MSX-4 is a selective adenosine A<sub>2A</sub> receptor antagonist used in scientific research. It is a water-soluble amino acid ester prodrug of MSX-2, the active metabolite of the drug. MSX-4 reverses the motivational deficits induced by the dopamine D<sub>2</sub> receptor antagonist eticlopride in animals and hence has the capacity to produce pro-motivational effects. MSX-4 was first described in the scientific literature by 2008.