Pharmacology of Quinine
Quinine is an important medicine in the treatment of malaria. While it has been touted as a potential treatment for restless legs syndrome, it should not be taken for this purpose – not least due to its side effect profile and increased risk of death.
Malaria is a serious illness brought on from infections caused by mosquito bites. Mosquitos infected with malaria are most common in Africa, South America and South-East Asia.
Quinine is active against the Plasmodium falciparum strain and may be used where other medicines – such as chloroquine or artesunate – are either ineffective or unavailable. Quinine is most effective at treating uncomplicated malaria. It becomes progressively less effective at treating more complicated, severe forms of malaria.
Let’s take a few minutes to learn how precisely quinine is thought to act.
Mechanism of action
The mechanism of action of quinine is not yet entirely understood.
Many current theories center around how a related medicine, chloroquine, works.
For example – chloroquine works by facilitating the aggregation of cytotoxic heme. In other words, when parasites feed off hemoglobin they produce digestive products – some beneficial, some harmful to the organism. Chloroquine facilitates the accumulation of cytotoxic heme which, when it builds, kills the parasite.
Again, this is not necessarily the mechanism of quinine. However, there is believed to be significant overlap. Take a few minutes to learn more about the medicinal chemistry of quinine.
Quinine has a notorious side effect profile, many of its effects being quite serious.
Side effects associated with quinine include:
- Deafness, which may be permanent
- Blindness, which may be permanent
- Hemolytic-uremic syndrome (HUS)
- Gastrointestinal upset
- QT interval prolongation
- Hypersensitivity reactions
- Serious skin reactions – Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN)
Quinine is also associated with cinchonism – a collection of symptoms which include:
- Hearing impairment
- Blurred vision
- Disturbance in color perception
Most patients taking quinine experience cinchonism to some degree, mostly mild.
Severe cinchonism is associated with the above effects,in severe form – plus vomiting, abdominal pain, deafness, blindness and arrhythmias.
When we talk about the clinical pharmacology of quinine, we need to think about the following effects:
- That quinine is relatively safe at recommended doses and, though it is associated with a serious side effect profile, that this safety must be taken into account.
- That quinine, despite the above caution, can cause serious side effects. Due caution should be taken with patients with established auditory or visual impairment.
- That quinine is teratogenic, harming the developing fetus. It should be entirely avoided in the first trimester of pregnancy. It is category C in the United States.
- That quinine can increase the risk of hemolysis in patients with glucose-6-phosphate deficiency (G6PD) though may, depending on the patient and their existing circumstances, be taken to offset more serious risks of malaria.
- That because quinine increases the risk of QT prolongation, caution is warranted to avoid other medicines that also increase this risk – macrolides, SSRIs, antipsychotics, fluoroquinolones and amiodarone, for example.
Though quinine is an effective medicine at treating malaria, it does come with a substantial side effect profile. These factors should be considered, along with the patient’s existing condition, to determine whether quinine is an appropriate course of treatment.
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