Ok-one more...I also found this..it's an older article but it tells how G551D and R117H are similar:
Potentiator drugs have also reached phase 1 and 2 clinical trials. This set of drugs targets mutated CFTR present in the apical membrane and is designed to augment the ion channel function of these proteins. The CFTR mutations that are especially amenable to this approach are the mutated channels that already reach the membrane but display less functional activity than wild-type. Two such mutations are R117H and G551D. R117H is present in 0.8 - 2% of the CF population (The Cystic Fibrosis Genetic Analysis Consortium) and results in mutated but mature CFTR protein that demonstrates a lower open probability (PO ~0.1) than wild type CFTR [PO ~0.35 - 0.40 [35, 36]. G551D also reaches the plasma membrane as a mature protein. However, it demonstrates severely diminished open probability, PO ~0.04 [37], and is present in 2 - 6% of CF patients (The Cystic Fibrosis Genetic Analysis Consortium). The plant isoflavone genistein, a phytoestrogen, has been well documented [37 - 39], at concentrations of ~50 µM or greater, to activate mutated CFTR in the epithelium. It is, however, difficult to achieve these concentrations, since genistein bioavailability by oral administration is very low [40].
Consequently, there has been a search to find more potent and specific CFTR potentiators than genistein. Two compounds are in development by Vertex Pharmaceuticals, in collaboration with Cystic Fibrosis Foundation Therapeutics, and they are currently in phase 1 and phase 2 clinical trials. Oral VX-770 is in phase 2 trials in G551D genotype CF patients. VX-809 is currently in phase 1A trials and expected to enter phase 1B trials in mid 2008. In vitro patch clamp studies, carried out on CFTR expressed in Fisher Rat Thyroid cells, have demonstrated that VX-770 increases ?F508 PO from 0.11 ± 0.05 to 0.45 ± 0.06, and G551D PO from 0.08 ± 0.03 to 0.52 ± 0.19 [41]**. These data suggest that VX-770 can increase ?F508 and G551D PO to values in the range reported for wild type CFTR. Chloride secretion across Fisher Rat Thyroid monolayers over-expressing ?F508 in Ussing chambers in response to VX770 was somewhat less robust, with an increase above a very low basal short circuit current of ~2 fold [41]. The data are not surprising, since the quantity of ?F508 CFTR protein in the membrane is presumably small. However, the increase in G551D current was 14-fold, representing 65% of the current carried by wild type CFTR in this system. Importantly, VX-770 is more potent than genistein, with an EC50 of ~600 nM, and is reported to be orally bioavailable [41]. Likely, for ?F508 CF patients, dual therapy will be required, with corrector to traffic mutated CFTR protein to the apical membrane and potentiators such as VX-770 to stimulate activation.
Here's the link to the complete article: <a target=_blank class=ftalternatingbarlinklarge href="http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2517236/">http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2517236/</a>