A compound of the formula: or a pharmaceutically acceptable salt thereof, wherein: R 1 is optionally substituted tetrazolyl; R 2 is optionally substituted phenyl, optionally substituted pyridinyl, optionally substituted pyrimidinyl, optionally substituted pyridazinyl or optionally substituted thienyl; R 3 is hydrogen; C 1-6 alkyl; hetero-C 1-6 alkyl or cyano; R 4 is hydrogen; C 1-6 alkyl or hetero-C 1-6 alkyl; or R 3 and R 4 together with the atom to which they are attached form a C 3-6 carbocyclic ring; R 5 is pyridinyl, pyrimidinyl, pyrazinyl, pyridazinyl, pyrazolyl or thiazolyl, each of which may be optionally substituted once or twice with a group or groups independently selected from methyl, ethyl, n-propyl, fluoro, chloro, amino, methylamino or dimethylamino; R 6 is H; C 1-6 alkyl; C 1-6 alkyloxy; halo; C 1-6 haloalkyl; or cyano; wherein optionally substituted, when used in association with "phenyl", "heteroaryl", or "heterocyclyl", means a, phenyl, heteroaryl, or heterocyclyl which is optionally substituted independently with one to four substituents selected from alkyl, cycloalkyl, cycloalkylalkyl, heteroalkyl, hydroxyalkyl, halo, nitro, cyano, hydroxy, alkoxy, amino, acylamino, mono-alkylamino, di-alkylamino, haloalkyl, haloalkoxy, heteroalkyl, -COR (where R is hydrogen, alkyl, phenyl or phenylalkyl), -(CR'R") n -COOR (where n is an integer from 0 to 5, R' and R" are independently hydrogen or alkyl, and R is hydrogen, alkyl, cycloalkyl, cycloalkylalkyl, phenyl or phenylalkyl), or -(CR'R") n -CONR a R b (where n is an integer from 0 to 5, R' and R" are independently hydrogen or alkyl, and R a and R b are, independently of each other, hydrogen, alkyl, cycloalkyl, cycloalkylalkyl, phenyl or phenylalkyl). R 1 is optionally substituted tetrazolyl; R 2 is optionally substituted phenyl, optionally substituted pyridinyl, optionally substituted pyrimidinyl, optionally substituted pyridazinyl or optionally substituted thienyl; R 3 is hydrogen; C 1-6 alkyl; hetero-C 1-6 alkyl or cyano; R 4 is hydrogen; C 1-6 alkyl or hetero-C 1-6 alkyl; or R 3 and R 4 together with the atom to which they are attached form a C 3-6 carbocyclic ring; R 5 is pyridinyl, pyrimidinyl, pyrazinyl, pyridazinyl, pyrazolyl or thiazolyl, each of which may be optionally substituted once or twice with a group or groups independently selected from methyl, ethyl, n-propyl, fluoro, chloro, amino, methylamino or dimethylamino; R 6 is H; C 1-6 alkyl; C 1-6 alkyloxy; halo; C 1-6 haloalkyl; or cyano; wherein optionally substituted, when used in association with "phenyl", "heteroaryl", or "heterocyclyl", means a, phenyl, heteroaryl, or heterocyclyl which is optionally substituted independently with one to four substituents selected from alkyl, cycloalkyl, cycloalkylalkyl, heteroalkyl, hydroxyalkyl, halo, nitro, cyano, hydroxy, alkoxy, amino, acylamino, mono-alkylamino, di-alkylamino, haloalkyl, haloalkoxy, heteroalkyl, -COR (where R is hydrogen, alkyl, phenyl or phenylalkyl), -(CR'R") n -COOR (where n is an integer from 0 to 5, R' and R" are independently hydrogen or alkyl, and R is hydrogen, alkyl, cycloalkyl, cycloalkylalkyl, phenyl or phenylalkyl), or -(CR'R") n -CONR a R b (where n is an integer from 0 to 5, R' and R" are independently hydrogen or alkyl, and R a and R b are, independently of each other, hydrogen, alkyl, cycloalkyl, cycloalkylalkyl, phenyl or phenylalkyl).