Potent inhibition of human phosphodiesterase-5 by icariin derivatives in an enzyme assay

The pharmacological treatment of erectile dysfunction relies mostly on the use of sildenafil (Viagra®) and other sildenafil-type selective inhibitors of phosphodiesterase 5 (PDE5). Adverse effects and the cost of the therapy represent a stimulus to search for alternative drugs. With the aim of searching for alternative pharmacological treatments, several plant extracts traditionally used for impotence (Tribulus terrestris L., Ferula hermonis Boiss, Epimedium brevicornum Maxim, Turnera diffusa var. aphrodisiaca (G.H. Ward) Urban, Cinnamomum cassia L.) were screened against PDE5A1 activity. Human recombinant PDE5A1 was prepared by expression of the full-length cDNA into COS-7 cells [1], and the enzyme activity was determined by a radioassay [1]. Statistical analysis was performed using GraphPad Prism 4. Only E. brevicornum extract (80% inhibition at 50µg/ml) and its active principle icariin (IC50 5.9µM) were highly active. In order to improve the inhibitory activity, icariin was submitted to modifications directed to remove the glycosidic moieties, and to replace them with the hydroxyethyl side chain as simplification of the sugar residues, or to cyclize the prenyl group. Compound 3,7-bis (2-hydroxyethyl) icaritin (5), where both sugars were replaced with hydroxyethyl residues, potently inhibited PDE5A1 with an IC50 very close to sildenafil (IC50 75 nM vs. 74 nM). The potency of 5 was 80 fold higher than the lead compound. The improved pharmacodynamic profile, and no sign of cytotoxicity in human fibroblasts make this compound a promising candidate for further development.