Once swallowed, orally-administered levodopa moves down the gastrointestinal tract – a process sometimes delayed in PD due to impaired gastric motility – until it is absorbed into the blood via the duodenum.2 From the circulation, levodopa eventually makes its way to the brain, crosses the blood–brain barrier, and is taken into nigrostriatal nerve terminals that convert it into biologically active dopamine.2
The role of dopamine is not confined to the brain, however; it is also a key component of the peripheral nervous system, and helps to regulate the immune system and the function of several organs.4 It is important, therefore, to prevent conversion of levodopa to dopamine outside the brain, both to conserve the drug for use by the brain, and to prevent unpleasant side effects.2 To this end, a dopa decarboxylase enzyme inhibitor is added to the formulation that preserves levodopa in its biologically-inert form until it reaches the brain.2
References:
1. Hornykiewicz O. A brief history of levodopa. J Neurol 2010; 257 (Suppl 2): S249–S252.
2. van Gerpen JA. Conventional treatment in Parkinson’s disease. In: Wolters & Baumann (eds). Parkinson Disease and Other Movement Disorders. VU University Press, 2014.