THE USE OF ANTIMALARIAL DRUGS

Resistance to antimalarial drugs arises as a result of spontaneously-occurring mutations that affect the structure and activity at the molecular level of the drug target in the malaria parasite or affect the access of the drug to that target (18). Mutant parasites are selected if antimalarial drug concentrations are sufficient to inhibit multiplication of susceptible parasites but are inadequate to inhibit the mutants, a phenomenon known as "drug selection" (11, 19). This selection is thought to be enhanced by subtherapeutic plasma drug levels and by a flat dose-response curve to the drug.

The evolution of drug resistance in Plasmodium is not fully understood although the molecular basis for resistance is becoming clearer. The development of resistance to chloroquine probably requires successive gene mutations and evolves slowly. Recent evidence indicates that for P. falciparum some of these mutations occur in a transporter-like gene on the surface of the parasite food vacuole (20). Preliminary reports suggest that a different set of mutations is probably involved in chloroquine resistance for P. vivax (20). The molecular basis for resistance to antifolates, such as sulfadoxine-pyrimethamine has been well characterized. P. falciparum resistance to sulfadoxine-pyrimethamine is primarily conferred by successive single-point mutations in parasite dhfr, the gene that encodes the target enzyme dihydrofolate reductase (DHFR), and by additional mutations in dhps, which encodes for the enzyme dihydropteroate synthetase (DHPS) (21).

Various factors relating to drug, parasite and human host interactions contribute to the development and spread of drug resistance. The molecular mechanism of drug action is a critical element in the speed at which resistance develops. In addition, drugs with a long terminal elimination half-life enhance the development of resistance, particularly in areas of high transmission. Similarly, increased drug pressure is a significant contributor to drug resistance. As increased amounts of a drug are used, the likelihood that parasites will be exposed to inadequate drug levels rises and resistant mutants are more readily selected (22). Parasite factors associated with resistance include the Plasmodium species concerned and the intensity of transmission. Human host factors include the widespread and/or irrational use of antimalarial drugs and possibly the level of host immunity. The role of host immunity in propagating resistance is unclear. However, immunity acts synergistically with chemotherapy and can enhance therapeutic effects and even parasite clearance of drug-resistant infections.

The increase in chloroquine resistance in East Africa has led to a rise in malaria mortality (4). Similarly, a significant rise in malaria mortality in children under 5 years of age has been obseved in Senegal in West Africa, coinciding with the emergence of chloroquine resistance in the area (23). The incidence of severe malaria has risen with increasing chloroquine resistance in Malawi and Democratic Republic of the Congo (24). Antimalarial drug resistance has also been implicated in the increasing frequency and severity of epidemics (3).

Conditions for the development and spread of drug resistance differ between the Asian and African continents. Migration of individuals carrying resistant gametocytes has probably been of major importance for the spread of chloroquine resistance between different endemic areas in Asia and Oceania and the initial introduction of chloroquine resistance to East Africa.

Parasite susceptibility to antimalarial drugs can be assessed by in vitro or in vivo techniques. In vitro techniques rely on the collection of parasitized blood from patients and the testing of parasite susceptibility to drugs in culture or by the use of molecular techniques such as PCR. In vivo techniques rely on monitoring of the symptoms associated with malaria, such as fever, and parasitaemia (25).

A major purpose of assessing the therapeutic efficacy of antimalarial drugs in confirmed malaria patients is to monitor efficacy over time, especially in vulnerable groups in highly endemic areas, and to guide treatment policy. Antimalarial drug responses are assessed clinically from rates of symptom resolution e.g. fever clearance, coma recovery, or parasitologically from parasite clearance and overall cure rates.

Until the end of the 1980s, most in vivo studies focused on the parasitological response to a given drug, and infections were classified as sensitive (S), or resistant (R) at one of three levels, RI, RII or RIII. An RI response corresponds to an initial clearance of parasitaemia and then recrudescence 8 or more days after treatment; an RII response is the clearance or substantial reduction of parasitaemia with recrudescence of parasitaemia on day 7; and an RIII response refers to a situation in which there is no initial reduction of asexual parasitaemia and the levels may actually increase (17). Follow-up of treated patients for evidence of recurrence of parasitaemia may continue for 7, 14 or 28 days, depending on the investigators’ interest in detecting lower levels of resistance and on budgetary limitations (26-28).

Protocols have been modified and simplified to facilitate their use in high-transmission areas in Africa, where populations may have asymptomatic parasitaemia in the absence of clinical manifestation. The generally accepted objective of malaria treatment in these areas is not so much the clearance of parasitaemia but the resolution of clinical symptoms and acute febrile illness as measured by the adequate clinical response (ACR) and early and late treatment failure (ETF and LTF) (29). The therapeutic response is classified as ETF if the patient develops clinical or parasitological symptoms during the first 3 days of follow up; and as LTF if the patient develops symptoms during the follow-up period from day 4 to day 14, without previously meeting the criteria for ETF. ACR is defined as either the absence of parasitaemia on day 14 (irrespective of axillary temperature), or the absence of clinical symptoms on day 14 (irrespective of parasitaemia), in patients who did not meet the criteria of ETF or LTF before. Although the measurement of clinical response is of value in areas of high transmission, the impact of asymptomatic residual parasitaemia on other malaria-related conditions, such as anaemia and malnutrition, has not been examined (2).

WHO has further adapted a protocol for use in areas with moderate or low endemicity (large areas in South-East Asia, the Western Pacific region, the Eastern Mediterranean region, South America and Central America, and parts of tropical Africa) using the same classification. However, in these areas, the objective of malaria treatment is the clearance of the parasitaemia as well as the resolution of clinical symptoms.

Experience in malaria control programmes has shown that in vitro tests of parasite susceptibility to antimalarial drugs cannot substitute for in vivo observations on malaria therapy. However, they are a useful research tool to provide background information for the development and evaluation of drug policies and can provide an early warning of the appearance of drug resistance. They are best used to define specific aspects of the temporal and geographical response to drugs: longitudinal follow-up of drug susceptibility of the parasites in areas where changes are introduced compared with those where such changes are not implemented; longitudinal follow-up of susceptibility to a drug previously withdrawn because of an unacceptable level of resistance; monitoring of cross-resistance patterns; and the establishment of baseline data on responses to a new antimalarial drug prior to its deployment for treatment. The application and usefulness of in vitro tests is restricted by the need for trained personnel and their labour-intensive nature.

A global picture of reduced susceptibility of P. falciparum to various antimalarial drugs is provided in figure 1.

Strains of P. falciparum resistant to chloroquine were first suspected in Thailand in 1957 and found in patients in Colombia and Thailand in 1960. Since then, resistance to this drug has spread widely and there is now high-level resistance to chloroquine in South Asia, South-East Asia, Oceania, the Amazon Basin and some coastal areas of South America. In Africa, chloroquine resistance was first documented in the United Republic of Tanzania in 1979 and has spread and intensified in the last 20 years. In most countries of East Africa and in Ethiopia more than 50% of patients currently experience a recurrence of parasitaemia with symptoms by day 14 after treatment. Moderate levels of resistance are found in central and southern Africa. In West Africa, reported rates of resistance vary widely but tend to be lower than in central and southern Africa. Strains of P. falciparum remain sensitive to chloroquine in Central America north of the Panama Canal, the island of Hispaniola (Haiti and the Dominican Republic) and in El Faiyûm governorate in Egypt.

Although amodiaquine is generally more effective than chloroquine against chloroquine-resistant strains of P. falciparum (30), there is cross-resistance and moderate-to-high levels of amodiaquine resistance have been reported from Papua New Guinea, East Africa and the Amazon Basin. This drug continues to be efficacious as a single drug in most of West and central Africa and on the northern Pacific Coast of South America where, in some countries, it is used in combination with sulfadoxine-pyrimethamine.

High levels of resistance to this drug are found in the Amazon Basin and throughout South-East Asia, with the possible exception of some areas in eastern Cambodia and northern Viet Nam. In East Africa resistance rates are variable, ranging from 10-50% in 14-day therapeutic efficacy trials. Low levels of resistance (< 10% ETF + LTF) are found on the Indian subcontinent, in central and southern Africa and in coastal areas of South America.

Fig. 1. Reduced susceptibility of Plasmodium falciparum to various antimalarial drugs (from published and unpublished sources using a variety of criteria)

Decreasing sensitivity to quinine has been detected in areas of South-East Asia where it has been extensively used as the first-line treatment for malaria and in some parts of South America. Patient adherence to a 7-day regimen as a single drug or in combination with other drugs such as tetracyclines is low, leading to incomplete treatment and parasite recrudescences. This may have led to the selection of resistant parasites. There is some cross-resistance between quinine and mefloquine, suggesting that the wide use of quinine in Thailand might have influenced the development of resistance to mefloquine in that country (31). Strains of P. falciparum from Africa are generally highly sensitive to quinine.

Recurrences of parasitaemia in over 50% of the patients treated with mefloquine alone have been reported from border areas between Cambodia, Myanmar and Thailand. Mefloquine resistance is uncommon in the remainder of South-East Asia. In the Amazon Basin, mefloquine resistance has been reported only from Brazil, where clinical failure rates remain below 5% (32). Existing data indicate that, in some endemic areas, mefloquine-resistant parasites may be found prior to the introduction of the drug. For example, isolates with reduced sensitivity to mefloquine have been reported from several sites in West and central Africa, although the drug has never been widely used there (33). In such cases, there is a potential for resistance to spread if mefloquine monotherapy is used on a large scale.

The recrudescence rate is high when these drugs are used in monotherapy, depending on the dose administered, the duration of treatment and the severity of disease but not at present on parasite resistance (34-38). Treatment regimens of less than 7 days gave unacceptably high recrudescence rates (39). In spite of reports of decreasing in vitro susceptibility so far, there is no confirmed in vivo evidence of resistance of P. falciparum to artemisinin and its derivatives.

P. vivax resistance to chloroquine was first reported from Irian Jaya (Indonesia) and Papua New Guinea in 1989. Nearly 50% of strains from these areas currently show evidence of reduced susceptibility in 28-day in vivo tests (40). Well-documented reports of resistance in individual patients or small case series have also appeared from Brazil, Guatemala, Guyana, India and Myanmar but the resistance appears to be focal and much less intense.

The current situation is summarized in Table 1. Since 1995, WHO and national malaria control programmes in the African Region have responded to the spread and intensification of chloroquine-resistant P. falciparum by strengthening national capacity in conducting 14-day in vivo drug efficacy studies in more than 30 countries south of the Sahara.

On the basis of the results from these studies, as of date nine countries have changed their antimalarial treatment policies: Botswana, Ethiopia, Kenya, Malawi, South Africa, Uganda, United Republic of Tanzania, Zambia and Zimbabwe. Burundi, Eritrea, Ghana, Mozambique, Rwanda and Zambia have started the process of change. In West Africa, rates of resistance vary, but tend to be lower than those in East and southern Africa and as yet no changes have been made in first-line treatment policy.

As shown in Table 2, chloroquine resistance was suspected in Asia as early as 1957. Chloroquine and sulfadoxine-pyrimethamine resistance are widespread in some parts of Cambodia, Lao PDR, Malaysia, Myanmar, Thailand and Viet Nam. In areas of sulfadoxine- pyrimethamine resistance, mefloquine has been the drug of choice. However, mefloquine resistance has spread rapidly in this region. In response, following a regional meeting of the Mekong Roll Back Malaria Initiative in May 2000, a standard of combination therapy including an artemisinin derivative was adopted for use following diagnosis by microscopy or rapid diagnostic testing. In this region, malaria is most prevalent in border areas; malaria control collaboration efforts therefore include antimalarial treatment policies. Combinations of quinine plus tetracycline or artemisinin derivatives plus mefloquine are being used. In western Cambodia, mefloquine resistance was first identified in 1995. The current policy is artemisinin combination therapy with mefloquine. Combination therapy is also being considered in the Philippines. One of the challenges to combination drug policy is that there are currently no formulations of the recommended combinations for use in children or during pregnancy.

Chloroquine resistance is widespread in Papua New Guinea, Solomon Islands and Vanuatu. At an interregional meeting in 1996 that considered drug resistance, the new protocol for in vivo testing was adopted. Combinations with and without artemisinins are increasingly being adopted in this region. The current first-line treatment is a combination of sulfadoxine- pyrimethamine and chloroquine (with the variation that children under 5 years of age are treated with amodiaquine) in Papua New Guinea and in Vanuatu, and a decision to adopt this combination is also being considered in Solomon Islands.

Chloroquine-resistant P. vivax has been found in Papua New Guinea (40) and the Solomon Islands (unpublished data).

Following a PAHO-sponsored meeting on antimalarial drug resistance in the Amazon region in Manaus in March 1998, several countries have undertaken in vivo drug efficacy testing using the revised WHO/PAHO protocol. Thus far, the only changes in drug policy have occurred in Peru (see country examples in section 4.7). The most commonly used replacement therapy for chloroquine was sulfadoxine-pyrimethamine. However, P. falciparum resistant to sulfadoxine- pyrimethamine rapidly emerged in Bolivia, Brazil, Colombia, Peru and Venezuela.

P. vivax remains sensitive to chloroquine in the Americas, but cases of vivax malaria that failed to respond to the standard dose of 25 mg of chloroquine base per kg have been reported from Brazil (41), Guatemala and Guyana. Despite occasional reports to the contrary, P. vivax resistance to chloroquine has not been confirmed in Peru and Venezuela (42, 43).

Since the early 1990s, the malaria situation has deteriorated considerably, owing to political and economic instability, massive population movements and large-scale development projects. In recent years, Azerbaijan, Tajikistan and Turkey have suffered explosive and extensive epidemics, while Armenia, Turkmenistan and Georgia have faced small-scale outbreaks. In 1995, a total of 92 048 malaria cases were reported in the Region, mostly vivax malaria. During 1996-2000, the reported total number of malaria cases declined from 91 723 to 32 724. Despite a substantial reduction in the reported incidence of malaria in the Region, the situation is complicated by the occurrence and spread of P. falciparum in Tajikistan where 773 cases were reported in 2000. Chloroquine-resistant malaria has not yet been found in countries of the Region where autochthonous cases are reported.

Imported malaria is a growing public health issue, and mortality due to malaria presents a challenging problem to medical professionals in countries of the Region. Since the beginning of 1970s the number of imported cases increased eight-fold: from 1 500 cases in 1972 to almost 13 000 in 1999. The majority (> 80%) of the imported cases of malaria reported in the European Region are acquired in Africa. The largest number of cases has been recorded in France, Germany, Italy and United Kingdom. At present P. falciparum accounts for almost 70% of cases. In the period 1989-1999, 680 people are known to have died from malaria in the European Region.