Projected 2018 sales:
$6.948 billion
2012 sales: $7.133 billion

Sometimes being at the top can be difficult, as Roche ($RHHBY) is finding out with its megablockbuster Rituxan. The chronic lymphocytic leukemia drug has a target on its back, with biosimilar developers preparing to vie for some of the $7 billion franchise's market share when the drug loses patent exclusivity later this year in Europe (and in 2018 in the U.S.). While some of them, like South Korea's Celltrion, have backed off knockoff programs for the drug, biologics makers such as Amgen ($AMGN) still pose serious threats.

Roche's solution? Get a replacement for Rituxan out there, and fast. The company believes it has one with obinutuzumab, which could directly terminate cancer cells while spurring the immune system to tackle leukemia. Roche has been pushing full speed ahead to bring the compound to market, earning the FDA's priority review status and a breakthrough drug designation that have resulted in an action date of Dec. 20, 2013.

The company is hoping to extend the success of the first monoclonal antibody treatment for cancer, whose sales continue to grow even after about a decade and a half on the market. That growth is partially due to an expanded market, as Rituxan over the years has bagged indications for several types of lymphoma as well as rheumatoid arthritis.

For more:
FDA cuts the regulatory fuse on Roche's heir to Rituxan
Roche's next-gen Rituxan candidate impresses in first look at PhIII data
Roche bets on PhIII contender to succeed its bestselling cancer drug
Report: Don't count Celltrion out of Rituxan biosim race