Projected 2018 sales: 
$1.702 billion
2012 sales: $172 million

Kalydeco may be the orphan drug of all orphan drugs. It is the first drug to target the underlying cause of cystic fibrosis, a genetic disorder that affects 70,000 worldwide and only 30,000 in the U.S. Moreover, Kalydeco is licensed solely for patients with the G551D mutation, which only accounts for about 4% of CF cases worldwide. Currently, drugmaker Vertex ($VTRX) gets by charging $294,000 per year for the treatment, which offsets the small patient pool. But all of that may be about to change.

Vertex is currently testing two combinations, each of which pair Kalydeco with a different experimental drug. And if those combinations prove successful, use of Kalydeco could skyrocket to up to 90% of CF patients, launching Kalydeco into blockbuster territory with peak annual sales that have been estimated to reach as high as $5 billion or $6 billion.

In April, Vertex announced that one of those combinations was already looking promising. The biotech tested Kalydeco with its VX-661 combo in patients with the most common CF mutation, carried by nearly half the disease's sufferers, and data showed statistically significant improvements in lung function over placebo for the two highest dose groups. 

For more:
Vertex skyrockets on positive Ph2 combo study in cystic fibrosis
EU OKs Vertex cystic fibrosis drug Kalydeco