Alkaloids: The Potential of Their Antimicrobial Activities of Medicinal Plants

*Mohammad Barati and Amir Modarresi Chahardehi*

## **Abstract**

Given the potential adverse effects of chemical drugs, utilizing natural products with diverse therapeutic and antimicrobial compounds is advisable. Countries can use indigenous flora from their regions in vegetation for medicinal purposes. Several nations exhibit distinctive indigenous flora owing to their geographic positioning and climatic conditions. These plants have been the subject of our research, which has explored their antimicrobial properties against fungi, parasites, bacteria, and viruses. Studies have investigated the therapeutic and antimicrobial effects of plants and their bioactive compounds, such as alkaloids, flavonoids, and terpenoids. Among them are alkaloids, a diverse class of naturally occurring chemicals, such as tropanes, terpenoids, and steroids. Some of these medicinal plants have been found to possess antioxidant and antiinflammatory properties in addition to their antimicrobial effects. This chapter explores the antimicrobial potential of alkaloids found in medicinal plants.

**Keywords:** alkaloids, medicinal plants, antimicrobial activity, secondary metabolite, bioactive compounds

## **1. Introduction**

Researchers are discovering infectious diseases are a major threat to world health [1, 2]. For millennia, medicinal plants have yielded an abundance of therapeutic compounds, which have been incorporated into traditional pharmacological practices across the globe [3, 4]. Since the dawn of time, people have known that plants have healing properties, making botanic medicine one of the first forms of therapy [5–7]. Antibacterial medications have traditionally been derived from natural materials. This avenue of inquiry declined in the 1980s as scientists shifted their focus to synthetic compound libraries because of their greater flexibility [8]. Antibiotic and antifungal medication discovery are crucial in the face of the rise of multidrug-resistant (MDR) fungi and bacteria [9]. The emergence of multidrug-resistant organisms is a significant worldwide health concern [10]. The incorrect use of antibiotics in human and animal health care is largely responsible for the rise of MDR strains. Consequently, the search for alternative, nonantibiotic-dependent solutions to this critical issue has become an urgent and imperative challenge [11]. In particular, each plant produces small quantities of secondary metabolites—tiny compounds like terpenoids, polyphenols, phenolics, alkaloids, essential oils, etc. [12]. The discovery of novel pharmacological compounds that can treat serious ailments has greatly benefited from research into medicinal plants [13]. Some plants, such as mustard, ginger, basil, garlic, cinnamon, sage, curry, and many other crude extracts, for instance, have antibacterial activity against many different forms of bacteria, including gram-positive and gramnegative [14]. Medicinal plants include phytochemicals, often responsible for their biological activity, commonly found in these plant sources [1]. Secondary metabolites found in plants include active chemical molecules with potential therapeutic applications for various diseases [15]. Isolated secondary metabolites in plants are thought to account for fewer than 10% of the total. Metabolites are commonly employed to safeguard against insects, herbivores, and microbes. The diverse range of aromatic substances and their oxygen-substituted derivatives plants synthesize accounts for the extensive variety observed [16]. Recently, drug resistance has emerged as a major issue in healthcare; the rate at which drug-resistant diseases are increasing is far higher than the rate at which new medications are being tested and authorized for human use. Thus, it is crucial to create new antimicrobial drugs [17–19].

Infectious illnesses caused by microorganisms significantly contribute to human suffering and death. About 60% of the biomass on Earth is thought to be composed of microbial species. This, together with their tremendous genetic, metabolic, and physiological variables, renders them a danger to the well-being and progress of human communities everywhere [20]. Hence, nature is the source of a significant proportion of the drugs currently used, derived from microorganisms, flora, or fauna. Identifying and synthesizing novel compounds possessing pharmacological properties depends on the natural environment's biodiversity [12, 21]. Many plant components are available without a prescription from herbal distributors and natural-food stores, and selfadministration of these drugs is common even though their purity is often questionable [22]. Chemical analysis of medicinal plants has uncovered various bioactive chemicals, including saponins, tannins, and alkaloids [1, 23]. Also, flavonoids, terpenoids, and alkaloids are the primary constituents of phytochemicals in the plant kingdom [24]. The pharmacologically active compounds encompass a variety of alkaloids that can be categorized into several classes, such as piperidines, pyrrolizidines, quinolizidines, imidazoles, tropanes, pyrrolidines, indoles, isoquinolines, and purines [15]. They belong to a vast group of naturally occurring chemical compounds that include at least one nitrogen atom (particularly in the form of an amino or amido group). The nitrogen atoms often form a ring shape [25]. Alkaloids are plant-derived bioactive compounds typically exhibiting alkaline properties due to their nitrogen atoms [26].

With many plants still waiting to be discovered and examined for their phytochemical compositions, the future of therapeutic plants seems bright. Synthetic medicine design and development have benefited from learning about medicinal plants [1]. Thus, alkaloids are the subject of intensive study because they may constitute a novel class of naturally occurring antibiotics with a broad antibacterial range, few side effects, and a low propensity to result in drug resistance. The present chapter centers on investigating the antimicrobial potential of alkaloids obtained from medicinal plants against human pathogenic microorganisms, specifically emphasizing multidrug-resistant clinical strains. The chapter elucidates the mechanism of action of these alkaloids when available and underscores their concentrations and usage.

## **2. Plant products as an antimicrobial agent**

Pathogenic bacteria create dangerous and potentially fatal infectious diseases that affect humans [27, 28]. On the other hand, antibiotic resistance is a significant issue in the twenty-first century, and infectious illnesses are still the second-greatest cause of mortality globally despite the success of antibiotic discoveries [1]. The growing incidence of antimicrobials-microbes resistance is causing growing alarm among scientists. The advent of drug-resistant bacteria has increased the difficulty and expense of creating newer antimicrobials from novel chemical compounds [15, 28]. Despite the approval of synthetic antimicrobial agents in numerous countries, using natural compounds derived from microbial, animal, or plant sources has garnered significant interest among researchers [29]. Numerous researchers are currently engaged in the investigation of plants to identify potential antimicrobial agents [15]. The quest for compounds possessing antimicrobial properties is common, and scholars have shown interest in medicinal plants due to their widespread use in traditional medicine as a treatment for various infectious ailments [30]. Hence, the demand for and research into plant-based pharmaceuticals and nutritional aids has increased rapidly in recent years [31]. Studies conducted on plants utilized in traditional medicine have been performed *in vitro* within the realm of microbiology, with a particular focus on the proliferation of infectious bacteria [30]. Betoni et al. found that plant compounds can either act as antimicrobial agents that complement antibiotics or increase a pathogen's susceptibility to an antibiotic that would have otherwise been ineffective [30].

Researchers from fields as diverse as ethnopharmacology, botany, microbiology, and natural products chemistry scour the planet in search of phytochemicals and "leads" that might be refined into effective antimicrobial drugs [31]. New medications can be developed by optimizing the structural makeup of phytochemicals present in plants [1]. Phytochemicals and other substances derived from plants have been used to treat a wide range of infectious diseases because they exhibit good antibacterial action against many human infections [29, 32]. However, it is widely established that several extracts and components of plants have antibacterial activity. Unfractionated extracts are typically used in these studies, despite their low *in vitro* antimicrobial activity. *In vivo* tests were rarely used to verify the results of these investigations [12]. Phytochemicals, which are bioactive organic chemical compounds, are present in medicinal plants [33, 34]. These compounds protect against chronic diseases, including those caused by metabolic or genetic disorders and infectious diseases. They are present in various foods made from plants, including cereals, veggies, and fruits [1]. There are several classes of phytochemicals, including carotenoids, alkaloids, phenolics, organosulfur compounds, and nitrogen-containing compounds [5].

## **3. Alkaloids**

Alkaloids are naturally occurring compounds sourced from various organisms, including plants (which comprise approximately 300 plant families), bacteria, fungi, and animals [12]. The compounds and biomolecules exhibit significant diversity, yet all these chemicals are byproducts of the amino acid biosynthesis process or the transamination reaction [35]. Alkaloids are predominantly solid compounds that are commonly found in higher plants. The aforementioned botanical families, namely Leguminoceae, Papaveraceae, Solanaceae, Ranunculaceae, Annonaceae,

Amaryllidaceae, Liliaceae, Apocynaceae, Boraginaceae, Loganiaceae, Magnoliaceae, Berberidaceae, Piperaceae, Gnetaceae, Rutaceae, Lauraceae, Menispermaceae, and Rubiaceae, are known to exhibit a high prevalence of the subject matter [36]. Certain plant species employ naturally occurring insecticides or pesticides to protect themselves against the harmful effects of select insect species. The synthesis of vegetal alkaloids primarily occurs in herbaceous and vascular plants [12]. The Arabic word alqali designates the source of soda. German scientist Carl F. W. Meissner developed the term "alkaloid" in 1819 to describe this compound [36]. One of the biggest groups of secondary metabolites in plants, alkaloids are present in some economically relevant plant families [37]. As mentioned, they are present in various kingdoms. However, their distribution is restricted within each domain [8]. Alkaloids are classified into multiple categories. The categorization is founded upon the compounds' heterocyclic ring structure and biosynthetic forerunners. The abovementioned compounds comprise indoles, pyrrolizidines, quinolizidines, pyrrolidines, piperidines, tropanes, isoquinoline, purines, and imidazoles [15]. The amino acids nicotinic acid, L-histidine, L-ornithine, L-tryptophan, L-lysine, L-tyrosine, acetate, L-phenylalanine, anthranilic acid, and L-phenylalanine are all precursors to the alkaloid phenylpropanoid [35]. Alkaloids also exhibit various pharmacological and biological properties and may be found in many herbal treatments [38]. Alkaloids have been the fundamental framework for advancing multiple antibiotics showing a broad activity spectrum [16]. Nicotine, caffeine, and cocaine are just a few examples of alkaloids incorporated into popular culture as drugs used for entertainment or abuse. Certain alkaloids have been identified as possessing high toxicity levels, resulting in numerous instances of human poisoning [16].

Alkaloids have a wide array of pharmacological activities, including antibacterial activity [12]. Most alkaloids exert their effects via efflux pump inhibitor (EPI) activity, which is considered a potential mechanism of antibacterial action [29]. In addition to their use as stimulant medications, alkaloids may be found in many of the foods and drinks we consume regularly. They have shown several pharmacological effects, including those of local anesthetic, anticancer, analgesic, pain-relieving, antifungal, anti-inflammatory, neuropharmacological, and antimicrobial, [25], antimalarial action, oxytocic and vasoconstrictor activity (ergometrine), activity against the central nervous system (brucine), and activity against the cholinergic system (atropine) [16]. Alkaloids, which derive their name from their resemblance to alkalis, can undergo salt formation upon reaction with acids, similar to inorganic alkalis. The nitrogen atoms exhibit basic properties in acid-base responses [25]. Alkaloids are characterized by a nitrogen atom that accepts protons and multiple amine hydrogens that donate protons. Hence, the biological activity of biomolecules is primarily attributed to their ability to establish hydrogen bonds with other biomolecules such as enzymes, receptors, and proteins [12, 24]. Thus, alkaloids can be used for a variety of pharmacological purposes [24]. Several antibiotics have been developed from alkaloids: the quinolones were discovered by accident during the production of quinine; the structure of metronidazole was altered from that of azomycin; and the quinoline scaffold was utilized to create bedaquiline [8]. Alkaloids can also be found in other medications like linezolid and trimethoprim scaffolding. Academic institutions, private companies, and public-private partnerships continue studying alkaloids to create effective antibacterial drugs [8].

A straightforward quantitative approach for identifying alkaloids in plants was developed by Li et al. [39]. Using tetrahydrofurfuryl methacrylate as the monomer, in situ radical polymerization was used to construct a polymer-based chromatographic

monolithic column. Based on the results of the technique validation, the accuracy of the spiking recovery measures is between 98.89 and 102.06%. These findings demonstrate the constructed monolithic column's viability for avoiding the lengthy analysis time required by conventionally packed C18 columns in quantitatively analyzing alkaloids from actual medicinal and culinary plant foods [39]. Alkaloids are used internally to improve health, physical performance, and the immune system. These entities are common in daily dietary intake, drinks, and supplementary products. Several compounds present in plants exhibit advantageous characteristics. Compounds such as caffeine, guaranine, and mateine, found in various plants, including coffee, have been observed to possess anti-inflammatory, antioxidant, and stimulatory properties. Additionally, cocoa contains theobromine and paraxanthine, which act as antioxidants. Ginger, conversely, contains gingerol and shogaols, which are phenolic alkenones that possess antioxidant, anti-inflammatory, antimicrobial, and antitumoral properties [37]. However, we provide a brief overview of the class of alkaloids concerning antimicrobial activity.

## **3.1 Alkaloids classification**

At present, the number of identified alkaloids exceeds 18,000 [15]. Natural antibacterial alkaloids have been the subject of research since the 1940s, although most of the earliest studies did not go far enough to determine minimum inhibitory concentrations (MICs). Despite this class's large number of chemicals, only a fraction of their biosynthesis routes have been determined [40]. The chemical makeup or inherent biological source of these entities determines their classification [16]. Chemical structure and characteristics are used to divide alkaloids into several classes. The feasibility of classifying alkaloids based on their natural origin arises because certain alkaloids are limited to specific sources [16]. The chemical structure or biological origin of alkaloids allows for two broad categories:


As mentioned above, there are primarily three classes of alkaloids [36]:

## *3.1.1 Protoalkaloids*

Alkaloids having a closed ring structure are protoalkaloids; they are chemically perfect but have a straightforward molecular structure. Among the alkaloids, they are in the minority [35]. The most notable examples of these alkaloids include yohimbine, mescaline, and hordenine (a phenethylamine) (**Figure 1**). Hordenine, a Tyr-derived

**Figure 1.** *Some examples of protoalkaloids.*

phenylethylamine alkaloid, was initially discovered in *Hordeum vulgare* (barley) [41]. They are prescribed for various conditions, from mental illness to chronic pain to neuralgia. The nitrogen atom in these alkaloids comes from a source other than the heterocyclic ring structure; instead, it is generated from an amino acid. Typically, Ltryptophan and L-tyrosine are the precursors to these alkaloids. Simple alkaloids make form the framework of this minor class [36]. Protoalkaloids are compounds where the heterocyclic bond does not include the N atom from an amino acid. One type of alkaloid consists of compounds derived from the amino acids L-tryptophan and Ltyrosine [35].

#### *3.1.2 True alkaloids*

These alkaloids and their precursor amino acids both have nitrogen in a heterocyclic ring. These entities exhibit high reactivity and possess significant biological efficacy [36]. These compounds can dissolve in water and form salts soluble in water. Additionally, many of these compounds exhibit a crystalline structure and can undergo conjugation with acids to form salts. Most authentic alkaloids are characterized by their solid state and bitter flavor, except nicotine, a brown liquid. Common true alkaloids include cocaine, morphine, and quinine [36]. Morphine, an alkaloid generated from tyrosine, has a nitrogen-containing heterocyclic ring and is used as a painkiller. It exhibits potent analgesic effects and is widely used as a painkiller in clinical settings [42]. Not all alkaloids show significant biological efficacy; some have no known pharmacological activity [43].

These subgroups have unique properties and uses, making them essential modern medicine and research components. Understanding the classification of alkaloids is an important step in understanding their potential therapeutic applications. For example, various pharmacological effects are associated with indole alkaloids found in plants, many of which are thought to be attributable to the indole nucleus [44]. Common plant families proven to contain indole alkaloids include Loganiaceae, Rubiaceae, Apocynaceae, and Nyssaceae. Preclinical and clinical research has shown that several of the discovered indole alkaloid compounds are particularly effective [44]. According to their antimicrobial activity, the most critical phytocompounds across all alkaloid chemical groups are shown in **Table 1**.

Monoterpenoid indole alkaloids are a class of widely recognized alkaloids that are derived from tryptamine and secologanin. Numerous alkaloids exhibit intricate structures and significant biological properties, rendering them intriguing. Various species belonging to the Apocynaceae family, including *Tabernanthe iboga*, *Voacanga africana*, and multiple *Tabernaemontana* species, synthesize alkaloids, including the ibogan type [116]. Antibiotic and well-known alkaloid tryptanthrin (TRYP) (indolo[2,1-b] quinazolin-6,12-dione) is found in *Candida lypolica*, higher plants, and numerous










**Table 1.** *Classification*

 *of alkaloids in plants family based on their* 

*antimicrobial*

 *activity.*

marine microbes [117]. Various biological and pharmacological qualities are related to the several structural scaffolds, and a wide variety of functional group modifications is found in the broad class of plant-specific metabolites known as benzylisoquinoline alkaloids. N-Methylation is a widely used modification technique that forms intermediates and final products in the tertiary and quaternary metabolic pathways [118].

#### **3.2 Some selected alkaloids with antimicrobial activity**

Various alkaloids found in nature have been shown to have antimicrobial effects against a wide range of diseases [15]. Some selected alkaloids with potent antimicrobial activity include berberine, quinine, and vincristine. The potential for these particular alkaloids' antibacterial action to expand therapy choices for infectious disorders caused by drug-resistant microbes or those not responding to conventional therapies has been widely discussed [16]. Hence, this review focuses on alkaloids with antibacterial activity against MDR microorganisms. Also, this article describes the most influential alkaloids with potent antibacterial properties. Here are some selected examples of these compounds:

#### *3.2.1 Berberine*

The natural isoquinoline alkaloid berberine has been shown to have minimal toxicity [119]. Berberine, derived from *Berberis* spp., is a prominent quaternary ammonium salt of protoberberines. It exhibits various antimicrobial properties, particularly against Gram-negative bacteria [24]. *Berberis vulgaris*, *Coptis chinensis*, *Hydrastis canadensis*, *Coptidis rhizoma*, *Xanthoriza simplicissima*, *Phellodendron amurense*, and *Chelidonium majus* all contain it, among many others, making them useful as therapeutic herbs [119]. Berberine is an effective antibacterial agent that may one day replace conventional antibiotics and help combat the problems caused by antibiotic resistance. Methanol extract of *Pancratium illyricum* L. bulbs yielded the isoquinoline alkaloid ungeremine. Its antimicrobial qualities have been well-praised. As mentioned earlier, the compound can induce a significant augmentation in DNA cleavage through its selective targeting and inhibition of bacterial topoisomerase IA [29]. Herpes, influenza, and respiratory syncytial viruses are susceptible to berberine's antiviral actions [34, 119]. Berberine's mechanism of action against *V. cholerae* and *E. coli*induced diarrhea has been thoroughly investigated. The effects of *E. coli* and *V. cholerae* enterotoxins were found to be directly inhibited by berberine *in vitro* as early as 1982 [120]. Berberine's antibacterial activity against *S. aureus* has been shown in *in vitro* investigations [121]. As reported in reference, berberine and CinA can undergo self-assembly, forming nanoparticles (NPs) that exhibit bacteriostatic properties against MRSA and potentially eliminate biofilms [40]. Cinnamaldehyde (CinA) is a principal constituent of the *Cinnamomi cortex*, a traditional spice that finds extensive usage in everyday routines [122].

The alkaloid berberine sulfate is harvested from the bark and roots of several plants. It exhibits antibacterial, antifungal, and antiprotozoal properties. Berberine sulfate disrupts fimbrial formation in *Streptococcus pyogenes*, impeding bacterial attachment to mucosal or epithelial surfaces [123]. On the other hand, L-Tyr is widely recognized as the biosynthesis precursor of berberine. 13 different enzymatic processes are involved in the production of berberine from L-Tyr. Notably, biochemical analysis has been performed on all of the enzymes in this pathway [24].

### *3.2.2 Caffeine*

Numerous plant species derive caffeine (1,3,7-trimethyl xanthine) from methylated alkaloids. It is structurally related to uric acid [124]. However, recent studies have shown that caffeine also has antimicrobial properties, which has led to increased interest in its potential use as an alternative to traditional antibiotics. Understanding caffeine's antimicrobial activity is crucial in developing new treatments for drugresistant infections, making it an important area of research. Another study by Ibrahim et al. found that growth inhibition was most noticeable at concentrations of 0.50% and above against *E. coli* [124]. Also, caffeine concentrations in coffee extracts are high enough to concern human health, with 50% antibacterial activity against *S. enterica* [125].

## *3.2.3 Capsaicin (CAP)*

The berries of virtually all peppers in the genus *Capsicum* contain capsaicin, also known as 8-methyl-N-vanillyl-6-nonenamide [12]. Peppers, especially chili peppers, are members of the Solanaceae plant family, responsible for their distinctive flavor [11]. *Capsicum annuum* powder is a commonly utilized seasoning in various culinary traditions across the globe. Apart from its gastronomic application, CAP is employed for analgesic purposes in different severe and persistent medical conditions [12]. Pepper fruits may contain capsaicin at a rate of up to 1% of their total weight. It is naturally produced in the epidermal cells of the placenta, which are located close to the seeds. The compound tends to accumulate in the form of "blisters" on the surface of the placenta. The molecule is a potent agonist of the transient receptor potential vanilloid ion-channel receptor 1 (TRPV1), eliciting its characteristic hot, burning sensation. However, the beneficial effects of capsaicin and the TRPV1 receptor cannot be attributed primarily to this interaction [11]. In an *in vitro* investigation [126], six capsaicin derivatives were developed, each possessing phenolic hydroxyl, a benzene ring, and amide structures. These derivatives were subsequently evaluated for their antibacterial properties against *E. coli* and *S. aureus*. Two powerful chemicals found in *Capsicum* species were shown to have antimicrobial capabilities, and Cichewicz and Thrope identified them. The experiment results showed that the plain and heated extracts displayed different levels of inhibition against *Streptococcus pyogenes*, *B. subtilis*, *B. cereus*, *Clostridium tetani*, and *Clostridium sporogenes* [127].

## *3.2.4 Colchicine*

Colchicine has been around longer than most other pharmaceuticals [128]. The use of colchicine as a pharmacological agent in humans has been permitted by the Food and Drug Administration (FDA). It is a safe and productive anti-inflammatory medication derived from the *Colchicum* and *Gloriosa* plant species. Colchicine has been utilized in treating cardiovascular ailments due to its distinctive effectiveness as an anti-inflammatory agent [24]. The chemical origins of colchicine have been the subject of extensive research, facilitated by numerous feeding studies utilizing isotopelabeled substrates in Colchicum plants. Furthermore, a well-defined biosynthetic hypothesis has been established thanks to structural study of colchicine-related alkaloids isolated from several members of the Colchicaceae family [24]. The first biosynthetic studies on colchicine were performed by Leete in 1960 [129].

The medical application of colchicine in cancer chemotherapy is restricted due to its comparatively high toxicity, despite its potency as an anticancer agent. Nevertheless, colchicine is currently utilized in therapy [130]. Colchicine's potential anticancer impact on hypo-pharyngeal carcinoma was studied. Colchicine dose-dependently suppressed hypo-pharyngeal human cell proliferation [128]. Colchicine inhibited adhesion, migration, and cell invasion via decreasing expression of MMP9, uPA, and FAK/SRC [128]. Researchers have shown that colchicine inhibits the reproduction of the Flaviviridae family of viruses by blocking microtubule polymerization. Researchers believe colchicine, a well-known anti-inflammatory medication, can cure COVID-19 by decreasing inflammation [131].

## *3.2.5 Piperine*

Piperine has been extracted from various species of the Piperaceae botanical family [132], as shown chemically in **Figure 2** [132]. Piperine is a major compound of black pepper (*Piper nigrum*) and long pepper (*Piper longum*), two species of the Piperaceae family. Studies suggest piperine exhibits bioavailability-enhancing properties for select nutritional substances [133]. The biting quality that is distinct from black pepper is attributed to piperine. Piperine exhibits numerous pharmacological properties and confers various health advantages, particularly for chronic ailments. These benefits include mitigation of anti-inflammatory effects, insulin resistance, amelioration of hepatic steatosis [134], anti-aging, antidiabetic, cardioprotective, antimicrobial, and anti-obesity [132]. When ciprofloxacin and a piperidine-type alkaloid from

**Figure 2.** *Piperine and its structural isomers (adapted from Ul-Haq et al. [132]).*

the plants. Together, *P. longum* and *P. nigrum* were able to inhibit the development of a mutant *S. aureus* and considerably reduce MIC values for *S. aureus* [135].

In the case of absorption, it is noteworthy that piperine exhibits no metabolic transformations upon absorption, as evidenced by its presence in both intestinal tissues and serosal fluid. This suggests that piperine remains unaltered throughout the absorption process [132].

#### *3.2.6 Reserpine*

Reserpine, an indole alkaloid extracted from the plant Rauwolfia serpentina, is well-known for its potent EPI action. The co-administration of reserpine has improved the antibiotic susceptibility of various bacterial species, such as *Micrococcus* spp., *Streptococcus* spp., and *Staphylococcus* spp. [29]. Combining reserpine with other commercially available antibiotics has been shown to improve the antibiofilm response and eradicate a sizable amount of bacterial biofilm in a urinary catheterization model, as reported by Parai et al. [136]. In another study, many acyl reserpine derivatives were made and tested for their antimycobacterial and antioxidant activities against *Mycobacterium* TB, strain H (37) Rv. This was done because reserpine is thought to have therapeutic benefits. According to the findings, 10 of 18 derivatives exhibited more significant suppression of antimycobacterial activity than reserpine [137]. On the other hand, reserpine inhibits AcrB. Acriflavine resistance protein B (AcrB) is an MDR efflux transporter that belongs to the Resistance-nodulation-division (RND) superfamily [138].

#### *3.2.7 Tomatidine*

Steroid alkaloid tomatidine is harvested from nightshade plants, including tomatoes, potatoes, and eggplant. As monotherapy or in combination with aminoglycosides, there is evidence that it is highly effective as an antibacterial agent against *S. aureus* [29]. Tomatoes and tomatidine, as found by Silva-Beltrán et al., have great promise as a source of several bioactive chemicals, antioxidants, and antibacterial agents [139]. Tomatidine exhibited bacteriostatic activity against smallcolony variants linked to their impaired electron transport system. The electron transport inhibitor 4-hydroxy-2-heptylquinoline-N-oxide (HQNO) increased the sensitivity of typical *S. aureus* strains to tomatidine [140].

#### *3.2.8 Conessine*

*Holarrhena antidysenterica*, a member of the Apocynaceae family, has a long history of medical usage for treating dysentery, diarrhea, fever, and bacterial infections [141]. Conessine is a steroidal alkaloid. The therapeutic actions of *H. antidysenterica* barks are due to the presence of alkaloids, specifically the steroidal alkaloid conessine. There is preliminary evidence that this compound can kill gram-positive and gramnegative bacteria [141]. Based on the existing evidence, it can be inferred that the steroidal crude extract of *H. antidysenterica* and conessine exhibit properties of efflux pump inhibitors (EPIs). Recently, it has been reported that the steroidal extract and alkaloid conessine can augment the efficacy of antibiotics by impeding the AdeIJK efflux pump in *A. baumannii* [142].

Other alkaloid classes, namely indolizidine, pyrrole-imidazole alkaloid, quinoline, aaptamine, indole, isoquinoline, piperazine, polyamine, bisindole, quinolone, indolequinoline, agelasine, aaptamine-indole, pyridoacridine, and bispyrrole have been reported to exhibit antibacterial activity [37].

## **4. Alkaloids derived from medicinal plants and their antimicrobial activities**

The distribution of alkaloids within plant tissues is heterogeneous, as mentioned previously, with varying concentrations observed across plant parts such as roots, seeds, leaves, fruits, and bark. Distinct alkaloid types may exist in various parts of a single plant [12]. The alkaloids are the most abundant secondary metabolites in the *Zanthoxylum* genus, and they exhibit a wide variety of biological functions due to their structural diversity [143]. A study by Farouk et al. indicated that *Eurycoma longifolia* leaf extracts were tested for antibacterial efficacy against *Pseudomonas aeruginosa* and *S. aureus* bacteria. The extracts were prepared using various solvents, including acetone, ethanol, phosphate buffer, and methanol at 5–100 mg/mL concentrations. Several extracts inhibited bacterial growth, with the widths of the inhibition zones ranging from 7 to 25 mm [144]. In addition to causing serious side effects, treating fungal infections with antifungal drugs often leads to drug-resistant strains of the fungus. This highlights the critical need to investigate potential new antifungal medicines. It has been shown that alkaloids isolated from the leaves of *Ruta graveolens* L. are fungi toxic [145]. Flavonoids and quinoline alkaloids isolated from the roots of *Waltheria indica* L. showed that to have antifungal activity against *Candida albicans* [146]. **Table 2** summarizes some selected medicinal plants that possess alkaloids with antimicrobial properties.

In a study by Erdemoglu et al. [154], capillary GC-MS identified 15 alkaloids. 13αhydroxylupanine (50.78%) and lupanine (23.55%) were assessed to be the significant alkaloids in the aerial parts of *L. angustifolius*. Ammodendrine, tetrahydrorhombifoline, isoangustifoline, α-isolupanine, 5,6-dehydrolupanine, 11,12 dehydrolupanine, 13α-tigloyloxylupanine, 13α-acetoxylupanine, angustifoline, 13α-isovaleroyloxylupanine, 13α-valeroyloxylupanine, 13α-*cis*-cinnamoyloxylupanine, and 13α-*cis*-cinnamoyloxy-17-oxolupanine were analyzed as the minor alkaloids of the substances in this plant. The alkaloid extract showed modest effectiveness against *E. coli,* while a strong point against *B. subtilis*, *S. aureus*, and *P. aeruginosa*. The extract was only moderately effective against *Candida albicans* and *C. krusei* [154]. Although native to the Middle East and Mediterranean regions, *Peganum harmala* has been introduced to Australia and the United States [155]. The alkaloids of *P. harmala* are concentrated in its roots and seeds. All 13 Gram-positive (*S. pyogenes*, *S. epidermidis*, *S. aureus*, *L. monocytogenes B. pumilus*, *B. cereus*, and *B. anthracis*) and Gram-negative (*Brucella melitensis*, *P. aeruginosa*, *Salmonella typhi*, *Klebsiela pneumoniae*, *E. coli*, and *P. mirabilis*) bacteria tested showed inhibition by methanol extract [155]. *Papaver somniferum,* belonging to the Papaveraceae botanical family, has been the subject of extensive research due to its benzylisoquinoline alkaloids (BIAs), which have been utilized for medicinal purposes since ancient times. It is notable for being the sole commercial source of morphine and codeine and is regarded as the model plant for BIA research. *P. somniferum* synthesizes vital alkaloids, such as sanguinarine, papaverine, and noscapine [162].

Native to Oman, *Ficus sycomorus* has had its leaf extracts investigated for their ability to eradicate *Haemophilus influenzae*, *S. aureus*, *E. coli*, and *Proteus* spp. [152]. *Ficus sycomorus* is abundant in flavonoids, alkaloids, tannins, and phenolic compounds.



*Medicinal Plants – Chemical, Biochemical, and Pharmacological Approaches*

#### **Table 2.**

*Selected medicinal plants possess antimicrobial activity based on their alkaloids as components.*

The leaves were subjected to methanol extraction, and subsequent extraction with various solvents. The disk diffusion technique results showed that at concentrations of 0.22–2.02 mg/mL, the crude leaf extracts showed antibacterial activity against *E. coli*, with inhibition diameters ranging from 0 to 9 mm [152].

The Apocynaceae plants, *Catharanthus roseus*, and *Rauwolfia serpentina* are known for their production of significant alkaloids, including serpentine, vinblastine, vincristine, ajmalicine, reserpine, and ajmaline. These plants are role models for understanding how monoterpene indole alkaloids (MIA) are synthesized. Considerable knowledge exists regarding the physiological and ecological factors producing MIA in *C. roseus* [37].

The date palm is widely distributed throughout the Arabian Peninsula and is recognized as a significant economic crop. Date palms possess various chemical compounds such as vitamins, flavonoids, steroids, alkaloids, tannins, and carbohydrates. Except for *E. faecalis*, both the methanol and acetone extracts showed potent antibacterial activity [156].

## **5. Alkaloids' antibacterial mechanism of action**

Alkaloids have been observed to affect various metabolic systems in animals, and their toxic mechanism of action can display considerable variability. Toxicity may present itself via enzymatic alterations that affect physiological functions, obstruction of DNA synthesis and repair mechanisms by intercalating with nucleic acids, or modulation of the nervous system. Various alkaloids can exert an influence on different physiological processes [37]. However, bactericidal drugs are those that, in the absence of confounding variables, result in a 99.9% reduction in bacterial viability at doses no higher than four times the MIC [96]. Most research shows that alkaloids are antibacterial, not bacteriostatic, though this might vary depending on the species of specific alkaloids (such as chelerythrine and prosopilosidine) [8, 15]. The MIC values of squalamine have been demonstrated to be bactericidal within 1–2 hours, killing 99.99% or more of gram-positive and gramnegative bacteria [8]. Their primary antibacterial methods involve blocking bacterial metabolism, altering membrane permeability, and blocking the creation of nucleic acids and proteins [17]. Techniques involving the controlled introduction of pathogens or herbivores, the physical or chemical stimulation of their presence, and the subsequent monitoring of gene expression, enzyme activity, and concentrations of precursors and the alkaloid itself have proven effective [37]. The distinct classes of alkaloids exhibit varying mechanisms of action as antibacterial agents [37]. The antibacterial properties of pergularinine and tylophorinidine, which belong to the indolizine class of alkaloids, are attributed to their ability to inhibit the dihydrofolate reductase enzyme, thereby impeding the synthesis of nucleic acids [153]. Agelasines alkaloids affect bacterial hemostasis by inhibiting the dioxygenase enzyme BCG 3185c, contributing to their antibacterial action. Agelasine D is an alkaloid with antimycobacterial activities, and its overexpression and binding affinity in studies led to the result mentioned above [163]. The respiratory inhibition effects of synthetic quinolone alkaloids, as well as the cell division inhibition effects of isoquinolines, including protoberberine, berberine, benzophenanthridine, and sanguinarine through perturbation of the Z-ring, have been documented. Additionally, the phenanthridine isoquinoline alkaloid ungeremine has been found to inhibit nucleic acid synthesis. In contrast, the indolizidine alkaloids pergularinine and tylophorinidine have been

shown to suppress nucleic acid synthesis by inhibiting dihydrofolate reductase [37]. The mechanisms of action about antibacterial activity exhibit variation across distinct alkaloids. The following examples are being examined [16]:

1.Disruption of the bacterial membrane.

Several alkaloids from herbal plants have been discovered to exhibit antimicrobial activity by disrupting the bacterial membrane. For example, herbal alkaloids like berberine and palmatine have been proven to cause bacterial cell death by rupturing their membrane [164, 165]. Additionally, squalamine is a polyamine alkaloid with a detergent-like mode of action, depolarizing Gram-positive bacteria membranes and disrupting Gramnegative bacteria's outer membranes [16]. The cytoplasmic membrane is disturbed by phenanthroindolizidine alkaloids [166]. For instance, berberine attacked the mitochondrial membrane of fungi and resulted in cytoplasmic damage in *Streptococcus agalactiae* (CVCC 1886 strain, obtained from the Microbiological Lab of Sichuan Agricultural University, Ya'an, China), whereas liriodenine caused cytoplasmic changes and cell wall destruction in *Paracoccidioides brasiliensis* [9].

2. Interfering with cell division.

Pergularinine and tylophorinidine, two phenanthroindolizidine plant alkaloids, can block the production of nucleic acids. Protein, RNA, and DNA synthesis rely on pyrimidine and purine precursors, produced by the crucial enzyme dihydrofolate reductase [16]. DNA-protein cross-linking and DNA cross-linking are two mechanisms through which certain alkaloids, such as aristolochic acids, can cause mutations [167]. Interaction with DNA is thought to be the primary mechanism by which quinoline alkaloids exert their antibacterial and antifungal effects [9]. Another example is berberine, which was effective against *Actinobacillus pleuropneumoniae* and *Streptococcus agalactiae* (CVCC 1886) by inhibiting DNA synthesis and preventing synthesis [168].

3.Bacterial enzyme and respiratory system inhibition:

Alkaloids from herbal plants have been reported to inhibit bacterial enzymes and respiratory systems. For example, inhibiting the respiratory system of bacteria, including *S. aureus*, has been demonstrated for the alkaloid tetrandrine, which is present in several medicinal plants [15]. Additionally, berberine can inhibit bacterial enzymes like DNA gyrase leading to cell death [15]. Also, the alkyl methyl quinolone alkaloids exhibit potent and selective antibacterial properties against *H. pylori* using respiratory inhibition [169].

4.Modulating the expression of virulence genes.

The regulatory protein ToxT has been identified in *V. cholerae.* It plays a crucial role in activating various virulence determinants, including the genes responsible for encoding virulence factors. Additionally, Yang et al. report that cholera toxin and ToxT co-regulated pilus [170]. The isoquinoline alkaloid known as virstatin has been found to effectively inhibit ToxT, which subsequently results in the inhibition of virulence factors. The research showed that it prevented *V. cholerae* from colonizing the intestines of newborn mice models [16].

On the other hand, the majority of quinoline and indole-based antifungal and antibacterial alkaloids discovered in Asian angiosperms, respectively, target DNA, topoisomerases, and the cytoplasmic membrane as their primary sites of action [9].

## **6. Conclusions and future**

Alkaloids comprise a vast and heterogeneous category of compounds that exhibit a broad-spectrum of biological functions that hold immense significance for plants, animals, and humans. These compounds possess remarkable pharmacological properties. The advent of antibiotic-resistant microorganisms has substantially compromised antibiotic effectiveness. To date, a new approach to tackling antibiotic resistance is urgently needed. In the coming years, bioactive compounds will likely be discovered using phytochemicals, which exhibit a variety of chemical structures and methods of action. Alkaloids exhibit varying primary functions across different plant species, and their metabolic profiles are often associated with distinct environmental factors and developmental cues, thereby providing evident adaptive advantages. Concerning potential toxicity to other organisms or the production of bioactive metabolites for therapeutic applications, the variation in plant alkaloid metabolism and accumulation is crucial. Alkaloids are effective in this review report as an alternate therapy for combating the emergence and spread of multidrug-resistant infections and the harmful effects of some antibiotics. The following compounds have been identified as primary candidates due to their MIC of less than 1 μg/mL: 8-Acetylnorchelerythrine, cryptolepine, sampangine, 8-hydroxydihydrochelerythrine, 6-methoxydihydrosanguinarine, 2<sup>0</sup> -nortiliacorinine, tiliacorine, rhetsisine, pendulamine A and B, tylophorinine, tryptanthrin, viroallosecurinine, and vallesamine.

## **Author details**

Mohammad Barati and Amir Modarresi Chahardehi\* Infectious Diseases Research Center, AJA University of Medical Sciences, Tehran, Iran

\*Address all correspondence to: amirmch@gmail.com

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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## Synthetic Alkaloids: Cantharidin Derivatives

*Nurhan Kishali*

## **Abstract**

Cantharidin is a naturally occurring cyclic anhydride found in many insect species, particularly Lytta vesicatoria, known as the Spanish fly. Although highly poisonous, dried Spanish fly has been used as an aphrodisiac since ancient Greeks and Romans. Spanish fly has been used in eastern medicine for many years as a natural anticancer agent, especially in the treatment of hepatoma and esophageal carcinoma. Over time, its stotoxicity was determined to be high and its use was limited. Later, alkaloid derivatives with no stotoxic effect were produced synthetically and evaluated as anticancer agents. Since cantharidin obtained from insects is not an alkaloid but its derivatives with lower stotoxicity, cantharimide and norcantharimide are cyclic imides, they can be evaluated in the class of alkaloids. Cantharimide and norcantharimide compounds have gained importance in terms of their stotoxic effect on many cancer cell lines. Many studies have been done on their synthesis and anticancer properties for many years.

**Keywords:** alkaloids, cantharidin, cantharimide, norcantharimide, anticancer

## **1. Introduction**

Alkaloids are chemical compounds that contain basic Nitrogen atoms and are produced naturally by various organisms. Alkaloids can contain some groups with neutral [1] or acidic properties [2]. Alkaloids are usually organic bases. They form salts when reacted with acid and form alkaline solutions when dissolved. Primary sources of alkaloids are flowering plants. Plants use alkaloids for defense against herbivores and pathogens. It has been determined that 20% of plants contain alkaloids. Alkaloids are cyclic compounds that contain nitrogen (**Figure 1**) [3].

Plenty of alkaloids have been used in medicine for ages and even nowadays, they are prominent medical compounds. Since primitive times, alkaloids obtained from plant extracts have been used in medicines and poisons. In ancient times, plant extracts containing alkaloids were used to treat numerous ailments, including snakebite, fever, and insanity. Generally, alkaloids are extremely toxic at low concentrations, even if they have a therapeutic effect. Defense chemicals of plants against microorganisms, insects, and herbivores, as well as other plants using allelopathic active chemicals [4]. Their taste is bitter. They are usually colorless crystals at room temperature and are optically active [5]. Purely isolated plant alkaloids and their

**Figure 1.** *Examples of alkaloids.*

synthetic derivatives are used as basic medicinal agents due to analgesic, antispasmodic, and bactericidal effects [6]. Alkaloids generally affect the nervous system in humans (especially acetylcholine, epinephrine, norepinephrine, gamma-aminobutyric acid, dopamine, and serotonin) [7]. Alkaloids such as berberine (in eye medications) and sanguinarine (in toothpaste) are used as antiseptics (**Figures 2** and **3**) [8].

While the search for new anticancer drugs continue, old drugs are viewed as new options. The dried body of Mylabris, the Chinese bubble beetle, has been the focus of attention for its anticancer properties, as it is known for traditional medicine in China for more than two thousand years, where it has been used as a traditional medicine. The oldest data in China on the use of Mylabris as a medicine dates back to 300– 168 BC. In Europe, it was found about 77 AD in a medical article published in Materia Medica. The active ingredient of Mylabris has been identified as cantharidin. Later, it

**Figure 2.** *Examples of alkaloids acting on the nervous system.*

**Figure 3.** *Chemical formula of berberine and sanguinarine alkaloid.*

*Synthetic Alkaloids: Cantharidin Derivatives DOI: http://dx.doi.org/10.5772/intechopen.111912*

#### **Figure 4.**

*The chemical formula of cantharidin, norcantharidin, cantharimide, and norcantharimide.*

was determined that cantharidin has both anticancer activity and leukocytosis and hemorrhagic cystitis properties [9]. When the natural product cantharidin was purified and used for a long time, its cytotoxic effects began to be observed, and norcantharidin, a demethylated derivative, was synthesized. Subsequently, nitrogencontaining derivatives cantharimide and norcantharimide were synthesized. Later, derivatives of these four analogs were synthesized both in the amide ring and in the cyclohexyl ring, and biological activity studies were carried out (**Figure 4**).

Norcantharidin slows the proliferation of tumors such as HeLa, CHO, CaEs-17, BEL-7402, SMMC7721 human hematoma, HEP-2, and human epidermoid larynx carcinoma [10–12]. Norcantharidin has been found to have fewer nephrotoxic and inflammatory effects [13–17]. Based on the structure–activity relationship, more analogs have been synthesized. For this reason, based on the structure–activity relationship, the researchers synthesized more analogs, and each synthesis was supported by biological activity and anticancer studies [18–28].

Disodium cantharidate and norcantharidin derivatives, among the compounds synthesized analogously, are among the derivatives synthesized in the earliest period by Wang et al. [10]. Because of their found stronger antihepatoma activities, these derivatives were more popular than cantharidin itself. However, they also cause minimal urinary irritation (**Figure 5**) [10].

Cantharidin has many derivatives besides disodium cantharidat and norcantharidin, some of which are hydrocantharidimite, methyl cantharidimite, and dehydronorkantharidin (**Figure 5**). Chinese scientists are collecting their medical literature by researching cantharidin and its derivatives, but there are still large gaps in researchers' knowledge about these drugs and their effects [29–32]. According to current information, norcantharidin, a demethylated analog of cantharidin, is stated to slow down the proliferation of tumors such as HeLa, CHO, esophageal carcinoma (CaEs-17), hepatoma (BEL-7402 and SMMC-7721), epidermoid larynx carcinoma (HEP2), and human epidermoid larynx carcinoma [10–12].

Two main methods were used in the synthesis of cantharimide and norcantharimides. One of them is the addition of furan and (2,3-dimethyl)maleic anhydride and then its conversion to the imide derivative. The other (2,3-

**Figure 5.**

**Figure 6.**

*General synthesis methods of cantharimide and norcantharimides (R*<sup>0</sup> *: Primary amine containing the desired derivative to be synthesized).*

dimethylmaleimide) is made by initially synthesizing maleimides and adding Diels-Alder with furan (**Figure 6**).

## **2. Cantharimides**

Cantharidine is found in *Cantharis vesicatory*, *Lytta caraganae*, *Mylabris phalerata*, *Meloidae*, *Oedemeridae*, and various other insect species. Derivatives of cantharidin are synthesized to reduce their blistering effects on the skin, reduce their toxic properties, and benefit from their antitumor activity. Since the 1950s, it has been determined that various anhydride and imide derivatives exhibit a wide range of biological activities (antidepressant, anticonvulsant) as well as antitumor properties [33–43]. For this reason, cantharimide, which has an imide structure, attracted the attention of Guang-Sheng WANG reported that he synthesized cantharimide and N-methyl cantharimide in an article he prepared in 1989 and conducted an activity study in KB cell culture. As a result of his study, he stated that both compounds showed antitumor activity [10].

Pen-Yuan Lin and his group synthesized 10 different cantharimide derivatives using various tryptamine, indolyl, naphthyl, and pyridyl amines [34]. Lin used Zhang's method in his synthesis. In this method, in the presence of triethyl amine at high pressure, the related amine compound gives cantharimides as a result of an addition reaction under pressure at 200°C (**Figures 7** and **8**) [44].

Another of the cantharimide derivatives is the derivatives obtained by the addition of heterocyclic groups. These derivatives were synthesized for cytotoxicity tests against human hepatocellular carcinoma cells. In this synthesis, N-thiazolyl and Nthiadiazolyl cantharimides were synthesized by the method used above. According to the results of the study, they observed that the side groups attached to N-Thiazolyl and N-Thiadiazolyl amine compounds showed higher cytotoxicity than cantharidin in

**Figure 7.** *Synthesis of tryptamine, indolyl, naphthyl, and pyridyl canthrimidines [44].* *Synthetic Alkaloids: Cantharidin Derivatives DOI: http://dx.doi.org/10.5772/intechopen.111912*

**Figure 8.** *Synthesis of cantharimides at high pressure [44].*

the area of electron-withdrawing (such as -NO2), whereas a decrease in cytotoxic activity was observed in methyl-substituted compounds (**Figure 9**) [45].

In 2007, a new cantharimide derivative was synthesized by Chan et al. Its structure was elucidated and its cytotoxicity on SK-Hep-1 hepatoma cells was determined. In this study, two cantharimides were synthesized with 2-amino benzothiazole derivatives. Cantharidin and 2-amino benzothiazole derivatives were added to a tube containing dry toluene and triethylamine (TEA), and related derivatives were synthesized at 200°C [45]. As a result of cytotoxic studies of the study group, it was determined that the compounds showed inhibition on SK-Hep-1 hepatoma. Using the results from the study, the group is also designing new (**Figure 10**) molecules [46].

Cantharimide derivatives were also synthesized using aliphatic primary amines, phenethyl amines, aniline, and pyridine components by high-pressure addition reaction method. In the synthesis, the product was obtained with a yield of 29—96%. It is thought that the primary factor affecting the efficiency is the inductive effect of electron-negative groups. Another factor affecting the yield is the nucleophilic and basicity strength of the amines. Again, cytotoxicity studies of the compounds obtained in this synthesis were performed in human hepatocellular carcinoma (Hep G2) and myeloid leukemia cells (HL-60). In the evaluation, it was determined that compounds (**10** and **16**) with electron-withdrawing NO2 groups in the pyridyl and benzene rings showed strong inhibitory effects in both cell lines. Methyl-containing compounds

**Figure 9.** *The synthesis of the N-thiazolyl- and N-thiadiazolyl cantharimides [45].*

**Figure 10.** *The synthesis of the (Me/-OMe)-2-amino benzothiazole cantharimides [46].*

**Figure 11.** *Cantharimides are synthesized by the high-pressure addition reaction method [33].*

**Figure 12.** *Dimeric-canthraimides isolated from M. Phaletata [34].*

(**6**, **7**, and **9**) have less effect, while halogen-containing compound (**8**) has a moderate effect. Compound **10** with the 2-(3-nitro pyridyl) group showed stronger cytotoxicity. Para-Pyridyl imide **4** showed greater potency than ortho- or meta-pyridyl imide. 3- Pyridyl (**5**) and N-phenyl imides (**14** and **16**) also showed strong cytotoxicity. Compounds with planar side chains (**12**, **14**, and **20**) and N-azaethyl or N-aryl compounds (**17**, **18**, **19**, and **21**) showed moderate cytotoxicity on both Hep G2 and HL-60. However, aliphatic chain imides (**1**, **2**, and **3**) were found to have very low effects in the studied cell lines (Hep G2, HL-60) (**Figure 11**) [33].

Until 2017, about thirty cantharidin derivativeshave been isolated from insects of the genus *Mylabris* and *Hycleus*. As cantharimide compounds are known to exist in different insects from *Mylabris* and *Hycleus* species, thirteen new cantharidin derivatives have been isolated from the whole body of *Mythicomyia phalerata* as part of studies to discover new potential antitumor agents [34]. During this purification, dimeric cantharimides were also obtained. The cytotoxic effects of the isolated derivatives against HepG2, MDA-MB-231, and A-549 cell lines were investigated, and it was determined that all isolated compounds showed high activity, except for the compound called Canthaminomide F (**Figure 12**) [34].

Due to the presence of dimeric products in cantharimides obtained from natural sources, some researchers have included these compounds in their synthesis. In a study conducted in this way, cantharimide dimers were synthesized and the structure was determined. The dimers synthesized in the same study were also isolated from natural sources. Along with these dimers, two different cantharimides were purified (**Figure 13**) [35].

*Synthetic Alkaloids: Cantharidin Derivatives DOI: http://dx.doi.org/10.5772/intechopen.111912*

**Figure 13.** *Cantharimide dimers from the Chinese* Blister Beetle *[35].*

## **3. Norcantharimides**

Studies on norcantharimides are examined by dividing them into two groups distribution methods and medicinal chemicals. Compounds such as norcantharimide [36] and norcantharidin dimer [37] belong to the class of medicinal chemicals. Norcantharimides are made from either the imide ring [38] or the cyclohexyl ring [39] in their medicinal chemical derivative synthesis. Then, new derivatives [40] were obtained by opening the imide ring in these derivatives (**Figures 14** and **15**).

In addition, when dimer products were detected in cantharimide derivatives purified in natural sources, dimer structures in norcantharimide derivatives were included in the synthesis [36]. Norcantharidin-dimer analogs as analogs of norcantharidin, norcantharidin with lactose acid, norcantharidin with amantadine, water-soluble norcantharidin with chitosan analogs, esterification of norcantharidin, amino acid norcantharimides, N-substituted dehydronortharimid analogs, immune liposomes, and modifications of norcantharidin show potential in the anticancer field [36].

Some norcantharidin analogs are known to have very good PP1 and PP2A inhibitory activity [22, 41–43]. In addition, studies by McCluskey et al. reported that some norcantharidin analogs, which have no toxic effect on human cell lines, kill trichostrongylid nematode *Trichostrongylus vitrinus* and *Haemonchus contortus larvae* [47].

McCluskey synthesizes a large number of norcantharimide in his studies and conducts various activity studies. In one of these studies, he synthesized 54 norcantharimide compounds and conducted a toxicity study against H. Contortus, which showed serine–threonine phosphatases (PP1 and PP2A) [48]. As a result of the study, it was determined that three of the 54 analogs synthesized were almost completely lethal against H. contortus and showed at least five times more inhibition

**Figure 14.** *Examples of norcantharimide as a medicinal chemical [36–40].*

#### **Figure 15.**

*(A) Reagents and conditions: (a) Et2O, rt., 48 h; (b) acetone, 10% Pd–C, H2(g) 50 psi, 18 h; (c) RNH2, PhCH3, reflux, 24–36 h (d) RNH2,THF, rt [41]. (B) Reagents and conditions: (a) CH3OH, rt. 30 min; (b) PhCH3, sealed tube 200°C, 36 h.*

than the control compounds. McCluskey' reported that he synthesized 54 compounds by the methods given below **Figure 14** [47].

In another study published by McCluskey in 2011, eighteen phosphate esters of the side group attached to the imide nitrogen were synthesized. It has bioscreened them in nine human cancer cell lines (HT29 ve SW480, MCF-7, A2780, H460, A431, DU145, BE2-C, SJ-G2). As a result of the study, he stated that he obtained a new series of norcantharidin analogs with broad-spectrum antiproliferative activity [41]. Another important finding from the study is the relative ease of Phosphate ester hydrolysis. Of the phosphates studied, diphenyl and bis-trichloroethyl analogs showed the highest level of cell death (**Figure 16**) [49].

McCluskey also conducted biological activity studies by synthesizing norcantharidin-dimer analogs. McCluskey explained the relationship between heterocyclic substituted (nor)cantharidin analogs and PP1 and PP2A as a result of his studies. McCluskey et al. synthesized many norcantharidin analogs, including two bis-norcantharimides. Among the compounds it synthesized were compounds containing 10, 12, and 14 alkyl chains attached to the imide nitrogen, 1,2-diol units, and two norcantharimide attached to dodecyl. Among these groups, two norcantharimide (Bis-Norcantharimide) bound to dodecyl showed the highest activity in the cell line [42].

Evaluating the results obtained from this study, Cuifang Cai showed that the compound containing N-C14H29 (**Figure 17**) side group has a longer half-life and *Synthetic Alkaloids: Cantharidin Derivatives DOI: http://dx.doi.org/10.5772/intechopen.111912*

**Figure 16.**

*Reagents and conditions: (a) H2N–X–OH, D, 18 h; (b) ClP(O)(OR)2, n-Bu2O, Et3N, rt [49].*

#### **Figure 17.**

*N-C14H29 and norcantharimide, whose physical and chemical stability was determined by Cuifang Cai [50].*

higher volume of distribution (Vss) compared to norcantharidin, as a result of the pharmacokinetic study performed by injecting it into rats. The results obtained from this study suggested that the formulation to be prepared with N-C14H29 is a promising alternative with its high encapsulation efficiency, significant physical and chemical stability, and long half-life (**Figure 17**) [50].

Dimeric compounds norcantharidin compounds have been synthesized and studied by many researchers for the treatment of cancer, HIV, Alzheimer's, malaria, and various parasitic diseases. Some of the scientists who carried out these studies are Gervais [51], McCluskey [42], T. Nakatani [35], Tang [52], and Tan [38].

When Xue-Jie Tan and his group realized that good results were obtained both in extract analysis and in many synthesis and activity studies, they used four dimeric norcantharimide directly synthesized in their studies. Two of these compounds were synthesized for the first time by Xue-Jie Tan and his group, while the remaining two were synthesized by Dominic V. McGrath's group. However, Xue-Jie Tan and his group presented the crystal structure, spectroscopic properties, and anticancer activities of these four unsaturated dimeric norcantharimides to the knowledge of researchers (**Figure 18**) [38].

**Figure 18.** *Dimer-norcantharimides synthesized by Tan et al. [38].*

## **4. Conclusion**

Alkaloids are defined as amine compounds naturally produced by plants. In addition, alkaloids are defined as secondary metabolites with important biological properties. It is also known that some alkaloids are beneficial for some diseases. In light of this information, we can evaluate cantharimide and norcantharimide derivatives in the alkaloid class, even if they are not of plant origin. Considering the information, we have briefly compiled above, the starting compound is a product of natural origin. Later, all of the synthesized derivatives contained nitrogen atoms, and positive test results were obtained on many disease-causing agents. After obtaining a large number of alkaloid cantharimide derivatives by extraction, it has also begun to be synthesized as a medicinal chemical. Researchers at the time sought to improve previous methods. In this review, it was concluded that medicinal chemical-synthesized derivatives of an insect-derived alkaloid can be used for pharmaceutical purposes.

## **Acknowledgements**

We would like to thank Atatürk University (Project number: 2012/470 and FBG-2018-6476) for its financial support of this work.

## **Author details**

Nurhan Kishali Faculty of Sciences, Department of Chemistry, Ataturk University, Erzurum, Turkey

\*Address all correspondence to: nhorasan@atauni.edu.tr

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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## Section 3
