A carboxylic acid containing analog of sildenafil (retains pharmacological activity but exhibits reduced cell entry.
The lipophilic design of conventional PDE5i’s such as sildenafil, tadalafil and vardenafil, enables entry into the circulation that is essential to reach the target tissues. To create a PDE5i with reduced systemic delivery, we sought to reduce the lipophilicity by adding groups that carry a charge at physiological pH. To accomplish this, we substituted malonate for the methyl group on the piperazine ring of the prototypical PDE5i sildenafil. This structure (malonyl-sildenafil) is predicted to exhibit much higher solubility and reduced membrane permeability at physiological pH values (Fig. 1A, B). An in vitroassay for PDE5i activity revealed an IC50 of 6.5 nM for malonyl-sildenafil that is similar to the parent compound (7.5 nM), indicating that the modification did not deleteriously affect the pharmacology (Fig. 1C). We used a novel cell-based assay to assess the ability of the PDE5i to enter colon epithelial cells and activate cGMP signaling (Fig. 2A). These results showed that malonyl-sildenafil was 18-fold less potent at increasing cGMP levels in the cells than the parent compound based upon EC50 (Fig. 2B). These observations were supported by a highly sensitive assay for activation of cGMP-dependent protein kinase (Fig. 2C).