Quinolizidine-Type Alkaloids: Chemodiversity, Occurrence, and Bioactivity

Abstract

Quinolizidine alkaloids (QAs) are nitrogen-containing compounds produced naturally as specialized metabolites distributed in plants and animals (e.g., frogs, sponges). The present review compiles the available information on the chemical diversity and biological activity of QAs reported during the last three decades. So far, 397 QAs have been isolated, gathering 20 different representative classes, including the most common such as matrine (13.6%), lupanine (9.8%), anagyrine (4.0%), sparteine (5.3%), cytisine (6.5%), tetrahydrocytisine (4.3%), lupinine (12.1%), macrocyclic bisquinolizidine (9.3%), biphenylquinolizidine lactone (7.1%), dimeric (7.1%), and other less known QAs (20.9%), which include several structural patterns of QAs. A detailed survey of the reported information about the bioactivities of these compounds indicated their potential as cytotoxic, antiviral, antimicrobial, insecticidal, anti-inflammatory, antimalarial, and antiacetylcholinesterase compounds, involving favorable putative drug-likeness scores. In this regard, research progress on the structural and biological/pharmacological diversity of QAs requires further studies oriented on expanding the chemical space to find bioactive scaffolds based on QAs for pharmacological and agrochemical applications.

Introduction

The quinolizidine alkaloids (QAs) are nitrogenous heterocycles with a 1-azabicyclo[4.4.0]decane moiety obtained from natural sources.¹ QAs are specialized metabolites biosynthesized from the amino acid l-lysine.² Their core structure can be built from one or two quinolizidines, differentiating them from other alkaloids derived from the l-lysine pathway, such as piperidine, indolizidine, and lycopodium alkaloids.³ QAs have been reported to possess various pharmacological effects such as sedative,⁴ anticonvulsant,⁵ anti-inflammatory,⁶ antiviral,⁷ antitumor,⁸ antipyretic,^9,10 antihepatitis B,¹¹ antifibrotic, antiallergic, antidiarrheal, analgesic,⁶ and antimicrobial.^12,13 Sparteine and lupinine were the first QAs to be isolated from Lupinus luteus leaves and stems at the onset of the 20th century.¹⁴ With the development of chromatographic and spectroscopic techniques, many naturally occurring QAs have been isolated and identified, and they still attract widespread attention for their plausible applications. In this sense, there are different natural sources for which QAs have been reported, whose structural variability is highlighted. Consequently, they can be classified according to the different QA moieties depending on the number of cycles and cyclic arrangement. The seven most-known structural QA types can be globally gathered (Figure 1), characterized by one quinolizidine moiety, as in the case of the substituted, fused bicycle quinolizidines (i.e., lupinine-type). Bridged tricycles, such as cytisine and tetrahydrocytisine, are also characteristic quinolizidine classes. Finally, the tetracyclic quinolizidines are divided into two groups, such as fused (i.e., matrine-type) and bridged (i.e., lupanine, anagyrine, and sparteine) heterocycles.

Figure 1.

Structural types of the most common quinolizidine alkaloids.

Considering that these most-known QA types are mainly associated with Fabaceae-related taxa, the present compilation was organized on the basis of three motivations: (1) lack of exclusive and comprehensive information on QAs reported to date from diversified sources, even incorporating animal sources such as sponges and frogs,¹⁵ since the available reviews on QAs are emphasized on occurrence in particular taxa, e.g., Sophora,^8,16,17 their bioactivities,¹⁸ biosynthesis,¹⁹⁻²¹ and diversity;²²⁻²⁴ (2) classification of QA structures beyond the plant-derived QAs; and (3) recognition of structural diversity and substituent variations within common QA skeletons to be described in light of their chemodiversity. Hence, this review provides comprehensive background information on the chemodiversity of QAs, focusing on their structural variants, substitutions, occurrences, and reported biological activities, harmonizing previous reviews.

Roles for Natural Producers

One of the leading chemical functions of alkaloids in plants is the defense against predators and herbivores.²⁵ QAs have antimicrobial properties but have also been reported to be teratogenic to some ruminants.²⁶ However, the importance of quinolizidine function in plants is essential. For instance, Lupinus plants use compounds such as QAs in periods of biotic stress as a repellent strategy against insects.²⁷ Likewise, humans have also used these QAs in mixtures with carotenoids and tannins for crop bioremediation purposes, taking advantage of the direct influence and allelopathic properties of the tannins and the toxicity of the alkaloids.^28,29

Another relevant function of these QAs is the chemical similarity to some molecules participating in signal transmission from the nervous system. Hence, they can block neuroreceptors, intermediaries of neuronal signal transduction, and ion channels in vertebrates and insects.¹ Additionally, they can serve as plant growth regulators since, in some cases, cadaverine- and putrescine-derived alkaloids increase significantly during germination.³⁰ Finally, the alkaloids are associated with fatty acids facilitating translocation within the plant since they can serve as storage products or transportation of nonmetabolized nitrogen. Indeed, QAs in Fabaceae can serve as nitrogen storage, especially the atmospherically fixed N₂.³¹⁻³³

General Distribution

Although QAs have predominantly been isolated from the Fabaceae family,²⁵ it is worth noting that other plant families, such as Saururaceae,²³ Acanthaceae,³⁴ Phyllanthaceae,³⁵ Rubiaceae,³⁶ Lycopodiaceae,³⁷ Lycopodiaceae,³⁸⁻⁴¹ Urticaceae,⁴² Ericaceae,⁴³ Euphorbiaceae,⁴⁴ and Connaraceae,²³ have also been investigated for the presence of relevant QAs. Additionally, QAs have been reported in families of terrestrial and marine animal species such as Dendrobatidae,^45,46 Mantellidae,^47,48 Formicidae,⁴⁹ Clavelinidae,⁵⁰ and Petrosiidae.¹⁵ Particularly, a series of petrosins, xestospongins, and araguspongines have also been identified from marine sponges belonging to the genera Petrosia, Xestospongia, and Oceanapia(15,51−54) but have also been identified in frog skins, specifically in the families Dendrobatidae and Mantellidae, highlighting species such as Phyllobates aurotaenia, Melanophryniscus moreirae, Melanophryniscus toads,⁴⁵Epipedobates tricolor,⁴⁶Mantella baroni,⁴⁸ and Mantella basileo.⁴⁷ These alkaloids constitute a unique type of macrocyclic QAs, formed by the union of two quinolizidines (precisely two 1-oxaquinolizidine fragments), called bisquinolizidines.¹⁵ They have exhibited biological activities such as cytotoxicity,⁵⁵ anti-inflammatory,⁵⁶ selective inhibition of the IP3 receptor, and HIV-1 RT inhibitory activity.⁷

The Fabaceae family, one of the world’s largest flowering plants (Angiosperms), is the primary source of QAs. It has cosmopolitan distribution but is also well-represented in the flora of the Andes.⁵⁷ It is the third largest family worldwide among the Angiosperms, surpassed by the Asteraceae and Orchidaceae families.⁵⁸ Remarkably, the QAs are biosynthesized and accumulated in the so-called primitive legumes of the tribes Genisteae, Lupinus, Sophoreae, Dalbergieae, Euchresteae, Thermopsidae, Bossiaeae, Brongniartieae, Podalyrieae, Liparieae, and Crotalarieae.¹ The previous tribes include the six most relevant genera due to the highest occurrence of QAs, such as Lupinus, Ulex, Cytisus, Sophora, Genista, and Orphanodendron.^1,19,59

The peripheral part of the seeds, bark, root, fruit, and leaf epidermis can majorly accumulate QAs.⁶⁰ Previous studies suggest that QAs play an essential role in plant defense against insects due to the bitter taste and toxicity conferred by QAs, especially in the seeds, where their most significant accumulation occurs.¹ The biosynthesis of most Fabaceae alkaloids (quinolizidines) is carried out in the green aerial parts of the plant, specifically in the chloroplast. These alkaloids are transported by the phloem to other plant organs and tissues and are predominantly accumulated in subepidermal cellular structures.^61,62 The organs necessary for survival and reproduction, such as flowers and seeds, store exceptionally high amounts of defensive alkaloids.¹ The seeds of Fabaceae plants are rich in alkaloids and can reach up to 3–4%, as they are moved from the senescent leaves during the growing season. In general, QAs are widely distributed in different plant parts/organs of QA-producing legumes, so there is an estimate of the alkaloid percentage gathered in each plant part, as depicted in Figure 2.

Figure 2.

General distribution of quinolizidines in different plant parts of QA-producing legumes.

Relevant Biosynthetic Remarks

QAs are biosynthesized from the amino acid l-lysine (Figure 3). This amino acid undergoes oxidative decarboxylation due to the action of the lysine decarboxylase (LDC), a pyridoxal phosphate (PLP)-dependent enzyme.³ This enzymatic process results in cadaverine, a precursor and intermediary between l-lysine and QAs. Thus, the nitrogen atoms of the quinolizidine skeleton (C₁₅N₂) are derived from l-lysine-derived cadaverine.¹⁹ However, despite the relevant abundance and distribution of QAs, there is a lack of knowledge of QA biosynthesis, although several hypotheses and proposals have been raised in recent decades. For more information, a recent review carefully discussed the mechanistic insights into the QA biosynthesis, particularly the pathway related to the sparteine formation based on the often-ignored precursor feeding studies.²⁰

Figure 3.

Biosynthesis of the most common quinolizidine alkaloids produced by legume plants.

The preferred hypothesis of the QA biosynthetic pathway starts with the copper amine oxidase (CAO)-catalyzed oxidative deamination of the precursor cadaverine and its subsequent cyclization to form the next important intermediary (i.e, Δ¹-piperidein-1-ium cation), whose step involves an aldol-like coupling between the two piperideine-related tautomers.²⁰ This dimerization occurs under plant physiological pH (pH = 6.5–7.0), forming two stereocenters having four potential stereochemical variants but being stereoselective to the product (2′R,3′R)-tetrahydroanabasine (THA) and, in turn, to the bicyclic (−)-lupinine.^63,64 In this transformation, the imine-containing ring of THA is hydrolyzed, and another oxidative deamination proceeds (Figure 3), forming the basic quinolizidine core, achieved through a Schiff base formation.¹⁹ Although the experimental evidence remains to be generated, it has been proposed that incorporating another Δ¹-piperideine molecule could produce the additional diazatetra(tri)cyclic moieties.^65,66 In addition, the cleavage of the fourth ring and the oxidation to a 2-pyridone system offer a potential route to cytisine, considering that any of the outermost rings could be cleaved to produce the same product.^3,67 Once the bicyclic, tricyclic, and tetracyclic structures are assembled, they can be modified by dehydrogenation, oxygenation, hydroxylation, glycosylation, or esterification (Figure 3). These transformations can afford a wide variety of structurally related quinolizidines.⁶⁸ For instance, acetylated products of 13α-hydroxylupanine/13α-hydroxymultiflorine and lupinine/epilupinine are produced by acyltransferases (e.g., HMT/HLT and ECT/EFT-LCT/LFT).⁶⁹ So far, only two enzymes have been identified in the QA pathway. Therefore, the discovery of biosynthetic genes remains as an opportunity for understanding the attractive chemistry and biology involved in QA biosynthesis.^19,21

Structural Variations and Occurrence of Quinolizidine Alkaloids

This review aimed to gather the chemical structures of naturally occurring QAs, reported between 1990 and 2023, to disclose the available QA-related chemical space. The presence of intriguing QA diversity and complexity has broadened our understanding of their structural characteristics, including their varied natural sources (plants and animals). Thus, the compiled compounds (n = 397) were mainly related to bridged or fused polycyclic QAs, including the most known QA types such as matrine, lupanine, anagyrine, sparteine, cytisine, tetrahydrocytisine, and lupinine-type compounds,^18,20,66 as depicted in Figure 1. In addition, other QAs with a more complex structure were also found, such as macrocyclic bisquinolizidine and biphenyl quinolizidine lactones, and widely reported as marine natural products⁵¹ and from frog skins.⁴⁶ In this context, Figure 4 shows the percentage of alkaloids subdivided into classified QA types. Matrine-type QAs are the most frequently isolated (13.6%), followed by lupinine (12.1%), lupanine-type (9.8%), macrocyclic bisquinolizidines (9.3%), and biphenyl(ether)quinolizidine lactones (7.1%). The remaining QA types encompass 48.1% of the total representatives and comprise 15 distinct structural variants (Figure 4). To disclose the QAs’ chemical diversity, the reported structural variants for each QA type and the global features of the compiled chemical space are expanded on below. To support such an expansion, the names and codified structural information on QAs (1–397) in the simplified molecular input line entry specification (SMILES) are presented in Table S1, and their structures are depicted in Figures S1–S22 (Supporting Information).

Figure 4.

Distribution of the compiled quinolizidines (n = 397) according to the structural type.

Matrine-Type QAs

The matrine-type QAs are significant representatives and abundant in species of the Sophora genus.¹⁶ This QA type and its sources have been used in traditional Chinese medicine for many years. Matrine-type QAs contain two condensed quinolizidine units, having a 6/6/6/6 diazatetracyclic building block and forming a fused, nonlinear bisquinolizidine. More than 50 matrine alkaloids have been isolated and described thus far, and they are disclosed in compounds 1–54 (Figure 5, Figure S1). The basic structure of this type of QA is the matrine alkaloid (7), having the mentioned tetracyclic moiety formed by two quinolizidine moieties and four contiguous stereogenic centers. The relative configurations of these chiral centers in matrine-related QAs have α- (at C-5, C-6, and C-7) and β-oriented (at C-11) hydrogens. Other basic structures have β-oriented hydrogens at C-6 (14) and C-7 (e.g., 15).

Figure 5.

Matrine-type quinolizidine alkaloids 1–54.

In addition, the matrine-type QAs are sometimes found in the N-oxide form (e.g., 1, 2, 17, 18, and 37), with N-1 being the most common point for such oxidation. The matrine-type QAs 1–54 differ mainly in their substitution patterns and stereochemistry.⁷⁰ They contain structural variations associated with double bonds (specifically in the D-ring) and α- or β-oriented substitutions around each tetracycle, such as hydroxyl (e.g., 1, 3, 4, 8, 18, 19, 21, 29, 30–43, 45, 47, 49, 50, 53, and 54), acetyl (e.g., 33, 35, 39, 51, and 52), epoxy (e.g., 40 and 41), (methylthio)methoxy (e.g., 34 and 36), methoxy (e.g., 38), and indolyl (e.g., 22) groups.⁷¹ Thus, nine different positions for matrine substitutions (R¹ to R⁹) were evidenced, the structural distribution of which is outlined in Figure 5.

The matrine-type compounds (1–54) have been extensively obtained from Sophora plants (96% of records) and mostly isolated from seeds and aerial parts from S. flavescens,¹⁶S. alopecuroides,⁷²S. tonkinensis,⁷³S. leachiana,⁷⁴S. velutina,⁷⁵ and Oxytropis plants, e.g., O. ochrocephala,^76,77 and even widely reported from seeds and leaves of Genista plants.²⁷ Additionally, according to Table 1, these compounds have been mainly isolated from the aerial parts, roots, and seeds, which comprise the plant parts where the QAs accumulate the most.¹

Table 1. Sources of Isolated Matrine-Type (1–54) and Lupanine-Type (55–93) Quinolizidine Alkaloids (QAs).

QAs	Species	Plant part	Ref
1–11	Sophora flavescens	roots	(16)
12–16	Sophora alopecuroides	roots	(17)
17–21	Sophoratonkinensis	roots	(73)
22	Sophora alopecuroides	seed	(78)
23–24	Oxytropis ochrocephala Bunge	whole plants	(76,77)
25–26	Sophora flavescens Ait., Subprostrate sophora	roots	(79)
27–28	Sophora flavescens	roots	(9)
29–31	Sophora flavescens Ait.	chipped roots	(80)
32–46	Sophora tonkinensis Gagnep	seeds	(71)
47–48	Sophora flavescens	roots	(11)
49–50	Sophora alopecuroides	aerial parts	(4)
51–54	Sophora alopecuroides, S. tonkinensis, S. viciifolia, Thermopsis lanceolata	fresh leaves	(81)
55–56	Sophora flavescens Ait.	roots	(16)
57–59	Sophora flavescens	roots	(78)
60	Lupinus albus L.	seeds	(67)
61	Cytisus purgans	aerial parts	(82)
62–64	Lupinus angustifolius	aerial parts	(83)
65	Sophora velutina subsp. zimbabweensis	fruits and pods	(75)
66–67	Lupinus lanatus	aerial parts	(84)
68	Lupinus albus, L. angustifolius	seeds	(85)
69	Lupinus sp.	leaves	(23)
70	Gonocytisus pterocladus	whole plant	(86)
71–73	Acosmium panamense	bark	(87)
74–75	Cytisus scoparius	seeeds	(88)
76	Lupinus lanatus	seeds	(89)
77–82	Genus pearsonia	aerial parts	(88)
83	Ormosia krugii	seeds	(90)
84	Lupinus polyphyllus	leaves	(91)
85	Lygos raetam	aerial parts	(92)
86–87	Lupinus sp.	aerial parts	(93)
88–93	Personia cajanifolia subsp. Cryptantha, P. sessilifolia subsp. marginata	aerial parts	(94)

Lupanine-Type QAs

Two condensed quinolizidine units constitute lupanine-type QAs. However, they differ from other QA types, e.g., sparteine-type, by the presence of a carbonyl group at C-2, forming a lactam moiety with N-1 as ring A. Lupanines also differ from the matrine-type compounds because they involve a 6/6/6/6 diazatetracyclic building block to form a bridged bisquinolizidine.⁹⁵ So far, almost 40 lupanine-like QAs have been reported, comprising a chemical diversification illustrated by compounds 55–93 (Figure 6, Figure S2). The central structural core agrees with the alkaloid lupanine (levo or dextrorotatory, i.e., 56 or 61, respectively), which is mostly substituted in rings A and D.⁸⁵ Compounds 55–93 exhibited C-3, C-4, and C-5 as preferred substitution positions in ring A, while the most substituted positions in ring D are associated with C-12, C-13, and C-15. In general, the most common substitutions are hydroxyl groups, affording alkaloids such as 5α-hydroxylupanine (57) or 13α-hydroxylupanine (63) but also ester moieties such as angeloyl (91), tigloyl (75), cinnamoyl (70), or pyrroyl (90). Common unsaturations are also present at positions Δ⁵⁽⁶⁾ (55) or Δ⁷⁽¹⁷⁾ (77), forming dehydrolupanines. Furthermore, the N-oxide derivatives at N-16 are also very common in these QA types. Ten distinct positions for lupanine substitutions (R¹ to R¹⁰) were then revealed, the structural distribution of which is depicted in Figure 6.

Figure 6.

Lupanine-type quinolizidine alkaloids 55–93.

On the other hand, compounds 55–93 (lupanine-type) show their most significant occurrence in the genus Lupinus, followed by Sophora and other genera such as Genista, Acosmium, Ormosine, Thermopsis, and Cytisus (Table 1). Thus, lupanine-type QAs are abundant mainly in the Lupinus genus and distributed in the aerial parts, leaves, seeds, bark, and root,²⁵ according to the gathered information in Table 1. More than 170 QAs have been identified in different species of the genus Lupinus.⁹⁵ Some species, such as L. argenteus,⁹⁶L. exaltatus,⁹⁷L. angustifolius,⁹⁸L. albus,⁶⁷L. mexicanus,⁹⁹ and L. lanatus,⁸⁹ are important representatives, where the QA diversity related to lupanine-type compounds has been widely investigated.

Anagyrine-Type QAs

Anagyrine-type alkaloids are structurally very similar to lupanine-type alkaloids since they have the 6/6/6/6 diazatetracyclic building block, forming a bridged bisquinolizidine, but differ in the 2-pyridone moiety in ring A as the typical feature of these QAs. There are currently 19 anagyrines reported as natural QAs, as represented by compounds 94–109 (Figure 7, Figure S3). The alkaloids anagyrine (94) and thermopsine (95) are the representatives of this QA type, which are epimerically related and differentiated by the relative configuration of C-11 (i.e., β-H for 94 and α-H for 95). Compounds 94–109 exhibit various substitutions on ring D at C-15, C-14, and C-12, while ring C appeared to be substituted at C-17 with a methoxyoxoethyl moiety (102). In addition, the C-7–C-8–C-9 bridge is mostly α-oriented. Like lupanines, an N-oxide thermopsine derivative at N-16 was also isolated (108), and other common substitutions, such as hydroxyl, epoxy, acetyl, alkanoyl, and indolyl, are also found in these QAs. An isolated alkaloid exhibited a double bond in Δ¹³⁽¹⁴⁾ (104), whereas other QA molecules exhibited an additional nitrogen instead of C-13 (100). Six substitution points in anagyrine-type QAs (R¹ to R⁶) are evidenced, as illustrated in Figure 7. These 2-pyridone-containing QAs (94–109) are typical alkaloids of many Papilionoideae subfamily-belonging genera,⁹⁵ such as Anagyris, Thermopsis, Genista, Clathrotropis, and Sophora (Table 2), and generally absent in Lupinus, excepting L. arboreus and L. argenteus.⁹⁶

Figure 7.

Anagyrine-type quinolizidine alkaloids 94–109.

Table 2. Sources of Isolated Anagyrine-Type (94–109) and Sparteine-Type (110–130) Quinolizidine Alkaloids (QAs).

QAs	Species	Plant Part	Ref
94	Anagyris fetida	aerial parts	(100)
95	Thermopsis rhombifolia, Genista sessilifolia, G. tinctoria L.	leaves	(101)
96	Sophora alopecuroides	seed	(78)
97–99	Clathrotropis glaucophylla	bark	(102)
100	Sophora grzfithii.	leafy shoots	(88)
101	Thermopsis chinensis	seeds	(93)
102–109	Thermopsis lanceolata	seeds	(103)
110	Lupinus sp.	aerial parts	(104)
111–113	Thermopsis chinensis, Laburnum watereri	leaves	(93)
114	Lupinus angustifolius	aerial parts	(83)
115–119	Lupinus sp.	aerial parts	(105)
120	Acosmium dasycarpum (Vog.)	root bark	(106)
121–124	Cytisus monspessulanus	leaves	(107)
125	Lupinus sericeus Purshl	aerial parts	(108)
126	Lygos ruetam var. surcocurpa	aerial part	(109)
127	Genista sesslifolia DC	aerial parts	(110)
128	Laburnum watereri	leaves	(93)
129–130	Lupinus varius	seeds	(111)

Sparteine-Type QAs

(+)- or (−)-Sparteine (110 or 111, respectively) constitutes the basic structure of this type of QA. They are also constituted by a bridged tetracyclic formed by two quinolizidine units, having a 6/6/6/6 building block similar to the lupanine moiety,⁹⁵ but differentiated by the absence of the carbonyl group at C-2 in ring A, mentioned above. Sparteine-type QAs appeared to have fewer reported chemical variants than lupanine-type QAs. Over 20 sparteine alkaloids have been currently reported, and their structural variations are represented by compounds 110–130 (Figure 8, Figure S4). Most of these sparteine-like compounds are characterized by an α-H at C-6 and a β-H at C-11. Generally, an α-orientation for C-7–C-8–C-9 is usually presented, with some β-oriented exceptions (121–124). Furthermore, some sparteines have substitutions at C-10 and C-17, specifically oxygenated groups (carbonyl and hydroxyl), and the presence of bulkier substitutions, such as piperidine derivatives. The substitution pattern (R¹ to R¹¹) of these QAs is depicted in Figure 8. Finally, unsaturations have also been observed, specifically at Δ²⁽³⁾ and Δ⁵⁽⁶⁾ in ring A and at Δ¹¹⁽¹²⁾, affording dehydrosparteine-like QAs (e.g., 114–116, 121, 123, 124, and 130).

Figure 8.

Sparteine-type quinolizidine alkaloids 110–130.

Regarding sparteine-type alkaloids (110–130), Lupinus plants produce and accumulate this QA type in seeds and leaves¹¹² and other genera such as Acosmium, Lygos, and Houttuynia (Saururaceae) (Table 2). Additionally, this QA type has also been isolated from seeds, leaves, flowers, and aerial parts of species belonging to the genera Cytisus,¹¹³Ormosia,¹¹⁴Ulex,¹¹⁵Genista,¹¹⁶Lupinus,²⁵ and Sophora,¹¹⁷ which contain structurally related sparteine-type compounds.

Cytisine-Type QAs

The cytisine-type QA is a class of bridged tricycle alkaloids containing a 2-pyridone moiety in ring A and mainly isolated from plants of the Faboideae subfamily.¹¹⁸ These QAs are characterized by having a 6/6/6 diazatricyclic building block, forming the base structure of 1,5-methanopyrido[1,2-a][1,5]diazocine, whose C-7–C-8–C-9 bridge has an α-orientation. Twenty-five cytisine-like alkaloids have been currently described, whose structural variations are represented by compounds 131–155 (Figure 9, Figure S5). Such variations comprise a particular substitution pattern on ring C, as shown as R¹ to R³ in Figure 9. In general, the different derivatives of cytisine (132) are mostly substituted at N-12, specifically with oxide, carbonyl, hydroxyl, alkanoyl, and alk(en)yl groups. Additionally, cytisine-like QAs can be hydroxylated at C-9 (140), while other cytisines have a carbonyl group at C-11 (e.g., 148–150) or an allyl group (137), which is generated if a sparteine-like tetracycle undergoes a ring D cleavage.¹¹⁹ The alkaloid 3-hydroxy-11-norcytisine (156) is a cytisine-like QA isolated from Laburnum anagyroides green pods,¹²⁰ having an unusual 7,11-diazatricyclo[7,2,1,0^2,7]dodeca-2,4-dien-6-one (6/6/5) moiety formed by the C-13 loss (Figure 9).

Figure 9.

Cytisine-type quinolizidine alkaloids 131–156.

Cytisine-type QAs (131–156) are also characteristic of the Fabaceae family plant species, and their distribution is widespread in various Fabaceae genera, widely distributed in the genera Sophora, Genista, Cytisus, Osyris, Spartium, Petteria, Euchresta, Dermatophyllum, and Styphnolobium and isolated from leaves, aerial parts, roots, seeds, and flowers (Table 3). Notably, these compounds accumulate mainly in the seeds and leaves and are obtained on a commercial scale from Laburnum anagyroides (=Cytisus laburnum),¹²¹Sophora alopecuroides,⁴Thermopsis alterniflora,¹²²Thermopsis lanceolata,¹²³ and Caragana sinica.¹²⁴

Table 3. Sources of Isolated Cytisine-Type (131–156) and Tetrahydrocytisine-Type (157–173) Quinolizidine Alkaloids (QAs).

QAs	Species	Part	Ref
131–133	Sophora flavescens	roots	(16)
134	Sophora flavescens	roots	(125)
135	Genista quadriflora Munby	roots and aerial parts	(126)
136–137	Dermatophyllum arizonicum, Dermatophyllum gypsophilum, Dermatophyllum secundiflorum, Styphnolobium affine, Styphnolobium japonicum	leaf tissue	(119)
138	Sophora velutina subsp. zimbabweensis	fruits and pods	(75)
139	Spartium junceum	fresh flowers	(127)
140	Osyris alba L.	aerial parts	(128)
141	Euchresta tubulosa Dunn	stem	(129)
142–144	Sophora exigua	aerial parts	(88)
145–146	Sophora griffithii	leaves	(130)
147	Petteria ramentacea	buds, leaves, and flowers	(131)
148–155	Thermopsis lanceolata	seeds	(103,132)
156	Laburnum anagyroides	green pods	(120)
157	Sophora flavescens	roots	(125)
158	Genista quadriflora	roots and aerial parts	(126)
159	Guianodendron praeclarum	leaves	(133)
160–161	Lupinus angustifolius	aerial parts	(83)
162	Lupinus sp.	leaves	(23)
163–164	Templetonia biloba	leaves	(134)
165	Lupinus termis	seeds	(135)
166	Thermopsis mongolica	aerial parts	(136)
167	Lupinus termis	seeds	(137)
168	Lupinus angustifolius	seeds	(85)
169	Lupinus angustifolius, L. campestris	aerial parts	(23)
170–171	Virgilia diuaricata, V. oroboides	left	(138)
172	Lupinus albus	aerial parts	(139)
173	Lupirnus polyphyllus	stems, leaves, and pods	(140)

Tetrahydrocytisine-Type QAs

The tetrahydrocytisine-type QAs are also characterized to be tricycles and differ from those of the cytisine type by the absence of a 2-pyridone moiety in the A ring. They also have a 6/6/6 building block, forming the base structure of a 1,5-methanopyrido[1,2-a][1,5]diazocine. The C-7–C-8–C-9 bridge can be α- or β-oriented (e.g., 158 and 157, respectively). Despite the fact that fewer chemical variants are reported for tetrahydrocytisine-type than for cytisine-type QAs, more substitutions were evidenced for the 17 alkaloids belonging to this QA class, illustrated by the structures of compounds 157–173 (Figure 10, Figure S6). Seven positions were observed to be substituted in tetrahydrocytisine-type QAs (R¹ to R⁹, Figure 10). Tetrahydrocytisine (158) has chemical variants commonly substituted at N-12, such as cytisine-like QAs. In addition, C-13 is also substituted with an allyl group, affording angustifoline (161) derivatives. On the carbonyl group, the position can occur at C-2 in this QA type as cytisine derivatives but also at C-4 in ring A, forming a cyclohexenone moiety, typical for albine (168) derivatives. Particularly, compound 167, a Δ⁵-dehydroalbine, exhibited a 4-pyridone moiety.

Figure 10.

Tetrahydrocytisine-type quinolizidine alkaloids 157–173.

These tetrahydrocytisine-type alkaloids (157–173) appear as the main QA in some lupin species such as L. angustifolius.^19,28 Very often, these alkaloids (such as 169) are considered minor components in Old World species (L. micranthus, L. albus), in South American species (L. gibertianus, L. mutabilis), and in North American plants (L. perennis, L. elegans, L. leucophyllus).¹⁴¹ The most significant accumulation of this QA type has been reported in leaves and flowers of Templetonia(134) and Lupinus(142) plants (Table 3).

Lupinine-Type QAs

Lupinine-type QAs represent the most basic quinolizidine unit, whose building block is a 6/6, forming the 1-azabicyclo[4.4.0]decane moiety, to comprise the quinolizidine core.¹⁴³ From this basic structure, based on (−)-lupinine and (+)-epilupinine (174 and 175, respectively), many substituted homologues have been identified in different plant and animal sources.²² Hence, more than 40 lupinine-like compounds have been reported, with structural variations illustrated with the reported compounds 174–220 (Figure 11, Figure S7). They are characterized by having oxygenated substitutions such as hydroxyl (177–178), hydroxyalkyl (183), acetyl (187 and 189), (substituted) benzoyl (207–209, 215), (substituted) cinnamoyl (210–212), furan-3-yl (216–220), and other substitutions associated with alk(en/in)yl (187–197, typically found in frog skins), phenyl (205–206), pyridyl (201–204), and (substituted) piperidin-1-yl (199–200, 213–214). These substitutions can be found in the different positions of the quinolizidine ring, comprising a substitution pattern represented by nine different positions (R¹ to R⁹, Figure 11). However, substitutions at C-6 have not yet been reported but involve chemical variants with α- or β-oriented hydrogens. In addition, two iminium salts between C-2 and N-1 have also been reported (182–183) as structural variations of this type of QAs. Finally, thermlanseedline A (221) is an alkaloid containing the quinolizidine moiety coupled with an acetylpiperidyl fragment, which was proposed to be derived from thermopsine after oxidative ring D cleavage/demethylation/acetylation biosynthetic steps.¹³²

Figure 11.

Lupinine-type quinolizidine alkaloids 174–221.

This type of alkaloid (174–221) is the most abundant in the Fabaceae family, and they generally occur in the genera Lupinus, Baptisia, Thermopsis, Maackia, Genista, Lycopodium, Ulex, Prosopis, Cytisus, and Sophora.²¹ The first reported structure of lupinine was carried out in 1938, isolated from the leaves of the Lupinus luteus,¹⁴⁴ and different Lupinus genotypes have shown the presence of lupinine-type compounds.¹⁴⁵ In addition, furan-3-yl-containing lupinine-type QAs were isolated from Nuphar plants. Additionally, 26% of records correspond to other genera of other families that can also biosynthesize QAs, such as Hypoestes (Acanthaceae), Flueggea (Phyllanthaceae), Myrioneuron (Rubiaceae), Huperzia (Lycopodiaceae), Heimia (Lythraceae), Boehmeria (Urticales), Vaccinium (Ericaceae), Croton (Euphorbiaceae), and Clavelina (Clavelinidae), whereas the other 16% of records correspond to animal species such as Solenopsis picea(49) and frog skin of the Dendrobates,¹⁴⁶Mantella,⁴⁸ and Epipedobates(46) genera (Table 4).

Table 4. Sources of Isolated Lupinine- (174-221) and Senepodine-Type (222–233) Quinolizidine Alkaloids (QAs).

QAs	Species	Part	Ref
174–175	Lupinus sp.	aerial parts	(147)
176	Maackia amurensis var. Buergeri, M. tashiroi	fresh stems	(148)
177	Virgilia divaricata, V. oroboides	leaves	(93)
178	Lycopodium cernuum var. sikkimense	whole plants	(149)
179–183	Lycopodium cernuum, L. chinense	club moss	(150)
184–185	Vaccinium myrtillus	aerial parts	(43)
186	Epipedobates tricolor	skin	(151)
187–189	Clavelina picta	leaves and aerial part	(93,152)
190–191	Melanophryniscus klappenbachi, M. cupreuscapularis	skin of poison frogs	(45)
192–193	Mantella basileo	skin	(47)
194–197	Mantella baroni	skin	(48)
198	Huperzia phlegmaria	club moss	(37)
199–200	Sophora chrysophylla	bark	(153)
201–203	Ulex jussiaei	aerial parts	(154)
204	Maackia amurensis var. buergeri	leaves	(155)
205	Heimia salicifolia	leaves	(41)
206	Pilea aff. martinii	aerial parts	(156)
207–211	Cylicomorpha solmsii	leaves	(157)
212	Lupinus varius ssp. orientalis	leaves	(81)
213	Sophora nuttalliana, S. stenophylla	leaf and stem tissue	(158)
214	Bongardia Chrysogonum	tubers	(159)
215	Lupinus varius ssp. orientalis	aerial parts	(81)
216–219	Nuphar pumilum	rhizomes	(160)
220	Nuphar japonicum	rhizomes	(161)
221	Thermopsis lanceolata	seeds	(132)
222–233	Lycopodium chinense	club moss	(23,38,150,162−164)

Senepodine-Type QAs

A group of alkaloids, also known for being part of theLycopodium QAs, comprise the C₂₂N₂ adduct formed between a lupinine-like moiety with an α- or β-oriented (1,7-dimethyldecahydroquinolin-5-yl)methyl moiety at C-10.³⁸ (+)- and (−)-Senepodine (222 and 223, respectively) can be considered the basic structure of this QA type.¹⁶⁵ Eleven senepodine-related compounds have been reported, having structural variations at six different positions (R¹ to R⁶, Figure 12), involving three variations for the lupinine moiety and the other three for the decahydroquinoline substitution. The four substituting positions involved α- or β-oriented methyl groups at C-2, C-4, N-1′, and C-8′ (224–225), formyl and acetyl groups at N-1′ (226–227, 229), and piperidin-2-yl at C-2 (230–232). Iminium salts between C-2 and N-1 have also been reported (224). In addition, α- or β-oriented hydrogens can be found at C-6 and C-10′ in the lupinine and decahydroquinoline moieties, respectively (Figure 12, Figure S8). Finally, a particular senepodine-like alkaloid, consisting of a fastigiatine-quinolizidine adduct (C₂₇N₃), himeradine A (233), was isolated from Lycopodium chinense (Table 4), having the fastigiatine moiety attached to C-2 through a methylene bridge.¹⁶⁴

Figure 12.

Senepodine-type quinolizidine alkaloids 222–233.

Uncommon Diazatetracyclic QAs (Aloperine, Multiflorine, Leontidine, and Cernuine Types)

These QA types (234–259) have structural similarities to sparteine or lupanine since they contain a diazatetracyclic moiety (Figure 13). However, they did not fall into the above-described QA types, but they can be gathered into four subclasses due to their structural similarities, such as aloperine, multiflorine, leontidine, and cernuine types. The first uncommon diazatetracyclic QA, i.e., aloperine type, have been isolated from Sophora and Oxytropis plants (Table 5). Thus, aloperine (234) is the base structure of this QA variant (234–237), sharing a sparteine-like structure but differing by the nitrogen position, i.e., N-12 in ring D. Aloperines have exclusively been reported with an α-oriented C-7–C-8–C-9 bridge. The other two related alkaloids have been isolated with formyl and oxide substitutions at N-12 (235 and 236, respectively)¹⁶⁶ and have a carbonyl at C-10 and an aromatic D ring (237) (Figure 13). Ochrocephalamine D (238) is an aloperine-like QA, isolated from Oxytropis ochrocephala,¹⁶⁶ having an additional pyrrolidin-2-one moiety formed by a further carbonyl group linked to N-12 and C-10 (Figure 13, Figure S9).

Figure 13.

Aloperine, multiflorine, leontidine, and cernuine-type quinolizidine alkaloids 234–259.

Table 5. Sources of Isolated Aloperine- (234–238), Multiflorine- (239–244), Leontidine- (245–251), and Cernuine-Type (252–259) Quinolizidine Alkaloids (QAs).

QAs	Species	Part	Ref
234	Sophora alopecuroides	seeds and leaves	(169)
235–238	Oxytropis ochrocephala Bunge	whole plant	(166,170)
239	Lupinus lanatus	aerial parts	(84)
240–242	Lupinus albus	seeds	(171)
243	Lupinus hirsutus Linn	seedlings	(172)
244	L. albus, L. varius, L. orientalis, L. hartwegii, L. densiflorus	whole plant	(173)
245	Leontice ewersmannii	leaves	(174)
246–248	Orphanodendron bernalii, O. grandiflorum	leaves	(133)
249	Guianodendron praeclarum	leaves	(133)
250	Maackia tashiroi	stems	(175)
251	Sophora velutina subsp. zimbabweensis	fruits and pods	(75)
252–253	Lycopodium cernuum, L. chinense	club moss	(150)
254–259	Lycopodium cernuum var. Sikkimense	club moss	(149)

On the other hand, (−)-multiflorine (239) comprises the basic structure of another related QA type, with a particularly high occurrence in the genus Lupinus(167) (Table 5), differing from lupanine by the C-4 carbonyl group, a nitrogen position at the D-ring (i.e., N-16), and a characteristic double bond at C-2. The β-oriented C-7–C-8–C-9 bridge has exclusively been reported for these QAs. Five additional multiflorine-like alkaloids have also been isolated (239–244), representing the structural variations of this QA type, which involve a further double bond at C-5 (240), a hydroxyl group at C-13 (241–242), an oxide at N-16 (243) or O-tigloyl ester (244) (Figure 13, Figure S10).

Another type of lupanine-like diazatetracyclic QA is related to leontidine (245), widely distributed in the genera Camoensia, Guianodendron, and Orphanodendron (Table 5), which differs from lupanine by the presence of a five-membered D-ring instead of a six-membered one, forming a quinolizidine/indolizidine adduct.⁵⁹ This type of QA exhibits an α- or β-oriented C-7–C-8–C-9 bridge and a carbonyl group at C-2 (e.g., 245–247, 250) but also at C-10 (e.g., 248–249). In addition, unsaturations at C-2 (248–249), C-3 and C-5 (245–246), and the oxide group at N-15 (250) can also be found. In addition, velutinine (251) is a highly unsaturated leontidine-like alkaloid isolated from Sophora velutina subsp. Zimbabweensis stem bark,⁷⁵ having a hydroxyl at C-8, an α-pyridone moiety at ring A, and a methylenedioxy group at C-13/C-14 (Figure 13, Figure S11).

Finally, cernuine (254) is the representative alkaloid of a QA type constituted by a tetradecahydro-2H-pyrido[1′,2′:3,4]pyrimido[2,1,6-de]quinolizine moiety, which is also known to be part of the Lycopodium alkaloids¹⁶⁸ (Table 5). Eight cernuine-type QAs (252–259) have been reported, characterized by having a carbonyl group at C-1 and can involve an α-oriented hydroxyl or acetyl group at C-12 (252, 256–259) or an α-hydroxyl at C-2 (255). They also have a Δ^C14(15) unsaturation (258–259) and an N-oxide at N-8 (252–253) (Figure 13, Figure S12).

Triazapolycyclic QAs (Ormosanine- and Homoormosanine-Type)

An interesting QA type involves those compounds having triazapolycyclic moieties (260–271) being related to (+)- or (−)-ormosanine (260 or 264, respectively) and homoormosanine (267) as the basic structures (Figure 14). The ormosanine-type alkaloids (260–266) have a diazatetracycle moiety bonded with a piperidine unit at C-9, commonly distributed in the Podopetalum(176) and Bowdichia(177) genera (Table 6). The other four ormosanine-like diastereomers have been reported due to the configuration of six chiral carbons (i.e., C-5, C-7, C-9, C-10, C-17, and C-18) (261, 263, 265, and 266). Finally, a Δ⁵⁽⁶⁾-containing structural variant of 264 was also reported ((−)-podopetaline, 262).²⁴ On the other hand, the homoormosanine-type QAs are based on a singular triazahexacyclic structure (267–271) involving an aloperine moiety fused with an additional azabicyclic fragment bonded at N-1 and C-9 of the aloperine moiety (Figure 14, Figure S13). The additional azabicyclic moiety can be formed through a methylenediaza bridge (267–270) or an N-1–C-20 linking (271). Homoormosanine (267) has these three epimeric variants (268–270) differentiated by the α/β-oriented hydrogen patterns at five chiral carbons, i.e., C-5, C-6, C-8, C-21, and C-22 (R¹ to R⁵, Figure 14, Figure S14).

Figure 14.

Ormosanine- and homoormosanine-type quinolizidine alkaloids 260–271.

Table 6. Sources of Isolated Ormosanine- (260–266) and Homoormosanine-Type (267–271) Quinolizidine Alkaloids (QAs).

QAs	Species	Part	Ref
260–261	Bowdichia uirgiloides	stem bark	(177)
262–263	Podopetalum ormondii F. Muell	leaves	(176)
264–266	Templetonia retusa	aerial parts	(24)
267–269	Bowdichia virgiloides	stem bark	(177)
270	Podopetalum ormondii F. Muell	leaves	(176)
271	Dasyprocta leporina	seeds	(178)

Phenanthroquinolizidine QAs

This QA type has a phenanthrene moiety fused with a quinolizidine, sharing the carbons C-3–C-4 of the quionolizidine fragments (272–279). The alkaloids (+)- or (−)-cryptopleurine (275 or 277, respectively) can be considered to be the basic structure of this QA type (Figure 15, Figure S15). Apart from 275/277, six phenanthroquinolizidine QAs have been additionally reported (272–274, 276, 278–279), varying by six different substitutions (R¹ to R⁶, Figure 15) involving hydroxyl or methoxyl groups and the α- or β-orientation of H-6. A representative of this QA type was first isolated in 1935 from the species Tylophora indica. In addition, the genus Tylophora, Pilea, Boehmeria, and Hypoestes are the reported plant sources of these particular QAs (Table 7).

Figure 15.

Phenanthroquinolizidine alkaloids 272–279.

Table 7. Sources of Isolated Phenanthroquinolizidines (272–279), Unusual Bridged Polycycles (280–297), Flavesine (298–301), and Alopecurine-Type (302–304) Quinolizidine Alkaloids (QAs).

QAs	Species	Part	Ref
272	Hypoestes forskaolii	aerial part	(34)
273–275	Pilea aff. martinii	leaves	(179)
276	Tylophora indica	aerial parts	(180)
277	Tylophora indica	leaves	(181)
278–279	Boehmeria siamensis	whole plants	(42)
280	Acosmium dasycarpum (Vog.) Yakovlev	root bark	(106)
281–282	Acosmium panamense	seed	(182)
283	Acosmium dasycarpum (Vog.) Yakovlev	root bark	(106)
284	Guianodendron praeclarum	leaves	(133)
285–293	Flueggea virosa	twigs and leaves	(35)
294–297	Myrioneuron faberi	aerial parts	(36)
298–303	Sophora flavescens	roots	(183)
304	Oxytropis ochrocephala Bunge	whole plant	(170)

Unusual Bridged Polycyclic QAs

Other QA types can be gathered into unique alkaloids containing unusually bridged polycycles (280–297). In this group, the N¹,N¹²-diaza-adamantane alkaloids are included (280–283), highly isolated from the genus Acosmium; therefore, acosmine (280) is the basic structure for this kind of alkaloid¹⁰⁶ (Figure 16, Figure S17). Few structural variants have been reported for the acosmine-type QAs, involving a substitution at C-6 (R¹), which comprises esterified 4-hydroxybutyl chains (282–282) or an allyl group (283). In addition, panacosmine (284) is a special diaza-adamantane alkaloid since it involves a 1-acetyl-1,4,5,6-tetrahydropyridin-3-yl substitution (284) instead of an acetamidomethylene (280) in the absence of the 4-hydroxybutyl substitution (Figure 16). On the other hand, neosecurinan (285) is an interesting hexahydro-2H,7H-5,10b-ethanofuro[2,3-a]quinolizine-containing QA, which was isolated for the first time in 1956 from the genus Securinegaen (Phyllanthaceae).³⁵ Eight securinol-type stereoisomers have been isolated (286–293) (Figure 16, Figure S17) from twigs and leaves of Flueggea virosa (Phyllanthaceae).³⁵ These stereoisomers differed from 285 by the presence of a carbonyl group at C-12, forming a furan-2(5H)-one moiety, whose differences between them are related to the absolute configuration of C-2, the C-7–C-15–C-14–C-10 bridge, and the carbinol carbon at C-8³⁵ (Figure 16). Finally, myrifabral-type QAs (294–297) are an uncommon group of QAs possessing a particular cyclohexane-bridged, tetrahydro-2H-pyran-fused quinolizidine skeleton (Figure 16, Figure S18), involving two pairs of epimers at C-13 (α- or β-OH as R²) and an α-oriented (diethylamino)methyl group at C-14 (R¹) (296–297). These QAs represent the first quinolizidine alkaloids of the genus Myrioneuron.³⁶

Figure 16.

Unusual bridged polycyclic quinolizidine alkaloids 280–297.

Modified Matrine-Related QAs (Flavesine- And Alopecurine-Type)

Flavesine-type QAs (298–301) represent a particular group of modified alkaloids isolated from Sophora and Oxytropis plants (Table 7). They have a matrine-like structure with an open-loop ring D, forming a 3-carboxypropyl moiety and having structural variations related to the unsaturation pattern in ring C (298–300) and a piperidine amide (301) (Figure 17, Figure S19).¹⁸³ Other modified matrine-related QAs involve alopecurine A (302) or B (303), which constitutes the first reported example of a matrine-type alkaloid with C-5–C-6 and C-6–C-7 bond fragmentations,¹⁸³ respectively, and the ochrocephalamine E (304), which was identified as a 14-nor methylene matrine with a unique 6/6/6/5 ring system¹⁷⁰ (Figure 17, Figure S19).

Figure 17.

Flavesine and alopecurine quinolizidine alkaloids 298–304.

Biphenyl and Phenyl Ether Quinolizidine Lactones

This QA type corresponds to a complex alkaloid class particularly occurring in Lythraceae plants,⁴¹ typified by having biphenyl or phenyl ether quinolizidine lactone skeletons. The structure contains quinolizidine (rings A and B) and biphenyl or phenyl ether (rings C and D), connected by C-4–C-2″ and C-2–O–C-12 bonds for both skeletons and the C1′–C1″ or C1′–O–C1″ bond for biphenyl or phenyl ether, respectively, forming the typical dodecano-12-lactone or 7-oxatridecane-13-lactone moiety, respectively. The basic structural core is represented by the alkaloid lythrine (319) or lagerine (331), although some of them are characterized by a hydroxyl group at C-14, involving those QAs related to lythridine (321).^39,40 In this regard, various phytochemical studies have led to the isolation of 21 biphenyl-containing (305–325) and seven phenyl-ether-bearing (326–332) QAs, whose structural variations are depicted in Figures 18 and S20, implying seven substituting positions (R¹ to R⁷) for biphenyl and four substituting positions (R¹ to R⁴) for phenyl ether quinolizidine lactones. Such variations comprise hydroxyl or methoxyl groups at C-2′, C-4″, C-5″, and C-6″ as the distinctive substitution pattern on biphenyl and phenyl ether substructures. In addition, hydroxyl substitutions at C-9 (e.g., 308) or C-10 (e.g., 326), having α- and β-orientation, respectively, also occurred. Some variants include an N-oxide group at N-5 (309–310, 315, 317, 329), which was reported for the first time from Lagerstroemia indica (Lyrthraceae).¹⁸⁴ Finally, reported quinolizidine lactones (90%) include α-hydrogen at C-10. In addition, these lactones have also been isolated from Heimia (Lythraceae) plants (Table 8).

Figure 18.

Biphenyl (305–325) and phenyl ether (326–332) quinolizidine lactones.

Table 8. Sources of Isolated Biphenyl (305–325), Phenyl Ether (326–332), and Macrocyclic (305–332) QAs.

QAs	Species	Part	Ref
305–314	Heimia salicifolia	leaves	(39,40)
315–317	Lagerstroemia indica	aerial parts	(184)
318–328	Heimia salicifolia	leaves	(41)
329	Lagerstroemia indica	aerial parts	(184)
330–332	Heimia salicifolia	leaves	(40)
333	Xestospongia muta	sponges	(15)
334	Neopetrosia chaliniformis	sponges	(185)
335	Xestospongia muta	sponges	(15)
336	Neopetrosia chaliniformis	sponges	(185)
337	Xestospongia muta	sponges	(15)
338	Neopetrosia chaliniformis	sponges	(185)
339–342	Xestospongia sp.	sponges	(186)
343–344	Xestospongia exigua	sponges	(187)
345	Neopetrosia exigua	sponges	(55)
346–349	Xestospongia muta	sponges	(15)
350	Haliclona exigua	sponges	(188)
351–354	Xestospongia exigua	sponges	(189)
355–359	Oceanapia sp.	sponges	(190)
360–362	Xestospongia sp.	sponges	(191,192)
363	Neopetrosia exigua	sponges	(193)
364–365	Xestospongia muta	sponges	(15)
366	Petrosia seriata	sponges	(51)
367	Xestospongia exigua	sponges	(194)
368–369	Neopetrosia chaliniformis	sponges	(185)

Macrocyclic Bisquinolizidines

Macrocycle-containing QAs are a class of marine natural products found mainly in sponges, which are considered to be biogenetically derived from bis-3-alkylpyridine units.¹⁸⁵ Araguspongins/xestospongins and petrosins are the two distinct macrocyclic QA subtypes, chemically characterized by possessing bis-1-oxaquinolizidine (6/6) and bisquinolizidine-2-one (6/6) moieties, respectively.⁵⁵ Thus, the bis-1-oxaquinolizidine units are connected by two six-carbon chains at C-2 and C-9 of each quinolizidine unit (i.e., C-2–(CH₂)₆–C-9′ and C-9–(CH₂)₆–C-2′), while the two bisquinolizidine-2-one units have the C-1–(CH₂)₅–C-9′ and C-9–(CH₂)₅–C-1′ connectivity, which comprise the respective macrocyclic substructure.⁵⁴ Thirty-seven chemical variants are reported for macrocyclic QAs, and the bis-1-oxaquinolizidine-containing QAs (n = 31) are more abundant than bis-quinolizidine-2-one-containing QAs (n = 6), the structures of which are depicted in Figures 19 and S21. In the case of araguspongins/xestospongins (333–363), the chemical diversity is mainly represented by stereochemical variations in six positions (R¹ to R⁶, Figure 19). Hence, hydroxyl groups at C-9(9′) (R¹ and R⁴), methyl groups at C-3(3′) (R² and R³), hydrogens at C-10(10′) (R⁵ and R⁶), and the connecting six-carbon chains can exhibit α- or β-orientation. A similar case is found for petrosins (364–369), having stereochemical variations at C-1(1′), C-9(9′), C-10(10′) (having α- or β-oriented hydrogens), and C-3(3′) (having α- or β-oriented methyl groups; Figure 19, Figure S21). These marine-origin macrocycles are mainly found in the genera Xestospongia, Neopetrosia, and Petrosia, belonging to the Petrosiidae family, but also from Haliclona (Chalinidae) and Oceanapia (Phloeodictyidae) (Table 8).

Figure 19.

Macrocyclic bisquinolizidines 333–369.

Dimeric QAs

Within this group are gathered those QAs that have unusual patterns to afford dimers or form interesting two-pair quinolizidine adducts (Figure 20, Figure S22), isolated from plants of the genera Thermopsis, Sophora, Oxytropis, and Nuphar (Table 9). In this sense, a series of QA adducts (370–376), namely, thermlanseedlines B–F and thermseedlines F–G, were isolated from Thermopsis lanceolata,^103,132 involving thermopsine or cytisine dimers or thermopsine/cytisine adducts. In this regard, a thermopsine dimer involved linking through an additional tetrahydrofuran ring (370), whereas the thermopsine/cytisine adducts (371–372) comprised an alkenyl chain between C-12 and N-12 of the 12-hydroxythermopsine and 11-oxocytisine moieties, respectively. In addition, the cytisine dimers contained an N¹²,N^12′-oxoalkyl (373–375) or N¹²,N^12′-alkoxyoxoalkyl (376) bridge between two 11-oxocytisine units. On the other hand, matrine-type dimers have also been discovered, mostly isolated from Sophora alopecuroides, which include different dimerization patterns, e.g., C-9–C-2′ (377, 379), C-10–C-3′ (378, 380), C-13–C-14′ (382), C-10–C-14′ (383), and C-11/C-12–C-13′/C-14′ (384–386), involving various substituted matrine units. Furthermore, other matrine adducts also involved nor-matrine derivatives, which contain loops formed by ring cleavage, such as ring A of the second matrine unit attached at C-3 of the first matrine unit (381) or ring B in the first unit linked to C-9 of a julolidine unit, representing rare epimeric nor-matrine/julolidine alkaloids (387–388) isolated from seeds.⁷⁸ Finally, the dimeric thiospirane quinolizidines (389–397), particularly isolated from Nuphar plants and known consequently as Nuphar alkaloids,¹⁶⁰ are composed of two lupinine-type units (usually substituted by α/β-methyl, α/β-furan-3-yl, and α/β-hydroxyl groups at C-6, C-3, and C-10, respectively) and connected by a spirocyclic tetrahydrothiophene ring. In addition, the sulfur can be oxygenated, forming a sulfoxide group (390, 394, and 396), or unfunctionalized (389, 391–393, 395, and 397).

Figure 20.

Dimeric quinolizidine alkaloids 370–397.

Table 9. Sources of Isolated Dimeric (370–397) Quinolizidine Alkaloids (QAs).

QAs	Species	Part	Ref
370–376	Thermopsis lanceolata	seeds	(103,132)
377–381	Sophora alopecuroides L.	aerial parts	(195)
382	Oxytropis ochrocephala Bunge	whole plant	(196)
383	Sophora alopecuroides L.	seeds	(78)
384–386	Sophora alopecuroides	leaves	(197)
387–388	Sophora alopecuroides L.	seeds	(78)
389–394	Nuphar pumilum	rhizomes	(160)
395	Nuphar Alkaloids	rhizomes	(198)
396–397	Nuphar pumilum	rhizomes	(160)

Bioactivity of Quinolizidine Alkaloids: An Overview

There is relevant chemical QA variability and a wide distribution in animal and plant sources, as described above (Figures 1–20, Tables 1–9). Apart from this generous chemodiversity and origin, these QA groups have great importance due to their biological activity¹⁸ but are also recognized as toxic agents.¹⁹⁹ For instance, the QAs present in seeds, pods, leaves, aerial parts, and roots of some genistoid plants have been broadly studied, and the alkaloids causing the lupin bittering contain mostly lupanine, lupinine, and hydroxylupanine.⁵³ Such bitter-related QAs have an excitatory effect on the CNS, depressing the respiratory and vasomotor centers, mainly observed in sheep,²⁰⁰ and exhibiting acute anticholinergic toxicity. These facts promoted lupin seed debittering or the research on low QA-containing lupin varieties since lupin seeds are good food options due to their protein content and quality.²⁰¹ Likewise, “twisted calf disease” cases have been reported in cattle due to the anagyrine presence in some herbaceous plants, which is responsible for teratogenic effects.^202,203 However, QAs have other relevant biological activities that can be exploited for several applications. In general, matrine-type QAs are mostly cytotoxic and anticancer bioactive (e.g., 7), whereas lupanine-type (e.g., 57–59) and sparteine-type (e.g., 110–113) QAs have potential against insects and microorganisms. Likewise, the cytisine and tetrahydrocytisine-type QAs have effects as cytotoxic and antiviral agents (e.g., 132–133, and 157), and in the case of the lupinine and macrocycle types, they exhibited relevant antiviral and anticancer properties. In this context, the bioactivities of the most abundant QAs, such as matrine, lupanine, sparteine, cytisine, lupinine, and other QA types, are described below and summarized in Table 10, focusing specifically on the most promising results of the alkaloid types.

Table 10. Overview of Biological Activities of Quinolizidine Alkaloids (QAs).

QAs	Activity tested	Outcome	Ref
4, 131–133	NO production in LPS-stimulated RAW 264.7 cells	IC50 = 22.1 μM	(16)
4–5, 27–28	Effect against HL-60, A-549, and SW480 cell lines	IC50 < 50 μM	(9)
7	Inhibition of Botryosphaeria dothidea mycelial growth	MIC = 1.682 mg/mL	(218)
7	Acaricidal (Tetranychus cinnabarinus) and aphicidal (Aphis citricola) activities	LC50 < 2 mg/mL	(217)
12–13	Cytotoxic activity (endothelial cells)	IC50 = 15.2 μM	(16,214)
12–16	Cytotoxic activity	IC50 = 57.8 μM (HepG-2) and 83.1 μM (CNE-2) for 12	(242)
17–21	In vivo anti-inflammatory activity	Significant inflammation reduction of 17 and 19	(73)
22	Antiviral activity against the hepatitis B virus	53.8% inhibition under the noncytotoxic concentration of 0.035 mM	(78)
23	Cytotoxic activity	IC50 = 20 μM (A-549)	(76)
32–46	Insecticidal activity	LC50 < 50 mg/mL	(71)
47–48	Antiviral activity against the hepatitis B virus	48.3–79.3% inhibition	(11)
47–48	Antiviral activity against the hepatitis B virus	41.3% inhibition	(11)
55–56	Cytotoxic activity	56 inhibit the growth of GSC-3# at 20 μg/mL	(16)
56	Glucose homeostasis	Improved glycemic control at 1 mM	(222)
57	Antiviral activity against the hepatitis B virus	53.8% inhibition	(125)
57–59, 134, 157	Antibacterial activities against Staphylococcus aureus and Escherichia coli	8 μg/mL < MIC < 32 μg/mL	(125)
65	Antibacterial activity against Pseudomonas aeruginosa and Enterococcus faecalis	10.9 μg/mL < MIC < 20.8 μg/mL	(75)
66–67, 239	Antibacterial activity against: Staphylococcus aureus, S. epidermides, S. saprophyticum, and Streptococcus pyogenes	25 μg/mL < MIC < 100 μg/mL	(84)
110–113	Insecticidal activity against Spodoptera frugiperda	Mortality = 83% at day 7 for 110	(82,223,243,244)
132	Cytotoxic activity	IC50 = 5.36 μM, inducing apoptosis.	(229,231)
132	Antiviral activity against human influenza virus A	IC50 < 200 μM	(235)
132	Cytotoxic activity	Cell percentage in the G2/M phase at 24 h increased from 21.1 to 50.0% (HEK293, Hep-G2, and Jurkat cell lines)	(229)
138	Antibacterial activity against E. faecalis	MIC = 208.3 μg/mL	(75)
148–155	Insecticidal activity against Aphis fabae	LC50 < 50 μg/mL	(103,132)
148–155	Inhibition against tomato spotted wilt virus	Protective effect >70%	(103,132)
161	Cytotoxic activity	IC50 = 10 μM (COLO-205)	(245)
179–183	Cytotoxic activity and acetylcholinesterase inhibition	IC50 < 330 μM (AChE)	(150)
		IC50 < 5.5 μg/mL (L1210)
189	Neuronal nicotinic acetylcholine receptors	IC50 = 1.5 μM (α4β2-nAChRs) and 1.3 μM (α7-nAChRs)	(152)
198	Cytotoxic activity HL-60 cells	IC50 = 39 μM (HL-60)	(37)
205, 311–314	Antimalarial activity	IC50 < 5 μg/mL (P. falciparum)	(40)
333–337	Cytotoxic activity	IC50 < 1.02 μM against all tested cancer cell lines	(15)
234	Cell growth and in vitro tumorigenesis of human thyroid cancer cells	IC50 < 161.7 μM (IHH-4)	(246)
234	Inhibition of human immunodeficiency virus 1	EC50 = 1.2 and 1.6 μM (NL4-3 and YU2)	(240)
234–237	Anticonvulsant effect	234 exhibited a better anticonvulsant effect	(17,240)
272	Antimalarial activity	IC50 < 6.11 nM (P. falciparum)	(34)
276	Cytotoxic activity	IC50 = 1.6 nM (A375), 2.5 nM (A549), 1.4 nM (HCT116) and 4.1 nM (Namalwa–Burkitt’s lymphoma)	(180)
278–279	Cytotoxic activity	0.2 ng/mL < IC50 < 100 ng/mL	(42)
294–297	Inhibition of hepatitis C virus replication	0.9 μM < IC50 < 4.7 μM	(36)
303	Antiviral activity against the hepatitis B virus	46.0–14.1% inhibition	(183)
311–314	Antimalarial activity	IC50 = 4.76 μg/mL (P. falciparum)	(40)
333, 335, 337	Cytotoxic activity	IC50 < 1.02 μM against all tested cancer cell lines	(15)
337–339	Cytotoxic activity	ED50 > 20 μM	(166)

The matrine-type alkaloids have been reported as the most biologically active QAs,⁶ exhibiting a wide spectrum of biological properties, including antitumor,²⁰⁴ antiviral,²⁰⁵ and anti-inflammatory¹⁰ activities. In addition, they have attracted attention due to their capacity to reduce hand and foot diseases caused by enterovirus (EV-71), which does not have an available vaccine. Hence, therapeutics based on derivatives of 1 have shown relevant results that could lead to the management and control of EV-71 (e.g., 34–35) in the future.²⁰⁵ Moreover, matrine derivatives (e.g., 3) have reduced disease symptoms by compensating for the decreased T-cell levels.²⁰⁶ It has also been suggested that 12 can inhibit and suppress the expression of TLR4, a pattern recognition receptor whose activation produces pro-inflammatory cytokines.²⁰⁷ Compounds 1–17 have been reported to inhibit the growth of malignant cells and tumors with promising results (IC₅₀ < 20 μM against different cancer cell lines) through proliferation inhibition and apoptosis induction, whose advances were compiled in a comprehensive review recently published on the anticancer properties of 1 and its derivatives.²⁰⁸ However, matrine and some naturally occurring derivatives are limited by various factors (i.e., toxicity, bioavailability, and low water solubility), and different matrine-inspired compounds have been synthesized to improve the inhibitory action against cancer cells.²⁰⁸ Compound 15 can improve the clinical signs of experimental autoimmune encephalomyelitis (EAE).^209,210 These studies have reported that 15 delays the disease progress, attenuates the clinical severity of EAE when tested in rats, decreases inflammation and demyelination generated in the brain, and suppresses apoptosis of oligodendrocytes (OLG) in the rat central nervous system.²¹¹

Compounds 1–11 exhibited cytotoxic, anti-inflammatory, and antianaphylactic activities. Recent studies have examined the expression of the hypoxia-inducible 1-alpha factor (HIF-1a) and endothelial vascular growth factor in different phases of human hemangioma (HA). At different concentrations (0–2 μg/μL) of 2, it was demonstrated that the HIF-1a expression increases significantly in the proliferation phase of HA but decreases in the involuntary phase of HA.²¹² On the other hand, alkaloid 9 has been widely studied for its anti-inflammatory properties. Recent studies have shown its excellent effects against lupus nephritis (LN) and lupus erythematosus (SLE) since it reduces the inflammatory response and inhibits the activation of the inflammatory NLRP3.^213,214 Additionally, compound 2 has also been reported to inhibit epidermal growth factor receptor (EGFR) related signaling pathways from suppressing the proliferation and invasion of malignant cells responsible for gastric cancer.²¹⁵ Alkaloid 2 significantly inhibited migration and invasion of human gastric cancer cells by decreasing phospho-cofilin (Ser3) and phospho-LIMK1 (Thr508) without changing the total expression of cofilin and LIMK1.²¹⁶ In addition, alkaloid 7 showed acaricidal and aphicidal effects on Tetranychus cinnabarinus and Aphis citricola,²¹⁷ respectively, and antifungal activity against Botryosphaeria dothidea(218) and Fusarium oxysporum.²¹⁹

It has been reported that the consumption of seeds from Lupinus plants containing alkaloid 62 has led to intoxication events in humans due to the acute anticholinergic toxicity of some lupanine-type QAs. The most common symptoms are blurred vision, dry mouth, easy flushing, and confusion.²⁰¹ Such symptoms are reported in a human who consumed 0.5 L of bitter water from Lupinus seeds. The immediate symptoms were weakness, accelerated palpitations, extrasystoles, and different anticholinergic symptoms.²²⁰ According to the antecedents, the lethal dose in rats for alkaloids 62 and 63 was investigated, determining a DL₅₀ = 1664 mg/kg.²²¹ However, other lupanines, e.g., 56, positively influenced pancreatic cells in an animal model of type-2 diabetes mellitus.²²² In the presence of glucose at 15 mM, insulin secretion was significantly elevated by compound 56 (0.5 mM). At the same time, the alkaloid did not stimulate insulin release with lower glucose concentrations, suggesting that 56 improved glycemic control in response to an oral glucose tolerance test in streptozotocin-diabetic rats.²²² In this context, the effect on insulin secretion of three alkaloids isolated from Lupinus has recently been studied, such as compounds 56, 59, and 70, along with a synthetic derivative involving in vitro evidence of an increase in glucose-induced insulin release, whose effect intensity depended on glucose concentration and ATP-sensitive K channel blocking.³² Also, various lupanine-type QAs have shown cytotoxic activities, such as 56 and 61 against human glioma stem cells GSC-3#,¹⁶ human breast cancer (MDA-MB-231), and human lung cancer (A549).⁸² Furthermore, esterified lupanines, such as 64 and 67, exhibited high deterrent effects against coleopteran and lepidopteran insects such as Spodoptera frugiperda(223) and Choristoneura fumiferana,²²⁴ as well as antibacterial activity by 65 and 66 against Pseudomonas aeruginosa, Enterococcus faecalis,⁷⁵Staphylococcus aureus, S. epidermides, S. saprophyticum, and Streptococcus pyogenes.⁸⁴

Sparteine-type alkaloids are relevant QAs since studies report their neuroprotective effects against cellular diseases associated with Alzheimer’s.²²⁵ Sparteine-type compounds, such as 110, may inhibit protein synthesis and acetylcholine receptors,²²⁶ while 125 presented nematicidal activity against Hemonchus contortus and Teladorsagia circumcincta.²²⁷ Similarly, 110 has toxic effects by inhibiting K⁺ channels and the tDNA synthesis and formation.⁹⁵ In addition, compound 110 and analogs have shown that subcutaneous administration of 25 mg/kg in neonatal rats decreases the mRNA levels of muscarinic acetylcholine receptors, specifically of M1–M3 subtypes, and generates an increase of M7 mRNA between 7 and 14 days after administration.²²⁸ Furthermore, the anticonvulsant effects of 110 on the behavior and electroencephalic activity were studied in three states of epilepsy (SE) models.⁵

Some studies have investigated the effect of some cytisine-type QAs (e.g., 131 and 132) on human lung and breast cancer. Results showed that 132 (i.e., cytisine) inhibited the growth of lung cancer cell lines, including A549 (IC₅₀ = 26.83 μM), NCI-H23 (IC₅₀ = 49.79 μM), and NCI-H460 (IC₅₀ = 32.45 μM) cells using the CCK-8 assay.²²⁹ Alkaloid 132 was influential in suppressing lung cancer cells through cell cycle arrest and the induction of mitochondrial-mediated apoptosis, suggesting that compound 132 may be a promising candidate for developing lung cancer treatments.²²⁹ In addition, compound 132 and homologues induce apoptosis of tumor cells via the endoplasmic reticulum (ER) pathway.²³⁰ The information suggested that calcium overload promotes ER stress-induced apoptosis in cytisine-induced HepG2 cells, modulating the CHOP/GADD153, JNK, and caspase-4 pathways.²³¹ Finally, the caspase cascade is activated to induce apoptosis of HepG2 cells, through the mitochondrial pathway, according to the reduction in the mitochondrial membrane potential.²³¹ Following cytisine treatment, mitochondrial permeability may increase, leading to mitochondrial matrix expansion, outer membrane rupture, and a cytochrome C release.²³¹

Alkaloid 132 and structurally related compounds (131–155) have been shown to have a high affinity for the neuronal nicotinic acetylcholine receptors (nAChR) and are essential probes in the investigation of central nervous system disorders.²³² Particularly, cytisine showed affinity to nAChRs and can activate α7-nAChR expression.²³⁰ Moreover, some synthetic derivatives (e.g., cytisine-12-carbamide and N-allylcytisine-12-carbamide) are acetylcholinesterase inhibitors and are toxic against Artemia salina at concentrations below 1000 ppm.²³³ Additionally, the antiviral activity of 132 was also evaluated against the human influenza A (H1N1) virus, the human parainfluenza virus type-3 (HPIV-3), and SARS-CoV-2 virus.²³⁴132–136 showed remarkable activity against HPIV-3 with a selectivity index (SI) of 58, calculated as the ratio of CC₅₀/IC₅₀.²³⁵ Compounds 148–155 isolated from seeds of Thermopsis lanceolata had moderate insecticidal activity against Aphis fabae (LC₅₀ = 43.15 and 46.47 mg/L, respectively),¹²³ and compound 132 showed antifungal activity against Fusarium oxysporum.²¹⁹ On the other hand, compounds 150–151 isolated from the rhizomes of the Chinese plant known as “Shan-Dou-Gen” (Sophora tonkinensis) were evaluated against the cancer lines T24 (human bladder cancer cell line), SPC-A2 (human lung adenocarcinoma), and A549 (human lung adenocarcinoma). The best results were obtained for 150 against the A549 cancer line, with an IC₅₀ = 10.36 μM.²³⁶

Alkaloid 174 is one of the most representative alkaloids of the genus Lupinus and can be considered the basic form of QAs, i.e., the 6/6 azabicycle quinolizidine moiety.⁹⁵ This QA type has had several biological activity records in recent years.⁸ In this regard, 179–180 inhibited acetylcholinesterase at IC₅₀ = 330 and 220 μM and cytotoxicity against murine lymphoma L1210 cells (IC₅₀ = 4.9 and 5.5 μg/mL),¹⁵⁰ while QA 189 was a potent blocker of α4β2- and α7-nAChRs (IC₅₀ = 1.5 and 1.3 μM, respectively). Alkaloid 199 was tested against HL-60 (human myeloid leukemia cell line), SMMC-7721 (human hepatocarcinoma cell line), and SW480 (human colon carcinoma), and the results were found to be promising (IC₅₀ < 10 μM).⁹ Compounds 206 and 207 displayed lower cytotoxicity compared with the commercial standard, being potent antiangiogenic agents.¹⁵⁶ Senepodines 222 and 227 showed moderate cytotoxicity against human blood promyelocytic leukemia (HL-60, 46% inhibition at 100 μM), whereas 224 and 225 did not show activity.³⁸

Those compounds structurally related to 234 (aloperine-type) show excellent anticancer, anti-inflammatory, antifibrotic, antiviral, and antiarrhythmic activities.¹⁷ In this regard, alkaloid 234 has been explored as an anti-inflammatory and antitumor agent.²³⁷ Recent studies have demonstrated that 234 generates protection against acute renal injury induced by ischemia-reperfusion.²³⁸ Additionally, studies have shown that 234 and its derivatives selectively repress IL-1β and IFN-α expression, regulating PI3K/Akt/mTOR signaling and NF-ob transcriptional activity.^8,239 In addition, 234 has also been one of the most important compounds because it inhibits HIV infection by blocking HIV-1 entry.²⁴⁰ This compound responded well by inhibiting cell–cell fusion mediated by the HIV envelope at low concentrations. This study demonstrated that the naturally occurring 234 and synthetic derivatives are key bioactives for inhibiting this globally problematic infection.²⁴⁰ Additionally, alkaloids 235–237 demonstrated potent antihepatitis B virus activities (HBV) and are more potent against the hepatitis B e-antigen (HBeAg) secretion than the hepatitis B surface antigen (HBsAg).¹⁶⁶ On the other hand, the antimicrobial activity of 239 against four Gram-positive bacteria (i.e., Staphylococcus aureus, S. epidermides, S. saprophyticum, and S. pyogenes), three Gram-negative bacteria (i.e., Escherichia coli, Klebsiela pneumonia, and Shigella sonei), and three yeasts (i.e., Candida albicans, Sacharomyes cerevisae, and Criptococcus neoformans) was evaluated, demonstrating moderate to good results (MIC < 50.0 μg/mL).⁸⁴ Regarding (homo)ormosanine-type QAs, compounds 260 and 267 showed good in vitro activity against chloroquine-sensitive and chloroquine-resistant strains of Plasmodium falciparum (IC₅₀ = 0.5 μg/mL).²⁴¹ Alkaloid 276 (a phenanthroquinolizidine QA) was evaluated against a panel of 30 cancer cell lines and found to inhibit the proliferation of all tested cell lines, including three multidrug-resistant cell lines (average IC₅₀ value of 2.1 nM), which is much lower than that of those previously reported for commercial standards.¹⁸⁰ Phenanthroquinolizidines 278 and 279 show cytotoxic activity against six cancer lines, including colon, lung, breast, prostate, kidney, and leukemia, with IC₅₀ between 0.2 and 100 ng/mL,⁴² whereas 272 exhibited high activity against malaria.³⁴ The myrifabrals 294–297 inhibited hepatitis C virus replication (HCV, IC₅₀ 0.9–4.7 μM) with cytotoxicity lower than reference standards.³⁶

Compendium of Biological Activities Reported for Quinolizidine Alkaloids

The studies conducted have primarily centered on investigating the biological properties of QAs, which can be categorized into eight classes, including cytotoxic, antiviral, antimicrobial, insecticidal, anti-inflammatory, antimalarial, antiacetylcholinesterase, and miscellaneous activities. A comprehensive compilation of each QA (1–397) can be found in Table S2 (Supporting Information), and the subsequent description highlights the most pertinent findings.

Cytotoxic Activity

Tumor cells are characterized by uncontrolled growth and unlimited proliferation. The primary function of many anticancer drugs is to damage these tumor cells directly. In the case of quinolizidine treatments, they have demonstrated the ability to inhibit the proliferation of tumor cells in various cancer types, including HL-60 (human promyelocytic leukemia cells), SMMC-7721 (hepatocellular carcinoma), human glioma stem cells (GSC), A-549 (adenocarcinomic human alveolar basal epithelial), MCF-7, and SW-480 (human colon adenocarcinoma). Considering the above, alkaloids have shown bioactivity against these specific cell lines depending on their respective types. For instance, matrine-type compounds (5,⁹6,²⁴⁷7,^248,24923,⁷⁷27,⁹ and 55(250)) have demonstrated cytotoxic activity with average IC₅₀ values greater than 50 μM against SMMC-7721, A549, HepG2, HL-60, MCF-7, and SW480 lines. Furthermore, lupanine- and cytisine-type compounds have also exhibited promising results in terms of cytotoxicity. Specifically, compounds 56,¹⁶94,²⁵¹135, and 139(236) have shown cytotoxic activity against GSC-3#, GSC-12#, GSC-18#, MCF-7, and HEPG-2 lines, with IC₅₀ values ranging between 117 and 20 μM.

In addition, an important group of lupinine-type quinolizidines, specifically compounds 179–184 (IC₅₀ > 8.2 μg/mL), demonstrated activity against murine lymphoma cells L1210.¹⁵⁰ Compound 186 exhibited activity against TE-671, SH-SY5Y, IMR-32, and K-177 with IC₅₀ values greater than 55 μM,⁴⁶ while 187–188 showed activity against P-388, A-549, U-251, and SNl2KI with IC₅₀ values above 24.7 μg/mL.²⁵² QA 198 was evaluated against HL-60 (IC₅₀ = 39 μM),³⁷ while 298 was evaluated against five cancer lines with IC₅₀ values below 100 μM.⁹ Compounds 206–211 were assessed against the HCT-116 line, demonstrating IC₅₀ > 80.2 μM.¹⁵⁷ On the other hand, compounds 216–219 were evaluated against B16 melanoma cells, resulting in inhibitions of less than or equal to 50%.¹⁶⁰ Regarding compounds 222–236, they were tested against L1210 lymphoma cells, exhibiting IC₅₀ values below 7.5 μg/mL.¹⁶³ Furthermore, QAs 230–236 were tested against various cancer lines, including MG-63, U2OS-OS, HepG2 2.2.15, papillary thyroid carcinoma (IHH-4), and anaplastic thyroid carcinoma (8505c and KMH-2), showing IC₅₀ values above 100 μM (inactive),^38,246,253 except for the L1210 line, which demonstrated IC₅₀ values below 10 μg/mL.¹⁶⁴ Compounds 244 and 245 displayed moderate activity (IC₅₀ > 50 μM) against U-87 (glioblastoma), 518-A2 (melanoma), and HCT-116 (colon cancer).¹⁷⁴ Finally, compounds 272–276 were successfully evaluated against KB (mouth epidermal carcinoma cells, CCL-17), HepG-2 (human liver hepatocellular carcinoma cells, HB-8065), LU-1 (human lung adenocarcinoma cells, HTB-57), and MCF-7 (human breast cancer cells, HTB-22), with IC₅₀ values above 1 μM, demonstrating promising potential.¹⁷⁹ QA 277 was evaluated against human gastric cancer AGS (hypoxia-inducible factor-1) with an IC₅₀ of 8.7 nM,^254,255 while 285 and 288 were evaluated against the P388 cell line; however, no promising activity was obtained.²⁵⁶ These results indicate that matrine- and lupinine-type quinolizidines show the most promising activity against the investigated cancer cell lines. Therefore, it is crucial to continue expanding the experimental knowledge regarding the biological activity of these QA types.

Antiviral Activity

Viral infections significantly threaten humans, animals, and economically important crops worldwide, leading to substantial mortality and disease-related losses. In order to mitigate the detrimental effects caused by various viruses, natural targets have been investigated, yielding noteworthy outcomes in both in vitro and in vivo studies. Several QAs have been examined for their ability to affect specific viruses, including the hepatitis B virus, hepatitis C, nonhuman influenza virus (H3N2), enterovirus EV-71, coxsackie B virus, human herpesvirus-6 (HHV-6), tobacco mosaic virus (TMV), tomato spotted wilt virus (TSWV), and others. In this regard, QAs 2,²⁵⁷7,⁷⁸13,¹⁸³15,¹¹21,²⁵⁸22,⁷⁸28,¹⁸³48,¹¹96,⁷⁸132,²⁵⁸235,¹⁶⁶237,¹⁷⁰238,¹⁶⁶298,¹⁸³304,¹⁷⁰383,⁷⁸387,⁷⁸ and 388(78) showed promising results against the hepatitis B virus, with inhibition percentages between 10 and 60% for the serologic marker (HBsAg) and between 10 and 40.5% for the antigen (HBeAg). On the other hand, compounds 294–297 were active against the hepatitis C virus with CC₅₀ values between 119 and 170 μM and EC₅₀ between 2 and 5 μM.³⁶ The importance of the effect of QAs 3, 14, 47, and 50 against the nonhuman influenza virus (H3N2) has also been reported, with mean inhibitory concentrations between 60 and 400 μM.^259,260 In addition, the matrine-type compounds 32–46 showed valuable results against the tobacco mosaic virus (TMV) with a protective effect above 50% and a curative effect between 20 and 65%. In another study, the effect of quinolizidines 4, 8, 21, and 202 against the Coxsackie B virus (pathogenic enterovirus) was evaluated, and the best result was obtained for compound 202 (IC₅₀ = 4.66 μM).²⁶¹ Finally, the inhibitory effect of 9 against the human herpes virus 6 (HHV-6) exhibited an IC₅₀ = 3.9 μM,²⁶² while compounds 150–155, 221, and 370–376 were evaluated against the tomato spotted wilt virus (TSWV) in Nicotiana tabacum cv.K326, whose results showed protective and curative effects between 16 and 60% and 18–50%, respectively.¹³²

Antimicrobial Activity

The antimicrobial effect of QAs has been evaluated against several microorganisms such as bacteria and fungi. Recent studies have evaluated the antifungal activity of compounds 2, 7, 15, 56, 59, 62, 63, 92, 94, 110, 132, 133, 139, 160, 161, 174, and 239 against the phytopathogen Fusarium oxysporum, involving IC₅₀ values between 10 and 400 μM.²¹⁹ On the other hand, some studies were performed on the effect of quinolizidines 351–354 against Candida albicans ATCC 14503, C. albicans UCD-FR1, C. glabrata, and C. krusei, with MIC values between 30 and 100 μg/mL.¹⁹¹ Other studies have evaluated the antibacterial activity of compounds 65, 133, 138, and 251 against Enterococcus faecalis, with MIC values between 20 and 200 μg/mL.⁷⁵ In addition, alkaloids 57–59, 66–67, 134, 157, and 202 were evaluated against two Gram-positive bacteria, i.e., Staphylococcus aureus and Escherichia coli, including a broad MIC range between 25 and 200 μg/mL,^84,125,261 whose best outcome was obtained for compound 202 (MIC = 8 μg/mL for S. aureus and MIC = 0.8 μg/mL for E. coli).²⁶¹ Finally, the antibacterial activity of 214 was evaluated against Proteus mirabilis, P. vulgaris, Klebsiella pneumoniae, Escherichia coli, and Shigellu dysenteriue, with MIC values between 100 and 150 μg/mL.¹⁵⁹

Insecticidal Activity

Insecticidal studies of QAs have also been investigated to counteract the problems caused by some insects on economically important crops, such as the black aphid (Aphis fabae), the common house mosquito (Culex pipiens), the red spider mite (Tetranychus urticae), and the brown leafhopper (Nilaparvata lugens). According to these studies, compounds 37, 103, 105–109, 148–155, 221, and 370–376 showed insecticidal activity with LC₅₀ between 25 and 32 ppm and inhibitions between 35 and 80%, demonstrating their promising protective capacity for Vicia faba crops.^71,103,132 Another study examined quinolizidines 94 and 133 against Culex pipiens, involving LC₅₀ ranging between 3.42 and 8.26 ppm and LC₉₀ between 43.83 and 154.18 ppm.²⁵¹ Insecticidal activity studies have also been conducted with the QAs 102–109, 103–108, and 375–376, involving an LC₅₀ between 49 and 65 ppm and inhibitions greater than 50% against Nilaparvata lugens and Tetranychus urticae.^103,132

Anti-inflammatory Activity

The anti-inflammatory activity of QAs on the tumor necrosis factor (TNF-α), associated with inflammation, apoptosis, and joint destruction, and the interleukin-6 factor (IL-6), associated with endothelial cells and fibroblasts, has also been evaluated. These studies have shown that compounds 1, 7, 17, 94, 133, and 377–381 could positively inhibit the TNF-α with values greater than 50%. The best result was obtained with compound 377, which showed an inhibition of 96.64%. In the case of the IL-6 factor, the studies reported 40–68% inhibitions.^{78,195,263−265}

Antimalarial Activity

Significant research efforts have been focused on exploring active compounds against tropical diseases with malaria being one of the primary targets. In this regard, the investigation of QAs against the parasitic protozoan Plasmodium falciparum has been conducted. Thus, QAs have shown promising results comparable to commercial standards, highlighting the promising activity of 260 (IC₅₀ = 5 μg/mL),²⁴¹267 (IC₅₀ < 20 μg/mL),²⁴¹ and 268 (IC₅₀ = 6.11 nM for KI strain and 5.13 nM for FCR3 strain of P. falciparum).³⁴318–325 were also active against D6 and W2 P. falciparum clones, and the best results were obtained with 322 and 325 with IC₅₀s between 2.80 and 4.76 μg/mL.⁴¹ Finally, compound 335 was also tested against the African clone D6 (IC₅₀ = 670 ng/mL) and Indochinese clone W2 (IC₅₀ = 280 ng/mL) of P. falciparum.¹⁸⁷

Antiacetylcholinesterase Activity

The antiacetylcholinesterase activity is a significant and medically relevant biological effect. This activity is particularly important because it has the potential to inhibit the degradation of acetylcholine, a neurotransmitter released in the synaptic clefts. Natural substances, including QAs, that possess the ability to inhibit the enzyme responsible for acetylcholine breakdown can enhance cholinergic neurotransmission by slowing acetylcholine degradation. With the aforementioned benefits in mind, studies were conducted to assess the inhibitory effects of compounds 110, 111, 126, 189, and 253–259 on acetylcholinesterase (AChE) and butyrylcholinesterase (BChE). These compounds exhibited inhibitions ranging from 15% to 73.9%, and their mean inhibitory concentrations ranged from 21 to 331 μM.^5,150,266 Finally, compound 189 blocked neuronal nicotinic acetylcholine receptors (α4β2 and α7) with IC₅₀ between 1.3 and 1.5 μM.¹⁵²

Miscellaneous Activities

Finally, some of the compiled QAs have undergone testing for specific types of biological activity that are less commonly observed. Despite their particularity, these reports provide valuable information regarding the bioactivity of these diverse and structurally intriguing compounds. Among these activities, the antiarrhythmic effect of QA 7 was tested on mice, whose lethal dose (LD₅₀) was 72.1 mg/kg,²⁶⁷ and the same compound was effective as a biopesticide against Diaphorina citri (LC₅₀ = 1247 ppm and LC₉₀ = 5712 ppm), Panonychus citri (LC₅₀ = 42 ppm and LC₉₀ = 73.7 ppm), Sitophilus zeamais (LC₅₀= 463.9 ppm and LC₉₀ = 1121 ppm), and Spodoptera frugiperda (LC₅₀ = 384.3 and LC₉₀ = 1034 ppm).¹⁹⁵ In the case of compound 174, dietary activity was investigated in rainbow trout, and no toxic effects were observed at a recommended dietary dose of 500 mg/kg.²⁶⁸ In addition, the inhibitory activity against hPTP1B (human protein tyrosine phosphatase 1B) of compounds 286–287 and the vasodilatory activity of compounds 339–342 have been documented. However, in both cases, these compounds were found to be inactive.^186,269 On the other hand, compound 352 was evaluated as a possible inhibitor of somatostatin and vasoactive intestinal peptide, and the results were very promising with IC₅₀ = 12 μM.²⁷⁰ Furthermore, a compound of the same QA type, i.e., 353, was a potent inhibitor of the inositol 1,4,5-trisphosphate receptor and endoplasmic reticulum Ca²⁺ pumps with an inhibition of 78%.²⁷¹ Finally, the RLAR (rat lens aldose reductase) activity of compounds 315–317 and 329 was evaluated with inhibitions between 25 and 32%¹⁸⁴ and the antimetastatic activity of compounds 389–397, whose best result was obtained for QAs 391–392 with inhibitions of 86.6% and 86.8%, respectively, classifying them as potent antimetastatic agents.¹⁶⁰

Structural and Drug-Likeness Comparison of Compiled Quinolizidine Alkaloids

The compiled information shows that QAs, a category of natural compounds, have garnered considerable attention in drug discovery research due to their wide range of pharmacological activities (Table 10, Table S2). QAs possess intricate structural complexity and distinctive properties, which position them as potential candidates for creating innovative therapeutics. However, an essential aspect to consider for this purpose, beyond biological properties, pertains to the physicochemical characteristics required for a compound to be suitable for drug development.²⁷² This aspect can be rationalized under the drug-likeness concept, which refers to a set of physicochemical properties that a compound should possess to effectively access body cells and perform physiological or pharmacological functions while maintaining safety within the host organism.²⁷³ Therefore, it can be considered a qualitative measure that aims to strike a balance between molecular and structural features, indicating how closely a substance resembles a potential drug regarding bioavailability.²⁷⁴ In this regard, structural factors come into consideration when evaluating the drug-likeness of QAs, considering that they exhibit a complexity with multiple stereocenters, which provides numerous variations and expands the range of potential drug-like properties that can be attributed to them. To explore such factors, the custom-made QA-based library (n = 397) was structurally compared using the similarity analysis module included in the DataWarrior ver. 5.5.0 program²⁷⁵ to visualize plausible structural relationships and patterns to define interesting QA-based scaffolds. Thus, the similarity plot (Figure 21a) on associating the FragFP descriptor (i.e., a substructure fragment dictionary-based binary fingerprint similar to the MDL keys) and the DataWarrior-based drug-likeness approach²⁷⁵ led to finding eight main clusters having similar fingerprints and positive drug-likeness score (d) (calculated values in Table S3). In this regard, the cluster with the best drug-likeness profile (d > 3, clusters 2 and 5) involved matrine and lupanine-type compounds having a 3-hydroxypiperidin-2-one moiety (related to 53 and 78, respectively) and the (homo)ormosanine-type QAs (e.g., 260–272), which seem to exhibit more drug-like properties. Compounds 53 and 78 have no reported activity, but they are related to 18 and 21, which exhibited promising anti-HBV activity.²⁵⁸ Other important clusters (i.e., 1, 3, and 5–8) involved bismacrocyclic (e.g., related to 355 and 336), piperidin-2-one-containing (e.g., related to darvasamine (12), camoensidine (247), N-formyltetrahydrocytisine (163), alopecuroide E (381)), cytisine-type (e.g., 132 and 133), and leontidine-type (e.g., 247) QAs with positive drug-likeness scores (d > 0).

Figure 21.

(a) Similarity plot combining the FragFP descriptor and drug-likeness. Colors were determined according to the drug-likeness score (d). Numbers in small boxes are related to QA numbering 1–397. Ball size is related to the structure–activity landscape index (SALI) of d/FragFp. Numbers in yellow rectangles are related to the relevant clusters 1–8. (b) Box plots of calculated values of c Log P, c Log S, and drug-likeness (d) values of QAs 1–397.

The d/FragFp ratio, which serves as the structure–activity landscape index (SALI), demonstrated values exceeding 200 (represented by the size of the balls in Figure 21a) for approximately 26% of the compiled QAs (SALI > 200). This enabled the prediction of the drug-like potential of these QAs based on their chemical structure, utilizing the principle of molecular similarity to known drugs. In essence, drug-likeness refers to the inherent characteristics of a chemical compound that are necessary to attain the desired optimal pharmacological properties.²⁷² Generally, QAs exhibited a favorable drug-likeness profile, with a positive d score observed for 62.7% of QAs (Figure 21a,b, Table S3). These facts suggested that most QAs contained fragments that are commonly found in commercially available drugs. However, some QAs were part of the exception (d < −5) with bad drug-like profiles, such as alkenyl-substituted lupinines (e.g., 188 and 189), biphenylquinolizidine lactones (e.g., 315–317), acosmine-type (e.g., 280), and dimeric quinolizidines (e.g., 377). In addition, most QAs also exhibited reasonable hydrophilicity, involving medians within the range of commercial drugs (c Log P > 0; 0 > c Log S > −4) (Figure 21b), rationalized by the presence of various H-acceptors (ca. four on average) (Table S3).

These findings indicated that the biological activities of QAs might be of paramount importance in drug discovery since their drug-like properties place QAs as attractive starting points for the development of drug candidates targeting various diseases and conditions. Although QAs present challenges and advantages due to their structural complexity, several offer immense potential as drug candidates. Continued research efforts focused on improving their drug-likeness through expanding the chemical space by isolation of more variants or synthetic modifications and computational modeling, and optimization strategies will pave the way for the development of novel therapeutics based on QAs.²⁷⁶

Perspectives

QAs have emerged as a fascinating class of naturally occurring compounds with diverse chemical structures and significant biological activities. The chemistry and biological activities of QAs have been the subject of extensive research, and their perspectives hold great promise for various scientific disciplines. From a chemical perspective, QAs exhibit remarkable structural complexity and diversity. Multiple substituted variants and stereocenters within the quinolizidine scaffold add to their structural intricacy. This complexity provides a fertile ground for studying stereochemistry, synthetic methodologies, and structure–activity relationships (SARs) in the context of drug discovery and natural product chemistry.¹⁸ In addition, the biological activity of QAs is another intriguing aspect that has attracted significant attention. These alkaloids have demonstrated a wide range of pharmacological properties, making them promising candidates for the development of therapeutics (Table 2). QAs have exhibited antimicrobial activity against various bacterial and fungal strains, including multidrug-resistant pathogens. Their cytotoxicity against cancer cells has also been investigated, showing potential as anticancer agents. Moreover, QAs have shown antiviral activity against several viral infections, such as hepatitis B and C viruses, and have been explored for their insecticidal and insect-repellent properties.

Understanding the mechanisms underlying the biological activities of QAs is crucial for their further development and utilization.¹⁷ Further studies can reveal that their bioactivities are often mediated through the modulation of specific molecular targets and cellular pathways, interacting with enzymes, receptors, ion channels, and signaling pathways, leading to their diverse pharmacological effects.²⁷⁷ However, several of them remain to be examined, and consequently, elucidating the molecular mechanisms of action can provide valuable insights into the design and optimization of QA-based therapeutics.

In recent years, advancements in analytical techniques, synthetic methodologies, and medicinal chemistry techniques have facilitated the obtention of diverse QA derivatives and analogs. These efforts to expand QA-related chemical space have contributed to SAR studies and structure-based drug design, enabling the development of potent and selective QA-based compounds.^278,279 Moreover, the discovery of natural sources of QAs, including plants, marine organisms, and animals, continues to expand the chemical space and offers new prospects for exploring QA chemistry and biological activity. In this context, the research perspectives on the chemistry, occurrence, and biological activity of QAs are multifaceted and hold immense potential. They provide opportunities for the discovery of novel drugs, exploration of natural product chemistry, and development of sustainable insecticides and antimicrobial agents. Furthermore, the unique structural features and diverse biological activities of QAs make them interesting subjects for interdisciplinary research, encompassing fields such as synthetic chemistry, pharmacology, biochemistry, and molecular biology.

Concluding Remarks

The present review encompasses a compilation of 397 quinolizidine alkaloids (QAs) representing the chemical diversity of these specialized metabolites isolated and reported over the past three decades. This compilation was organized into various QA types; as such, categorization had not been previously undertaken but was necessary. These QA types exhibit a high degree of structural complexity, characterized by intricate stereoisomerism, which renders them attractive as leads and scaffolds for various purposes. Most of these compounds have been isolated from the seeds, leaves, and aerial parts of Fabaceae plants, although other families, such as Lythraceae, also contain an intriguing group of macrocycle-type QAs. Additionally, macrocycle-type QAs have been identified in marine sponges (Petrosia, Xestospongia, and Oceanapia), frogs (Dendrobatidae), and ants (Formicidae). Despite initially being recognized for their natural defensive properties, various types of QAs have attracted considerable attention for research and utilization due to their wide range of biological activities. These activities include cytotoxic, antiviral, antimicrobial, insecticidal, anti-inflammatory, antimalarial, and antiacetylcholinesterase effects. Importantly, these QAs also exhibit a favorable putative drug-likeness profile, making them promising candidates to be considered in drug discovery endeavors. Accordingly, the chemistry, occurrence, and biological activity perspectives of QAs offer a rich landscape for scientific exploration and innovation. Continued research efforts, including synthetic studies, structure–activity relationship investigations, and mechanistic studies, will undoubtedly contribute to unlocking the full potential of QAs as valuable chemical entities with significant therapeutic applications.

Thus, the information gathered in this review underscores the need for further research to expand the chemodiversity and identify more potent bioactive compounds based on QAs as valuable scaffolds for pharmacological and agrochemical applications.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c02179.

• Names and SMILES of 397 compiled quinolizidine alkaloids (QAs) (Table S1), individual structures of quinolizidine alkaloids 1–397 (Figures S1 to S22), compendium of reported biological activity of the 397 quinolizidine alkaloids (Table S2), and calculated properties of compiled QAs 1–397 (Table S3) (PDF)

This study was funded by the Vicerrectoría de Investigaciones at the Universidad Militar Nueva Granada (UMNG) through the project IMP-CIAS-2924, validity 2020.

The authors declare no competing financial interest.