Source: Pharmacyclics

After Gilead Sciences won FDA approval for its cancer drug Zydelig last summer, some investors worried that sales of Johnson & Johnson (NYSE:JNJ) and Pharmacyclics(UNKNOWN:PCYC.DL) Imbruvica, which is approved to treat the same indications as Zydelig, would falter. However, those worries appear overblown given that at the JP Morgan Healthcare Conference today Pharmacyclics issued rosy Imbruvica sales estimates for 2015 and forward looking guidance that suggests that Imbruvica is on track to be a top seller for J&J and Pharmacyclics for years to come.

First, a bit of background
Imbruvica's efficacy in early stage clinical trials prompted J&J to agree to pay Pharmacyclics $975 million in milestones, including $150 million upfront, as well as split profits on eventual sales, to co-develop the drug in 2011.

That appears to have been money well spent.

Imbruvica has gone on to notch FDA approval as a second-line treatment for both mantle cell lymphoma, or MCL, and chronic lymphocytic leukemia, or CLL; however, it's the larger CLL indication that is more important to investors.

There are 15,680 Americans diagnosed with CLL and 4,580 Americans die from CLL every year, so the need for new treatment options such as Imbruvica are high. As a result, Imbruvica was approved last February under the FDA's accelerated approval process, a process that allows for an FDA green-light ahead of late stage data if data that is presented is compelling. In the case of Imbruvica, the CLL approval was based on trial results showing that 58% of Imbruvica patients saw their cancer tumors shrink. In July, the FDA adjusted Imbruvica's labeling to show that the clinical benefit behind its early approval had been verified in late stage trials.

Imbruvica's success in helping patients diagnosed with this tough-to-treat disease is resulting in substantial quarterly sales growth. At the JP Morgan conference, the company reported that Imbruvica has become the market share leading drug used to treat relapsed/recurring CLL and MCL. Pharmacyclics also reported that preliminary fourth quarter sales of the drug were $185 million, up from $141.6 million in the third quarter, and that Imbruvica's revenue likely totaled $492 million in 2014.

Stronger tailwinds
Pharmacyclics expects that Imbruvica's current approved indications and the potential approval for its use in Waldenstrom's Macroglobulinemia this year will result in Imbruvica net product revenue of approximately $1 billion in 2015, up 103% from 2014.

Looking further out, J&J and Pharmacyclics, which share revenue and profit on Imbruvica, could see the drug's sales head even higher. That's because the two companies are conducting a slate of studies that are designed to extend Imbruvica's reach across hematology, as well as allow for Imbruvica's use as solid tumor treatment.

Source: Pharmacyclics

Overall, Imbruvica is being studied across 58 clinical trials, including 13 that are in phase 3, which are expected to yield one to two new indications to Imbruvica's label per year. Across all potential hematology indications that are being studied, Imbruvica's total addressable patient population could expand from roughly 55,000 people today to 374,000 patients over time.

A similarly large market opportunity may exist for Imbruvica as a solid tumor treatment. The companies are doing preclinical work to evaluate Imbruvica's potential use in lung, breast, colon, and pancreatic cancer.

Additionally, Imbruvica could become more widely used as part of combination therapies that include some of the globe's best selling medicines, such as Rituxan and Revlimid, and some of the planet's most intriguing clinical stage drugs, including PD-L1 and PD-1 drugs.

Looking ahead
Imbruvica's opportunity for these two companies is undeniably large, and it wouldn't be shocking to see Imbruvica become a multibillion dollar therapy in keeping with other top-selling oncology drugs, especially given that its sales ramp so far is eclipsing most of cancer's most widely used medicines.

However, Pharmacyclics' future may not be limited solely to Imbruvica. The company is also studying a next-generation version of Imbruvica as a treatment for various autoimmune diseases. Results from its phase 1 trial studying this new BTK inhibitor in rheumatoid arthritis are expected by year's end. Regardless, both J&J and Pharmacyclics appear to have a big winner on their hands in the form of Imbruvica and that makes both increasingly attractive to investors.

For more Foolish coverage of the JP Morgan Healthcare Conference, click here.