“…Overall, among the R,R enantiomers tested, only a few showed IC 50 values similar to those mentioned above. ,,, Other studies focused exclusively on 3,4-methylenedioxyphenyl analogues. Interestingly, the tolerance for N -substitution was lesser in this context, and small, aliphatic moieties were found more promising for anti-PDE5 activity than aromatic or extended groups (e.g., IC 50 = 16 nM for 76 vs. 5 nM for 8 ). ,, In addition, the introduction of polar groups, such as OH or NH 2 , was detrimental, compared to hydrophobic equivalents . Finally, 6-(3,4-methylenedioxyphenyl) compounds bearing ( R )-pyrrolidine-type substituents at the amido position of the piperazinedione ring were developed as the most potent β-carboline-based PDE5 inhibitors to date.…”