Generality
Antiparasitic drugs, as you can easily guess, are medicines used in the treatment of parasite infestations.
In this regard, among the most common parasitic diseases, we remember:
- Amebiasis, giardiasis, Chagas disease, leishmaniasis, malaria, sleeping sickness, toxoplasmosis, pneumocystosis and trichomoniasis (all caused by protozoan infestations);
- Helminthiasis caused by nematodes, tapeworms and trematodes;
- Scabies and pediculosis caused by ectoparasites.
Naturally, the type of pharmacological strategy to be implemented to eradicate the infestation varies according to the type of parasite responsible for the disease.
The main antiparasitic drugs used in the treatment of the aforementioned infestations by protozoa, helminths and ectoparasites will be briefly described below.
Treatment of protozoal infestations
As mentioned, protozoan infestations are responsible for the onset of many of the best known parasitic diseases.
The best therapeutic approach to combat this type of infestation remains prevention, which involves compliance with common hygiene rules, avoiding the consumption of water and foods that may be contaminated by the parasite, the use of adequate protective clothing and the use of of insecticidal and / or insect repellent products (which often represent the vectors of parasites).
However, in cases where prevention is not sufficient, early diagnosis and prompt drug therapy are fundamental tools.
The active ingredients most used in the treatment of protozoal infestations will be briefly illustrated below.
In any case, to obtain more detailed information, we recommend reading the dedicated articles already present on this site: "Amebiasis - drugs and treatment of" Amebiasis ";" Giardia - Drugs for the treatment of Giardiasis ";" Malaria - Drugs for the treatment and prevention of Malaria ";" Sleep Sickness - Drugs for the Treatment of Sleeping Sickness ".
Nitroimidazoles
Nitroimidazoles (such as metronidazole and tinidazole) are drugs with antibiotic activity, but they are also particularly effective in fighting protozoal infestations. More specifically, these drugs are used in the treatment of amoebiasis, giardiasis and trichomoniasis.
However, the exact mechanism of action by which metronidazole and tinidazole are able to eradicate parasitic infestations has not yet been fully elucidated.
The most accredited hypothesis is that according to which these active principles act as prodrugs which - once taken - undergo structural changes within the parasite itself. These modifications lead to the synthesis of reactive oxygen species (ROS) which, in turn, damage the parasitic cells and DNA, leading these organisms to certain death.
Pentamidine
Pentamidine (Pentacarinat®) is an antiparasitic drug - more precisely, an antiprotozoal - particularly useful in the treatment of malaria, pneumocystosis, leishmaniasis and African trypanosomiasis.
Furthermore, this active ingredient is also endowed with fungicidal activity.
The mechanism of action through which pentamidine carries out its action involves interaction with the parasite's DNA and RNA.
Atovaquone
Atovaquone (Malarone®, Malarone bambini®) is among the antiparasitic drugs most used in the treatment of malaria, but it has also proved effective in the treatment of pneumocystosis.
The antiparasitic mechanism of this active principle is expressed through the inhibition of the mitochondrial respiratory chain which leads, irreversibly and inexorably, to the death of the parasite.
4-substituted quinolines
Within the class of 4-substituted quinolines we find antiparasitic drugs such as quinine, chloroquine, hydroxychloroquine, halofantrine and mefloquine. All these active principles are antimalarial agents widely used to counteract the parasite responsible for the onset of malaria. .
The mechanism of action by which all these drugs carry out their activity is roughly the same.
More specifically, the host's hemoglobin is transported to the vacuole of the malaria plasmodium, inside which it is digested with the formation of free heme. The latter is toxic to the parasite and, for this reason, immediately undergoes a process of polymerization that transforms it into hemozoin. Quinolines intervene during this phase of the parasite's life cycle.
More precisely, quinolines bind to hemozoin through a heme-drug complex that blocks its polymerization. In doing so, inside the parasite's vacuole there is an ever greater increase in the levels of toxic free heme. The continued increase in these levels will then lead to the death of the parasite itself.
Treatment of Helminthiasis
Helminthiases (or worm infestations, if you prefer) are probably the most common parasitic infestations in the world.
The worms capable of infesting humans can basically be divided into two groups: flatworms (or flatworms, which include cestodes and flukes) and aschelminths (or roundworms, which include nematodes).
Helminths represent a vast category of parasites that differ in size, life cycle and site of infestation.
For this reason, the drugs used to combat them can be, from a chemical point of view, very different from each other and carry out their action with different mechanisms of action.
Benzimidazoles
Albendazole (Zentel®), mebendazole (Vermox®) and thiabendazole belong to this class of drugs.
These active ingredients are broad spectrum antiparasitic drugs, particularly effective in the treatment of intestinal helminth infestations.
Benzimidazoles seem to exert their antiparasitic action through two different mechanisms of action.
These molecules are, in fact, able both to bind to the parasite's tubulin, preventing its polymerization and thus hindering cell division, and to inhibit fumarate reductase, a fundamental enzyme for the metabolic processes of the same helminths. Thanks to this double mechanism of action, benzimidazoles lead the parasite towards certain death.
Praziquantel
From a chemical point of view, praziquantel is an isoquinoline derivative which is particularly effective against cestodes and trematodes (flat worms), but inactive against nematodes.
This active principle exerts its activity through various mechanisms of action. More precisely, praziquantel is able to cause the contraction and paralysis of the parasite (thus facilitating its expulsion from the organism), it is capable of affecting its deposits of glycogen and its energy balance, and is capable of causing damage to the helminth's integuments, with consequent activation of the immune response by the host's antibodies which ultimately leads to the death of the parasite itself.
Treatment of ectoparasite infestations
Among the ectoparasites that most commonly infest humans, we find lice (both of the head, pubis and body) and Sarcoptes scabiei (or itch mite), responsible for the onset of scabies.
The antiparasitic drugs used for the treatment of the aforementioned parasites are roughly the same.
These drugs will be briefly explained below. However, to obtain more information about the treatment of lice and scabies, please refer to the dedicated articles "Lice - Drugs to Eliminate Lice" and "Scabies - Drugs for the Treatment of Scabies".
Permethrin
Permethrin (Nix®) is a pesticide belonging to the category of synthetic pyrethroids. The latter are synthetic molecules with a chemical structure similar to pyrethrum, a natural insecticide contained in some plants belonging to the family of Asteraceae.
Both in the treatment of pediculosis and in the treatment of scabies, permethrin is available in the form of pharmaceutical formulations for topical use.
Permethrin exerts its action by acting as a neurotoxin that blocks the opening of the sodium channels present on the membranes of excitable cells.
By doing so, permethrin is able to increase the opening time of the aforementioned sodium channels, leading to nerve paralysis and death of the parasites.
Malathion
Malathion (or malathione) is one of the most widely used antiparasitic drugs in the treatment of lice and itch mite infestations. More specifically, malathion is an organophosphate insecticide with low toxicity for humans, but extremely toxic for the parasites in question.
The mechanism of action through which this active principle is able to cause the death of lice and itch mites provides for the irreversible inhibition of acetylcholinesterase, the enzymes responsible for the metabolism of acetylcholine (a neurotransmitter of vital importance for parasites , as for man).
Therefore - similarly to what has been said for permethrin - malathion also exerts its antiparasitic activity by behaving as a sort of neurotoxin.
Malathion is available in topical pharmaceutical formulations and is effective in killing both parasites and their eggs.
