Shares of Vertex Pharmaceuticals (Nasdaq: VRTX) spiked yesterday after the company released interim phase 2a study results for its potential cystic fibrosis treatment VX-770. What makes these results so interesting is that cystic fibrosis is a very debilitating progressive disease that causes early death in all patients, and there are no good treatments on the market to fight it.

Right now, Gilead Sciences (Nasdaq: GILD) is the drugmaker whose cystic fibrosis treatment is closest to getting on the market. The company's aztreonam lysine antibiotic compound is awaiting FDA regulatory review, with a PDUFA date of Sept. 16, and Gilead also filed a European Union marketing application for the drug earlier this month.

Versus the competition
Gilead's aztreonam lysine data has generally been very positive, and I predict that it will receive FDA approval later this year. So how does Vertex's VX-770 stack up against aztreonam lysine?

One of the main ways to test a patient's lung functioning is with what is called an FEV1 test (basically, a one-second test of the strength of a person's breath). Clinical trials testing drugs that treat lung disorders like cystic fibrosis -- as well as drugs such as InterMune's (Nasdaq: ITMN) idiopathic pulmonary fibrosis drug candidate, pirfenidone -- use FEV1 or a similar breath test to gauge whether a patient's lung disorder is progressing or getting better.

In one of Gilead's two pivotal phase 3 studies for aztreonam lysine (the results of which were released last year), the drug produced a 6.3% relative improvement in cystic fibrosis patients' FEV1 versus placebo after 28 days of treatment. In another study, it produced a 10.3% improvement in FEV1 for cystic fibrosis patients versus placebo after 28 days.

Before continuing, note that there are some huge caveats here. Gilead's drug was tested in different cystic fibrosis patient populations, had different study inclusion criteria and a different mechanism of action, and FEV1 wasn't the primary endpoint in its phase 3 studies. But in the subpopulation of cystic fibrosis patients that Vertex's VX-770 was tested in, one of the VX-770 patient groups experienced at least a 10.1% relative improvement in their FEV1 versus the control group in a much shorter portion of the study (only 14 days).

A few points to consider
Currently, cystic fibrosis treatments on the market don't generally improve the underlying disease; most are simply stopgap attempts at slowing the decline in patients' lung functioning, treating their lung infections, and hopefully giving them several more years of life (the median age of death for U.S. cystic fibrosis patients is 37). Gilead's antibiotic aztreonam lysine, and now Vertex's VX-770, are very exciting potential additions to doctors' cystic fibrosis treatment arsenals. But there are a few addenda to the Vertex study results announced yesterday that need to be mentioned.

Estimates vary on the number of cystic fibrosis sufferers in the U.S.; the number ranges from 30,000, according to the Cystic Fibrosis Foundation (and Vertex), to roughly 85,000 (one out of every 3,500 people in North America), according to the World Health Organization. The data Vertex produced yesterday, however, was from a very small subpopulation of these cystic fibrosis patients, all of whom had a specific gene mutation that affects only about 4% of cystic fibrosis sufferers in the United States.

If VX-770 is limited to treating just 4% of the already small cystic fibrosis patient population, then that will be quite a tiny potential market for the drug. The good news is that Vertex plans on testing VX-770 in much broader cystic fibrosis patient populations in the future, but it's difficult to know how the positive efficacy data announced yesterday will translate to other, widely diverse, cystic fibrosis patient groups.

Vertex's data yesterday was also very light on the safety and adverse-events info. Indeed, in such a short and small (only 20-person) study, it would be hard for many adverse-event signals to even pop up. So investors shouldn't forget about the potential for safety issues to derail VX-770, or to limit its application to only sicker cystic fibrosis patients.

Also, in phase 1 testing, VX-770 did cause an increased incidence of a rash in some treated patients. On the flip side, though, with cystic fibrosis being such a serious progressive disease, regulatory agencies will give VX-770 a longer adverse-event leash compared to drugs that treat less serious conditions.

The future of VX-770
Currently, Vertex holds the worldwide rights to VX-770, but it will have to pay an undisclosed royalty and milestone payments to the Cystic Fibrosis Foundation if the drug eventually gets approved.

Considering the orphan indication that VX-770 treats and the relatively low cost of running clinical trials with the drug, Vertex will likely not have to find a commercial partner for VX-770, which is what it had to do when it out-licensed its hepatitis C compound, telaprevir, to Johnson & Johnson (NYSE: JNJ).

There's another interesting possible wrinkle in the cystic fibrosis drug race, too: Gilead and Vertex both have multiple compounds for the disease in the pipeline that have different drug targets. It will be interesting to see how the potential for combo treatments develops with all these new and revolutionary cystic fibrosis compounds in the pipeline.

Vertex's next step with VX-770 is to finish this phase 2a study, which it will do by enrolling 16 patients in 28 days of testing. Vertex guided for this part of the study to start in the second quarter. After the very preliminary results announced yesterday, I can't wait to see what sort of data the next chapter of this study produces.

More Foolishness on drugs that treat small patient populations:

Vertex and InterMune are active picks of our market-beating Rule Breakers newsletter. You can check out all our other recommendations, as well as get access to our message boards and exclusive content, with a 30-day free trial.

Johnson & Johnson is an Income Investor pick.

Fool contributor Brian Lawler does not own shares of any company mentioned in this article. The Fool has an A+ disclosure policy.