12. Rituxan

Anti-CD20 MAb
2015 sales: $7.32 billion
2020 sales: $5.05 billion

Rituxan, developed by Roche's ($RHHBY) Genentech, is one of several drugs in the top 20 that are seeing sales wane: It is expected to be down 6% in 2020 compared to 2014. But even with its slow fade, it is expected to generate more than $5 billion in annual sales in 2020.

Approved nearly 20 years ago, MabThera as it is known in some markets, has the distinction of being the first monoclonal antibody treatment for cancer and has held the title as the best-selling cancer drug. It will lose that distinction to some newer treatments like Bristol-Myers Squibb's ($BMY) Opdivo, but Rituxan continues to be a go-to treatment in many instances.

Because it is so successful, there are a host of biosimilar developers hoping to steal some of those sales when it goes off patent in 2018. But Roche did not wait around to see how that would play out. It developed a followup, Gazyva, which has outperformed Rituxan in the treatment of chronic lymphocytic leukemia (CLL), and for which the FDA and the EU have approved it as a first-line treatment. Gazyva, however, is not expected to see the kind of success that Rituxan has had, with peak sales projected to reach only $2.5 billion, if that much. -- Eric Palmer (email | Twitter)

For more:
Special Reports: The 10 best-selling drugs of 2013 - Rituxan/MabThera | Top 20 orphan drugs by 2018 - Rituxan
Roche gets thumbs up for CLL use of Gazyvaro in EU
Roche nabs European approval for time-saving formulation of Rituxan
Roche's new Gazyva beats predecessor Rituxan at staving off CLL

12. Rituxan