Chapter 34: Anthelmintic Drugs

ORGANIZATION OF CLASS

These drugs are effective against worms (helminths). In humans, worms may remain within the intestinal lumen or may have complex life cycles that involve movement through the body. The infective form may be either an adult worm or an immature worm.

The worm life cycle is strongly dependent on neuromuscular coordination, energy production, and microtubular integrity. Most antiworm drugs target one of these three areas.

The easiest way to organize these drugs is to consider a reasonable organization of the worms. Helminths (worms) are classified into three groups: cestodes (flatworms), nematodes (roundworms), and trematodes (flukes). If you look at the table of worms and the drug of choice for each worm, several patterns emerge.

DRUGS USED AGAINST CESTODES AND TREMATODES

First, here’s a quick review of the worms. Cestodes are the tapeworms. They are flat and segmented. The head has suckers. Larvae develop into adults in the small intestine. Therefore, treatment can be confined to the small intestine.

The trematodes are the flukes. If you recall, the flukes move about the body; there are blood flukes and liver flukes, and so on. Therefore, the treatment needs to reach the systemic circulation in order to affect the fluke.

The mechanism of action of praziquantel is unknown. It is postulated to alter membrane function of the worm and increase membrane permeability. It is absorbed after oral administration. That’s why it can have an action on the trematodes (flukes) that cause schistosomiasis.

DRUGS USED AGAINST NEMATODES

The nematodes are a more diverse set of worms. Overall, they are the roundworms because they are elongated and cylindrical (round). This group includes whipworm, pinworm, and hookworm. In the United States, the most common helminth infections are ascaris lumbricoides, ancylostoma duodenale (roundworm), and necator americanus (hookworm). Most patients with nematode infections can be treated using mebendazole or pyrantel. A special group of nematodes can be considered separately: the filaria. Patients with filariasis are treated using two other drugs.

These drugs bind to β-tubulin and inhibit tubulin polymerization, which disrupts motility and replication. They can be given orally, and very little is absorbed from the gastrointestinal (GI) tract.

DRUGS USED AGAINST FILARIA