Antiparasitics are a group of compounds that kill parasitic organisms or inhibit their growth. Parasites can be divided into unicellular parasites (protozoa) and multicellular parasites (helminths, arthropods) according to their structure. Protozoa have intricate life cycles often involving more than one host, while helminths and arthropods have highly developed organ systems. In general, antiparasitics exert antiparasitic activity by interfering with parasites’ carbohydrate, protein and lipid metabolism, DNA replication, cellular membrane integrity, or motility. Common targets of antiparasitic agents include genes, cytoskeleton proteins, intracellular signal transduction proteins, cell membrane components and enzymes involved in intermediate metabolism.
Antiprotozoal agents include amodiaquine, amphotericin B, antifolates (such as proguanil, pyrimethamine, and trimethoprim), artemisinin and its derivatives, atovaquone, chloroquine, clindamycin, difluoromethylornithine, diloxanide furoate, fumagillin, furazolidone, halofantrine, etc. Artemisinins (such as artesunate, artemether, and dihydroartemisinin) are sesquiterpene lactone endoperoxides that bind to heme and decompose its endoperoxide bridge, leading to the release of toxic free radicals that damage the parasite. Antifolates act at various steps in the folic acid cycle in protozoa and are most used in combination. Chloroquine and other antimalarial quinolones inhibit heme detoxification in parasites.
The compounds against multicellular parasites include salicylanilides, albendazole, bithionol, diethylcarbamazine, flubendazole, ivermectin, mebendazole, metrifonate, oxamniquine, piperazine, praziquantel, pyrantel pamoate, thiabendazole, triclabendazole, etc. Mebendazole is a benzimidazole carbamate with a broad range of anthelmintic activity. It binds to parasite tubulin to block microtubule assembly and to interfere with glucose absorption. Salicylanilides include closantel, niclosamide, oxyclozanide, rafoxanide, and resorantel. They interfere with energy metabolism of parasites by uncoupling oxidative phosphorylation. Pyrantel pamoate belongs to tetrahydropyrimidines whose mechanism of action is similar to levamisole. It acts as cholinergic receptor agonists, inducing spastic paralysis and muscle contracture of parasites and allowing expulsion from their host.
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