Pyrazolo[3,4-b] pyridine compounds, and their use as phosphodiesterase inhibitors

The invention relates to a compound of formula (I) or a salt thereof: wherein: R1 is C1-4alkyl, C1-3fluoroalkyl, -CH2CH2OH or -CH2CH2CO2C1-2alkyl; R2 is a hydrogen atom (H), methyl or C1fluoroalkyl; R3 is optionally substituted C3-8cycloalkyl or optionally substituted mono-unsaturated-C5-7cycloalkenyl or an optionally substituted heterocyclic group of sub-formula (aa), (bb) or (cc); in which n1 and n2 independently are 1 or 2; and in which Y is O, S, SO2, or NR10; or R3 is a bicyclic group (dd) or (ee): and wherein X is NR4R5 or OR5a. The compounds are phosphodiesterase (PDE) inhibitors, in particular PDE4 inhibitors. Also provided is the use of a compound of formula (I), or a pharmaceutically acceptable salt thereof, in the manufacture of a medicament for the treatment and/or prophylaxis of an inflammatory and/or allergic disease in a mammal such as a human, for example chronic obstructive pulmonary disease (COPD), asthma, or allergic rhinitis.