Pharmacology and mechanism of action

Artemisinin (qinghaosu) is an antimalarial compound first isolated in pure form in 1972 by Chinese scientists from the herb qinghao (Artemisia annua). This herb (worm wood) has been used in Chinese traditional medicine to control fever for over 2000 years [1]. Artemisinin is a compound with a peculiar structure, low toxicity and high efficacy even in severe chloroquine resistant P. falciparum malaria. Unlike current antimalarial drugs which have a nitrogen-containing heterocylic ring system, it is a sesquiterpene lactone with an endoperoxide linkage. The endoperoxide linkage is essential for the antimalarial activity of the drug. Artemisinin has been shown to be a potent schizontocidal drug both in vitro and in experimental animal models, but it has no practical effect against the exoerythrocytic tissue phase, the sporozoites and the gametocytes[2]. The mechanism of action of artemisinin is not clearly understood. The drug selectively concentrates in parasitized cells by reacting with the intraparasitic hemin (hemozoin). In vitro this reaction appears to generate toxic organic free radicals causing damage to parasite membranes [2-4]. The derivatives of artemisinin are more potent than the parent drug and have apparently a similar mechanism of action [1,2].

Indications

A sesquiterpene peroxide derived from A. annua, chiefly used in the form of artemether, the methyl ester synthesized from dihydroartemisinin, or artesunate, the water-soluble hemisuccinate. Formulated for administration by the oral, intramuscular or intrarectal routes; artesunate can also be given intravenously. Artemisinin and its derivatives are valuable drugs for the management of malaria. They should not be used unnecessarily or with incomplete dosage regimens. They are indicated only in areas where multidrug resistant P. falciparum malaria is prevalent [5].

Side effects

Artemisinin and its derivatives are exceptionally safe drugs. Millions of people have taken them and serious side effects have yet to be reported. The most commonly reported side effects include mild and transient gastrointestinal problems (such as nausea, vomiting, abdominal pain and diarrhoea), headache, and dizziness particularly after oral administration. Transient first degree heart block and bradycardia were reported in a few individuals, who received artesunate or artemether at the standard doses. Brief episodes of drug-induced fever have also been observed in a few studies [6,5]. After rectal administration the patients may experience tenesmus, abdominal pain and diarrhoea. A transient dose-related decrease in circulating reticulocytes has been reported following high doses of artesunate above 4 mg/kg for 3 days. All values returned to pre-treatment values within 14 days [6,5]. Neurotoxicity has been observed in animal studies but has never been documented in man [7].

Contraindications

There are no known contraindications. However, artemisinin and its derivatives should only be used when other antimalarial drugs do not work.

Preparations

Artemether ? Paluther? (Rh?ne-Poulenc Rorer). Solution for injection 80 mg/ml. ? Artenam? (Dragon Pharmaceuticals Ltd, Wales UK). Solution for injection 100 mg/ml. ?Several other preparations containing artemisinin derivatives are manufactured in China and Vietnam. The availability of these preparations is presently uncertain.

Pharmaceutical Applications

The genus Artemisia of the family Asteraceae is comprised of more than 500 species which are found all over the world. Many members of the genus are used in various traditional therapies including East Asian medicine and Ayurveda. Some important species which have been studied for their various therapeutic potentials are A. asiatica for inflammation, infection, and ulcerogenic disorders; A. annua for fevers specially malaria; A. afra for cough, cold, headache, dyspepsia, colic, diabetes, and kidney disorders; A. judaica for gastrointestinal disorders; A. tripartite for sore throat, tonsillitis, cold, headache, and wounds; A. vulgaris as analgesic, anti-inflammatory, and antispasmodic; and A. verlotorum for hypertension (Bora and Sharma 2011). Artemisinin is the major bioactive compound, which is rich in mono- and sesquiterpenes, and is a new class of potential antimalarial drug used throughout the globe. The combination therapies of artemisinin are considered to be the best treatment for Plasmodium falciparum malaria (He et al. 2009). Apart from antimalarial activity, the oil has antibacterial and antifungal (Bilia et al. 2014), immunosuppressive, anti-inflammatory, antioxidant (Cavar et al. 2012), and antiviral (Alesaeidi and Miraj 2016) activities. A. annua has also been studied against diabetes, heart diseases, arthritis, eczema, and cancer. In vitro and in vivo studies on artemisinin have given good evidence of its anticancer activity. The mechanism of action of its antineoplastic activity has also been exhaustively studied and reviewed. Artemisinin is described to induce oxidative stress and nitric oxide production; cause DNA damage and repair; induce apoptosis, autophagy, and necrosis; and inhibit angiogenesis and mitogen-activated protein kinases (MAPK) pathway, metastatic pathway, etc. (Efferth 2017). Phase I and II clinical trials for the molecule have also been done; but hepatotoxicity caused by artemisinin combination therapy is a limitation as of now. The anticancer activity of artemisinin has been studied in breast cancer, in lung cancer, and in prostate carcinoma (Lai and Singh 2006; Sun et al. 2014; Michaelsen et al. 2015).

Drugs for treatment of malaria

Artemisinin is the drug for the treatment of malaria with the most excellent efficacy, being a kind of sesquiterpene lactone containing peroxide group extracted from the traditional Chinese medicine Artemisia annua. It is characterized with high efficiency, rapid efficacy, clearing summer-heat, clearing deficiency heat, protozoa-killing effect and low toxicity. Currently, the efficacy of the artemisinin-based combination therapy (ACT) for the treatment of malaria worldwide has reached over 90%. ACT has been already widely applied to the treatment of malaria in many countries around the world. It has a strong and rapid killing effect on the erythrocytic stage of plasmodium, being able to rapidly control the clinical seizures and symptoms. Meanwhile, it also has prominent efficacy in the treatment of chicken coccidiosis, Mycoplasma Suis, toxoplasmosis, weakness and fever, damp heat jaundice, tertian malaria, falciparum malaria, cerebral malaria and chloroquine malaria. Artemisinin was first successfully developed by Chinese scientists, being effective monomer originated from the folk malaria-treatment herbs Artemisia annua. China is one of the major resource countries for the growth of such plants. The demands for research and development originated from the Vietnam War in 1960s when Malaria parasites had been resistant to special drug chloroquine at that time. In Vietnam War, many soldiers are not killed in the war, but instead died of malaria. Because of the presence of artemisinin resistance in the border areas of Cambodia and Thailand, the World Health Organization advocates the use of combinations rather than monomeric formulation. In this environment, the world's first artemisinin-based compound antimalarial drug-compound artemether has been successfully developed in China. However, due to the lack of attention on the importance of intellectual property of Chinese pharmaceutical companies, currently in the world, only Novartis Company has the authority of foreign selling of artemether compound that has been recognized by the World Health Organization. The Novartis Company has offered the drug to the WHO at the cost price, wining the wide acclaim from international community. However, in this trade war, China can only play a role of major drug producing countries.

Production method

It can be extracted from the leaves of Artemisia annua L. (Compositae). In addition to artemisinin, China also produces both artemether and sodium artemisinin.

History

The discovery of artemisinin dramatically changes the landscape to combat malaria and leads to a paradigm shift in antimalarial drug development.However, the discovery of artemisinin is the first stage; the development of artemisinin derivatives and their compound preparations is another important stage. Based on artemisinin, scientists obtained artemisinin ether derivatives by semisynthetic method. After screening of antimalarial activity, artemether was found. To further improve the solubility of artemisinin derivatives, artesunate was also found. The discovery of artesunate makes artemisinin and its derivatives much easier to promote, and more convenient dosage forms to treat malaria enriched the clinic application of artemisinin and its derivatives .

Antimicrobial activity

Artemisinins are active against the erythrocytic and gametocyte stages of chloroquine-sensitive and chloroquine-resistant strains of P. falciparum and other malaria parasites. Two anomers of artemether are produced on synthesis, α-artemether and β-artemether, of which the latter has higher antimalarial activity. Activity against the protozoa Tox. gondii and Leishmania major and the helminth Schistosoma mansoni has been demonstrated in experimental models.

Acquired resistance

Resistance caused, for example, by changes in the plasmodial endoplasmic reticulum ATPase has been shown in experimental models. There have been clinical reports of reduced susceptibility to treatment with artesunate in Cambodia.

Biological Activity

Antimalarial agent; interacts with heme to produce carbon-centred free radicals, causes protein alkylation and damages parasite microorganelles and membranes. Also selectively inhibits the P-type ATPase (PfATP6) of Plasmodium falciparum (K i ~ 150 nM). Displays antiangiogenic effects in mouse embryonic stem cell-derived embryoid bodies.

Biochem/physiol Actions

Artemisinin (Qinghaosu), a sesquiterpene lactone, is a highly active anti-malarial (falciparum malaria) drug. Artemisinin is also an anthelmintic (parasitic worm) effective against the blood fluke, schistosomiasis.

Pharmacokinetics

Oral absorption: Incomplete Cmax 500 mg oral: 0.4 mg/L after 1.8 h Plasma half-life (dihydroartemisinin): 40–60 min Volume of distribution: c. 0.25 L/kg Plasma protein binding (artemether): 77% Artemisinins are concentrated by erythrocytes and are rapidly hydrolyzed to dihydroartemisinin. They are hydroxylated by cytochromes 2B6, 2C19 and 3A4; the derivatives induce this metabolism. After injection, peak plasma concentrations are reached within 1–3 h, when levels of dihydroartemisinin are included. The elimination half-life of intravenous artesunate is <30 min; artemether appears to have a much longer half-life (4–11 h).

Pharmacology

The mechanism of artemisinins is not known, but the most widely accepted theory is that they are first activated through cleavage after reacting with haem and iron(II) oxide, which results in the generation of free radicals that in turn damage susceptible proteins, resulting in the death of the parasite .Artemisinin and its derivatives also show a good antitumor effect , which is mainly via (1) apoptosis, ferroptosis, or necrosis; (2) anti-angiogenesis; (3) oxidative stress; (4) tumor suppressor genes; and (5) protein targeting. In addition, artemisinin can exhibit antiarrhythmic, anti-fibrotic, and immunomodulating effects.

Safety Profile

Moderately toxic by ingestion,intramuscular, and intraperitoneal routes. When heated todecomposition it emits acrid smoke and fumes.

Synthesis

Quinghaosu, octahydro-3,6,9-trimethyl-3,12-epoxy-12Hpyrano-(4,3-di)-1,2-benzodioxepin-10-(3H)-one (37.1.1.57), is isolated from the plant Artemisia annua. It also has been made synthetically.

Chemical properties

It appears as colorless needle crystal with a melting point being156-157 °C. It is easily soluble in chloroform, acetone, ethyl acetate and benzene, being soluble in methanol, ethanol and insoluble in water.

Physical properties

Appearance: colorless needles or white crystalline powder. Solubility: practically insoluble in water, very soluble in dichloromethane, freely soluble in acetone and ethyl acetate, and soluble in glacial acetic acid, methanol, and ethanol. Melting point: 150–153?°C. Specific optical rotation: +75 to +78°.

Definition

ChEBI: A sesquiterpene lactone obtained from sweet wormwood, Artemisia annua, which is used as an antimalarial for the treatment of multi-drug resistant strains of falciparum malaria.

General Description

The artemisinin series are the newest of the antimalarialdrugs and are structurally unique when comparedwith the compounds previously and currently used. Theparent compound, artemisinin, is a natural product extractedfrom the dry leaves of Artemisia Annua (sweetwormwood). The plant has to be grown each year fromseed because mature plants may lack the active drug. The growing conditions are critical to maximize artemisininyield. Thus far, the best yields have been obtained fromplants grown in North Vietnam, Chongqing province inChina, and Tanzania.