David H.W. Lau, Sashi Kommu, Faiz H. Mumtaz, Robert J. Morgan, Cecil S. Thompson and Dimitri P. Mikhailidis Pages 89 - 93 ( 5 )
The oral phosphodiesterase type 5 (PDE5) inhibitors have made a valuable contribution to the treatment of erectile dysfunction (ED). PDE5 inhibitors enhance cavernosal smooth muscle relaxation, vasodilatation and penile erection. However, PDE5 inhibitors are not always effective. Decreased efficacy, cost, incorrect administration, lack of sexual stimulation, vascular risk factors associated with ED and vascular or neurogenic diseases are causes of PDE5 inhibitor failure. Tachyphylaxis may also occur. This is defined as reduced tissue responsiveness to a drug in the presence of a constant concentration of this drug. Treatment failure may cause considerable distress. If dose titration, more attempts and continuous dosing of PDE5 inhibitors (taken on a daily basis) fail to resolve the initial PDE5 inhibitor failure, clinicians need to consider alternative treatments. These include sublingual apomorphine, intracavernosal/intraurethral pharmacotherapy, vacuum devices, the insertion of a prosthesis and penile vascular surgery. Combination therapy like prostaglandin E1 (PGE1) with doxazosin (dox; an alpha-1-blocker) or ketanserin (ketan; a 5-HT2 antagonist) as well as other pro-erection agents, like Endothelin-1 antagonists, angiotensin II antagonists (valsartan/ losartan), adrenomedullin, Rho kinase inhibitors and nitric oxide (NO) donors may be beneficial in the treatment of ED. However, these combination therapies need to be validated. Adding an androgen to a PDE5 inhibitor may help when circulatory testosterone levels are low. The early use of PDE5 inhibitors in patients with hypertension, hyperlipidaemia or diabetes with concomitant ED and treating these risk factors may improve corporeal blood flow and lead to long-term preservation of cavernosal function. Therefore, the efficacy of PDE5 inhibitors may be maintained. Targeting the risk factors of ED (similar to those for arteriosclerosis) in the early stages of the disease may prevent the development or decrease the severity of ED.
PDE5 inhibitors, tachyphylaxis, corpus cavernosum, erectile dysfunction
Dept. of Clinical Biochemistry,Royal Free Hospital, Royal Free and University College School ofMedicine, University of London, Pond Street, London NW3 2QG, UK.