A compound of structural formula 1-1 or 1-2: or a pharmaceutically acceptable salt thereof; wherein "a" is absent; X is independently selected from N and CH; each R 1 is independently selected from the group consisting of: -(CH 2) 0-2 C 2-6 cycloheteroalkyl, and -(CH 2) 0-2 aryl, wherein each CH 2, cycloheteroalkyl and aryl is unsubstituted or substituted with one to three groups independently selected from R a; R2 is hydrogen; each R 3 is independently selected from the group consisting of: -(CH 2) n aryl, and -(CH 2) n heteroaryl, wherein each CH 2, aryl and heteroaryl is unsubstituted or substituted with one to three substituents selected from R c; each R 4 is independently selected from the group consisting of: -(CH 2) s aryl, wherein each CH 2 and aryl is unsubstituted or substituted with one to three substituents selected from R d; each R a is independently selected from the group consisting of: hydrogen, -OH, oxo, -C 1-6 alkyl, -OC 1-6 alkyl, halogen, -CF 3, -OCF 3, -CN, -C C 1-6 alkyl, -CO 2 H, -CO 2 C 1-6 alkyl, -(CH 2) t C 3-7 cycloalkyl, -(CH 2) t C 2-6 cycloheteroalkyl, -(CH 2) 0-1 aryl, -CH(phenyl) 2, -(CH 2) t heteroaryl, -SO 2 C 1-6 alkyl, -SO 2 N(R f) 2; -SO 2 -C 3-7 cycloalkyl, -SO 2 -C 2-6 cycloheteroalkyl, -SO 2 -aryl-, and -SO 2 -heteroaryl; each R c is independently selected from the group consisting of: hydrogen, -OH, oxo, -C 1-6 alkyl, -OC 1-6 alkyl, halogen, -CF 3, -OCF 3, -CN, -CO 2 H, and -CO 2 C 1-6 alkyl; each R d is independently selected from the group consisting of: hydrogen, -OH, oxo, -C 1-6 alkyl, -OC 1-6 alkyl, halogen, -CF 3, -OCF 3, -CN, -CO 2 H, and -CO 2 C 1-6 alkyl; each R f is independently selected from the group consisting of: hydrogen, and -C 1-6 alkyl; n is selected from 0, 2, and 3; r is selected from 0, 1, 2 and 3; s is selected from 0, 2 and 3 and t is selected from 0, 1, 2 and 3. "a" is absent; X is independently selected from N and CH; each R 1 is independently selected from the group consisting of: -(CH 2) 0-2 C 2-6 cycloheteroalkyl, and -(CH 2) 0-2 aryl, wherein each CH 2, cycloheteroalkyl and aryl is unsubstituted or substituted with one to three groups independently selected from R a; -(CH 2) 0-2 C 2-6 cycloheteroalkyl, and -(CH 2) 0-2 aryl, R2 is hydrogen; each R 3 is independently selected from the group consisting of: -(CH 2) n aryl, and -(CH 2) n heteroaryl, wherein each CH 2, aryl and heteroaryl is unsubstituted or substituted with one to three substituents selected from R c; -(CH 2) n aryl, and -(CH 2) n heteroaryl, each R 4 is independently selected from the group consisting of: -(CH 2) s aryl, wherein each CH 2 and aryl is unsubstituted or substituted with one to three substituents selected from R d; each R a is independently selected from the group consisting of: hydrogen, -OH, oxo, -C 1-6 alkyl, -OC 1-6 alkyl, halogen, -CF 3, -OCF 3, -CN, -C C 1-6 alkyl, -CO 2 H, -CO 2 C 1-6 alkyl, -(CH 2) t C 3-7 cycloalkyl, -(CH 2) t C 2-6 cycloheteroalkyl, -(CH 2) 0-1 aryl, -CH(phenyl) 2, -(CH 2) t heteroaryl, -SO 2 C 1-6 alkyl, -SO 2 N(R f) 2; -SO 2 -C 3-7 cycloalkyl, -SO 2 -C 2-6 cycloheteroalkyl, -SO 2 -aryl-, and -SO 2 -heteroaryl; hydrogen, -OH, oxo, -C 1-6 alkyl, -OC 1-6 alkyl, halogen, -CF 3, -OCF 3, -CN, -C C 1-6 alkyl, -CO 2 H, -CO 2 C 1-6 alkyl, -(CH 2) t C 3-7 cycloalkyl, -(CH 2) t C 2-6 cycloheteroalkyl, -(CH 2) 0-1 aryl, -CH(phenyl) 2, -(CH 2) t heteroaryl, -SO 2 C 1-6 alkyl, -SO 2 N(R f) 2; -SO 2 -C 3-7 cycloalkyl, -SO 2 -C 2-6 cycloheteroalkyl, -SO 2 -aryl-, and -SO 2 -heteroaryl; each R c is independently selected from the group consisting of: hydrogen, -OH, oxo, -C 1-6 alkyl, -OC 1-6 alkyl, halogen, -CF 3, -OCF 3, -CN, -CO 2 H, and -CO 2 C 1-6 alkyl; hydrogen, -OH, oxo, -C 1-6 alkyl, -OC 1-6 alkyl, halogen, -CF 3, -OCF 3, -CN, -CO 2 H, and -CO 2 C 1-6 alkyl; each R d is independently selected from the group consisting of: hydrogen, -OH, oxo, -C 1-6 alkyl, -OC 1-6 alkyl, halogen, -CF 3, -OCF 3, -CN, -CO 2 H, and -CO 2 C 1-6 alkyl; hydrogen, -OH, oxo, -C 1-6 alkyl, -OC 1-6 alkyl, halogen, -CF 3, -OCF 3, -CN, -CO 2 H, and -CO 2 C 1-6 alkyl; each R f is independently selected from the group consisting of: hydrogen, and -C 1-6 alkyl; hydrogen, and -C 1-6 alkyl; n is selected from 0, 2, and 3; r is selected from 0, 1, 2 and 3; s is selected from 0, 2 and 3 and t is selected from 0, 1, 2 and 3.