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Antiparasitic Drugs: Types and Mechanisms

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  • 0:05 Parasites: Protozoans…
  • 1:01 Antimalarial Drugs
  • 2:21 Other Antiprotozoan Drugs
  • 3:23 Antihelminthic Drugs
  • 4:11 Lesson Summary
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Lesson Transcript
Instructor: Katy Metzler

Katy teaches biology at the college level and did her Ph.D. work on infectious diseases and immunology.

Parasites can cause nasty infections, but they are actually very similar to our own cells. In this lesson, learn about the types of drugs commonly used to eradicate eukaryotic pathogens.

Parasites: Protozoans and Worms

Parasites, such as protozoans and worms, or helminths, are some of the freakiest pathogens around. Is it because they're bigger than bacteria and can look like miniature animals that they're so gross? Is it because just the idea of worms living in our body is disgusting? Or, is it the terrible symptoms they can cause, such as severe intestinal problems, skin lesions and major damage to organs such as the brain, heart, liver, eyes and lungs?

At any rate, we don't want parasites in our bodies. So, we can kill them off with drugs, right? Since protozoans and worms are eukaryotes like us, there are limited options for treating parasitic infections without harming the host too much. In this lesson, we'll learn about some of the most common antiparasitic drugs and how they work. You're going to hear a lot of drug names in this lesson. For many of you, it may be more important just to learn the general ways that parasites can be treated rather than remember all of the drug names.

Antimalarial Drugs

You've probably heard of malaria, an important disease caused by a protozoan parasite called Plasmodium. Malaria is estimated to affect 300 to 500 million people in the world and is especially prevalent in Africa, where a child dies about every 30 seconds from this devastating disease.

The most widely-used and cheapest drug for malaria is Chloroquine, which is derived from a plant-based traditional medicine called quinine. Chloroquine causes the buildup of a toxic molecule called heme inside the parasite, and it is also thought to block the parasite's DNA synthesis. However, nowadays, there is widespread resistance to this drug, so alternatives are needed.

One alternative to chloroquine is Artemisinin Combination Therapies (ACTs). Artemisinin had been used for a long time in traditional Chinese medicine to control fevers before it was discovered to be effective against malaria. It inhibits the parasite's nucleic acid and protein synthesis, essentially shutting down the factories that produce the molecules the parasite needs to reproduce. ACTs are expensive compared to chloroquine, which has unfortunately led to cheap counterfeit versions that contain very little of the actual drug. The main result of these counterfeit drugs is to increase resistance to ACTs, which is very bad news for the future of malaria treatment.

Other Antiprotozoan Drugs

Have you ever heard of Giardia? This waterborne parasite causes severe intestinal disease, as does the related parasite Cryptosporidium. These two bugs are particularly problematic because they are somewhat resistant to chlorine, so they can sometimes contaminate drinking water and swimming pools. Amoebas can also cause a nasty intestinal disease called amebic dysentery.

There are a few drugs that are commonly used to treat these kinds of protozoan diseases. The first one, Metronidazole, fights protozoans by damaging their DNA and inhibiting DNA synthesis. It is selectively toxic because it's a pro-drug, meaning that it starts off as a harmless molecule that must be converted into an active drug. Lucky for us, this conversion doesn't happen in our own cells, only inside of protozoans and also anaerobic bacteria.

Nitazoxanide is another antiprotozoan drug that disrupts the anaerobic metabolism that these parasites use to make energy, so treatment with this drug is like turning off the parasite's electricity source.

Antihelminthic Drugs

Next, we'll talk about the drugs that are used to treat helminth infections, that is, parasitic worms. Niclosamide is a drug that pulls the plug on worms' energy source by stopping production of ATP, a major energy carrier in all cells. Mebendazole also cuts off worms' energy, this time by preventing them from absorbing nutrients from their environment. Can you imagine if a drug prevented you from eating? You wouldn't last long, would you?

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