GSK investors cheer as Gilead's HIV candidate bictegravir fails to beat their own Tivicay

Gilead is developing candidate bictegravir to take on Glaxo's dolutegravir in the HIV space.

It was less than three weeks ago that a major investor was pulling out of GlaxoSmithKline, in part on worries about HIV competition from Gilead. But Tuesday, GSK shareholders were celebrating.

The British drugmaker’s shares rose after Gilead unveiled new data from studies of its candidate bictegravir, which is in line to compete with Glaxo’s dolutegravir—marketed as Tivicay—if it wins approval. And while four studies of the Gilead med hit their primary endpoints by measuring up to Tivicay and other antiretroviral agents, they didn’t prove that bictegravir is superior to the older meds.

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That’s not to say they never will, because Gilead's studies are ongoing. Three of the four announced Tuesday pitted the combination of bictegravir, emtricitabine and tenofovir alafenamide—also known as BIC/FTC/TAF—against regimens containing GSK's dolutegravir.

One examined the trio against a pairing of dolutegravir and Gilead’s Descovy in patients new to treatment, and one compared BIC/FTC/TAF with GSK’s Triumeq—a fixed-dose combo of dolutegravir, abacavir and lamivudine—in the same population. A third study looked at switching patients to BIC/FTC/TAF versus keeping them on antiviral meds containing dolutegravir.

Related: Chief GSK breakup advocate Woodford, tired of being 'ignored,' pulls out

Gilead says it now plans to submit its approval application in this year's second quarter, but GSK investors apparently figure the threat to ViiV, Glaxo's HIV unit, isn’t as great as they’d imagined. And that’s a good thing for Glaxo, considering how much cash Tivicay and Triumeq have been pumping into the company’s top line, making up for cancer revenue the company traded away to Novartis a couple years back.

Earlier this month, the mere possibility of an assault on Glaxo’s hot HIV sellers was enough to help send high-profile investor Neil Woodford packing. In a blog post titled “Glaxit,” the longtime company critic cited Gilead's forthcoming HIV data as one reason he had decided to exit the stock.

Related: Glaxo's two-drug HIV bet looks good in phase 3 studies—and turns up the heat on Gilead

Meanwhile, though, GSK isn’t just sitting back and waiting for Gilead to jeopardize its market position. It’s gambling with a two-drug, dolutegravir-containing regimen that could potentially cut down on the side-effect burden of Gilead’s three-drug approaches, and their cost, too. Some industry watchers say a two-drug success could help the British pharma giant poach share from its market-leading nemesis.