Health Articles

B816679j 435.454

www.rsc.org/csr Chemical Society Reviews Artemisinin and its derivatives: a novel class of anti-malarialand anti-cancer agentsw Devdutt Chaturvedi, Abhishek Goswami, Partha Pratim Saikia, Nabin C. Barua*and Paruchuri G. Rao Received 3rd February 2009First published as an Advance Article on the web 24th August 2009DOI: 10.1039/b816679j In this tutorial review, an effort towards presentation of a comprehensive account of the recentdevelopments on various kinds of artemisinin derivatives including artemisinin dimers, trimersand tetramers has been made and their efficacy towards malaria parasites and different cancercells lines was compared with that of artemisinins, and various other anti-malarial and anti-cancerdrugs. It is expected that this review will provide first-hand information on artemisinin chemistryto organic/medicinal chemists, and pharmacologists working on anticancer and anti-malarial drugdevelopment.
including P. berghii and P. yeolii are specific to other groupsof the mammalian class. This disease is transmitted from Malaria still remains one of the most dangerous widespread person to person through the bite of female anopheles parasitic diseases of the developing world although it is mosquito. Out of the above four species of the malarial known to humankind since ancient times in different forms, parasites of human host, Plasmodium vivax, P. malariae and and exists over 100 countries, including the United States.1 It P. ovale, are the causes of intermittent high fevers making is caused by the Plasmodium parasite and kills approximately a person very ill but they are rarely fatal. The remaining species 1–3 million people and causes disease in 300–500 million P. falciparum, is the cause of malignant tertian, falciparum people annually. The malaria parasite is a Plasmodium malaria which has a substantial mortality if it is untreated, protozoan species, which evolved with time differentiating intofour distinct species: P. falciparum, P. vivex, P. malarae and especially in the first or an early attack. Among the four human P. ovale, specific to humans. Some other related species malaria parasites, P. falciparum has developed resistance to allof our available drugs, therefore it is an overwhelming causeof serious disease and death. In patients with severe andcomplicated disease, the mortality rate is between 20–50%.
Natural Products Chemistry Division, North-East Institute of Science& Technology, Assam, Jorhat-785006, India.
The increasing resistance of malaria parasites to quinoline E-mail: [email protected]; Fax: 91-376-2370011 based anti-malarial drugs is a major contributor to the w This article is dedicated to the achievements of the North-East re-emergence of this disease as a major public health problem Institute of Science and Technology (formerly RRL), Jorhat (CSIR),on the eve of its 50th anniversary.
and its spread to new locations and populations.
Abhishek Goswami was born obtained his PhD degree in in Jorhat, Assam (India) in Medicinal Chemistry, from the 1981. He completed his BSc Dr B. R. Ambedkar University, (2002) degree from Science carried out at CDRI, Lucknow), University and MSc (2005) in 2003. After working as a degree form Gauhati University, Postdoctoral Fellow at the Assam (India) with specia- University of Georgia, USA lization in Physical Chemistry.
and University of Go¨ttingen, At present he is pursuing his Germany, he returned to India and worked shortly as a Senior Products Chemistry Division, Postdoc at the Department of Chemistry, IIT, Madras and (India) under the guidance of Devdutt Chaturvedi as a Scientist (Fellow) at Dr Nabin C. Barua. His area Indian Institute of Integrative of research interest is partial Medicine, Jammu. He has worked on several areas of organic and total synthesis of natural products of biological significance synthesis and medicinal chemistry. He is presently working at and development of new synthetic methodologies for target NEIST, Jorhat on artemisinin chemistry.
oriented synthesis.
This journal is  c The Royal Society of Chemistry 2010 Chem. Soc. Rev., 2010, 39, 435–454 435

reducing the incidence of malaria among the troops servingSoutheast Asia. Because of its many undesirable side effects it Commonly used drugs (Fig. 1) in single drug therapy for the is no longer used in clinics.4 early diagnosed malaria are given below: (d) Mefloquine (4): Structurally related to quinine it (a) Quinine (1): Originally isolated from the bark of the is effective against many resistant strains of Plasmodium.
Cinchona tree, quinine is the only drug which over a long Initially it was considered as a good prophylactic because of period of time has remained largely effective in treating the its long half life. Widespread resistance and undesirable side disease. A number of its derivatives are known to be good effects (mainly acute brain syndrome) associated with this drug anti-malarials. However, it is now used only for treating severe have resulted in decline of its use.5 Because of its structural falciparum malaria, partly because of undesirable side effects.2 similarity to quinine the two are not recommended together.
(b) Chloroquine (2): This is effective in curing all forms of (e) Halofantrin (5): This is an effective anti-malarial, malaria with few side effects when taken in a prescribed dose.
however, due to its short half life of 1 to 2 days and its high It is still an effective and cheap drug both from prophylactic cost, it is not suitable for use as a prophylactic. Unfortunately and chemotherapeutic point of view. Unfortunately, most strains resistant forms are increasingly being reported and there is some of falciparum malaria are now resistant to chloroquine and more concern about its side effects. Halofantrin has been associated recently resistance of vivax malaria has also been reported.3 with neuropsychiatric disturbances. It is contra-indicated (c) Mepacrine (Alebrine) (3): This was developed in the early 1930s and used as a prophylactic on a large scale during the during pregnancy and is not advised to women who are Second World War (1939–45) and had a major influence in breast-feeding. Abdominal pain, diarrhoea are some of thecommon side effects.6 (f) Azithromycin (6): This is a macrocyclic glycosylated lactone and is mainly used for the chemoprophylaxis. It also Partha Pratim Saikia was shows limited toxicity but the studies are limited to date.7 (g) Atovaquone (7): This is an important antifolate drug for (India) in 1980. After obtaining malaria treatment and used in combination with proguanil his BSc (Chemistry Honors) which is a prodrug and metabolically converted to cycloguanil, from Science College, Jorhat, in 2002, he moved to GauhatiUniversity, Guwahati, where To combat the rapid spread of drug resistant malaria, he finished his MSc in 2004.
He effective therapeutic agents are continuously being sought, towards his PhD at Natural especially against those strains which are resistant to Products Chemistry Division, conventional quinoline and acridine based drugs. Wars have North-East Institute of Science many times led not only to the development of new technology & Technology (CSIR), Jorhat, but also new medicaments. The antimalarial drugs are typical examples. Chloroquine resulted from the World War II.
Partha Pratim Saikia guidance of Dr Nabin C.
Mefloquine resulted from the Vietnam War on the American Barua. His areas of research side. However, what many do not know is that artemisinin interest are stereoselective total synthesis of natural products also resulted from the Vietnam War only as a result of large- of biological significance and development of new syntheticmethodologies for target oriented synthesis.
scale research launched by the Chinese Government.
Dr Nabin C. Barua obtained Dr Paruchuri Gangadhar Rao, his PhD degree in Natural obtained his MTech and PhD Products Chemistry in 1981, under the supervision of Dr He joined NEIST, Jorhat, R. P. Sharma. Later on, he initially in 1976 and moved to CLRI, Chennai in 1991. He doctoral Fellow with Prof. Dr returned to NEIST, Jorhat, in 2002 as Director of the the University of Konstanz, institute. His research areas Germany, where he worked of interest are chemical process on the chemistry of functionally development of agrochemicals, substituted vinyl carbanions.
process design and engineering He joined NEIST, Jorhat in to provide basic engineering 1981, where he is currently packages and applications of working as a Head of the ultrasound. He is presently Natural Products Chemistry coordinating all the divisions Division. He has trained many masters, doctoral and post- of chemical sciences area of NEIST, Jorhat. Dr Rao is a Fellow doctoral students. He has been actively involved on synthesis of Indian Institute of Chemical Engineers and was also its past of biologically active natural products/drug intermediates and artemisinin chemistry as well.
436 Chem. Soc. Rev., 2010, 39, 435–454 This journal is  c The Royal Society of Chemistry 2010 Structures of commonly available anti-malarial drugs.
Discovery of artemisinin sensitive molecule for large scale derivatization. Fortunately,it was found that the carbonyl group of artemisinin 8, can In 1972, a group of Chinese researchers isolated a new anti- be easily reduced to dihydroartemisinin 9 ($3500 kg1) in malarial drug, (+)-artemisinin 8, a sesquiterpene lactone of high yields using sodium borohydride, which has in turn led the amorphene sub-group of cadinene9 from the hexane to the preparation of a series of semi-synthetic first-generation extract of a traditional Chinese medicinal plants, Artemesia analogues including the oil-soluble artemether 10 and annua (Asteraceae) a plant which has been used for arteether 11, and water-soluble sodium artesunate 12 and the treatment of fever and malaria since ancient times.10 sodium artelinate 13. These three analogs become very Artemisinin is a sesquiterpene lactone containing an endoper- potent anti-malarial drugs effective against chloroquine- oxide linkage in it. This highly oxygenated sesquiterpene resistant strains of P. falciparum. Artemether 10 ($3600 kg1), lactone peroxide, unlike most other anti-malarials, lacks has been included in the WHO lists of Essential Drugs for the nitrogen containing heterocyclic ring systems and was found treatment of severe MDR malaria. In this family, the Walter to be a superior plasmocidal and blood schizontocidal Reed Institute of research has patented a stable, water-soluble agent compared to conventional anti-malarial drugs, such as derivative called artelinic acid 12 which is now being tested in chloroquine, quinine etc against malaria strains, without animals. A key advantage of these endoperoxides containing obvious adverse effects in patients.
anti-malarial agents, which have been used for nearly two Artemisinin is obtained from Artemisia annua in a maximum decades, is the absence of drug resistance.
yield of 0.1%. This plant is peculiar in its behavior. Carefully Although a number of excellent review articles have grown plants may be devoid of artemisinin and in order that been published14 on different aspects of artemisinin, we the plant synthesizes the product, special agricultural conditions will concentrate our discussion on the recent and most must be adopted. Best results have been reported in plantations important work carried out to study the structure–activity in North Vietnam, mainly in the vicinity of Hanoi. Highest relationship of artemisinin derivatives which, in recent years content was found about two weeks before flowering.
have emerged as a novel class of anti-malarial and anti-cancer Artemisinin 8 ($420 kg1) (Fig. 2) is active at nanomolar concentrations in vitro both against chloroquine sensitive andresistant P. falciparum strains. However, the practical valuesof artemisinin, nevertheless, is impaired by (i) its poor solubilityeither in oil or water,11 (ii) the high rate of parasite recrudescenceafter treatment12 and (iii) its short-plasma half life (3–5 h) andits poor oral activity.13 However, a low level of resistance hasrecently been observed using artemisinin, which disappearedas soon as the drug-selection pressure has been withdrawn.
However, artemisinin with an endoperoxide linkage is a Structure of artemisinin and its analogs.
This journal is  c The Royal Society of Chemistry 2010 Chem. Soc. Rev., 2010, 39, 435–454 437 Artemisinin derivatives C-12 ether/ester derivatives Artemisinin is only sparingly soluble in water or oil and notwell absorbed by the gastro-intestinal tract. Search for morepotent analogues of artemisinin with better bioavailability wasinitiated in China focusing attention on ethers and estersof dihydro-artemisinin i.e. arteether, artemether, artesunate,artelinate etc. Although these derivatives are potentialantimalarial agents in vitro, they have poor bioavailability,principally as a result of metabolic instability of the acetalfunction.15 One of the principal routes for metabolism ofartemether 10, for example, involves oxidative dealkylationto give DHA 9, a compound associated with toxicity and shorthalf life (Scheme 1).
An approach to increasing metabolic stability of artemisinin derivatives involves incorporation of a phenyl group in placeof alkyl group (in the ether linkage) of arteether and artemether.
afforded the desired sugar derivatives 19a–d (Scheme 3). On This modification would be expected to block oxidative in vitro anti-malarial bioassay of the derivatives against metabolic formation of DHA in vivo. With this idea in mind, P. falciparum, they were found to be more effective against O'Neill's group synthesized16 a series of C-12 phenoxy W-2 and W-6 clones and were not cross-resistant with existing derivatives by reacting DHA with 4 equivalents of the phenol in anhydrous ether at room temperature in presence of The trimethylsilyl derivative 17 was more active than derivatives 18a–d which possess activity comparable to or 3-etherate. This reaction is believed to proceed via an oxonium intermediate as shown in Scheme 2.
better than that of artemisinin 8. However, the deacetylated Several C-12 phenoxy derivatives were evaluated against compounds 19a–d were substantially less active than the malaria parasites and found to possess excellent in vitro acetylated ones 18a–d. The anti-malarial activity results anti-malarial activity. On the basis of the excellent yield and suggested that the in vivo activity of these sugar derivatives stereoselectivity obtained from the p-trifluoromethyl derivative parallel those observed in in vitro tests and that the increase in polarity or water solubility tends to decrease anti-malarial 3, IC50 = 3.90 nM), this compound and the parent phenyl substituted derivative 15 (R = H) were subjected to in vivo biological evaluation by the authors on P. berghei in a In search of water-soluble and potent artemisinin derivatives, mouse model and metabolism studies in rats. Compound 15 Li et al. have reported18 syntheses and anti-malarial activities of new 30 dihydroartemisnin derivatives (Table 1), containing 3) demonstrated excellent in vivo antimalarial potency an amino group (Scheme 4). Syntheses of targeted compounds 50 value of 2.12 mg kg1 (cf. artemether = 6 mg kg1) vs P. berghei. Furthermore, from preliminary were achieved by treatment of dihydroartemisinin 9, with the metabolic studies they have reported that this compound allylic alcohol in the presence of BF3Et2O in dry CH2Cl2 was not metabolized to dihydroartemisinin, suggesting it solution to furnish compounds 20, 23 in quantitative yield.
should have a longer half-life and potentially lower toxicity Compound 21 was obtained through the epoxidation of 20 than arteether and artemether.
using m-chloroperbenzoic acid. A series of amine derivatives As discussed earlier, one of the major disadvantages of using 22, 24 were prepared by treating compounds 21 or 23 with artemisinins is their poor water solubility. To overcome this various amines. Treatment of these basic compounds with difficulty, sugar derivatives of DHA were prepared17 (18a–d) organic acids (oxalic acid, maleic acid, etc.) yielded the by condensing 12-O-(trimethylsilyl)dehydroartemisinin 17 corresponding salts. Generally, the maleates have better solubility with 1-hydroxypolyacetylated sugars in presence of catalytic in water than the corresponding oxalates. Compounds 24f (SD50 = 1.61 mg kg1 day1), 24h (SD50 = 1.74 mg kg1 day1), and 24r (SD 2Cl2 at 78 1C. Deacetylation of intermediates 18a–d 50 = 1.82 mg kg1 day1) showed 4–5 fold higher activity against P. berghei infected mice by oral adminis-tration than artesunic acid 12 (SD50 = 6.33 mg kg1 day1),although their activities drastically decrease (30–60 times)when administered via subcutaneous injection. Compounds24f, 24h and 24r and artesunic acid 12 in a dose of3.16 mg kg1 day1 and compounds 24f and 12 in a dose of10.0 mg kg1 day1 were given orally in P. knowlesi infectedmonkeys for 7 days.
Compounds 24f, 24h and 24r reduced parasites more rapidly than artesunic acid 12, but a dose of 3.16 mg kg1 24f did notcleanse all parasites. Compounds 24h and 24r recrudescence in 5–10 days after administration, whereas artesunic acid 12 can 438 Chem. Soc. Rev., 2010, 39, 435–454 This journal is  c The Royal Society of Chemistry 2010 against P388 cells in vitro, and that a pair of isomers(compound 26a and 27a) in test of antiproliferative potentialdisplayed in vitro cytotoxicity against P388 and L1210 murineleukemia cell lines. They prepared the compound 26b (22%yield) and 27b (25% yield), as a pair of isomers, in the presenceof BF3Et2O with 2-(4-bromophenyl)-2-hydroxyacetonitrileand KCN. These compounds were tested for the anti-proliferative effect against P388 and A549 tumor cell lines(Scheme 5). In order to test whether the peroxy group isessential for anti-tumor activity, compound 28 (60% yield)was also prepared from compound 27b. Compound 26b and27b showed the potent and similar activity to inhibit theproliferation of P388 and A549 cells, but compound 28 wasnot active. It is noteworthy that the peroxy group appears to be essential for cytotoxicity as in the case of anti-malarialactivity, and the configuration of C-16 has insignificant influence cleanse parasites at a dose of 10.0 or 3.16 mg kg1; no on the activity. Compounds 26b and 27b were equipotent for recrudescence within 105 days was observed. Its contradictory the inhibition of the proliferation and cell cycle progression.
results in mice and monkeys explain that their water-soluble The immunosuppressive action of artemisinin and its artemisinin derivatives have different types of absorbtion, derivatives has also been studied in China for many years.
excretion and metabolism in different species.
Many experimental results in vitro and in vivo suggested that Li et al. found19 that cyano artemether 25, possessed these new type of antimalarial drugs, such as artemisinin 8, inhibitory effect against P. falciparum and no cytotoxic effect dihydroartimisinin 9, artemether 10, and artesunic acid 12, Series of synthesized compounds HNR1R2 = morpholine (A), piperazine (B), N-methylpiperazine (C), N-diphenylmethylpiperazine (D), pyrrolidine (E). a As oxalate salt.
b As maleate salt. c As fumerate salt.
This journal is  c The Royal Society of Chemistry 2010 Chem. Soc. Rev., 2010, 39, 435–454 439 possessed definite immunosuppressive activity.20 In search fornew potential immunosuppressive agents with much higherefficacy and lower toxicity, Yang et al. have synthesized21 aclass of novel artemisinin derivatives (30–37) starting fromdihyroartemisinin acetate 29 (Scheme 6) and found thatintroduction of phen(oxy)yl aliphatic acid and ester intoartemisinin nucleus enhanced their immunosuppressive activity.
These compounds (30–37) were assayed in their cytotoxicity oflymphocyte, inhibition activity on cancanavalin A (ConA)induced T cell proliferation and lipopolysaccharide (LPS)induced B cell proliferation. Among them, 31b, 33b, 34d,35b, 36, and 37 remarkably exhibited lower cytotoxicity andhigher inhibition activity on the mitogen-induced T-cell andB-cell proliferation in comparison with artemisinin, artesunate,and artemether in vitro. More significantly, compound 31b-displayed reduced cytotoxicity by over 100-fold comparedwith cyclosporine A (CsA) and comparable inhibition 38j (IC50 = 2.2  107), 39a (IC50 = 5.0  107), 39b activity (SI = 848) on ConA-induced T cell proliferation to (IC50 = 1.8  107), 39c (IC50 = 1.0  107), and 39e CsA (SI = 963) and more than 4000 times the inhibitory effect (IC50 = 1.7  107), exhibited 30- to 88-fold higher bioactivity (SI = 28473) on LPS-induced B-cell proliferation compared than artemisinin (IC50 = 9.0  106), artemether (IC50 = with CsA (SI = 7) in vitro. The in vivo experimental results 1.8  106), and artesunate (IC50 = 9.9  107) respectively.
showed that compound 36 could inhibit 2,4-dinitrofluorobenzene Yang et al. have also synthesized23 a series of artemisinin (DNFB)-induced delayed type hypersensitivity (DTH) reaction derivatives bearing Mannich base groups (40a and 40b) starting and sheep red blood cells (SRBC) induced antibody, production from dihydroartemisinin 9 and tested for their anti-malarial activity against P. berghei and P. falciparum in K1 and NF54 Based on their previous work, Yang et al. have further cells. Compound 40a (IC50 = 0.18 and 0.36 ng mL1) and 40b extended the study of immunosuppressive activity of a new (IC50 = 0.25 and 0.17 ng mL1) were found to be more series of substituted phenoxy propionic acids and ester active in mice than artesunic acid (IC50 = 1.20 and derivatives (38,39).22 The synthesis of targeted compounds 1.20 ng mL1). These derivatives 40a and 40b (dose 3.16 and were achieved using dihydroartemisinin 9 (Scheme 7). These 10 mg kg1 day1) were also examined for their anti-malarial new dihydroartemisinin derivatives were tested in vitro for activity against P. knowles in rhesus monkeys (Scheme 8) their cytotoxicity on murine spleen cells and inhibitory activity of 7 days treatment using artesunic acid as standard drug on ConA-induced T cell proliferation or lipopolysaccharide(LPS) induced B cell proliferation with artemisinin, artemether,and artesunate as the controls. The cytotoxicity of eachcompound was expressed as the concentration of compoundthat reduced cell viability to 50% (CC50). The immuno-suppressive activity of each compound was expressed as theconcentration of compound that inhibited ConA-induced Tcell proliferation and LPS-induced B cell proliferation to 50%(IC50) of the control value. Among the whole series ofcompounds, 38a (IC50 = 6.8  107), 38e (IC50 = 4.6 107), 38h (IC50 = 7.0  107), and 38j (IC50 = 8.4  107)had 5- to 9-fold higher bioactivity than artemisinin (IC50 =4.4  106), artemether (IC50 = 3.8  106), artesunate(IC50 = 4.8  106) in the ConA-induced T cell proliferation.
In the inhibition of LPS-induced B cell proliferation 38e (IC50 =2.8  107), 38f (IC50 = 1.6  107), 38g (IC50 = 3.0  107), 440 Chem. Soc. Rev., 2010, 39, 435–454 This journal is  c The Royal Society of Chemistry 2010 Although, the target for anti-malarial action of artemisinins is controversial, recent evidence suggest that an Fe2+-activatedform of the drug potentially inhibits PfATP, a key parasite (dose 3.16 and 10 mg kg1 day1) and thus found to be better Ca2+ transporter. On the other hand, the mode of action of than artesunic acid.
another anti-malarial drug quinine 1, has been suggested as a Recently, Singh et al. have synthesized24 a new series of result of interference with host hemoglobin digestion. Due to ether derivatives (41a–k) of dihydroartemisinin and their anti-malarial synergism between artemisinin and quinine, antimalarial activity was evaluated against multidrug-resistant Walsh et al. have recently prepared26 a covalently linked novel P. yoelii nigeriensis in mice. The synthesis of the targeted artemisinin–quinine hybrid compound 45 through the coupling compounds was achieved through the Lewis acid catalyzed of dihydroartemisinin 9 with a carboxylic acid derivative of (i.e. BF3Et2O) coupling reaction between dihydroartemisinin quinine 44 via an ester linkage (Scheme 11). This hybrid 9 with the corresponding alcohol in CH2Cl2 at subzero molecule had potent activity against the 3D7 and (drug- temperature (10 1C to 5 1C) furnishing the corresponding resistant) FcB1 strains of P. falciparum in culture. The activity ether derivative in 65–99% yields as diastereomeric mixtures of this hybrid molecule 45 (IC50 = 8.95 nM) was superior to of a and b-isomers, with the b-isomers as the major products that of artemisinin 8 (IC50 = 49.4 nM) or, quinine 44 (IC50 = (Scheme 9). These new derivatives are highly lipophilic (log P 149 nM) alone, or a 1 : 1 mixture of artemisinin and quinine.
in the range of 5.51 to 7.19) as compared with b-arteether Hybrid molecules (known as reversed chloroquine) of (log P 3.84), and several of them are two- to four-fold more chloroquine was earlier made by Peyton et al. and found to active than b-arteether. Among, the ether derivatives, the be effective against chloroquine resistant parasites.
a-isomers are more active than the b-isomers. The a-etherbiphenyl derivatives 41f (log P = 6.91), and 41h (log P = 6.85) C-12 sulfur derivatives are most active compounds of the series, provided 100% Angiogenesis, the formation of new blood vessels from existing protection to infected mice at 12 mg kg1  4 days as host capillaries stimulated by biochemical stimulators, in compared to b-arteether, i.e. 100% and 20% protection at normal vascular systems is involved in wound healing, 48 mg kg1  4 days and 24 mg kg1  4 days, respectively.
embryonic development, and the female reproductive cycle More recently, Singh et al. have also synthesized25 a series under elaborate regulations. In particular, tumor angiogenesis (42a–j) of ester derivatives starting from DHA 9, incorporating is caused by angiogenic inducers playing a key role in the pharmacologically privileged substructure, such as biphenyl, growth of the solid tumors, their invasion, and metastasis.
admantane and fluorene (Scheme 10) and evaluated for Therefore, the control of angiogenesis may be a promising therapeutic strategy for the related disease. Strategies for P. yoeli nigeriensis by oral route. Several of these compounds regulating angiogenesis have been carried out mainly in molecular 42a (log P = 6.95), 42b (log P = 6.89), 42c (log P = 6.53), biology. However, in spite of the settlement of bioavailability, 42d (log P = 6.53), 42e (log P = 6.05), 42f (log P = 5.99), 42g biostability, and effectiveness, it has been insufficiently carried (log P = 5.85), 42h (log P = 6.41), 42i (log P = 6.61), 42j out to develop small molecule anti-angiogenic agents. Therefore, (log P = 6.79), were found to be more active than the it is important to discover anti-angiogenic small molecules that anti-malarial drugs b-arteether 12 (log P = 3.84) and artesunic might be suitable as clinical therapies.
acid 13 (log P = 3.04). Compound 42i was found to be mostactive of this series, providing 100% and 80% protection tothe infected mice at 24 mg kg1  4 days and 12 mg kg1 4 days, respectively.
This journal is  c The Royal Society of Chemistry 2010 Chem. Soc. Rev., 2010, 39, 435–454 441 Recently, Chen et al. have reported27 that artemisinin and dihydroartemisinin and C-12 acetal type of artemisininderivatives display anti-angiogenic-activity. Consequently, Oh et al. have synthesized28 C-12 sulfur derivatives of artemisinin(46–52) starting from dihydroartemisinin 9 (Scheme 12) and Latter, Ziffer's group has synthesized30 another series of tested against HUVEC proliferation at the concentration level C-12 carba analogues (58a–60 and 61) using key intermediate of 1 mM using artemisinin 8, and DHA 9. Compounds 46 (IC50 = aldehyde 57, prepared through the O'Neill's compound 53 0.93 mM), 52 (IC50 = 1.74 mM), and 51 (IC50 = 1.29 mM), (Scheme 14). The aldehyde 57 was then reacted with a variety have displayed potent growth inhibitory activity as compared of Grignard reagents to produce 58a–d. These Grignard to 8 (IC50 4 50 mM), and 9 (IC50 = 8.91 mM).
products (58a–d) were oxidized to their corresponding ketones(61) using Jones' reagent. Aldehyde 57 was also reacted with a C-12 carbon analogues Wittig reagent to afford 59. They observed that the peroxidemoiety essential for anti-malarial activity was not altered The poor bioavailability and rapid clearance observed with under the reaction conditions of the Wittig or Grignard first-generation analogues of DHA is due to the acetal function reactions. The reaction of 57 with trimethyl(trifluoromethyl)- present in these derivatives. As discussed earlier in this article, silane yielded a pair of isomeric alcohols 60a and 60b.
one of the principle routes of metabolism of artemether, for The in vitro anti-malarial activities of all the synthesized example, involves oxidative dealkylation to DHA 9, a compound compounds (58a–d, 59, 60, and 61) were determined against associated with toxicity and short half life (Scheme 1).
two drug resistant clones (W-2 is chloroquine resistant, Replacement of the oxygen at the C-12 position with carbon mefloquine sensitive while D-6 is mefloquine resistant and would be expected to produce compounds not only with chloroquine sensitive) of P. falciparum. Out of all the synthesized greater hydrolytic stability but also with a longer half life compounds tested, compounds 58bb (IC50 = 4.8 and 5.8) and and potentially lower toxicity. Consequently, several groups 58db (IC50 = 5.4 and 6.8) were 5–7 times more potent than have developed synthetic and semisynthetic approaches to artemisinin 8.
O'Neill's group has synthesized29 several novel second- generation fluorinated ether and ester analogues of arteetherand artemether and evaluated for their anti-malarial potency(Scheme 13). All of their derivatives demonstrated high anti-malarial potency in vitro against the chloroquine sensitive HB3and resistant K1 strains of P. falciparum. The fluorinatedaromatic ring systems selected were linked to alcohol 54 byeither an ester linkage, 56 or an ether linkage 55, starting fromthe key intermediate allyl deoxo-derivative 53. In vitro, themost potent derivative 55a (IC50 = 0.22) was 15 times moreactive than artemisinin (IC50 = 3.04) and 5 times more potentthan DHA 9 (IC50 = 1.04) against HB3 strain of P. falciparum.
However, in vitro against P. berghei in the mouse, selectedderivatives were generally less potent than DHA 9 (ED50 =1.15) with ED50 values between 5 to 8 mg kg1. On the basis ofthe products obtained from the in vitro biomimetic Fe(II)-mediated decomposition of 55a, they believe that the radicalmediator of biological activity of this series may be differentfrom that of the parent drug artemisinin 8.
442 Chem. Soc. Rev., 2010, 39, 435–454 This journal is  c The Royal Society of Chemistry 2010 Posner et al. reported31 a series of C-12 carba analogues by treating organometallic reagents with aldehyde 62 under con-trolled conditions (Scheme 15). Treatment of aldehyde 62 withorganometallic reagents produces the allylic alcohol 63 whilealdehyde 62 reacts with Wittig reagent to form a mixture ofgeometric isomers of exocyclic alkene 65 without cleaving theendoperoxide linkage. Anti-malarial testing of these analoguesin vitro against P. falciparum NF54 malaria parasites, showedthat C-9,10 unsaturated, C-10 carbon substituted heteroarylartemisinin analogues ketones 64 (IC50 = 4.3 nM whereR = n-Bu and 4.6 nM where R = Ph), tertiary alcohol 66(IC50 = 4.5 nM) and exocyclic alkene 65 [IC50 = 28 nM(R = CHQCH2), 16 nM (R = (E)-CHQCHPh), 8.1 nM(R = (Z)-CHQCHPh), 11 (R = (E)-CHQCHPhNO2-p)] areall similar to clinically used natural artemisinin (IC50 = 10.1 1.3 nM) analogues.
It is generally accepted now that the carbon centered free radicals generated in the course of degradation-rearrangement of artemisinin and the like may play a major role in thekilling of malaria parasites. Thus, Wu et al.32 have reported the peroxide bond is cleaved may directly be related to the their findings on the Fe(II) induced cleavage of the peroxide anti-malarial potency of trioxane.
bond in artemisinin and its derivatives and the DNA damage Recently, Khac et al. have reported33 the synthesis of C-12 associated with this process. In order to afford a sounder basis amino and ester/acid analogues using Ziffer's key intermediate for probing the chemical and biochemical processes that aldehyde 57 (Scheme 17). The key intermediate 57 was subjected artemisinin derived compounds may participate in, they to condensation with a variety of aliphatic, substituted aromatic designed a few C-12 carba-derivatives that carry a UV and substituted heterocyclic amines to afford the corresponding chromophore through a C–C s bond (Scheme 16). The Schiff base derivatives, which upon borohydride reduction isomer 68 which has the normal configuration (i.e. the same afforded corresponding C-12 amine derivatives 70a–d. For configuration as in artemisinin) at C-12 showed high the synthesis of C-12 esters/acid derivatives, aldehyde 57 was anti-malarial activity in the preliminary in vivo test on mice reduced to alcohol, and further treated with a variety of against P. berghei IC173 strain. The abnormal isomer 69 carboxylic acid anhydrides affording the corresponding ester/ (ED50 = 7.08 mg kg1, ED90 = 60.99 mg kg1) is obviously acid derivative 71a–c. Among the synthesized compounds, 71a much less potent than 68 (ED50 = 0.58 mg kg1, ED90 = (IC50 = 0.4 and 0.5 nM), 71b (IC50 = 0.4 and 0.5 nM), 1.73 mg kg1) against P. berghei K173 strain administered and 71c (IC50 = 0.4 and 0.5 nM) have shown very strong orally to mice as suspensions in Tween 80 as compared to anti-malarial potency against strains (W-2 and Ghana) of artemether 10 (ED50 = 1.00 mg kg1, ED90 = 3.10 mg kg1).
P. falciparum. These compounds (71a, 71b, 71c) are about The marked difference in the anti-malarial potencies of 68 and 25 times more potent to the resistant clone (W-2) and 20 times 69, due to their difference in stereochemistry only at C-12 in to the sensitive clone (Ghana) than artemisinin (IC50 = 10 and the structures, has been claimed by these workers as an entirely 9 nM). In addition, other derivatives containing amino-alkyl new observation and according to them the ease with which and heterocycles were also highly potent against P. falciparum.
Compound 70d (IC50 = 1.0 and 1.0 nM) is about 10 timesmore potent than artemisinin.
Recently, Magueur et al. have reported34 that introduction of gem-difluoromethylene group at the C-12 position of theartemisinin resulted in better in vitro antimalarial activity. They This journal is  c The Royal Society of Chemistry 2010 Chem. Soc. Rev., 2010, 39, 435–454 443 Artemisinin dimers, trimers and tetramers: novel leads in It has been established through the structure–activity relation-ship (SAR) studies of artemisinin and its various kinds ofC-12/C-13 ether/ester derivatives that only peroxide-linkageaffects the anti-malarial and anti-cancer activity. Furthermore,several drawbacks associated with these compounds viz solubility, thermal and hydrolytic stability, bioavailability,and short half life etc, have led to development of second have synthesized gem-difluoromethylene deoxo-artemisinin 72 generation C-12/C-13 trioxane-derivatives. Furthermore, it through artemisinin 8 in three steps (Scheme 18). The in vitro was thought worthwhile that the extent of anti-malarial antimalarial activity of artemisinin 8 (IC50 = 8.9) and activity depends upon the extent of the number of peroxide of compound 72 (IC50 = 4.6) were determined using the units, which can be increased by adding of additional artemisinin chloroquine resistant FCB1 strain of P. falciparum.
moiety through careful chemical manipulations. Thus, researchershave directed their efforts for the synthesis of various kinds of C-13 substituted derivatives from artemisitene artemisinin dimers, trimers and tetramers of various lengthand flexibility. Artemisinin dimers reported till date, have The majority of derivatives of artemisinin prepared so far were displayed structural diversity, separated through artemisinin derivatized through C-12 either as acetal and non-acetal monomer units with or without linkers of various length and type, and only a few C-13 derivatives were reported.35 flexibility with diverse stereochemistry. Several of these C-12/ Artemisitene 73 exists in the same plant in much lower C-13 carbon artemisinin dimers have shown outstanding yield and has less anti-malarial activity than artemisinin 8.36 anti-malarial and anti-cancer activity and are better than However, artemisitene 73 can be easily prepared from C-12 ether/ester dimers. Artemisinin trimers and tetramers artemisinin 8 in a single-step reaction in 73% yield. It contains of C-12/C-13 non-acetal derivatives have also been reported in an a,b-unsaturated lactone moiety, which can be used recent years, wherein artemisinin units are connected through as a Michael receptor for derivatization. Thus, Li et al.
linkers of various kinds with diverse length and stereochemistry.
prepared37 74a–c and 75a–c with 1,2,4-triazole, benzotriazole However, the number of artemisinin dimers synthesized so far is far-ahead of the number of artemisinin trimers and (Scheme 19), heating 1,2,4-triazole as its salts and artemisitene tetramers. Many of these dimers, trimers and tetramers have 73 in CH3CN at 60 1C gave a mixture of 74a and 75a shown outstanding anti-malarial and anti-cancer activities (40% yield), refluxing benzotriazole and artemisitene 73 in compared to artemisinin and related compounds, and are aqueous ethanol gave 74b and 75b (40 and 29% yield), in various phases of clinical trials. These compounds may reacting benzimidazole and artemisitene 73 in THF in become future potential leads in anti-malarial and anti-cancer the presence of KF-Al2O3 gave 74c and 75c (26 and 38% yield). In in vivo anti-malarial tests against K173 strain ofP. berghei, the mixture of 74a and 75a showed nine times Artemisinin dimers (SD50 = 2.38 mg kg1 day1, SD90 = 6.52 mg kg1 day1)more potency than artemisinin 8 (SD50 = 5.13 mg kg1 day1, Based on the C-12/C-13 linkage between artemisinin units SD90 = 11.50 mg kg1 day1). Compound 74b (SD50 = through the linker, whether acetal or non-acetal, artemisinin 5.48 mg kg1 day1, SD90 = 11.41 mg kg1 day1) dimers have been classified as follows: showed activity comparable to that of the artemisinin 8.
(a) C-12 oxa dimers: Wherein two artemisinin monomers Compound 74c (SD50 = 3.94 mg kg1 day1, SD90 = units are linked through the C-12 acetal (i.e. ether–ester 10.05 mg kg1 day1) and 75c (SD50 = 1.59 mg kg1 day1, linkage) linker.
SD90 = 56.03 mg kg1 day1) were more active than (b) C-12 carba dimers: Wherein two artemisinin monomers artemisinin 8. Compound 74c had higher activity than its units are linked through the C-12 non-acetal (i.e. carbon– carbon linkage) linker.
444 Chem. Soc. Rev., 2010, 39, 435–454 This journal is  c The Royal Society of Chemistry 2010 (c) C-13 carba dimers: Wherein two artemisinin monomers units are linked through the C-13 non-acetal (i.e. carbon–carbon linkage) linker.
(a) C-12 oxa dimers. In search for new potential artemisinin derivatives, several new C-12 oxygen derivatives have beenreported by various groups. Thus, Beekman et al. havereported38 the anti-cancer activity of C-12 oxa-dimers where two artemisinin units were connected through an ether-linkage(Scheme 20). The cytotoxicity of these derivatives were determined lines (e.g. leukemia, HL-60, non-small cell lung cancer against EN2 tumor cells using the MTT assay. They realized NCI-H226, colon cancer COLO 205, and KM-12, CNS cancer that artemisinin 8 (IC50 = 0.98) was more cytotoxic than the corresponding deoxyartemisinin 76 (IC50 = 111), whichlacks an endo-peroxide linkage. Ether–linked dimers of (b) C-12 carba dimers. Although C-12 oxa dimers have deoxyartemisinin with defined stereochemistry were found to displayed high antimalarial, antiproliferative and anti-tumor differ in extent of cytotoxic effect on EN2 cells. The activities in vitro, often they are hydrolytically unstable. In non-symmetrical dimer 77 (IC50 = 0.11) was more cytotoxic order to enhance the hydrolytic stability, researchers have than the symmetrical dimer 78 (IC50 = 2.0). Similarly, the directed their efforts to synthesize a variety of C-12 olefinic symmetrical dimer 80 (IC50 = 99.8) was less effective than 79 carba dimers. Thus, Posner et al. have first reported41 syntheses (IC50 = 8.9).
of C-12 olefinic carba dimers of m-xylene by reacting their In order to study the role of linker in affecting the biological previously prepared aldehyde 62, with Wittig reagent 83 to activity of dimers, Posner et al. have synthesized39 a series of afford three isomers with different stereochemistry 84a–c artemisinin C-12 oxa dimers linked through two artemisinin (Scheme 23). The in vitro anti-malarial potencies of m-xylene units either by a polyethylene glycol or carbon chain link dimers 84a–c against chloroquine sensitive P. falciparum or disulfide linker, with varying length and flexibility (NF54) parasites was measured, wherein 84a (IC50 = 77 nM), (Scheme 21). The syntheses of targeted dimers were achieved 84b (IC50 = 35 nM), 84c (C50 = 18 nM) were found less starting from dihydroartemisinin 9 using the required linker.
potent than artemisinin 8 (IC50 = 9.7 nM). They have further They have tested the anti-proliferative activities in normal extended the study and synthesized C-12 non-acetal saturated murine keratinocytes using calcitriol as a standard drug and dimers 85a.b starting from artemether 10 via novel titanium- anti-tumor activity in various cancer cells in vitro. Of the promoted condensation (Scheme 24). They have further dihydroartemisinin polyethylene-glycol dimers 81a–c, the one synthesized the C-12-non-acetal saturated C-12 dimers with b-stereochemistry at both of the lactol acetal positions (87–89) by coupling of recently prepared artemisinin derived (i.e. 81b, IC50 = 1.9 nM) is more active than artemisinin 8 fluoride 86 via Friedel–Crafts or aluminium acetylide (IC50 = 10 nM). Likewise, the b,b-dimers 81d and 81e are condensations (Scheme 25). Although benzoylmethylene dimers 85a and 85b and acetylenic dimers 89a and 89b are Recently, Grellepois et al. have synthesized40 another C-12 stereochemically b-linked to C-12 of the artemisinin, aryl oxa dimer 82 through olefinic metathesis reaction using dimers 87 and 88 are a-linked; the basis for this difference in compound 20 (Scheme 22). Preliminary growth inhibitory stereochemistry of attachment is not fully understood. Unlike activities were evaluated in vitro using a diverse panel of the bis-acetylene dimers 89a and 89b, aryl dimer 87 and furan 60 human cancer cell lines. This dimer 82 was efficient in dimer 88 are considerably more potent anti-malarial agents cancer cell growth inhibition with a GI50 less than 10 nM. In (IC50 = 1.3–3.2 nM) than natural artemisinin (IC50 = 9.7 nM) particular, TGI (total growth inhibition) data shows the as mentioned in Table 2. Compounds 85a–c, 87, 88, 89a–c selectivity and potency of dimer 82 against a few cancer cell showed good to excellent antiproliferative activity. Dimers85a, 87 and 89a were specially potent and selective in This journal is  c The Royal Society of Chemistry 2010 Chem. Soc. Rev., 2010, 39, 435–454 445 Antimalarial activity of C-12 deoxyartemisinin dimers 85–89 Antimalarial activity, IC50 = 1.7 nM; 90c, IC50 = 3.9 nM) than artemisinin 8(IC50 = 7.6 nM), whereas tetrafluorinated dimers 91a–b are2-4 times less potent (91a, IC50 = 28 nM; 91b, IC50 = 15 nM)than artemisinin 8. Dimer 92 (IC50 = 1030 nM) withoutperoxide linkage posseses very weak or no antimalarial activity.
Antiproliferative activities were measured in vitro usingmurine keratinocytes for new non-fluorinated dimers 90band 90c and for new fluorinated dimers 91a and 91b. It isnoteworthy that these dimers are more effective at 1 mMconcentration than calcitriol used as a standard drug. Growthinhibitory activities at nanomolar to micromolar concentrations,measured in vitro using a diverse panel of 60 human cancer celllines, indicates that non-fluorinated dimers 90b and 90c areparticularly inhibitory to leukemia cells, and these dimers arevery selectively potent in a few other cancer cell lines (e.g.
colon 205, ovarian cancer OVCAR-4, non-small cell lungEKVX). The highly selective and powerful anticancer activitiesof dimers 90b and 90c, coupled with lack of cytotoxicity, makethese promising lead compounds for further preclinical study in dual action chemotherapy of both malaria and cancer.
Later on, Jung et al. reported43 a series of C-12 non-acetal inhibition of growth of some human cancer cell lines in the amido (97a–b, 100, 101) and sulfide/sulfoxide (102, 103, 104, NCI in vitro 60-cell line assay.
105) dimers (Scheme 27). Synthesis of target compounds were Later on, Posner's group has also synthesized42 C-12 non- achieved from the key intermediates 94 and 96, which were acetal dimers 90a–c starting from dihyroartemisinin acetate 29 easily synthesized from compound 93. The syntheses of 97a,b and their ketone functionality was converted into difluoro were achieved through the direct coupling of acid 94 with derivative 91a.b using bis(2-methoxyethyl)amino sulfur trifluoride amino compound 96 using EDC–HOBt system. Synthesis of (BAST) (Scheme 26). The peroxide group of C-12 90a dimer dimers 100 and 101 was achieved by coupling of compound was reduced using Zn/AcOH to afford 92. Anti-malarial 96a with protected glutarate 98 using the EDC–HOBt system activity of these compounds were tested against chloroquine- to afford compound 99, which upon further coupling with 96a sensitive NF54 strain of P. falciparum. Dicarbonyl dimers using EDC–HOBt afforded t-Boc protected amido-dimer 90a–c are 2–5 times more potent (90a, IC50 = 1.9 nM; 90b, 100. Compound 100 upon treatment with TFA afforded 446 Chem. Soc. Rev., 2010, 39, 435–454 This journal is  c The Royal Society of Chemistry 2010 unprotected amido dimer 101. Compound 95a on treatmentwith Na2S afforded compound 102 which upon further treatmentwith m-CPBA afforded compound 103. Compound 95a ontreatment with 1,3-thiol afforded dimer 104, which uponfurther treatment with m-CPBA afforded compound 105.
The in vitro cytotoxicity of artemisinin and related dimersagainst murine and human cancer cells. Sulfide dimer 102(IC50 = 0.40 mg mL1) is active comparable to aldriamycin(IC50 = 0.39 mg mL1) and four times more active thanmitomycin (IC50 = 1.50 mg mL1) against-mouse fibroblastleukemia (P388). Sulfone-linked dimer 103 (IC50 = 1.04 mg mL1)is active comparable to mitomycin (IC50 = 0.85 mg mL1) andsix times more active than adriamycin (IC50 = 6.24 mg mL1)and taxol (IC50 = 7.39 mg mL1) against human placentalchoricarcinoma cells (Bewo). Dimer 97a (IC50 = 0.005 mg mL1),particularly, is 24 times more active than aldriamycin(IC50 = 0.12 mg mL1) and 200 times more active thanmitomycin (IC50 = 0.93 mg mL1), but 50 times less activethan taxol (IC50 = 0.0001 mg mL1).
Later on, Posner's group has synthesized44 a series of artemisinin dimers (106–118) starting from dihydroartemisininacetate 29 (Scheme 28). Compound 29 upon treatment withallylic bis-silane using tin chloride afforded dimer 106, whichwas used as a key intermediate for the transformation to adiverse series of dimer derivatives (107–116). Anti-malarialactivity of these dimer derivatives were carried out in vitrousing chloroquine-sensitive P. falciparum (NF 54) parasites. Insharp contrast to the potencies of the natural trioxane artemisinin 8 (IC50 = 9.0 nM) and of the initial olefinic dimer106 (IC50 = 24 nM), alcohol and diol dimers 107 (IC50 = all are several times more potent than artemisinin 8. Anti-cancer 0.87 nM) and 110 (IC50 = 0.59 nM) and ketone dimer 112 growth inhibitory activities of these dimers were measured (IC50 = 0.91 nM), all have substantially enhanced potencies, in vitro using a diverse panel of human cancer cell lines, with IC50 values below 1 nM. Also, water-soluble carboxylic indicates that alcohol and diol dimers 107 and 110 are strongly acid dimers 108 (IC50 = 2.0 nM), 109 (IC50 = 1.7 nM), 111 growth inhibitory but not cytotoxic towards several human (IC50 = 3.0 nM), 114 (IC50 = 2.1 nM), and 118 (IC50 = 2.4 nM) cancer cell lines. Moreover, water-soluble dimers 108, 111 and This journal is  c The Royal Society of Chemistry 2010 Chem. Soc. Rev., 2010, 39, 435–454 447 118 are also potent inhibitors of cancer cell growth without that both phosphate ester dimers 122a (IC50 = 0.14 mM) and being cytotoxic. These semi-synthetic new chemical entities 122b (IC50 = 0.24 mM) are more potent than the anti-cancer 107 and 110, and especially the easily synthesized dimer agent doxorubicin (IC50 = 0.51 mM).
carboxylic acids, 108 and 111, therefore, deserve further Posner's group has further synthesized46 isobutyric acid preclinical evaluation as potential drug candidates for chemo- dimer 124 and isonicotinate N-oxide 125 dimer starting from therapy of malaria and cancer.
alcohol dimer 107 (Scheme 30). Antimalarial potencies of Later on, Posner and O'Neill's group synthesized45 a new dimer 124 and 125 against chloroquine-sensitive P. falciparum series of artemisinin dimers (119, 121–123) starting from their (NF 54) parasites were determined. Both isonicotinate N-oxide previously prepared key intermediate 54 (Scheme 29). All dimer 125 (ED50 = 0.53 nM) and isobutyric acid dimer 124 of these dimers prepared displayed potent low nanomolar (ED50 = 2.4 nM) were considerably more antimalarially anti-malarial activity vs. the K1 and HB3 strains of efficacious than clinically used sodium artesunate 12 (ED50 = P. falciparum. The most potent compound assayed was 1.5 nM) via both oral and intravenous administration. Both phosphate dimer 122a (IC50 = 0.2 nM), which was greater alcohol dimer 107 and N-oxide dimer 125, but not carboxylic than 50 times more potent than the parent drug artemisinin 8 acid dimer 124, very strongly inhibit the growth of prostate (IC50 = 12.3 nM), and about 15 times more potent than cancer cells.
the clinically used acetal artemether 10. All of the dimers Recently, Posner et al. have also reported47 the syntheses of expressed poor anticancer activity apart from the trioxane phosphate ester dimer 122a and 122b, which expressed nano- (127–131) starting from conjugated dimer 126, obtained molar growth inhibitory (GI50) values vs. a range of cancer cell directly from dihydro-artemisinin acetate 29 (Scheme 31).
lines in the NCI 60 human cell line screen. Furthermore, Thus, conjugated new dimer 126 undergoes 4 + 2-cycloaddition detailed studies on these dimers in vitro in HL60 cells demonstrate reaction to afford compound 127. Dimer 127 through a series 448 Chem. Soc. Rev., 2010, 39, 435–454 This journal is  c The Royal Society of Chemistry 2010 of other synthetic-transformations affords dimer 128–131.
Anti-malarial activities of these dimers (126–131) were determinedin vitro against chloroquine-sensitive P. falciparum (NF 54)parasites. Except for the water-soluble phthalic acid dimer 128(IC50 = 360 nM), all of other dimers are considerably morepotent anti-malarials [(126, IC50 = 2.9 nM), (127, IC50 = 1.6 nM),(129, IC50 = 0.77 nM), (130, IC50 = 3.0 nM), (131, IC50 =3.7 nM)] than artemisinin 8 (IC50 = 6.6 nM).
Preliminary growth inhibitory activities at nanomolar to micromolar concentrations were measured in vitro using adiverse panel of 60 human cancer cell lines. It has been realizedthat trioxane phthalate dimer 127 (IC50 = 500 nM) is preclinical evaluation as potential drug candidates for effective approximately 10–20 times more potent than trioxane-monomer chemotherapy of malaria and cancer.
DHA 9 and that trioxane diol dimer 129 (IC50 = 46.5 nM) Later on, Posner's group synthesized48 C-12 dimers is approximately 110–220 times more potent than DHA 9, (132–134) starting from his previously reported alcohol dimer without being toxic to primary normal cervical cells. The most 107 (Scheme 32). The mechanism of action of these dimers potent and selective two dimers 127 and 129 deserve further were examined in human (LNCaP) and mouse (TRAMP-C1A This journal is  c The Royal Society of Chemistry 2010 Chem. Soc. Rev., 2010, 39, 435–454 449 and -C2H) prostate cancer cell lines. All these dimers(132–134) inhibited cell growth with the 134 being most potentin C1A (GI50 = 18.0 nM), C2H (GI50 = 17 nM) and LNCaP(GI50 = 17.9 nM) cells in comparison to the standard drugdoxorubicin (GI50 = 45.3 nM). These trioxane dimers inducedG0/G1 cell cycle arrest in LNCaP cells and decreased cells in Sphase. These changes correlated with modulation of G1 phasecycle proteins, including decreased cyclin D1, cyclin E and cdk2, and increased P21wafl and p27kipl. These dimers(132–134) also promoted apoptosis in LNcaP cells with (134, 138, 139, 140a) cure malaria-infected mice after only a increased expression of proapoptotic BAX. These results single subcutaneous dose, and two other dimers (140b, 140c) demonstrate that these dimers (132–134) are potentially useful cure after three oral doses in P. Berghei infected mice. These agents that warrant further preclinical development for the dimer compounds have become lead drug candidates to enter treatment of prostate cancer.
in advanced preclinical evaluation and would ultimately be Simultaneously, Jung et al. have also reported49 a new series used in human studies.
of trioxane–artemisinin dimers (136, 137) starting from their More recently, Posner's group has also reported51 another previously prepared key intermediate alcohol 135. Thus, the series of trioxane dimers (141–163) starting from their alcohol 135 was first converted into a malonate dimer 136, previously reported dimers 106, 107, 110, 112, 124 as a key obtained by treatment of alcohol 135 with malonyl chloride.
intermediates (Scheme 35). Thus, hydrazone dimer 141 was This malonate dimer 136 was further converted to Bingel obtained from the corresponding keto-compound 112, ketal adduct trioxane dimer 137 (i.e. fullerene conjugate dimer) by dimers (142–149) were synthesized from the diol dimer 110, treatment with fullerene C60 (Scheme 33). They have tested the various alkylated ether/esters dimers (150, 152) were synthesized in vivo antiangiogenesis activity of these dimers (136, 137) by selective alkylation of diol dimer 110, various amide dimers along with the standard drug fumagillin, thalidomide and were synthesized (156–161), and oxadiazoles dimers (162, 163) artemisisinin 8. It has been realized that the anti-angiogenic from the corresponding acid dimer 124, which was obtained effect of fullerene dimer 137 is similar to fumagillin and through the oxidation of alcohol dimer 107. This alcohol thalidomide and double in comparison with artemisinin 8.
was further converted Recently, Posner's group has synthesized50 another new (153–155). Out of 23 dimers synthesized, 11 of these new series of trioxane dimers (138–140) starting from their trioxane dimers (142–145, 147–150, 153, 155 and 156) cure previously prepared dimer compounds 106, 107, 110, 124 as malaria-infected mice via oral dosing at 3  30 mg kg1. These mentioned in references (Scheme 34). Four of these dimers trioxane dimers are stable both thermally and hydrolytically.
Furthermore, chemical structure–biological activity relationship(SAR) is ongoing, aimed at developing trioxane dimers able toachieve single oral dose cure.
(c) C-13 carba dimers. In order to search for more hydro- lytically stable and potent compounds, researchers becameinterested to synthesize the C-13 dimers of artemisinin 8,starting from the key intermediate artemistine 73.
Ekthawatchai et al. have reported52 synthesis of C-13 carba-dimers (164, 166–168) starting from the key intermediateartemisitene 73 (Scheme 36). The synthesis of dimers 164a–dwas achieved through the base-catalyzed coupling of 73 withthe disulfide linkers of different length. However, compounds164a–d were not found to be very stable and spontaneouslydecomposed in solutions or upon storage at room temperaturegiving complex mixtures. Synthesis of C-13 dimers 167 and168 was achieved through Grignard reagent of suitable lengthwith the key intermediate 73. Dimer 166 was achieved throughthe nucleophilic addition of 165 with 73. Anti-malarial activitiesof dimers 164a–d, 167 and 168 were tested in an in vitro malaria screening system against P. falciparum (K1, multidrug 450 Chem. Soc. Rev., 2010, 39, 435–454 This journal is  c The Royal Society of Chemistry 2010 resistant strain). It has been observed that these dimers own key intermediate artemisitene 73 (Scheme 38). Anti- (164a–d, 167 and 168) do not show promising anti-malarial malarial activities of these trimers [(173a, EC50 = 2.4 nM), activity, perhaps since the stereochemical orientations at C-13 (173b, EC50 = 3.1 nM)] and tetramers [(174a, EC50 = 5.8 nM), and C-130 of the two artemisinin molecules in these dimers (174b, EC50 = 20 nM)] are quite impressive and higher (except might play almost insignificant roles with regard to their 174b) in comparison to artemisinin 8 (EC50 = 12.1 nM).
biological activities. Anticancer activities of artemisinin dimers Later on, Jung et al. synthesized43 another artemisinin 166a–g show high potency against vero cells only.
trimer 175 through the coupling of their previously synthesized Recently, Grellepois et al. have reported40 synthesis of C-13 key intermediates 94 and 96a (Scheme 39). The in vitro carba dimer 170 (Scheme 37) starting from C-13 allylic ether cytotoxicity of this trimer 175 was tested against murine and compound 169 using Grubbs metathesis reaction. They have human cancer cells. The trimer 175 (IC50 = 0.12 mg mL1) further synthesized C-13 olefinic dimer containing hydroxy is three times more active than adriamycin (IC50 = groups 172 through a metathesis approach, starting from C-13 0.39 mg mL1), 12 times more active than mitomycin allylic alcohol 171.
((IC50 = 1.50 mg mL1), and 20 times more active than taxol Preliminary growth-inhibitory activity of these dimers (170, (IC50 = 2.27 mg mL1) against P388. Furthermore, it has been 172) was evaluated in vitro using a diverse panel of 60 human realized by these researchers that this trimer 175 is most potent cancer cell lines. Compound 170 was efficient in cancer cell in almost all the human cancer cell lines tested, and should growth inhibition with a GI50 less than 10 nM in many cases.
receive more attention as a possible anti-cancer drug candidate.
Particularly, TGI data shows the selectivity and potency ofdimer 170 against a few cancer cell lines (e.g. leukemia HL-60,non-small cell lung cancer NCI-H-226, colon cancer COLO Mechanism of action of artemisinins 205, and KM-12, CNS cancer SF-295).
Mode of anti-malarial activity The entry of the malaria parasites into their human host is Artemisinin trimers and tetramers through a mosquito bite. They first enter the liver and replicate In order to search for more potent, more bioavailable, hydro- there for two weeks, before beginning a cycle of red blood cell lytically stable, and less toxic compounds of artemisinin invasion, followed by growth, replication and red cell destruction derivatives, researchers have directed their efforts towards that leads to the symptoms of the disease. The artemisinin the synthesis of trimer and tetramer derivatives of artemisinin.
drugs are known to act specifically during this blood stage.
Ekthawatchai et al. have first reported52 synthesis of trimers Although the mechanism of action of artemisinins is still not 173a,b and tetramers 174a,b of artemisinin, starting from their conclusive, there are strong evidences to suggest that an This journal is  c The Royal Society of Chemistry 2010 Chem. Soc. Rev., 2010, 39, 435–454 451 endoperoxide linkage of artemisinins and a heme iron playcritical roles in their mechanism of action, which is comprisedof two distinct steps.53 In the first step (activation step), theheme iron attacks and breaks the endoperoxide linkage ofartemisinin to produce an oxy free radical, which is thenrearranged to give a carbon free radical. In the second step,the carbon free radical produced from the first step willalkylate specific malarial proteins causing lethal damage tomalarial parasites (Scheme 40).
For the activation step, there are two possible pathways 1 and 2 (Scheme 40). In pathway 1, the heme iron attacks the 452 Chem. Soc. Rev., 2010, 39, 435–454 This journal is  c The Royal Society of Chemistry 2010 Proposed mechanism of action of artemisinins endoperoxide moiety at the O2 position, giving the free radical tetramers as potential ‘leads' for anti-malarial and anticancer at the O1 position (176). This process is followed by an drugs have been carried out, where the activities have been intramolecular 1,5-H shift and the C4 free radical (178) is correlated with artemisinin and various other standard anti- obtained. In pathway 2, the heme iron, on the other hand, malarial and anti-cancer drugs. From the current degree of attacks the endoperoxide moiety at the O1 position, giving the development, it is understandable that several new potent free radical at the O2 position (177). This process is followed dimers and trimer ‘lead' molecules have been discovered in by a hemolytic cleavage of the C3–C4 bond, also resulting in recent years which are in the various phases of clinical trials.
the C4 free radical (179). Hence, it could be concluded that the Such drug candidates include compounds 90b, 90c, 107, 110, C4 free radical product is very critical for antimalarial activity 108, 111, 127, 129, 132–134, 138–140 and 175 as novel leads in of artemisinins.
anti-malarial and anti-cancer drug discovery. In view ofdevelopment of new ‘lead' compounds, it is hoped that interest Mode of anti-cancer activity in this rapidly growing area will continue further to yield In the mid-1990s selective cytotoxicity of artemisinin-derived exciting results in the coming years.
peroxides towards cancer cells also became known. Cancercells require much iron to assist their rapid proliferation and indeed, human cancer cells are known to be richer than normalhuman cells in receptors for transferrin, an iron transporting The authors thank Director, North-East Institute of Science protein. Most cancer cells express higher cell surface concen- and Technology, Jorhat, for providing the necessary facility tration of transferrin receptors than normal cells and have during the preparation of the manuscript. We also thank Mr high rates of iron intake via transferring receptors. Efforts to Suman K. Sen, of IIT Kharagpur for providing several explore the molecular mechanism of action of these monomeric references during the preparation of the manuscript. A. G.
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