The Food and Drug Administration approved two new HIV drugs last week -- Tybost and Vitekta -- but Gilead Sciences (NASDAQ:GILD) didn't even bother to issue a press release on its website. It might be a sign of the times: Hepatitis C has become more important to the big biotech than HIV. Last week, Gilead Sciences announced that the European CHMP gave its hepatitis C cocktail, Harvoni, a positive opinion, so it wasn't like the press office was on vacation.

But there's more to it than that. Tybost and Vitekta are already on the market as part of Gilead Sciences' quad pill Stribild, which was approved more than two years ago.

Source: Gilead Sciences.

Around the same time, Gilead asked that the individual components be approved so they could be combined with other medications; but in April 2013, the FDA turned down the drugs because there were issues with Gilead's quality control tests. A year and a half later, Gilead Sciences fixed the problems, and the FDA signed off on the marketing of the drugs individually.

More cocktails
Neither of the drugs will be used by themselves. Gilead is making them available so they can be used in combination with other company's HIV drugs.

Tybost is a boosting agent, designed to increase the blood levels of protease inhibitors, such as Bristol-Myers Squibb's (NYSE:BMY) Reyataz, and Johnson & Johnson's (NYSE:JNJ) Prezista. The increased concentration, which is currently achieved by taking AbbVie's (NYSE:ABBV) booster Norvir, allows the protease inhibitors to be dosed just once a day.

In a clinical trial, Tybost was shown to be non-inferior to Norvir when the drugs were combined with Reyataz and Gilead's Truvada. Without any efficacy advantage, it's hard to see patients switching from Norvir to Tybost if the former is working for them. Gilead might be able to use its marketing muscle to promote sales for new patients, but most of them start on all-in-one combination pills like Stribild and Atripla.

Vitekta is an integrase inhibitor, which prevents the HIV DNA from being integrated into the DNA of the cells it infects. Merck (NYSE:MRK) also has an integrase inhibitor, Isentress, which worked as well as Vitekta in a phase 3 trial with other medications. Vitekta only has to be taken once a day, which is more convenient than Isentress' twice-a-day regimen. Unfortunately, AbbVie's Tivicay, which is in the same class, only has to be taken once a day, also, so it's not clear Vitekta has much to offer over the drugs doctors have been prescribing.

Gilead might be able to generate some sales of the individual drugs from patients coming off of Stribild because of side effects. Doctors may be interested in changing just one component at a time to figure out the issue, which they can do now that the individual drugs are available.

Of course, that probably doesn't include a lot of patients. Combine that fact with its limited use in other combinations, and you'll likely find the explanation as to why Gilead didn't bother to announce the approvals.