1H-pyrazolo[4,3-b]pyridines as PDE1 inhibitors

A compound of formula (I):

• or a pharmaceutically acceptable salt thereof, wherein:
• L is NH;
• R1 is H or C1-C5 alkyl;
• R2 is C1-C4 alkyl;
• R3 is methyl substituted with a 5-membered heteroaryl which is substituted with methyl, or is methyl substituted with a pyridinyl which is substituted with C1-C3 fluoroalkyl or C1-C3 alkoxy;
• and
• R4 is pyridinyl which is substituted with C1-C3 alkoxy.