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).