A comprehensive review on phytochemistry and pharmacology of genus Kopsia: monoterpene alkaloids – major secondary metabolites

Kopsia belongs to the family Apocynaceae, which was originally classified as a genus in 1823. Kopsia consists of medicinal plants that can be traditionally used to treat rheumatoid arthritis, pharyngitis, tonsillitis, and dropsy. More than one hundred and twenty-five publications have been documented relating to the phytochemical and pharmacological results, but a systematic review is not available. The goal of this study is to compile almost all of the secondary metabolites from the plants of genus Kopsia, as well as the coverage of their pharmacological research. The document findings were conducted via reliable sources, including Web of Science, Sci-Finder, Science Direct, PubMed, Google Scholar, and publishers, while four words “Kopsia”, “monoterpene alkaloids”, “Phytochemistry” and “Pharmacology” are key factors to search for references. Most Kopsia secondary metabolites were collected. A total of four hundred and seventy-two, including four hundred and sixty-six monoterpene alkaloids, five triterpenoids, and one sterol, were summarized, along with their resource. Kopsia monoterpene alkaloids presented in various skeletons, but aspidofractinines, eburnamines, and chanofruticosinates are the three major backbones. Mersinines and pauciflorines are new chemical classes of monoterpene alkaloids. With the rich content of monoterpene alkaloids, Kopsia constituents were also the main objects in pharmacological studies since the plant extracts and isolated compounds were proposed for anti-microbial, anti-inflammatory, anti-allergic, anti-diabetic, anti-manic, anti-nociceptive, acetylcholinesterase (AChE) inhibitory, cardiovascular, and vasorelaxant activities, especially cytotoxicity.


Introduction
Natural products are chemical substances created by living organisms and found in nature. In the medicinal chemistry eld, this concept is usually limited to secondary metabolites. 1 The pharmacological studies on potential bioactive agents tend to nd that lead molecules for drug development could arise from natural resources.
Kopsia belongs to the subfamily Rauvoloideae of the family Apocynaceae. 2 This genus, containing about 30 species, is widely distributed in Southeast Asia, China, Australia, and some islands of the Western Pacic. 3,4 Kopsia plants are recognized as a fertile reservoir of novel and bioactive secondary metabolite type alkaloids. Therefore, they have been traditionally used in each country. Chinese folk medicine deals with the use of parts of K. officinalis Tsiang & P. T. Li to treat rheumatoid arthritis, pharyngitis, tonsillitis, and dropsy. 4 In Malaysia, the roots of four species, K. larutensis King & Gamble, K. macrophylla Hook.f., K. singapurensis Ridl., and K. paucifora Hook.f., were applied as a poultice to ulcerate noses in tertiary syphilis. 2,5 Kopsia constituents are also well-known in pharmacological discoveries, in which they have a wide spectrum of pharmacological effects such as anticancer and anti-manic activities. 6,7 Recently, the search for bioactive molecules from the genus Kopsia has drawn lots of interest to natural product chemists and pharmacists. [8][9][10][11][12][13] Although there have been a variety of experimental studies, an overview of phytochemical and pharmacological assessments is not available now. The current review provides notes on basic knowledge about phytochemical research and sheds light on the pivotal role of Kopsia constituents in pharmacological examinations. More than one hundred twenty-ve relevant publications have been used, as well as the data collection is from the 1950s to now.

Skytanthines, rhazinilams, and lundurines
It is recognized that the unique chemical class of skytanthines can be arranged as a new group of alkaloids. These phytochemicals were isolated from Apocynaceae Skytanthus acutus for the rst time in 1960. 123 From Table 1 and Fig. 7, ten new skytanthines 391-400 have been summarized. The extracts of K. dasyrachis and K. macrophylla, especially K. pauciora, are accompanied by the presence of this type. 27,30,112,113 Two publications in 1996 and 1997 by Kam and partners successfully reported the structures of serial new skytanthines kinabalurines Fig. 7 Skytanthines, rhazinilams and lundurines from genus Kopsia.
A-F (391-396) from K. pauciora leaves. 111,112 while their following congener kinabalurine G (397) was derived from K. dasyrachis leaf. 30 Signicantly, the novel alkaloidal kopsirachine (399) isolated from K. dasyrachis leaves was determined to be a hybrid compound by the combination of catechin and skytanthine. 113 Aer being run Sephadex LH-20 and silica gel CC, a new monoterpene alkaloids containing a lactone ring, kopsilactone (398), and other new monoterpene alkaloids possessing 2-azabicyclo[3.3.1] backbone, kopsone (400), were isolated from the MeOH extract of K. macrophylla bark. 27 Based on these ndings, skytanthines can be seen as chemical evidence to determine the close relationship among Apocynaceae plants, especially between genera Skytanthus and Kopsia.
Kopsia lundurines 410-426 have generally been formed by the combination of an indole ring and a lactam ring through an eight-ring member (Fig. 7). Notably, all of these seventeen compounds were novel in nature, and the three plants, K. lapidilecta, K. grandifolia, and K. tenuis, are the main reservoirs (Table 1).
Catharinensines, which belong to the group of oxindole alkaloids, can be found in several higher plants, such as Peschiera catharinensis. 126 In Kopsia plants, ve catharinensines 432-436 were detected (Table 1 and Fig. 8). Phytochemical research conducted by Gan and partners revealed that the use of mobile phase CHCl 3 -MeOH is appropriate to isolate alkaloidal catharinensines. 22 By this approach, three new compounds, kopsirensines A-C (433-435), together with known analog catharinensine (432), have been successfully puried from the acidic EtOH extract of K. pauciora leaves. 22 New catharinensine kopsiyunnanine B (436) was rst collected as a light yellow solid from the alcoholic extract of K. officinalis aerial part, and then was detected in the K. arborea aerial part. 96,97 Phytochemical studies on Kopsia plants have also led to the isolation of alkaloid leuconoxines 437-442, and their structures were compiled in Fig. 8. Leuconoxine (441) was described as a major component since it occurred in K. arborea stem bark, K. griffithii leaf and stem bark, K. pauciora stem, stem bark and leaf, K. singapurensis stem bark, K. teoi stem bark. 15,17,19,22,23,43 Arboloscine (437) was one of the new compounds in K. arborea stem bark, while melodinine E (442) was a known metabolite extracted from its twigs. 10,21,118 New compound arboloscine A (438) isolated from K. pauciora leaf has a similarity in structural feature with compound 437, but the methyl group of 437 was replaced by the ethyl group in 438. 22 In the genus Kopsia, leuconodine D (439) was only detected in K. officinalis stems, whereas leuconodine F (440) was characteristic of K. griffithii leaves and K. pauciora leaves. 22,43,75 To nd bioactive molecules from medicinal plants, four alkaloids type pericines, including two new compounds pericidine (443) and pericine N-oxide (445) and two known analogs pericine (444) and valparicine (446) were isolated (Table 1 and Fig. 8). All of these isolates originated from K. arborea stem bark. 10,118,119 To the best of our knowledge, only three compounds 447-449 were classied as alkaloid alstonines (Table 1 and Fig. 8). Oxayohimban-16-carboxy acid (447) derived from K. officinalis stem has never been isolated from the genus Kopsia before. 75 The major component (À)-tetrahydroalstonine (448) appeared in K. arborea stem bark, K. dasyrachis stem, K. griffithii leaf, K. officinalis root, twig and leaf, K. larutensis stem bark and leaf, K. pauciora stem, stem bark and leaf, K. singapurensis stem bark; K. teoi stem bark. 10,15,[17][18][19]22,23,25,32,42,43,66 Compound 449, a pseudoindoxyl derivative of compound 448, was identied to be a new constituent from the acidic EtOH extract of K. pauciora leaves. 22 In the same manner, there are only three quebrachamines from the genus Kopsia till now (Table 1 and Fig. 8). (À)-Quebrachamine (452) is now abundant in nature and can be found in K. arborea aerial parts, K. hainanensis twigs and leaves, K. officinalis roots, and K. pauciora leaves. 9,22,69,114 However, kopsiyunnanines D and H (450-451) from K. arborea aerial part were conrmed to be two new analogs. 90,114

Others indole alkaloids and non-alkaloids
Phytochemical studies on Kopsia plants also recorded the appearance of other alkaloidal types (Table 1 and Fig. 9). Chromatographic procedure on the acidic MeOH extract of K. arborea bark has resulted in the isolation of three new metabolites, arbophyllinines A-B (453-454) and arbophyllidine (463). 59 Arbo-orine (455) from K. arborea stem bark was a known alkaloid type arboorine, but its new analog kopsiyunnanine E (456) was detected in the aerial part of K. arborea and K. officinalis. 10,96,99,121 Besides the main constituents, the EtOH extract of K. pauciora leaves has composed of a new component, andransinine A (458), along with a known one andransinine (457). 22 New corynantheines arboricine (459) and arboricine (460) were found in both the leaves and stem of K. arborea. 10,120 The new carboline harmane (461) was presented in both leaves and stem of K. grif-thii, but the new congener harmicine (462) was only detected in its leaves. 15,17 To nd bioactive compounds from plants, mersicarpine (464) was rst isolated from K. arborea stem bark. 10 It was then further found in K. pauciora leaves and K. singapurensis stem bark. 22,23 Two nal alkaloids, a new alkaloid type, azepanefused tetrahydro-b-carboline kopsiyunnanine K (465) and a known alkaloid type andranginine (466), were constituents of K. arborea aerial part. 102 To date, there have not been many results on the separation of non-alkaloidal constituents from the plants of the genus Kopsia. A phytochemical report from Shan and partner (2017) identied that the n-hexane extract of K. singapurensis dried leaf and bark has accompanied with the existence of ve triterpenoids b-amyrin (467), b-amyrin acetate (468), b-amyrone (469), lupeol (470), lupeol acetate (471), and one sterol stigmasterol (472) ( Table 1 and Fig. 10). 122 This is the rst time to observe these compounds in the genus Kopsia.
Taken together, despite the fact that there have been preliminary chemotaxonomic and synthetic reviews. 127,128 This is the rst time that we provide fully information on phytochemical separation, a detailed list of almost isolated compounds, chemical classication, botanical resource, and the great value of Kopisa monoterpene alkaloids in botanical and chemical relationship.

Pharmacological activities
Cytotoxic, antimicrobial, anti-inammatory, anti-diabetic, cardiovascular, vasorelaxant, and other positive properties have been studied utilizing Kopsia secondary metabolites and extracts in pharmacological research. In Table 2, a summary of prior pharmacological appraisals on Kopsia plant materials is presented in detail.

Anti-microbial activity
Nowadays, microbial resistance to well-known antibiotics has caused major concern about the treatment of infectious diseases. A vast amount of studies has recently been conducted to determine possible answers. Phytochemicals have been shown to exhibit antibacterial activity against sensitive and resistant infections through various approaches. To have a look at the IZ (inhibitory zone) and MIC values of Kopsia constituents (Table 2), compounds 43-47, 48-53, and 76 are not only potential anticancer molecules but also useful antimicrobial agents. 52,53 Especially, kopsiafrutine E (47) with the MIC values of 0.15-1.14 mM established a remarkable antimicrobial effect against twelve pathogenic microorganisms, including two Gram positive bacteria Staphylococcus aureus and S. epidermidis, ve Gram negative bacteria Escherichia coli, Enterobacter cloacae, Pseudomonas aeruginosa, Klebsiella pneumoniae, and Shigella dysenteriae, three fungi Candida albicans, C. tropicalis, and C. glabrata, and two oral pathogens Streptococcus mutans and S. viridans. 52 Likewise, compounds 48-49 showed strong antimicrobial activity with MIC values of less than 0.3 mM against seven bacteria E. cloacae, E. coli, K. pneumoniae, P. aeruginosa, S. aureus, S. dysenteriae, and S. epidermidis. 53 In another assessment, kopsiorine (74) and kopsihainins D-F (85-87) showed suppression towards the Gram positive bacterium Staphylococcus aureus with IZ values ranging from 9.7 to 11.2 mm, but compounds 3, 17, 73, 109, 124, 405, and 406 were inactive. 12 In an antimicrobial assay against E. coli, Erwinia carotovra, Bacillus subtilis, B. cereus, and S. aureus, two best agents N-decarbomethoxykopsamine (14) and N 1 -decarbomethoxy chanofruticosinic acid (206) were associated with the MIC values of 7.8-15.5 and 15.5-31.3 mg mL À1 , respectively. 7 These two molecules further showed antifungal activity against In vivo To decrease arterial blood pressure and heart rate 131 208 In vivo 13% Relaxation occurred rat aorta ring 84 210 In vivo 24% Relaxation occurred rat aorta ring 84 211 In vivo 26% Relaxation occurred rat aorta ring 84 216 In vivo 28% Relaxation occurred rat aorta ring 84 219 In vivo 40% Relaxation occurred rat aorta ring 84 225 In vivo 41% Relaxation occurred rat aorta ring 84 227 In vivo 15% Relaxation occurred rat aorta ring 84 228 In vivo 37% Relaxation occurred rat aorta ring 84 229 In vivo 19% Relaxation occurred rat aorta ring 84 230 In vivo 19% Relaxation occurred rat aorta ring 84 239 In vivo 23% Relaxation occurred rat aorta ring 84 Rhizoctonia solani, Penicillium italicum, Fusarium oxysporum f. sp. Cubense, and F. oxysporum f. sp. Niveum (Table 2). 7 Lastly, two eburnamines 19-hydroxy-(À)-eburnamonine (267) and phutdonginin (297) showed moderate activity against the growth of E. coli with the same MIC value of 32 mg mL À1 . 21

Conclusion and future perspectives
To a certain extent, our comprehensive review establishes a panel of useful information on phytochemistry and pharmacology of the genus Kopsia. Since the 1950s, about nineteen Kopsia plants were used in phytochemical investigations, and more than four hundred seventy secondary metabolites have been isolated. Among 472 isolated compounds, monoterpene alkaloids (466 compounds) accounted for 98.73%. Kopsia monoterpene alkaloids have been fallen into about 30 structural skeletons, but aspidofractinines (204 compounds),