Questions

What are the drug targets for antiprotazoan agents?

What are examples of different classes of antiprotozoan agents?

There are four major groups of protozoal parasites: sporozoates, flagellates, ciliates and amoebae. Unlike helminths, protozoa are single cell organisms that rapidly divide in the infected host.

Malaria

Malaria is transmitted by the Anopheles mosquito, which carries sporozoites of Plasmodium spp. that infect the human host when a mosquito bites (Fig. 3.48.1). The parasites infect liver cells and begin asexual division before being released into the blood as merozoites, which invade and multiply within red blood cells. Cell lysis releases the parasite into the bloodstream and gives rise to the clinical symptoms of malaria. Alternatively, some merozites differentiate into male and female gametocytes, which reenter a mosquito that bites an infected host and thus continue the parasite’s life cycle. P. falciparum causes the most deaths from malaria. A number of different drugs are available for the treatment of this disease (Table 3.48.1).

Fig. 3.48.1 The life cycle of the malarial parasite and site of action of drugs (see Table 3.48.1).

Table 3.48.1 Action of Antimalarial Drugs

Stage	Effect	Drugs
Merozoites	Block the link between the exoerythrocytic and erythrocytic stages (chemoprophylactic)	4-Aminoquinolines, antifolates, antibiotics, arteminsin
Erythrocytic forms	Can cure P. falciparum forms or P. malariae infections as these have no exoerythrocytic stage; suppress acute attack for P. vivax or P. ovale but the exoerythrocytic forms can cause later relapses	Quinoline-methanols, 4-aminoquinolines,halofantrine,arteminsin derivatives,antibiotics, antifolates
Tissue schizonts	Effect a radical cure by acting on the parasites inthe liver	8-Aminoquinoline (primaquine), antibiotic (tetracycline),antifolates
Gametocytes	Reduce the spread of nfection	8-Aminoquinoline (primaquine), arteminsin derivatives