Artemisinin: A Revolutionary Antimalarial Agent

1. Introduction

In the fight against malaria, artemisinin and its derivatives are an essential and potent class of medications. Extensive worldwide study has improved our knowledge of this extraordinary phytochemical, including its distinct chemical and pharmacological characteristics, since its discovery in the early 1970s.

Today, while artemisinin remains the cornerstone of antimalarial therapy, several challenges have emerged in its ongoing use and development. These include the rise of delayed treatment responses to artemisinin in malaria patients and attempts to repurpose these drugs for non-malaria applications.

A crippling illness that has affected people all over the world since ancient times, malaria remains one of the most pervasive, destructive, and deadly epidemics in human history [1]. Malaria, which frequently emerged and spread in the humid regions surrounding marshes and swamps, was once thought to be caused by the “bad air” preventing it in these areas [2]. This misconception gave rise to the term “malaria”, derived from the medieval Italian words ma (bad) and aria (air). For centuries, the disease was wrongly attributed to foul air, delaying the discovery of its true transmissible and parasitic nature. It was not until the late 1800s that the groundbreaking discoveries of Charles Louis Alphonse Laveran and Roland Ross uncovered the true cause of malaria. Their discoveries revealed that malaria is caused by protozoa of the Plasmodium genus and is primarily transmitted by Anopheles mosquitoes. These groundbreaking findings earned Laveran and Ross recognition as among the earliest recipients of the Nobel Prize in Physiology or Medicine [3].

Only five of the more than 100 Plasmodium species can affect humans. The main causes of malaria are Plasmodium malariae, Plasmodium ovale, Plasmodium vivax, and the extremely deadly Plasmodium falciparum, but Plasmodium knowlesi is not very dangerous to people.

Malaria treatment depends on chemotherapy with medication that targets various stages of the Plasmodium parasite’s life cycle. These consist of mefloquine (Larium), lumefantrine, doxycycline, quinoline compounds, sulfadoxine/pyrimethamine, and artemisinin-based combination treatment (ACTs). Mefloquine (ASMQ), lumefantrine (Coatem), amodiaquine (ASAQ), piperaquine (Duo-Cotecxin), pyronaridine (Pyramax), and other antimalarial medications are often combined with an artemisinin derivative in the most widely used ACTs. Additionally, preventative medicines like chloroquine, doxycycline, mefloquine (Lariam), primaquine, and the atovaquone-proguanil combination (Malarone) may be used for prophylaxis when necessary [4].

The World Health Organization (WHO) started the Emergency Response to Artemisinin Resistance in the Greater Mekong Subregion and the Global Plan for Artemisinin Resistance Containment (GPARC) to combat the growing threat of artemisinin-tolerant and resistant malaria. These programs support a systemic, multi-tiered strategy aimed at preventing, reducing, and eventually eradicating the emergence and transmission of malaria that is resistant to artemisinin [5]. Significantly lowering the worldwide malaria burden has been made possible by increasing access to artemisinin-based combination treatments (ACTs) in areas where malaria is endemic. At present, no alternative antimalarial treatment matches the efficacy and tolerability of ACTs.

2. Artemisinin

A naturally occurring sesquiterpene lactone substance with a distinct chemical structure is artemisinin, and its derivatives, which are collectively referred to as ‘artemisinin’ unless otherwise noted. It comes from the Asteraceae family’s sweet wormwood plant (Artemisia annua L.). Artemisinin has been proven effective against different forms of Plasmodium parasites, making it a focal point of significant scientific and medical interest [6, 7]. The discovery of artemisinin traces back to a Chinese government initiative in the late 1960s aimed at finding a cure for malaria. Tu Youyou won the 2015 Nobel Prize in Physiology or Medicine for her revolutionary discovery in 1972, which was the result of her hard work. The current global standard for treating malaria caused by Plasmodium falciparum and other Plasmodium species is artemisinin-based combination treatments (ACTs).

Since the 1980s, artemisinin derivatives have increasingly become the focus of research due to their affordability, effectiveness, and ease of use [8, 9, 10]. In 2006, the World Health Organization (WHO) officially recommended artemisinin-based combination therapies (ACTs) as the first-line treatment for Plasmodium falciparum malaria [11, 12]. Artemisinin and its derivatives remain the most crucial and effective drugs for malaria treatment [13]. However, the emergence of drug-resistant Plasmodium falciparum has driven continuous advancements in antimalarial drug research, with particular focus on enhancing artemisinin-based combination therapies (ACTs) [13, 14, 15].

3. Chemical structure of artemisinin and its derivatives

Artemisinin is a sesquiterpene trioxane lactone distinguished by the presence of a unique peroxide bridge (-O-O-) within a seven-membered ring; when its peroxide bridge interacts with iron in the parasite, leading to the destruction of the parasite’s cellular structure, which is crucial for its antimalarial activity (Figure 1) [16]. The lactone group in artemisinin can be readily reduced using sodium borohydride, yielding dihydroartemisinin. This derivative demonstrates even greater antimalarial activity in vitro compared to artemisinin itself [17].

To enhance its effectiveness, numerous derivatives (Figures 2–6) have been synthesized from dihydroartemisinin. Among them, artemether, arteether, artesunate, and artelinic acid are either currently in use or under evaluation. These derivatives play a crucial role in artemisinin-based combination therapies (ACTs), which the WHO recommends as the first-line treatment for Plasmodium falciparum malaria [18].

1. Dihydroartemisinin (DHA): It is a semisynthetic derivative or a reduced form of artemisinin where the lactone is converted to a hydroxy group. Known for enhanced solubility in water and greater antimalarial potency compared to artemisinin. It kills parasites by damaging their membranes and disrupting their mitochondrial function. DHA is low in toxicity and has saved many lives [19].

2. Artemether: A peroxide-containing lactone, a derivative of dihydroartemisinin formed by methylation of the hydroxy group. Lipophilic, making it suitable for intramuscular injection or oral use. It is used to treat uncomplicated malaria.

3. Arteether: Arteether is an ethyl ether derivative of dihydroartemisinin. Its lipophilic nature allows for intramuscular administration in long-acting formulations. Being oil soluble, it has an extended elimination half-life (greater than 20 hours), enhanced chemical stability, and greater overall stability compared to other artemisinin compounds [20].

4. Artesunate: A water-soluble derivative of dihydroartemisinin achieved through hemisuccinate esterification. Commonly used in severe malaria cases due to its rapid action and intravenous or intramuscular administration.

5. Artelinic acid: It is a semisynthetic derivative of artemisinin, specifically a triaxone dicarboxylic acid derivative of dihydroartemisinin (DHA). It was developed to improve the pharmacokinetic properties and stability of artemisinin derivatives for antimalarial therapy. Artelinic acid retains the core sesquiterpene trioxane lactone structure of artemisinin but introduces a dicarboxylic acid group, which enhances its water solubility and chemical stability compared to other derivatives like artesunate. The peroxide bridge (-O-O-) critical for antimalarial activity is preserved.

Each of these derivatives retains the critical peroxide bridge, which is activated upon interaction with iron from the parasite’s heme, generating free radicals that inflict damage on the parasite. These structural modifications enhance solubility, bioavailability, and pharmacokinetics, optimizing the derivatives for specific clinical applications.

4. Extraction of artemisinin

In 1969, Professor Youyou Tu was appointed to lead a research team as part of a project dedicated to screening Traditional Chinese Medicine (TCM) for novel antimalarial drugs. The China Academy of Chinese Medical Sciences’ Institute of Chinese Materia Medica is where this study was carried out. A list of more than 2000 herbal medicines was developed by Tu and her team using a wealth of TCM knowledge, including folklore, ancient literature, and practitioner interviews. They then reduced this list to about 640 viable options. From this refined selection, they tested more than 380 extracts from around 200 herbs, including Qinghao (Artemisia annua), though most failed to produce satisfactory results [21, 22]. The Qinghao extract, however, gained significant attention around 1971 due to its promising yet inconsistent results. This inconsistency prompted a meticulous re-examination of the traditional literature, ultimately leading to a pivotal breakthrough in the discovery process.

Drawing from classical literature, particularly Ge Hong’s Zhouhou Beiji Fang (Handbook of Prescriptions for Emergency), and her deep knowledge of Traditional Chinese Medicine (TCM), Tu proposed a crucial modification to the extraction process using low-temperature conditions to preserve the active compounds. The extracts obtained through this refined method were further purified by separating the acidic and neutral phases, a technique designed to retain the active components while reducing toxicity. This strategy produced a breakthrough: in tests carried out in or around October 1971, the resultant material demonstrated 100% efficacy against rodent malaria. In late December of the same year, this astounding discovery was confirmed in full in monkey malaria trials, conclusively demonstrating the effectiveness of the Qinghao extract [21].

This success confirmed the safety profile of the Qinghao extract, allowing clinical trials to proceed without delay in the latter half of 1972. The extract’s effectiveness in treating malaria was further confirmed by the extremely positive outcomes of the studies, which were carried out in Hainan Province and at the 302 Hospital PLA (now a component of the fifth Medical Center of the Chinese PLA General Hospital) in Beijing. These results helped propel Qinghao's research to the national level, driving further investigation and development. A concerted effort by the Chinese scientific community further propelled the research and development of Qinghao. In November 1972, Tu’s team at the Institute of Chinese Materia Medica successfully isolated the active component from the Qinghao extract; artemisinin (also known as Qinghaosu).building on this discovery the team later developed dihydroartemisinin (DHA), a derivative that remains one of the most pharmacologically significant antimalarial agents in use today.

Subsequently, artemisinin-based therapies significantly improved parasite clearance and quickly reduced symptoms in both mild and severe Plasmodium falciparum malaria infections, particularly when paired with slower-acting antimalarials like mefloquine or piperaquine. Additionally, these treatments demonstrated excellent tolerability with minimal reports of toxicity or safety concerns [22].

The exceptional effectiveness and safety of artemisinin-based treatments became more evident after more than 10 years of independent randomized clinical trials and meta-analyses. In 2006, the WHO officially revised its treatment guidelines to fully implement artemisinin-based combination therapies (ACTs) as the first-line treatment for malaria [23]. To this day, ACTs remain the most effective and widely recommended antimalarial treatments [24].

5. Biosynthesis of artemisinin

Two C₅ isoprenoid units from the cytosolic mevalonate (MVA) pathway and one isoprenoid unit from the non-mevalonate (MEP or DXP) pathway [25] are used to manufacture Farnesyl pyrophosphate (FPP, C₁₅), a crucial precursor of artemisinin [19, 20, 26]. FPP then undergoes cyclization through the enzymatic action of amorpha-4,11-diene synthase (ADS), leading to the formation of amorpha-4,11-diene [27, 28, 29]. This process involves the generation of bisabolyl and 4-amorphenyl cation intermediates (Figure 7) [25, 30, 31].

Amorpha-4,11-diene 12-monooxygenase (CYP71AV1) then oxidizes amorpha-4,11-diene to artemisinin alcohol [32]. Additionally, this enzyme makes it easier for artemisinic alcohol to continue oxidizing into artemisinic aldehyde and subsequently artemisinic acid. Additionally, alcohol dehydrogenase 1 (ADH1) catalyzes the oxidation of artemisinic alcohol to artemisinic aldehyde, while aldehyde dehydrogenase 1 (ALDH1) converts artemisinic aldehyde into artemisinic acid [33, 34]. Initially, artemisinic acid was thought to be the final precursor of artemisinin [25]. However, studies have revealed that it undergoes non-enzymatic conversion into arteannuin B and related compounds rather than directly forming artemisinin [25, 35].

Artemisinic aldehyde Δ11(13) reductase (DBR2) catalyzes the reduction of artemisinic aldehyde to dihydroartemisinic aldehyde [25], which is the subsequent stage in the production of artemisinin [36]. As shown in Figure 7, aldehyde dehydrogenase 1 (ALDH1) subsequently oxidizes dihydroartemisinic aldehyde to dihydroartemisinic acid, which subsequently goes through a non-enzymatic conversion to artemisinin [33, 36]. The enzyme dihydroartemisinic aldehyde reductase 1 (RED1), which converts dihydroartemisinic aldehyde to dihydroartemisinic alcohol, was discovered by Rydén et al. [37]. Although the exact function of RED1 in the biosynthesis of artemisinin is yet unknown, research indicates that Artemisia annua may produce more artemisinin if RED1 is silenced.

Artemisinin is primarily synthesized in glandular secretory trichomes (GSTs), and as the plant ages, its accumulation decreases. According to Olofsson et al. [38], Artemisia anna has GSTs in all aerial tissues but not in roots or hairy roots. The highest density of GSTs is seen in flower buds and young leaves, progressive decreasing as the leaves age [25].

6. Antimalarial mechanism of artemisinin/mechanism of action of artemisinin

Artemisinin and its derivatives exhibit potent and rapid antimalarial activity by reducing malaria parasite levels and alleviating symptoms. They act by disrupting the erythrocytic stage of the parasite’s life cycle, particularly inhibiting merozoite formation in red blood cells. The activation of artemisinin relies on the cleavage of its endoperoxide bridge, leading to the generation of reactive free radical species. Two primary activation pathways have been proposed: the mitochondrial pathway and the heme-mediated degradation pathway [39, 40].

6.1 Bioactivation of artemisinin in parasite

A number of protease enzymes in the malaria parasite’s host break down hemoglobin, releasing peptides and amino acids necessary for the parasite’s growth and freeing up room in the digesting vacuole. Hematine accumulates during this process, which can be quite harmful to the parasite, as shown in Figure 8 [42]. To mitigate this threat, the parasite has developed a detoxification mechanism wherein hematin undergoes biomineralization, forming insoluble and non-toxic hemozoin, commonly referred to as malaria pigment.

Detoxification of hemoglobin: The malaria parasite transforms toxic hematin, which is created when heme monomers hydrogen bond [42], into hemozoin, an insoluble, non-toxic substance. Recent studies suggest that during hemozoin formation, the propionate group of each Fe(III) PPIX molecule coordinates with the Fe(III) center of an adjacent molecule, stabilizing the crystalline structure and reducing toxicity [43].

6.3 Heme-mediated pathway

In the heme-mediated activation pathway, two models have been proposed: the reductive scission model and the open peroxide model, both of which lead to the formation of an active carbon-centered radical [41, 49]. Additionally, while some studies suggest that heme plays a major role in artemisinin activation, outweighing the contribution of Fe²⁺ ions, despite some studies suggesting that non-heme Fe²⁺ ions can bind to and activate artemisinin [50, 51]. As shown in Figure 9, heme is generated in Plasmodium species by hemoglobin breakdown at the trophozoite stage and endogenous production during the early ring stage. However, the amount of heme produced by hemoglobin digestion is much greater than that produced by endogenous heme, indicating that hemoglobin-derived heme plays a dominating role in artemisinin activation [51, 53].

Hemozoin, a parasite pigment, is deposited within the food vacuole following hemoglobin digestion. Although the plasmodium stages are most vulnerable to artemisinin action, which happens too early in development to show visible pigment, despite the fact that it has long been suggested that artemisinins target it [54, 55]. In infected erythrocytes, excess heme is converted into hematin, a toxic molecule capable of inducing oxidative damage and lysing cell membranes [56]. To neutralize hematin toxicity, malarial parasites employ a detoxification mechanism that converts hematin into inert, crystallized hemozoin through a biocrystallization process [43]. Activated artemisinin has been reported to inhibit hemozoin formation by alkylating heme, functioning similarly to other antimalarial drugs that target hemozoin synthesis, such as chloroquine (CQ) [57, 58, 59, 60]. Consequently, free heme from hemoglobin digestion serves as both the activator and target of artemisinin, reinforcing its potent antimalarial effect [57].

In in vitro studies, the endoperoxide bridge of artemisinin is proposed to be activated by ferrous iron, generating oxygen- or carbon-centered free radicals, which subsequently alkylate heme. Since iron is a key component of hemozoin, the digestion of hemoglobin by the parasite is believed to make them particularly vulnerable to locally activated artemisinins. Heme and iron produced by hemozoin can activate artemisinin to produce free radicals. The activated artemisinin disrupts the physiological functions of Plasmodium by targeting proteins, lipids, and nucleic acids, leading to the parasites’ death. The asterisk signifies the activated form of ART.

6.4 The intra-erythrocytic parasite and proposed targets of artemisinins

Plasmodiun falciparum replicates within red blood cells, relying on hemoglobin digestion for survival during its 48-hour asexual life cycle (Figure 10) [61]. Artemisinin have long been proposed to target the parasite’s hemoglobin digestion process within the ‘food vacuole’ (Figure 10b). Additionally, studies suggest that artemisinins may also act on the parasite mitochondrion, the translationally controlled tumor protein (TCTP), and PfATP6, parasite-encoded sarcoplasmic-endoplasmic reticulum calcium ATPase (SERCA).

The intricate life cycles of parasites that cause human malaria involve three cycles of asexual reproduction and one cycle of sexual reproduction, and they depend on both human hosts and mosquito vectors. Within the host’s red blood cells, one of the asexual stages takes place (Figure 10a). Invasive forms of the parasite, known as merozoites, enter red blood cells and remain relatively metabolically inactive for 10–15 hours during the ring stage. This is followed by a rapid growth phase over the next 25 hours, forming the trophozoite stage, during which the parasite digests most of the host cell’s hemoglobin and expands to occupy more than 50% of the cell’s volume. Hemoglobin digestion occurs within a specialized organelle called the food vacuole (Figure 10b), leading to the formation of heme. As heme is generated, it associates through one of its peripheral carboxyl groups with the Fe³⁺ of an adjacent heme molecule, forming insoluble hemozoin. It has been suggested that a protein called histidine-rich protein II aids in this process, albeit this has not been proved. The parasite goes through several rounds of division during the schizont stage after the trophozoite stage. The cycle is continued when the host cell lyses 48 hours after invasion, releasing freshly produced merozoites.

Over the years, it has been suggested that artemisinins target a variety of pathways, some of which may not require activation by Fe2+ (Figure 10b). These include the endoplasmic reticulum-localized calcium pump known as PfATP6, the mitochondrion, the translational controlled tumor protein (TCTP), and the heme detoxification route.

7. Artemisinin resistance

Clinically, artemisinin resistance is characterized by the delayed clearance of malaria parasites from the bloodstream following treatment with artemisinin derivatives or artemisinin-based combination therapies (ACTs) [62]. The delayed clearance of malaria parasites following artemisinin treatment is more precisely termed “partial resistance”, as it is restricted to specific timeframes and stages within the parasite’s life cycle. Research indicates that resistance mechanisms predominantly impact the ring stage, allowing parasites to temporarily withstand drug exposure before progressing through later developmental stages [63]. The emergence of partial artemisinin resistance is worrisome, as it could progressively extend to other stages of the parasite’s life cycle. If this evolution continues, it may ultimately result in complete resistance, compromising the efficacy of artemisinin-based treatments.

To mitigate the risk of artemisinin resistance, it is essential to use a potent partner drug in artemisinin-based combination therapies (ACTs). Furthermore, careful evaluation of the ACT partner drug’s efficacy is necessary to minimize the likelihood of treatment failure [49, 50]. Resistance to ACT partner drugs has been detected in the Greater Mekong Subregion (GMS) [51]. Effective management of drug resistance necessitates continuous monitoring of the efficacy of both artemisinins and their partner drugs, alongside a thorough investigation of the underlying resistance mechanisms. Mutations in the K13 gene, along with alterations in other associated genes, have been identified as key contributors to the emergence of artemisinin resistance in Plasmodium parasites (Figure 11).

The PfK13 mutation decreases the endocytosis of host hemoglobin, which lowers hemoglobin catabolism levels and inhibits the activation of artemisinin medications. Furthermore, the K13 mutation improves DNA repair processes and reduces protein damage, increasing resistance to artemisinin. Other mutations, including Pfcoronin, AP-2μ, and UBP1, may further inhibit artemisinin activation, thereby reducing its parasiticidal effect.

8. Therapeutic properties of artemisinin and its derivatives

Ongoing research has led to the widespread clinical adoption of artemisinin compounds, extending their use beyond malaria treatment. These compounds have shown promising potential in treating other parasitic diseases, as well as in combating tumors, inflammation, and various other medical conditions [64, 65, 66, 67].

9. Conclusions

Adequate knowledge and further research on the biological activities of Artemisia annua, particularly its key constituents such as artemisinin and other crucial metabolites, will provide deeper insights into their therapeutic applications and potential medical benefits. Alongside improved diagnostics and effective vector control measures, artemisinin-based combination therapies (ACTs) have the potential to significantly reduce the burden of malaria in tropical regions. However, to maximize their impact, it is crucial to ensure greater affordability and accessibility. Despite its widespread use, the precise antimalarial mechanism of artemisinin and its derivatives remains unclear, creating challenges for drug development, clinical applications, and malaria control efforts. Further research is crucial to fully understand how artemisinin exerts its effects against malaria. Notably, artemisinin has shown superior antimalarial efficacy compared to many other plant-derived secondary metabolites, highlighting its unique pharmacological properties. Furthermore, extensive research has revealed the potential of artemisinin and its derivatives in managing metabolic disorders and obesity-related diseases. Beyond their well-established antimalarial effects, these compounds exhibit diverse biological activities, including anti-proliferative, anti-angiogenic, antifungal, anti-helminthic, anti-protozoal, anti-tumor, and antibacterial properties. Their broad therapeutic potential continues to drive interest in their application across various medical fields. Key derivatives of artemisinin, including dihydroartemisinin (DHA), artesunate, artemisone, artemisinin, artemiside, artemether, and arteether, have all been identified in Artemisia annua. Future research should prioritize the development of novel pharmaceutical formulations incorporating targeted transport systems. These advancements could enhance the anticancer efficacy of artemisinin and its derivatives, opening new avenues for therapeutic applications and treatment strategies across various medical fields.

Acknowledgments

SP wants to thank MT for the concept of the content. Both the authors are grateful to IntechOpen and Dr. Yash Gupta for giving us the opportunity.

Conflict of interest

The authors declare no conflict of interest.

Abbreviations