GlaxoSmithKline plc  (GSK) - Get Report   shares outperformed the market Tuesday after clinical trials data for Gilead's (GILD) - Get Report  Bictegravir HIV treatment appeared to reduce the threat of a superior rival to GSK's Dolutegravir.

Gilead said Tuesday that Bictegravir met its end point of non-inferiority in four separate phase III clinical trials, when combined with emtricitabine, as a treatment for HIV-1. The U.S. drug maker has been running its Bictegravir treatment head to head as a combo with Glaxo's Dolutegravir in trials and has said it aims to file for approval with the Food & Drug Administration in the current quarter.

Glaxo stock rose just more than 1% during noon trading in London, to change hands at an intraday high of 1,665.0 pence, which was in contrast to the 0.03% loss of the Stoxx Europe 600 TMI Pharmaceuticals index.

Analysts had assumed that Gilead's drug could achieve non-inferiority status but, at the same time, feared the emergence of a potentially superior drug to GSK's Dolutegravir.

Gilead can continue to test for superiority of its Bictegravir against Glaxo's Dolutegravir. It said Tuesday that its trials are still ongoing.

"Given that both drugs are highly efficacious, we think it is unlikely that the data show that bictegravir/F/TAF is superior, but if this does happen the competitive threat to ViiV will be dramatically increased," said Laura Sutcliffe, an analyst at Berenberg.

Dolutegravir is a drug produced by Glaxo's HIV business ViiV, which is a joint venture with Pfizer (PFE) - Get Report , and has emerged as a white knight for the British drugmaker in recent years.

Better-than-expected growth of ViiV sales have helped to offset the impact of a decline in Glaxo's respiratory franchise, which has been facing a cliff edge after it lost the patents to asthma treatment Advair.

Multiple rivals have filed for approval for generic competitors to Advair, a drug that still accounts for around 15% of the Glaxo's top line, although none have secured approval so far.

HIV treatments, on the other hand, now account for more than 20% of sales at Glaxo and income from these is still growing at double digit rates.