Stereoelectronic properties of antimalarial artemisinin analogues in relation to neurotoxicity

The Artemiside-Artemisox-Artemisone-M1 Tetrad: Efficacies against Blood Stage P. falciparum Parasites, DMPK Properties, and the Case for Artemiside

Because of the need to replace the current clinical artemisinins in artemisinin combination therapies, we are evaluating fitness of amino-artemisinins for this purpose. These include the thiomorpholine derivative artemiside obtained in one scalable synthetic step from dihydroartemisinin (DHA) and the derived sulfone artemisone. We have recently shown that artemiside undergoes facile metabolism via the sulfoxide artemisox into artemisone and thence into the unsaturated metabolite M1; DHA is not a metabolite. Artemisox and M1 are now found to be approximately equipotent with artemiside and artemisone in vitro against asexual P. falciparum (Pf) blood stage parasites (IC₅₀ 1.5–2.6 nM). Against Pf NF54 blood stage gametocytes, artemisox is potently active (IC₅₀ 18.9 nM early-stage, 2.7 nM late-stage), although against the late-stage gametocytes, activity is expressed, like other amino-artemisinins, at a prolonged incubation time of 72 h. Comparative drug metabolism and pharmacokinetic (DMPK) properties were assessed via po and iv administration of artemiside, artemisox, and artemisone in a murine model. Following oral administration, the composite C_max value of artemiside plus its metabolites artemisox and artemisone formed in vivo is some 2.6-fold higher than that attained following administration of artemisone alone. Given that efficacy of short half-life rapidly-acting antimalarial drugs such as the artemisinins is associated with C_max, it is apparent that artemiside will be more active than artemisone in vivo, due to additive effects of the metabolites. As is evident from earlier data, artemiside indeed possesses appreciably greater efficacy in vivo against murine malaria. Overall, the higher exposure levels of active drug following administration of artemiside coupled with its synthetic accessibility indicate it is much the preferred drug for incorporation into rational new artemisinin combination therapies.

Quantum Chemical Lipophilicities of Antimalarial Drugs in Relation to Terminal Half-Life

According to the WHO, artemisinin-based combination therapies (ACTs) have been integral to the recent reduction in deaths due to Plasmodium falciparum malaria. ACT-resistant strains are an emerging problem and have evolved altered developmental stages, reducing exposure of the most susceptible stages to artemisinin drugs in popular ACTs. Lipophilicity, log K ow, is a guide in understanding and predicting pharmacokinetic properties such as terminal half-life which alters drug exposure. Consistent log K ow values are not necessarily available for artemisinin derivatives designed to extend terminal half-life, increase bioavailability, and reduce neurotoxicity. For other drugs used in ACTs, an assortment of experimental and computational log K ow values are available in the literature and in some cases, do not account for subtle but important differences between closely related structures such as between diastereomers. Quantum chemical methods such as density functional theory (DFT) used with an implicit solvent model allow for consistent comparison of physical properties including log K ow and distinguish between closely related structures. To this end, DFT, B3LYP/6-31G(d), with an implicit solvent model (SMD) was used to compute ΔG ow o and ΔG vow o for 1-octanol-water and olive oil-water partitions, respectively, for 21 antimalarial drugs: 12 artemisinin-based, 4 4-aminoquinolines and structurally similar pyronaridine, and 4 amino alcohols. The computed ΔG ow o was close to ΔG ow o calculated from experimental log K ow values from the literature where available, with a mean signed error of 2.3 kJ/mol and mean unsigned error of 3.7 kJ/mol. The results allow assignment of log K ow for α-and β-diastereomers of arteether, and prediction of log K ow for β-DHA and five experimental drugs. Linear least square analysis of log K ow and log K vow versus terminal elimination half-life showed strong linear relationships, once the data points for the 4-aminoquinoline drugs, mefloquine and pyronaridine were found to follow their own linear relationship, which is consistent with their different plasma protein binding. The linear relationship between the computed log K vow and terminal elimination half-life was particularly strong, R 2 = 0.99 and F = 467, and can be interpreted in terms of a simple pharmacokinetic model. Terminal elimination half-life for β-DHA and four experimental artemisinin drugs were estimated based on this linear relationship between log K vow and terminal t 1/2. The computed log K ow and log K vow values for epimers α- and β-DHA and α and β-arteether provide physical data that may be helpful in understanding their different pharmacokinetics and activity based on their different molecular geometries. Relative solubility of quinine and quinidine are found to be sensitive to thermal corrections to enthalpy and to vibrational entropy and do not follow the general trend of longer terminal t 1/2 with greater predicted log K ow. Geometric relaxation of α- and β-DHA in solvent and inclusion of thermal correction for enthalpy and entropy results in correct prediction that α-DHA is favored in aqueous environments compared to β-DHA. Predictions made regarding experimental drugs have implications regarding their potential use in response to artemisinin drug-resistant strains.

Nanomedicines for Malaria Chemotherapy: Encapsulation vs. Polymer Therapeutics

Malaria is one of the oldest infectious diseases that afflict humans and its history extends back for millennia. It was once prevalent throughout the globe but today it is mainly endemic to tropical regions like sub-Saharan Africa and South-east Asia. Ironically, treatment for malaria has existed for centuries yet it still exerts an enormous death toll. This contradiction is attributed in part to the rapid development of resistance by the malaria parasite to chemotherapeutic drugs. In turn, resistance has been fuelled by poor patient compliance to the relatively toxic antimalarial drugs. While drug toxicity and poor pharmacological potentials have been addressed or ameliorated with various nanomedicine drug delivery systems in diseases like cancer, no clinically significant success story has been reported for malaria. There have been several reviews on the application of nanomedicine technologies, especially drug encapsulation, to malaria treatment. Here we extend the scope of the collation of the nanomedicine research literature to polymer therapeutics technology. We first discuss the history of the disease and how a flurry of scientific breakthroughs in the latter part of the nineteenth century provided scientific understanding of the disease. This is followed by a review of the disease biology and the major antimalarial chemotherapy. The achievements of nanomedicine in cancer and other infectious diseases are discussed to draw parallels with malaria. A review of the current state of the research into malaria nanomedicines, both encapsulation and polymer therapeutics polymer-drug conjugation technologies, is covered and we conclude with a consideration of the opportunities and challenges offered by both technologies.

Facile Preparation of N-Glycosylated 10-Piperazinyl Artemisinin Derivatives and Evaluation of Their Antimalarial and Cytotoxic Activities

According to the precepts that C-10 amino-artemisinins display optimum biological activities for the artemisinin drug class, and that attachment of a sugar enhances specificity of drug delivery, polarity and solubility so as to attenuate toxicity, we assessed the effects of attaching sugars to N-4 of the dihydroartemisinin (DHA)-piperazine derivative prepared in one step from DHA and piperazine. N-Glycosylated DHA-piperazine derivatives were obtained according to the Kotchetkov reaction by heating the DHA-piperazine with the sugar in a polar solvent. Structure of the D-glucose derivative is secured by X-ray crystallography. The D-galactose, L-rhamnose and D-xylose derivatives displayed IC₅₀ values of 0.58–0.87 nM against different strains of Plasmodium falciparum (Pf) and selectivity indices (SI) >195, on average, with respect to the mouse fibroblast WEHI-164 cell line. These activities are higher than those of the amino-artemisinin, artemisone (IC₅₀ 0.9–1.1 nM). Notably, the D-glucose, D-maltose and D-ribose derivatives were the most active against the myelogenous leukemia K562 cell line with IC₅₀ values of 0.78–0.87 µM and SI > 380 with respect to the human dermal fibroblasts (HDF). In comparison, artemisone has an IC₅₀ of 0.26 µM, and a SI of 88 with the same cell lines. Overall, the N-glycosylated DHA-piperazine derivatives display antimalarial activities that are greatly superior to O-glycosides previously obtained from DHA.

Methylene Homologues of Artemisone: An Unexpected Structure-Activity Relationship and a Possible Implication for the Design of C10-Substituted Artemisinins

We sought to establish if methylene homologues of artemisone are biologically more active and more stable than artemisone. The analogy is drawn with the conversion of natural O- and N-glycosides into more stable C-glycosides that may possess enhanced biological activities and stabilities. Dihydroartemisinin was converted into 10β-cyano-10-deoxyartemisinin that was hydrolyzed to the α-primary amide. Reduction of the β-cyanide and the α-amide provided the respective methylamine epimers that upon treatment with divinyl sulfone gave the β- and α-methylene homologues, respectively, of artemisone. Surprisingly, the compounds were less active in vitro than artemisone against P. falciparum and displayed no appreciable activity against A549, HCT116, and MCF7 tumor cell lines. This loss in activity may be rationalized in terms of one model for the mechanism of action of artemisinins, namely the cofactor model, wherein the presence of a leaving group at C10 assists in driving hydride transfer from reduced flavin cofactors to the peroxide during perturbation of intracellular redox homeostasis by artemisinins. It is noted that the carba analogue of artemether is less active in vitro than the O-glycoside parent toward P. falciparum, although extrapolation of such activity differences to other artemisinins at this stage is not possible. However, literature data coupled with the leaving group rationale suggest that artemisinins bearing an amino group attached directly to C10 are optimal compounds.

DEVELOPMENT OF A FORCE FIELD FOR ARTEMISININ AND MOLECULAR DYNAMICS SIMULATION OF THE DISSOLUTION OF ARTEMISININ IN DIFFERENT SOLVENTS

Artemisinin is widely employed to treat malaria. A variety of experiments have been done to research the dissolution property of artemisinin in different solvents. To have an in-depth understanding of the property, it is essential to explore the dissolution property from molecular level with molecular dynamics (MD) simulation, which needs a satisfactory force field of artemisinin. Therefore in the research a quantum chemistry based force field was developed. The quantum chemical calculation at different levels was done and Hartree–Fock (HF) level calculation gives satisfactory results. The charge distribution was then determined successfully. The van der Waals (VDW) parameters of the C unit with sp3-C were tuned according to the difference between the dissolution enthalpy of artemisinin in ethanol and ethyl acetate. With the developed force field, MD method was employed to successfully simulate the dissolution property of artemisinin in different solvents. The simulation results show that artemisinin molecules tends to aggregate in water, while in the aqueous solution of ethanol, the same number of artemisinin molecules tends to disperse. Furthermore, simulation results show that 8 M ethyl acetate aqueous solution has better dissolution ability than 8 M ethanol aqueous. The simulation gave agreements with the experimental results.

Pharmacophore mapping and electronic feature analysis for a series of nitroaromatic compounds with antitubercular activity

A five point pharmacophore was generated using PHASE for a series of nitroaromatic compounds and their congeners as antitubercular agents. The generated pharmacophore yielded significant 3D-QSAR model with r(2) of 0.890 for a training set of 92 molecules. The model also showed excellent predictive power with correlation coefficient Q(2) of 0.857 for a test set of 31 compounds. The pharmacophore indicated that presence of a nitro group, a piperazine moiety, one aromatic ring feature and two acceptor features are necessary for potent antitubercular activity. The pharmacophore was supported by electronic property analysis using density functional theory (DFT) at B3LYP/3-21*G level. Molecular electrostatic profile of the compounds was consistent with the generated pharmacophore model, particularly appearance of localized negative potential regions near both the oxygen atoms of nitro group extending laterally to the isoxazole ring system/amide bond in the most active compounds. Calculated data further revealed that all active compounds have smaller LUMO energies located over the nitro group, furan ring, and isoxazole ring/amide bond attached to it. Higher negative values of LUMO energies concentrated over the nitro group are indicative of the electron acceptor capacity of the compounds, suggesting that these compounds are prodrugs and must be activated by TB-nitroreductase. The results obtained from this study should aid in efficient design and development of nitroaromatic compounds as antitubercular agents.

In silico pharmacophore model for tabun-inhibited acetylcholinesterase reactivators: a study of their stereoelectronic properties

Organophosphorus (OP) nerve agents that inhibit acetylcholinesterase (AChE; EC 3.1.1.7) function in the nervous system, causing acute intoxication. If untreated, death can result. Inhibited AChE can be reactivated by oximes, antidotes for OP exposure. However, OP intoxication caused by the nerve agent tabun (GA) is particularly resistant to oximes, which poorly reactivate GA-inhibited AChE. In an attempt to develop a rational strategy for the discovery and design of novel reactivators with lower toxicity and increased efficacy in reactivating GA-inhibited AChE, we developed the first in silico pharmacophore model for binding affinity of GA-inhibited AChE from a set of 11 oximes. Oximes were analyzed for stereoelectronic profiles and three-dimensional quantitative structure-activity relationship pharmacophores using ab initio quantum chemical and pharmacophore generation methods. Quantum chemical methods were sequentially used from semiempirical AM1 to hierarchical ab initio calculations to determine the stereoelectronic properties of nine oximes exhibiting affinity for binding to GA-inhibited AChE in vivo. The calculated stereoelectronic properties led us to develop the in silico pharmacophore model using CATALYST methodology. Specific stereoelectronic profiles including the distance between bisquarternary nitrogen atoms of the pyridinium ring in the oximes, hydrophilicity, surface area, nucleophilicity of the oxime oxygen, and location of the molecular orbitals on the isosurfaces have important roles for potencies for reactivating GA-inhibited AChE. The in silico pharmacophore model of oxime affinity for binding to GA-inhibited AChE was found to require a hydrogen bond acceptor, a hydrogen bond donor at the two terminal regions, and an aromatic ring in the central region of the oximes. The model was found to be well-correlated (R = 0.9) with experimental oxime affinity for binding to GA-inhibited AChE. Additional stereoelectronic features relating activity with the location of molecular orbitals and weak electrostatic potential field over the aromatic rings were found to be consistent with the pharmacophore model. These results provided the first predictive pharmacophore model of oxime affinity for binding toward GA-inhibited AChE. The model may be useful for virtual screening of compound libraries to discover and/or custom synthesize more efficacious and less toxic reactivators that may be useful for GA intoxication.

Homology modeling and atomic level binding study of Leishmania MAPK with inhibitors

The current therapy for leishmaniasis is not sufficient and it has two severe drawbacks, host-toxicity and drug resistance. The substantial knowledge of parasite biology is not yet translating into novel drugs for leishmaniasis. Based on this observation, a 3D structural model of Leishmania mitogen-activated protein kinase (MAPK) homologue has been developed, for the first time, by homology modeling and molecular dynamics simulation techniques. The model provided clear insight in its structure features, i.e. ATP binding pocket, phosphorylation lip, and common docking site. Sequence-structure homology recognition identified Leishmania CRK3 (LCRK3) as a distant member of the MAPK superfamily. Multiple sequence alignment and 3D structure model provided the putative ATP binding pocket of Leishmania with respect to human ERK2 and LCRK3. This analysis was helpful in identifying the binding sites and molecular function of the Leishmania specific MAPK homologue. Molecular docking study was performed on this 3D structural model, using different classes of competitive ATP inhibitors of LCRK3, to check whether they exhibit affinity and could be identified as Leishmania MAPK specific inhibitors. It is well known that MAP kinases are extracellular signal regulated kinases ERK1 and ERK2, which are components of the Ras-MAPK signal transduction pathway which is complexed with HDAC4 protein, and their inhibition is of significant therapeutic interest in cancer biology. In order to understand the mechanism of action, docking of indirubin class of molecules to the active site of histone deacetylase 4 (HDAC4) protein is performed, and the binding affinity of the protein-ligand interaction was computed. The new structural insights obtained from this study are all consistent with the available experimental data, suggesting that the homology model of the Leishmania MAPK and its ligand interaction modes are reasonable. Further the comparative molecular electrostatic potential and cavity depth analysis of Leishmania MAPK and human ERK2 suggested several important differences in its ATP binding pocket. Such differences could be exploited in the future for designing Leishmania specific MAPK inhibitors.

Stereoelectronic properties of spiroquinazolinones in differential PDE7 inhibitory activity

A detailed computational study on a series of spiroquinazolinones showing phosphodiesterase 7 (PDE7) inhibitory activity was performed to understand the binding mode and the role of stereoelectronic properties in binding. Our docking studies reproduced the essential hydrogen bonding and hydrophobic interactions for inhibitors of this class of enzymes. The N1 proton of the quinazolinone scaffold was involved in H-bonding to an amide side chain of the conserved glutamine residue in the active site. The central bicyclic ring of the molecules showed hydrophobic and pi-stacking interactions with hydrophobic and aromatic amino acid residues, respectively, present in the PDE7 active site. The docked conformations were optimized with density functional theory (DFT) and DFT electronic properties were calculated. Comparison of molecular electrostatic potential (MEP) plots of inhibitors with the active site of PDE7 suggested that the electronic distribution in the molecules is as important as steric factors for binding of the molecules to the receptor. The hydrogen bonding ability and nucleophilic nature of N1 appeared to be important for governing the interaction with PDE7. For less active inhibitors (pIC(50) < 6.5), the MEP maximum at N1 of the spiroquinazolinone ring was high or low based on the electronic properties of the substituents. All the more active molecules (pIC(50) > 6.5) had MEP highest at N3, not N1. Efficient binding of these inhibitors may need some rearrangement of side chains of active-site residues, especially Asn365. This computational modeling study should aid in design of new molecules in this class with improved PDE7 inhibition.

Pharmacophoric features of Pseudomonas aeruginosa deacetylase LpxC inhibitors: an electronic and structural analysis

Various electronic properties of structurally diverse synthetic LpxC inhibitors containing oxazoline, aroylserine and thiazoline rings were calculated and correlated with biological activity. These electronic features include the magnitude and locations of 3-dimensional molecular electrostatic potentials, hydrogen bond acceptor/donor density, lowest unoccupied molecular orbital, and highest occupied molecular orbital. Strong correlation of these stereo-electronic properties with LpxC inhibitory potency reveals the potential pharmacophoric features of specific LpxC inhibitors. Thus, these pharmacophoric features of LpxC inhibitors based on electronic and surface analysis could be successfully exploited for designing more potent LpxC inhibitors.

Comparative protein modeling and surface analysis of Leishmania sirtuin: A potential target for antileishmanial drug discovery

Homology model of Leishmania SIR2 shed new light on the ligand binding features of this enzyme. The molecular electrostatic potentials (MESP), the cavity depth analysis, and LmSIR2-hSIRT2 models' superposition suggested that the nicotinamide binding catalytic domain has several minor but potentially important structural differences. These differences could be exploited for designing antileishmanial compounds.

Comparative protein modeling of methionine S-adenosyltransferase (MAT) enzyme from Mycobacterium tuberculosis: a potential target for antituberculosis drug discovery

Mycobacterium tuberculosis (Mtb) is a successful pathogen that overcomes the numerous challenges presented by the immune system of the host. In the last 40 years few anti-TB drugs have been developed, while the drug-resistance problem is increasing; there is thus a pressing need to develop new anti-TB drugs active against both the acute and chronic growth phases of the mycobacterium. Methionine S-adenosyltransferase (MAT) is an enzyme involved in the synthesis of S-adenosylmethionine (SAM), a methyl donor essential for mycolipid biosynthesis. As an anti-TB drug target, Mtb-MAT has been well validated. A homology model of MAT has been constructed using the X-ray structures of E. coli MAT (PDB code: 1MXA) and rat MAT (PDB code: 1QM4) as templates, by comparative protein modeling principles. The resulting model has the correct stereochemistry as gauged from the Ramachandran plot and good three-dimensional (3D) structure compatibility as assessed by the Profiles-3D score. The structurally and functionally important residues (active site) of Mtb-MAT have been identified using the E. coli and rat MAT crystal structures and the reported point mutation data. The homology model conserves the topological and active site features of the MAT family of proteins. The differences in the molecular electrostatic potentials (MEP) of Mtb and human MAT provide evidences that selective and specific Mtb-MAT inhibitors can be designed using the homology model, by the structure-based drug design approaches.

Convenient Access Both to Highly Antimalaria-Active 10-Arylaminoartemisinins, and to 10-Alkyl Ethers Including Artemether, Arteether, and Artelinate

An economical phase-transfer method is used to prepare 10-arylaminoartemisinins from DHA and arylamines, and artemether, arteether, and artelinate from the corresponding alcohols. In vivo sc screens against Plasmodium berghei and P. yoelii in mice reveal that the p-fluorophenylamino derivative 5 g is some 13 and 70 times, respectively, more active than artesunate; this reflects the very high sc activity of 10-alkylaminoartemisinins. However, through the po route, the compounds are less active than the alkylaminoartemisinins, but still approximately equipotent with artesunate.

Structure-activity relationship study of steroidal 1,2,4,5-tetraoxane animalarials using computational procedures

A three-dimensional QSAR pharmacophore model for antimalarial activity of steroidal 1,2,4,5-tetraoxanes was developed from a set of 17 substituted antimalarial derivatives out of 27 analogues that exhibited remarkable in vitro activity (below 100 ng/mL) against sensitive and multidrug-resistant Plasmodium falciparum malaria. The pharmacophore, which contains two hydrogen bond acceptors (lipid) and one hydrophobic (aliphatic) feature, was found to map well onto the potent analogues and many other well-known antimalarial trioxane drugs including artemisinin, arteether, artesunic acid, and tetraoxanes. The presence of at least one hydrogen bond acceptor in the trioxane or the tetraoxane moiety appears to be necessary for potent activity of this class of compounds. Docking calculations of some of these compounds with heme are consistent with the above observation as the proximity of the heme iron to the oxygen atom of the trioxane or the tetraoxane moiety favors potent activity of the compounds. Electron transfer from the oxygen of trioxane or the tetraoxane appears to be crucial for mechanism of action of the compounds. This information together with the pharmacophore should enable search for new peroxide containing antimalarial candidates from databases and custom designed synthesis of more efficacious and safer analogues.

Alkylation of manganese(II) tetraphenylporphyrin by antimalarial fluorinated artemisinin derivatives

The alkylating properties of two artemisinin derivatives bearing a trifluoromethyl substituent at C10 were evaluated toward manganese(II) tetraphenylporphyrin, considered as a heme model. Chlorin-type covalent adducts were obtained by alkylation of the porphyrin ring by C-centered radicals derived from reductive activation of the peroxide function of the drugs.

Demonstrating the validity of natural products as anti-infective drugs

This presentation reviews the synthetic or classical development pathway of drug development and contrasts it with developing natural products as drugs. Also presented is an example of a traditional medicine that has been developed from a natural product and has become a "new/old" antiparasitic drug used in the treatment of malaria. The classic paradigm of synthetic drug development breaks down into drug discovery, drug design, preclinical studies, and clinical studies. This paradigm, constructed to weed out failures, results in a drug-development process that is high risk, time consuming, and expensive. The process requires screening an average of 10,000 active compounds to find a single compound that successfully makes its way through validation to drug approval and the marketplace. Following this paradigm, researchers progress from identifying a chemical lead to testing the compound in humans. The World Health Organization (WHO) Guidelines for the Assessment of Herbal Medicines are based on the classical guidelines and follow the classical approach to validating quality, safety, and efficacy--with one major difference. The starting point is to look at the natural product in humans. By taking into account the traditional experience with the product, the validation standard for safety and efficacy of natural products allows for the prolonged and apparently uneventful use of a substance to offer testimony of its safety. The reliance, then, is on experience--or what Western regulatory agencies would call "anecdotal information." Since most phytomedicines are a combination of several active ingredients, the WHO guidelines cover two kinds of combination products: Combinations that are already used in traditional medicine are considered "old" combination products. "New" combination products are well-known substances that are now being used in combination. Artemisia annua, a pervasive weed, has been referred to in Chinese medicine for thousands of years as a treatment for fever. In 1971, an extraction of artemisia yielded activity against Plasmodium berghei, a mouse model for malaria. The isolated compound, artemisinin, is an example of a traditional medicine that started out in humans, but which then provided a lead structure for a standard drug-development paradigm. Today, artemisinin derivatives are being used widely in combination therapy, especially in areas of the world where there is multidrug-resistant malaria.

Alkylating capacity and reaction products of antimalarial trioxanes after activation by a heme model

The reactivity of 1,2,4-trioxane molecules 2-5, structurally related to the antimalarial drug artemisinin, with a heme model, manganese(II) tetraphenylporphyrin, is reported. With the pharmacologically active drugs 2-4, covalent adducts were obtained by addition of a drug-derived radical onto the porphyrin macrocycle, whereas no reaction was obtained with the nonactive compound 5. This confirms that alkylation is probably one of the key factors of the pharmacological activity of endoperoxide-based antimalarial drugs.

NMR studies on novel antitumor drug candidates, deoxoartemisinin and carboxypropyldeoxoartemisinin

Artemisinin and its derivatives, which have been known as antimalarial drugs, have also demonstrated their cytotoxicity against tumor cells. It has been proposed that antitumor activity depends on the lipophilicity of functional group on artemisinin derivatives. Solution structures of two artemisinin derivatives as antitumor drug candidates, deoxoartemisinin and carboxypropyldeoxoartemisinin, were determined by NMR spectroscopy to elucidate structure-activity relationship. According to biological assay, antitumor efficiencies are not dependent upon lipophilicity. Instead, these compounds demonstrated their distinctive structural features of boat/chair conformation and capability to interact with receptors, as they have different efficiencies on antitumor activity. Especially, carboxypropyl moiety or carbonyl moiety in artemisinin derivatives influences the conformation and stability of ring structure. Although the detailed mechanism of antitumor activity by artemisinin derivatives has not been addressed, we suggest that antitumor activity is not determined only with lipophilicity and that artemisinin derivatives have specific target proteins in each type of cancer.