¹Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, TN, USA

Correspondence: Professor SH Francis, Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, 21st and Garland, Nashville, TN 37232-0615, USA. E-mail: Sharron.francis@vanderbilt.edu

Received 21 December 2007; Revised 30 January 2008; Accepted 30 January 2008; Published online 17 April 2008.

Abstract

Cyclic guanosine monophosphate (cGMP) in penile vascular smooth muscle cells (VSMC) plays a key role in promoting penile erection. Phosphodiesterase-5 (PDE5) in VSMC breaks down cGMP to counter this effect. Sildenafil (Viagra), vardenafil (Levitra) and tadalafil (Cialis), treatments for erectile dysfunction, inhibit PDE5 action. Many men with erectile dysfunction have improved erectile function after plasma inhibitor concentration falls below therapeutic levels. Maximum effect plus onset and duration of action of inhibitor determines its efficacy. The rate and extent of cellular drug accumulation and efflux of drug from smooth muscle cells plus persistence of drug effects in these cell impact these parameters. We propose possible molecular mechanisms that could account for prolonged action of PDE5 inhibitors including (1) persistence of biochemical effects after inhibitor is cleared from cells, and (2) retention of drug in VSMC beyond plasma clearance.