The Genus Eranthis: Prospects of Research on Its Phytochemistry, Pharmacology, and Biotechnology

This review summarizes information about the chemical composition and beneficial properties of species of the genus Eranthis Salisb. from the world’s flora. To date, seven out of ~14 species found in Asia and parts of Europe have been studied to various degrees. Here, data are presented on the diversity of sets of chromones, furochromones, triterpene saponins, coumarins, and other classes of secondary metabolites of Eranthis species according to the literature. For new compounds—isolated from Eranthis for the first time—structural formulas are also provided. Among the new compounds, chromones and coumarins predominate, as do triterpene saponins of the olean and cycloartane series and lectin. The results of pharmacological studies are presented showing anti-inflammatory, antioxidant, antiviral, and other types of biological activities found in extracts, in their fractions, and in individual compounds of the aboveground and underground organs and parts of Eranthis species. Despite the limited geographic range of Eranthis plants, it is possible to search for active substances, develop methods for biological and chemical synthesis of the isolated substances, and create a finished therapeutic substance based on them. In addition, it is feasible to obtain the desired standardized pure materials from Eranthis species grown in vitro.

Plants 2023, 12, x FOR PEER REVIEW 2 of 20 subdivided into two sections: Eranthis sect.Eranthis and E. sect.Shibateranthis (Nakai) Tamura [12].The type section Eranthis is characterized by plants with tubers, yellow sepals, and emarginate or slightly bilobate upper petal margins without pseudonectaries (Figure 1) [6,11].The section Eranthis in Europe includes E. hyemalis (L.) Salisb.and E. bulgarica (Stef.)Stef., whereas in Southwest and West Asia, it includes E. cilicica Schott et Kotschy, E. kurdica Rukšāns, E. longistipitata Regel, and E. iranica Rukšāns et Zetterl.[13][14][15][16].The section Shibateranthis has long-lived tubers, white sepals, and bilobate or forked petal margins with pseudonectaries (Figure 1) [6,17].Representatives of this section have a natural geographic range in temperate North and East Asia (E.albiflora Franch., E. byunsanensis B.Y. Sun, E. lobulata W.T.Wang, E. pinnatifida Maxim., E. pungdoensis B.U. Oh, E. sibirica DC., E. stellata Maxim., and E. tanhoensis Erst) [10,11].Plants of the tribe Cimicifugeae are some of the richest sources of various active ingredients and of therapeutic and health-promoting substances.The value of the constituents has been confirmed by many years of use in East Asian countries in folk medicine.Thus, it is important to integrate new technologies into research on Cimicifugeae, both for the sustainable use of pharmaceutical resources from Cimicifugeae and for a search for new compounds with potential clinical efficacy and fewer adverse effects [18][19][20].In the tribe Cimicifuga, representatives of the genus Actaea are the most frequently studied plants in the world of science.Nonetheless, little is known about the chemical profile and biological activity of other representatives of Cimicifugeae: Beesia and Anemonopsis.In recent decades, new information has been obtained about the chemical profiles of (and biological effects of extracts and individual compounds from) Eranthis species, which are early flowering geophytes with a limited geographic range.No reviews on plants of this genus have previously been carried out to the best of our knowledge.In this brief review,   Since the 1960s, from some Eranthis species, a series of substances has been isolated that represents an important class of oxygen-containing heterocyclic compounds that are derivatives of benzo-γ-pyrone: chromones.Their isolation has been performed by various chromatographic methods, and the structures of individual compounds have been investigated by 1-dimensional (H-NMR) and 2-dimensional nuclear magnetic resonance (C-NMR) spectroscopy.In structure, chromones are similar to flavonoids and coumarins but are substantially less common in the wild.Chromones can give rise to hydroxy-and methoxy-derivatives and can attach a sugar moiety, whereas after condensation with benzene, pyran, or furan rings, they can be transformed into a variety of benzo-, pyrano-, or furochromones, respectively.Compounds from the class "simple furochromones and chromones" are most often found in Eranthis species; chromones have been detected in underground parts, whereas furochromones have been found in underground and aboveground parts (Table 1).E. longistipitata (L) [25] Visamminol-3 -O-glucoside (4-hydroxy-2-(2-hydroxypropan-2yl)-methyl-2,3-dihydrofuro [3,2-g] chromen-5-one)   E. stellata (L) E. tanhoensis (L) [25,31,32]   Structurally, the chromones found in Eranthis species can be categorized into several classes (Figure 2).Structures of compounds 1-6 are similar in the carbon backbonecontaining an oxepin ring-but differ from one another in substituents at positions C-2 and C-9.These substituents can be methyl and hydroxymethyl groups as well as monoand diglycosides.These compounds have been registered in the underground parts of E. cilicica Schott & Kotschy and E. hyemalis (L.) Salisb.[21][22][23].The first publications on the isolation of chromones of this subclass date back to the end of the 1970s, when 5-hydroxy-9-hydroxymethyl-2-methyl-8,11-dihydro-4H-pyrano[2,3-g][1]benzoxepin-4-one [named eranthin (2)] and its β-D-glucoside (3) were isolated [22].
Structures similar to this type (7 and 8), in contrast to the above compounds, have a displaced double bond [from C-9(10) to C-10(11)] on the oxepin ring; in addition, at position C-9, there is both an oxymethylene group bearing various substituents and a hydroxyl group.These compounds have so far been found only in the underground part of E. cilicica [21]. 1 H and 13 C nuclear magnetic resonance data have allowed us to identify the structural features of compounds 9-14 and to determine that their oxepin ring is open; a 4 -hydroxy-3 -methylbut-2 -enyl group has been found at position C-8.Compounds 11 and 12 additionally contain a D-glucose residue at the C-7 position, whereas compounds 13 and 14 contain it at the C-4 position.These chromones have been detected in the underground parts of E. cilicica and E. hyemalis [21,22].All of the above chromones are specific to the genus Eranthis.

Furochromones
These compounds of Eranthis species are formed by the condensation of a simple chromone with a furan ring at positions C-6 and C-7 and, in contrast to the aforementioned chromones, are relatively common in the plant kingdom.For instance, the first representative of this subclass of compounds called khellin ( 16) has long been used in folk medicine to relieve ureteral pain during colic.For the first time, khellin was found in a seed extract of Ammi visnaga (L.) Lam. and was isolated as far back as the end of the 19th century [36].Currently, khellin's ability to act directly on smooth-muscle fibers is widely used in clinical practice [37].Khellin, aside from species of the genus Ammi, has been found in other representatives of the family Apiaceae Lindl., for example, in Dioscorea L. sp. and Pimpinella L. sp.[38][39][40], and among Eranthis species, in E. hyemalis and E. longistipitata Regel [24,25].The diversity of the structures in the furochromone subclass, which includes khellin, is mostly determined by the presence of substituents at the C-4, C-7, and C-9 positions.At the C-4 position, methoxy or hydroxyl groups can serve as a substituent; at position C-7, methoxy groups and glucose; and at position C-9, a methoxy group, or-as in 15, 17, 19, and 21-the substituent may be absent.Khellol (17) represents an aglycone of khellol glucoside (18), in which the sugar moiety is attached at position C-7.In the genus Eranthis, most research on furochromones of this subclass has been conducted on samples of the aerial parts (leaves, stems, and flowers) of E. pinnatifida Maxim., E. hyemalis, and E. longistipitata [24][25][26][27], and only compound 21 has been detected in an underground part (tubers) of E. cilicica [21].Structures similar to this type (7 and 8), in contrast to the above compounds, have a displaced double bond [from C-9(10) to C-10(11)] on the oxepin ring; in addition, at position C-9, there is both an oxymethylene group bearing various substituents and a hydroxyl group.These compounds have so far been found only in the underground part of E. cilicica [21]. 1 H and 13 C nuclear magnetic resonance data have allowed us to identify the structural features of compounds 9-14 and to determine that their oxepin ring is open; a 4′-hydroxy-3′-methylbut-2′-enyl group has been found at position С-8.Compounds 11 and 12 additionally contain a D-glucose residue at the C-7 position, whereas compounds 13 and 14 contain it at the C-4′ position.These chromones have been detected in the un- Recently, new compounds not previously found in Eranthis species were discovered in samples of E. longistipitata from Central Asia (Kyrgyzstan): methoxsalen ( 22), 5-Omethylvisammioside (26), and visamminol-3 -O-glucoside (27) [25].Methoxsalen ( 22) is often seen in the plant extracts of such families as Apiaceae, Rutaceae Juss., Fabaceae Lindl., and Brassicaceae Burnett [41,42], whereas the last two compounds of this subclass (26 and 27) have been registered only in extracts from an underground part of Saposhnikovia divaricata (Turcz.)Shischk.(Apiaceae) [43][44][45].
Because Eranthis species synthesize chromones during normal physiological processes, preliminary conclusions have been made that the genus Eranthis is closest to the genera Cimicifuga and Actaea (in whose extracts, chromones have also been found), and not Helleborus L. (for example), whose species do not synthesize chromones but are distinguished by the accumulation of cardenolides and bufadienolides [23,46].

Triterpene Saponins
A phytochemical analysis of a methanol extract from tubers of E. cilicica has revealed two new bisdesmosidic triterpenes, named eranthisaponins A (28) and B (29) [28].The new saponins are based on the structural backbone of hederagenin, which is a triterpenoid first isolated from seeds and leaves of Hedera helix L. [47].A distinctive feature of eranthisaponin A ( 28) is a branched tetraglycoside attached at the C-3 position of the aglycone, whereas a feature of eranthisaponin B ( 29) is a linear hexaglycoside attached at the C-28 position of the aglycone.Such sugar forms in triterpene saponins have not been described previously.
In addition, as one of the substituents, eranthisaponin A (28) contains D-allopyranose: a monosaccharide that is extremely rare in plant saponins [28].
Furthermore, in Eranthis plants, a number of known triterpene saponins (30-35) have been discovered that are (just as eranthisaponins B and A) based on the backbone of hederagenin with substituents at positions C-3 and C-28; extremely rarely (only in 31), the substituent (a hydroxyl group) is located at the C-23 position.Other substituents include di-and triglycosides composed of glucose, arabinose, and rhamnose residues.
The research continued by K. Watanabe with coauthors [29] has allowed to subsequently isolate a new oleanane glycoside (34) from the tubers of E. cilicica.Another oleanane glycoside (35) had been discovered earlier in the underground part of Anemone coronaria L. (Ranunculaceae) [48].These substances (34 and 35) contain triglycosides only at position C-3 of the carbohydrate part of the molecule.All of the above triterpene saponins belong to the oleanan type.
The compounds of both subclasses differ among themselves in the presence of various sugar moieties at the C-3 position of the aglycone as well as in the presence of a hydroxy, oxo, or methyl group at position C-28.

Alkaloids
In the tubers and aerial parts of E. hyemalis, trace amounts of an alkaloid called corytuberine (49) have been found [30].

Coumarins
In ongoing studies on E. longistipitata, coumarins have been discovered in aqueousethanol extracts from the leaves of this species: this class of compounds was registered in the genus Eranthis for the first time [25] but is widespread in the family Ranunculaceae [49].5,7-Dihydroxy-4-methylcoumarin (50), scoparone (51), and fraxetin ( 52) are affiliated with the subclass "simple coumarins", which are based on a coumarin molecule with substituents in the form of methyl, hydroxy, and methoxy groups.Luvangetin (53) can be assigned to linear pyranocoumarins, in which-aside from various substituents-a pyran ring is present in the backbone.
The research continued by K. Watanabe with coauthors [29] has allowed to subsequently isolate a new oleanane glycoside (34) from the tubers of E. cilicica.Another oleanane glycoside (35) had been discovered earlier in the underground part of Anemone coronaria L. (Ranunculaceae) [48].These substances (34 and 35) contain triglycosides only at position C-3 of the carbohydrate part of the molecule.All of the above triterpene saponins belong to the oleanan type.
Although the chemical composition of seed oil has been investigated only in E. hyemalis, this class of compounds deserves special attention because in most other genera of Ranunculaceae it is taxonomically significant.For instance, in most of Ranunculus L. species, hexadecadienoic acid (16:2n − 6) is dominant and constitutes 2-10% of seed oil.For the genera Pulsatilla Mill., Adonis L., and Aconitum L. and some Anemone L. species, the major fatty acid (up to 80% of total) is linoleic acid, whereas the relative abundance of eicosadienoic acid reaches 7-8% in some Anemone species, and its concentration in the species of Cimicifuga Wernisch., Helleborus L., Actaea L., and Caltha L. is the lowest [50].In E. hyemalis, cis-13,16-docosadienoic acid (109) serves as a major fatty acid, constituting up to 57% of seed oil [33].In terms of the total set of fatty acids in seed oil, E. hyemalis is close to the genera Cimicifuga and Actaea, but more detailed conclusions require additional investigation.

Lectins
The name lectin was proposed by W. Boyd in 1954 [51] for proteins that can agglutinate red blood cells and selectively bind to carbohydrates [52].So far, more than 500 lectins have been isolated from higher and lower plants [53] and can accumulate in roots, leaves, fruits, seeds, and wood [54,55].It is believed that they provide protection to plants from phytopathogenic microorganisms and phytophages, play a decisive role in the establishment of symbiotic relationships with nitrogen-fixing bacteria, and participate in the transport of hormones and glycoproteins [55][56][57].
To date, in the family Ranunculaceae, only in Clematis montana Buch.-Ham and Eranthis hyemalis have researchers demonstrated the presence of lectins.The E. hyemalis lectin, called EHL, was first isolated in the second half of the 1980s by B.P. Cammue [34].This lectin represents the most widespread type of lectin among plants: ribosome-inactivating proteins [58][59][60].EHL, just like other ribosome-inactivating proteins, consists of two chains: chain A is responsible for enzymatic activity, and chain B binds carbohydrates, thereby helping the lectin molecule get inside the cell.In terms of its specificity to carbohydrates, EHL belongs to type II, that is, it can bind to D-galactose and N-acetyl-D-galactosamine.The role of bound carbohydrates is probably to increase the water solubility of a given glycoprotein [61].Structurally, EHL resembles the lectin of Bryonia dioica Jacq.(Cucurbitaceae), but the latter possesses moderate activity, its relative abundance does not exceed 0.4% of the total soluble protein, and this lectin is found in all vegetative organs [62].On the contrary, EHL is located in underground organs, its relative abundance reaches 2% of the total amount of soluble proteins, and in terms of activity, EHL exceeds the lectin of B. dioica 20-fold [34].The research of B.P. Cammue was expanded in 1993 by M.A. Kumar and coworkers, who characterized in detail physicochemical properties of the lectin and determined a part of amino acid sequence of chain A [35].

Compounds from Other Classes
In a leaf extract of E. longistipitata, by means of liquid chromatography combined with high-resolution mass spectrometry, the presence of amino acid-related compounds

Prospects for the Pharmaceutical Use of Eranthis Plants
Eranthis species are not well investigated from phytochemical and pharmaceutical points of view, most likely owing to their limited endemic occurrence.Nevertheless, there are substantial prospects for the practical application of these species because plants of the tribe Cimicifugeae are some of the richest sources of various biologically active compounds as well as health-promoting and therapeutic agents [18,20].There is evidence that an entire Eranthis plant can be applied against urolithiasis and diuresis [63].The analysis of the literature data on biological activity of Eranthis plants showed that there are few in vitro studies on extracts from the plants of this genus (Table 2).In vivo studies on these plants have not been conducted yet; hence, they are interesting and topical for further investigation.
The biological activity of compounds from the underground organs of Eranthis plants is actively being studied.Triterpene glycosides of the oleanane series found in E. cilicica tubers (compounds 34 and 35, Table 1) have a cytotoxic effect on the HL-60 cell line, with average IC 50 values of 10.5 µM [29].The mechanism of the cytotoxicity may be related to their ability to induce apoptosis.
Extracts from representatives of Eranthis are expected to have antioxidant properties because they contain compounds with high antioxidant activity, e.g., flavonoids, phenolcarboxylic acids, and chromones.The antioxidant activity of chromones extracted from the tubers of E. cilicica has been evaluated against the superoxide anion radical in a chemiluminescent assay [21].In that work, some chromone glycosides did not exert any obvious superoxide anion-scavenging ability, even at the sample's concentration of 1000 µg/mL.Derivatives of 2-hydroxymethylchromone showed moderate activity (IC 50 values were in the range of 179-274 µg/mL, respectively), which may be attributed to the presence of a 2-hydroxymethyl group.Epigallocatechin gallate, serving as a positive control, had an IC 50 of 3.0 µg/mL [21].
The chromones found in Eranthis plants are of great interest to the pharmaceutical industry because they have high pharmacological activity but are not widespread in the plant kingdom.Some time ago, a study was conducted on the biosynthesis of individual chromones, for example, of khellol isolated from E. hyemalis [65].The chemical synthesis of therapeutically active chromones from the plants of this genus is possible as well.For example, eranthin (compound 2, Table 1) has already been synthesized artificially [66].Investigation into the biosynthesis and subsequent chemical synthesis of such substances offers good prospects for obtaining new pharmacologically active compounds based on substances isolated from Eranthis.
Some papers on the lectin of E. hyemalis (EHL) [34,35,61] have shown that further research in this field is promising, as is the use of EHL for plant protection in studies on insecticidal, fungicidal, and bactericidal properties.EHL has an antiviral effect against alfalfa mosaic virus as well as a larvicidal effect against southern corn rootworm [35].The research has been continued by a group led by McConnell [67], who have found that EHL impedes the development and reproduction of the nematode Caenorhabditis elegans.By conjugating EHL with gold nanoparticles, other researchers have obtained a drug called AuNPs@EHL, which is capable of inhibiting the reproduction of C. elegans [68].
Eranthis and some species of Helleborus-H.niger L., and H. orientalis Lam.(Ranunculaceae)-are often called poisonous plants because they contain cardiac glycosides [69,70].Nonetheless, cardiac glycosides have not yet been found in Eranthis plants (see Section 2); these compounds are not detectable even in cardioactive fractions from E. hyemalis tubers [23,27,28].It is likely that some other substances are simultaneously cardioactive and toxic in Eranthis plants.This phenomenon may be explained, for example, by the levels of chromones.Pharmacological trials of eranthin-β-D-gentiobioside (compound 5, Table 1) have revealed a negative inotropic effect on an isolated guinea pig papillary muscle [23].Cytotoxic effect Despite the limited geographic range of Eranthis plants and the small amount of plant raw material in existing populations, it is possible to search for active ingredients, develop methods for the biological and chemical synthesis of the isolated substances, and to create a finished therapeutic substance based on them.

Prospects for the Propagation of Eranthis Plants by Biotechnological Methods
Eranthis plants are geophytes, bloom in early spring, and-in addition to the abovementioned pharmacological potential-have high decorative value.Some representatives of this genus reproduce well vegetatively.For example, the E. hyemalis growing in Europe exhibits a high rate of tuber propagation and is cultivated widely [71].This species is capable of self-renewal outside its range.Mayorov and Vinogradova [72] classify E. hyemalis as a potentially invasive species in the city of Moscow.Nevertheless, most representatives of Eranthis are endemics and have a limited geographic range.The cultivation of these plants in open ground is problematic.For instance, E. sibirica is considered unsuitable for introduction into Siberia [73,74].The seed reproduction of Eranthis representatives is limited by the morphophysiological dormancy of seeds owing to underdeveloped embryos [75][76][77].

Introducing E. stellata into In Vitro Cultures
It is now possible to obtain the desired standardized, clean, uninfected plant material propagated in vitro.A protocol has been developed for introducing E. stellata into in vitro culture; this is an endemic species with limited occurrence in the Far East of Russia and in North Korea and Northeast China [78].It has been revealed that peduncles with underdeveloped buds are the most efficient as the primary explants of E. stellata.It is reported that during introduction into in vitro cultivation, such explants should be cultivated at a low temperature (17 • C) in the dark to overcome phenolic oxidation.The half-strength Murashige-Skoog nutrient medium containing 5 µM 6-benzylaminopurine is reported to be optimal for this species' propagation [78].

Implementation of E. tanhoensis Callus Cultures
Callus cultures can be employed for the sustainable large-scale production of secondary metabolites in the pharmaceutical, cosmetics, and food industries.On the basis of the callus cultures of medicinal plants, it is possible to produce phytochemicals for treating cancer and cardiovascular, neurodegenerative, and infectious diseases [79,80].Conditions have been found for setting up and carrying out E. tanhoensis callus cultures [81].For this purpose, those authors used etiolated tuber seedlings collected in the Mamai River Valley in Irkutsk Oblast (Siberia, Russia).Explants were cultured in test tubes containing the Murashige-Skoog agar medium supplemented with growth regulators: 2,4-dichlorophenoxyacetic acid (2 mg/L) and 6-benzylaminopurine (0.5 mg/L).Before transfer into the test tubes, sterile explants were immersed for 5 min in a liquid culture medium supplemented with 0.1 g/L ascorbic acid and 0.5 g/L polyvinylpyrrolidone. Two weeks after the planting, a thickening of the explants was observed, followed by callus formation.The resultant callus had a bright yellow color and dense consistency [81].
Further refinement of the protocol for the cultivation of Eranthis species in vitro and for the implementation of callus cultures of these plants will help to obtain large quantities of this material for preserving rare species and at the same time for using them in more in-depth phytochemical and pharmacological research.

Materials and Methods
The scientific literature was searched in various databases, including PubMed, Scopus, Google, Google Scholar, e-Library, and Web of Science, by means of the keyword "Eranthis".All articles published in English from 1961 to August 2023 inclusively were found, as were articles in Russian, Chinese, Polish, Bulgarian, or German with an English abstract.Papers that have not addressed the phytochemistry, biological activity, and/or ethnopharmacology of Eranthis species were excluded.The chemical structures of the phytocomponents were found in the PubChem database, and the ChemDraw 19.0 software was used to draw selected structures.

Conclusions
The present review outlines the results of studies on the chemical composition and biological effects of species of the genus Eranthis.Our analysis of the literature data showed that there is information about the chemical profile and biological activity for only seven out of >14 Eranthis species of the world's flora.In the extracts of the analyzed plants, more than 125 compounds have been identified: flavonoids, chromones, coumarins, phenolcarboxylic acids, and other classes of substances.For some stand-alone compounds, various biological activities have been demonstrated experimentally.Although Eranthis species have rarely been a subject of phytochemical and pharmacological investigation, the above information about their chemical composition and biological activity implies the fairly high potential usefulness of the medicinal properties of these species and justifies the interest in these plants among chemists studying new natural compounds.Furthermore, the analysis of the chemical composition of Eranthis species serves as a striking example of the fruitfulness of comprehensive research; this analysis helps to solve-inter alia-the taxonomic problems facing systematic botanists during studies on Ranunculaceae in general and Eranthis in particular.

Figure 1 .
Figure 1.Species of the genus Eranthis.(A) E. longistipitata, (B) E. cilicica, (C) E. hyemalis, (D) E. sibirica, (E) E. tanhoensis, and (F) E. stellata.Plants of the tribe Cimicifugeae are some of the richest sources of various active ingredients and of therapeutic and health-promoting substances.The value of the constituents has been confirmed by many years of use in East Asian countries in folk medicine.Thus, it is important to integrate new technologies into research on Cimicifugeae, both for the sustainable use of pharmaceutical resources from Cimicifugeae and for a search for new compounds with potential clinical efficacy and fewer adverse effects[18][19][20].In the tribe Cimicifuga, representatives of the genus Actaea are the most frequently studied plants in the world of science.Nonetheless, little is known about the chemical profile and biological activity of other representatives of Cimicifugeae: Beesia and Anemonopsis.In recent decades, new information has been obtained about the chemical profiles of (and biological effects of extracts and individual compounds from) Eranthis species, which are early flowering geophytes with a limited geographic range.No reviews on plants of this genus have previously been carried out to the best of our knowledge.In this brief review, we summarize research on the phytochemistry, pharmacology, and biotechnology of Eranthis plants of the world flora since 1961.

Table 1 .
Chemical constituents of the genus Eranthis (all classes of metabolites identified to date: vertical subdivisions in the table).

Table 2 .
Bioactive effects of Eranthis species and compounds isolated from them: preclinical studies (in vitro).