This was from November of the Phase IIa clinical trial of 770+809. As you can see, the sweat chloride from baseline didn't decrease very dramatically after 21 days, on average. Mean decrease was - 9.10 mmol/L
It's not a complete fail - but it's definitely not what we saw with 770 and G551D. They had a mean decrease of -59.5 mmol/L.
It's thought that sweat chloride reduction will correlate well with increase in FEV1. So before doing a full on, huge, very costly study, they study a few patients with study endpoints of just sweat chloride.
http://investors.vrtx.com/releasedetail.cfm?ReleaseID=620638