Targeting bacterial and human levodopa decarboxylases for improved drug treatment of Parkinson's disease: Discovery and characterization of new inhibitors

Parkinson's disease (PD) is a common neurodegenerative disease, typically treated with levodopa, which alleviates the motor symptoms of the disease. However, levodopa metabolism in peripheral tissues hampers its bioavailability and leads to undesired side-effects. Therefore, co-administration of amino acid decarboxylase (AADC) inhibitors is necessary, but still, up to 50 % of levodopa may not reach the brain. Recent evidence suggests that gut microbes, especially Enterococcus faecalis, are also able to metabolize levodopa and affect the bioavailability by utilizing microbial tyrosine decarboxylase (TyrDC) enzyme. The main aim of this study was to develop inhibitors targeting gut microbial and host decarboxylation of levodopa. First, a virtual screen of a library of 158,000 compounds against E. faecalis TyrDC was conducted, combining three methods: molecular docking against the E. faecalis TyrDC homology model, structure-based pharmacophore model, and shape similarity searches based on levodopa, carbidopa (AADC inhibitor) and (S)-α-fluoromethyltyrosine (TyrDC inhibitor). A total of 394 compounds were selected and tested in vitro by using a cell-based E. faecalis assay measuring inhibition of levodopa metabolism. Three most active compounds (49-92% inhibition at 100 µM) sharing a similar scaffold and a set of commercially available and in-house synthesized analogs were then assessed against purified TyrDC and AADC enzymes. The IC₅₀ values for the most potent compounds for TyrDC and AADC inhibition were 23 µM / 102 µM (compound 1), 42 µM / 154 µM (compound 2) and 51 µM / 182 µM (compound 3), respectively. These compounds also displayed cytotoxic effects on HeLa cells and modest antibacterial activity against E. faecalis at the same concentration range. The core structure of the compounds presented here can serve as a starting point for the development of a new inhibitor class against TyrDC and AADC, and offers a promising avenue for personalized PD treatment, particularly for patients with high levels of gut microbes expressing the levodopa metabolizing TyrDC enzyme.