Category:Antiparasitics

An antiparasitic is a medicine used to treat infection by a parasitic organism: protozoa, helminths, or ectoparasites. The category is divided by the biology of the target organism into the antiprotozoals, the anthelmintic medicines (which act against the parasitic worms, divided clinically into nematode, cestode, and trematode infections), and the ectoparasitic agents that act on lice, scabies, mites, and fleas on the skin surface. Each sub-category has its own characteristic medicines, its own resistance pattern, and its own clinical-epidemiology profile that reflects the geographic distribution of the parasitic diseases.

The pharmacological history of the category is older than the bacterial-infection one. The Spanish Jesuit missionaries in Peru in the 1630s observed indigenous treatment of fever with the bark of the cinchona tree (Cinchona officinalis), and the bark, carried to Europe under the name "Jesuit's powder" or "Peruvian bark", was the first effective treatment for malaria. The active alkaloid, quinine, was isolated in 1820 by Pelletier and Caventou in Paris; the same chemists who, the same year, isolated colchicine from autumn crocus and emetine from ipecac. Quinine dominated antimalarial therapy for a century. The synthetic 4-aminoquinoline chloroquine (developed at IG Farben in 1934 as Resochin, then re-developed at the U.S. Army research programme during the Second World War) replaced quinine for routine use in the post-war decades; chloroquine resistance, however, has been progressive in Plasmodium falciparum since the 1960s, and contemporary first-line antimalarial therapy is an artemisinin-based combination treatment. Artemisinin was isolated in 1972 by the Chinese researcher Tu Youyou of Project 523 (the secret antimalarial research programme commissioned by Mao Zedong at the request of Ho Chi Minh during the Vietnam War) from the plant Artemisia annua (sweet wormwood), used in traditional Chinese medicine for fever for two thousand years; Tu received the Nobel Prize in 2015.^[1]

The transformative antiparasitic of the late twentieth century was the avermectin family, isolated by Satoshi Ōmura at the Kitasato Institute in Tokyo from a soil sample sent by William Campbell of Merck in 1974 and developed into the broad-spectrum antiparasitic ivermectin (originally an animal-health medicine, then approved for human use in 1987 as an oral medicine for onchocerciasis and strongyloidiasis).^[2] Ivermectin's clinical use was extended by donation programmes by Merck (the Mectizan Donation Program of 1987 onward, the largest medicine donation programme in history) that have largely eliminated onchocerciasis-caused blindness from large areas of equatorial Africa. Ōmura and Campbell shared the 2015 Nobel Prize in Physiology or Medicine with Tu Youyou. The pyrazinoisoquinoline praziquantel (Bayer, 1980), a similarly transformative anthelmintic against the trematode and cestode parasites (schistosomes, tapeworms), has been the foundation of the World Health Organization's schistosomiasis-control programmes that have substantially reduced the global burden of schistosomiasis.

The other antiparasitic classes complete the category. The benzimidazole anthelmintics (albendazole, mebendazole, thiabendazole), introduced at Smith Kline & French in the 1960s and 1970s, are the standard treatment for the soil-transmitted helminthiases (ascariasis, trichuriasis, hookworm) and for cysticercosis and echinococcosis. The piperazine-derivative pyrantel pamoate is used for pinworm and roundworm in the over-the-counter setting. The nitroimidazoles (metronidazole, tinidazole) are used against the anaerobic protozoa (Trichomonas vaginalis, Entamoeba histolytica, Giardia lamblia), with cross-listing to antibacterials for their anaerobic-bacterial use. The pentavalent antimonials (sodium stibogluconate, meglumine antimoniate) and the second-line miltefosine remain the principal medicines for visceral leishmaniasis, although amphotericin B (a polyene antifungal repurposed) is now first-line in many endemic areas. The arsenicals (melarsoprol) and the newer fexinidazole are used in human African trypanosomiasis. The amebicides paromomycin and iodoquinol treat luminal amebic infection.

The ectoparasitic medicines are a smaller sub-category. Permethrin (a synthetic pyrethroid) is the standard topical agent for scabies and head lice. The ivermectin oral and topical formulations are used for scabies and for rosacea (off-label, on the demodex-mite hypothesis of rosacea). The newer spinosad and abametapir topical preparations have been added for head lice resistant to permethrin and malathion.

The contemporary clinical pharmacology of the antiparasitics is dominated by access and resistance issues. Many of the parasitic diseases are concentrated in low-income tropical settings, and the development and pricing of antiparasitic medicines have historically been driven less by commercial return than by donation programmes, by public-private partnerships (DNDi, the Medicines for Malaria Venture), and by World Health Organization elimination targets. Artemisinin partial resistance has emerged in southeast Asia and is spreading to Africa, threatening the gains of the last twenty years. The eradication of dracunculiasis (guinea worm) is nearing completion through a combination of water filtration and case management rather than through a medicine; the elimination of onchocerciasis and lymphatic filariasis, by contrast, depends on continued ivermectin and albendazole mass drug administration.

• Antiprotozoals:
  • Antimalarials: chloroquine, hydroxychloroquine, mefloquine, primaquine, tafenoquine, the artemisinin-based combination therapies (artemether-lumefantrine, artesunate-amodiaquine, artesunate-mefloquine, dihydroartemisinin-piperaquine), atovaquone-proguanil (Malarone), quinine and quinidine (the latter intravenously for severe malaria)
  • Antitrichomonal, antigiardial, antiamoebic: metronidazole, tinidazole, nitazoxanide, paromomycin, iodoquinol, fumagillin
  • Antileishmanial: liposomal amphotericin B, sodium stibogluconate, meglumine antimoniate, miltefosine
  • Antitrypanosomal: pentamidine, suramin, melarsoprol, eflornithine, nifurtimox, benznidazole, fexinidazole
  • Anti-toxoplasma: pyrimethamine + sulfadiazine, the trimethoprim-sulfamethoxazole alternative
• Anthelmintic (parasitic worms):
  • Nematode: albendazole, mebendazole, pyrantel pamoate, ivermectin for soil-transmitted helminthiases and strongyloidiasis
  • Cestode (tapeworm): praziquantel, niclosamide, albendazole for cysticercosis
  • Trematode (fluke): praziquantel for schistosomiasis and most other fluke infections, triclabendazole for liver fluke
  • Filarial: ivermectin (onchocerciasis), diethylcarbamazine (lymphatic filariasis), doxycycline (which kills the Wolbachia endosymbionts of filariae)
• Ectoparasitic:
  • Scabies: permethrin (topical), ivermectin (oral)
  • Head lice: permethrin, malathion, ivermectin (topical), spinosad, dimethicone, abametapir
  • Body lice and pubic lice: similar agents

The boundary of this category is "medicine that treats infection by a parasitic organism." The antimalarials, although collected as a sub-category here, also serve as immunomodulators (hydroxychloroquine for autoimmune disease) and were historically used for non-parasitic indications; their primary indication is what places them in this category. The antimycobacterials (isoniazid, rifampin, the medicines for tuberculosis and leprosy) treat infections by bacteria, not parasites, and are listed under antibacterials in their conventional classification. Bacteriophage preparations targeting parasites have been investigated but are not in routine clinical use. The medicines for the protozoa Pneumocystis jirovecii (reclassified as a fungus in recent decades but historically treated as a parasite) are listed under antibacterials when the agent is trimethoprim-sulfamethoxazole or atovaquone or pentamidine in this indication.

This category page is an encyclopedia article about its subject. The actual index of medicines belonging to the category is generated automatically by the wiki engine, from category-membership declarations on the individual medicine pages, and appears at the foot of the page below the references.