Exploring the Biomedical Potential of Terpenoid Alkaloids: Sources, Structures, and Activities

Terpenoid alkaloids are recognized as a class of compounds with limited numbers but potent biological activities, primarily derived from plants, with a minor proportion originating from animals and microorganisms. These alkaloids are synthesized from the same prenyl unit that forms the terpene skeleton, with the nitrogen atom introduced through β-aminoethanol, ethylamine, or methylamine, leading to a range of complex and diverse structures. Based on their skeleton type, they can be categorized into monoterpenes, sesquiterpenes, diterpenes, and triterpene alkaloids. To date, 289 natural terpenoid alkaloids, excluding triterpene alkaloids, have been identified in studies published between 2019 and 2024. These compounds demonstrate a spectrum of biological activities, including anti-inflammatory, antitumor, antibacterial, analgesic, and cardioprotective effects, making them promising candidates for further development. This review provides an overview of the sources, chemical structures, and biological activities of natural terpenoid alkaloids, serving as a reference for future research and applications in this area.


Introduction
Alkaloids, a diverse class of secondary metabolites, are widely distributed in nature, with more than 27,000 species identified to date, predominantly originating from the plant kingdom, though relatively few are found in the animal and microbial kingdoms [1].They typically exhibit strong biological activities, including antitumor, antibacterial, insecticidal, and analgesic effects [2][3][4].Among the numerous classes of alkaloids, terpenoid alkaloids (TeAs) occupy a pivotal position.These alkaloids are formed from terpenoids through amination reactions, making them aminated terpenes [5].TeAs are classified as pseudo alkaloids primarily because their biosynthetic origins do not involve the amino acid pathway.Instead, terpenoid moieties in TeAs are biosynthesized from isoprene through the methylerythritol phosphate (MEP) pathway, while nitrogen atoms are typically introduced into the structures of terpenoids in the form of β-aminoethanol, ethylamine, or methylamine [1].
Despite the vast variety of alkaloids and terpenoids isolated from nature, only a tiny proportion of them conform to the structural features of TeAs.TeAs, as a natural product with diverse structures, are primarily divided into monoterpene, sesquiterpene, diterpene, and triterpene alkaloids according to the differences in their skeletons [6].Among them, monoterpene alkaloids are derived from iridoid compounds, mainly concentrated in the plants of Bignoniaceae, Lamiaceae, Gentianaceae, and Scrophularia [7].Sesquiterpene alkaloids are the least abundant class of TeAs, which are narrowly distributed in the plant kingdom and mainly concentrated in plants such as Dendrobium [5].Diterpenoid alkaloids (DAs) are the most complex and numerous compounds in TeAs, mainly concentrated in the Aconitum and Delphinium plants of Ranunculaceae [8].In addition, marine sponges are also an important source of diterpenoid alkaloids.
Although small in number, these alkaloids are widely bioactive.For example, incarvillateine, a monoterpene alkaloid with strong analgesic activity, isolated from the traditional Chinese medicine Incarvillae sinensis LAM., has become a significant lead compound in the development of new non-narcotic pain medications [9].DAs have been used for many years as traditional medicines in China, Japan, Russia, Mongolia, and India [10].Because of their severe toxicity, in ancient times, Aconitum roots were often used to hydrolyze highly toxic DAs (e.g., aconitine) into less toxic derivatives (e.g., benzylaconine) by soaking, boiling, or other processing methods [8,11].Modern pharmacological studies have shown that diterpene alkaloids have significant anti-inflammation, analgesia, anticancer, and anti-arrhythmia effects [8].Moreover, as a diterpenoid alkaloid, Crassicauline A has been clinically utilized as an anti-arrhythmic drug [12].Similarly, cyclovirobuxine-D, a triterpene alkaloid, is also used clinically as an antiarrhythmic drug [13] and has been recognized as a lead compound for innovative analgesics [14].
The research significance and medical value of TeAs as a class of natural products with unique structures and a wide range of biological activities are clear.Given the complexity and variability of triterpenoid alkaloids' structures and the constraints of space, this paper will focus on the sources, chemical structures, and biological activities of natural TeAs, excluding triterpene alkaloids, discovered in the past five years, hoping to provide a reference for the further research and application of TeAs.

Classes of Terpenoidal Alkaloids 2.1. Monoterpenoid Alkaloids
Monoterpenoid alkaloids represent a distinct class of alkaloids derived from iridoid glycosides, typically originating from loganin and secologanin after amination.According to Wang's classification of monoterpene alkaloids, they can be divided into two categories: iridoids and secoiridoids [15].This section discusses 26 monoterpenoid alkaloids isolated from plants, including 24 iridoid-type alkaloids  and two secoiridoid-type alkaloids (25)(26).Specific plant sources are listed in Table 1.The chemical structure details are shown in Figure 1.
2.1.1.Iridoid-Type Alkaloids  The biosynthetic precursors of these alkaloids are iridoid glycosides.Based on the level of hydrogenation within their nitrogen-containing six-membered rings, they can be classified into four subtypes: pyridine ring type, piperidine ring type, dihydropyridine ring type, and tetrahydropyridine ring type [15].
Isoxerine (13), isolated from the roots of Scrophularia ningpoensis, was named due to its absolute configuration of C-7 being 7S, differing from oxerine [20].Forsyqinlingines C (14) and D (15) were isolated from the ripe fruits of Forsythia suspensa (Oleaceae), with the structures determined by analysis of spectra including HR-ESI and NMR.Both alkaloids belong to a rare class of planar C9-monoterpenoid alkaloids [21].

Sesquiterpene Alkaloids (27-32)
Sesquiterpene alkaloids represent the least abundant class of TeAs derived from sesquiterpenes and incorporate nitrogen atoms in the basic skeleton of sesquiterpenes [27].This subsection mainly describes six sesquiterpene alkaloids from nature, including a rare alkaloid from the ocean.The plant sources are listed in Table 2, and the chemical structure details are shown in Figure 2.
This kind of alkaloid is mainly distributed in Gentianaceae plants and derived from secoiridoid glycosides [15].Longiflorine (25), isolated from the leaves of Uncaria longiflora var.Pteropoda (Rubiaceae), is a monoterpenoid alkaloid with a lactam ring derived from secologanin [26].Lomatogonin C (26), isolated from dried whole plants of Lomatogonium carinthiacum (Gentianaceae), is a natural monoterpene alkaloid derived from secoiridoid [25].(27)(28)(29)(30)(31)(32) Sesquiterpene alkaloids represent the least abundant class of TeAs derived from sesquiterpenes and incorporate nitrogen atoms in the basic skeleton of sesquiterpenes [27].This subsection mainly describes six sesquiterpene alkaloids from nature, including a rare alkaloid from the ocean.The plant sources are listed in Table 2, and the chemical structure details are shown in Figure 2.  Commipholactam A (27) was isolated from the dried myrrh of Resina Commiphora and represented a rare cadinane-type sesquiterpenoid.Unlike typical cadinane sesquiterpenoids, where ring C is usually present as a lactone, compound 27 appears as a lactam ring [28].

Diterpenoid Alkaloids (DAs) (33-289)
DAs are the most abundant and structurally complex TeAs, characterized by numerous stereocenters.They typically originate from the amination of tetracyclic or pentacyclic diterpenes, forming heterocyclic systems possessing β-aminoethanol, methylamine, or ethylamine nitrogen atoms [32].Based on the number of carbon atoms in the skeleton of DAs, they can be classified into three major categories: C18, C19, and C20 [33].Shen Yong comprehensively reviewed the classification of diterpenoid alkaloids in 2020 [8].Consequently, this article will not delve into an extensive discussion of this classification but will focus only on the classification of new members discovered in the past five years.This section describes 257 newly discovered natural DAs, including 11 C18-DAs, 139 C19-DAs, 84 C20-DAs, 14 Bis-DAs, and 9 other types of DAs.These DAs were predominantly isolated from the plants in the Aconitum and Delphinium genera, with only two new DAs isolated from the ripe fruits of Forsythia suspensa.Additionally, five new DAs were obtained from microorganisms and marine animals.(33)(34)(35)(36)(37)(38)(39)(40)(41)(42)(43) Without C18 in the structure, these alkaloids predominantly feature a 4-OH or ester substitution, with a few compounds having 3,4-epoxy substitution.According to the presence or absence of oxygen-containing groups at C7, they are classified into lappaconitines and ranaconitines [32].Eleven C18-DAs (33)(34)(35)(36)(37)(38)(39)(40)(41)(42)(43) are described in this subsection.Plant sources are shown in Table 3. Detailed chemical structures are shown in Figure 3.    Compounds 33-37 are identified as lappaconine-type alkaloids, with four originating from Aconitum and only Naviconine (33) derived from Delphinium [34][35][36].Amino groups are generally uncommon in DAs, whereas Leucostosine C (35) is the first naturally occurring DA to feature an amino group attached at C-7 [36].
Compounds 41 and 42 are rearranged C18-DAs, where the C7-C17 bond was rearranged to a C8-C17 bond [37].Compounds 41 and 42 are derived from the rearrangement of ranaconitine-type DAs, and both contain an oxygen-containing hydroxyl group at C7. Additionally, Barpubenine A (41) is the first reported N-oxide in C18-DAs [37].
According to the different oxygen-containing groups at C-7 and C-8, they can be divided into two subtypes.Eleven new compounds (115-125) feature a C-7 and C-8 diol.Compounds 114 and 126 have rare methoxy and acetoxy groups at C-8, respectively [34,69].Compounds 115 and 116, isolated from Aconitum sczukinii, have very similar chemical structures, with the only difference being the presence of double bonds between C-2 and C-3 in compound 115 [65].Compound 117 contains a carboxyl group attached to the nitrogen.Compounds 118 and 119 are identified as a pair of regioisomers [62].Compound 121 from Delphinium ajacis is notable for its rare hydroxyl group at C-12 [70].Compounds 124-126 all featured a characteristic N=CH fragment, with compound 126 also possessing an additional nitrone group [68].

Biological Activity
Terpenoid alkaloids, a class of compounds with far-reaching pharmacological significance, exhibit unique pharmacological effects and extensive biological activities.This section provides an overview of the biological activities of TeAs that have been newly discovered in the past five years, including anti-inflammatory activity, analgesic effect, anticancer activity, and antibacterial and antiviral properties.A table of TeAs' biological activities is provided (Table 19).

Biological Activity
Terpenoid alkaloids, a class of compounds with far-reaching pharmacological significance, exhibit unique pharmacological effects and extensive biological activities.This section provides an overview of the biological activities of TeAs that have been newly discovered in the past five years, including anti-inflammatory activity, analgesic effect, anticancer activity, and antibacterial and antiviral properties.A table of TeAs' biological activities is provided (Table 19).

Anti-Inflammatory Activity
The anti-inflammatory activity of TeAs has been well documented, with compounds like gentianine and benzoylaconitine among those reported [105,106].This paper highlights 16 new anti-inflammatory members in TeAs over the past five years, including 4 monoterpene alkaloids and 12 DAs.

Analgesic Activity
Opioids and non-steroidal anti-inflammatory drugs (NSAIDs) are the primary drugs for pain treatment [107].However, both drug classes can cause severe adverse reactions in clinical use.As an essential class of TeAs, the analgesic activity of DAs has been widely studied.Several new compounds with analgesic activity have been reported in the past five years of research.Compounds 89, 91, 176, 180, 182, 241, 259-264, and 288 showed analgesic activity by inhibiting acetic acid-induced abdominal contractions in mice.
The transient receptor vanilloid 1 (TRPV1) channel is a crucial target in developing new analgesics for pain management [108].Acosinomonine B (178) showed a strong inhibitory effect on the activation of the TRPV1 channel in HEK-293 cells mediated by capsaicin (0.5 µM), with an inhibition rate of 31.78% at the concentration of 10 µM, making compound 178 a promising analgesic lead structure [79].

Antitumor Activity
TeAs have proven to be effective chemotherapeutic drugs for various cancers.For example, paclitaxel and its derivatives docetaxel and cabazitaxel have been clinically used for cancer treatment [109].Over the past five years, studies have identified eight new members of TeAs with potential anticancer activity.
Incarvine G (12) showed cytotoxicity with the IC 50 value of 60.29 µM against MDA-MB-231 cells and inhibited the migration and invasion of breast cancer cells.Further mechanistic studies showed that Incarvine G inhibited the migration and invasion of MDA-MB-231 cells by inhibiting actin cytoskeleton formation [19].(±)-Caryopterisines A (19) and B (20) reduced kynurenine (Kyn) biosynthesis in HeLa cells by inhibiting indoleamine 2,3-dioxygenase (IDO) at doses of 10 µM with inhibition ratios of 25.7% and 29.8%, respectively [22].Given the role of IDO cancer immunotherapy [110], compounds 19 and 20 are highlighted for their potent anticancer activities via IDO inhibition.

Cardioprotective Activity
Cardioprotective activity is a unique biological activity of DAs, such as Guan fu base A, which has been clinically developed to treat arrhythmias.Two new DAs with cardioprotective activity have been discovered in the past five years.
Smirnotine A (94) has some preventive effects on aconitine-induced arrhythmia in mice.The occurrence of ventricular tachycardia and ventricular flutter was significantly prolonged at 8 mg/kg, and ventricular flutter, ventricular fibrillation, and survival time of mice were prolonged considerably at 16 mg/kg [58].Gyalanunine A (253) showed significant cardiotonic activity after perfusion in frog hearts and significantly inhibited myocardial contraction when combined with β-blockers in isolated frog hearts, suggesting that its mechanism of action may be related to epinephrine β receptors [89].The existence of a hemiacetal moiety might be the critical structural feature necessary for the cardiac effect of 253 [89].

Antimicrobial Activity
Natural alkaloids have been proven to possess excellent antimicrobial activity.In the past five years, it has been found that several TeAs exhibit antimicrobial activity, such as antibacterial, antiviral, and antiplasmodial activities.

Other Activity
In addition to the widely recognized anti-inflammatory, analgesic, antitumor, and antimicrobial activities mentioned above, recent research reported over the past five years has uncovered several TeAs with other significant biological activities, including vascular relaxation activity, antifibrosis activity, and neuroprotective activities.

Conclusions
This review summarizes the sources, chemical structures, and biological activities of 289 TeAs discovered between 2019 and 2024, including 26 monoterpenoid alkaloids, 6 sesquiterpenoid alkaloids, and 257 DAs.DAs are the most abundant class of terpenoid alkaloids widely distributed in Aconitum and Delphinium.Seven novel DAs (283-289) were obtained from Spongia sp., Trichoderma koningii A729, and Forsythia suspensa in the last five years, respectively.Monoterpene alkaloids are mainly distributed in Apocynaceae, Scrophulariaceae, and Gentianaceae, while sesquiterpene alkaloids, the least common class, are found in Dendrobium.
The majority of TeAs exhibit anti-inflammatory, antitumor, and antimicrobial properties.Among the terpenoid alkaloids discovered in the past five years, the analgesic activity is unique to diterpenoid alkaloids.Distinctively, the analgesic activity has been identified exclusively in diterpenoid alkaloids, with several DAs demonstrating analgesic effects in mice superior to standard drugs like aspirin and acetaminophen, including Episcopaline B, Pseudostapine C, and Austroyunnanine B. These findings support the potential for developing novel analgesics.Moreover, TeAs show promise as therapeutic agents for various cancers, exhibiting inhibitory effects on breast, intestinal, liver, lung, and cervical cancer cells in vitro.The efficacy of these compounds in vivo remains an area for future research.TeAs also hold potential as cardiovascular medications, exemplified by compounds like Smirnotine A, which has shown heart-protective activity.
While current research has revealed a broad spectrum of TeAs' biological activities, most of these studies have been limited to in vitro cell viability assessments.There is a significant need for further in vivo pharmacological studies to understand the therapeutic

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Lactone-type C19-DAs (154-158) Lactone-type C19-DAs are generally formed by oxidation of the 14-ketone in the C ring of aconitine-type DAs to form a six-membered lactone C ring.Only five new members (154-158) belong to this type.The plant sources are shown in Table 6.Detailed chemical structures are shown in Figure 6.Molecules 2024, 29, x FOR PEER REVIEW 13 of Lactone-type C19-DAs (154-158) Lactone-type C19-DAs are generally formed by oxidation of the 14-ketone in the C ring of aconitine-type DAs to form a six-membered lactone C ring.Only five new members (154-158) belong to this type.The plant sources are shown in Table 6.Detailed chemical structures are shown in Figure 6.

Table 1 .
Names and plant sources of monoterpene alkaloids.
/: did not report.
/: did not report.

Table 4 .
Names and plant sources of aconitine-type C19-DAs.