Atovaquone/Proguanil-Resistant Plasmodium falciparum Malaria Appears

Abstract & Commentary

Synopsis: A returning traveler from West Africa developed falciparum malaria, which was treated with the combination agent atovaquone/proguanil. Relapse provided an opportunity to use molecular techniques to confirm parasite resistance due to a genetic mutation affecting parasite cytochrome b in the original infecting strain of Plasmodium falciparum. When such a mutation occurs in a strain of P falciparum, which has already experienced a mutation conferring resistance to folic acid antagonists, it spells potential trouble for atovaquone/proguanil.

Source: Schwartz E, et al. Genetic confirmation of atovaquone-proguanil-resistant Plasmodium falciparum malaria acquired by a nonimmune traveler to East Africa. Clin Infect Dis. 2003;37:450-451.

A 24-year-old Israeli woman acquired falciparum malaria during a 1-week vacation to Mombassa, Kenya, in January 2002. She had not taken malaria chemoprophylaxis, and her symptoms developed 10 days after return home, with her admission blood smears showing 3% Plasmodium falciparum parasitemia. She was treated with atovaquone 1000 mg and proguanil 400 mg daily for 4 days, with resolution of fever. Her malaria smears showed parasite clearance by day 4. Fever recurred 30 days later, and malaria blood smears again showed P falciparum. The patient recovered with administration of oral quinine 600 mg t.i.d. for 3 days and doxycycline 100 mg b.i.d. for 7 days.

Molecular techniques were used to compare the malaria parasites observed during her initial presentation and at recurrence. This included extraction of DNA from malaria parasites, PCR amplification of the gene encoding the merozoite surface protein-1, and genetic fingerprinting with single-strand conformational polymorphism analysis, which showed the isolates to have identical genetic fingerprints. The initial isolate had wild-type sequence of cytochrome b, but the recrudescent isolate had a mutation at position 268 resulting in a substitution of tyrosine with serine. Both isolates had mutations in the dihydrofolate reductase gene associated with resistance to cycloguanil, the active metabolite of proguanil. These techniques confirmed the true basis for this malaria treatment failure.

Comment by Lin H. Chen, MD

Drug resistance occurring among P falciparum organisms are well-recognized problems that arise from chromosomal mutations. Polymorphisms in the pfcrt gene, which lead to impaired parasite vacuole uptake of chloroquine and polymorphisms in the pfmdr1 gene, are both associated with chloroquine resistance.1,2 Point mutations in the dhps gene and the dhfr gene lead to reduced drug-binding affinities for dihydropteroate synthetase and dihydrofolate reductase, respectively, and are associated with sulfadoxine-pyrimethamine resistance.2 Mefloquine resistance is quite significant in Southeast Asia and appears to be associated with mutations in the pfmdr1 gene, as does quinine resistance.2 Doxycycline had been the only chemoprophylactic option in areas with multidrug resistance until the combination of atovaquone and proguanil became available. Atovaquone/proguanil is also a recent addition to the therapeutic options for falciparum malaria, with cure rates > 96% at the dose of 1000 mg/400 mg daily for 3 days.3,4

Atovaquone acts on malaria parasites by inhibiting mitochondrial electron transport at the cytochrome b level.3 Treatment of P falciparum with atovaquone alone unfortunately results in rapid emergence of resistance. Proguanil, with its metabolite cycloguanil, is a parasite dihydrofolate reductase inhibitor. Proguanil enhances atovaquone’s effect on mitochondrial membrane potential, and the combination is synergistic.5 However, point mutations in the parasite dhfr gene lead to proguanil resistance, which is well established.2

Resistance to atovaquone is attributed to point mutations in the P falciparum cytochrome b gene. Resulting amino acid substitutions caused by such gene mutations lead to decreased drug binding to malaria parasite cytochrome b, thereby reducing atovaquone’s ability to inhibit the cytochrome b complex role in electron transport.6 Mutations at codon 268 of the cytochrome b gene produce the amino acid change from tyrosine to serine and have been associated with atovaquone/proguanil treatment failure.7 The case report by Schwartz and associates documents clinical failure of atovaquone/proguanil in the treatment of a traveler with P falciparum malaria and clearly demonstrates the genetic alteration. 

Travelers have played significant roles as both couriers and disease transmitters in the history of infectious diseases.8 The dramatic increase in the movement of people across international borders provides opportunities for travelers to be sentinels of emerging infections, including drug-resistant malaria. Surveillance networks such as GeoSentinel and TropNetEurope collect and analyze data on travel-related illnesses, especially imported infectious diseases.9,10 Analysis of imported falciparum malaria in the TropNetEurope database from 1999 to 2000 showed that the majority of European travelers (58.8%) and immigrants (68.2%) acquired their malaria infections in West Africa, and only 40% of European travelers and 28% of the immigrants used malaria chemoprophylaxis during travel.10 Such information is useful in focusing preventive strategies on travelers going to West Africa and reaching more immigrants traveling to visit friends and relatives.

Another informative study assessed the efficacy of malaria chemoprophylaxis in returning Danish travelers with malaria from 1997 to 1999.11 Breakthrough malaria occurred in travelers taking atovaquone/proguanil, as well as other chemoprophylactic regimens. The incidence of imported malaria differed greatly according to the country visited, from 1 per 140 travelers to Ghana to almost 1 per 40,000 in travelers to Thailand; the estimated failure rates against P falciparum, based on cases per prescription, in compliant patients, were: chloroquine/ proguanil (1:599), mefloquine (1:2232), and atovaquone/proguanil (1:1943).11 Thirty-seven percent of the malaria cases took all doses of their chemoprophylactic regimen, indicating a combination of poor compliance and significant failures. This particular study in travelers demonstrated a lower efficacy of chloroquine/proguanil against P falciparum, likely from increasing parasite resistance.11

Other case reports of travelers with P falciparum malaria resistant to atovaquone/proguanil have been published in travelers returning from the Ivory Coast (Reviewed in Travel Medicine Advisor Update May/June 2003) and Nigeria.12,13 Molecular studies showed mutations that led to changes from tyrosine to serine12 or tyrosine to asparagine13 in codon 268 of cytochrome b. The case report is further evidence that P falciparum can rapidly develop resistance to atovaquone and proguanil following treatment and provides an example of travelers as sentinels for the emergence of drug resistance. Malaria should remain in the differential of febrile returning travelers even in those receiving atovaquone/proguanil for prophylaxis. Travel medicine specialists should be watchful for the possible emergence of atovaquone and proguanil resistance in the evaluation of travelers, if use either as malaria chemoprophylaxis or as malaria therapy.

Dr. Chen is Clinical Instructor, Harvard Medical School, Director, Travel Resource Center, Mt. Auburn Hospital, Cambridge, Mass.


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