A comprehensive review of non - alkaloidal metabolites from the subfamily Amaryllidoideae ( Amaryllidaceae

: Amaryllidoideae is a subfamily belonging to the Amaryllidaceae and is widely distributed in the southern hemisphere. The subfamily is well known for its content of pharmacologically active alkaloids and represents an important epicenter of Amaryllidaceae - alkaloid diversity. Other metabolites from Amaryllidoideae such as pheno - lics including ﬂ avonoids, lignans, chromones, and acet - ophenones, in addition to terpenoids and ceramides, have been reported and received less attention compared to alkaloids. Currently, 224 non - alkaloidal compounds have been isolated and identi ﬁ ed from ∼ 7% of the subfamily members. Many of the isolated compounds showed inter - esting biological activities. Isolation of certain compounds such as ﬂ avans and phytosterols from Amaryllidoideae has signi ﬁ cant taxonomical value among the Amaryllidaceae subfamilies. This article represents an extensive review of the non - alkaloidal chemical constituents of Amaryllidoideae and their biological activities including a brief discussion of their chemotaxonomical importance.


Introduction
The Amaryllidaceae is a family of monocotyledonous flowering plants in the order of Asparagales and was established in 1805; according to the current phylogenetic work, Chase et al. [1] developed the broader (sensu lato) concept of the family.According to the plant list website [2], it contains about 2,362 accepted species, divided into about 80 genera, 17 tribes, and 3 subfamilies; the Agapanthoideae (only one genus, Agapanthus), Allioideae (∼20 genera) and Amaryllidoideae (∼59 genera) [3].The Amaryllidoideae (formerly recognized as a separate family, Amaryllidaceae J.St.-Hil.) is a widely distributed subfamily of about 850 species.It has a center of diversity in the southern hemisphere, especially South Africa, South America, and the Mediterranean [4,5].Plants of the Amaryllidoideae were recognized by traditional healers in South Africa and were used to treat different diseases such as, inter alia, cancer and mental health issues.The traditional use of the plant family has been related to their unique alkaloidal contents [4,[6][7][8][9].The plant's extracts and chemical constituents "mainly alkaloids" are reported to have different biological activities such as antiproliferative, antiinflammatory, antihypertensive, neuroprotection, anticancer, and antimicrobial [4,5,7,[10][11][12].On the other hand, only relatively few reports have been published on other chemical constituents such as terpenoids and flavonoids [4,13,14].In continuation of the previous reviews on the genera Crinum and Zephyranthes, which reflected the presence of non-alkaloidal chemical constituents [15,16], this review was designed to compile chemical and biological information on non-alkaloidal chemical constituents from plants belonging to the Amaryllidoideae subfamily and to discuss the chemotaxonomical importance concerning other subfamilies among the Amaryllidaceae.
2 Chemistry and biological activities of Amaryllidoideae subfamily constituents 2.1 Flavonoids higher plants.Flavonoids play a variety of biological roles, such as the color and aroma of flowers, protection from different biotic and abiotic stresses and act as unique UV filters, function as signal molecules, allelopathic compounds, phytoalexins, detoxifying agents, and antimicrobial defensive compounds [17,18].Chemically, phenylpropanoid formation is a key step in the biosynthesis of flavonoids which react further with malonyl-CoA to form the basic carbon skeleton C6-C3-C6.Different sub-classes of these compounds include flavans, flavanols, chalcones, flavanones, flavones, flavonols, and isoflavones.

Flavonols
The glycosidic forms of kaempferol, quercetin, and isorhamnetin constituted the majority of these isolated compounds in which it was found that the flowers are the main source of flavonols (Table 1 and Figure 3).None of the isolated compounds contain a methyl group at C 6 and/or C 8, which indicates that the flowers accumulate different types of flavonoids from the bulbs.Rutin (73) was the most common flavonol glycoside isolated.The glycosylation pattern, in general, occurs at C 3 ; however, C 7 (60), C 3 ′ (69), and C 4 ′ (63) were isolated as well [26,27].

Anthocyanins
The color chemistry of Amaryllidoideae flowers has been subjected to some chemical investigation and found that they are linked with certain compounds belonging to the carotenoids, flavones, and/or anthocyanins [28,29].The colors of different Hippeastrum cultivar flowers have been attributed to the presence of cyanidin 3-O-rutinoside (80) and pelargonidin 3-O-rutinoside (81).According to the CIELab analysis, 80 contributes to the red color while 81 contributes to the orange color [30].
Studies using mainly LC-MS identified different anthocyanins in different Amaryllidoideae flowers.Glycosides of pelargonidin and/or cyanidin have been identified in the genera Lycoris [31,28], Nerine [32], and Hippeastrum [33].Different color hues such as red, purple, or blue depend not only on the chemical constituents (the pigments) but also on the pH of the media and additionally due to copigmentation among others [34,35].(Continued) Non-alkaloidal metabolites from the subfamily Amaryllidoideae  5

Homoisoflavanones
The detection of isoflavonoids using LC-MS in different species of Amaryllidoideae showed the presence of daidzein in addition to 14 common isoflavonoids in different parts of the studied plants [36].The rare homoisoflavanone derivatives (82-85) were isolated from Cyrtanthus obliquus bulbs (Table 1 and Figure 3) [37].
Flavonoids are one the most important food constituents.Plants with high flavonoid contents have a great potential for the neutralization of many human pathologies.Flavans 13, 14, and 26 were isolated from Z. candida collected in China and displayed significant inhibitory effects on the LPS-induced NO production in RAW264.7 mouse macrophages with IC 50 values of 17.34, 16.14, and 21.52 μM, respectively.The compounds (13,14,26) have low cytotoxicity when tested against the same cell line and indicated a high safety margin and the potential as a potent scaffold for the treatment of inflammation [19].Another flavan derivative (15) was isolated from Z. candida collected in Nigeria and demonstrated antipoliovirus activity with an IC 50 of 0.2384 μg/mL and a selectivity index >151 and confirm the potent activity and safety margin of this class of compounds [38].
Compounds 61, 70, and 72 were isolated from C. bulbispermum and showed moderate antibacterial activity C. obliquus bulbs have been used by South African traditional healers for treatment of different diseases such as pregnancy-related ailments, cystitis, dementia, and leprosy.The antioxidant evaluation of the isolated compounds (82-85) was performed using DPPH and FRAP chemical assays and showed strong activity for compounds 82 and 84 [37].The detection of homoisoflavanones in C. obliquus may need special attention and further investigation for the biological activities associated with this rare skeleton.
C. latifolium is a rare species and grows in Vietnam.The local people used the plant for the treatment of cancer.The coumarin derivative 4-[(senecioyloxy)methyl]-6,7-dimethoxycoumarin (104) was isolated as a bioactive constituent, which showed strong antiangiogenic activity and inhibited 76.6% of the tube-like formation of HUVECs at 3.0 μg/mL with no toxicity against B16F10 and HCT116 cell lines [49].The obtained results justify the traditional uses of the plant for the treatment of cancer.

Phenylethanoids and phenylpropanoids
Phenylethanoids and phenylpropanoids are important building blocks in the biosynthesis of many natural products.Table 1 and Figure 6 illustrate the compounds reported from Amaryllidoideae.Non-glycosylated free forms such as tyrosol (105) and/or glucosyl/methyl/ethyl ethers have also been isolated.Mono (108, 109, 112, 113, 118, and 121) and diglucoside (110,111,114,119,120) derivatives were among the isolated compounds.

Flavanones (3)
Chalcones (4)  Non-alkaloidal metabolites from the subfamily Amaryllidoideae  9 The extract of flowers of N. tazetta var.chinensis showed antimelanogenic activity.Pure compounds 108, 111, 114, 118, and 120 were isolated as active constituents and exhibited potent activity against theophylline-stimulated melanogenesis in B16 melanoma 4A5 cells at nontoxic concentrations [50], the isolated compounds have both lipophilic and hydrophilic characteristics and represent an ideal model for drug discovery with potent activity and selectivity.

Phenolic acids
The majority of isolated phenolic acids have the phenylpropanoid skeleton (Table 1 and Figure 7).The bulbs of C. asiaticum L. var.sinicum have been used to treat abscesses, aching joints, and sores in China.Phytochemical studies resulted in the isolation of different derivatives of benzoic (125,126) and cinnamic (137, 139, and 140) acids [51,52], while caffeic (137) and dihydrocaffeic (131) acids were isolated from the flowers of H. vittatum [53].
The chlorogenic acid family belongs to the caffeoyl acid esters of quinic acid, which is widely distributed in nature and showed interesting biological activities [56,57].

Terpenoids
Terpenoids have been poorly studied among the Amaryllidoideae, especially mono-, sesqui-, and diterpenoids.A few compounds belonging to different classes of triterpenes were reported in addition to the sesquiterpene (184).To the best of our knowledge, 28 terpenoids have been reported so far.Triterpenes having lupane, ursane, and oleanane  skeletons have been identified.In addition, terpenes with different skeletons of the phytosterols having different substitution patterns at C 4 , C 14, and C 24 have also been reported (Table 1 and Figure 9).Phytosterols (C 24 alkyl-substituted sterols), e.g., ergostane and stigmastane, are the most abundant sterols in the plant kingdom [69,70].Cholestane is commonly accumulating in animals; however, some related compounds have been reported from certain species of higher plants [71].The first examples of cholestanes (158, 162) isolated from Amaryllidoideae have recently been isolated from B. haemanthoides [72].
Compounds 165 and 169 were isolated from B. disticha bulbs and both compounds showed low activity against human neuroblastoma (SH-SY5Y) cells with IC 50 values of 173.0 and 223.0 μM, respectively [73].
The sesquiterpene, parthenicin (184), was isolated from C. ensifolium and it showed strong cytotoxic activity against a selection of cancer cell lines and strongly inhibited NF-κB activity with an IC 50 value of 1.82 μM [75].Non-alkaloidal metabolites from the subfamily Amaryllidoideae  15

Conclusion
This review has brought comprehensive information about the non-alkaloidal chemical composition and biological activities of the plants belonging to the subfamily Amaryllidoideae.Such knowledge is of importance in the understanding of how plants can have an impact on human health or diseases (Figure 12).Plants belonging to the Amaryllidoideae subfamily have a unique alkaloidal chemical composition making them different from the two Amaryllidaceae subfamilies, Agapanthoideae and Allioideae.
Considering the high alkaloidal content in the bulbs of the studied plant, it is interesting to note that the majority of the isolated compounds have been documented from the bulbs as well (45 plants, 81%) and contribute synergistically to the final biological effects of the bulbs, the most widely used plant part by traditional healers.
Currently, about 224 non-alkaloidal compounds (as listed above, and Figures 13 and 14) have been identified and belong to different classes of compounds.Most importantly, different chemical trends among the isolated compounds have been identified, which make this subfamily unique not only for its alkaloidal content but also for the other metabolites such as flavonoids, terpenoids, and ceramides.
Although relatively fewer terpenoids have been isolated, the presence of different skeletons of phytosterols including cholestane is very significant.The phytosteroidal saponins were reported from the subfamily Allioideae as active constituents and taxonomical biomarkers.The presence of saponin aglycons in the Amaryllidoideae subfamily may represent a limited but important chemical bridge between the two subfamilies.
Among the flavonoids, many lipophilic flavans/flavanols were isolated from both aerial parts and bulbs.The lipophilic character increases the bioavailability of the flavans in the human diet and thereby boosts the therapeutic effects.On the other hand, mono-, di-, and triglycosides of flavonol and acetophenones were isolated mainly from flowers.
The activities of 1, 7, 15, 19, 90, and 146 against HL-60 (human promyelocytic leukemia cells) with IC 50 ranged from 13.8 to 42.6 μM indicating the importance of discovery/design derivatives with the same chemical scaffold for treatment of such grave disease.
The updated knowledge on Amaryllidoideae chemistry and pharmacological activity has considerable opportunity for future discoveries.Relatively few species have been studied extensively for non-alkaloidal contents, and are mainly from China and Japan.Currently, there is little consistency in the compounds studied in different species.While few studies attempt to characterize the non-alkaloidal active components, it appears to be universally assumed that the alkaloids "only" are responsible for the pharmacological activity.Further research on different aspects of Amaryllidoideae chemistry is also necessary to discover the new chemical/ biological aspects of the subfamily, especially non-alkaloidal contents.Currently, from the 55 plant species under this review, limited numbers of biological studies are performed and represent ∼23% (13 plants).It is possible that other compounds are overlooked due to the alkaloidoriented studies.For example, there are ∼1,000 research articles on alkaloids while only 40 on flavonoids.The reported 224 non-alkaloidal metabolites were isolated from less than 7% (55 plants) of the subfamily species, which indicates that a reasonable concentration of these metabolites is present in comparison to alkaloids and the method of phytochemical studies needs to be changed for more generic schemes than the alkaloid-focus method.
A more holistic approach to the study of Amaryllidoideae chemistry is necessary to further research on the subfamily.The current research tends to focus on alkaloids in different bulbs but fails to look at other parts such as leaves and flowers.
The presence of such a wide range of important biologically active non-alkaloidal compounds should encourage more investigation in the near future to fully understand the chemical profile of this subfamily and thereby discover their biological potential for human health benefits.

Figure 12 :
Figure 12: Different types of secondary metabolites from Amaryllidoideae.The general profile of the secondary metabolites distribution among Amaryllidoideae shows the domination of alkaloids with 74%.Other metabolites like flavanoids, terpenoids, etc., share a lower percentage.

Figure 13 :
Figure 13: Non-alkaloidal secondary metabolites distribution among Amaryllidoideae.The distribution of the non-alkaloidal secondary metabolites among different genus studied so far from Amaryllidoideae shows the highest number of isolated compounds coming from Crinum genus followed by Narcissus.Also, the figure shows that flavonoids (yellow blocks) are the most distributed class of compounds.

Figure 14 :
Figure 14: Non-alkaloidal secondary metabolites of Amaryllidoideae.The profile of the non-alkaloidal secondary metabolites distribution shows the domination of flavonoids with 38%.