In vitro activity: Sildenafil citrate is a potent PDE type 5 reversible and selective inhibitor that blocks cGMP hydrolysis effectively (Ki ~3 nM). Sildenafil exhibited high affinity for PDE type 5 and type 6 with inhibition constants (Ki) of ~3.5 and 33 nM, respectively. Sildenafil enhances sodium nitroprusside- or transmural electrical stimulation-induced relaxation of precontracted corpus cavernosum muscle strips in organ baths, suggesting that sildenafil augments the activity of NO-mediated relaxation. Sildenafil citrate increases intracellular cGMP concentrations in cultured smooth muscle cells treated with sodium nitroprusside and in rabbit corpus cavernosum in vitro. Sildenafil is metabolized in the liver by cytochrome P450 and is converted into an active metabolite with characteristics similar to the parent compound.

Kinase Assay: Pretreatment with 1 μM Sildenafil citrate potentiates the phosphorylation of ERK1/ERK2, an increase in the percentage of cells in S phase and cell proliferation, compared with serotonin stimulation alone (P<0.05). Pretreatment with 1 μM Sildenafil citrate followed by serotonin stimulation leads to dramatic increase in OD value to 0.33, significantly different compared with serotonin stimulation alone (P<0.05). 1 μM Sildenafil obviously enhances the upregulation of ERK1/ERK2 phosphorylation induced by serotonin.

Cell Assay: Cells at approximately 90% confluence are harvested with 0.1% trypsin/0.01% ethylene diamine tetraacetic acid (EDTA) solution and seeded into a 96-well plate at a density of 2×104 cells/well and grown in RPMI-1640 containing 10% FBS for three days, followed by serum starvation for three days. Cells are then incubated for different time with various concentration of serotonin or 1 μM Sildenafil followed by serotonin with or without U0126, as indicated. Control cells are treated in the same way except sterile PBS replaced the drug. After treatment, medium is changed to fresh medium, and cells are incubated with 5 g/L of MTT for four hours. MTT is then dissolved with 150 μL of 10% DMSO for 20 minutes. The optical densities (OD) in the 96-well plates are determined using a microplate reader at 570 nm.