A drug used to treat Parkinson's disease has significant anticancer effects in human cells and could pave the way for novel treatments against the deadly disease, scientists have found.
The findings may also explain the low incidence of many cancers in patients with Parkinson's disease.
"Carbidopa is an FDA-approved drug for treating Parkinson's disease. Hence, clinical trials can be conducted right away to evaluate its efficacy in humans as an anticancer drug," said Yangzom Bhutia from Texas Tech University in the US.
Parkinson's disease is a degenerative neurological disorder that mostly influences a person's movement and motor skills, with symptoms including shaking, rigidity and difficulty in walking.
These effects are a result of lower than normal production of dopamine, a chemical that sends behavioural signals from the brain to the body.
While there is currently no cure for the condition, there are a number of treatments that act to reduce the severity of the symptoms of Parkinson's disease. Dopamine itself cannot be used as a drug, as it will not cross the blood-brain barrier.
However, one of the chemicals that forms dopamine - levodopa or L-DOPA - can cross into the brain and is converted into dopamine once there.
L-DOPA has been used to treat Parkinson's disease symptoms for many years, but when used alone can result in side effects such as nausea.
This is because only 5-10 per cent of levodopa taken as a drug crosses the blood-brain barrier, with the rest being converted into dopamine elsewhere in the body.
The drug carbidopa does not cross into the brain, but when taken with L-DOPA prevents its conversion into dopamine outside the brain and reduces side effects for patients.
Many studies show that patients with Parkinson's disease have a lower rate of most cancers compared with the general population.
As most patients in these studies were treated with a combination of L-DOPA and carbidopa, it is possible that one or both of these drugs could exhibit anti-cancer properties and contribute to the lower incidence of cancer observed in these patients.
Earlier studies showed that L-DOPA does not have anticancer properties, but until now the potential anticancer properties of carbidopa have not been investigated.
"Carbidopa is never used by itself as a drug for any disease. But our data show that carbidopa by itself possesses the anticancer effect," said Bhutia, lead author of the study published in the Biochemical Journal.
"We believe that the reduced incidence of most cancers in Parkinson's disease patients is due to carbidopa," she said.
"We also postulate that the increased incidence of melanoma in Parkinson's disease patients is most likely due to L-DOPA, and not due to carbidopa, because L-DOPA is the precursor for melanin synthesis, a metabolic pathway that occurs exclusively in melanocytes," she added.
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