Source: Aerie Pharmaceuticals,

What: Shares in Aerie Pharmaceuticals (NASDAQ:AERI) jumped by nearly 50% earlier today after the company reported that the FDA is allowing it to change a primary endpoint on a key phase 3 trial evaluating its Rhopressa eye drops.

So What: Aerie Pharmaceuticals shares took it on the chin in April when the company reported that Rhopressa failed to achieve statistical significance in one phase 3 trial that was evaluating Rhopressa's ability to lower interocular pressure in patients with glaucoma or ocular hypertension.

The failure of Aerie Pharmaceuticals' most advanced clinical stage therapy cast doubt on a second phase 3 trial that is also evaluating whether or not once-daily Rhopressa works as well as, or better than, twice-daily timolol, a commonly-used therapy for this condition.

However, the FDA's allowing Aerie Pharmaceuticals to narrow the baseline range of interocular pressure when calculating statistical significance offers investors new hope. Statistical significance will now be determined based on interocular pressure that is above 20 mmHg and below 25 mmHg, rather than the range of above 20 mmHg and below 27 mmHg that was used to evaluate the first phase 3 trial. If the 20 mmHg to 25 mmHg range had been used in the first trial, Rhopressa would have achieved statistical significance.

Now What: The FDA is also allowing Aerie Pharmaceuticals to use a particular statistical approach that could mean that its trial doesn't need to enroll any additional patients. That could speed up the timeline for data collection and reduce costs associated with further patient recruitment. Aerie Pharmaceuticals anticipates it will deliver phase 3 efficacy data from this trial to investors in Q3 and safety data to investors next year.

Although the FDA decision suggests that Aerie Pharmaceuticals' chances for delivering positive results from this second phase 3 trial are better than they would otherwise have been, investors may still want to exercise some caution. That's because the lasting effect of Rhopressa seemed to wear off over time during the prior phase 3 trial. The number of patients reporting a loss of efficacy was cut in half when considering the new interocular pressure range, but it's still something to keep in mind. Because of the risk associated with this trial and the fact that it's unlikely that a Rhopressa filing for approval would come until the end of 2016, I'm content to sit on the sidelines and see how this plays out.