8. Soliris

Rare diseases including atypical hemolytic uremic syndrome
2015 sales:
$2.59 billion
2020 projection: $5.60 billion

Soliris is one of those treatments for ultra-rare diseases with an ultra-high price tag. In fact, for several years it has topped the ranks of the world's most expensive meds--and its list price keeps getting higher. At last word, its U.S. list price was $669,000 per year, according to CBC News.

Because the diseases Soliris treats are so rare, payers tend to be willing to cover the costs. Alexion ($ALXN) has seen some pushback lately: Canada tried to crack down on its price, and the company sued, while cost-effectiveness watchdogs in England demanded information on R&D and manufacturing spending to justify the cost.

Still, the National Institute for Health and Care Excellence decided to cover Soliris anyway. Its sales continue to climb, to almost $2.6 billion in 2015 from $2.2 billion in 2014. Three potential new indications, including one in myasthenia gravis, could give Soliris an even bigger boost, analysts say. Some not only predict the drug will top $5 billion by 2020, but also figure it can burgeon to $7.5 billion by 2025.

One potential spoiler: A rival drug from Alnylam ($ALNY), ALN-CC5. Phase II data on the drug will be unveiled at a conference in June. It's an RNAi therapy designed to go after a particular gene implicated in the diseases Soliris now treats. -- Tracy Staton (email | Twitter)

For more:
Alexion sues Canada over Soliris price cut attempt as drug cost debate explodes on center stage
Leaping Soliris sales send Alexion's bottom line soaring
Orphan drug approvals are up, but prices are up more

8. Soliris