Interpretation of CF Gene Therapy Trial Results
Authors: Peadar G. Noone, Michael R. Knowles
Journal: Molecular Therapy (2000)

To the Editor: The quantitation and interpretation of biological outcome measures of gene transfer in CF are challenging, and the comments and questions of Professor Alton and his colleagues address important considerations. We note with interest the comments of the UK group in relation to our study (1Noone P.G. et al.Safety and biological efficacy of a lipid-CFTR complex for gene transfer in the nasal epithelium of adult patients with cystic fibrosis.Mol. Ther. 2000; 1: 105-114Abstract Full Text Full Text PDF PubMed Scopus (120) Google Scholar) and offer the following responses. 1We agree with Alton et al. that study design is an important feature of any clinical trial and that it is difficult to make comparisons among different protocols with differing methodologies and outcome measures. Given the current status regarding gene transfer studies in CF, we believed that a review of the available data from all liposomal and viral trials was timely and important. We did not mean to impugn the results of any specific study, particularly those that were well-designed and placebo-controlled. Rather, we wished to call attention to the relatively small magnitude of reported PD responses (an index of CFTR-mediated Cl– conductance) and raise the level of awareness of newly recognized variables that may facilitate the design and interpretation of future studies.2We agree with Alton et al. that the optimal discriminator between CF and normal is the CFTR-mediated Cl– conductance, i.e., the voltage response (delta PD) to perfusion with zero (or low, 6 mmol/liter) chloride-and beta agonist (isoproterenol/terbutaline)-containing solutions, in the presence of amiloride. The mean normal CFTR-mediated Cl– conductance in the nose of normal subjects has been reported to range from ~–19 to –32 mV (2Middleton P.G. Geddes D.M. Alton E.W. Protocols for in vivo measurement of the ion transport defects in cystic fibrosis nasal epithelium.Eur. Respir. J. 1994; 7: 2050-2056Crossref PubMed Scopus (67) Google Scholar, 3Knowles M.R. Paradiso A.M. Boucher R.C. in vivo nasal potential difference: Techniques and protocols for assessing efficacy of gene transfer in cystic fibrosis.Hum. Gene Ther. 1995; 6: 445-455Crossref PubMed Scopus (285) Google Scholar, 4Hay J.G. McElvaney N.G. Herena J. Crystal R.G. Modification of nasal epithelial potential differences of individuals with cystic fibrosis consequent to local administration of a normal CFTR cDNA adenovirus gene transfer vector.Hum. Gene Ther. 1995; 6: 1487-1496Crossref PubMed Scopus (127) Google Scholar, 5Noone P.G. Zhou Z. Winders J.W. Hofmann T. Boucher R.C. Knowles M.R. CFTR-mediated Cl- and Na+ transport and CFTR mRNA expression in the nasal epithelia of normal subjects and carriers of a CF mutation.Ped. Pulmonol. Suppl. 1996; 13: 226Google Scholar, 6Porteous D.J. et al.Evidence for safety and efficacy of DOTAP cationic liposome mediated CFTR gene transfer to the nasal epithelium of patients with cystic fibrosis.Gene Ther. 1997; 4: 210-218Crossref PubMed Scopus (290) Google Scholar), which we used as the denominator for our comparisons. In that context, we considered a delta PD of –2 mV to be small (7Caplen N.J. et al.Liposome mediated CFTR gene transfer to the nasal epithelium of patients with cystic fibrosis.Nat. Med. 1995; 1: 39-46Crossref PubMed Scopus (711) Google Scholar, 8Gill D.R. et al.A placebo-controlled study of liposome mediated gene transfer to the nasal epithelium of patients with cystic fibrosis.Gene Ther. 1997; 4: 199-209Crossref PubMed Scopus (211) Google Scholar, 9Alton E.W. et al.Cationic lipid-mediated CFTR gene transfer to the lungs and nose of patients with cystic fibrosis: A double blind placebo-controlled trial.Lancet. 1999; 353: 947-954Abstract Full Text Full Text PDF PubMed Scopus (382) Google Scholar). We did note that Alton et al. showed a larger increase in the lower airways (–4 mV), although the sequence of perfusates (low chloride followed by amiloride) differed from the nasal protocol.3As mentioned above, we agree that the optimal discriminator between normal and CF airway epithelial CFTR-mediated Cl– conductance is perfusion with zero (or low) chlorides- and beta agonist-containing solutions (in the presence of amiloride) (2Middleton P.G. Geddes D.M. Alton E.W. Protocols for in vivo measurement of the ion transport defects in cystic fibrosis nasal epithelium.Eur. Respir. J. 1994; 7: 2050-2056Crossref PubMed Scopus (67) Google Scholar, 3Knowles M.R. Paradiso A.M. Boucher R.C. in vivo nasal potential difference: Techniques and protocols for assessing efficacy of gene transfer in cystic fibrosis.Hum. Gene Ther. 1995; 6: 445-455Crossref PubMed Scopus (285) Google Scholar, 4Hay J.G. McElvaney N.G. Herena J. Crystal R.G. Modification of nasal epithelial potential differences of individuals with cystic fibrosis consequent to local administration of a normal CFTR cDNA adenovirus gene transfer vector.Hum. Gene Ther. 1995; 6: 1487-1496Crossref PubMed Scopus (127) Google Scholar, 5Noone P.G. Zhou Z. Winders J.W. Hofmann T. Boucher R.C. Knowles M.R. CFTR-mediated Cl- and Na+ transport and CFTR mRNA expression in the nasal epithelia of normal subjects and carriers of a CF mutation.Ped. Pulmonol. Suppl. 1996; 13: 226Google Scholar, 6Porteous D.J. et al.Evidence for safety and efficacy of DOTAP cationic liposome mediated CFTR gene transfer to the nasal epithelium of patients with cystic fibrosis.Gene Ther. 1997; 4: 210-218Crossref PubMed Scopus (290) Google Scholar). We did not mean to suggest that other studies did not employ this protocol. Rather, we were attempting to contrast our use of zero Cl– plus isoproterenol as the primary indicator, whereas other studies have emphasized the results of delta PD in the nose in response to low Cl– alone (7Caplen N.J. et al.Liposome mediated CFTR gene transfer to the nasal epithelium of patients with cystic fibrosis.Nat. Med. 1995; 1: 39-46Crossref PubMed Scopus (711) Google Scholar, 8Gill D.R. et al.A placebo-controlled study of liposome mediated gene transfer to the nasal epithelium of patients with cystic fibrosis.Gene Ther. 1997; 4: 199-209Crossref PubMed Scopus (211) Google Scholar, 9Alton E.W. et al.Cationic lipid-mediated CFTR gene transfer to the lungs and nose of patients with cystic fibrosis: A double blind placebo-controlled trial.Lancet. 1999; 353: 947-954Abstract Full Text Full Text PDF PubMed Scopus (382) Google Scholar).4Regarding the possibility that “inflammation” might have been related to the delta PD recorded in the study of gene transfer to the lower airways (9Alton E.W. et al.Cationic lipid-mediated CFTR gene transfer to the lungs and nose of patients with cystic fibrosis: A double blind placebo-controlled trial.Lancet. 1999; 353: 947-954Abstract Full Text Full Text PDF PubMed Scopus (382) Google Scholar), we noted that all subjects treated with lipid–DNA had a systemic illness different from controls (lipid alone). The authors speculate as to the etiology of this reaction, and the favored hypothesis was that a macrophage-mediated mechanism might initiate a range of proinflammatory secretory responses (9Alton E.W. et al.Cationic lipid-mediated CFTR gene transfer to the lungs and nose of patients with cystic fibrosis: A double blind placebo-controlled trial.Lancet. 1999; 353: 947-954Abstract Full Text Full Text PDF PubMed Scopus (382) Google Scholar). It may be difficult to demonstrate an increase in the inflammatory response in the airways of CF patients, who usually have a striking degree of airway inflammation at baseline.5We note that the UK studies of EDMPC are congruent with our finding that this liposome is an inefficient gene transfer vector in respiratory (nasal) epithelium. The field of gene transfer faces ongoing challenges to develop useful study designs and protocols and quantitative measurements of biological efficacy. We hope that the comments and questions of Alton et al., and our responses, have clarified some of these important issues.