Recent advances in antimalarial compounds and their patents

Malaria is one of the most severe tropical parasitic disease causing 1-3 million deaths annually. In the last 25 years very few new antimalarial molecules have been developed and only a limited number of them are currently in various stages of clinical development. The presently available antimalarial drugs include artemisinin analogs, quinoline derivatives and antifolates. This review summarizes recent advances in antimalarial drug development and world patents published between 2000-2006 claiming new synthetic antimalarial compounds and their activities. The most over-represented classes of compounds in malaria patent literature in order of frequency are artemisinin analogs, quinoline derivatives, DOXP reductoisomerase inhibitors, antifolates and febrifugine analogues. Many of these patents describe the novelty and potential of these synthetic derivatives with an attempt to identify the next generation antimalarials that may have potential commercial advantages.     

Artemisinin derivatives: a patent review (2006 – present)

Introduction: The isolation of artemisinin from an ancient Chinese remedy in the early 1970s heralded the beginning of a new era in antimalarial drug therapy culminating in artemisinin-based combination therapies currently being the mainstay of malaria treatment worldwide. Ongoing research on this compound and its derivatives has revealed its potential use in treating other infectious and noninfectious diseases.

Areas covered: This review provides a summary of patents published globally from January 2006 to June 2012 covering promising artemisinin derivatives and artemisinin-based drug combinations developed for use in various therapeutic areas.

Expert opinion: The diversity of semi-synthetic artemisinin derivatives has been limited to the same design strategy of modifying the artemisinin molecule at the same positions due to inherent synthetic challenges. To address this, future endeavors should include: the use of biotransformation strategies to modify other positions in the sesquiterpene ring while retaining the endoperoxide bridge; the design and synthesis of synthetic ozonides based on the pharmacophoric endoperoxide motif and drug repositioning approaches to artemisinin-based combination therapy. A better understanding of the mechanism of action of artemisinin derivatives and their biomolecular targets may provide an invaluable tool for the development of derivatives with a wider array of activity and greater clinical utility than currently appreciated.     

青蒿素及其衍生物的研究进展

青蒿素类化合物是含过氧桥的化合物,在治疗多药抗药性恶性疟疾方面卓有成效。除此之外,该类化合物还具有抗肿瘤、抗真菌、抗心律失常、抗寄生虫等活性。本文就近年来国内外学者对青蒿素及其衍生物的结构改造及生物活性等方面的研究进行概述。     

Preparation of chemically robust new artemisinin derivatives

The resurgence of malaria, largely through emergence of drug-resistant strains of the malaria parasite, Plasmodium falciparum, has in recent times substantially increased public and private focus on the development of new treatments for the disease. However, in the absence of any vaccine, major reliance continues to be placed on chemotherapy involving both traditional, largely quinoline-based, antimalarial drugs and the newer class of antimalarial drug based on artemisinin, the remarkable peroxidic sesquiterpene isolated by the Chinese in 1972. Derivatives of this compound, artesunate and artemether, are now used in routine therapy in conjunction with a longer half-life quinoline or related antimalarial. However, problems of instability and neurotoxicity associated with the current artemisinin derivatives has resulted in a vigorous search for new, more stable derivatives with better pharmacological profiles. The Centre Nationale de la Recherche Scientifique patent describes a class of relatively readily accessible new artemisinin derivatives, which in incorporating the trifluoromethyl group, appear to be more stable than the current derivatives.     

The therapeutic potential of semi-synthetic artemisinin and synthetic endoperoxide antimalarial agents

Artemisinin derivatives such as artesunate, dihydroartemisinin and artemether are playing an increasing role in the treatment of drug-resistant malaria. They are the most potent antimalarials available, rapidly killing all asexual stages of the parasite Plasmodium falciparum. This review highlights the recent developments in the area of improved second-generation semi-synthetic artemisinin derivatives and fully synthetic antimalarial endoperoxide drugs. In pursuit of synthetic analogues of the artemisinins, one of the major challenges for chemists in this area has been the non-trivial development of techniques for the introduction of the peroxide bridge into candidate drugs. Although chemical research has enabled chemists to incorporate the endoperoxide 'warhead' into synthetic analogues of artemisinin, significant drawbacks with many candidates have included comparatively poor antimalarial activity, non-stereoselective syntheses and chemical approaches that are not readily amenable to scale up. However, very recent progress with synthetic 1,2,4-trioxolanes provides a new benchmark for future medicinal chemistry efforts in this area.     

Searching for artemisinin production improvement in plants and microorganisms

The endoperoxide sesquiterpene lactone artemisinin which is isolated from the plant Artemisia annua, and its semi-synthetic derivatives, are potent, novel, antimalarial drugs. They are effective against multidrug-resistant Plasmodium strains and have become essential components of the so-called Artemisinin-based Combination Therapy, that is recommended by the World Health Organization as the treatment of choice for malaria tropica. Moreover, artemisinin and its derivatives show additional anti-parasite, antitumor, and anti-viral properties. The plants, however, are very poor resources for the drug, as the content of artemisinin is low (from 0,1 to 1,5 % of dried leaves) and dependent on seasonal and somatic variations as well as the infestation of bacteria, fungi and insects. A chemical synthesis of the compound is complex and uneconomic. Therefore, artemisinin is in short supply and remains unaffordable for most people in malaria-endemic countries. Thus, many researchers have focused on enhancing the production of artemisinin, first, through traditional breeding and in in vitro plant tissue cultures and, then, by heterologous expression systems (a semi-synthetic approach) with the use of genetically-modified or transgenic microbes. In this review, we summarize the progress made in the production of artemisinin by the biotechnological approach.     

Artemisia annua L.: a source of novel antimalarial drugs

Artemisia annua L. contains artemisinin, an endoperoxide sesquiterpene lactone, mainly in its leaves and inflorescences. This compound and a series of derivatives have attracted attention because of their potential value as antimalarial drugs. In this review a survey of the currently available literature data is given. It includes phytochemical aspects, such as constituents ofA. annua, the artemisinin content during the development of the plant and its biosynthesis, isolation, analysis and stability. Total chemical synthesis of artemisinin is referred to, as well as structure—activity relationships of derivatives and simplified analogues. Pharmacological studies are summarized, including the mechanism of action, interaction of the antimalarial activity with other drugs, possible occurrence of resistance to artemisinin, clinical results, toxicological aspects, metabolism and pharmacokinetics. Finally, plant cell biotechnologyy is mentioned as a possible means to obtain plants and cell cultures with higher artemisinin contents, allowing an industrial production of pharmaceuticals containing this novel drug.     

Medicinal chemistry perspectives of trioxanes and tetraoxanes

Trioxane based compounds such as artemisinin and its synthetic and semi-synthetic analogues constitute promising class of antimalarial agents. The pharmaceutical development of artemisinin was started in 1971 after the isolation from Chinese medicinal plant Artemisia annua and this compound has drawn much attention from medical chemist and pharmacologist worldwide. Researchers from across the globe have independently and collaboratively conducted various studies on the artemisinin system in an attempt to identify lead molecules for malaria chemotherapy. This systematic study led to the discovery of artemether, arteether, dihydroartemisinin, and sodium artesunate which are being used as antimalarial drug for the treatment of Plasmodium falciparum related infections. These studies also revealed that the trioxane bridge is essential for the antimalarial activity of this class of compounds. Another class of structurally simple peroxides that emerged from these studies was the 1,2,4,5-tetraoxanes. Some of the tetraoxane based compounds have shown promising antimalarial potential, and much of work has been done on this type of compound in recent years. Apart from their antimalarial activity, these classes of compounds have also shown promising anticancer and antibacterial activity. To this end, an attempt has been made to describe the medicinal potential of trioxane and tetraoxane-based compounds. Literature from 1999 has been critically reviewed and an attempt has been made to discuss structure activity relationship study among the series of trioxane and tetraoxane based compounds.     

New trends in anti-malarial agents

Malaria is the major parasitic infection in many tropical and subtropical regions, leading to more than one million deaths (principally young African children) out of 400 million cases each year (WHO world health report 2000). More than half of the world's population live in areas where they remain at risk of malaria infection. During last years, the situation has worsened in many ways, mainly due to malarial parasites becoming increasingly resistant to several antimalarial drugs. Furthermore, the control of malaria is becoming more complicated by the parallel spread of resistance of the mosquito vector to currently available insecticides. Discovering new drugs in this field is therefore a health priority. Several new molecules are under investigation. This review describes the classical treatments of malaria and the latest discoveries in antimalarial agents, especially artemisinin and its recent derivatives as well as the novel peroxidic compounds.     

The therapeutic potential of semi-synthetic artemisinin and synthetic endoperoxide antimalarial agents

Artemisinin derivatives such as artesunate, dihydroartemisinin and artemether are playing an increasing role in the treatment of drug-resistant malaria. They are the most potent antimalarials available, rapidly killing all asexual stages of the parasite Plasmodium falciparum. This review highlights the recent developments in the area of improved second-generation semi-synthetic artemisinin derivatives and fully synthetic antimalarial endoperoxide drugs. In pursuit of synthetic analogues of the artemisinins, one of the major challenges for chemists in this area has been the non-trivial development of techniques for the introduction of the peroxide bridge into candidate drugs. Although chemical research has enabled chemists to incorporate the endoperoxide ‘warhead’ into synthetic analogues of artemisinin, significant drawbacks with many candidates have included comparatively poor antimalarial activity, non-stereoselective syntheses and chemical approaches that are not readily amenable to scale up. However, very recent progress with synthetic 1,2,4-trioxolanes provides a new benchmark for future medicinal chemistry efforts in this area.     

Artemisinin, promising lead natural product for various drug developments

Artemisinin and its synthetic derivatives are widely used for antimalarial agents in the world. Moreover, they are discovered to additionally use as anticancer, antiangiogenesis, antiviral, immunosuppressive, and antifungal agents. Recent research results supported that it is a very promising field in drug discovery. In this review, it will discuss the structural and biological features of artemisinin and its derivatives that have published since 2003 except antimalarial, and show that they are useful lead compounds for novel drug discovery.     

Artemisia annua L.: a source of novel antimalarial drugs

Artemisia annua L. contains artemisinin, an endoperoxide sesquiterpene lactone, mainly in its leaves and inflorescences. This compound and a series of derivatives have attracted attention because of their potential value as antimalarial drugs. In this review a survey of the currently available literature data is given. It includes phytochemical aspects, such as constituents ofA. annua, the artemisinin content during the development of the plant and its biosynthesis, isolation, analysis and stability. Total chemical synthesis of artemisinin is referred to, as well as structure—activity relationships of derivatives and simplified analogues. Pharmacological studies are summarized, including the mechanism of action, interaction of the antimalarial activity with other drugs, possible occurrence of resistance to artemisinin, clinical results, toxicological aspects, metabolism and pharmacokinetics. Finally, plant cell biotechnologyy is mentioned as a possible means to obtain plants and cell cultures with higher artemisinin contents, allowing an industrial production of pharmaceuticals containing this novel drug.     

Current progress in the chemistry, medicinal chemistry and drug design of artemisinin based antimalarials

This review covers developments in relation to artemisinin-based antimalarial agents. Topics covered include a brief introduction to the history and treatment of malaria, and more recently, drug resistant malaria; the discovery of the naturally occurring novel peroxidic antimalarial artemisinin; artemisinin biosynthesis, metabolism and biotransformations; the diversity of proposed mechanisms of action; pharmacokinetics; the insight into structure-toxicity relationships; the total syntheses and the progress made in the syntheses of its analogs; and, ultimately the contribution of these efforts towards rational drug design in order to access potent, non-toxic antimalarial drugs based on artemisinin.     

Structure-activity relationships of the antimalarial agent artemisinin. 8. design,synthesis, and CoMFA studies toward the development of artemisinin-based drugs against leishmaniasis and malaria

Artemisinin (1) and its analogues have been well studied for their antimalarial activity. Here we present the antimalarial activity of some novel C-9-modified artemisinin analogues synthesized using artemisitene as the key intermediate. Further, antileishmanial activity of more than 70 artemisinin derivatives against Leishmania donovani promastigotes is described for the first time. A comprehensive structure-activity relationship study using CoMFA is discussed. These analogues exhibited leishmanicidal activity in micromolar concentrations, and the overall activity profile appears to be similar to that against malaria. Substitution at the C-9beta position was shown to improve the activity in both cases. The 10-deoxo derivatives showed better activity compared to the corresponding lactones. In general, compounds with C-9alpha substitution exhibited lower antimalarial as well as antileishmanial activities compared to the corresponding C-9beta analogues. The importance of the peroxide group for the observed activity of these analogues against leishmania was evident from the fact that 1-deoxyartemisinin analogues did not exhibit antileishmanial activity. The study suggests the possibility of developing artemisinin analogues as potential drug candidates against both malaria and leishmaniasis.     

Combination therapy for malaria: the way forward?

Unless new strategies are deployed to combat malaria, the already enormous health and economic burden related to the disease in tropical countries is bound to worsen. The main obstacle to malaria control is the emergence of drug resistant strains of Plasmodium falciparum. As for HIV/AIDS and tuberculosis, the use of combinations of antimalarial drugs reduces the risk of selecting for resistant mutants of the plasmodial parasites. In large field trials, the combination of an artemisinin derivative and a partner drug with an unrelated mode of action (in this case mefloquine), has shown a remarkable double effect: preventing the emergence and spread of drug resistance, and interrupting the transmission of P. falciparum. This has opened the way for a new approach to the deployment of antimalarial drugs. Coupled with early detection and confirmed diagnosis, this strategy represents the only way forward in the chemotherapy of malaria. Massive economic assistance will be needed to detect and treat adequately the estimated 500 million cases of malaria per year, but without radical action there is no prospect of 'Rolling Back' malaria.     

Preventing antimalarial drug resistance through combinations.

Throughout the tropical world antimalarial drug resistance is increasing, particularly in the potentially lethal malaria parasite Plasmodium falciparum. In some parts of Southeast Asia, parasites which are resistant to chloroquine, pyrimethamine-sulfadoxine, and mefloquine are prevalent. The characteristics of a drug that make it vulnerable to the development of resistance are a long terminal elimination half-life, a shallow concentration-effect relationship, and that one or two base-pair mutations confer a marked reduction in susceptibility. The development of resistance can be delayed or prevented by drug combinations. The artemisinin derivatives are the most potent of all antimalarial drugs. They reduce the infecting parasite biomass by approximately 10 000-fold per asexual life cycle. There are good arguments for combining, de novo, an artemisinin derivative with all newly introduced antimalarial drugs.     

Recent developments in the chemotherapy of malaria

Malaria remains a major infectious disease and is worsening in some areas, partly because of the spread of resistance to established antimalarial drugs. New drugs are urgently needed to combat the protozoan parasite, Plasmodium. This review covers new developments, including artemisinin derivatives, synthetic peroxides, folate pathway inhibitors, primaquine analogs and proteinase inhibitors. However, few of these agents are in clinical trials and many are derived from chemical classes already used extensively against malaria. The emerging understanding of parasite biology and new technological developments in drug discovery offer hope of improvement, but this will require increased interest from the pharmaceutical industry.     

Artemisinin drugs: novel antimalarial agents

Artemisinin and its derivatives, artesunate and artemether, represent a new class of antimicrobial drug with potent activity against Plasmodium falciparum. Although they show excellent efficacy in both severe and uncomplicated malaria, dosage regimens still need to be optimised and pharmacokinetic profiles defined. In the treatment of uncomplicated malaria, the artemisinin drugs should be used in combination with a long acting antimalarial to protect both drugs against the emergence of resistance. In the treatment of severe malaria, parenteral artemether is at least as effective as quinine and is simpler to use. The use of rectal preparations of artesunate and artemisinin at the rural health level will facilitate early initiation of the treatment of falciparum malaria and this may reduce the proportion of patients progressing to severe disease. All of the artemisinin drugs have comparable efficacy; the choice of derivative should be based upon availability, cost and quality of the preparation. Artemisinin, artesunate and artemether are well-tolerated in both adults and children, with no evidence to date of serious clinical toxicity.     

Artemisinin drugs: novel antimalarial agents

Artemisinin and its derivatives, artesunate and artemether, represent a new class of antimicrobial drug with potent activity against Plasmodium falciparum. Although they show excellent efficacy in both severe and uncomplicated malaria, dosage regimens still need to be optimised and pharmacokinetic profiles defined. In the treatment of uncomplicated malaria, the artemisinin drugs should be used in combination with a long acting antimalarial to protect both drugs against the emergence of resistance. In the treatment of severe malaria, parenteral artemether is at least as effective as quinine and is simpler to use. The use of rectal preparations of artesunate and artemisinin at the rural health level will facilitate early initiation of the treatment of falciparum malaria and this may reduce the proportion of patients progressing to severe disease. All of the artemisinin drugs have comparable efficacy; the choice of derivative should be based upon availability, cost and quality of the preparation. Artemisinin, artesunate and artemether are well-tolerated in both adults and children, with no evidence to date of serious clinical toxicity.     

Trioxaquines: hybrid molecules for the treatment of malaria

Artemisinin, with its 1,2,4-trioxane as active motif, is now the first-line treatment for multidrug-resistant malaria. The endoperoxide ring is essential for the antimalarial activity of artemisinin. Based on its mechanism of action, new hybrid molecules named trioxaquines with a dual mode of action have been designed. Trioxaquines are made by the covalent attachment of a trioxane, having alkylating ability, to a quinoline, known to easily penetrate within infected erythrocytes. This review discusses the importance of various hybrid molecules of artemisinin and 4-aminoquinoline in the treatment of malaria and the evolution of a trioxaquine hybrid as a promising antimalarial drug candidate.