Antimalarial Activity of Cryptolepine and Some Other Anhydronium Bases

Indoloquinoline Alkaloids as Antimalarials: Advances, Challenges, and Opportunities

Malaria infections affect almost half of the world's population, with over 200 million cases reported annually. Cryptolepis sanguinolenta, a plant native to West Africa, has long been used across various regions of Africa for malaria treatment. Chemical analysis has revealed that the plant is abundant in indoloquinolines, which have been shown to possess antimalarial properties. Cryptolepine, neocryptolepine, and isocryptolepine are well-studied indoloquinoline alkaloids known for their potent antimalarial activity. However, their structural rigidity and associated cellular toxicity are major drawbacks for preclinical development. This review focuses on the potential of indoloquinoline alkaloids (cryptolepine, neocryptolepine, and isocryptolepine) as scaffolds in drug discovery. The article delves into their antimalarial effects in vitro and in vivo, as well as their proposed mechanisms of action and structure-activity relationship studies. Several studies aim to improve these leads by reducing cytotoxicity while preserving or enhancing antimalarial activity and gaining insights into their mechanisms of action. These investigations highlight the potential of indoloquinolines as a scaffold for developing new antimalarial drugs.

Intramolecular trapping of an iminium salt: rapid construction of quindoline derivatives

Construction of the pyridine ring is a practical and streamline way to construct a variety of quindoline derivatives. We have developed a novel method for synthesis of quindoline derivatives by means of intramolecular ring-closure reactions of 3-N-methylphenylindoles via an iminium salt intermediate. This practical method has the advantages of a short reaction time, operational simplicity, and nearly quantitative yields; and it can be used for the rapid synthesis of a variety of valuable quindoline derivatives.

Unveiling antiplasmodial alkaloids from a cumulative collection of Strychnos extracts by multi-informative molecular networks

Malaria, a disease known for thousands of years and caused by parasites of the Plasmodium genus, continues to cause many deaths throughout the world today, particularly due to the emergence of parasite resistance to the current therapeutic arsenal. Plants of the Strychnos genus, remarkable due to their multiple traditional uses as well as their alkaloid content, are promising candidates to develop new antimalarial treatments. Indeed, previous research on this plant group has shown promising (≤ 5 µg/ml) or good (between 5 and 15 µg/ml) antiplasmodial activities. Using the chloroquine-sensitive strain of Plasmodium falciparum (3D7), and artemisinin as positive control, a screening of antiplasmodial activities from 43 crude methanolic extracts from 28 species of the Strychnos genus was carried out in three independent assays. A total of 12 extracts had good (6 extracts) or promising (6 extracts) antiplasmodial activities. These results allowed both to confirm known activities but also to detect new ones. These extracts were then analyzed by HPLC-ESI(+)-Q/TOF, and the processed MS/MS data allowed to generate a molecular network in which the antiplasmodial activities were implemented as metadata. The exploration of the molecular network revealed the presence of alkaloids still unknown, and potentially active against malaria, in particular alkaloids close to usambarensine and its derivatives. This study shows that the emergence of molecular networking offers new leads for identifications of alkaloids from the Strychnos genus. The presence of unknown alkaloids potentially active against malaria confirms all the interest to continue in studying the Strychnos genus. Bioassay- and mass-guided fractionations as well as various dereplication tools would allow to identify and characterize these interesting alkaloids further.

Semi-Synthetic Analogues of Cryptolepine as a Potential Source of Sustainable Drugs for the Treatment of Malaria, Human African Trypanosomiasis, and Cancer

The prospect of eradicating malaria continues to be challenging in the face of increasing parasite resistance to antimalarial drugs so that novel antimalarials active against asexual, sexual, and liver-stage malaria parasites are urgently needed. In addition, new antimalarials need to be affordable and available to those most in need and, bearing in mind climate change, should ideally be sustainable. The West African climbing shrub Cryptolepis sanguinolenta is used traditionally for the treatment of malaria; its principal alkaloid, cryptolepine (1), has been shown to have antimalarial properties, and the synthetic analogue 2,7-dibromocryptolepine (2) is of interest as a lead toward new antimalarial agents. Cryptolepine (1) was isolated using a two-step Soxhlet extraction of C. sanguinolenta roots, followed by crystallization (yield 0.8% calculated as a base with respect to the dried roots). Semi-synthetic 7-bromo- (3), 7, 9-dibromo- (4), 7-iodo- (5), and 7, 9-dibromocryptolepine (6) were obtained in excellent yields by reaction of 1 with N-bromo- or N-iodosuccinimide in trifluoroacetic acid as a solvent. All compounds were active against Plasmodia in vitro, but 6 showed the most selective profile with respect to Hep G2 cells: P. falciparum (chloroquine-resistant strain K1), IC₅₀ = 0.25 µM, SI = 113; late stage, gametocytes, IC₅₀ = 2.2 µM, SI = 13; liver stage, P. berghei sporozoites IC₅₀ = 6.13 µM, SI = 4.6. Compounds 3-6 were also active against the emerging zoonotic species P. knowlesi with 5 being the most potent (IC₅₀ = 0.11 µM). In addition, 3-6 potently inhibited T. brucei in vitro at nM concentrations and good selectivity with 6 again being the most selective (IC₅₀ = 59 nM, SI = 478). These compounds were also cytotoxic to wild-type ovarian cancer cells as well as adriamycin-resistant and, except for 5, cisplatin-resistant ovarian cancer cells. In an acute oral toxicity test in mice, 3-6 did not exhibit toxic effects at doses of up to 100 mg/kg/dose × 3 consecutive days. This study demonstrates that C. sanguinolenta may be utilized as a sustainable source of novel compounds that may lead to the development of novel agents for the treatment of malaria, African trypanosomiasis, and cancer.

Discovery of Highly Potent Serotonin 5-HT2 Receptor Agonists Inspired by Heteroyohimbine Natural Products

The serotonin 5-HT2 receptors are important pharmaceutical targets involved in signaling pathways underlying various neurological, psychiatric, and cardiac functions and dysfunctions. As such, numerous ligands for the investigation of these receptors' activity and downstream effects have been developed synthetically or discovered in nature. For example, the heteroyohimbine natural product alstonine exhibits antispychotic activity mediated by 5-HT2A/2C agonism. In this work, we identified a heteroyohimbine metabolite containing a serotonin pharmacophore and truncated the scaffold, leading to the discovery of potent agonist activity of substituted tetrahydro-β-carbolines across the 5-HT2 receptor family. Extensive SAR development resulted in compound 106 with EC50 values of 1.7, 0.58, and 0.50 nM at 5-HT2A, 5-HT2B, and 5-HT2C, respectively. Docking studies suggest a π-stacking interaction between the tetrahydro-β-carboline core and conserved residue Trp6.48 as the structural basis for this activity. This work lays a foundation for future investigation of these compounds in neurological and psychiatric disorders.

Liposomes for malaria management: the evolution from 1980 to 2020

Malaria is one of the most prevalent parasitic diseases and the foremost cause of morbidity in the tropical regions of the world. Strategies for the efficient management of this parasitic infection include adequate treatment with anti-malarial therapeutics and vaccination. However, the emergence and spread of resistant strains of malaria parasites to the majority of presently used anti-malarial medications, on the other hand, complicates malaria treatment. Other shortcomings of anti-malarial drugs include poor aqueous solubility, low permeability, poor bioavailability, and non-specific targeting of intracellular parasites, resulting in high dose requirements and toxic side effects. To address these limitations, liposome-based nanotechnology has been extensively explored as a new solution in malaria management. Liposome technology improves anti-malarial drug encapsulation, bioavailability, target delivery, and controlled release, resulting in increased effectiveness, reduced resistance progression, and fewer adverse effects. Furthermore, liposomes are exploited as immunological adjuvants and antigen carriers to boost the preventive effectiveness of malaria vaccine candidates. The present review discusses the findings from studies conducted over the last 40 years (1980-2020) using in vitro and in vivo settings to assess the prophylactic and curative anti-malarial potential of liposomes containing anti-malarial agents or antigens. This paper and the discussion herein provide a useful resource for further complementary investigations and may pave the way for the research and development of several available and affordable anti-malarial-based liposomes and liposomal malaria vaccines by allowing a thorough evaluation of liposomes developed to date for the management of malaria.

Botanical Medicines Cryptolepis sanguinolenta, Artemisia annua, Scutellaria baicalensis, Polygonum cuspidatum, and Alchornea cordifolia Demonstrate Inhibitory Activity Against Babesia duncani

Human babesiosis is a CDC reportable disease in the United States and is recognized as an emerging health risk in multiple parts of the world. The current treatment for human babesiosis is suboptimal due to treatment failures and unwanted side effects. Although Babesia duncani was first described almost 30 years ago, further research is needed to elucidate its pathogenesis and clarify optimal treatment regimens. Here, we screened a panel of herbal medicines and identified Cryptolepis sanguinolenta, Artemisia annua, Scutellaria baicalensis, Alchornea cordifolia, and Polygonum cuspidatum to have good in vitro inhibitory activity against B. duncani in the hamster erythrocyte model. Furthermore, we found their potential bioactive compounds, cryptolepine, artemisinin, artesunate, artemether, and baicalein, to have good activity against B. duncani, with IC₅₀ values of 3.4 μM, 14 μM, 7.4 μM, 7.8 μM, and 12 μM, respectively, which are comparable or lower than that of the currently used drugs quinine (10 μM) and clindamycin (37 μM). B. duncani treated with cryptolepine and quinine at their respective 1×, 2×, 4× and 8× IC₅₀ values, and by artemether at 8× IC₅₀ for three days could not regrow in subculture. Additionally, Cryptolepis sanguinolenta 90% ethanol extract also exhibited no regrowth after 6 days of subculture at doses of 2×, 4×, and 8× IC₅₀ values. Our results indicate that some botanical medicines and their active constituents have potent activity against B. duncani in vitro and may be further explored for more effective treatment of babesiosis.

Recent Advances in the Chemistry and Pharmacology of Cryptolepine

Cryptolepine, the principal constituent of the West African climbing shrub Cryptolepis sanguinolenta, continues to be of interest as a lead to new therapeutic agents, especially for the treatment of protozoal infections and cancer. This contribution reviews the research published in the last decade, highlighting new synthesis routes to cryptolepine and to analogs of this alkaloid, as well as their pharmacology. Studies relating to the use of C. sanguinolenta as an herbal medicine for the treatment of malaria are discussed, as well as the development of analogs of cryptolepine as leads to new agents for the treatment of malaria, trypanosomiasis, and cancer with an emphasis on the pharmacological mechanisms involved. Other potential therapeutic applications include antimicrobial, antidiabetic, and anti-inflammatory activities; the pharmacokinetics and toxicity of cryptolepine are also reviewed.

N-Methyl-β-carboline alkaloids: structure-dependent photosensitizing properties and localization in subcellular domains

N-Methyl-β-carboline (βC) alkaloids, including normelinonine F (1b) and melinonine F (2b), have been found in a vast range of living species playing different biological, biomedical and/or pharmacological roles. Despite this, molecular bases of the mechanisms through which these alkaloids would exert their effect still remain unknown. Fundamental aspects including the photosensitizing properties and intracellular internalization of a selected group of N-methyl-βC alkaloids were investigated herein. Data reveal that methylation of the βC main ring enhances its photosensitizing properties either by increasing its binding affinity with DNA as a biomolecular target and/or by increasing its oxidation potential, in a structure-dependent manner. As a general rule, N(9)-substituted βCs showed the highest photosensitizing efficiency. With the exception of 2-methyl-harminium, all the N-methyl-βCs investigated herein induce a similar DNA photodamage profile, dominated largely by oxidized purines. This fact represents a distinctive behavior when comparing with N-unsubstituted-βCs. On the other hand, although all the investigated compounds might accumulate mainly into the mitochondria of HeLa cells, methylation provides a distinctive dynamic pattern for mitochondrial uptake. While rapid (passive) diffusion is most probably reponsible for the prompt uptake/release of neutral βCs, an active transport appears to mediate the (reatively slow) uptake of the quaternary cationic βCs. This might be a consequence of a distinctive subcellular localization (mitochondrial membrane and/or matrix) or interaction with intracellular components. Biomedical and biotechnological implications are also discussed herein.

Salient features of the aza-Wacker cyclization reaction

The intramolecular aza-Wacker reaction has unparalleled potential for the site-selective amination of olefins, but it is perhaps underappreciated relative to other alkene oxidations. The first part of this review makes the distinction between classical and tethered aza-Wacker cyclization reactions and summarizes examples of the latter. The second portion focuses on developments in asymmetric aza-Wacker cyclization technology. The final part of the review summarizes applications of all classes of aza-Wacker cyclization reactions to natural product assembly.

Synthesis and In Vitro Antiproliferative Activity of 11-Substituted Neocryptolepines with a Branched ω-Aminoalkylamino Chain

Neocryptolepine, which is a kind of tetracyclic indoloquinoline alkaloid, exhibits the inhibition of topoisomerase II and shows antiproliferative activity. The present study describes the synthesis and antiproliferative evaluation of several neocryptolepine analogues carrying a branched, functionalized dibasic side chain at C11. These 2-substituted 5-methyl-indolo[2,3-b]quinoline derivatives were prepared by nucleophilic aromatic substitution (S_NAr) of 11-chloroneocryptolepines with appropriate 1,2- and 1,3-diamines. Some of the 11-(ω-aminoalkylamino) derivatives were further transformed into 11-ureido and thioureido analogues. Many of the prepared neocryptolepine derivatives showed submicromolar antiproliferative activity against the human leukemia MV4-11 cell line. Among them, 11-(3-amino-2-hydroxy)propylamino derivatives 2h and 2k were the most cytotoxic with a mean IC₅₀ value of 0.042 μM and 0.057 μM against the MV4-11 cell line, 0.197 μM and 0.1988 μM against the A549 cell line, and 0.138 μM and 0.117 μM against the BALB/3T3 cell line, respectively.

Phytochemical and Pharmacological Review of Cryptolepis sanguinolenta (Lindl.) Schlechter

Ethnopharmacological Relevance

Cryptolepis sanguinolenta is a scrambling thin-stemmed shrub found in Africa. Traditionally in West Africa, it is employed in the treatment of malaria, diarrhea, and respiratory conditions. This review discusses the traditional importance as well as the phytochemical, ethnomedical, pharmacological, and toxicological importance of this plant.

Materials and Methods

Excerpta Medica Database, Google Scholar, Springer, and PubMed Central were the electronic databases used to search for and filter primary studies on Cryptolepis sanguinolenta.

Results

The detailed review of various studies conducted on C. sanguinolenta and some of its constituents gives an important body of proof of its potential therapeutic benefits and also of its use as a source of lead compounds with therapeutic potentials.

Conclusion

The review on C. sanguinolenta is important in identifying grey areas in the research on this medicinal plant and also provides comprehensive data thus far to continue research on this plant.

Hybrid Monoterpenoid Indole Alkaloids Obtained as Artifacts from Rauvolfia tetraphylla

Five new hybrid monoterpenoid indole alkaloids bearing an unusual 2,2-dimethyl-4-oxopiperidin-6-yl moiety, namely rauvotetraphyllines F-H (1, 3, 4), 17-epi-rauvotetraphylline F (2) and 21-epi-rauvotetraphylline H (5), were isolated from the aerial parts of Rauvolfia tetraphylla. Their structures were established by extensive spectroscopic analysis. The new alkaloids were evaluated for their cytotoxicity in vitro against five human cancer cell lines.

Aza-Diels-Alder reaction between N-aryl-1-oxo-1H-isoindolium ions and tert-enamides: Steric effects on reaction outcome

The synthesis of 5-substituted 6,6a-dihydroisoindolo[2,1-a]quinolin-11(5H)-ones via [4 + 2] imino-Diels-Alder cyclization from N-aryl-3-hydroxyisoindolinones and N-vinyl lactams under Lewis acid-catalysed anhydrous conditions is reported. Reactions of N-(2-substituted-aryl)-3-hydroxyisoindolinones with N-vinylpyrrolidone under identical conditions resulted in the formation of 2-(2-substitued-aryl)-3-(2-(2-oxopyrrolidin-1-yl)vinyl)isoindolin-1-one analogues indicating steric hinderance as the cause of deviation. The probable mechanism of the reaction based on the results from X-ray crystallography and molecular modelling is discussed.

Searching for new derivatives of neocryptolepine: synthesis, antiproliferative, antimicrobial and antifungal activities

A series of novel amino acid and dipeptide derivatives of neocryptolepine were synthesized and tested for their antimicrobial, antifungal and antiproliferative activity in vitro against cancer cell lines (KB, A549, MCF-7, LoVo) and normal mice fibroblast cells (BALB/3T3). Biological evaluation revealed that almost all of the new compounds displayed high antiproliferative activity against the tested cells and moderate to potent antibacterial activities. Interestingly, these compounds were active against Candida albicans biofilms at doses significantly lower than those required against free-floating planktonic fungal cells. The most promising compounds are derivatives with glycine and L-proline as a substituent both at 2 and at 9 position of 5H-indolo[2,3-b]quinoline. In general, these new compounds (2a, 3a, 6a and 7a) showed the highest dual action against cancer lines and infectious pathogenic microbes in vitro.

Synthesis and in vitro antiproliferative activity of new 11-aminoalkylamino-substituted chromeno[2,3-b]indoles

To search for new biological activities of the chromeno[2,3-b]indoles, the 5-oxa analog of the indolo[2,3-b]quinolines that are known to have a potent antitumor activity, a series of 11-amino derivatives with various substituents at the C-2 position were prepared. The synthesis of the chromeno[2,3-b]indole structure involves the cyclization of 2-phenoxy-3-indolecarboxylates 3, accessible from the indole-3-carboxylate 1 and phenols 2, producing the chromeno[2,3-b]indol-11(6H)-ones 4, which is followed by dehydroxychlorination with phosphorus oxychloride to afford the 11-chlorochromeno[2,3-b]indoles 5. The treatment of 5 with various amines produced the corresponding 11-aminated chromeno[2,3-b]indoles 6, while some of the 11-ω-aminoalkylamino derivatives 6 were transformed into the 11-ω-sulfonylaminoalkylamino derivatives 8. The antiproliferative activity of these 11-aminochromeno[2,3-b]indoles 6 and 8 in vitro were tested using MV4-11 (human leukemia), A549 (lung cancer), HCT116 (colon cancer), and the normal mice fibroblast (BALB/3T3) and their potencies were described.

Synthesis and in vitro antiproliferative activity of new 11-aminoalkylamino-substituted 5H- and 6H-indolo[2,3-b]quinolines; structure-activity relationships of neocryptolepines and 6-methyl congeners

The present report describes the synthesis and antiproliferative evaluation of certain 11-aminoalkylamino-substituted 5H- and 6H-indolo[2,3-b]quinolines and their methylated derivatives. These 5-Me- and 6-Me-indolo[2,3-b]quinoline derivatives 10-14, 20 were prepared by amination at the C-11 position of the 11-chloro-5-methyl-5H- and 11-chloro-6-methyl-6H-indolo[2,3-b]quinolines with different substituents on the quinoline ring. The 11-aminoalkylaminomethylated 23, the homologue of 11, was prepared from the same intermediate for a further SAR study. These intermediates are accessible from 4-substituted anilines or their N-methylated analogues and methyl indole-3-carboxylate as a counterpart. The in vitro antiproliferative assay indicated that the 5-methylated derivatives 10-14 are more cytotoxic than their respective 6-methylated 6H-indolo[2,3-b]quinoline derivatives 20. Among them, N-(3-aminopropyl)-2-bromo-5-methyl-5H-indolo[2,3-b]quinolin-11-amine 12f was the most cytotoxic with a mean IC(50) value of 0.12 μM against human leukemia MV4-11 cell line, and also exhibited selective cytotoxicities against A549 (lung cancer), HCT116 (colon cancer) cell lines and normal fibroblast BALB/3T3 with IC(50) values of 0.543, 0.274 and 0.869 μM, respectively. The binding constant of products 12f and 20f to salmon fish sperm DNA were also evaluated using UV-vis absorption spectroscopy, indicating intercalation binding with a constant of 2.93×10(5) and 3.28×10(5)Lmol(-1), respectively.

Review of the phytochemical, pharmacological and toxicological properties of Alstonia Scholaris Linn. R. Br (Saptaparna)

The use of ethnornedical information has immensely contributed to health care, and scientific studies have shown that the evaluation of traditionally used medicines may provide leads towards effective drug discovery. Since antiquity, Alstonia scholaris connmonly known as devil's tree has been used for the treatment of many human ailments. Literature suggests that Alstonia scholaris is useful in treating malaria, abdominal disorders, dyspepsia, leprosy, skin diseases, tumors, chronic and foul ulcers, asthma, bronchitis, helminthiasis, agalactia, and debility. Preclinical studies have shown that it possesses anti-microbial, anti-diarrhoeal, anti-plasmodial, anti-oxidant, anti-inflammatory hepatoprotective, nootrophic, anti-stress, anti-fertility, immunomodulatory, analgesic, anti-ulcer, wound healing, anti-cancer, chemopreventive, radiation protection, radiation sensitization, and chemosensitization activities. The diverse pharmacological observations are supposed to be due to the presence of alkaloids, flavonoids and phenolic acids. The bark and leaf extract when administered orally did not induce lethality or adverse affects at the limit doses of 2 000 mg/kg body weight. However when administered intraperitoneally at high concentrations, the extract showed systemic and developmental toxicities. This review addresses the experimentally authenticated facts and also suggests the need for research on chemical and pharmacological properties of Alstonia scholaris.

Rauvotetraphyllines A-E, new indole alkaloids from Rauvolfia tetraphylla

Five new indole alkaloids rauvotetraphyllines A–E (1–5), together with eight known analogues, were isolated from the aerial parts of Rauvolfia tetraphylla. The structures were established by means of spectroscopic methods.

Three new indole alkaloids from Rauvolfia yunnanensis

One rare tetracyclic macroline-type indole alkaloid, named rauvoyunine A (1), and two new picraline-type alkaloids rauvoyunines B and C (2 and 3) were isolated from the aerial parts of Rauvolfia yunnanensis. Their structures were elucidated on the basis of extensive spectroscopic analysis. Compounds 2 and 3 were evaluated for their in vitro cytotoxicity against five human tumor cell lines.

Metabolism of cryptolepine and 2-fluorocryptolepine by aldehyde oxidase

Objectives

To investigate the metabolism of cryptolepine and some cryptolepine analogues by aldehyde oxidase, and to assess the implications of the results on the potential of cryptolepine analogues as antimalarial agents.

Methods

The products resulting from the oxidation of cryptolepine and 2-fluorocryptolepine by a rabbit liver preparation of aldehyde oxidase were isolated and identified using chromatographic and spectroscopic techniques. The antiplasmodial activity of cryptolepine-11-one was assessed against Plasmodium falciparum using the parasite lactate dehydrogenase assay.

Key findings

Cryptolepine was oxidized by aldehyde oxidase give cryptolepine-11-one. Although 2-fluorocryptolepine was found to have less affinity for the enzyme than cryptolepine, it was a better substrate for aldehyde oxidase than the parent compound. In contrast, quindoline, the 11-chloro- , 2,7-dibromo- and 2-methoxy analogues of cryptolepine were not readily oxidized. Cryptolepine-11-one was found to be inactive against P. falciparum in vitro raising the possibility that the effectiveness of cryptolepine as an antimalarial, may be compromised by metabolism to an inactive metabolite by liver aldehyde oxidase.

Conclusions

Cryptolepine and 2-fluorocryptolepine are substrates for aldehyde oxidase. This may have implications for the design and development of cryptolepine analogues as antimalarial agents.

Natural products as starting points for future anti-malarial therapies: going back to our roots?

Background

The discovery and development of new anti-malarials are at a crossroads. Fixed dose artemisinin combination therapy is now being used to treat a hundred million children each year, with a cost as low as 30 cents per child, with cure rates of over 95%. However, as with all anti-infective strategies, this triumph brings with it the seeds of its own downfall, the emergence of resistance. It takes ten years to develop a new medicine. New classes of medicines to combat malaria, as a result of infection by Plasmodium falciparum and Plasmodium vivax are urgently needed.

Results

Natural product scaffolds have been the basis of the majority of current anti-malarial medicines. Molecules such as quinine, lapachol and artemisinin were originally isolated from herbal medicinal products. After improvement with medicinal chemistry and formulation technologies, and combination with other active ingredients, they now make up the current armamentarium of medicines. In recent years advances in screening technologies have allowed testing of millions of compounds from pharmaceutical diversity for anti-malarial activity in cellular assays. These initiatives have resulted in thousands of new sub-micromolar active compounds – starting points for new drug discovery programmes. Against this backdrop, the paucity of potent natural products identified has been disappointing. Now is a good time to reflect on the current approach to screening herbal medicinal products and suggest revisions. Nearly sixty years ago, the Chinese doctor Chen Guofu, suggested natural products should be approached by dao-xing-ni-shi or ‘acting in the reversed order’, starting with observational clinical studies. Natural products based on herbal remedies are in use in the community, and have the potential unique advantage that clinical observational data exist, or can be generated. The first step should be the confirmation and definition of the clinical activity of herbal medicinal products already used by the community. This first step forms a solid basis of observations, before moving to in vivo pharmacological characterization and ultimately identifying the active ingredient. A large part of the population uses herbal medicinal products despite limited numbers of well-controlled clinical studies. Increased awareness by the regulators and public health bodies of the need for safety information on herbal medicinal products also lends support to obtaining more clinical data on such products.

Conclusions

The relative paucity of new herbal medicinal product scaffolds active against malaria results discovered in recent years suggest it is time to re-evaluate the ‘smash and grab’ approach of randomly testing purified natural products and replace it with a patient-data led approach. This will require a change of perspective form many in the field. It will require an investment in standardisation in several areas, including: the ethnopharmacology and design and reporting of clinical observation studies, systems for characterizing anti-malarial activity of patient plasma samples ex vivo followed by chemical and pharmacological characterisation of extracts from promising sources. Such work falls outside of the core mandate of the product development partnerships, such as MMV, and so will require additional support. This call is timely, given the strong interest from researchers in disease endemic countries to support the research arm of a malaria eradication agenda. Para-national institutions such as the African Network for Drugs and Diagnostics Innovation (ANDi) will play a major role in facilitating the development of their natural products patrimony and possibly clinical best practice to bring forward new therapeutics. As in the past, with quinine, lapinone and artemisinin, once the activity of herbal medicinal products in humans is characterised, it can be used to identify new molecular scaffolds which will form the basis of the next generation of anti-malarial therapies.

The Quaternary Indole Alkaloids from Two Sumatran Lerchea species

The Sumatran forest plants Lerchea cf. bracteata and L. parviflora were found to contain alkaloids and their extract showed siginificant activity toward some testing pathogenic microbes. Isolation work on L.cf. bracteata yielded known quaternary alkaloid N(b)-methylantirhine (2) while L. parviflora gave 5,6-dihydroflavopereirine (3).

Antimalarial and antitubercular nostocarboline and eudistomin derivatives: synthesis, in vitro and in vivo biological evaluation

The synthesis of nine nostocarboline derivatives with substitutions of the 2-methyl group by alkyl, aryl and functionalized residues, 10 symmetrical bis cationic dimers linking 6-Cl-norharmane through the 2-position and fifteen derivatives of the marine alkaloids eudistomin N and O is reported. These compounds were evaluated in vitro against four parasites (Trypanosoma brucei rhodesiense STIB 900, Trypanosoma cruzi Tulahuen C2C4, Leishmania donovani MHOM-ET-67/L82 axenic amastigotes, and Plasmodium falciparum K1 strain), against Mycobacterium tuberculosis H37Rv, Mycobacterium smegmatis mc(2)155 and Corynebacterium glutamicum ATCC13032, and cytotoxicity was determined against L6 rat myoblast cells. Nostocarboline and derivatives displayed potent and selective in vitro inhibition of P. falciparum with weak cytotoxicity. The dimers displayed submicromolar inhibition of L. donovani and T. brucei, and nanomolar activity against P. falciparum, albeit with pronounced cytotoxicity. One dimer showed a MIC(99) value against M. tuberculosis of 2.5 microg/ml. The alkylated eudistomin N and O derivatives displayed activities down to 18 nM against P. falciparum for N-Me Eudistomin N. Four dimers, nostocarboline and three eudostomin derivatives were evaluated in an in vivo Plasmodium berghei mouse model. No significant activity was observed for the dimers, but a 50% reduction in parasitaemia was observed at 4 x 50 mg/kg ip for nostocarboline.

Structure-activity relationship of antiparasitic and cytotoxic indoloquinoline alkaloids, and their tricyclic and bicyclic analogues

Based on the indoloquinoline alkaloids cryptolepine (1), neocryptolepine (2), isocryptolepine (3) and isoneocryptolepine (4), used as lead compounds for new antimalarial agents, a series of tricyclic and bicyclic analogues, including carbolines, azaindoles, pyrroloquinolines and pyrroloisoquinolines was synthesized and biologically evaluated. None of the bicyclic compounds was significantly active against the chloroquine-resistant strain Plasmodium falciparum K1, in contrast to the tricyclic derivatives. The tricyclic compound 2-methyl-2H-pyrido[3,4-b]indole (9), or 2-methyl-beta-carboline, showed the best in vitro activity, with an IC(50) value of 0.45 microM against P. falciparum K1, without apparent cytotoxicity against L6 cells (SI>1000). However, this compound was not active in the Plasmodium berghei mouse model. Structure-activity relationships are discussed and compared with related naturally occurring compounds.

Genomewide expression profiling of cryptolepine-induced toxicity in Saccharomyces cerevisiae

We have used the budding yeast Saccharomyces cerevisiae to identify genes that may confer sensitivity in vivo to the antimalarial and cytotoxic agent cryptolepine. Five S. cerevisiae strains, with different genetic backgrounds in cell permeability and DNA damage repair mechanisms, were exposed to several concentrations of cryptolepine. Cryptolepine showed a relatively mild toxicity for wild-type strains, which was augmented by either increasing cell permeability (Deltaerg6 or ISE2 strains) or disrupting DNA damage repair (Deltarad52 strains). These results are compatible with the ability of cryptolepine to intercalate into DNA and thus promote DNA lesions. The effects of low concentrations of cryptolepine (20% and 40% inhibitory concentrations [IC(20) and IC(40)]) were analyzed by comparing the gene expression profiles of treated and untreated Deltaerg6 yeast cells. Significant changes in expression levels were observed for 349 genes (117 upregulated and 232 downregulated). General stress-related genes constituted the only recognizable functional cluster whose expression was increased upon cryptolepine treatment, making up about 20% of upregulated genes. In contrast, analysis of the characteristics of downregulated genes revealed a specific effect of cryptolepine on genes related to iron transport or acid phosphatases, as well as a significant proportion of genes related to cell wall components. The effects of cryptolepine on the transcription of iron transport-related genes were consistent with a loss of function of the iron sensor Aft1p, indicating a possible disruption of iron metabolism in S. cerevisiae. Since the interference of cryptolepine with iron metabolism is considered one of its putative antimalarial targets, this finding supports the utility of S. cerevisiae in drug-developing schemes.

Are West African plants a source of future antimalarial drugs?

Ethnopharmacology is a very interesting resource in which new therapies may be discovered. In the case of malaria, two major antimalarial drugs widely used today came originally from indigenous medical systems, that is quinine and artemisinin, from Peruvian and Chinese ancestral treatments, respectively. There is an urgent need for the discovery of new drugs due to the critical epidemiological situation of this disease. New inexpensive therapies that are simple to use and that will limit the cost of drug research are good justifications for this ethnopharmacological approach. Therefore, the aim of this review is to empirically analyse plants that are used for antimalarial treatment in West Africa, and to determine those with real promising antimalarial activity. The major leads such as those extracted from Cochlospermum, Cryptolepsis, Guiera and Azadirachta have been highlighted. Indeed, some extracts seem to be promising in future research, but development of new isolation and characterization techniques, for designing new derivatives with improved properties need to be discussed.

Recent developments in naturally derived antimalarials: cryptolepine analogues

Increasing resistance of Plasmodium falciparum to commonly used antimalarial drugs has made the need for new agents increasingly urgent. In this paper, the potential of cryptolepine, an alkaloid from the West African shrub Cryptolepis sanguinolenta, as a lead towards new antimalarial agents is discussed. Several cryptolepine analogues have been synthesized that have promising in-vitro and in-vivo antimalarial activity. Studies on the antimalarial modes of action of these analogues indicate that they may have different or additional modes of action to the parent compound. Elucidation of the mode of action may facilitate the development of more potent antimalarial cryptolepine analogues.

Synthesis of cryptolepine analogues as potential bioreducible anticancer agents

A series of 10 novel nitro-analogues of cryptolepine (1) has been synthesised and these compounds were evaluated for their in-vitro cytotoxic properties as well as their potential for reductive activation by the cytosolic reductase enzymes NQO1 and NQO2. Molecular modelling studies suggest that cryptolepine is able to fit into the active site of NQO2 and thus raising the possibility that nitro-analogues of 1 could act as bioreductive prodrugs and be selectively reduced by NQO1 and NQO2 to more toxic species in cancer cells in which these enzymes are over-expressed. Analogues were screened against the RT112 cell line (high in NQO2), in the presence and absence of the essential cofactor dihydronicotinamide riboside (NRH), whereby all analogues were shown to be cytotoxic (IC50<2microM) in the absence of NRH. With the addition of NRH, one analogue, 2-fluoro-7,9-dinitrocryptolepine (7), exhibited a 2.4-fold increase in cytotoxic activity. Several nitro-derivatives were also evaluated as substrates for purified human NQO1 and analogues that were found to be substrates were subsequently tested against the H460 (high NQO1) and BE (low NQO1) cell lines to detect in-vitro activation by NQO1. The analogue 8-chloro-9-nitrocryptolepine (9) was found to be the best substrate for NQO1 but it was not more toxic to H460 than to BE cells. Fluorescence laser confocal microscopy of 1 and several analogues showed that in contrast to 1 the analogues were not localised into the nucleus suggesting that their cytotoxic mode(s) of action are different. This study has identified novel substrates for both NQO1 and NQO2 and further work on nitrocryptolepine derivatives as a lead towards novel anticancer agents would be worthwhile.

Identification of bis-quindolines as new antiinfective agents

Several N-substituted quindolines were made to further evaluate the role of N-alkylation on the activity of indoloquinolines as antifungal agents. While N-5 substitution is required for these activities, N-10 alkylation alone leads to inactive products but is tolerated in the presence of N-5 alkyl groups. It was also discovered that bis-quindolines appear to have a more expanded antimicrobial spectrum and lower cytotoxicity than their monomeric counterparts.

In vitro genotoxicity of the West African anti-malarial herbal Cryptolepis sanguinolenta and its major alkaloid cryptolepine

Cryptolepine (CLP), the major alkaloid of the West African anti-malarial herbal Cryptolepis sanguinolenta (Periplocaceae) is a DNA intercalator that exhibits potent toxicity to a variety of mammalian cells in vitro. We have hypothesized that the DNA intercalating properties of cryptolepine could trigger genetic damage in mammalian cells. The objective of the present study was therefore to assess the ability of both cryptolepine (CLP) and the traditional anti-malarial formulation, the aqueous extract from the roots (CSE) to induce mutation at the hprt locus and micronuclei (MN) formation in V79, a Chinese hamster fibroblast cell line commonly used in genetic toxicity studies. CSE at a high concentration (50 microg/ml) induced an apparent significant ten fold increase in mutant frequency compared to vehicle control (mean of 38 versus 4 mutant clones/10(6) surviving cells) but, this concentration of CSE was very toxic (<15% cell survival). CLP did not appear to be mutagenic in the dosage range used (up to 2.5 microM, equivalent to 1.1 microg/ml). However, after 24h treatment of V79 cells both CSE and CLP induced a dose-dependent increase in micronuclei of 4.15% and 6.43% (25 microg/ml CSE and 2.5 microM, equivalent to 1.1 microg/ml CLP, respectively) compared to 0.36% in vehicle control. These results show that treatment of mammalian cells with CSE and CLP can lead to DNA damage and we suggest that the routine use of CSE and the potential use of CLP derivatives in malaria chemotherapy could carry a genotoxic risk.

Synthesis and Evaluation of β-Carbolinium Cations as New Antimalarial Agents Based on π-Delocalized Lipophilic Cation (DLC) Hypothesis

Several beta-carbolines including naturally occurring substances and their corresponding cationic derivatives were synthesized and evaluated for antimalarial (antiplasmodial) activity in vitro and in vivo. A tetracyclic carbolinium salt was elucidated for antileishmanial and antitrypanosomal activities in vitro as well as antiplasmodial activity. Quarternary carbolinium cations showed much higher potencies in vitro than electronically neutral beta-carbolines and a good correlation was observed between pi-delocalized lipophilic cationic (DLC) structure and antimalarial efficacy. beta-Carbolinium compounds exhibit medium suppressive activity in vivo against rodent malaria.

Jusbetonin, the first indolo[3,2-b]quinoline alkaloid glycoside, from Justicia betonica

A new indolo[3,2-b]quinoline alkaloid glycoside, jusbetonin (1), and three known alkaloids, namely, 10H-quindoline (2), 6H-quinindoline (3), and 5H,6H-quinindolin-11-one (4), have been isolated from the leaves of Justicia betonica. The structure of 1 was established on the basis of 1D and 2D NMR ((1)H-(1)H COSY, HMQC, and HMBC) and HRFABMS data. Compound 1 is the first example of a glycosylated indolo[3,2-b]quinoline alkaloid, while compound 4 was isolated for the first time from a natural source.

Anhydronium bases from Rauvolfia serpentina

Five anhydronium bases were isolated by preparative HPLC from a methanolic extract of Rauvolfia serpentina roots. For the first time 3,4,5,6-tetradehydroyohimbine, 3,4,5,6-tetradehydro-(Z)-geissoschizol, 3,4,5,6-tetradehydrogeissoschizol and 3,4,5,6-tetradehydrogeissoschizine-17-O-beta-D-glucopyranoside were isolated from a natural source. In addition, the well-known anhydronium base serpentine was isolated. The structures of the compounds were determined by 1H and 13C NMR, MS and UV.