Drug resistance in malaria—A perspective

Triple Combination Therapy and Drug Cycling-Tangential Strategies for Countering Artemisinin Resistance

PURPOSE OF REVIEW

This review attempts to understand the reasons for the successes and failures of the two novel strategies that have slowly begun to emerge as potential counters for anti-malarial drug resistance-"Triple Combination Therapy" and "Drug Cycling."

RECENT FINDINGS

Recent reports have suggested that increasing the heterogeneity within the parasite's environment, both at an individual and the population level, may help raise the probabilistic barrier of development of resistance in the parasite. The encouraging results following the implementation of a few experimental triple combination therapies like atovaquone-proguanil-artesunate along with the re-emergence of chloroquine sensitive Plasmodium falciparum parasites in the sub-Saharan African nations have re-kindled mankind's hope of curbing anti-malarial drug resistance. The addition of a third drug with traits like a medium half-life and benign safety profile is crucial to achieving SERCAP (single encounter radical cure and preventive therapy), the principle of a triple combination therapy. Simultaneously, the plausible reasons behind the re-emergence of chloroquine sensitive Plasmodium falciparum malaria in the high transmission regions could be the re-expansion of an existing chloroquine susceptible parasite reservoir and a greater predisposition towards the development of polyclonal infections. Another potential reason for this observation could be an impaired deoxyribonucleic acid (DNA) repair mechanisms in the south-east Asian Plasmodium falciparum parasites. These strategies may potentially emerge as the key players in warding off anti-malarial drug resistance in the near future. However, their implementation would be dictated by a host of factors like the epidemiological knowledge, population pharmacokinetics, drug-resistance patterns, cost, availability, and ease of adherence.

Malaria and quinine resistance: a medical and scientific issue between Brazil and Germany (1907-19)

This article addresses the discussion about quinine-resistant malaria plasmodium in the early decades of the twentieth century. Observed by Arthur Neiva in Rio de Janeiro in 1907, the biological and social resistance of malaria sufferers to preventive and curative treatment with quinine was corroborated three years later by Oswaldo Cruz during the construction of the Madeira-Mamoré Railway in the Brazilian Amazon. Likewise in 1910, ailing German workers were transferred from Brazil to Hamburg's Institute for Maritime and Tropical Diseases, where quinine resistance was confirmed by Bernard Nocht and Heinrich Werner. When the First World War saw failures in treating and preventing malaria with quinine along with violent outbreaks of the disease on the Turkish and Balkan fronts, resistance to this alkaloid became the topic of the day within the field of experimental medicine in Germany. New attempts were made to account for the resistance, especially by the physician Ernst Rodenwaldt, who explored the topic by applying modern theories on heredity. The present article offers a preliminary survey and analysis of pronouncements about quinine resistance, shedding new light on the circulation of knowledge in the field of tropical medicine.

Antimalarial drug resistance

Malaria, the most prevalent and most pernicious parasitic disease of humans, is estimated to kill between one and two million people, mainly children, each year. Resistance has emerged to all classes of antimalarial drugs except the artemisinins and is responsible for a recent increase in malaria-related mortality, particularly in Africa. The de novo emergence of resistance can be prevented by the use of antimalarial drug combinations. Artemisinin-derivative combinations are particularly effective, since they act rapidly and are well tolerated and highly effective. Widespread use of these drugs could roll back malaria.

The de novo selection of drug-resistant malaria parasites

Antimalarial drug resistance emerges de novo predominantly in areas of low malaria transmission. Because of the logarithmic distribution of parasite numbers in human malaria infections, inadequately treated high biomass infections are a major source of de novo antimalarial resistance, whereas use of antimalarial prophylaxis provides a low resistance selection risk. Slowly eliminated antimalarials encourage resistance largely by providing a selective filter for resistant parasites acquired from others, and not by selecting resistance de novo. The de novo emergence of resistance can be prevented by use of antimalarial combinations. Artemisinin derivative combinations are particularly effective. Ensuring adequate treatment of the relatively few heavily infected patients would slow the emergence of resistance.

A review on vaccination against protozoa and arthropods of veterinary importance

The recent advances in immunology and biotechnology have stimulated much research on the control of parasitic diseases through vaccination. This is a review of the state of the art regarding important protozoan and arthropod veterinary parasites. A live oocyst vaccine for avian coccidiosis is still in use but much work has been done on the identification, cloning, and assay of protective antigens. The sporozoites of Eimeria tenella have been the preferred subject and at least four recombinant antigens have already been tested with partial success. Premunization against babesiosis is still widely used in Latin America as is a live vaccine with attenuated parasites in Australia. At least three Babesia bovis and three Babesia bigemina antigens that generate partial protection have been produced as recombinant proteins. A vaccine against canine babesiosis is being commercialized in France. Infection-treatment is still used to vaccinate against Theileria parva and a schizont vaccine against Theileria annulata. Recombinant sporozoite antigens have been assayed with partial success against both species but the identification and administration of protective schizont antigens, regarded as the most important, still requires considerable work. The immunological control of African trypanosomoses is still impaired by the antigenic variation that the parasites experience during the infection. Although some possibilities exist, most specialists are pessimistic about the promise of developing a vaccine in the near future. Control of Boophilus ticks with an occult tick intestine recombinant antigen seems to have potential in inhibiting reproduction of the tick but salivary antigens appear to be more effective at inhibiting feeding and pathogen transmission. Vaccination with a Hypoderma protein, recently cloned, has induced 90% protection against subsequent infestations. It is very likely that effective vaccines against veterinary parasites will become available in the near future.

The dynamics of drug resistance in Plasmodium falciparum

Drug resistance of Plasmodium falciparum is not a recent phenomenon, but it became a major problem when the parasite became resistant to chloroquine, the cheapest and initially the most effective antimalarial compound that could be used for treatment and suppression. In some areas this problem is compounded by resistance to the first line of alternative drugs, and rapid loss of sensitivity to the next line. The dynamics of drug resistance are regulated mainly by drug related selection pressure and intensity of malaria transmission. Mass drug administration in its various forms, and insufficient treatment are obviously the most important motors of selection.

In vitro activity of artemisinin, its derivatives, and pyronaridine against different strains of Plasmodium falciparum

The activities of artemisinin (qinghaosu), dihydroartemisinin (dihydroqinghaosu), artemether and pyronaridine were tested in a 48 h in vitro assay against 3 chloroquine-sensitive and one chloroquine-resistant strains of Plasmodium falciparum. Growth inhibition was modelled with a sigmoid Emax model. All compounds markedly inhibited the growth of all strains although, for the chloroquine-resistant strain, merozoites were detected at concentrations as high as 10(-4) M of artemisinin, dihydroartemisinin and artemether. Dihydroartemisinin, artemether and pyronaridine appeared to be more potent than artemisinin, with EC50 values of 4.7-23 nM, 0.98-6.1 nM and 4.4-18 nM respectively, while the EC50 value of artemisinin was 3-108 nM against all 4 strains.

Strategies for the prevention of malaria in travellers: comparison of drug regimens by means of risk-benefit analysis

A risk-benefit analysis has been used to balance the risks to travellers of death from malaria against the toxicity of various drug regimens. The mortality of travellers who take no prophylaxis is estimated to be surprisingly low, and it is concluded that most travellers, even in areas of drug resistance should take only non-toxic drugs, such as a combination of chloroquine and proguanil.

[Resistance of Plasmodium falciparum to fansidar, quinine and tetracycline]

Descreve-se pela primeira vez uma cêpa amazônica de Plasmodium falciparum resistente ao Fansidar (pirimetamina mais sulfadoxina) e quinino mais tetraciclina simultaneamente. O paciente, com 13 anos de idade, residia há cinco anos em Ariquemes, Estado de Rondônia, na Amazônia brasileira. A infecção foi curada com dose elevada de cloroquina administrada em dose única intravenosa (IV). É evidenciado o alto valor da cloroquina na cura da malária falciparum resistente, quando administrada em doses maiores que as usadas nos esquemas convencionais de tratamento.

Sensitivity of Plasmodium falciparum to chloroquine in Busia District, Kenya

Individuals infected with Plasmodium falciparum were randomly divided into two groups; one group was treated with a single dose of 10 mg chloroquine base per kg. body-weight and the other with 25 mg base of chloroquine per kg body-weight given over three days, followed by an observation period of seven days. By Day 3 of observation complete parasite clearance had occurred in all the 125 triple dose recipients and 113 of 114 (99.1%) of those who had the single dose. 94.4% of 36 isolates tested in vitro by the macrotechnique were sensitive to drug concentration of 0.75 nmol/ml blood or less. On isolate was relatively less sensitive and required a concentration of chloroquine of 1.50 nmol/ml to inhibit schizont growth. However, the same isolate responded well to 25 mg base of chloroquine. These findings have demonstrated that, at present, isolates of P. falciparum in Busia District are sensitive to the standard dose of 10 mg chloroquine base and there is no reason therefore to resort to alternative antimalaria drugs. These should be reserved for special cases only.