Pharmacology · Antiparasitic Drugs

Antiprotozoal Agents

Nitroimidazoles, trypanosomiasis, Chagas disease, leishmaniasis, and toxoplasmosis

Metronidazole is a prodrug activated exclusively in anaerobic and microaerophilic organisms by ferredoxin-dependent nitroreduction — the resulting radical anion causes DNA strand breaks; mammalian cells lack this reducing environment, which is the basis of the drug's selective protozoal and antibacterial toxicity.
Treatment of human African trypanosomiasis is strictly stage-dependent — first-stage (blood/lymphatic) and second-stage (CNS) disease require completely different agents, and the subspecies (T. b. gambiense vs. T. b. rhodesiense) determines which second-stage regimen is used; choosing the wrong agent risks fatal treatment failure.
The trypanothione system is the unifying pharmacological target across all kinetoplastid parasites (Trypanosoma, Leishmania) — trypanothione replaces glutathione as the primary antioxidant defense, and its absence from mammalian cells makes it an ideal selective drug target for antimonials, nifurtimox, melarsoprol, and eflornithine.
Miltefosine is the only oral agent for visceral leishmaniasis — its long half-life (~150 hours) creates a selection pressure window during the tail of drug exposure, which is why strict contraception is required not just during treatment but for 5 months after the last dose.
The standard treatment for toxoplasmosis (pyrimethamine + sulfadiazine) must always be supplemented with folinic acid (leucovorin) — not folic acid — to protect human bone marrow from DHFR inhibition; folic acid cannot bypass the DHFR block and is ineffective as a rescue agent.

Abbreviations: NECT = nifurtimox-eflornithine combination therapy · HAT = human African trypanosomiasis · DHFR = dihydrofolate reductase · DHPS = dihydropteroate synthase · ODC = ornithine decarboxylase · PCR = polymerase chain reaction · ROS = reactive oxygen species

Metronidazole — Nitroimidazole Class

Mechanism

Anaerobic Reductive Activation

Prodrug: ferredoxin-dependent reductases in anaerobic organisms reduce the nitro group → cytotoxic radical anion
Radical anion causes single- and double-strand DNA breaks → cell death
Mammalian cells lack the low-redox ferredoxin system — selective toxicity to anaerobes and microaerophilic protozoa
Tinidazole: same mechanism, longer half-life (~13 hours vs. 8 hours), fewer GI effects, single-dose options for giardiasis and trichomoniasis

Spectrum and Key Interactions

Protozoal and Anaerobic Coverage

Protozoa: Giardia lamblia, Entamoeba histolytica (intestinal and hepatic), Trichomonas vaginalis
Anaerobic bacteria: Bacteroides fragilis, Clostridium species, oral anaerobes
Amebic liver abscess: metronidazole clears invasive disease but does not eliminate intestinal cysts — must add a luminal agent (diloxanide furoate or iodoquinol) to prevent relapse
Alcohol: disulfiram-like reaction — inhibits aldehyde dehydrogenase → acetaldehyde accumulation; avoid alcohol during treatment and 48 hours after
Warfarin: inhibits CYP2C9 → ↑ INR; monitor INR closely

Human African Trypanosomiasis — Stage-Dependent Treatment

Stage	T. b. gambiense	T. b. rhodesiense	Rationale
Stage 1 — Blood and Lymph	Pentamidine	Suramin	No blood-brain barrier penetration needed; both agents effective in blood stage
Stage 2 — CNS Invasion	NECT (nifurtimox + eflornithine)	Melarsoprol (arsenic; post-treatment reactive encephalopathy risk ~10%)	Blood-brain barrier penetration required; eflornithine active only in gambiense (slower ODC turnover); fexinidazole now approved for gambiense stage 2

Eflornithine Selectivity for T. b. gambiense — Why It Does Not Work for T. b. rhodesiense

Eflornithine irreversibly inhibits ornithine decarboxylase (ODC) — the first enzyme in polyamine biosynthesis. Polyamines are essential for trypanosome cell division and are incorporated into the trypanothione system. The basis for its species selectivity is enzyme turnover rate: T. b. gambiense ODC has a much slower turnover than mammalian or T. b. rhodesiense ODC. Irreversible inhibition of a slowly-replaced enzyme leads to sustained depletion of polyamines; in rhodesiense (where ODC turns over more rapidly), the enzyme is replaced before sufficient depletion occurs and the drug fails. NECT (nifurtimox + eflornithine) allows a shorter eflornithine course (7 days vs. 14 days monotherapy), substantially reducing toxicity while maintaining efficacy. Fexinidazole is now WHO-recommended for all stages of gambiense HAT and represents the first fully oral treatment for stage 2 disease.

Chagas Disease and the Trypanothione System

Chagas Disease — T. cruzi

Benznidazole and Nifurtimox

Benznidazole (nitroimidazole): reductively activated by T. cruzi nitroreductases → covalent damage to DNA, RNA, and proteins
Nifurtimox (nitrofuran): generates reactive oxygen species → overwhelms the trypanothione-based antioxidant system unique to kinetoplastids
Acute phase: both agents achieve parasitological cure rates of 70–80%; highly effective for congenital disease if treated early
Chronic cardiomyopathy (established): BENEFIT trial showed no reduction in cardiac events despite PCR conversion to negative — treatment benefit in chronic disease is uncertain
Both agents are teratogenic — contraindicated in pregnancy

Shared Biochemical Vulnerability

Trypanothione System — Absent from Human Cells

Kinetoplastid parasites (Trypanosoma, Leishmania) use trypanothione (glutathione linked to spermidine) as their primary thiol-based antioxidant — not glutathione
Trypanothione is absent from mammalian cells → selective drug target across the entire kinetoplastid family
Melarsoprol (arsenic): directly binds and inactivates trypanothione → lethal oxidative stress
Nifurtimox: generates ROS that overwhelm trypanothione reductase capacity
Antimonials (Sb³⁺): inhibit trypanothione reductase → parasite cannot recycle oxidized trypanothione
Eflornithine: depletes polyamine precursors for trypanothione synthesis

Leishmaniasis — Drug Selection by Form and Region

First-Line — Visceral (India and Americas)

Liposomal Amphotericin B

Binds Leishmania membrane ergosterol-like sterols → lethal ion flux and membrane disruption
Liposomal formulation preferentially accumulates in reticuloendothelial cells where amastigotes reside → high therapeutic drug concentration at the infection site
Preferred in India (antimonial resistance >90% in Bihar state) and Europe
HIV coinfection: high relapse rate; indefinite secondary prophylaxis with L-AmB or pentamidine required after initial treatment

First Oral Agent — Visceral

Miltefosine

Alkylphosphocholine analog — disrupts Leishmania membrane phospholipid bilayer and modulates cell signaling
Only oral drug for visceral leishmaniasis; first-line in South Asia
Highly teratogenic — strict contraception mandatory during treatment and for 5 months after last dose (long half-life ~150 hours creates selection pressure tail)
Long half-life increases resistance selection risk — miltefosine resistance emerging in South Asia

Legacy and Region-Specific Agents

Antimonials and Pentamidine

Pentavalent antimonials (sodium stibogluconate): converted to trivalent Sb³⁺ → inhibits trypanothione reductase + purine transporter; still first-line in East Africa and parts of South America
Cardiotoxicity (QTc prolongation), pancreatitis, hepatotoxicity — significant adverse effect burden
Pentamidine: disrupts kinetoplast DNA structure; used for cutaneous leishmaniasis in certain regions
Pentamidine: severe hypoglycemia (destroys pancreatic beta cells) followed by diabetes with prolonged use

Toxoplasmosis — Sequential Folate Blockade

Standard Treatment

Pyrimethamine + Sulfadiazine + Folinic Acid

Sulfadiazine: inhibits DHPS — blocks early folate synthesis step (sulfonamide mechanism)
Pyrimethamine: inhibits DHFR — blocks downstream folate reduction step
Sequential double blockade: synergistic suppression of folate pathway → synergistic antiprotozoal effect
Folinic acid (leucovorin) is mandatory — bypasses the DHFR block in human bone marrow cells to prevent myelosuppression; pyrimethamine causes significant bone marrow toxicity without it
Folic acid does NOT work — it cannot bypass the DHFR block; cannot substitute for folinic acid
Treatment duration: 6 weeks acute; then chronic suppression until immune reconstitution

Prophylaxis and Special Situations

TMP-SMX and Pregnancy

Primary prophylaxis in HIV/AIDS with CD4 <100 cells/μL: TMP-SMX (double-strength) — also simultaneously prevents Pneumocystis jirovecii pneumonia
Discontinue prophylaxis after sustained CD4 count recovery above 200 cells/μL on antiretroviral therapy × 3 months
Alternatives if sulfa-intolerant: dapsone + pyrimethamine + folinic acid, or atovaquone
Pregnancy: spiramycin is used to reduce placental transmission risk in acute maternal infection — does not treat established fetal infection; add pyrimethamine + sulfadiazine after first trimester if fetal infection confirmed

Suggested References

Author / Source	Title	Publication
Katzung BG, ed.	Basic and Clinical Pharmacology, 15th ed. — Chapter 52: Antiprotozoal Drugs	McGraw-Hill, 2021
Brunton LL, Knollmann BC, eds.	Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed. — Chapter 50: Chemotherapy of Protozoal Infections	McGraw-Hill, 2023
Brunton L, Knollmann B, Hilal-Dandan R, eds.	Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed. — Chapter 50: Chemotherapy of Protozoal Infections	McGraw-Hill; 2023
World Health Organization	Control and surveillance of human African trypanosomiasis	WHO Technical Report Series No. 984; 2013
Priotto G, Kasparian S, Mutombo W, et al.	Nifurtimox-eflornithine combination therapy for second-stage African Trypanosoma brucei gambiense trypanosomiasis (NECT): a multicentre, randomised, phase III, non-inferiority trial	Lancet. 2009;374(9683):56–64
Morillo CA, Marin-Neto JA, Avezum A, et al.	Randomized trial of benznidazole for chronic Chagas cardiomyopathy (BENEFIT)	N Engl J Med. 2015;373(14):1295–1306
Sundar S, Chakravarty J	Leishmaniasis: an update of current pharmacotherapy	Expert Opin Pharmacother. 2013;14(1):53–63
Bhattacharya SK, Sinha PK, Sundar S, et al.	Phase 4 trial of miltefosine for the treatment of Indian visceral leishmaniasis	J Infect Dis. 2007;196(4):591–598
Bern C	Chagas disease	N Engl J Med. 2015;373(5):456–466
Panel on Opportunistic Infections in Adults and Adolescents with HIV	Guidelines for the prevention and treatment of opportunistic infections in HIV-infected adults and adolescents: Toxoplasma gondii encephalitis	AIDSinfo; 2023. aidsinfo.nih.gov
Montoya JG, Liesenfeld O	Toxoplasmosis	Lancet. 2004;363(9425):1965–1976
Muller I, Baker J, Mesu V, et al.	Fexinidazole for late-stage African Trypanosoma brucei gambiense sleeping sickness	Lancet. 2021;397(10279):1152
Drugs for Parasitic Infections	Treatment guidelines for parasitic infections	The Medical Letter on Drugs and Therapeutics; 2013 special issue
Fairlamb AH, Cerami A	Metabolism and functions of trypanothione in the Kinetoplastida	Annu Rev Microbiol. 1992;46:695–729