl glycolide. An enantiopure aluminum salen catalyst with binaphthyl backbone facilitates the regioselective ring-opening of this unsymmetrical cyclic diester exclusively at the glycolide acyl-oxygen bond site. This living, chain-growth polymerization is able to reach low dispersities with tailored molecular weights. Quantitative regioselectivity calculations and sequence error analysis have been established for this sequence-controlled polymer.

Simultaneous Detection of Two Tumor Marker Proteins Using Dual-Colored Signaling Molecularly Imprinted Polymers Prepared via Multi-Step Post-Imprinting Modifications

Abstract We report simultaneous detection of tumor marker proteins using a molecularly imprinted polymer-based fluorescence sensing system, in which prostate-specific antigen (PSA) recognition cavity, labeled with Alexa Fluor 594, and α-fetoprotein (AFP) recognition cavity, labeled with Alexa Fluor 647, exist together in the polymer matrix. The individually fluorescent-labeled PSA- and AFP-imprinted polymer was prepared by a dual imprinting method, followed by multi-step post-imprinting modifications (PIM). A polymerizable group, conjugated with PSA or AFP via a disulfide bond, was prepared and immobilized on a phenylboronic acid moiety-introduced substrate by the formation of cyclic diester between phenylboronic acid and glycans on proteins. The polymer matrix was prepared using surface-initiated atom transfer radical polymerization. After the reduction of the disulfide bond and hydrolysis of the cyclic diester, PSA- and AFP-imprinted nano-cavities were generated simultaneously. In multi-step PIM, thiol-reactive fluorescent dyes were introduced via a dynamic protection procedure using the target protein, which yielded dual fluorescence-labeled imprinted nano-cavities. Fluorescence signaling abilities were assessed, and each AFP and PSA-imprinted nano-cavity was confirmed to transduce the protein binding events into specific fluorescence signals, with lower values of limit of detection (<2.0 ng/mL). Therefore, the proposed methodology could be a novel platform for the simultaneous detection of multiple proteins.

Toxic Prediction of Pyrrolizidine Alkaloids and Structure-Dependent Induction of Apoptosis in HepaRG Cells.

Pyrrolizidine alkaloids (PAs) are common phytotoxins and could cause liver genotoxicity/carcinogenicity following metabolic activation. However, the toxicity of different structures remains unclear due to the wide variety of PAs. In this study, the absorption, distribution, metabolism, excretion, and toxicity (ADMET) of 40 PAs were analyzed, and their toxicity was predicted by Komputer Assisted Technology (TOPKAT) using Discovery Studio software. The in silico results showed that all PAs except retronecine had good intestinal absorption, and all PAs were predicted to have different toxicity ranges. To verify the predictive results, 4 PAs were selected to investigate cell injury and possible mechanisms of the differentiation in HepaRG cells, including retronecine type of twelve-membered cyclic diester (retrorsine), eleven-membered cyclic diester (monocrotaline), noncyclic diester (retronecine), and platynecine type (platyphylline). After 24 h exposure, retronecine-type PAs exhibited concentration-dependent cytotoxicity. The high-content screening assay showed that cell oxidative stress, mitochondrial damage, endoplasmic reticulum stress, and the concentration of calcium ions increased, and neutral lipid metabolism was changed notably in HepaRG cells. Induced apoptosis by PAs was indicated by cell cycle arrest in the G2/M phase, disrupting the mitochondrial membrane potential. Overall, our study revealed structure-dependent cytotoxicity and apoptosis after PA exposure, suggesting that the prediction results of in silico have certain reference values for compound toxicity. A 1,2-membered cyclic diester seems to be a more potent apoptosis inducer than other PAs.

Kinetic Study of Depolymerization of Lactic and Glycolic Acid Oligomers in the Presence of Oxide Catalysts.

For glycolic acid oligomers (GAO): l-lactic acid oligomers (LAO) mixtures, as an example, it was shown that the nature of the active sites of the catalysts significantly affect the depolymerization process. So, ZnO, which has mainly basic sites, leads to the highest yield of cyclic diesters. On the contrary, depolymerization in the presence of acidic γ-Al2O3 and without a catalyst is characterized by lower diester yields due to the prevalence of a side polycondensation reaction. Using GAO:LAO mixtures, it was shown that in the case of three studied systems (with ZnO, γ-Al2O3, and without catalyst), mixed interactions occurred, including homo-paired and hetero-paired intermolecular interactions, as well as intramolecular interactions of oligomeric molecules. Kinetic models of the processes under study were determined by isoconversional thermal analysis. In the case of depolymerization of oligomers in the presence of ZnO, the kinetic model was between the kinetic models of the first (F1) and second (F2) orders, while in depolymerization in the presence of γ-Al2O3 and without a catalyst, the process was described by diffusion models such as the Jander equation (D3) and Zhuravlev, Lesokin, Tempelman equation (D5).

Depolymerization of lactic acid oligomers into lactide: Epimerization, stereocomplex formation, and nature of interactions of oligomers

This research considers the features of depolymerization of lactic acid oligomers in the presence of oxide catalysts, including the study of their epimerization and the nature of interactions in the synthesis of cyclic diesters. It was shown by the polarimetric method and under depolymerization conditions that the epimerization of L-lactic acid oligomers (L-LAO) proceeds predominantly on strong basic sites of catalysts, while Lewis acid sites promote further polycondensation of oligomers. The nature of the interactions of oligomers was studied using mixtures of oligomers of different chirality. It was shown by IR spectroscopy and thermal analysis that in the case of the studied mixtures of L-LAO and D-lactic acid oligomers (D-LAO), stereocomplexes are formed. During depolymerization, meso-lactide is formed as a result of intermolecular interactions of oligomers and their epimerization. A small amount of meso-lactide can be associated with the formation of stereocomplexes based on L-LAO and D-LAO with a parallel arrangement of oligomeric molecules complicated intermolecular interactions and its lower thermodynamic stability compared to other lactide isomers.

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.