GlaxoSmithKline execs touted their HIV growth drive this week, pointing to next-generation medications Triumeq and Tivicay as the fuel. But both drugs actually fell short of analyst forecasts—and that's thanks to archrival Gilead Sciences and its newly launched Biktarvy.

For the first quarter, the company's HIV unit ViiV Healthcare put up 14% growth for a total of £1.05 billion ($1.47 billion). That's not a statistic to sneeze at. But at least one analyst didn’t share management's optimism about the franchise.

In a note to investors Wednesday, Bernstein analyst Tim Anderson, M.D., named Gilead competition "a key concern" for ViiV this year and beyond. The sign? Triumeq and Tivicay, both key to the company's HIV growth, posted sales that were "softer" than analysts had estimated. Triumeq’s £606 million and Trivicay’s £348 million each fell £13 million short, Anderson pointed out.

That was probably the work of Gilead’s single-tablet, three-drug regimen Biktarvy. Tivicay (dolutegravir) and its sister combo medication Triumeq have lost new-to-brand share—about 6 points—Bank of America Merrill Lynch analyst Graham Parry noted during the company's earnings call on Wednesday. Though Gilead's Genvoya itself lost 6 points, the brand-new Biktarvy—just approved in February—has already grabbed about 19%.

Glaxo Chief Strategy Officer David Redfern assured investors that the British pharma has seen "very little impact if anything at all on the dolutegravir franchise from the competitive launch." In terms of total prescriptions, Redfern said it has maintained a leading position of just over 28% in the overall single-tablet and core-agent market, and its share among previously untreated patients has held steady in the low 30s.

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He did admit, however, that Glaxo is capturing fewer patients switching from other drugs—but insisted that a lot of patients switching to Biktarvy were moving over from other Gilead drugs.

Of course, it is still early days, as Biktarvy was just put on the stand. But one of the reasons the drug is expected to hit $6 billion in peak sales is that it's targeting both groups of patients: the treatment-naïve and the switch-overs.

In contrast, Glaxo’s Juluca, the first complete HIV treatment regimen containing just two drugs, is only approved for patients who are already virologically suppressed. In its first full quarter, Juluca recorded sales of £10 million.

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The two companies are still in the thick of studies testing their respective drugs in various groups of patients and at differing dosing schedules. Soon after winning its Juluca nod last November, Glaxo started a phase 3 trial to assess bimonthly injections of its candidate cabotegravir with Janssen’s Edurant (rilpivirine) in comparison with monthly dosing. And that was on top of two other phase 3 studies of the monthly regimen, dubbed FLAIR and ATLAS, that will read out during the latter half of this year.

Gilead recently struck back with data from a head-to-head study comparing Biktarvy to a combo of Tivicay, Epivir and Ziagen. It not only showed that Biktarvy matched up to GSK's three-drug regimen but also demonstrated a lower incidence of side effects.

Both companies really need their HIV drugs to deliver, given what’s lagging in other departments. For Gilead, the company estimates its once-high-flying hepatitis C franchise will likely plummet by more than half to around $3.5 billion to $4 billion. And Glaxo's facing pressure on its respiratory star Advair, which dropped 20% to £566 million in the first quarter, thanks to competition from newer brands and generics—with more copycats to come, likely later this year.

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