Source: Gilead Sciences.

Gilead Sciences (NASDAQ:GILD) hepatitis C drugs may be capturing all the attention, but its Gilead's HIV drug franchise that's been responsible for the company's long-standing success. Gilead is racking up billions in global revenue thanks to a slate of HIV drugs that includes two therapies on their way to billion-dollar blockbuster sales success. Those two drugs, Stribild and Complera, have seen their sales spike this year, so investors shouldn't discount the impact they may have on Gilead's profit next year.

First, a bit of background
Innovative treatment and increased testing are driving significant advances in HIV treatment. The average lifespan of newly diagnosed HIV patients has more than doubled in the past 20 years; but despite those advances, more work remains.

According to the World Health Organization, more than 1.5 million people died from HIV globally in 2012, and more than 33 million people remain infected with the disease worldwide. Many of those cases are in far-flung regions of the world, but HIV remains an important and under-diagnosed disease in the United States, too.

According to the Centers for Disease Control, or CDC, there are 50,000 new cases of HIV diagnosed annually. Yet, roughly 200,000 of the 1.1 million Americans suspected of having HIV remain undiagnosed and, therefore, untreated.

Because so many people remain undiagnosed, many states are increasing their efforts toward identifying and testing at-risk patients. New York announced a bold plan earlier this year that it hopes will effectively eliminate new HIV cases. And earlier this month, Washington state launched a program that would provide Gilead's preventative HIV drug, Truvada -- a combination of Gilead's Viread and Emtriva -- at no charge to some at-risk patients.

As more states follow the leads of New York and Washington and embrace early intervention, demand for HIV therapies will likely climb. That may already be happening given that Gilead -- the biggest HIV drugmaker -- is on pace to have five different drugs eclipse the $1 billion sales mark this year, including Stribild and Complera.

Growing quickly
Stribild is a single pill combination of Gilead's Vitekta, Tybost, Viread, and Emtriva. Since winning FDA approval in 2012, Stribild has gone on to become the leading HIV therapy for newly treated HIV patients. It's also become the second most-prescribed regimen, behind Gilead's Atripla, across all HIV patients.

Market-share growth for Stribild, which costs roughly $28,000 per year, is starting to have a much larger impact on Gilead's financials. In the third quarter, Stribild's sales jumped 128% year over year, to $328 million. In addition to rapid growth in America, where sales climbed from $135 million a year ago to $279 million last quarter, sales overseas are jumping, too.

Thanks to recent launches, such as in France, Stribild's European sales increased dramatically from $7.9 million in the third quarter of 2013 to $38.3 million last quarter. Sales in markets outside the U.S. and EU are also heading higher, increasing from $1.3 million last year to $10.8 million. Stribild's success is impressive, and would likely be capturing more investor attention if not for Sovaldi and Harvoni; but Complera shouldn't be ignored either.

Demand for Complera, which combines Gilead's Viread and Emtriva with Johnson & Johnson's (NYSE:JNJ) Edurant, is also growing. U.S. sales of Complera, which costs about $20,000 per year, climbed from $127 million a year ago to $183 million in the third quarter, and sales of Complera in Europe, where it's sold under the brand name Eviplera, grew from $74 million to $134 million year over year.

Source: Gilead Sciences and author's calculation.

Advancing treatment
Based on those numbers, both Stribild and Complera are likely to eclipse sales of $1 billion this year. However, Gilead thinks that it can improve upon that success and win more scripts if the FDA issues a green light for TAF, its new formulation of Viread.

In clinical trials, replacing Viread with TAF in a Stribild regimen performed as well as Stribild with less risk of kidney damage. Because HIV patients are increasingly living longer, toxicity is a major concern for both doctors and patients. Therefore, if the FDA approves TAF, and Gilead can effectively transition its combination therapies from Viread to TAF, patients may be able to stay on those therapies longer. If Gilead can execute on that strategy, it may find that these drugs remain top sellers for years to come.