Complex sentence: PDE3 inhibitors are a type of phosphodiesterase inhibitors, and as such increase intracellular concentrations of the second messenger cyclic adenosine monophosphate (cAMP).
Split into simpler sentences: PDE3 inhibitors are a type of phosphodiesterase inhibitors. Inhibition of the PDE isoenzyme 3 leads to an increase of intracellular concentrations of the second messenger cyclic adenosine monophosphate (cAMP).