A compound of the formula I: ** Formula ** wherein: R1 is selected from -C1-10 alkyl substituted with 1 or 2 groups -ONO2, -CH2O-R10, -C1-6-OCH2- CH alkylene (ONO2 ) -10 C1-6 alkyl, -phenylene-R10, -C 1-6 alkylene-SO2NH (OH), ** Formula ** and R10 is -C2-10 alkyl substituted with 1 or 2 -ONO2 groups； and a is an integer from 2-5； R2 is selected from -OH, -CH2OH, -OP (O) (OH) 2 and -CH2OP (O) (OH) 2； and R3 is selected from H and -CH3； or R2 is taken together with R3 to form -CH2-O-CH2- or -CH2-CH2-； or R2 and R3 are both -CH3； Z is -CH- or -N-； X is -COOR4 or -C1-9 heteroaryl substituted with R5 and R6； R5 is absent or is selected from H； halo； -C0-5-OH alkylene； -NH2； -C 1-6 alkyl； -CF3； -C3-7 cycloalkyl； -C0-2-O-C 1-6 alkyl； -C (O) H； -C (O) -C 1-6 alkyl； -C0-1-COOR50 alkylene； -C (O) NR51R52； -NHC (O) R53； >； = O； -NO2； -C (CH3)>； = N (OH)； phenyl optionally substituted with one or two groups independently selected from halo, -OH, -CF3, -OCH3, -NHC (O) CH3 and phenyl； naphthalenyl； pyridinyl； pyrazinyl； pyrazolyl optionally substituted with methyl； thiophenyl optionally substituted with methyl or halo； furanyl； and -CH2-morpholinyl； and R5, when present, is attached to a carbon atom； wherein R51 and R52 are independently selected from H, -C1-6 alkyl, -CH2COOH, - (CH2) 2OH, - (CH2) 2OCH3, - (CH2) 2SO2NH2, - (CH2) 2N (CH3) 2 , -C0-1 alkylene-C3-7 cycloalkyl and - (CH2) 2-imidazolyl； or R51 and R52 are taken together to form a saturated or partially unsaturated C3-5 heterocycle optionally substituted with halo, -OH, -COOH or -CONH2； and optionally, containing an oxygen atom in the ring； and R53 is selected from -C 1-6 alkyl； -C 0-1 -C 1-6 alkyl alkylene； phenyl optionally substituted with halo or -OCH3； and C1-9 heteroaryl； R6 is absent or is selected from H； -OH； -C 1-6 alkyl； -C1-2-COOR60 alkylene； -CH2OC (O) CH (R61) NH2； -OCH2OC (O) CH (R61) NH2； -OCH2OC (O) CH3； -CH2OP (O) (OH) 2； -CH2CH (OH) CH2OH -CH [CH (CH3) 2] -NHC (O) O-C1-6 alkyl； pyridinyl； and phen
