Sales of Vertex Pharmaceuticals' (NASDAQ:VRTX) cystic fibrosis drug, Kalydeco, increased 25% year over year in the third quarter, but that's nothing compared with the sales potential for its cystic fibrosis drug franchise.

It just has to get the drugs approved and persuade countries to pay for it.

Source: Vertex Pharmaceuticals.

About 2,600 patients are currently eligible for reimbursement for Kalydeco, but Vertex Pharmaceuticals is working toward getting the drug approved to treat additional mutations and gain reimbursement coverage, which should bring that number to around 4,000 patients.

In the U.S., the FDA approved its use in eight additional mutations earlier this year, which accounts for much of the third-quarter growth. In Europe, sales increased as coverage for the G551D came online. The third-quarter sales also benefited from a one-time $7 million bump as Vertex expanded its distribution network in anticipation of expanding on its cystic fibrosis drug franchise. Keep those sales in mind when comparing quarter-over-quarter sales in the fourth quarter.

To get to around 4,000 patients eligible for Kalydeco, Vertex needs the FDA to approve the drug for use in patients with the R117H mutation. Last week an FDA advisory committee voted 13-2 recommending that Kalydeco be approved to treat patients with the R117H mutation. If the FDA follows its outside experts' advice, the drug could be approved on or before its PDUFA date of Dec. 30. Combined with the EU -- where an application is also pending -- and Australia, the R117H mutation could add another 1,100 patients or more.

Kalydeco is also up for approval in the U.S. and EU to treat patients aged 2-5, which could expand its use by another 300 patients.

Expanding from 2,600 patients to 4,000 patients sounds like a lot until you look at the 28,000 patients who could be eligible for treatment with Kalydeco combined with ivacaftor. After releasing positive phase 3 data earlier this year, Vertex is on track to file marketing applications for the combination with the FDA and EU regulators this quarter, which could result in a U.S. approval in the middle of next year and approval in the EU a little bit after that.

If regulators approve the drugs for all the new indications and payers agree to cover the drugs, Vertex is going to look like a much different company a year from now. Until then, investors will have to settle for only a 25% increase in sales.