Antimalarial Medication - Resistance

Resistance

Antimalarial resistance is common.

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.