7. Herceptin

Anti-HER2 for breast cancer
2015 sales: $6.79 billion
2020 sales: $5.66 billion

Roche ($RHHBY) has had a good, long run for Herceptin, a drug that remains its third-best seller after about 15 years on the market, coming in between two other Roche drugs on this list, Avastin and Rituxan. The company's first targeted cancer drug, aimed at patients whose tumors have specific genetic characteristics, Herceptin has paved the way for what has become a huge part of the cancer treatment market. In this case, the appropriate patients have tumors that overexpress the HER2 gene.

But Herceptin's days as an unchallenged leader are waning. Mylan ($MYL) and Indian partner Biocon have launched their own versions in India, and South Korea's Celltrion has a biosimilar of the drug approved in its home country. U.S. drugmaker Hospira ($HSP), which has an aggressive biosimilar program underway, is laying the groundwork for attacking the U.K. market, convincing a court to set aside two Roche patents on the drug. Unless Roche wins an appeal, the ruling could open the large market to biosimilar competition when Herceptin's main patent expires on July 28. -- Eric Palmer (email | Twitter)

For more:
Special Report: The 10 best-selling drugs of 2013 - Herceptin
Hospira wins Herceptin patent suit in U.K., paving way for biosimilar
Roche legal action messes with Mylan's India launch of its Herceptin biosimilar

7. Herceptin