Doxycycline
Doxycycline (Vibramycin, Pfizer), a tetracycline, continues to be the preferred agent for persons unable to tolerate mefloquine or for those traveling to areas where mefloquine resistance is present. The drug is taken in a dose of 100 mg daily during exposure and continued for 4 weeks after the traveler returns home. No loading dose is required. Doxycycline has been shown to have comparable prophylactic efficacy to mefloquine, but the need for daily dosing may reduce adherence and, therefore, the drug’s effectiveness. Side effects of doxycycline include gastrointestinal upset, esophagitis, and vaginal candidiasis. Furthermore, since the drug can be photosensitizing, its use requires adequate sunscreen protection. Doxycycline is contraindicated in pregnant or lactating women and in children younger than 8 years of age. Long-term administration of tetracyclines has generally been well tolerated.
Atovaquone/Proguanil
The U.S. Food and Drug Administration recently approved a fixed combination tablet of atovaquone and proguanil (Malarone, Glaxo Wellcome) for the chemoprophylaxis of malaria. Travelers need only take the medication during periods of exposure and for 1 week after departure. This is an advantage over mefloquine and doxycycline, which need to be taken for 4 weeks following exposure. Atovaquone/proguanil has been found to be useful against strains of malaria that are resistant to other agents. The adult dosing regimen for prophylaxis consists of one tablet (250 mg atovaquone/100 g proguanil) daily starting 2 days before travel, one tablet per day during travel, and one tablet daily for 7 days after leaving an endemic area. The most common adverse effects are abdominal pain, nausea, and headaches. Since insufficient data exist on its safety in pregnancy, it should not be taken by pregnant or lactating women. Atovaquone/proguanil is also contraindicated in those with severe renal impairment. Overall, this combination is well tolerated and efficacious in the prevention of P. falciparum. More data are needed to confirm its efficacy against the other malaria species.
Primaquine
Primaquine (base, Sanofi-Synthelabo) has activity against both the blood and liver stages of malaria parasites. In patients who have been infected with P. vivax or P. ovale, relapses can occur far after chemoprophylaxis is discontinued because the standard drugs fail to eliminate the liver hypnozoites. To prevent relapses, a course of primaquine may be given as terminal prophylaxis (to eliminate latent intrahepatic stages) at the conclusion of the standard post-travel chemoprophylaxis regimen. One 26.3 mg tablet is administered daily for 14 days after the trip. Terminal prophylaxis is generally indicated only for people who have a prolonged exposure in malaria endemic areas or who have visited areas of intense P. vivax transmission. Primaquine causes acute intravascular hemolysis in those with an inborn deficiency of glucose-6-phosphate-dehydrogenase (G6PD). The drug should not be prescribed without confirming a normal G6PD level. Primaquine should not be used in pregnant patients.
Future Options
Since primaquine is effective against the hypnozoites developing in the liver, it has recently been reconsidered for causal prophylaxis (to interfere with the early intrahepatic development) of malaria, particularly if the exposure is short. Travelers would need to take primaquine only during exposure and for 1 week after. Randomized controlled trials of the use of primaquine as a chemoprophylactic agent (30 mg base/day) have demonstrated protective efficacy against both P. falciparum and P. vivax infections, but at this time primaquine is not licensed for this indication. Tafenoquine is a long-acting primaquine analogue with the same mechanism of action, but this drug is not yet licensed in the United States. Like primaquine, it can be used for causal prophylaxis. It has a long elimination half-life (14-28 days for tafenoquine versus 4-6 hours for primaquine) that allows for infrequent dosing regimens. Preliminary reports indicate that a once-weekly dose is effective against P. falciparum, and a 3-day loading dose alone provides protection for several weeks. Tafenoquine remains only a future option for the prevention of malaria, but it may improve medication compliance with malaria chemoprophylaxis.
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