Initial results from clinical trials suggest that vardenafil, Bayer's new drug for erectile dysfunction (ED), will be an effective treatment for ED in men with diabetes.

Diabetic men are three times more likely than the general population to have erectile difficulties, and between 50% to 60% of diabetic men over the age of 50 have some degree of ED. Patients with diabetes mellitus are also less responsive to available oral ED therapies, so an effective new treatment option is needed and welcome.

The newest data on this therapeutic challenge were presented in June at the 61st Annual Meeting of the American Diabetes Association in Philadelphia. The study compared Bayer's vardenafil with a placebo ("sugar pill") in a population of 452 men with Type 1 or Type 2 diabetes mellitus and a history of ED. Subjects were randomly assigned to use either vardenafil or placebo for 12 weeks. The study was of a double-blind design, meaning neither the volunteers nor the researchers knew which patients received the active drug and which received placebo until after the study ended.

Overall, 72% of the men who used vardenafil reported an improvement in their ability to obtain an erection, compared with 13% who received placebo. In 64% of the vardenafil group, erections were adequate for vaginal penetration, compared with 36% in the placebo group. Similarly, 54% of vardenafil users were able to sustain erections for the completion of intercourse, compared with 23% in the placebo group.

Causes of ED

Physicians recognize three major categories of ED: organic, when the dysfunction is due purely to anatomic or biological causes such as circulatory or neurologic (nerve) problems; psychogenic, when the cause is psychological, such as fear, anxiety, and/or depression; and mixed, when physical and psychological factors are both present.

In an interview with SexHealth.com, Irwin Goldstein, MD, the principal investigator for this study, characterized the typical diabetic ED patient he sees as having multiple risk factors for ED. These include an age of 50 or older, a history of cigarette smoking, obesity, high cholesterol, and cardiovascular disease.

"The response to vardenafil is as good in psychogenic ED as in organic ED," says Goldstein. "It's only when the organic issues are excessively severe that vardenafil doesn't work."

Viagra and vardenafil: how do they compare?

Like Viagra, vardenafil works by inhibiting the action of the enzyme phosphodiesterase 5 (PDE5). Simply put, PDE5 helps a man's erection subside after intercourse, but in men with impaired erectile function, PDE5 overwhelms the natural process of erection. Temporary inhibition of PDE5 with a drug like Viagra or vardenafil helps men with reduced erectile capacity obtain and maintain an erection.

One Viagra study in 268 diabetic men with ED reported a rate of successful completion of intercourse of 48%. While it's tempting to compare that 48% rate with the 54% rate in the vardenafil study, the relative effectiveness (and safety) of Viagra and vardenafil can only be determined in head-to-head clinical comparisons enrolling identical groups of patients. To date, no such trials have been completed. There are, however, some important differences between these two drugs that have been scientifically established and may well translate into genuine clinical benefits for vardenafil.

Vardenafil has a faster onset of action than Viagra and its effect lasts longer. It's also substantially more potent an inhibitor of PDE5 than Viagra, which means it takes less drug to do the same job. But the most promising difference is in specificity: there are at least 13 different phosphodiesterases (PDEs) that perform a number of different functions at various sites in the human body. Inhibition of PDEs other than PDE5 increases the potential for unwanted side effects. One of the more troublesome side effects of Viagra, for example, is a temporary disturbance of color vision, most often in the form of objects appearing to have a bluish tinge or halo. This effect is attributed to Viagra's inhibition of PDE6, an enzyme found in the retina.

Vardenafil has greater specificity for PDE5 than Viagra and therefore less potential for inhibition of other PDEs. This potential seems to have been confirmed in the recent study. According to Goldstein, no color vision disturbances were observed with vardenafil, and the rates of other side effects normally associated with PDE5 inhibition (headache, facial flushing, nasal congestion, dyspepsia, and occasional back pain) were about 50% lower than what would be expected with Viagra. He also noted that most of the men in the present study had used Viagra in the past and expressed a preference for vardenafil.

Faster acting, longer lasting, fewer side effects--these potential advantages with vardenafil have yet to be borne out in one-on-one clinical comparisons. If they are, however, and if no unexpected toxicity problems emerge and the effectiveness of vardenafil proves equal (or superior) to that of Viagra, it appears Pfizer's "little blue pill" will be facing some stiff competition in the not so distant future.