Pfizer Inc said that results from clinical studies investigating the use of Viagra (sildenafil citrate) as a treatment for female sexual arousal disorder (FSAD) do not support a regulatory filing.

Pfizer began its FSAD research programme in 1996. To date the company has conducted several large-scale placebo-controlled studies involving approximately 3,000 women. While Viagra was found to be safe in all studies, efficacy data were inconclusive.

A highly prevalent condition with a major impact on quality of life, FSAD is a complex constellation of symptoms that are difficult to identify, measure and treat. The American Foundation for Urological Diseases defines FSAD as a persistent inability to attain or maintain sufficient sexual excitement that causes personal distress. The condition, which may occur alone or in conjunction with other female sexual disorders, is believed to result from a broad range of medical and psychological conditions.

"FSAD is an emerging area of research and is far more complex than male erectile dysfunction," said Dr Joe Feczko, president of Worldwide Development at Pfizer. "Diagnosing FSAD involves assessing physical, emotional and relationship factors, and these complex and interdependent factors make measuring a medicine's effect very difficult."

Pfizer discovered and over the next decade developed Viagra, the world's leading oral treatment for male erectile dysfunction. Since Viagra's approval in 1998, more than 23 million men have been prescribed the medicine worldwide.

"While we are disappointed that the FSAD programme was not more successful, this is the nature of drug development," said Karen Katen, president of Pfizer Global Pharmaceuticals, executive vice president, Pfizer Inc. "We make substantial financial investments, marshal the talents of our best scientists and clinicians, and spend years studying potential treatments only to find that they do not meet the requirements for regulatory approval".

Pfizer continues to invest in sexual health research and is studying other treatment approaches for women.

"Pfizer has made a number of major contributions to the emerging science of female sexuality," Dr Feczko said. "Because of Pfizer's innovative research, we now have a better understanding of the biochemical and physiological factors involved in female sexual function. In addition, we've created and validated tools to allow doctors to more accurately identify patients with FSAD subtypes and to measure the effect of potential treatments."

Pfizer is the world's largest private medical research and development organization. This year the company expects to invest $7.9 billion in research and development.