Source: Gilead Sciences

When you hear "Gilead Sciences," I know many people think, "AbbVie" -- the two have certainly been connected in headlines lately. And The FDA approval of AbbVie's (NYSE:ABBV) hepatitis C drug Viekira Pak could mean that AbbVie takes a big bite out of Gilead Sciences (NASDAQ:GILD)  massive hepatitis C sales moving forward.

But there's a stock already making Gilead look pretty bad...and it's not even in hepatitis C.

This company's product is outperforming Gilead's.

Oh, and its shares grew faster than Gilead's over the past year.

Have you guessed it yet?

It's Pharmacyclics (UNKNOWN:PCYC.DL). Pharmacyclics and Johnson & Johnson's (NYSE:JNJ) cancer drug Imbruvica remains the market share leading second line treatment for chronic lymphocytic leukemia, or CLL, even though Gilead Sciences has begun ramping up its competitor Zydelig.

Inching along
I can't say I blame investors for having high hopes for Gilead Sciences' blood cancer drug Zydelig when it launched last year. After Gilead's stratospheric sales success with its first hepatitis C drug Sovaldi, it became much harder to bet against Gilead's ability to dominate the market for CLL, too.

But despite the best efforts and marketing might of Gilead's sales team, Zydelig's revenue appears to be growing much slower than Imbruvica was at the same point following its launch.

Zydelig won FDA approval on July 23rd and its sales totaled $5.9 million in the third quarter. In the fourth quarter, Gilead reported that Zydelig's sales totaled $17 million.That's not bad for Zydelig's second full quarter on the market, but it's a far cry from the $56 million that Imbruvica sold in its second full quarter on the market.

Zydelig's run rate seems even less spectacular when you consider that until February of 2014, Imbruvica was only approved to treat the very small mantle cell lymphoma indication. That means that the much larger and commercially important CLL indication only contributed to Imbruvica's second quarter on the market for about six weeks.

Imbruvica's momentum hasn't appeared to slow since Zydelig's launch either. Despite competing against Zydelig for the entire fourth quarter, Imbruvica's quarter-over-quarter sales growth actually accelerated from 29% in the third quarter to 31% in the fourth quarter.

Building on its lead
Pharmacyclics thinks that Imbruvica's approval for use in mantle cell lymphoma and CLL, along with its recent approval for use in Waldenstrom's macroglobulinemia patients, will lead to Imbruvica net product revenue of nearly $1 billion in 2015, up 103% from 2014.

However, that could prove to be just the tip of the iceberg for Pharmacyclics and J&J. The two companies are studying Imbruvica in 58 ongoing clinical trials that cut across hematology and solid tumor treatment, including as a therapy for use in treating lung, breast, colon, and pancreatic cancer.

Of those trials, 13 are in phase 3, and that could mean that additional label expansions occur in the coming year or two. If so, Pharmacyclics and J&J believe that the total patient population that could benefit from Imbruvica could grow from 55,000 people today to 374,000 patients in the future.

Establishing a beachhead
The sheer volume of activity surrounding Imbruvica has analysts expecting that the drug could easily become a $3 billion-a-year drug. Analyst estimates for Zydelig's peak sales of $1.2 billion are far more timid.

Regardless, Gilead Sciences views Zydelig as the first of a series of cancer-fighting compounds that it intends to launch in the coming years. Other therapies that Gilead Sciences is developing include momelotinib, a JAK inhibitor for the treatment of myelofibrosis and pancreatic cancer, GS-9973 and GS-9901 for blood cancers, and GS-5745 for solid tumor indications.

Source: Gilead Sciences.

Looking ahead
Imbruvica's opportunity as a monotherapy already appears to be bigger than Zydelig's, but Imbruvica's sales potential isn't limited to its use as a stand-alone drug.

Pharmacyclics and J&J are studying Imbruvica's efficacy alongside a host of top-selling oncology drugs, such as Revlimid. Additional studies are also being conducted to evaluate whether Imbruvica can improve outcomes when paired up with some of cancer's most promising new treatments, too. For example, Imbruvica is being combined with Bristol-Myers Squibb's (NYSE:BMY) recently approved PD-1 inhibitor Opdivo as a potential therapy for non-Hodgkin lymphoma.

Based on all of the activity surrounding Imbruvica, it wouldn't be surprising if Pharmacyclics maintains its leadership over Gilead Sciences in oncology. But with Gilead Sciences raking in billions of dollars in sales from its market-leading HIV and hepatitis C drugs, investors shouldn't count it out -- at least not yet.