As with bacteria, most protozoa are usually harmless. However, certain protozoa may be harmful in any amount, and for them the goal of treatment is to achieve a full microbiological cure. (As we will see later, this is different from certain gastrointestinal helminths.)
Cinchona bark was used in Europe for the treatment of malaria beginning in the 1600s. Its active ingredient is a quinoline alkaloid called quinine. Synthesis of new quinolines was stimulated by the interruption of quinine supplies during the World War I and World War II and, after 1961, by the growing impact of drug-resistant falciparum malaria in several areas of the world. Among the most effective agents are those that share the double-ring structure of quinine.
✺ Quinine and quinoline analogs active against malaria
Current analogs fall into three major groups: 4-aminoquinolines (including chloroquine), 8-aminoquinolines (including primaquine), and 4-quinolinemethanols (including mefloquine). All of them selectively destroy intracellular parasites by accumulating in parasitized host cells. Most of these agents appear to inhibit heme polymerase, leading to the buildup of toxic hemoglobin metabolites within the malarial parasite.
Accumulate in parasitized cells, block heme metabolism
Quinine, chloroquine, and mefloquine concentrate in parasitized erythrocytes and rapidly destroy the erythrocytic stage of the parasite that is responsible for the clinical manifestations of malaria. Thus, these agents can be used either prophylactically to suppress clinical illness if infection occurs or therapeutically to terminate an acute attack. They do not concentrate in tissue cells, and thus organisms sequestered in exoerythrocytic sites, particularly the liver, survive and may later reestablish erythrocytic infection and produce a clinical relapse. In contrast, primaquine accumulates in tissue cells, destroys hepatic parasites, and effects a full “radical” cure.
✺ Quinine, 4-aminoquinolines (eg, chloroquine), and 4-quinolinemethanols suppress malarial infection in the human red blood cell
✺ 8-Aminoquinolines (eg, primaquine) effect radical cure by treating the liver
Chloroquine phosphate was the most widely used of the blood schizonticidal drugs for decades. In the doses used for long-term malaria prophylaxis it was remarkably free of untoward effects. Unfortunately, its heavy use led to widespread resistance in Plasmodium falciparum, and thus it is no longer recommended for prevention or treatment of falciparum malaria in most parts of the world (see Resistance below). Primaquine phosphate, the 8-aminoquinoline used to eradicate persistent hepatic parasites, has toxic effects related to its oxidant activity. Methemoglobinemia and hemolytic anemia are particularly frequent in patients with glucose-6-phosphate dehydrogenase deficiency, because they are unable to generate sufficient quantities of the reduced form of nicotinamide adenine dinucleotide to respond to this oxidant stress. Typically, the anemia is severe in patients of Mediterranean and Far Eastern ancestry and mild in patients of African ancestry.
✺ Chloroquine no longer effective against malaria in most regions
✺ Primaquine may have hematologic toxicity
Quinine is the oldest and most toxic of the quinolines. It is currently used primarily to treat strains of drug-resistant P falciparum that are spreading rapidly through Asia, Latin America, and Africa. Quinidine, a less cardiotoxic optical isomer of quinine, is more readily available in the United States and is preferred to quinine when parenteral administration is required. Mefloquine, an oral 4-quinolinemethanol analog, originally displayed a high level of activity against most chloroquine-resistant parasites; however, mefloquine-resistant strains of P falciparum are now widespread in Southeast Asia and are present to a lesser degree in South America and Africa. Concerns regarding psychiatric side effects of mefloquine have been generally overblown, but serve as another reason for the waning use of this medication.
Quinine is active against many chloroquine-resistant malarial strains
Phenanthrene methanols are not in the strict sense quinine analogs. Nevertheless, they are structurally similar to this group of agents and, together with them, were discovered to have antimalarial activity during the World War II. Halofantrine*, the most effective of the group, is a blood schizonticide effective against both sensitive and multidrug-resistant strains of P falciparum. Unfortunately, because of rare cases of fatal heart arrhythmias, it is not available in the United States. A related drug, Lumefantrine, is much safer, but is unreliable when dosed alone. It is always administered as a coformulation with artemisinins (see later).
Phenanthrene methanols active against multidrug-resistant malaria
This natural extract of the plant Artemisia annua (qing hao, sweet wormwood) is a sesquiterpene lactone peroxide that is structurally distinct from all other known antiparasitic compounds. Extracts of qing hao were recommended for the treatment of fevers in China as early as AD 341; their specific antimalarial activity was defined by Chinese investigators in 1971. Although it has also been shown to be active against the free-living amoeba Naegleria fowleri and several trematodes, including Schistosoma japonicum, Schistosoma mansoni, and Clonorchis sinensis, its greatest impact to date has been in the treatment of malaria. Extensive investigations showed it to be schizonticidal for both chloroquine-sensitive and chloroquine-resistant strains of P falciparum. Several derivatives, among them artemether and artesunate, are significantly more active than the parent compound. All are concentrated in parasitized erythrocytes, where they decompose and release free radicals, which are thought to damage parasitic membranes. Artemisinin compounds act more rapidly than other antimalarial agents, stopping parasite development and preventing cytoadherence in falciparum malaria. Because of their relatively short half-life, they should be administered in coformulations with longer-acting agents such as lumefantrine. This “artemisinin combination therapy (ACT)” is so safe and effective that it has become the standard of care for treatment of acute malaria worldwide. Unfortunately, resistance has already been detected, especially among P falciparum isolates from the Thai–Myanmar border. Although depression of reticulocyte counts has been noted, these agents appear significantly less toxic than quinoline antimalarials. Because there is some evidence that they may possess teratogenic properties, they should be avoided in pregnancy. They may be given orally, rectally (by suppository), or parenterally.
Plant derivative active against malaria, amoebas, and Schistosoma
Concentrated in parasitized erythrocytes
✺ ACT now treatment of choice for falciparum malaria
Atovaquone is a novel hydroxynaphthoquinone that shows promise in the treatment of malaria and toxoplasmosis. Its antiparasitic activity appears to result from the specific blockade of pyrimidine biosynthesis secondary to the inhibition of the parasite’s mitochondrial electron transport chain.
Efficacy trials established its capacity to affect rapid clearance of parasitemia in patients with chloroquine-resistant falciparum malaria. Frequent parasitic recrudescences were eliminated when atovaquone was administered in combination with the folate antagonist proguanil (see later). This coformulation (Malarone) is popular in malaria prophylaxis because it is effective, well tolerated, and protects against liver infection, thus can be dosed for just a week following exposure. Subsequently, this agent was shown to be effective for the treatment of toxoplasmosis in patients with acquired immunodeficiency syndrome (AIDS). Unlike other antitoxoplasma agents, atovaquone is active against Toxoplasma gondii cysts as well as tachyzoites, suggesting that this agent may produce radical cure. Supporting this is the infrequency with which cessation of atovaquone treatment of toxoplasmic cerebritis in AIDS patients has resulted in relapse. Relapse after atovaquone treatment of the fungus Pneumocystis jirovecii in this same patient population appears similarly uncommon.
✺ Atovaquone stable and active against malaria and toxoplasmosis
Folic acid is a critical coenzyme for the synthesis of purines and ultimately DNA. In protozoa, as in bacteria, the active form of folic acid is produced in vivo by a simple two-step process. The first step, the conversion of para-aminobenzoic acid to dihydrofolic acid, is blocked by sulfonamides. The second step, the transformation of dihydro- to tetrahydrofolic acid, is blocked by folic acid antagonists, which competitively inhibit dihydrofolate reductase. Used together with sulfonamides, folate antagonists are very effective inhibitors of protozoan growth.
Sulfonamide and folate antagonists inhibit protozoa
Trimethoprim, an inhibitor of dihydrofolate reductase, is used in combination with sulfamethoxazole to treat toxoplasmosis. Another folate antagonist, pyrimethamine, has a high affinity for sporozoan dihydrofolate reductase and has been particularly effective, when used with a sulfonamide, in the management of clinical malaria and toxoplasmosis. A third folate antagonist, proguanil, is commonly taken in combination with atovaquone for malaria prophylaxis. Acquired protozoal resistance to sulfonamides coformulated with folate antagonists has greatly diminished their effectiveness for malaria prevention and treatment.
✺ Sulfonamides effective in Toxoplasma infections
Folate antagonists may result in folate deficiency in individuals with limited folate reserves, such as newborns, pregnant women, and the malnourished. This is of greatest concern when large doses are used for prolonged periods, as in the treatment of acute toxoplasmosis. When folate antagonists are used with sulfonamides, the entire range of sulfonamide toxic effects may be seen. Patients with advanced AIDS may suffer an unusually high incidence of toxic side effects to trimethoprim–sulfamethoxazole.
Folate deficiency and sulfonamide toxicities may occur during treatment
Metronidazole, a nitroimidazole, was introduced in 1959 for the treatment of trichomoniasis. Subsequently, it was found to be effective in the management of giardiasis, amebiasis, and a variety of infections produced by obligate anaerobic bacteria. Energy metabolism in all of them depends on the presence of low-redox–potential compounds, such as ferredoxin, to serve as electron carriers. These compounds reduce the 5-nitro group of the imidazoles to produce intermediate products responsible for the death of the protozoal and bacterial cells, possibly by alkylation of DNA. Resistance, though uncommon, has been noted in strains of Trichomonas vaginalis lacking nitroreductase activity. Nausea, dysgeusia (taste perversion), and peripheral neuropathy are notable potential side effects.
Tinidazole, a newer nitroimidazole, appears to be both a more effective and better tolerated antiprotozoal agent. Its greater lipid solubility improves cerebrospinal fluid levels and in vitro activity. Either drug can be used for trichomoniasis, invasive amebiasis, and giardiasis.
Benznidazole, another member of this drug family, is used for the treatment of Chagas disease. A related medication in the nitrofuran class, Nifurtimox, is also used for this condition. Both may be toxic to the gastrointestinal and neurological systems and these treatments have limited efficacy in chronic Chagas. This disease is an important cause of morbidity and mortality in Latin America, and newer treatments are urgently needed.
Active against protozoa at low-redox–potential
A member of the thiazolide class, nitrazoxanide provides an unusually broad spectrum of activity. It is effective not only against gastrointestinal protozoa such as giardia and amoeba, but in trials has also killed helminths such as human hookworm. In fact, it has demonstrated in vitro activity against certain anaerobic bacteria and even some viruses, although it is not used clinically for those purposes. In diarrhea due to giardia or cryptosporidium, for which it is approved in the United States, its mechanism seems to be interfering with the cell’s electron transfer enzymes. Unfortunately, its clinical usefulness in cryptosporidiosis among immunocompromised patients is limited, and new medications for this condition are urgently needed.
Alternative option for giardiasis
Eflornithine is an enzyme-activated, irreversible inhibitor of ornithine decarboxylase (ODC). In mammalian cells, decarboxylation of ornithine by ODC is a mandatory step in the synthesis of polyamines, compounds thought to play critical roles in cell division and differentiation. Originally developed as an antineoplastic agent, eflornithine proved ineffective in cancer chemotherapy trials. It was also marketed as a topical depilatory agent (anti-hair growth). With the discovery that polyamines of Trypanosoma species were also synthesized from ornithine, eflornithine was successfully tested in the treatment of animal trypanosomiasis. It has been used to treat advanced cases of human West African sleeping sickness due to T brucei gambiense. However, it is not effective against the more virulent T brucei rhodesiense, it is dosed intravenously, and it remains expensive. Eflornithine appears to be cytostatic and requires an intact host immune system for maximum effect.
Originally an anticancer drug
Active against West African sleeping sickness
Arsenic and antimonial compounds have been used since ancient times. They form stable complexes with sulfur compounds and probably exert their biologic effects by binding to sulfhydryl (–SH) groups. They are toxic to the host as well as to the parasite, and have their greatest impact on cells that are metabolically active such as neuronal, renal tubular, intestinal epithelial, and bone marrow stem cells. Their differential toxicity and therapeutic value are due to enhanced uptake by the parasite and its intense metabolic activity. However, significant host toxicity remains. Only one trivalent arsenical, melarsoprol (Mel B), is now used for African trypanosomiasis of the central nervous system, because of its penetration of the blood–brain barrier. Due to its toxicity, including a roughly 10% chance of fatal arsenic poisoning, it is used only when less toxic agents have failed or when the central nervous system is involved. Safer agents are urgently needed for this deadly disease.
Arsenic and antimonial compounds inactivate –SH groups
Some differential toxicity based on enhanced uptake by parasite, but still very toxic to humans
✺ Melarsoprol active against all stages of trypanosomiasis, but highly toxic
Antimonial agents are now restricted to the management of leishmanial infections. Two pentavalent compounds, sodium stibogluconate (Pentostam) and meglumine antimoniate† (Glucantime), may be used for all forms of leishmaniasis. In disseminated visceral disease, prolonged therapy is usually required, and relapses often occur. In localized cutaneous leishmaniasis, cure is usually achieved with a relatively brief course. Toxic side effects are similar to those of the arsenicals, although less severe. However, visceral leishmaniasis is usually treated using intravenous amphotericin-lipid formulations, which are typically used as antifungal medications. In fact, these drugs are being used more frequently for cutaneous leishmaniasis as well, where they are often effective and better tolerated than the antimonials.
✺ Antimonials used only for leishmania infections
A recent advance in antiprotozoal treatment is the introduction of miltefosine. This alkylphosphocholine compound belongs to the phospholipid family. It appears to target protein kinase B, a molecule involved in cellular apoptosis regulation. Blockade of protein kinase B seems to trigger programmed death of infected cells, including many strains of leishmania and free-living amoebas. As clinical experience grows with this medication, it seems to hold great promise for the treatment of these neglected infections.
Miltefosine useful in visceral and cutaneous leishmaniasis and amebic encephalitis