Naxagolide
Naxagolide, also known as PHNO, dopazinol, L-647339, and MK-458 among other synonyms, is a dopamine [receptor agonist] which was developed for the treatment of Parkinson's disease but was never marketed. A radiolabeled form has been used for brain imaging. The drug was developed for use both orally and transdermally.
It acts as a potent dopamine D2 and D3 receptor agonist. Naxagolide was described in the 1990s as the most potent dopamine D2 receptor agonist that had been used. It shows about 50-fold selectivity for the dopamine D3 receptor over the dopamine D2 receptor. The drug is a naphthoxazine derivative. It is structurally similar to ergolines such as pergolide and cabergoline but is a non-ergoline itself.
Naxagolide was first described in 1984 and was under development by Merck & Co in the 1980s and 1990s. It was developed for treatment of Parkinson's disease and reached Phases of [clinical research#Phase II|phase 2] clinical trials for this indication. The drug was discontinued due to inadequate effectiveness and/or due to toxicity.