i'm not in the medical or scientific field - just pulling from the internet. It looks to me like some patients were one tropism and became dual tropism or were dual and went to one tropism. remember - pro 140 is for the ccr5 strain only - which is a vast majority of cases. I'm not sure but i think "finding out why" is more about patient screening and tropism changes than pro 140 function. I think Nader is more interested in making sure the trial is testing pro 140 in the correct patients and not having incorrect patients skew the data in the trial.
I asked a similar question.. Tropism failures (low VL screening problem) does not account for why 13 patients from the 'n=28' cohort were not rolled over.