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Fansidar

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Fansidar

CLINICAL PHARMACOLOGY

Microbiology

Mechanism of Action

Sulfadoxine and pyrimethamine, the constituents of Fansidar (sulfadoxine and pyrimethamine) , are folic acid antagonists. Sulfadoxine inhibits the activity of dihydropteroate synthase whereas pyrimethamine inhibits dihydrofolate reductase.

Activity in vitro

Sulfadoxine and pyrimethamine are active against the asexual erythrocytic stages of Plasmodium falciparum. Fansidar (sulfadoxine and pyrimethamine) may also be effective against strains of P. falciparum resistant to chloroquine.

Drug Resistance

Strains of P. falciparum with decreased susceptibility to sulfadoxine and/or pyrimethamine can be selected in vitro or in vivo. P. falciparum malaria that is clinically resistant to Fansidar (sulfadoxine and pyrimethamine) occurs frequently in parts of Southeast Asia and South America, and is also prevalent in East and Central Africa. Therefore, Fansidar (sulfadoxine and pyrimethamine) should be used with caution in these areas. Likewise, Fansidar (sulfadoxine and pyrimethamine) may not be effective for treatment of recrudescent malaria that develops after prior therapy (or prophylaxis) with Fansidar (sulfadoxine and pyrimethamine) .

Pharmacokinetics

Absorption

After administration of 1 tablet, peak plasma levels for pyrimethamine (approximately 0.2 mg/L) and for sulfadoxine (approximately 60 mg/L) are reached after about 4 hours.

Distribution

The volume of distribution for sulfadoxine and pyrimethamine is 0.14 L/kg and 2.3 L/kg, respectively.

Patients taking 1 tablet a week (recommended adult dose for malaria prophylaxis) can be expected to have mean steady state plasma concentrations of about 0.15 mg/L for pyrimethamine after about four weeks and about 98 mg/L for sulfadoxine after about seven weeks. Plasma protein binding is about 90% for both pyrimethamine and sulfadoxine. Both pyrimethamine and sulfadoxine cross the placental barrier and pass into breast milk.

Metabolism

About 5% of sulfadoxine appears in the plasma as acetylated metabolite, about 2 to 3% as the glucuronide. Pyrimethamine is transformed to several unidentified metabolites.

Elimination

A relatively long elimination half-life is characteristic of both components. The mean values are about 100 hours for pyrimethamine and about 200 hours for sulfadoxine. Both pyrimethamine and sulfadoxine are eliminated mainly via the kidneys.

Characteristics in Patients

In malaria patients, single pharmacokinetic parameters may differ from those in healthy subjects, depending on the population concerned. In patients with renal insufficiency, delayed elimination of the components of Fansidar (sulfadoxine and pyrimethamine) must be anticipated.

Last reviewed on RxList: 10/10/2008
This monograph has been modified to include the generic and brand name in many instances.

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