Metronidazole is the prototype nitroimidazole antiprotozoal and is also one of the most widely used antibacterial agents for anaerobic infections. Its selective activity against anaerobic and microaerophilic organisms depends on a mechanism of reductive bioactivation that is uniquely feasible in low-oxygen environments, making it both highly effective in its target organisms and essentially non-toxic to aerobic mammalian cells at standard therapeutic concentrations.
Mechanism of Action. Metronidazole is a prodrug that requires intracellular activation. In anaerobic and microaerophilic organisms, reduced electron-carrier proteins (ferredoxin (Fd) in anaerobic bacteria and Giardia, pyruvate:ferredoxin oxidoreductase (PFOR) in Trichomonas and Entamoeba) donate electrons to the nitro group of metronidazole, reducing it to a reactive nitroso radical anion intermediate. This reactive intermediate causes deoxyribonucleic acid (DNA) strand breaks, disrupts DNA helical structure, and inhibits nucleic acid synthesis, leading to rapid cell death. Because aerobic mammalian cells lack these low-potential electron donors, the drug is not activated in host tissues at therapeutic concentrations, providing its selective antiprotozoal and antianaerobic activity. The short half-life of the toxic radical intermediate ensures that DNA damage is highly localized to the organism generating it.1
Pharmacokinetics. Metronidazole is nearly completely absorbed orally, achieving bioavailability above 90 percent; the oral and intravenous (IV) routes produce essentially equivalent plasma concentrations, making IV use appropriate primarily when oral administration is not possible. It distributes widely, achieving therapeutic concentrations in virtually all body compartments including cerebrospinal fluid (CSF), brain, bone, hepatic abscesses, and vaginal secretions. The volume of distribution (Vd) is approximately 0.6 to 0.8 L/kg. Metronidazole is metabolized primarily in the liver by oxidative side-chain hydroxylation (cytochrome P450 3A4 (CYP3A4) and CYP2C9 (cytochrome P450 2C9)) to a hydroxy-metabolite that retains antiprotozoal activity, and by glucuronide conjugation. The elimination half-life is 6 to 10 hours; the hydroxy-metabolite half-life is approximately 10 hours. Dose reduction is required in severe hepatic impairment but not in renal impairment. Metronidazole crosses the placenta and is excreted in breast milk; it is classified as safe in the second and third trimesters but avoided in the first trimester for non-life-threatening indications.1
Antiprotozoal Spectrum. The protozoal organisms susceptible to metronidazole include Giardia lamblia (giardiasis), Entamoeba histolytica (intestinal and extraintestinal amebiasis), and Trichomonas vaginalis (trichomoniasis). For giardiasis, metronidazole at 250 mg three times daily for 5 to 7 days achieves cure rates of 85 to 95 percent. Tinidazole, a second-generation nitroimidazole with a longer half-life of 12 to 14 hours, achieves equivalent or superior cure rates for giardiasis and trichomoniasis with single-dose administration (tinidazole 2 g single dose), improving adherence. For amebic liver abscess, metronidazole 500 to 750 mg three times daily for 7 to 10 days is the treatment of choice; it must always be followed by a luminal agent (diloxanide furoate or iodoquinol) to eliminate intestinal cyst carriage, because metronidazole alone does not reliably clear the intestinal lumen.112
Antibacterial Spectrum. Beyond protozoal infections, metronidazole has broad activity against anaerobic bacteria including Bacteroides fragilis and the Bacteroides group, Clostridium difficile (though oral vancomycin or fidaxomicin is now preferred for most C. difficile infection (CDI) presentations), fusobacteria, peptostreptococci, and anaerobic gram-positive cocci. It has no activity against aerobic or facultative organisms and must be combined with agents covering aerobic gram-negatives when polymicrobial infections are suspected. Standard antibiotic uses include intra-abdominal infections, pelvic inflammatory disease, bacterial vaginosis, brain abscess (combined with a beta-lactam), aspiration pneumonia, and surgical prophylaxis in colorectal procedures.1
Adverse Effects and Drug Interactions. The most clinically significant adverse effect is a disulfiram-like reaction with alcohol: metronidazole inhibits acetaldehyde dehydrogenase, causing accumulation of acetaldehyde when ethanol is ingested, producing flushing, nausea, vomiting, headache, and tachycardia. Patients must be counseled to avoid all alcohol and alcohol-containing products (including mouthwashes and many liquid medications) during treatment and for 48 hours after the last dose (72 hours for tinidazole due to its longer half-life). Other adverse effects include metallic taste (common, often dose-limiting in prolonged courses), peripheral neuropathy (with prolonged high-dose use), and rarely reversible cerebellar toxicity or encephalopathy. Metronidazole potentiates the anticoagulant effect of warfarin through CYP2C9 inhibition; international normalized ratio (INR) monitoring is required when the two drugs are co-administered.1
Resistance. Metronidazole resistance in T. vaginalis is clinically significant and increasing in prevalence, estimated at 2 to 5 percent for low-level resistance. Resistance mechanisms include decreased nitroreductase activity (reducing drug activation), upregulation of oxygen scavenging (raising intracellular oxygen levels and preventing radical formation), and reduced Fd expression. Low-level resistance is often overcome by higher doses or longer courses; high-level resistance may require desensitization protocols or alternative agents (tinidazole at higher doses may retain activity against low-level metronidazole-resistant strains). Resistance in Giardia and Entamoeba remains clinically rare.23
Metronidazole vs. tinidazole: both nitroimidazoles; tinidazole has longer half-life (12–14 hrs), better GI tolerability, and allows single-dose therapy for giardiasis and trichomoniasis. Amebic liver abscess: metronidazole PLUS luminal agent (diloxanide furoate or iodoquinol) mandatory — metronidazole alone does not eradicate intestinal cysts. Alcohol interaction: disulfiram-like reaction; avoid alcohol during treatment and 48 hrs (metronidazole) or 72 hrs (tinidazole) after. Warfarin: inhibits CYP2C9; monitor INR. First trimester pregnancy: avoid for non-life-threatening indications.
Human African trypanosomiasis (HAT), caused by Trypanosoma brucei gambiense (West and Central Africa, responsible for over 95 percent of cases) and Trypanosoma brucei rhodesiense (East Africa), is a vector-borne disease transmitted by the tsetse fly (Glossina species). Without treatment, HAT is uniformly fatal. Drug selection is determined by the infecting subspecies and by whether the disease has progressed to the central nervous system (CNS) stage, which entirely determines the required treatment regimen.
Disease Staging and Its Pharmacological Implications. HAT progresses through two stages. Stage 1 (hemolymphatic stage) involves parasitemia with fever, lymphadenopathy (Winterbottom sign: posterior cervical lymphadenopathy, characteristic of T. b. gambiense), and constitutional symptoms. Stage 2 (encephalitic stage) is defined by cerebrospinal fluid (CSF) pleocytosis (above 5 white blood cells per microliter or the presence of trypanosomes in CSF) and is characterized by the neuropsychiatric symptoms that give sleeping sickness its name: disrupted circadian rhythm, personality change, tremor, ataxia, and, in late stage, coma. Stage determination requires lumbar puncture in all cases and directs treatment: stage 1 drugs need not cross the blood-brain barrier (BBB), while stage 2 treatment absolutely requires CNS-penetrating agents.4
Suramin. Suramin is a polysulfonated naphthylurea used for stage 1 T. b. rhodesiense infection. Its mechanism involves inhibition of multiple glycolytic and other metabolic enzymes essential to trypanosome energy production, particularly glycerol-3-phosphate oxidase. It is administered intravenously because it is not absorbed orally; after IV administration it binds extensively to plasma proteins and achieves a half-life of approximately 50 days due to slow release from protein-bound stores. It does not cross the BBB and is therefore ineffective against stage 2 disease. Suramin causes a spectrum of adverse effects including nephrotoxicity (urinalysis must be performed before each dose; proteinuria above 2+ is a contraindication to further doses), skin reactions, peripheral neuropathy, adrenal insufficiency with prolonged use, and, rarely, fatal anaphylaxis on first exposure (a small test dose of 100 to 200 mg is given before the first full dose). Pentamidine is the preferred agent for stage 1 T. b. gambiense, with suramin reserved for T. b. rhodesiense due to differential susceptibility.45
Melarsoprol. Melarsoprol is an organoarsenic compound used for stage 2 HAT caused by both T. brucei subspecies, though it is now largely replaced for T. b. gambiense by better-tolerated options. Melarsoprol crosses the BBB in therapeutic concentrations and kills trypanosomes by reacting with trypanothione (the parasite-specific dithiol equivalent of glutathione) through its trivalent arsenic moiety, inhibiting trypanothione reductase and disrupting the parasite redox homeostasis. Despite its efficacy, melarsoprol carries a 5 to 10 percent incidence of reactive encephalopathy (post-treatment reactive encephalopathy, PTRE), a poorly understood inflammatory syndrome that is fatal in approximately 50 percent of affected patients, yielding an overall treatment-attributable mortality of 2 to 5 percent. Prednisolone co-administration at 1 mg/kg/day reduces but does not eliminate PTRE risk. Melarsoprol is given as a series of intravenous injections dissolved in propylene glycol; thrombophlebitis at the injection site is universal. Resistance to melarsoprol, mediated by loss of the adenosine transporter 1 (AT1) that imports the drug into trypanosomes, has been documented particularly in foci in Uganda and the Democratic Republic of Congo.5
Eflornithine and Combination Therapy. Eflornithine (difluoromethylornithine (DFMO)) is an irreversible inhibitor of ornithine decarboxylase (ODC), the rate-limiting enzyme in polyamine biosynthesis. Polyamines (putrescine, spermidine, spermine) are essential for trypanosome cell division and differentiation. T. b. gambiense has a much slower turnover of ornithine decarboxylase (ODC) than T. b. rhodesiense, meaning that irreversible ODC inhibition is sustained much longer in gambiense infections, accounting for the selective efficacy of eflornithine against this subspecies. Eflornithine crosses the BBB and is used for stage 2 T. b. gambiense HAT. The nifurtimox-eflornithine combination therapy (NECT) regimen, adopted by the World Health Organization (WHO) in 2009, combines oral nifurtimox with a shortened eflornithine course, achieving equivalent efficacy to eflornithine monotherapy with a reduced drug burden and better operational feasibility. NECT has substantially reduced HAT mortality and is the current standard of care for stage 2 T. b. gambiense. Eflornithine adverse effects include bone marrow suppression (thrombocytopenia, leukopenia), seizures, and gastrointestinal (GI) disturbance; full blood count (FBC) monitoring is required during treatment.45
| Agent | Subspecies | Stage | Route | Key Toxicity |
|---|---|---|---|---|
| Pentamidine | T. b. gambiense | Stage 1 only | IM or IV | Hypoglycemia, nephrotoxicity, hypotension |
| Suramin | T. b. rhodesiense | Stage 1 only | IV | Nephrotoxicity, anaphylaxis (test dose required) |
| Melarsoprol | Both | Stage 2 | IV (propylene glycol) | PTRE (5–10%), 50% fatal; thrombophlebitis |
| Eflornithine | T. b. gambiense only | Stage 2 | IV | Bone marrow suppression, seizures |
| NECT (nifurtimox + eflornithine) | T. b. gambiense only | Stage 2 | Oral + IV | Current standard of care; reduced eflornithine burden |
| Fexinidazole | T. b. gambiense | Stage 1 and 2 | Oral | First all-oral HAT regimen; psychiatric AEs; approved 2019 |
Never treat HAT without staging: lumbar puncture is mandatory before treatment selection. Suramin does not cross the BBB — it will fail in stage 2 disease. Melarsoprol PTRE (post-treatment reactive encephalopathy): give prednisolone 1 mg/kg/day concurrently; monitor neurological status closely for 10–14 days post-treatment. Eflornithine is selective for T. b. gambiense due to slow ODC turnover — it is ineffective for T. b. rhodesiense stage 2 (use melarsoprol). Fexinidazole (2019 approval): first oral regimen covering both stages of T. b. gambiense HAT; changing clinical practice in endemic regions.
Chagas disease, caused by Trypanosoma cruzi and transmitted by triatomine insects (the reduviid or "kissing" bug) in Latin America, affects an estimated 6 to 7 million people worldwide. The disease progresses from an acute phase characterized by high-level parasitemia to a chronic phase in which parasites persist in cardiac and gastrointestinal tissue at low density, causing progressive cardiomyopathy and megasyndromes (megaesophagus, megacolon) in approximately 30 percent of infected individuals decades after initial infection. Treatment efficacy is highly stage-dependent: cure rates in the acute phase exceed 80 percent, while chronic-phase treatment is controversial and provides uncertain benefit for established cardiac disease.
Benznidazole. Benznidazole is the preferred first-line agent for Chagas disease globally. It is a nitroimidazole derivative (structurally related to metronidazole) that undergoes reductive activation by T. cruzi nitroreductases to generate reactive intermediates that cause oxidative and nitrosative damage to parasite deoxyribonucleic acid (DNA), proteins, and lipids. Benznidazole is given orally at 5 to 7 mg/kg/day in two divided doses for 60 days in adults; children receive weight-based dosing over the same duration. In the acute phase (including congenitally acquired infection in neonates, where treatment is strongly indicated), benznidazole achieves parasitological cure in 60 to 80 percent of treated patients. For chronic-phase adult infection, the BENEFIT (Benznidazole Evaluation for Interrupting Trypanosomiasis) trial demonstrated that benznidazole reduces parasite detection by polymerase chain reaction (PCR) but does not reduce the incidence of cardiac events or mortality over 5 years of follow-up in patients with established Chagas cardiomyopathy, raising questions about the utility of treatment in this population. However, current guidelines from the Pan American Health Organization (PAHO) recommend treatment for all patients up to 50 years of age with chronic indeterminate or early cardiac Chagas disease, recognizing that PCR negativity may still offer long-term benefit not captured by shorter trials.6
Nifurtimox. Nifurtimox is a nitrofuran compound that undergoes one-electron reduction by T. cruzi to generate superoxide and other reactive oxygen species (ROS) that overwhelm parasite antioxidant defenses (the trypanothione system). It is used as a second-line agent when benznidazole is not tolerated or unavailable, and in the nifurtimox-eflornithine combination therapy (NECT) regimen for African trypanosomiasis (where it acts by a complementary mechanism). The standard adult dose for Chagas disease is 8 to 10 mg/kg/day in three to four divided doses for 60 to 90 days. Nifurtimox has a less favorable adverse effect profile than benznidazole, with higher rates of neuropsychiatric effects (headache, insomnia, dizziness, irritability, and polyneuropathy with prolonged use), nausea, and anorexia that cause treatment discontinuation in 10 to 30 percent of adult patients. Pediatric patients tolerate both agents substantially better than adults, which reinforces the emphasis on treating acute pediatric and congenital Chagas disease.7
Adverse Effects and Monitoring for Both Agents. Benznidazole adverse effects include dermatological reactions (pruritic rash, occurring in 20 to 30 percent of patients, often manageable with antihistamines or dose reduction), peripheral neuropathy (dose- and duration-dependent, typically reversible), and bone marrow suppression (leukopenia, requiring complete blood count (CBC) monitoring at weeks 2, 4, and 8 of therapy). Absolute neutrophil count (ANC) below 1,500 cells/mm³ warrants dose reduction or treatment interruption. Both benznidazole and nifurtimox are teratogenic and contraindicated in pregnancy; nifurtimox is also contraindicated in severe hepatic or renal impairment. Neither drug is available through conventional commercial pharmacy channels in most countries outside Latin America; in the United States, they are available through the Centers for Disease Control and Prevention (CDC) drug service.7
Treatment Response Assessment. Parasitological cure is defined as sustained negative PCR at least 12 months after completion of treatment. Serology (enzyme-linked immunosorbent assay (ELISA), indirect fluorescent antibody (IFA), indirect hemagglutination assay) becomes negative 3 to 5 years after cure in adult patients; in congenitally treated neonates, serological conversion occurs within 12 months. Because serology reflects long-lived immunoglobulin G (IgG) antibodies and does not rapidly reflect treatment response, PCR is the preferred early marker of treatment efficacy. Patients with negative baseline PCR (common in indeterminate chronic phase) cannot be assessed by PCR alone; sustained serological negativity over 5 to 10 years is the ultimate endpoint. Echocardiography and electrocardiography (ECG) surveillance remains essential in all patients with Chagas cardiomyopathy regardless of antiparasitic treatment status.7
Acute phase (any age): treat immediately with benznidazole first-line; nifurtimox if intolerant. Congenital infection (neonate): treat regardless of PCR status; cure rates above 90% if treated in first year of life. Chronic indeterminate phase (age ≤50, no advanced cardiomyopathy): treat per PAHO guidelines; uncertain benefit in advanced cardiac disease. Monitoring: CBC at weeks 2, 4, 8; PCR at 12 months post-treatment for cure assessment. BENEFIT trial: benznidazole reduces PCR positivity but does not reduce cardiac events in established cardiomyopathy. US access: both drugs available through CDC drug service only.
Leishmaniasis is caused by more than 20 species of Leishmania protozoa transmitted by the bite of female phlebotomine sandflies, and exists in three major clinical forms: visceral leishmaniasis (VL), also called kala-azar, caused primarily by Leishmania donovani and Leishmania infantum, which is uniformly fatal if untreated; cutaneous leishmaniasis (CL), caused by numerous species including Leishmania major and Leishmania tropica; and mucocutaneous leishmaniasis (MCL), caused principally by Leishmania braziliensis, which involves destructive mucosal invasion. Drug selection is driven by disease form, geographic region (which determines species and resistance patterns), patient immune status, and available formulations.
Liposomal Amphotericin B. Liposomal amphotericin B (L-AmB, AmBisome) is the treatment of choice for visceral leishmaniasis in immunocompetent patients, adopted as first-line therapy by the WHO for most settings. The leishmanicidal mechanism exploits the presence of ergosterol-like sterols in the Leishmania membrane: L-AmB binds to these sterols, disrupts membrane integrity, and causes lethal ion flux in the parasite. The liposomal formulation achieves high concentrations in the reticuloendothelial system (liver, spleen, bone marrow), which is precisely where Leishmania amastigotes reside within macrophages, while minimizing nephrotoxic exposure. The WHO-recommended regimen for VL in immunocompetent patients is L-AmB 3 mg/kg/day on days 1 to 5 plus day 10 (total dose 18 mg/kg), or alternatively 10 mg/kg as a single dose in resource-limited settings (proven efficacy in South Asian VL). In human immunodeficiency virus (HIV)-coinfected patients, higher total doses (40 mg/kg or more) are required and secondary prophylaxis with monthly L-AmB infusions is recommended indefinitely, because relapse rates in immunosuppressed hosts approach 100 percent without prophylaxis.8
Miltefosine. Miltefosine is the first oral agent with proven efficacy against visceral leishmaniasis and represents a major advance in treatment accessibility. Originally developed as an anticancer agent, it is an alkylphosphocholine that disrupts Leishmania membrane phospholipid composition and apoptotic signaling, inhibiting alkyl-lysophospholipid metabolism and ether-lipid biosynthesis. The adult dose is 2.5 mg/kg/day (maximum 150 mg/day) in two to three divided doses for 28 days; pediatric weight-based dosing is standardized. Miltefosine is well absorbed orally and has an elimination half-life of approximately 7 days, with drug detectable for more than 5 weeks after the last dose. This prolonged tissue half-life carries a risk of subtherapeutic exposure at the end of treatment (subMIC tail), which may contribute to resistance selection. Adverse effects include nausea and vomiting (most common, reduced by taking with food and split dosing), teratogenicity (Category X; highly effective contraception is mandatory during and for 5 months after treatment due to the long half-life), and transaminase elevation. Miltefosine is now first-line for VL in South Asia (where pentavalent antimonial resistance is widespread) and is effective against CL caused by several South American species.89
Pentavalent Antimonials. Meglumine antimoniate (Glucantime) and sodium stibogluconate (Pentostam) are pentavalent antimony compounds that were for decades the mainstay of leishmaniasis treatment. Their mechanism involves reduction of pentavalent antimony (Sb(V)) to the active trivalent form (Sb(III)) within macrophage phagolysosomes, where Sb(III) inhibits trypanothione reductase and disrupts Leishmania energy metabolism (adenosine triphosphate (ATP) and guanosine triphosphate (GTP) synthesis). They must be given parenterally (intramuscular (IM) or IV) for 28 to 30 days, making administration logistically demanding. Serious toxicities include pancreatitis (most common cause of treatment-limiting adverse events), hepatotoxicity, QTc prolongation (cardiac monitoring required), nephrotoxicity, and bone marrow suppression. In the Bihar region of India and neighboring areas, pentavalent antimonial resistance in L. donovani has reached 60 percent or higher, rendering these agents effectively obsolete for South Asian VL; they retain utility in East African, European, and South American settings.8
Pentamidine. Pentamidine isethionate, a diamidine compound, acts against Leishmania by accumulating in the parasite mitochondria and interfering with kinetoplast deoxyribonucleic acid (kDNA) structure and mitochondrial membrane potential. It is used for cutaneous leishmaniasis (especially L. guyanensis in South America) and as a second-line agent for VL when other options are not feasible. As noted in Module 02 context, pentamidine is also a first-line agent for stage 1 T. b. gambiense African trypanosomiasis. Its adverse effects include severe hypoglycemia (direct pancreatic beta-cell toxicity causing insulin release followed by beta-cell destruction), nephrotoxicity, hypotension, cardiac arrhythmias, and diabetes mellitus with prolonged use. Blood glucose must be monitored before and after each administration. Administration is by IV or deep IM injection; aerosolized pentamidine is used for Pneumocystis jirovecii pneumonia (PJP) prophylaxis in a completely different clinical context.8
| Agent | Disease Form | Route | Key Toxicity | Notes |
|---|---|---|---|---|
| L-AmB | VL (all regions), MCL | IV | Nephrotoxicity (less than AmBd), infusion reactions | WHO first-line VL; monthly prophylaxis in HIV |
| Miltefosine | VL, CL (South America) | Oral | GI, teratogenic (Category X), prolonged half-life | First oral agent; contraception 5 months after |
| Antimonials | VL (non-South Asia), CL, MCL | IM or IV | Pancreatitis, QTc, nephrotoxicity, hepatotoxicity | Resistant in Bihar/India; monitor ECG |
| Pentamidine | CL (L. guyanensis), VL (2nd line) | IV or IM | Hypoglycemia, nephrotoxicity, DM with prolonged use | Monitor BG before/after each dose |
South Asia (India, Nepal, Bangladesh): L-AmB or miltefosine first-line (high antimonial resistance). East Africa: antimonials + paromomycin combination; L-AmB alternative. Mediterranean/South America: antimonials retain activity; L-AmB for severe VL or HIV-coinfected. HIV-coinfected VL: L-AmB higher total dose (40+ mg/kg); monthly secondary prophylaxis mandatory. MCL: antimonials or L-AmB; never withhold treatment (destruction is progressive). Teratogenicity: miltefosine is Category X; confirm negative pregnancy test and counsel on 5-month contraception requirement before prescribing.
Toxoplasma gondii is an obligate intracellular parasite with a worldwide seroprevalence of 10 to 80 percent, with the vast majority of immunocompetent primary infections being self-limited or asymptomatic. Clinical disease of pharmacological significance occurs in three settings: in immunocompromised hosts (particularly those with acquired immunodeficiency syndrome (AIDS) and CD4 (cluster of differentiation 4) lymphocyte counts below 100 cells/mm³), where reactivation of latent tissue cysts causes toxoplasmic encephalitis (TE); in congenital infection transmitted transplacentally during primary maternal infection; and in ocular disease producing chorioretinitis.
Pyrimethamine-Sulfadiazine: The Standard Regimen. The combination of pyrimethamine and sulfadiazine exploits sequential blockade of the folate synthesis pathway in T. gondii. Sulfadiazine is a sulfonamide that competitively inhibits dihydropteroate synthase (DHPS), blocking the conversion of para-aminobenzoic acid (PABA) to dihydropteroate, an early step in folate synthesis. Pyrimethamine inhibits dihydrofolate reductase (DHFR), blocking the conversion of dihydrofolate to tetrahydrofolate, a further downstream step. Because T. gondii synthesizes its own folate (unlike mammals, which rely on dietary folate), this sequential blockade produces synergistic antiparasitic activity while mammalian cells are protected by exogenous folate supplementation. Folinic acid (leucovorin) at 10 to 25 mg/day must be co-administered with every pyrimethamine-containing regimen; it bypasses the DHFR block in mammalian cells and prevents the hematological toxicities (leukopenia, thrombocytopenia, megaloblastic anemia) that are the primary dose-limiting adverse effects of pyrimethamine. Folic acid (the oxidized form) does not bypass the block and must not be substituted for folinic acid.10
Dosing for Acute Toxoplasmic Encephalitis. For acute TE in adults, the standard induction regimen is pyrimethamine 200 mg loading dose on day 1, then 50 to 75 mg/day plus sulfadiazine 1,000 to 1,500 mg every 6 hours plus folinic acid 10 to 25 mg/day, continued for at least 6 weeks. Clinical and radiological response is expected by week 2; failure to respond by 2 weeks should prompt brain biopsy to exclude alternative diagnoses (primary central nervous system (CNS) lymphoma in human immunodeficiency virus (HIV)-infected patients has a similar imaging appearance). After acute treatment, lifelong secondary prophylaxis is required at reduced doses (pyrimethamine 25 to 50 mg/day plus sulfadiazine 500 to 1,000 mg twice daily plus folinic acid) until immune reconstitution occurs (CD4 count above 200 cells/mm³ sustained on antiretroviral therapy (ART) for at least 6 months).1011
Alternative Regimens. When sulfadiazine is not tolerated (typically due to sulfonamide hypersensitivity, rash, or nephrotoxicity from crystalluria), pyrimethamine plus clindamycin at 600 mg every 6 hours is the preferred alternative and has demonstrated equivalent efficacy to the standard regimen in controlled trials. Trimethoprim-sulfamethoxazole (TMP-SMX) at high doses (5 mg/kg trimethoprim component every 12 hours) is an alternative supported by observational data and is widely used when pyrimethamine is unavailable (as it often is outside Europe and the United States); the combination achieves DHFR and DHPS inhibition similar to pyrimethamine-sulfadiazine. Atovaquone, alone or in combination with pyrimethamine or sulfadiazine, has been used in sulfa-intolerant patients with activity against tissue cysts (cysticidal activity) as well as tachyzoites, making it potentially advantageous for complete eradication.1011
Primary Prophylaxis in Immunocompromised Hosts. Primary prophylaxis to prevent initial TE reactivation is indicated in HIV-infected patients with CD4 below 100 cells/mm³ who are seropositive for T. gondii immunoglobulin G (IgG). TMP-SMX one double-strength tablet daily is the preferred prophylactic regimen and confers the additional benefit of preventing Pneumocystis jirovecii pneumonia (PJP) simultaneously. Dapsone plus pyrimethamine plus folinic acid, or atovaquone, are alternatives when TMP-SMX is not tolerated. Prophylaxis can be safely discontinued when CD4 rises above 200 cells/mm³ for more than 3 months on effective ART.11
Congenital Toxoplasmosis and Ocular Disease. Congenital toxoplasmosis, transmitted during primary maternal infection in pregnancy, requires treatment to reduce severity of fetal and neonatal disease. Pyrimethamine-sulfadiazine-folinic acid for one year is the standard treatment for infected neonates; spiramycin is used in pregnancy to reduce vertical transmission (it concentrates in placental tissue but does not treat established fetal infection). Ocular toxoplasmosis (chorioretinitis) is treated with pyrimethamine-sulfadiazine-folinic acid combined with systemic corticosteroids to reduce the inflammatory response around the retinal lesion; treatment duration is typically 4 to 6 weeks. Ophthalmology co-management is essential as vision-threatening lesions may require surgical or laser adjuncts. Recurrent ocular disease is managed with long-term TMP-SMX prophylaxis at three times weekly dosing.10
Pyrimethamine-sulfadiazine is ALWAYS paired with folinic acid (leucovorin) — never folic acid. Folinic acid bypasses the DHFR block in mammalian cells; folic acid does not. No response at 2 weeks: re-evaluate diagnosis (CNS lymphoma mimics TE on MRI). Secondary prophylaxis: mandatory in HIV until CD4 above 200 cells/mm³ for 6+ months on ART. TMP-SMX primary prophylaxis: CD4 below 100 with positive T. gondii IgG; also covers PJP. Congenital: spiramycin reduces transmission in pregnancy; pyrimethamine-sulfadiazine-folinic acid treats infected neonate for 12 months.
The antiprotozoal agents covered in this module treat diseases that collectively affect hundreds of millions of people worldwide yet receive a fraction of the drug development investment of common infectious diseases. The practical prescribing challenges are therefore distinct: drug availability through conventional pharmacy channels is limited or absent for many agents; treatment durations are prolonged; adverse effect profiles are often severe; and the clinical context frequently involves severely ill, immunocompromised, or nutritionally compromised patients in whom toxicity thresholds are lower.
Drug Access and Regulatory Status. Many of the agents in this module are available only through specialized channels. In the United States, benznidazole and nifurtimox for Chagas disease are available through the CDC drug service. Suramin and melarsoprol for human African trypanosomiasis (HAT) are similarly accessed through specialized tropical medicine programs. Pentavalent antimonials are not FDA-approved and are available only through the CDC in the United States, though they remain on formulary in endemic countries. Miltefosine and L-AmB are FDA-approved and commercially available. For clinicians in non-endemic countries encountering these diseases in travelers or immigrants, early consultation with an infectious disease specialist experienced in tropical medicine and, when needed, direct contact with the CDC Emergency Operations Center are appropriate first steps. The CDC provides 24-hour access to antiparasitic drugs not commercially available in the US through the CDC Emergency Operations Center.
The Immunocompromised Host. Human immunodeficiency virus (HIV)-infected and otherwise immunocompromised patients present additional complexity in managing protozoal infections. In Chagas disease, HIV coinfection is associated with reactivation of chronic infection presenting as meningoencephalitis or myocarditis, which can mimic toxoplasmic encephalitis (TE) or other central nervous system (CNS) infections; both benznidazole treatment and antiretroviral therapy (ART) are indicated. In leishmaniasis, visceral leishmaniasis (VL) in HIV-infected patients requires higher drug doses, longer treatment durations, and indefinite secondary prophylaxis as noted above; loss of immune control is the fundamental driver of relapse. Toxoplasmic encephalitis is almost exclusively a reactivation disease in patients with advanced HIV infection; immune reconstitution inflammatory syndrome (IRIS) can paradoxically worsen neurological status within weeks of initiating ART in treated TE, requiring corticosteroid management. In solid organ transplant recipients, toxoplasmosis may arise from donor-derived primary infection (particularly cardiac transplant from T. gondii-seropositive donor to seronegative recipient) and requires trimethoprim-sulfamethoxazole (TMP-SMX) prophylaxis for 6 to 12 months post-transplant.1112
Shared Mechanisms Across Antiprotozoal Classes. A clinically useful organizing principle is that several antiprotozoal drug classes exploit the trypanothione system, a parasite-specific redox pathway that is absent in mammalian cells. Trypanosoma and Leishmania species use trypanothione (a conjugate of glutathione and spermidine) rather than glutathione as their primary intracellular antioxidant; trypanothione reductase (the enzyme that maintains trypanothione in its reduced form) is inhibited by melarsoprol (via arsenic binding to trypanothione), by pentavalent antimonials (after reduction to Sb(III)), and is overwhelmed by the reactive oxygen species generated by nifurtimox. This shared vulnerability means that drugs targeting the trypanothione system have activity against both trypanosomes and leishmania, though clinical applications differ by pharmacokinetics and CNS penetration. Understanding this shared pathway clarifies why cross-resistance between melarsoprol and nifurtimox has been observed in some HAT isolates.5
Emerging Therapies and the Changing Landscape. The antiprotozoal pharmacopeia is expanding after decades of stagnation. Fexinidazole, approved by the European Medicines Agency (EMA) in 2018 and by the Democratic Republic of Congo national authority in 2019, is the first all-oral regimen for both stages of T. b. gambiense HAT and is progressively replacing the nifurtimox-eflornithine combination therapy (NECT) regimen in endemic regions with appropriate healthcare infrastructure. Acoziborole, a boron-containing compound active against HAT in a single oral dose, was approved for stage 1 and early stage 2 HAT in 2023 and represents a potential elimination tool. For Chagas disease, research into new nitroheterocyclic compounds with improved pharmacokinetic and safety profiles is ongoing. For leishmaniasis, combination therapy strategies (combining L-AmB with miltefosine or with paromomycin) are being evaluated to shorten treatment duration and reduce costs while preventing resistance emergence.12
Metronidazole mechanism: PFOR-mediated reductive activation to DNA-damaging radical anions; selective for anaerobes/microaerophiles. Amebic liver abscess: metronidazole PLUS luminal agent mandatory. Alcohol: disulfiram-like reaction; 48 hrs clearance. HAT staging: lumbar puncture mandatory; suramin/pentamidine for stage 1 only; NECT or fexinidazole for stage 2 gambiense; melarsoprol for stage 2 rhodesiense. Chagas: benznidazole first-line; 60-day course; CBC at weeks 2, 4, 8; teratogenic; CDC access in US. VL treatment: L-AmB or miltefosine; antimonials limited by South Asian resistance; miltefosine teratogenic (5-month contraception). Toxoplasmosis: pyrimethamine-sulfadiazine always with folinic acid (not folic acid); secondary prophylaxis until CD4 above 200 on ART.
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