Cystic fibrosis gene therapy trials and tribulations.

isthe most recent example of the complexity of the prob-lem (1). Noone and colleagues outline the progress of acarefully constructed nasal epithelial gene transfer studyutilizing a cationic lipid delivery vehicle and high doses ofplasmid CFTR cDNA in adult CF patients. Despite thepromise of preclinical studies that established the optimallipid/plasmid formulation in cell culture, conﬁrmed thefunction of recombinant human CFTR protein after trans-fection, and assessed the safety and efﬁcacy of singledosing to lungs of rhesus monkeys, the investigators wereunable to detect recombinant CFTR mRNA in nasal epi-thelial biopsies or to detect CFTR-mediated chloride trans-port by nasal potential difference measurements. Previousstudies of different cationic lipid formulations by otherinvestigators (2–5) have reported successful gene deliveryby DNA-PCR and gene expression by RT-PCR, but onlysmall changes in CFTR-mediated chloride conductance of2–4 mV. Although all these studies were conducted withdifferent formulations, study designs, and nasal potentialdifference protocols, the outcomes remain similar—tran-sient and barely detectable changes in CFTR-mediatedchloride transport and no clear evidence of a dose-depen-dent effect. Why is it so difﬁcult to achieve CFTR genetransfer and CFTR-mediated ion transport?The nasal epithelium appears to be an ideal surrogatesite for testing novel pulmonary gene transfer vectors orpharmaceuticals designed to restore airway epithelial iontransport balance. The inferior turbinate is a relativelysafe site for testing agents that might trigger an unwantedinﬂammatory response. Regulated ion transport can bemeasured along the inferior turbinate relatively easilycompared to the proximal airways by a technique calledtransepithelial potential difference. The nasal techniqueor nasal potential difference is a sensitive measure ofsodium and chloride transport that can be reasonablyspeciﬁc for CFTR when the cAMP-dependent response isincluded in the protocol. It is not, however, completelyspeciﬁc, as alternative chloride conductances that re-spond to changes in intracellular calcium, cell volume, orextracellular pH, to name a few, could play a role when-ever the chemical gradient for chloride exit is favored bya low chloride superfusate. The authors constructed theirstudy so that one nares served as the placebo control forthe treated nares in a double-blind, randomized design.The unexpected ﬁnding was a small increase in CFTR-mediated chloride conductance on the third baseline visit(day 21) suggesting that repeated daily measurements ofnasal epithelial chloride transport induced changes of 2–3mV. While the three predose measurements remainedﬁrmly within the CF range, the investigators point outthat this magnitude of change is what has been reportedas evidence of recombinant CFTR gene expression inother gene therapy trials. Are there comparable changesin CFTR function from pharmacological trials that mightguide the interpretation of these results? What should weexpect from sufﬁcient CFTR gene expression in the nasalpotential difference measurement?The magnitude of the change or repolarization of thepotential difference after nasal superfusion of non-CFsubjects with a chloride-free, amiloride-containing solu-tion followed by the addition of a b-adrenergic agonistsuch as isoproterenol has been reported to be at least 21mV with a mean response of 230 mV (6, 7). CF patientvolunteers ranged from 21or22 mV to about 110 mVwith a mean of around 15 mV. This low chloride/isopro-terenol response in CF subjects can be partially restored inspeciﬁc situations. Pharmacologic correction of prema-ture-translation termination stop codon mutants in CFTR(e.g., W1282X) by topical treatment with aminoglyco-sides to promote readthrough to a full-length CFTR hasbeen reported to accomplish a change of 0 to 210 mV (8).Daily oral 4-phenylbutyrate therapy for 1 week to ho-mozygous DF508 CFTR patients induced a 22- to 27-mVdifference between pre- and postdrug values (9). In theabsence of any pharmacologic intervention, partiallyfunctional CFTR mutants such as those from Class IV(partial conduction) or V (reduced levels of CFTR) may becapable of generating small changes in potential differ-ence. Several of the subjects in the paper by Noone