Gilead Sciences has been boosted by the results of a Phase III study showing that its four-drug HIV treatment was as effective as its top-selling drug Atripla, setting up a regulatory filing for the new product early next year.

The 'quad' regimen consists of Gilead’s investigational drugs elvitegravir and cobicistat with the two active ingredients in Truvada (emtricitabine and tenofovir), all formulated into a single, fixed-dose tablet.

It is seen as a successor Atripla (efavirenz/tenofovir/emtricitabine), Gilead's biggest-selling product with sales of $1.57 billion in the first half of 2011. Quad is tipped to be a big earner for Gilead if it secures approval because the drugmaker owns all four compounds in the formulation. In the case of Atripla, Gilead needs to share revenues with partner Bristol-Myers Squibb.

In the comparative study, 88% of HIV patients starting antiretroviral therapy for the first time saw a reduction in viral load to less than 50 copies per ml after 48 weeks' treatment, compared to 84% of those receiving Atripla.

The new drug combination also showed some signs it may be superior to the triple therapy. It achieved increases in CD4 cell counts of 239 cells/ml after 48 weeks, compared to 206 cells/ml with Atripla, which was a statistically-significant difference. Meanwhile, therapy discontinuations due to side effects were also comparable between the two treatment arms.

"Achieving non-inferiority to the current standard of care in HIV therapy is a major developmental milestone for our Quad regimen," said Norbert Bischofberger, Gilead's chief scientific officer.

A second Phase III trial comparing Quad to a regimen of Truvada plus ritonavir-boosted atazanavir is due to give results in the next few weeks.  Meanwhile, cobicistat, in the new integrase inhibitor class, is also being evaluated as a stand-alone boosting agent for other antiretrovirals such as atazanavir.