Best guess is that the difference in improvement in FEV-1 between the Phase II data and the Phase III data is primarily a consequence of the difference in study participants.
One imagines that when the Company recruited for the Phase II studies, the patients who volunteered were those most in need of treatment.
If I'm understanding the presentation correctly, the average FEV-1 of the patients in the Phase II extension study was just 19.3% of what would normally be expected from the same number of healthy patients of the same age, sex, weight etc. By contrast, the Phase III study excluded patients whose FEV-1 was less than 25% of what would normally be expected.
Chances are that as all of the low-hanging fruit had been picked for the earlier studies, the need to find 300 patients as quickly as possible resulted in the enrolment of many young patients whose FEV-1 was still much closer to what would normally be expected. From memory, 10% were expected to be below the age of twelve.
This was a conscious decision on the part of the Company, with the objective of a label to treat younger patients for whom Tobi isn't approved. Needless to say, once Arikace is established as the main antibiotic in the rotational regimen there seems no reason for that to change any time soon.
The downside is that a patient with a poor FEV-1 has the potential to evidence a far higher percentage improvement than a patient with a high FEV-1.