The next generation of malaria treatments: the great expectations

“ The specificity of the candidate compound treating malaria should be based on the biological specific characteristics of this parasite. ”

parasite. Some argue that fragments or parts of hemozoin may interact with these drugs, partially inhibiting parasite reproduction and; therefore, relieving the symptoms of malaria [5].
We have to point out here that one of the most important strategies to combat infection with the malaria parasite is to stop hemozoin crystallization. There are many studies involved in describing the mechanism of action of these drugs. New generations of these drugs are being developed, which we will discuss later in this article.
These drugs have earned great popularity for various reasons, the most significant of which are their high antimalarial efficacy, low toxicity and low cost. However, despite this, there are harmful side effects for some of these drugs, particularly chloroquine. More crucially, in my opinion, the ambiguity still surrounds the mechanism of action of this unique drug. Especially, since it is used as well to treat other human diseases.
Chloroquine is an old treatment not only for malaria but also for some autoimmune diseases, and it has been used partially to treat cancer [6]. Additionally, it was also used to treat SARS-CoV-2 infections in the 2020 pandemic [7]. For all these life-threatening diseases, the mechanism of action of chloroquine is still controversial. In malaria, the suggested mechanism of action is interference with hemozoin crystallization. Those who support this mechanism used physiochemical approaches to prove the high affinity of chloroquine to hemozoin which is supported by the relative efficiency of chloroquine against Plasmodium spp. However, this suggested mechanism does not answer the question: how does a poorly lipid-soluble chloroquine reach and inhibit hemozoin formation inside the digestive vacuole of the parasite which is filled with lipids and covered by the lipid layers?
In addition, if the interference of hemozoin crystallization is the correct mechanism; what about the progressively resistant strains of Plasmodium (the first reports of such resistance were since 1950)?
The answer was that resistant strains can remove the chloroquine drug from the digestive vacuole using a transmembrane pump. Resistant strains are designated as P. falciparum chloroquine resistance transporter (PfCRT) gene, ABC transporter P. falciparum multidrug resistance (PfMDR1) gene and altered chloroquine-transporter protein, CG2 has been associated with chloroquine resistance. Subsequently, a treatment combination of chloroquine with other drug agents has been proposed to avoid this resistance [8,9]. This proposed mechanism is not the only one, among other suggested mechanisms: inactivation of hemepolymerase, weakening hydrogen bonds in DNA, impairment of metabolic processes, damage of the parasite's biological membranes, inhibition of DNA and RNA polymerase of the parasite or an anti-inflammatory role [10,11].
Increased resistance to chloroquine among other factors is a great motivation to search for a more ideal treatment. There are already some similar compounds such as artemisinin, mefloquine or ferroquine, that work hypothetically as chloroquine against heme biocrystallization. But the challenge here is to change the approach (targeting hemozoin) instead of searching for similar chloroquine derivatives.
Sooner or later, the classical treatments for malaria will be less efficient or ineffective. The level of prevention such as vaccination and curation is on the way to producing the next generation of malaria treatment. All treatments or vaccines of course are based on a deep knowledge of the parasite life cycle, interaction with the immune system, hemozoin nucleation mechanisms and Plasmodium spp. interspecies capabilities resistant against chloroquine and similarly acting compounds.
Let us now briefly review the most promising vaccines and drugs that are being used or those in the process of being used against malaria. At present, many vaccines are based on live, attenuated P. falciparum parasites (sporozoites), obtained from the salivary glands of mosquitoes. The attenuated (chemically, genetically or by radiation) P. falciparum sporozoite (PfSPZ) is the recent type of these vaccines. The PfSPZ vaccine consists of active, attenuated sporocysts that migrate to liver cells where CD8 + T cells that produce IFNγ are activated. The frequency of PfSPZ-specific CD3 + CD4 + , CD3 + CD8 + and CD3 + γδ T cells is dose-dependent [12]. The successful vaccine should be efficient and durable and pass a series of clinical trials. Developed by the company Sanaria , the results were safe and extremely tolerated and efficient. At the 6 month follow-up, vaccine efficacy was up to 48%. At the 18 month follow-up, vaccine efficacy was up to 46% and complete protection was observed after 10 weeks with three doses of PfSPZ-CVac [13].
Another option in malaria vaccination is the use of recombinant protein as a stimulant to the immune system, mostly this type of recombinant protein is used with adjuvant stimulants [14]. The problem with this parasite is its wonderful ability to evade the immune system through a variety of mechanisms. The human immune system produces many types of antibodies against malaria, but these defense tools are not sufficient despite many repeated infections. Thus, choosing the perfect antigen is critical to make immune system memory to prevent parasite evasion mechanisms. On this base, the recombinant proteins as vaccines are used currently for these purposes. One of the currently proposed antimalarials is ganaplacide It belongs to the class of imidazolopiperazines, Unexpectedly, the exact mechanism is unidentified but suspected to be hemozoin inhibition. In addition to some adverse side effects, certain strains of Plasmodium spp. are resistant to ganaplacide. Due to the partial efficacy of ganaplacide, it is recommended to combine it with lumefantrine, which has been a classic antimalarial drug in China since the 1970s. The commonality of the two compounds is that both are thought to be hemozoin inhibitors.
Cipargamin is another synthetic molecule used as antimalarial, which structurally imitates GNF439, the latter was identified as a P. falciparum ATP4 (PfATP4 protein) inhibitor. With rapid efficacy in killing the parasite, cipargamin could potentially be used as a treatment for malaria soon [15].
Ivermectin is one of the controversial compounds. It has been known for decades as an antiparasitic agent but not specifically for malaria. Now, this compound and its derivatives are being investigated as a prospective antimalarial based on its action by binding the glutamate-gated chloride channel of the parasite tissues causing the channels to open and disrupt the parasite cell membrane's polar stability [16]. But until now, no notable efficacy trials have been done against malaria.
Atovaquone is a hydroxy-1,4-naphthoquinone, which is recognized as an antimicrobial, an anti-parasitic (certainly for Toxoplasma gondii), and is also used as an antimalarial. The mechanism of action is not completely described. But it is thought to inhibit the mitochondrial electron transport chain of the parasite. Usually, atovaquone is used in combination with proguanil (chlorguanide) which acts as an inhibitor of dihydrofolate reductase [17]. GABA-A receptor antagonists are also now under investigation to be an antimalarial drug candidate [18].
An exciting notice of most malaria treatments is that the precise mechanism of action is not perfectly understood. This lack of understanding presents a challenge in enhancing the efficacy of antimalarial drug candidates. This shortage of knowledge originates from our poor understanding of some biological behavior of Plasmodium itself, certainly the hemozoin biocrystallization. An additional shortage too is our poor understanding of Plasmodium neutrophiles-monocytes interactions. Despite this, drugs such as artemisinin and chloroquine are very old medications, and of course malaria, itself is a well-known disease.
Separately from vaccination, the description of new compounds against malaria depends on experimentation of the disease manifestation not understanding the behavior of Plasmodium and the phenomena related to it. Two phenomena should be elucidated very well before the dream of malaria elimination: the hemozoin biocrystallization and Plasmodium interaction with neutrophils and monocytes. And since we can target the hemozoin nucleation with unique compounds in the infected patients, this will be the best choice for treatment.
Targeting any other parts of the Plasmodium such as ion channels of the parasite or the nerve receptors of parasites will not be specific and consequently result in adverse side effects for the infected persons. The specificity of the candidate compound treating malaria should be based on the biological specific characteristics of this parasite.
The phenomenon of feeding on hemoglobin is not exclusive to Plasmodium, as it is a common feature of many protozoa and arthropods. Advancement has been achieved exploring the hemozoin nucleation mechanisms, but this is not sufficient. It is necessary to study this phenomenon carefully in comparative studies within Plasmodium species and within hemoglobin-feeding parasites, to know the mechanisms of drug-hemozoin binding as a method of inhibiting the growth of the parasite.
What's more, it is important to investigate the exact structural transformations of this parasite in its different life stages in mosquitoes and humans. In future, it will be vital to probe the biological mechanisms of the parasitic apicoplast, as the organelle demonstrates the ability to circumvent hemozoin toxicity? Full comprehension of this process on a molecular level will open up new druggable targets and advance antimalarial drug discovery.

Author contributions
The authors were fully responsible for all content, were involved at all stages of manuscript development and approved the final version.