The naturally occurring compounds including anthraquinones (AQs) have been under intensive investigations because of the fast development of drug resistance towards conventional chemo-drugs and routine therapeutics [1]. Moreover, there is a general tendency of back to the nature as a main source of low toxicity substances with comparable biological activity as the synthetic analogues. Worldwide, ~80% plants are used to treat diseases in everyday life [2]. As a result of the recognition of the numerous traditional routines in healthcare systems, these medicinal plants and herbs have increasingly been in free usages. Most of the natural AQ derivatives that have been isolated from plants are introduced to be photosensitive compounds in the ultraviolet (UV)-visible spectrum [3]. Among the natural photosensitizers (PS) only few of them such as chlorophyll, curcumin, pheophorbide A, hypocreline and AQs aloe-emodin, and hypericin have been studied for photodynamic therapy (PDT) method [4]. Nonetheless that most of the natural occurring-PSs have been evaluated with properties apart of being ideal PS, their research and development as photosensitive compounds for PDT are still under research interests because of other advantages of their curative actions [5]. A better solution to the main problem with harsh dark and phototoxicity of synthetic PSs are the photosensitive compounds of natural origin.

Currently, only synthetic PDT drugs are in clinical practice [6]. The properties of the known AQs such as low water-solubility, insufficient selectivity, long lasting skin phototoxicity are leading to many complications and undesirable side effects including genotoxicity and mutagenicity. These drugs are under further improvement through different structural modifications. It is worth to refer to the nature for PDT drugs starting with the known as well as the novel natural photoactive compounds. Among the natural AQs, only hypericin (HYP) found in the plant species of the genus Hypericum L., especially Hypericum perforatum L. has already been clinically investigated in cancer PDT [7]. Chemically HYP is a polymeric 1,8- dihydroxyanthraquinone (4,5,7,4${}^{\prime }$,5${}^{\prime }$,7${}^{\prime }$-hexahydroxy-2,2-dimethylnaphtodi anthrone) with a restriction of absorbance which is in UV and short visible range (590 nm), situated out of the “optical PDT window” (630–850 nm) [8]. A limitation of natural PSs which includes the spectra of absorption within the sun light spectrum might lead to undesirable phototoxicity. Another one such as a minimal or lack of selectivity can provoke the harmful phototoxicity on normal cells as well together with skin phototoxicity [9].

One of the largest groups of natural quinones that can be found in plants, but also in fungi, insects and lichens are 1,8-dihydroxyanthraquinones [10]. Initially, AQs are discovered as natural colorants with variety of colors that make them usable as dyes and pigments in industrial coloration [11]. Their polycyclic planar, rigid aromatic structure contains 14 carbons in the fused benzene rings which is tricyclic anthracene with two carbonyl groups including two benzene rings. The largest class of quinones are oxidized forms of anthracene derivatives which include 1,4- and 9,10-anthraquinones creating dioxoanthracene with characteristics of natural pigments.

The most studied plants containing AQs are part of mainly three families (81%): Fabaceae, Rubiaceae and Polygonaceae [12]. Additional families that contain AQ-derivatives are Asphodelaceae, Rhamnaceae, Hypericaceae, Scrophulariaceae, Bignoniaceae, Pedaliaceae, Verbenaceae and Valerianaceae. However, in higher amount AQs can be found in Fabaceae (Cassia spp. and Senna spp.), Rubiaceae (Morinda spp. and Rubia spp.), Polygonaceae (Rheum spp. and Rumex spp.), Rhamnaceae (Rhamnus spp. and Berchemia spp.) and Asphodelaceae (Aloe spp. and Asphodeline spp.). Among the most identified AQ-derivatives are aloe emodin, chrysophanol, emodin, physicion, rhein, rubiadin and soranjidiol, and damnacanthal (Supplementary Table 1).

Recent literature reviews suggested that the plants’ occurring AQs possess a broad spectrum of medical actions, including antimicrobial, antiviral, cathartic, anticancer, diuretic, vasorelaxant, phytoestrogen, autoimmune disorders and many other clinical applications [13, 14]. The wavelength of HYP (~590 nm) used for photo-experiments is considered as limitation for PDT due to the lesser light penetration in disordered cells [15]. This determines the development of the synthetic AQs [16]. PDT procedure includes a multi-factorial step that generally lead to production of toxic oxygen species (Fig. 1). The photodynamic compounds aim to produce different reactive oxygen species (ROS) by two mechanisms including oxygen radicals and anions (Type I) and singlet oxygen (Type II). The both are initiated by photon energy and next the electron/ proton or an energy transfer reaction due to formation of excited-state PSs. The produced reactive species further affect the pathologic target cells by several mechanisms [17]. There are limited numbers of studies on natural AQs, which are related to possible mechanisms of PDT action [18, 19, 20]. The PDT response might lead to apoptosis or necrosis, degeneration of the tumor vasculature, stimulation of anti-tumor immune response, and inflammatory reactions in the treated lesion (Fig. 1).

Fig. 1.

Mechanisms of anthraquinone (AQ) photo-excitation and the possible pathways of the photodynamic therapy (PDT) action and the targets. *: singlet or triplet excited state of AQ; **: lowest energy excited state of AQ.

The plant-occurring AQs are among the natural producers of anthracycline drugs which are observed as the most effective antineoplastic agents with planar tetracyclic molecules based on AQ core linked to a sugar moiety and phenolic groups [21]. The first clinically accepted compounds in cancer chemotherapy were daunorubicin and doxorubicin (DOX) with DNA binding by intercalation and the trapping of DNA Topoisomerase II complexes [22]. As a DNA intercalator, DOX forms hydrogen-bonds with guanine and insert into the DNA strand at sites with adjacent guanine-cytosine (GC) base pairs. This process tends to cause DNA damage and induce cell death. A second, Topoisomerase II poisoning by DOX interrupts DNA replication and transcription, resulting in apoptosis mediated cell death. The oxidation of DOX consequences in radical formation and then, the superoxide and H${}_2$O${}_2$ production that leads to raised oxidative stress, which may lead to DNA damage [23]. The treatment also outcomes in an increased cellular ceramide level, activating growth arrest, apoptosis, and senescence. A catalytic action was observed as a result of undergoing cyclic one electron oxidation-reduction reactions. The produced ROS are capable of induction of oxidative stress causing membrane lyses and cellular damage, which consequently results in cell death [24, 25].

This review summarizes on the recent studies of natural AQs originated from higher plants and the main plants’ families with plant-originated AQs which are already studied as therapeutics. This is an attempt for a critical overview on the AQs as photosensitizers and their phototoxicity and photodynamic action. Additionally, we made a discussion on the chemical structure, and the correlation between the photosensitizing properties and biological PDT functions of the most investigated plants extracts and the isolated AQs. The limited AQ-PDT studies on different pathological targets (tumors, viruses, pathogenic bacteria, fungi and others) are described and discussed in respect to the potential usage of AQs in clinical PDT practice for an alternative curative therapy due to resistance and in emergency cases.

The article is based on the systematic overview on the research papers and reviews published in the last years considering the research field of our interest. The literature sources taken from the PubMed, Scholar Google, Web of Science and Scopus databases are issued during last ten years (till end of 2023) but not limited to this time for some themes. Supplementary Material summarizes in a Table the data about AQs with their structures and the plants of their origin during the first appearance in the literature until the time of manuscript preparation. The searches were made using the keywords “anthraquinones”, “plants and herbs”, “photosensitizers” and “photosensitization”, “photodynamic therapy”, “phototoxicity”. The searches were limited to the most popular AQs isolated from higher plants which are already in usage as medicinal herbs with focus on the photo-properties and PDT studies.

A great number of anthraquinones (AQs) have been isolated from higher plant species and subsequently characterized. Those higher plant speaces are spreading mainly in angiosperm of limited number of families [26]. In plants, fungi, and lichens, there are around 700 representatives species of this group. Compounds with origin from flowering plants which contain AQs as was lately described are approximately 200 structures [27]. Different quantities of AQs have been found in various parts of plant tissues namely flowers, roots, stems, rhizomes, bark, fruits and leaves like rhizomes. In those plants, where the biosynthesis goes by the shikimate-mevalonate pathway, the AQs predominate as aglycones [28, 29]. Much more derivatives of AQs (~700) are described as useful dyes and pigments, making them the most significant group of naturally occurring colorants [30]. Natural AQs are well-studied secondary metabolites, products of biosynthesis that occurs via two major metabolic pathways, namely polyketide pathway (acetate-malonate pathway) and the shikimate pathway. The main difference between the both biosynthetic pathways is that one produces 1,8-dihydroxy anthraquinones (acetate-malonate pathway) and the other (shikimate) pathway never generates substitution at position 8 (Fig. 2). The cathartic activity of some AQs is related to the 1,8-diOH substitution [26, 30]. Anthraquinones from Rubia spp. are produced through the chorismite/o-succinylbenzoic acid or shikimate pathways [31], while the emodin type AQs (among them are the most effective photoactive compounds) are predominantly synthesized through the polyketide pathway [32].

Fig. 2.

Anthraquinones from higher plants with properties for PDT. Their common substitution groups are in color and close to the plants.

The natural photosensitive compounds such as chlorophylls and chlorophyll metabolites, curcumin, thiophenes, alkaloids, as well as anthraquinones have been featured of importance for investigations as photosensitizers for PDT applications (Fig. 3). Recent work about the phototoxic terrestrial plants including the plants containing AQs, reviews the phototoxic and photosensitizing metabolites of 250 vascular plants that have been documented as phototoxic or with photosensitive properties [100]. This review presents an in-depth investigation with the list of phytochemicals synthesized by terrestrial plants with the photosensitive compounds, and the basic molecules for the further development of new PDT-PSs. The compounds originated from different plant species are under intensive research due to their promising photocytotoxicity in the treatment of cancer and microbial infections [101]. This indicates the future for natural compounds among the class of AQs as anticancer and antiinfection therapeutics (Fig. 3). Moreover, PDT is recognized as a method with a high impact on the resistance inhibition.

Fig. 3.

Green PDT approach due to the spectra and plants’ origin of the photosensitive anthraquinone compounds.

Presently, the application of quinones as PDT agents is limited because of their intensive absorption in the whole UV spectrum, the lack of selectivity, fast clearance and strong hydrophobicity. However, there are few derivatives that can be used as potential PDT agents after properly performed structural modification, so that the absorption relocates to the longer wavelength ($>$500 nm), improves selectivity and also reduces hydrophobicity [102]. The positive aspect of AQs’ mechanism of action is their ability to produce toxic species without loss of cytotoxic abilities in pathologic sites with a low oxygen level, such as hypoxic tumors or oxygen-free pathogenic bacteria [103]. Moreover, the production of the reactive species might happen without light exposure of AQs. The appropriate photosensitization mechanisms responsible for PDT outcome require a sufficient oxygen surrounding atmosphere. The early studies suggested that the natural AQ compounds such as aloe emodin and hypericin can be considered as promising PS candidates for PDT [104]. The recent advances in anticancer activity of emodin, aloe emodin, hypericin, chrysophanol, rhein, and physcion showed these AQs as effective individually but also in combination with other chemotherapeutic agents on in vitro and in vivo basal carcinoma (BC) models [105].

Currently, several chemotherapeutic anticancer agents containing AQ-nucleus are the Food and Drug Administration (FDA) approved drugs, namely daunorubicin, idarubicin, doxorubicin (DOX), epirubicin, valrubicin, pixantrone and mitoxantrone all for clinical use in cancer therapy [106]. The appearance of drug resistance and some possible side effects as for example, the heart muscle injure after DOX treatment, lower the use of doxorubicin-based cancer treatments. Mechanisms of antitumor action of these anthracycline drugs are associated with hyperproduction of ROS and inhibition of glycolysis [107]. However, there is an emerging limitation related to the poor efficacy of chemotherapeutic agents, due to the resistance of tumor cells to these drugs, as well as due to toxicity-related side effects [108]. The score of new drug discovery is known as very low with the estimation that only ten of ten thousand novel tested compounds have been passed the regulations to enter clinical stage, and then only one may be approved through regulatory valuation and licensed for the medical application. The biologically active substances require a long term and complicated procedure including many stages of research, pre- and clinical trials on humans to develop medicine for the pharmacy shelf [109].

Gollnick et al. [110] (1992) showed in an initial study on AQs that the compounds bearing hydroxyl groups in their structure which are distributed on the benzene rings of both sides may have promising potential for photodynamic action. The further study on the photosensitizing properties of some natural AQs isolated from leaves and stems of Heterophyllaea pustulata Hook. f. (Rubiaceae) showed the both mechanisms of photosensitization for the studied rubiadin, soranjidiol, soranjidiol-1-methyl ether, damnacanthal and damnacanthol [111]. The photosensitized generation of superoxide anion radical (Type I mechanism) was presented only for rubiadin-1-methyl ether as was determined in leukocyte suspensions. The singlet oxygen generation (Type II mechanism) was studied in solution for damnacanthal (0.70) followed by soranjidiol (0.47) which were evaluated with relatively very high yields.

Although there are drugs with AQ structure that have already been approved for clinical practice, a large number of AQ analogues have been synthesized and the synthesis continued [112]. The structure of AQ is suitable for metal/non-metal coordination, but there is a limited number of studies on synthetic AQs prepared as a complexe [113]. The coordination of metals was studied with the synthetic ligands through the carbonyl group at AQ-ring and nitrogen atom of amide group. The antibacterial inactivation of both Gram-positive and Gram-negative pathogenic bacterial species showed the high activity for complexes types [Cd(L1)2Cl2] and [Cd(L2)2Cl2]. The clinical AQs have been found to have positive cardiotoxicity, but after the formation of complexes with metals such as Cu, Co, Fe and Mn was shown to limit the ROS production and reduce their toxic and side effects [114]. This can be a starting point for design of novel AQ complexes with low side effect and the superior mechanism Type II photosensitization for PDT.

A very recent study summarized the phototoxic effect of AQs and the plants of their origin and the compounds that are with promising efficiency on different experimental cancer cells [115]. The research and review papers present the AQs such as emodin, aloe-emodin, parietin, rubiadin, hypericin, and soranjidiol as photosensitive compounds for cancer PDT [116, 117].

Generally, depending on drug and light administration conditions, the PDT effect induces the cell death through several mechanims, which can be the initiation of apoptosis, autophagy-associated cell death or necrosis [118, 119]. Another possible mechanism could be an immunogenic cell death (ICD) [120]. A recent study on aloe emodin (AE) from Rheum palmatum’s rhizome reported a mechanism of the PDT of apoptosis in target cells [121]. The mechanisms of cell-death have been mostly investigated for the natural hypericin [122]. Hypericin shows absorption with maximum close to therapeutic region (630 nm–850 nm), which by the irradiation leads to generation of ROS as well as singlet oxygen. In cells, HYP primarily accumulates in the hydrophobic core of the membrane by forming non-covalent persistent interactions and the lipid double bonds, plus in the endoplasmic reticulum (ER), mitochondria, lysosomes and Golgi apparatus [123]. Recently, Kessel reported the capacity of HYP to target the ER for photodamage, which provokes a mainly unexplored manner of photokilling that implicates extensive cytoplasmic vacuole formation but does not represent autophagy [124]. This phenomenon called ‘paraptosis’ seems to be a response to the appearance of misfolded ER proteins as important to eradicate the malignant cell population with a reduced apoptotic pathway [125, 126].

A review paper about “anthraquinones as a promising scaffold for the discovery and development of therapeutic agents in cancer therapy” concluded that in silico approaches (molecular modeling and docking studies) have been recognized as an extensively useful support for progress of novel anticancer compounds based on AQ-moieties [127, 128]. The knowledge on the binding interactions of AQs with amino acids located in active site pocket of targeting enzymes such as CK2$\alpha$, topoisomerase II $\beta$, human Ser/Thr kinase pim1 and reacted quinone reductase might lead to further promising modifications [128]. The AQ core molecule gives the idea to develop an advantaged scaffold in medicinal chemistry in targeting various biological proteins. The genetic reports including topoisomerases, telomerase, protein kinases and matrix metalloproteinases (MMPs) have been widely highlighted. Since the natural AQs and their synthetic derivatives show an inclusive variety of biological activities. Some of them are with high social importance suggesting their probable medical application and surely might serve as a source of effective medications for the progress in development of anticancer drugs [129].

The conformational analysis with three-dimensional quantitative structure-activity relationships (3D-QSAR) of compounds and thus the design of new synthetic photosensitizers could provide a methodological reference for the design and synthesis studies of novel photosensitizers [130, 131]. The AQ moiety provides a potential substrate for DNA insertion and bio-reduction activation, and the alkylation group confers cytotoxic activity [130]. DNA intercalators that typically contain coplanar aromatic backbones such as doxorubicin, daunorubicin, and mitoxantrone (MTZ) can be reversibly inserted into base pairs of DNA. The scientists reported that AQ moieties may serve as a base for preparation of synthetic hydroxyanthraquinone derivatives as the recently reported with N-mustards, thiophene and benzene ring as substituents. The molecular docking studies of compounds demonstrated that the new compound (A1) could interact with the catalytic active site in topoisomerase II. The recent paper studied five pH-responsive anthraquinone photosensitizers (A1 to A5) synthesized through addition, elimination, and substitution mechanisms using chrysophanol as the raw material [131]. The compounds were shown to have good PDT antibacterial activity and selectivity in weak acid environments. The cytotoxicity (3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyl tetrazolium bromide, MTT test) on the cancer cell line HeLa demonstrated the photo- and dark toxicity effects of compounds (for A3 at 0.625 µM) similar to the reference photosensitizer Chlorin e6 (Ce6). Nevertheless these findings, it is still very questionable to find a very powerful PS among the plants-originated AQ-compounds because of the spectral properties of short-wavelength absorption (600 nm). One obvious explanation of the low toxicity and gentle photodynamic action after irradiation can be explained with the evolution stage under the sun light and oxygen atmosphere, and both are involved in photosensitization but also they are critical for the plants’ growth and their being in material sense.

Green approach to PDT includes the PS isolated from plants or other natural source and the specific visible light spectra (400–600 nm), and sufficient oxygen surrounding [132]. Presently, there are no clinically approved plants isolated photosensitizers (PSs) for application as drugs in medical PDT. It is only hypericin which pass throughout many years of research and development and pre-clinical studies so that it has been featured with good perspectives for clinical applications [133]. The scientific society expect that it is the near future when the natural PSs and their synthetic analogues to be invented in cancer PDT. The natural AQs can serve as a core molecule for design of novel more powerful anticancer agents with photosensitive properties that are tailored for PDT.

Practically the natural compounds exhibited the poor solubility, high body clearance rate and restricted tumor targeting capability together the expected lower efficiency as PDT drugs. In order to solve these problems, natural PSs and their derivatives might be involved in molecular assembly technique to enhance uptake, selectivity and PDT efficacy in addition to minimal toxicity in the presence of visible light as the main advantage of these PSs. Also, since natural compounds are usually universal, and some of them are already in usage as herbs and therapeutic drugs, they may be more readily-accessible and easy for therapeutic invention.

In future, researchers may discover these photoactive plants in nature and compare the effects with clinical drugs. Furthermore, owning on the well-accepted structures of PSs for PDT, the natural AQs’ might serve as a template for development of variety of novel target-specific PSs containing definite functional groups as substituents for the target specific PDT. The novel compounds may have the tunability of the photochemical and the photobiological properties, high biocompatibility, and selective cellular uptake in the tumors or other target pathologic tissue and cells, and not the last the better solubility in natural fluids, and absorbance in the optical PDT window (630–850 nm). In addition, the far-red fluorescence emission will be advantageous for the non-invasive optical diagnosis.

As researchers, we must consider the enquiry whether or not is realistic the anthraquinone—associated PDT to get permission for clinical practice? The crucial aspect about plants’ PS is that they have not characteristics of typical photosensitizers due to the natural exposure to sunlight which could be harmful and destructive during the plants growing. However, there are good opportunities for natural anthraquinone hypericin, as it is already on clinical level in cancer PDT. Since the progress in PDT involves the applications of the method to the actual diseases due to resistance to present drugs and for the other diseases without alternative treatments or with insufficient treatment consequence. The decisive stage for further development of natural plant-occurring AQ for PDT applications is to find the unique plant or plants that are having high quantity appropriate compounds and substances with valuable photosensitizing properties. This fact will allow to develop a procedure to obtain the photoactive compound(s) in a large scale. Moreover, the plant cultures hold great potential for the manufacture of phytochemicals to obtain natural compounds on a large scale, in an economically viable and eco-friendly manner.

AQ, anthraquinones; AE, aloe emodin; HYP, hypericin; ROS, reactive-oxygen species; LED, light-emitting diode; PS, photosensitizers; PDT, photodynamic therapy.

VM designed, made the searches, wrote original draft, made the corrections, supervised, and answered the reviewers. DB made the searches, wrote Supplementary, prepared original Figures and made critical corrections. IL made searches, wrote paragraphs, and reviewed. TG made searches, wrote draft, and made the critical corrections. All authors read and approved the final manuscript. All authors have participated sufficiently in the work and agreed to be accountable for all aspects of the work.

Not applicable.

Thanks to Bulgarian Academy of Sciences.

This research received no external funding.

Given her role as Guest Editor, Vanya Mantareva had no involvement in the peer-review of this article and has no access to information regarding its peer review. Full responsibility for the editorial process for this article was delegated to Jen-Tsung Chen. The authors declare no conflict of interest.