(Adds sales forecast, more details)

LONDON, Aug 13 (Reuters) - AstraZeneca, which fended off a $118 billion takeover approach from Pfizer in May, reported positive results for an experimental gout drug on Wednesday that could boost its sales modestly.

The drug, lesinurad, is not forecast by analysts to be a major seller but its success in three late-stage clinical trials is a vindication of AstraZeneca's ability to develop new medicines and comes after recent progress in cancer treatments.

The three Phase III studies all showed the potential of lesinurad, a selective uric acid re-absorption inhibitor, for use in combination with existing medicines as a treatment for gout - a painful joint condition caused by the build-up of uric acid in the blood.

However, the drug did also cause more side effects, especially at higher doses, than seen with the older drugs used on their own.

The company said detailed results would be submitted to a scientific meeting later this year, and it would prepare a regulatory submission for a 200 mg dose of the drug in combination therapy.

Industry analysts forecast annual sales of lesinurad to reach $281 million by 2019, according to consensus forecasts compiled by Thomson Reuters Pharma.

AstraZeneca acquired lesinurad after buying U.S. company Ardea Biosciences for $1.26 billion two years ago.

(Reporting by Paul Sandle and Ben Hirschler; editing by James Davey and Tom Pfeiffer)