Pathways

Amodiaquine, a 4-aminoquinoline similar to chloroquine in structure and activity, is an antimalarial drug. It has also been used as an anti-inflammatory agent. Amodiaquine is at least as effective as chloroquine, and is effective against some chloroquine-resistant strains, although resistance to amodiaquine has been reported. 4-Aminoquinolines depress cardiac muscle, impair cardiac conductivity, and produce vasodilatation with resultant hypotension. They depress respiration and cause diplopia, dizziness and nausea. The mechanism of action of Amodiaquine is not certain, but like other quinoline derivatives, it is thought to inhibit heme polymerase activity. This results in the accumulation of free heme, which is toxic to the parasite. The drug binds to the heme which prevents the parasite from converting it to a less toxic form. This complex is toxic and disrupts membrane function. This eventually causes parasite death.

Artenimol is an artemisinin derivative and antimalarial agent used in the treatment of uncomplicated Plasmodium falciparum infections. It is used in combination with Piperaquine. Artemisinins, including Artenimol which is a major active metabolite of many artemisinins, are believed to bind to haem within the P. falciparum parasite. The source of this haem varies with the life stage of the parasite. When the parasite is in the early ring stage artemisinins are believed to bind haem produced by the parasite's haem biosynthesis pathway. In later stages artemisinins likely bind to haem released by haemoglobin digestion. Once bound to haem, artemisinins are thought to undergo activation involving ferrous iron via reductive scission which splits the endoperoxide bridge to produce a reactive oxygen. This reactive oxygen is thought to undergo a subsequent intramolecular hydrogen abstraction to produce a reactive carbon radical. The carbon radical is believed to be the source of the drugs potent activity against P. falciparum by alkylating a wide array of protein targets. The nature and magnitude of the effect on specific protein function as a result of this alkylation is unknown. One target which has been the focus of research is the sarco/endoplasmic reticulum Ca2+ ATPase pump of P. falciparum. Artemisinins have been found to irreversably bind to and inhibit this protein at a binding site similar to that of Thapsigargin.

Artesunate is artesunate is an artemesinin derivative indicated for the initial treatment of severe malaria. Artesunate is an artemisinin derivative that is metabolized to DHA, which generates free radicals to inhibit normal function of Plasmodium parasites. Artesunate is metabolized in the liver to the active DHA. DHA enters the Plasmodium parasite. Once in the parasite iDHA reacts with heme, which generats free radicals which inhibit protein and nucleic acid synthesis of the Plasmodium parasites. Reactions with these free radicals can also lead to alkylation of parasitic proteins such as a calcium adenosine triphosphatase and EXP1, a glutathione S-transferase.

The electron transport chain in mitochondria leads to the transport of hydrogen ions across the inner membrane of the mitochndria, and this proton gradient is eventually used in the production of ATP. Electrons travel down a chain of electron carriers in the inner mitochondrial membrane, ending with oxygen. The outer membrane of the mitochondrion is permeable to ions and other small molecules and nothing in this pathway requires a specific transporter to enter into the intermembrane space. However, the inner membrane is only permeable to water, oxygen and carbon dioxide, and all other molecules, including protons, require transport proteins. Phosphate is able to enter the mitochondrial matrix via the glucose-6-phosphate translocase, and ADP is able to enter the matrix as ATP leaves it via the ADP/ATP translocase 1 protein. Electrons donated by NADH can enter the electron transport chain as NADH dehydrogenase, known as complex I, facilitates their transfer to ubiquinone, also known as coenzyme Q10. As this occurs, the coenzyme Q10 becomes reduced to form ubiquinol, and protons are pumped from the intermembrane space to the matrix. Lower energy electrons can also be donated to complex II, which includes succinate dehydrogenase and contains FAD. These electrons move from succinic acid to the FAD in the enzyme complex, and then to coenzyme Q10, which is reduced to ubiquinol. Throughout this, succinic acid from the citric acid cycle is converted to fumaric acid, which then returns to the citric acid cycle. This step, unlike the others in the electron transport chain, does not result in any protons being pumped from the matrix to the intermembrane space. Regardless of which complex moved the electrons to coenzyme Q10, the cytochrome b-c1 complex, also known as complex III, catalyzes the movement of electrons from ubiquinol to cytochrome c, oxidizing ubiquinol to ubiquinone and reducing cytochrome c. This process also leads to the pumping of hydrogen ions into the intermembrane space. Finally, the transfer of electrons from the reduced cytochrome c is catalyzed by cytochrome c oxidase, also known as complex IV of the electron transport chain. This reaction oxidizes cytochrome c for further electron transport, and transfers the electrons to oxygen, forming molecules of water. This reaction also allows protons to be pumped across the membrane. The proton gradient that is built up through the electron transport chain allows protons to flow through the ATP synthase proteins in the mitochondrial inner membrane, providing the energy required to synthesize ATP from ADP.

Atovaquone is an hydroxynaphthoquinone antimicrobial indicated for the prevention and treatment of Pneumocystis jirovecii pneumonia (PCP) and for the prevention and treatment of Plasmodium falciparum malaria. Atovaquone is a highly lipophilic drug that closely resembles the structure [ubiquinone]. Its inhibitory effect being comparable to ubiquinone, atovaquone can act by selectively affecting mitochondrial electron transport and parallel processes such as ATP and pyrimidine biosynthesis in atovaquone-responsive parasites. The mechanism of action against Pneumocystis carinii has not been fully elucidated. In Plasmodium species, the site of action appears to be the cytochrome bc1 complex (Complex III). Several metabolic enzymes are linked to the mitochondrial electron transport chain via ubiquinone. Inhibition of electron transport by atovaquone will result in indirect inhibition of these enzymes. The inhibition of the electron transport chain prevents the release of energy, therefore it inhibits nucleic acid and ATP synthesis. This eventually leads to cell death..

Chloroquine is an antimalarial drug used to treat susceptible infections with P. vivax, P. malariae, P. ovale, and P. falciparum. It is also used for second line treatment for rheumatoid arthritis. Chloroquine inhibits the action of heme polymerase, which causes the buildup of toxic heme in Plasmodium species, preventing the conversion of heme to hemazoin. Plasmodium species continue to accumulate toxic heme, killing the parasite. Chloroquine passively diffuses through cell membranes and into endosomes, lysosomes, and Golgi vesicles; where it becomes protonated, trapping the chloroquine in the organelle and raising the surrounding pH. The raised pH in endosomes, prevent virus particles from utilizing their activity for fusion and entry into the cell.

Halofantrine is an synthetic antimalarial used for the treatment of severe malaria. It is effective against multi drug resistant (including mefloquine resistant) P. falciparum malaria. The mechanism of action of Halofantrine may be similar to that of chloroquine, quinine, and mefloquine; by forming toxic complexes with ferritoporphyrin IX that damage the membrane of the parasite. The plasmodium falciparum invades the erythrocytes in blood. Halofantrine accumulates in the parasite’s food vacuole and inhibits the enzyme heme ligase. Heme ligase is involved in hemoglobin breakdown. Hemoglobin from the erythrocyte is broken down in the digestive vacuole of the parasite. Hemoglobin is first broken down into heme and globin. Globin is further broken down to amino acids which are used by the parasite for nutrition and protein synthesis. Therefore, hemoglobin breakdown is essential for the parasite survival. The heme from hemoglobin is toxic to the parasite and is further broken down by heme ligase to detoxify heme. By halofantrine inhibiting heme ligase, there is a build up of heme in the parasite vacuole which becomes toxic to the parasite, thereby killing it.

Hydroxychloroquine is an antimalarial medication used to treat uncomplicated cases of malaria and for chemoprophylaxis in specific regions. Also a disease modifying anti-rheumatic drug (DMARD) indicated for treatment of rheumatoid arthritis and lupus erythematosus. Hydroxychloroquine is an aminoquinoline like chloroquine. Hydroxychloroquine is not effective against malaria in areas where chloroquine resistance has been reported. Hydroxychloroquine is also used for the prophylaxis of malaria in regions where chloroquine resistance is unlikely. It was developed during World War II as a derivative of quinacrine with less severe side effects. The FDA emergency use authorization for hydroxychloroquine and chloroquine in the treatment of COVID-19 was revoked on 15 June 2020. The exact mechanisms of hydroxychloroquine are unknown. It has been shown that hydroxychloroquine accumulates in the lysosomes of the malaria parasite, raising the pH of the vacuole. This activity interferes with the parasite's ability to proteolyse hemoglobin, preventing the normal growth and replication of the parasite. Altering the pH of the lysozomes also reduces low affinity self antigen presentation in autoimmue diseases and interferes with the ability of plasmodia to proteolyse hemoglobin for their energy requirements. It can also inhibit heme polymerase which causes an accumulation of toxic heme or ferriprotoporphyrin IX in the vacuole of the malaria cell. This leads to cell and parasite death.

Lumefantrine is an antimalarial agent used in combination with artemether for the treatment of acute uncomplicated malaria caused by Plasmodium falciparum and unidentified Plasmodium species, including infections acquired in chloroquine-resistant areas. It is thought that administration of lumefantrine with artemether results in cooperate antimalarial clearing effects. Artemether has a rapid onset of action and is rapidly cleared from the body. It is thus thought to provide rapid symptomatic relief by reducing the number of malarial parasites. Lumefantrine has a much longer half life and is believed to clear residual parasites. The exact mechanism by which lumefantrine exerts its antimalarial effect is unknown. However, available data suggest that lumefantrine inhibits the formation of β-hematin by forming a complex with hemin and inhibits nucleic acid and protein synthesis, as well as causing the accumulation of toxic heme.

Piperaquine is an antimalarial agent first synthesized in the 1960's and used throughout China. Its use declined in the 1980's as piperaquine resistant strains of Plasmodium falciparum appeared and artemisinin derivatives became available. It has come back into use in combination with the artemisinin derivative Artenimol as part of the combination product Eurartesim. The mechanism of piperaquine inhibition of the haem detoxification pathway is unknown but is expected to be similar to that of Chloroquine.