Antiprotozoal Agents

Metronidazole is a prodrug. In anaerobic and microaerophilic organisms, reduced electron-carrier proteins — ferredoxin in anaerobic bacteria and Giardia, pyruvate:ferredoxin oxidoreductase in Trichomonas and Entamoeba — donate electrons to the nitro group of metronidazole, generating a reactive nitroso radical anion. This radical causes deoxyribonucleic acid strand breaks and inhibits nucleic acid synthesis, killing the organism. Aerobic mammalian cells lack these low-potential electron donors and cannot activate the drug, which is the basis for its selective toxicity. Tinidazole is a second-generation nitroimidazole with the same mechanism but a longer half-life, allowing single-dose treatment of several indications where metronidazole requires multi-day courses.

The principal protozoal targets are Giardia lamblia (giardiasis), Entamoeba histolytica (intestinal and extraintestinal amebiasis), and Trichomonas vaginalis (trichomoniasis). For amebic liver abscess, metronidazole is the treatment of choice and must always be followed by a luminal agent — either diloxanide furoate or iodoquinol — to eliminate intestinal cyst carriage. Metronidazole penetrates poorly into the intestinal lumen at therapeutic tissue concentrations, so luminal cysts survive and serve as a source of relapse and transmission if not separately treated.

Metronidazole has broad activity against anaerobic bacteria including Bacteroides fragilis, Clostridium species, fusobacteria, and anaerobic gram-positive cocci. It has no activity against aerobic or facultative organisms and must be combined with agents covering aerobic gram-negatives in polymicrobial infections. It is used in intra-abdominal infections, pelvic inflammatory disease, bacterial vaginosis, aspiration pneumonia, and brain abscess.

The most important adverse effect at the Step 1 level is the disulfiram-like reaction with alcohol: metronidazole inhibits acetaldehyde dehydrogenase, causing acetaldehyde accumulation when ethanol is ingested, producing flushing, nausea, vomiting, and tachycardia. Patients must avoid all alcohol during treatment and for 48 hours after the last dose. Metronidazole also inhibits cytochrome P450 2C9 (CYP2C9), potentiating warfarin anticoagulation. Peripheral neuropathy occurs with prolonged high-dose use. A metallic taste is common.

Metronidazole resistance in Trichomonas vaginalis is clinically significant, mediated by decreased nitroreductase activity and upregulation of oxygen scavenging that prevents drug activation. Resistance in Giardia and Entamoeba remains uncommon. Low-level Trichomonas resistance is sometimes overcome by higher doses; tinidazole may retain activity against low-level resistant strains.

Stage 1 (hemolymphatic) involves blood and lymph parasitemia without central nervous system involvement. Stage 2 (encephalitic) is defined by central nervous system invasion, confirmed by cerebrospinal fluid pleocytosis or the presence of trypanosomes in cerebrospinal fluid. This staging distinction is the single most important determinant of drug choice: stage 1 drugs need not cross the blood-brain barrier, while stage 2 treatment requires central nervous system-penetrating agents. Lumbar puncture for staging is therefore mandatory in all cases before treatment begins.

Pentamidine is the first-line agent for stage 1 Trypanosoma brucei gambiense infection. It accumulates in trypanosome mitochondria and disrupts kinetoplast deoxyribonucleic acid structure and mitochondrial membrane potential. It does not cross the blood-brain barrier and is ineffective in stage 2 disease. Key toxicities include severe hypoglycemia (direct pancreatic beta-cell stimulation followed by exhaustion), nephrotoxicity, and hypotension.

Suramin is used for stage 1 Trypanosoma brucei rhodesiense. It inhibits multiple glycolytic enzymes essential to trypanosome energy production. It is given intravenously, binds extensively to plasma proteins, and does not cross the blood-brain barrier. Its principal toxicity is nephrotoxicity.

Eflornithine is an irreversible inhibitor of ornithine decarboxylase, the rate-limiting enzyme in polyamine biosynthesis. Polyamines are essential for trypanosome cell division. Trypanosoma brucei gambiense has a much slower turnover of ornithine decarboxylase than Trypanosoma brucei rhodesiense, so irreversible inhibition is sustained far longer in gambiense infections — this accounts for the species-selective efficacy of eflornithine. It crosses the blood-brain barrier and is used for stage 2 Trypanosoma brucei gambiense disease. The nifurtimox-eflornithine combination therapy (NECT) regimen combines oral nifurtimox with a shortened eflornithine course, achieving equivalent efficacy with reduced drug burden. NECT is now the World Health Organization standard of care for stage 2 Trypanosoma brucei gambiense human African trypanosomiasis.

Melarsoprol is an organoarsenic compound active against both subspecies in stage 2 disease. It kills trypanosomes by reacting with trypanothione — the parasite-specific dithiol redox molecule — through its trivalent arsenic moiety, inhibiting trypanothione reductase and disrupting parasite redox homeostasis. Despite its efficacy, melarsoprol causes post-treatment reactive encephalopathy in a significant minority of treated patients, with high fatality among those affected. It has been largely replaced for Trypanosoma brucei gambiense by NECT but remains used for Trypanosoma brucei rhodesiense stage 2 disease.

Fexinidazole, a nitroimidazole approved by the European Medicines Agency in 2018, is the first all-oral regimen effective for both stages of Trypanosoma brucei gambiense human African trypanosomiasis and is progressively replacing NECT where healthcare infrastructure allows.

Benznidazole is the preferred first-line agent for Chagas disease. It is a nitroimidazole derivative that undergoes reductive activation by Trypanosoma cruzi nitroreductases, generating reactive intermediates that cause oxidative and nitrosative damage to parasite deoxyribonucleic acid, proteins, and lipids. It achieves high parasitological cure rates in the acute phase, including congenitally acquired infection in neonates, where treatment is strongly indicated. In the chronic phase, benznidazole reduces parasite detection by polymerase chain reaction (PCR) but has not been shown to reduce cardiac events or mortality in patients with established cardiomyopathy, as demonstrated in the BENEFIT (Benznidazole Evaluation for Interrupting Trypanosomiasis) trial. Current guidelines nonetheless recommend treatment for patients with chronic indeterminate or early cardiac disease up to a certain age, recognizing that PCR negativity may provide long-term benefit not captured in shorter trials.

Key adverse effects of benznidazole are dermatological reactions (pruritic rash, common and often manageable), peripheral neuropathy (dose- and duration-dependent, typically reversible), and bone marrow suppression. Both benznidazole and nifurtimox are teratogenic and contraindicated in pregnancy.

Nifurtimox is a nitrofuran compound that undergoes one-electron reduction by Trypanosoma cruzi to generate superoxide and other reactive oxygen species that overwhelm the parasite trypanothione antioxidant system. It is used as a second-line agent for Chagas disease when benznidazole is not tolerated, and — at a different dose and in combination with eflornithine — for stage 2 Trypanosoma brucei gambiense human African trypanosomiasis (NECT). Pediatric patients tolerate both agents substantially better than adults, reinforcing the emphasis on treating acute and congenitally acquired disease in children.

Liposomal amphotericin B is the treatment of choice for visceral leishmaniasis in immunocompetent patients. The leishmanicidal mechanism exploits ergosterol-like sterols in the Leishmania membrane: the drug binds these sterols, disrupts membrane integrity, and causes lethal ion flux. The liposomal formulation concentrates in the reticuloendothelial system — liver, spleen, and bone marrow — precisely where Leishmania amastigotes reside within macrophages, while minimizing the nephrotoxicity that limits conventional amphotericin B. In human immunodeficiency virus (HIV)-coinfected patients, visceral leishmaniasis relapse rates without secondary prophylaxis approach 100 percent, because the fundamental driver is loss of immune control; indefinite secondary prophylaxis with liposomal amphotericin B is required in this population.

Miltefosine is the first oral agent with proven efficacy against visceral leishmaniasis. Originally developed as an anticancer agent, it is an alkylphosphocholine that disrupts Leishmania membrane phospholipid composition and apoptotic signaling. It is highly effective for visceral leishmaniasis in South Asia, where pentavalent antimonial resistance is widespread, and for cutaneous leishmaniasis caused by several South American species. Its long tissue half-life carries a risk of subtherapeutic drug levels at the end of the treatment course, potentially contributing to resistance selection. Miltefosine is teratogenic — effective contraception is mandatory during and for several months after treatment given the drug's prolonged tissue persistence. Nausea and vomiting are the most common adverse effects and are reduced by taking the drug with food.

Meglumine antimoniate and sodium stibogluconate are pentavalent antimony compounds that were long the mainstay of leishmaniasis treatment. Their mechanism involves reduction of pentavalent antimony to the active trivalent form within macrophage phagolysosomes, where trivalent antimony inhibits trypanothione reductase and disrupts Leishmania energy metabolism. Serious toxicities include pancreatitis, hepatotoxicity, corrected QT interval prolongation, and nephrotoxicity. In South Asia, antimonial resistance in Leishmania donovani has reached levels that render these agents effectively obsolete for visceral leishmaniasis there; they retain utility in East Africa and parts of South America.

Pentamidine acts against Leishmania by accumulating in the parasite mitochondria and disrupting kinetoplast deoxyribonucleic acid structure and mitochondrial membrane potential — the same mechanism as in human African trypanosomiasis. It is used for cutaneous leishmaniasis caused by Leishmania guyanensis in South America and as a second-line agent for visceral leishmaniasis. Its major toxicity is severe hypoglycemia from direct pancreatic beta-cell effects, followed by diabetes mellitus with prolonged use.

The standard treatment for toxoplasmic encephalitis combines pyrimethamine and sulfadiazine in a regimen that exploits sequential blockade of the Toxoplasma folate synthesis pathway. Sulfadiazine is a sulfonamide that competitively inhibits dihydropteroate synthase (DHPS), blocking an early step in parasite folate synthesis. Pyrimethamine inhibits dihydrofolate reductase (DHFR), blocking a downstream step. Because Toxoplasma synthesizes its own folate — unlike mammals, which rely on dietary folate — this sequential double blockade produces synergistic antiparasitic activity while mammalian cells are protected by dietary folinic acid supplementation.

Folinic acid (leucovorin) must be co-administered with every pyrimethamine-containing regimen. It bypasses the dihydrofolate reductase block in mammalian cells and prevents the hematological toxicities — leukopenia, thrombocytopenia, and megaloblastic anemia — that are the primary dose-limiting adverse effects of pyrimethamine. Folic acid (the oxidized form) is not a substitute and must not be used in its place — it cannot bypass the dihydrofolate reductase block. This distinction is high-yield.

When sulfadiazine is not tolerated, pyrimethamine plus clindamycin is the preferred alternative, with equivalent efficacy to the standard regimen in controlled trials. Trimethoprim-sulfamethoxazole at high doses is an alternative when pyrimethamine is unavailable; it achieves dihydrofolate reductase and dihydropteroate synthase inhibition similar to the standard combination and is widely used where pyrimethamine is not accessible. Atovaquone has cysticidal activity against tissue cysts in addition to activity against the active tachyzoite form, making it useful in sulfonamide-intolerant patients.

Primary prophylaxis against toxoplasmic encephalitis reactivation is indicated in HIV-infected patients with advanced immunosuppression who are seropositive for Toxoplasma gondii immunoglobulin G. Trimethoprim-sulfamethoxazole is the preferred prophylactic agent and confers the additional benefit of simultaneously preventing Pneumocystis jirovecii pneumonia. After immune reconstitution on antiretroviral therapy — sustained recovery of cluster of differentiation 4 lymphocyte counts — prophylaxis can be safely discontinued. Secondary prophylaxis at reduced treatment doses is required indefinitely until immune reconstitution occurs, because relapse rates without maintenance therapy are very high in this population.

For congenital toxoplasmosis, spiramycin is used in pregnancy when maternal primary infection is detected but fetal infection is not yet confirmed. Spiramycin concentrates in placental tissue and reduces the risk of vertical transmission but does not treat established fetal infection. When fetal infection is confirmed by amniocentesis, pyrimethamine-sulfadiazine-folinic acid is used to treat the infected fetus, with pyrimethamine avoided in the first trimester due to its anti-folate teratogenic potential.