Antimalarial drug

Antimalarial drugs are designed to prevent or cure malaria. Some antimalarial agents, particularly chloroquine and hydroxychloroquine, are also used in the treatment of rheumatoid arthritis and lupus associated arthritis. There are many of these drugs currently on the market. Quinine is the oldest and most famous anti-malarial.

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Quinine

Quinine has a long history stretching from Peru, and the discovery of the Cinchona tree, and the potential uses of its bark, to the current day and a collection of derivatives that are still frequently used in the prevention and treatment of malaria. Quinine is an alkaloid that acts as a blood schizonticidal and weak gametocide against Plasmodium vivax and Plasmodium malariae. As an alkaloid, it is accumulated in the food vacuoles of plasmodium species, especially Plasmodium falciparum. It acts by inhibiting the heme polymerase enzyme, thus facilitating an aggregation of cytotoxic heme. Quinine is less effective and more toxic as a blood schizonticidal agent than Chloroquine; however it is still very effective and widely used in the treatment of acute cases of severe P. falciparum. It is especially useful in areas where there is known to be a high level of resistance to Chloroquine, Mefloquine and sulfa drug combinations with pyrimethamine. Quinine is also used in post-exposure treatment of individuals returning from an area where malaria is endemic.

The treatment regimen of Quinine is complex and is determined largely by the parasite’s level of resistance and the reason for drug therapy (i.e. acute treatment or prophylaxis). The World Health Organization recommendation for Quinine is 8mg/kg three times daily for 3 days (in areas where the level of adherence is questionable) and for 7 days (where parasites are sensitive to Quinine). In areas where there is an increased level of resistance to Quinine 8mg/kg three times daily for 7 days is recommended, combined with Doxycycline, Tetracycline or Clindamycin. Doses can be given by oral, intravenous or intramuscular routes. The recommended method depends on the urgency of treatment and the available facilities (i.e. sterilised needles for IV or IM injections).

Use of Quinine is characterised by a frequently experienced syndrome called cinchonism. Tinnitus (a hearing impairment), rashes, vertigo, nausea, vomiting and abdominal pain are the most common symptoms. Neurological effects are experienced in some cases due to the drug’s neurotoxic properties. These actions are mediated through the interactions of Quinine causing a decrease in the excitability of the motor neuron end plates. This often results in functional impairment of the eight cranial nerve; resulting in confusion, delirium and coma. Quinine can cause hypoglycaemia through its action of stimulating insulin secretion, this occurs in therapeutic doses and therefore it is advised that glucose levels are monitored in all patients every 4-6 hours. This effect can be exaggerated in pregnancy and therefore additional care in administering and monitoring the dosage is essential. Repeated or over-dosage can result in renal failure and death through depression of the respiratory system.

Other Alkaloids

Quinimax and Quinidine are the two most commonly used alkaloids related to Quinine, in the treatment or prevention of Malaria. Quinimax is a combination of four alkaloids (namely Quinine Quinidine Cinchoine and Cinchonidine). This combination has been shown in several studies to be more effective than Quinine, supposedly due to a synergistic action between the four Cinchona derivatives. Quinidine is a direct derivative of Quinine. It is a distereoisomer, thus having similar anti-malarial properties to the parent compound. Quinidine is recommended only for the treatment or severe cases of malaria.

Chloroquine

Chloroquine was until recently the most widely used anti-malarial. It was the original prototype from which most other methods of treatment are derived. It is also the most inexpensive, best tested and safest of all available drugs. The emergence of drug resistant parasitic strains is rapidly decreasing its effectiveness; however it is still the first-line drug of choice in most sub-Saharan African countries. It is now suggested that it is used in combination with other antimalarial drugs to extend it’s effective usage.

Chloroquine is a 4-aminoquinolone compound with a complicated and still unclear mechanism of action. It is believed to reach high concentrations in the vacuoles of the parasite, which, due to it’s alkaline nature, raises the internal pH. This causes subsequent clumping of haemoglobin. It also acts to inhibit haem-polymerase (controls the conversion of toxic haem to haemozoin) thus poisoning the parasite through excess levels of toxicity. Other potential mechanisms through which it may act include interfering with the biosynthesis of parasitic nucleic acids, the formation of a chloroquine-haem or chloroquine-DNA complex. The most significant level of activity found is against all forms of the schizonts (with the obvious exception of chloroquine-resistant P. falciparum and P. vivax strains) and the gametocytes of P. vivax, P. malariae, P. ovale as well as the immature gametocytes of P. falciparum. Chloroquine also has a significant anti-pyretic and anti-inflammatory effect when used to treat P. vivax infections, thus it may still remain useful even when resistance is more widespread.

Children and adults should receive 25mg of chloroquine per kg given over 3 days. A pharmacokinetically superior regime, recommended by the WHO, involves giving an initial dose of 10mg/kg followed 6-8 hours later by 5mg/kg, then 5mg/kg on the following 2 days. For chemoprophylaxis: 5mg/kg/week (single dose) or 10mg/kg/week divided into 6 daily doses is advised. It should be noted that chloroquine is only recommended as a prophylactic drug in regions only affected by P. vivax and sensitive P. falciparum strains. Chloroquine has been used in the treatment of malaria for many years and no abortifacient or teratogenic effects have been reported during this time, therefore it is considered very safe to use during pregnancy. However, itching can occur at intolerable level.

Amodiaquine

Amodiaquine is a 4-aminoquinolone anti-malarial drug similar in structure and mechanism of action to Chloroquine. It is most frequently used in combination with Chloroquine, but is also very effective when used alone. It is thought to be more effective in clearing parasites in uncomplicated malarial than Chloroquine, thus leading to a faster rate of recovery. However, some fatal adverse effects of the drug were noted during the 1980’s, thus reducing it’s usage in chemoprophylaxis. The WHO’s most recent advice on the subject still maintains that the drug should be used when the potential risk of not treating an infection outweighs the risk of developing side effects. It has also been suggested that it is particularly effective, and less toxic than other combination treatments in HIV positive patients.

The drug should be given in doses between 25mg/kg and 35mg/kg over 3 days in a similar method to that used in Chloroquine administration. Adverse reactions are generally similar in severity and type to that seen in Chloroquine treatment. In addition, bradycardia, itching, nausea, vomiting and some abdominal pain have been recorded. Some blood and hepatic disorders have also been seen in a small number of patients.

Pyrimethamine

Pyrimethamine is used in the treatment of uncomplicated malaria. It is particularly useful in cases of chloroquine-resistant P. Falciparum strains when combined with Sulphadoxine. It acts by inhibiting dihydrofolate reductase in the parasite thus preventing the biosynthesis of purines and pyrimidines. Therefore halting the processes of DNA synthesis, cell division and reproduction. It acts primarily on the schizonts during the hepatic and erythrocytic phases.

Sulphadoxine

The action of Sulphadoxine is focused on inhibiting the use of para-aminobenzoic acid during the synthesis of dihydropteroic acid. When combined with Pyrimethamine the two key stages in DNA synthesis in the plasmodia are prevented. It also acts on the schizonts during the hepatic and erythrocytic phases. It is mainly used for treating P. falciparum infections and is less active against other Plasmodium strains. However usage is restricted due to the long half life of the combination which exerts a potentially large selection pressure on the parasite hence encouraging the possibility of resistance developing. This combination is not recommended for chemoprophylaxis because of the severe skin reactions commonly experienced. However it is used frequently for clinical episodes of the disease.

Proguanil

Proguanil (Chloroguanadine) is a biguanide; a synthetic derivative of pyrimidine. It was developed in 1945 by a British Antimalarial research group. It has many mechanisms of action but primarily is mediated through conversion to the active metabolite cycloguanil pamoate. This inhibits the malarial dihydrofolate reductase enzyme. It’s most prominent effect is on the primary tissue stages of P. falciparum, P. vivax and P. ovale. It has no known effect against hypnozoites therefore is not used in the prevention of relapse. It has a week blood schizonticidal activity, although not recommended for therapy currently, when combined with Atovaquone (a hydroxynaphthoquinone) it has been shown to be effective against multi-drug resistant strains of P. falciparum. Proguanil is used as a prophylactic treatment in combination with another drug, most frequently Chloroquine. 3mg/kg is the advised dosage per day, (hence approximate adult dosage is 200mg). The pharmacokinetic profile of the drugs indicates that a half dose, twice daily maintains the plasma levels with a greater level of consistency, thus giving a greater level of protection. It should be noted that the Proguanil- Chloroquine combination does not provide effective protection against resistant strains of P. falciparum. There are very few side effects to Proguanil, with slight hair loss and mouth ulcers being occasionally reported following prophylactic use.

Mefloquine

Mefloquine was developed during the Vietnam War and is chemically related to quinine. It was developed to protect American troops against multi-drug resistant P. falciparum. It is a very potent blood schizonticide with a long half-life. It is thought to act by forming toxic heme complexes that damage parasitic food vacuoles. It is now used solely for the prevention of resistant strains of P. falciparum despite being effective against P. vivax, P. ovale and P. marlariae. Mefloquine is effective in prophylaxis and for acute therapy. It is now strictly used for resistant strains (and is usually combined with Artesunate). Chloroquine/Proguanil or sufha drug-pyrimethamine combinations should be used in all other Plasmodia infections.

A dose of 15-25mg/kg is recommended, depending on the prevalence of Mefloquine resistance. The increased dosage is associated with a much greater level of intolerance, most noticeably in young children; with the drug inducing vomiting and oesophagitis. The effects during pregnancy are unknown, although it has been linked with an increased number of stillbirths. It is not recommended for use during the first trimester, although considered safe during the second and third trimesters. Mefloquine frequently produces side effects, including nausea, vomiting, diarrhea, abdominal pain and dizziness. Several associations with neurological events have been made, namely affective and anxiety disorders, hallucinations, sleep disturbances, psychosis, toxic encephalopathy, convulsions and delirium. Cardiovascular effects have been recorded with bradycardia and sinus arrhythmia being consistently recorded in 68% of patients treated with Mefloquine (in one hospital-based study).

Halofantrine

Halofantrine is a relatively new drug developed by the Walter Reed Army Institute of Research in the 1960s. It is a phenanthrene methanol, chemically related to Quinine and acts acting as a blood schizonticide effective against all plasmodium parasites. Its mechanism of action is similar to other anti-malarials. Cytotoxic complexes are formed with ferritoporphyrin XI that cause plasmodial membrane damage. Despite being effective against drug resistant parasites, Halofantrine is not commonly used in the treatment (prophylactic or therapeutic) of malaria due to its high cost. It has very variable bioavailability and has been shown to have potentially high levels of cardiotoxicity. It is still a useful drug and can be used in patients that are known to be free of heart disease and are suffering from severe and resistant forms of acute malaria. The level of governmental control and the prescription-only basis on which it can be used contributes to the cost, thus Halofantrine is not frequently used.

A dose of 8 mg/kg of Halofantrine is advised to be given in three doses at six hour intervals for the duration of the clinical episode. It is not recommended for children under 10 kg despite data supporting the use and demonstrating that it is well tolerated. The most frequently experienced side-effects include nausea, abdominal pain, diarrhoea, and itch. Severe ventricular dysrhythmias, occasionally causing death are seen when high doses are administered. This is due to prolongation of the QTc interval. Halofantrine is not recommended for use in pregnancy and lactation, in small children, or in patients that have taken Mefloquine previously.

Primaquine

Primaquine is a highly active 8-aminoquinolone that is used in treating all types of malaria infection. It is most effective against gametocytes but also acts on hypnozoites, blood schizonticytes and the dormant plasmodia in P. vivax and P. ovale. It is the only known drug to cure both relapsing malaria infections and acute cases. The mechanism of action is not fully understood but it is thought to mediate some effect through creating oxygen free radicals that interfere with the plasmodial electron transport chain during respiration.

For the prevention of relapse in P. vivax and P. ovale 0.15 mg/kg should be given for 14 days. As a gametocytocidal drug in P. falciparum infections a single dose of 0.75mg/kg repeated 7 days later is sufficient. This treatment method is only used in conjunction with another effective blood schizonticidal drug. There are few significant side effects although is has been shown that Primaquine may cause anorexia, nausea, vomiting, cramps, chest weakness, anaemia, some suppression of myeloid activity and abdominal pains. In cases of over-dosage granulocytopenia may occur.

Artemesinin and derivatives

Artemesinin is a Chinese herb (Qinghaosu) that has been used in the treatment of fevers for over 1,000 years, thus predating the use of Quinine in the western world. It is derived from the plant Artemisia annua, with the first documentation as a successful therapeutic agent in the treatment of malaria is in 340 AD by Ge Hong in his book Zhou Hou Bei Ji Fang (A Handbook of Prescriptions for Emergencies).^[1] The active compound was isolated first in 1971 and named Artemsinin. It is a sesquiterpene lactone with a chemically rare peroxide bridge linkage. It is this that is thought to be responsible for the majority of its anti-malarial action. At present it is strictly controlled under WHO guidelines as it has proven to be effective against all forms of multi-drug resistant P. falciparum, thus every care is taken to ensure compliance and adherence together with other behaviours associated with the development of resistance. It is also only given in combination with other anti-malarials.

• Artemesinin has a very rapid action and the vast majority of acute patients treated show significant improvement within 1-3 days of receiving treatment. It has demonstrated the fastest clearance of all anti-malarials currently used and acts primarily on the trophozite phase, thus preventing progression of the disease. It is converted to active metabolite dihydroartemesinin that then inhibits the sarcoplasmic/endoplasmic reticulum Calcium ATPase encoded by P. falciparum. On the first day of treatment 20 mg/kg should be given, this dose is then reduced to 10mg/kg per day for the 6 following days. Few side effects are associated with artemesinin use. However, headaches, nausea, vomiting, abnormal bleeding, dark urine, itching and some drug fever have been reported by a small number of patients. Some cardiac changes were reported during a clinical trial, notably non specific ST changes and a first degree atrioventricular block (these disappeared when the patients recovered from the malarial fever).

• Artemether is a methyl ether derivative of Dihydroartemesinin. It is similar to Artemesinin in mode of action but demonstrates a reduced ability as a hypnozoiticidal compound, instead acting more significantly to decrease gametocyte carriage. Similar restrictions are in place, as with Artemesinin, to prevent the development of resistance, therefore it is only used in combination therapy for severe acute cases of drug-resistant P. falciparum. It should be administered in a 7 day course with 4mg/kg given per day for 3 days, followed by 1.6 mg/kg for 3 days. Side effects of the drug are few but include potential neurotoxicity developing if high doses are given.

• Artesunate is a hemisuccinate derivative of the active metabolite Dihydroartemisin. Currently it is the most frequently used of all the Artemesinin-type drugs. Its only effect is mediated through a reduction in the gametocyte transmission. It is used in combination therapy and is effective in cases of uncomplicated P. falciparum. The dosage recommended by the WHO is a 5 or 7 day course (depending on the predicted adherence level) of 4mg/kg for 3 days (usually given in combination with Mefloquine) followed by 2mg/kg for the remaining 2 or 4 days. In large studies carried out on over 10,000 patients in Thailand no adverse effects have been shown.

• Dihydroartemisinin is the active metabolite to which Artemisinin is reduced. It is the most effective Artemesinin compound and the least stable. It has a strong blood schizonticidal action and reduces gametocyte transmission. It is used for therapeutic treatment of cases of resistant and uncomplicated P. falciparum. 4mg/kg doses are recommended on the first day of therapy followed by 2mg/kg for 6 days. As with Artesunate, no side effects to treatment have thus far been recorded.

• Arteether is an ethyl ether derivative of Dihydroartemisinin. It is used in combination therapy for cases of uncomplicated resistant P. falciparum. The recommended dosage is 150mg/kg per day for 3 days given by IM injections. With the exception of a small number of cases demonstrating neurotoxicity following parenteral administration no side effects have been recorded.

Other agents

Doxycycline

Doxycycline is a Tetracycline compound derived from Oxytetracycline. The tetracyclines were one of the earliest groups of antibiotics to be developed and are still used widely in many types of infection. It is a bacteriostatic agent that acts to inhibit the process of protein synthesis by binding to the 30S ribosomal subunit thus preventing the 50s and 30s units from bonding. Doxycycline is used primarily for chemoprophylaxis in areas where quinine resistance exists. It can be used in resistant cases of uncomplicated P. falciparum but has a very slow action in acute maleria, therefore it should never be used in monotherapy.

When treating acute cases and given in combination with Quinine; 100mg/kg of Doxycycline should be given per day for 7 days. In prophylactic therapy, 100mg (adult dose) of Doxycycline should be given every day during exposure to malaria.

The most commonly experienced side effects are permanent enamel hypoplasia, transient depression of bone growth, gastrointestinal disturbances and some increased levels of photosensitivity. Due to its effect of bone and tooth growth it is not used in children under 8, pregnant or lactating women and those with a known hepatic dysfunction.

Tetracycline is only used in combination for the treatment of acute cases of P.Falciparum infections. This is due to its slow onset. Unlike Doxycycline it is not used in chemoprophylaxis. For Tetracycline, 250mg is the recommended adult dosage (it should not be used in children) for 5 or 7 days depending on the level of adherence and compliance expected. Oesophageal ulceration, gastrointestinal upset and interferences with the process of ossification and depression of bone growth are known to occur. The majority of side effects associated with Doxycycline are also experienced.

Clindamycin

Clindamycin is a derivative of Lincomycin, with a slow action against blood schizonticides. It is only used in combination with Quinine in the treatment of acute cases of resistant P. falciparum infections and not as a prophylactic. Being more expensive and toxic than the other antibiotic alternatives, it is used only in cases where the Tetracyclines are contraindicated (for example in children).

Clindamycin should be given in conjunction with Quinine as a 300mg dose (in adults) four times a day for 5 days. The only side effects recorded in patients taking Clindamycin are nausea, vomiting and abdominal pains and cramps. However these can be alleviated by consuming large quantities of water and food when taking the drug. Pseudomembranous colitis (caused by Clostridium difficile} has also developed in some patients; this condition may be fatal in a small number of cases.

Drug regimens

The following regimens are recommended by the WHO, UK HPA and CDC for adults and children aged 12 and over:

• chloroquine 300 to 310 mg once weekly, and proguanil 200 mg once daily (started one week before travel, and continued for four weeks after returning);
• doxycycline 100 mg once daily (started one day before travel, and continued for four weeks after returning);
• mefloquine 228 to 250 mg once weekly (started two-and-a-half weeks before travel, and continued for four weeks after returning);
• Malarone® 1 tablet daily (started one day before travel, and continued for 1 week after returning).

Other chemoprophylactic regimens that are available:

• Dapsone 100 mg and pyrimethamine 12.5 mg once weekly (available as a combination tablet called Maloprim® or Deltaprim®): this combination is not routinely recommended because of the risk of agranulocytosis;
• Primaquine 30 mg once daily (started the day before travel, and continuing for seven days after returning): this regimen is not routinely recommended because of the need for G-6-PD testing prior to starting primaquine (see the article on primaquine for more information).
• Quinine sulphate 300 to 325 mg once daily: this regimen is effective but not routinely used because of the unpleasant side effects of quinine.

Resistance to antimalarials

Anti-malarial drug resistance has been defined as: "the ability of a parasite to survive and/or multiply despite the administration and absorption of a drug given in doses equal to or higher than those usually recommended but within tolerance of the subject. The drug in question must gain access to the parasite or the infected red blood cell for the duration of the time necessary for its normal action." In most instances this refers to parasites that remaining following on from an observed treatment. Thus excluding all cases where anti-malarial prophylaxis has failed. In order for a case to be defined as resistant, the patient under question must have received a known and observed anti-malarial therapy whilst the blood drug and metabolite concentrations are monitored concurrently. The techniques used to demonstrate this are: in vivo, in vitro, animal model testing and the most recently developed molecular techniques.

Drug resistant parasites are often used to explain malaria treatment failure. However, they are two potentially very different clinical scenarios. The failure to clear parasitemia and recover from an acute clinical episode when a suitable treatment has been given and anti-malarial resistance in its true form. Drug resistance may lead to treatment failure, but treatment failure is not necessarily caused by drug resistance despite assisting with its development. A multitude of factors can be involved in the processes including problems with non-compliance and adherence, poor drug quality, interactions with other pharmaceuticals, poor absorption, misdiagnosis and incorrect doses being given. The majority of these factors also contribute to the development of drug resistance.

The generation of resistance can be complicated and varies between plasmodium species. It is generally accepted to be initiated primarily through a spontaneous mutation that provides some evolutionary benefit, thus giving an anti-malarial used a reduced level of sensitivity. This can be caused by a single point mutation or multiple mutations. In most instances a mutation will be fatal for the parasite or the drug pressure will remove parasites that remain susceptible, however some resistant parasites will survive. Resistance can become firmly established within a parasite population, existing for long periods of time.

The first type of resistance to be acknowledged was to Chloroquine in Thailand in 1957. The biological mechanism behind this resistance was subsequently discovered to be related to the development of an efflux mechanism that expels Chloroquine from the parasite before the level required to effectively inhibit the process of haem polymerization (that is necessary to prevent build up of the toxic by products formed by haemoglobin digestion). This theory has been supported by evidence showing that resistance can be effectively reversed on the addition of substances which halt the efflux. The resistance of other quinolone anti-malarials such as amiodiaquine, mefloquine, halofantrine and quinine are thought to have occurred by similar mechanisms.

Plasmodium have developed resistance against antifolate combination drugs, the most commonly used being sulfadoxine and pyrimethamine. Two gene mutations are thought to be responsible, allowing synergistic blockages of two enzymes involved in folate synthesis. Regional variations of specific mutations give differing levels of resistance.

Atovaquone is recommended to be used only in combination with another anti-malarial compound as the selection of resistant parasites occurs very quickly when used in mono-therapy. Resistance is thought to originate from a single-point mutation in the gene coding for cytochrome-b.

Spread of resistance

There is no single factor that confers the greatest degree of influence on the spread of drug resistance, but a number of plausible causes associated with an increase have been acknowledged. These include aspects of economics, human behaviour, pharmokinetics, and the biology of vectors and parasites.

The most influential causes are examined below:

1. The biological influences are based on the parasites ability to survive the presence of an anti-malarial thus enabling the persistence of resistance and the potential for further transmission despite treatment. In normal circumstances any parasites that persist after treatment are destroyed by the host’s immune system, therefore any factors that act to reduce the elimination of parasites could facilitate the development of resistance. This attempts to explain the poorer response associated with immunocompromised individuals, pregnant women and young children.
2. There has been evidence to suggest that certain parasite-vector combinations can alternatively enhance or inhibit the transmission of resistant parasites, causing ‘pocket-like’ areas of resistance.
3. The use of anti-malarials developed from similar basic chemical compounds can increase the rate of resistance development, for example cross-resistance to chloroquine and amiodiaquine, two 4-aminoquinolones and mefloquine conferring resistance to quinine and halofantrine. This phenomenon may reduce the usefulness of newly developed therapies prior to large-scale usage.
4. The resistance to anti-malarials may be increased by a process found in some species of plasmodium, where a degree of phenotypic plasticity was exhibited, allowing the rapid development of resistance to a new drug, even if the drug has not been previously experienced.
5. The pharmokinetics of the chosen anti-malarial are key; the decision of choosing a long-half life over a drug that is metabolised quickly is complex and still remains unclear. Drugs with shorter half-life’s require more frequent administration to maintain the correct plasma concentrations, therefore potentially presenting more problems if levels of adherence and compliance are unreliable, but longer-lasting drugs can increase the development of resistance due to prolonged periods of low drug concentration.
6. The pharmokinetics of anti-malarials is important when using combination therapy. Mismatched drug combinations, for example having an ‘unprotected’ period where one drug dominates can seriously increase the likelihood of selection for resistant parasites.
7. Ecologically there is a linkage between the level of transmission and the development of resistance, however at present this still remains unclear.
8. The treatment regime prescribed can have a substantial influence on the development of resistance. This can involve the drug intake, combination and interactions as well as the drug’s pharmokinetic and dynamic properties.

Prevention of resistance

The prevention of anti-malarial drug resistance is of enormous public health importance. It can be assumed that no therapy currently under development or to be developed in the foreseeable future will be totally protective against malaria. In accordance with this, there is the possibility of resistance developing to any given therapy that is developed. This is a serious concern, as the rate at which new drugs are produced by no means matches the rate of the development of resistance. In addition, the most newly developed therapeutics tend to be the most expensive and are required in the largest quantities by some of the poorest areas of the world. Therefore it is apparent that the degree to which malaria can be controlled depends on the careful use of the current drugs to limit, insofar as it is possible, any further development of resistance.

Provisions essential to this process include the delivery of fast primary care where staff are well trained and supported with the necessary supplies for efficient treatment. This in itself is inadequate in large areas where malaria is endemic thus presenting an initial problem. One method proposed that aims to avoid the fundamental lack in certain countries health care infrastructure is the privatisation of some areas, thus enabling drugs to be purchased on the open market from sources that are not officially related to the health care industry. Although this is now gaining some support there are many problems related to limited access and improper drug use, which could potentially increase the rate of resistance development to an even greater extent.

There are two general approaches to preventing the spread of resistance: preventing malaria infections and, preventing the transmission of resistant parasites.

Preventing malaria infections developing has a substantial effect on the potential rate of development of resistance, by directly reducing the number of cases of malaria thus decreasing the requirement for anti-malarial therapy. Preventing the transmission of resistant parasites limits the risk of resistant malarial infections becoming endemic and can be controlled by a variety of non-medical methods including insecticide-treated bed nets, indoor residual spraying, environmental controls (such as swamp draining) and personal protective methods such as using mosquito repellent. Chemoprophylaxis is also important in the transmission of malaria infection and resistance in defined populations (for example travellers).

A hope for future of anti-malarial therapy is the development of an effective malaria vaccine. This could have enormous public health benefits, providing a cost-effective and easily applicable approach to preventing not only the onset of malaria but the transmission of gametocytes, thus reducing the risk of resistance developing. Anti-malarial therapy could be also be diversified by combining a potentially effective vaccine with current chemotherapy, thereby reducing the chance of vaccine resistance developing.

Combination therapy

The problem of the development of malaria resistance must be weighed against the essential goal of anti-malarial care; that is to reduce morbidity and mortality. Thus a balance must be reached that attempts to achieve both goals whilst not compromising either too much by doing so. The most successful attempts so far have been in the administration of combination therapy. This can be defined as, ‘the simultaneous use of two or more blood schizonticidal drugs with independent modes of action and different biochemical targets in the parasite’. There is much evidence to support the use of combination therapies, some of which has been discussed previously, however several problems prevent the wide use in the areas where its use is most advisable. These include: problems identifying the most suitable drug for different epidemiological situations, the expense of combined therapy (it is over 10 times more expensive than traditional mono-therapy), how soon the programmes should be introduced and problems linked with policy implementation and issues of compliance.

The combinations of drugs currently prescribed can be divided into two categories: Non-artemesinin and Quinine based combinations and, Artemesinin based combinations.

Non-Artemesinin based combinations

• Sulfadoxine-Pyrimethamine (SP)–This combination has been used for many years and has wide-spread resistance. It has serious adverse effects but is cheap and is available in a single dose, thus decreasing problems associated with adherence and compliance. The recommended dose is 25mg/kg of sulfadoxine and 1.25mg/kg of pyrimethamine.

• Sulfadoxine-pyrimethamine plus Chloroquine–This is another cost-effective combination, which benefits from the drugs having similar pharmacokinetic profiles but different biochemical parasitic targets. There is already some level of parasite resistance present and numerous side-effects are associated with the use of SP. Chloroquine is recommended at 25mg/kg over 3 days with a single dose of SP as described above.

• Sulfadoxine-pyrimethamine plus Amodiaquine–This combination has been shown to produce a faster rate of clinical recovery than SP and Chloroquine, however there are serious adverse reactions associated with use that have limited its distribution. It is thought to have a longer therapeutic lifetime than other combinations and may be a more cost-effective option to introduce in areas where resistance is likely to develop. This is unlikely to occur until more information regarding its safety has been obtained. The recommended dose is 10mg/kg of Amodiaquine per day for 3 days with a single standard dose of SP.

• Sulfadoxine-Pyrimethamine plus Mefloquine–This is produced as a single dose pill and has obvious advantages over some of the more complex regimes. This combination of drugs has very different pharmokinetic properties with no synergistic action. This characteristic is potentially thought to delay the development of resistance, however it is counteracted by the very long half life of Mefloquine which could exert a high selection pressure in areas where intensive malaria transmission occurs. It is also an expensive combination and has not been recommended for used since 1990 due to Mefloquine resistance.

• Tetracycline or Doxycycline plus Quinine–Despite the increasing levels of resistance to Quinine this combination has proven to be particularly efficacious. The longer half-life of the Tetracycline component ensures a high cure rate. Problems with this regime include the relatively complicated drug regimen, where Quinine must be taken every 8 hours for 7 days. Additionally, there are severe side effects to both drugs (Cinchonism in Quinine) and Tetracyclines are contraindicated in children and pregnant women. For these reasons this combination is not recommended as first-line therapy but can be used for non-responders who remain able to take oral medication. Quinine should be taken in 10mg/kg doses every 8 hours and Tetracycline in 4mg/kg doses every 6 hours for 7 days.

Artemesinin-based combinations

Artemesinin has a very different mode of action than conventional anti-malarials (see information above), this makes is particularly useful in the treatment of resistant infections, however in order to prevent the development of resistance to this drug it is only recommended in combination with another non-artemesinin based therapy. It produces a very rapid reduction in the parasite biomass with an associated reduction in clinical symptoms and is known to cause a reduction in the transmission of gametocytes thus decreasing the potential for the spread of resistant alleles. At present there is no known resistance to Artemesinin and very few reported side-effects to drug usage, however this data is limited.

• Artesunate and Chloroquine–This combination has been thoroughly tested in randomised controlled trials and has demonstrated that it is well tolerated with few side effects. However, in one study there was less than 85% cure in areas where Chloroquine resistance was known. It is not approved for use in combination therapy and is unadvised in areas of high P. falciparum resistance.

• Artesunate and Amodiaquine–This combination has also been tested and proved to be more efficacious and similarly well tolerated to the Chloroquine combination. The cure rate was greater than 90%, potentially providing a viable alternative where levels of Chloroquine resistance are high. The main disadvantage is a suggested link with neutropenia. Dosage is recommended as 4mg/kg of Artesunate and 10mg/kg of Amodiaquine per day for 3 days.

• Artesunate and Mefloquine–This has been used as an efficacious first-line treatment regimen in areas of Thailand for many years. Mefloquine is known to cause vomiting in children and induces some neuropsychiatric and cardiotoxic effects, interestingly these adverse reactions seem to be reduced when the drug is combined with Artesunate, it is suggested that this is due to a delayed onset of action of Mefloquine. This is not considered a viable option to be introduced in Africa due to the long half-life of Mefloquine, which potentially could exert a high selection pressure on parasites. The standard dose required is 4mg/kg per day of Artesunate plus 25mg/kg of Mefloquine as a split dose of 15 mg/kg on day 2 and 10 mg/kg on day three.

• Artemether and Lumefantrine–(Coartem, Riamet, and Novartis) This combination has been extensively tested in 16 clinical trials, proving effective in children under 5 and has been shown to be better tolerated than Artesunate plus Mefloquine combinations. There are no serious side effects documented but the drug is not recommended in pregnant or lactating women due to limited safety testing in these groups. This is the most viable option for widespread use and is available in fixed-dose formulas thus increasing compliance and adherence.

• Artesunate and Sulfadoxine/Pyrimethamine–This is a well tolerated combination but the overall level of efficacy still depends on the level of resistance to Sulfadoxine and Pyrimethamine thus limiting is usage. It is recommended in doses of 4mg/kg of Artesunate per day for 3 days and a single dose of 25mg/kg of SP.

Other combinations

There are several anti-malarial combinations currently being developed that are hoped to be highly efficacious, cost-effective, safe and well tolerated. These are to be newly developed compounds and not derivatives of currently used drugs, thus decreasing the likelihood of resistance.

• Piperaquine-dihydroartemisinin-trimethoprim (Artecom) and Artecom combined with Primaquine has been studied in resistant areas of China and Vietnam. The drug has been shown to be highly efficacious (greater than 90%) even to strains resistant to Primaquine. Prior to introduction more information is required on safety and tolerability in pregnant women and children and toxicology data.

• Pyronaridine and Artesunate has been tested and demonstrated a clinical response rate of 100% in one trial in Hainan (an area with high levels of P. falciparum resistance to Pyronaridine).

• Chlorproguanil-Dapsone and Artesunate (Lapdap plus) is the most tested drug currently under development and could be introduced in African countries imminently. It is not recommended as a monotherapy due to concerns of resistance developing thus threatening the future use of related compounds.

See also

• Malaria prophylaxis

References

• 2007 guidelines are available from the UK Health Protection Agency website as a PDF file and includes detailed country-specific information for UK travellers.
• The World Health Organisation provides country-specific advice on malaria prevention.
• The Centers for Disease Control and Prevention website hosts constantly updated country-specific information on malaria. The advice on this website is less detailed, is very cautious and may not be appropriate for all areas within a given country. This is the preferred site for travellers from the US.

HPA and WHO advice are broadly in line with each other (although there are some differences). CDC guidance frequently contradicts HPA and WHO guidance.