Erectile dysfunction (ED) was largely an unmet medical need prior to the introduction of sildenafil [Viagra (1)] in 1998. Sildenafil is a potent inhibitor of phosphodiesterase type 5 (PDE5) but has several notable side effects such as headache, nausea, cutaneous flushing, and visual disturbances, which may be attributed to its limited selectivity against other PDE isozymes, most notably PDE1 and PDE6. Thus, there is a need for improved PDE5 inhibitors with greater PDE isozyme selectivity and fewer side effects. In this study, using two rounds of parallel synthesis, we identified a series of potent PDE5 inhibitors based on a pyrazolopyridine template (i.e., 4) obtained via directed screening. This series demonstrated potent PDE5 inhibition in vitro. Compound 5 distinguished itself by its selectivity profile, with superior PDE1 and PDE6 isozyme selectivities compared to sildenafil (1), albeit with decreased PDE4 selectivity. Compound 5 also showed comparable in vitro functional PDE5 inhibition (in rabbit corpus cavernosal tissue strips) when compared with 1. Additional studies demonstrated that 5 had a good pharmacokinetic profile in rats and dogs (equal or better exposure than 1, short terminal half-life suitable for on-demand dosing) with no apparent safety issues (no drug-related effects at 10 and 50 mg/kg in rats, only lower body weight in male rats at 250 mg/kg). Thus, because of its improved PDE isozyme selectivity profile compared with sildenafil, compound 5 might be expected to have fewer PDE-related side effects if used for the treatment of ED.