Antihelmintic Drugs

Helminth infections are a major problem in the lives of many millions of people on the earth, especially in the subtropics. Depending on the type and localization of the causative agent of helminthosis, it can run asymptomatic, or it can be the cause of anemia, or damaged blood vessels, liver, or eyes. There is intestinal and nonintestinal helminthosis. The causative agents are classified as roundworms (nematodes) and flatworms. These groups include tapeworms (cestoda) and fluke worms (trematoda). Most nematode infections are localized in the intestinal tract, although a few of them can pass into other organs, including the heart, liver, lungs, muscles, and so on, from which removal is significantly harder. Cestode infections are usually localized in the gastrointestinal tract, but there have been cases of them passing into the circulatory system. Trematodes cause chronic infection, called schistosomiasis, in which the blood vessels are attacked and various organ structures (liver, intestines, urinary tract) are damaged.

Antihelmintic drugs are intended for exterminating helminthes and removing them from the host organism. Examples include albendazole, diethylcarbamazine, mebendazole, nicolsamide (against tapeworms), suramin, and thiabendazole. Many members of the piperazine family are successful anthelmintics. Natural anthelmintics include black walnut, wormwood (Artemisia absynthium), clove (Syzygium aromaticum), tansy tea (Tanacetum vulgare), and the male fern (Dryopteris filix-mas). They are subdivided into drugs that damage neuromuscular coordination of helminthes, drugs that have an effect on the energetic processes of helminthes (in particular on the metabolism of glucose), and drugs that affect the enzymatic system, laying of eggs by helminthes, and so on. Most anti-helmintic drugs are intended to have an effect on certain helminthes. Historically, halo-genated carbohydrates, naphthoquinones, phenothiazine, a number of natural compounds isolated from leaves of sagebrush and ferns, ether oils (derivatives of pinene), alkaloids (arecoline group), alkaloids of the emetine group, and many others were used to treat helminthosis. However, the currently essential drugs for treating helminthosis are those described below.

Mebendazole: Mebendazole, methyl-[5-(benzoyl)-1H-benzoimidazol-2-yl]carbamate (38.1.5), is a derivative of benzoimidazole, which is made by reacting 3,4-diaminobenzo-phenone (38.1.3) with N-methoxycarbonyl-S-methylthiourea (38.1.4) [1,2].

The necessary reagents are made in the following manner. Nitration of 4-chlorobenzo-phenone with nitric acid at a temperature lower than 5°C gives 4-chloro-3-nitrobenzophenone (38.1.1), in which the chlorine atom is replaced with an amino group by heating it to 125°C in a solution of ammonia in methanol to make 4-amino-3-nitrobenzophenone (38.1.2). Reducing the nitro groups in this compound with hydrogen using a palladium on carbon catalyst gives 3,4-diaminobenzophenone (38.1.3).

The second reagent, N-methoxycarbonyl-S-methylthiourea (38.1.4), is made by reacting methyl chloroformate with S-methylthiourea.

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