Mevidalen, also known as LY3154207, is a novel, potent, and subtype selective human D1 positive allosteric modulator (PAM) with minimal allosteric agonist activity (EC50 = 3 nM). In contrast to orthosteric agonists, LY3154207 showed a distinct pharmacological profile without a bell-shaped dose-response relationship or tachyphylaxis in preclinical models. Identification of a crystalline form of free LY3154207 from the discovery lots was not successful. Instead, a novel cocrystal form with superior solubility was discovered and determined to be suitable for development. LY3154207 is now in clinical development as a potential first-in-class D1 PAM for Lewy body dementia..