The present invention relates to a compound of formula I or a pharmaceutically acceptable salt thereof, wherein ring A is a monocyclic heteroaryl selected from oxazolyl, isoxazolyl, pyridinyl, pyrimidinyl, pyrazinyl and thiazolyl； wherein ring A is optionally substituted with up to two substituents； ring B is pyridinyl, pyrimidinyl, pyridazinyl and pyrazinyl； wherein ring B is optionally substituted with up to two substituents； R1 and R3 are each independently selected from hydrogen, C1-C4 alkyl, C1-C4 haloalkyl, -O-C1-C4 alkyl and CN, wherein any alkyl portion of R1 is optionally substituted with -OH, NH2, NH(C1-C4 alkyl) or N(C1-C4 alkyl)2； R2 is selected from -(C1-C6 alkyl), -(C2-C6 alkenyl or alkynyl), -(C1-C6 alkylene)-N(R6)-(C1-C6 alkylene)-O-(C1-C6 alkyl), -(C1-C6 alkylene)-N(R6)-(C0-C6 alkylene)-Q, -(C1-C6 alkylene)-N(R6)(R6), etc., wherein any alkyl or alkylene moiety present in R2 is optionally substituted with one or more -OH, -O(C1-C4 alkyl) or halo； any terminal methyl moiety present in R2 is optionally substituted with -CH2OH, CF3, -CH2F, -CH2Cl, C(O)CH3, C(O)CF3, CN or CO2H； each R6 is independently selected from hydrogen and C1-C6 alkyl； and Q is selected from aryl, heteroaryl, C3-C6 cycloalkyl and heterocyclyl, any of which is optionally substituted； etc., as disclosed in the claims. The invention further relates to a compound of formula II or a pharmaceutically acceptable salt thereof, wherein ring A' is pyridin-2-yl optionally substituted with one or two substituents independently selected from chloro, fluoro, -CF3, -CHF2, -CH3, -CH2CH3, -CF2CH3, -OH, -OCH3, -OCH2CH3, -NH2, -NH(CH3) and -N(CH3)2； ring B' is selected from pyridin-3-yl, pyridin-4-yl and pyrimidin-5-yl, wherein ring B' is optionally substituted with one to two substituents independently selected from halo； -CN； -OH； C1-C4 alkyl optionally substituted with halo, CN or -OH； -S(O)2-C1-C4 alkyl； -S(O)-C1-C4 alkyl； -S(O)2-NH-C1-C4 alkyl； -S(O)2-N(C1-C4 alkyl)2； -S(O)2-azetidin-1-yl； -O-C1
