The identification of a unique series of compounds which possesses special advantages in terms of drug-like properties due to their possessing advantageous properties in terms of potency and/or pharmacokinetic and/or selectivity and/or in vivo receptor occupancy properties. Specifically, the selection of a 1,3-thiazol-2-yl ring member linked by an ethynylene to the 3 position of a pyridyl ring or the 5 position of a pyrimidinyl ring, wherein the ring is substituted with selected substituents, results in a compound having superior drug-like properties. The invention includes pharmaceutically acceptable salt forms of these heterocyclic compounds, in particular chloride salts and trifluoroacetate salts.