A compound of formula wherein R 1 is hydrogen, alkyl, cycloalkyl, aryl, heteroaryl or heterocycloalkyl, wherein each instance of cycloalkyl, aryl, heteroaryl and heterocycloalkyl is optionally substituted with one, two, three or four substituents independently selected from R 4; R 2 is hydrogen, cycloalkyl, alkenyl, cyano, amino, hydroxyl, halogen, alkyl, haloalkyl, haloalkoxy or alkoxy; R 3 is alkyl, alkoxy, hydrogen, halogen or haloalkyl; R 4 is halogen, alkyl, heterocycloalkyl, heterocycloalkylalkyl, alkylheterocycloalkyl, haloheterocycloalkyl, cycloalkyl, cycloalkylalkyl, alkylcycloalkyl, halocycloalkyl, cycloalkylamino, aryl, arylalkyl, alkylaryl, haloaryl, cyano, hydroxy, oxo, haloalkyl, alkylcarbonyl, alkoxy, haloalkoxy, alkoxyalkyl, alkoxycarbonyl, amino, alkylamino, dialkylamino, aminoalkyl, alkylaminoalkyl, dialkylaminoalkyl, aminoalkylamino, alkoxyalkylamino, alkylcarbonylamino, alkoxycarbonylamino, hydroxyalkyl, hydroxyalkoxyalkyl or hydroxyalkylamino; and A 1 is -CH- or -N-; or a pharmaceutically acceptable salt thereof; provided that 7-(8-fluoro-2-methyl-imidazo[1,2-a]pyridin-6-yl)-2-(1-methyl-4-piperidyl)-[1,3,4]thiadiazolo[3,2-a]pyrimidin-5-one; 7-(8-fluoro-2-methyl-imidazo[1,2-a]pyridin-6-yl)-2-(4-piperidyl)-[1,3,4]thiadiazolo[3,2-a]pyrimidin-5-one; 7-(8-fluoro-2-methyl-imidazo[1,2-a]pyridin-6-yl)-2-(4-methylpiperazin-1-yl)-[1,3,4]thiadiazolo[3,2-a]pyrimidin-5-one; 7-(8-fluoro-2-methyl-imidazo[1,2-a]pyridin-6-yl)-2-piperazin-1-yl-[1,3,4]thiadiazolo[3,2-a]pyrimidin-5-one; 7-(8-fluoro-2-methyl-imidazo[1,2-a]pyridin-6-yl)-2-(2,2,6,6-tetramethyl-4-piperidyl)-[1,3,4]thiadiazolo[3,2-a]pyrimidin-5-one; 2-(4,7-diazaspiro[2.5]octan-7-yl)-7-(8-fluoro-2-methyl-imidazo[1,2-a]pyridin-6-yl)-[1,3,4]thiadiazolo[3,2-a]pyrimidin-5-one; and 1,1-dimethylethyl 7-[7-(8-fluoro-2-methylimidazo[1,2-a]pyridin-6-yl)-5-oxo-5H-1,3,4-thiadiazolo[3,2-a]pyrimidin-2-yl]-4,7-diazaspiro[2.5]octane-4-carboxylate; are excluded. R 1 is hydrogen, alkyl, cycloalkyl, aryl, heteroaryl or heterocycloalkyl, wherein each instance of cycloalkyl, aryl, heteroaryl and heterocycloalkyl is optionally substituted with one, two, three or four substituents independently selected from R 4; R 2 is hydrogen, cycloalkyl, alkenyl, cyano, amino, hydroxyl, halogen, alkyl, haloalkyl, haloalkoxy or alkoxy; R 3 is alkyl, alkoxy, hydrogen, halogen or haloalkyl; R 4 is halogen, alkyl, heterocycloalkyl, heterocycloalkylalkyl, alkylheterocycloalkyl, haloheterocycloalkyl, cycloalkyl, cycloalkylalkyl, alkylcycloalkyl, halocycloalkyl, cycloalkylamino, aryl, arylalkyl, alkylaryl, haloaryl, cyano, hydroxy, oxo, haloalkyl, alkylcarbonyl, alkoxy, haloalkoxy, alkoxyalkyl, alkoxycarbonyl, amino, alkylamino, dialkylamino, aminoalkyl, alkylaminoalkyl, dialkylaminoalkyl, aminoalkylamino, alkoxyalkylamino, alkylcarbonylamino, alkoxycarbonylamino, hydroxyalkyl, hydroxyalkoxyalkyl or hydroxyalkylamino; and A 1 is -CH- or -N-; or a pharmaceutically acceptable salt thereof; provided that 7-(8-fluoro-2-methyl-imidazo[1,2-a]pyridin-6-yl)-2-(1-methyl-4-piperidyl)-[1,3,4]thiadiazolo[3,2-a]pyrimidin-5-one; 7-(8-fluoro-2-methyl-imidazo[1,2-a]pyridin-6-yl)-2-(4-piperidyl)-[1,3,4]thiadiazolo[3,2-a]pyrimidin-5-one; 7-(8-fluoro-2-methyl-imidazo[1,2-a]pyridin-6-yl)-2-(4-methylpiperazin-1-yl)-[1,3,4]thiadiazolo[3,2-a]pyrimidin-5-one; 7-(8-fluoro-2-methyl-imidazo[1,2-a]pyridin-6-yl)-2-piperazin-1-yl-[1,3,4]thiadiazolo[3,2-a]pyrimidin-5-one; 7-(8-fluoro-2-methyl-imidazo[1,2-a]pyridin-6-yl)-2-(2,2,6,6-tetramethyl-4-piperidyl)-[1,3,4]thiadiazolo[3,2-a]pyrimidin-5-one; 2-(4,7-diazaspiro[2.5]octan-7-yl)-7-(8-fluoro-2-methyl-imidazo[1,2-a]pyridin-6-yl)-[1,3,4]thiadiazolo[3,2-a]pyrimidin-5-one; and 1,1-dimethylethyl 7-[7-(8-fluoro-2-methylimidazo[1,2-a]pyridin-6-yl)-5-oxo-5H-1,3,4-thiadiazolo[3,2-a]pyrimidin-2-yl]-4,7-diazaspiro[2.5]octane-4-carboxylate; are excluded.