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Sulfadoxine + pyrimethamine: mechanism and uses
This combination is an antifolate regimen. It inhibits two sequential steps in the protozoal folate pathway, producing a synergistic effect.
1. Sequential blockade of folate synthesis
Parasites need folate derivatives to make DNA and multiply.
| Drug | Enzyme blocked | Effect |
|---|---|---|
| Sulfadoxine | Dihydropteroate synthase (DHPS) | Prevents formation of dihydrofolic acid |
| Pyrimethamine | Dihydrofolate reductase (DHFR) | Prevents conversion of dihydrofolate to active tetrahydrofolate |
Pathway:
PABA → dihydropteroic acid → dihydrofolic acid → tetrahydrofolate → DNA synthesis
↑ sulfadoxine blocks here ↑ pyrimethamine blocks here
↑ sulfadoxine blocks here ↑ pyrimethamine blocks here
Because the medicines act at consecutive steps, the parasite has much less ability to produce tetrahydrofolate. Without tetrahydrofolate, it cannot synthesize purines, thymidylate, and ultimately DNA.
Pyrimethamine preferentially inhibits protozoal DHFR, but it is not perfectly selective. It can also inhibit human DHFR, particularly at higher or prolonged doses. This accounts for its bone-marrow toxicity. A sulfonamide plus pyrimethamine gives synergistic activity through this sequential inhibition of folate metabolism. Katzung's Basic and Clinical Pharmacology, 16th Edition, Ch. 46 and Ch. 52.
2. Why folinic acid is given
Folinic acid, also called leucovorin or calcium folinate, is a reduced, usable form of folate.
It is administered with pyrimethamine, especially in toxoplasmosis treatment, to reduce toxicity to human bone marrow, such as:
- Megaloblastic anemia
- Leukopenia or neutropenia
- Thrombocytopenia
- Pancytopenia
It is called rescue therapy because it bypasses the DHFR-blocked step in human cells. It helps protect the patient without materially rescuing the parasite. Pyrimethamine plus sulfadiazine plus leucovorin is a standard regimen for toxoplasmosis. Murray & Nadel's Textbook of Respiratory Medicine, toxoplasmosis section.
Uses
Toxoplasmosis
The classic treatment is:
Pyrimethamine + sulfadiazine + folinic acid
Note the distinction:
- Sulfadiazine is classically paired with pyrimethamine for toxoplasmosis.
- Sulfadoxine is a long-acting sulfonamide that has been paired with pyrimethamine mainly for malaria prevention programs and older malaria treatment regimens.
Falciparum malaria
Sulfadoxine-pyrimethamine (SP) is also known as Fansidar.
Historically, it was used to treat uncomplicated Plasmodium falciparum malaria, including chloroquine-resistant malaria. However, resistance is now widespread, so it is generally not used as routine treatment or prophylaxis for travelers.
Its continuing public-health role is mainly intermittent preventive treatment in pregnancy in eligible malaria-endemic areas, according to local/WHO policy. A recent systematic review confirms that resistance can reduce its effectiveness in pregnancy, though SP remains policy-relevant in many African settings (2025 systematic review). Goldman-Cecil Medicine, section “Inhibitors of Folate Metabolism.”
Leishmaniasis
The screenshot states that pyrimethamine and sulfadiazine have been used for leishmaniasis. This is not a usual modern first-line treatment. Pyrimethamine-containing regimens have had historical or occasional roles, but routine treatment depends on the type and region of leishmaniasis and typically uses agents such as liposomal amphotericin B, miltefosine, paromomycin, or antimonials.
Adverse effects
Serious cutaneous reactions
This is the major warning on your slide, and it is especially associated with the sulfonamide component.
Possible reactions include:
- Rash and urticaria
- Photosensitivity
- Erythema multiforme
- Stevens-Johnson syndrome (SJS)
- Toxic epidermal necrolysis (TEN)
A rash, mucosal sores, blisters, skin peeling, fever, facial swelling, or eye involvement after taking this drug requires urgent medical assessment. WHO product information warns that severe skin reactions, liver injury, and blood disorders are rare but serious (WHO patient information).
Other important adverse effects
- Bone marrow suppression from pyrimethamine
- Nausea, vomiting, stomatitis
- Hepatitis or liver injury
- Hypersensitivity reactions due to sulfadoxine
- Avoidance/caution in people with previous severe sulfonamide reactions
Concurrent use with other folate antagonists or sulfonamides, such as trimethoprim-sulfamethoxazole, can increase hematologic and cutaneous toxicity.
Exam summary
Sulfadoxine inhibits DHPS and pyrimethamine inhibits protozoal DHFR. Together they cause sequential blockade of folate synthesis, impairing DNA synthesis. Pyrimethamine can also inhibit human DHFR and cause marrow suppression, so folinic acid is co-administered, particularly in toxoplasmosis. Major toxicity of sulfadoxine-pyrimethamine is rare but severe cutaneous hypersensitivity such as SJS/TEN.
Current-evidence note: Resistance to sulfadoxine-pyrimethamine, including in India, remains a major limitation (2024 India resistance review).
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