The metabotropic glutamate receptor 5 (mGlu5) is a glutamate-activated class C G protein-coupled receptor widely expressed in the central nervous system and clinically investigated as a drug target for a range of indications, including depression, Parkinson's disease, and fragile X syndrome. CTEP is a novel, potent, selective and orally bioavailable allosteric antagonist of the metabotropic glutamate receptor subtype mGluR5. CTEP binds mGlu5 with low nanomolar affinity and shows >1000-fold selectivity when tested against 103 targets, including all known mGlu receptors. In animal studies CTEP was found to have a high oral bioavailability and a long duration of action, lasting 18 hours after a single dose, giving it considerably improved properties over older mGluR5 antagonists such as MPEP and fenobam.