Determination of a new phosphodiesterase V inhibitor, DA-8159, in plasma and urine by high-performance liquid chromatography

A high-performance liquid chromatographic (HPLC) method using liquid-liquid extraction for sample preparation was developed for the determination of a new phosphodiesterase V inhibitor, DA-8159, in rat plasma and urine using sildenafil citrate as an internal standard. A 100 μl aliquot of 0.1 M Na₂CO₃ (containing sildenafil citrate, 3 μg/ml as free sildenafil) and a 1 ml aliquot of ether were added to a 100 μl aliquot of biological samples (urine samples were diluted 20 times with distilled water). After vortex centrifugation at 9000×g for 3 min, the ether layer was collected and dried under nitrogen gas. The residue was reconstituted with a 150 μl aliquot of the mobile phase, centrifuged, and a 100 μl aliquot of the supernatant was injected onto a reversed-phase column. The mobile phases, 20 mM KH₂PO₄ (pH 4.7):acetonitrile (70:30, v/v for plasma and tissue samples, and 75:25, v/v for urine samples), were run at a flow rate of 1.0 ml/min. The column effluent was monitored by an ultraviolet detector set at 292 nm. The retention times for DA-8159 and the internal standard were approximately 10.7 and 9.1 min, respectively, in plasma and tissue samples and the corresponding values in urine samples were 47 and 33 min. The detection limits for DA-8159 in rat plasma and urine were 20 and 100 ng/ml, respectively. The coefficients of variation of the assay were generally low: below 10% for plasma and 9.9% for urine. No interferences from endogenous substances were found.