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The U.S. Food and Drug Administration cleared the combination of dolutegravir and lamivudine, to be branded as Dovato, for use in newly diagnosed adults.
GSK showed in drug trials last year that the once-a-day Dovato pill was as good as a standard three-drug cocktail in suppressing the virus that causes AIDS and also in terms of tolerability.
GSK's HIV drugs division ViiV, in which Pfizer Inc and Shionogi & Co Ltd have small stakes, will use the lower drug burden as its main selling point to patients and physicians, while hoping that longer-term studies will yield hard evidence of fewer side effects over time.
As HIV positive patients grow older thanks to highly effective treatment, the focus shifts to long-term side effects as nearly half of all HIV patients in the profitable markets of North America and Europe are now over 50.
"We are trying to establish a new normal for people with HIV. We absolutely believe that people living with HIV should not take more medicines than they need," said Deborah Waterhouse, who heads the British drugmaker's HIV unit.
ViiV accounted for about 39 percent of GSK's group operating profit last year, according to UBS analysts. With a 23 percent share in the $26 billion-a-year HIV market, it is seeking to catch up with Gilead's 53 percent.
Gilead will keep up the pressure with new anti-retroviral drug Biktarvy, a combination of three HIV medicines in a daily tablet, which was approved early last year. Market researcher Evaluate pharma expects sales from the product to reach $6.1 billion in 2024.
Hopes for growth - despite harsh competition from Gilead - rest on Dovato as well as on another two-drug combination that will be given as a monthly injection but regulatory approval of the latter is not expected before next year.
The company's growth prospects are clouded by the recent U.S. launch of cheap generic copies of blockbuster asthma treatment Advair, generated 2.4 billion pounds in revenues for GSK last year.
GSK already has two-drug regimen Juluca on the market, but only for the smaller market of HIV patients that have received prior treatment. Dovato is the first such product for new cases.
(Additional reporting by Manas Mishra in Bengaluru; Editing by Shounak Dasgupta and Marguerita Choy)