Kevin Southern. November, 2001. North American CF Conference, Orlando 2001 [online]. University of Liverpool, Institute of Child Health, Alder Hey Children's Hospital, Liverpool, UK. Available from http://www.cysticfibrosismedicine.com

A subdued atmosphere at the 2001 NACFC was not surprising given the current international situation. However, the conference was able to report significant advances in our understanding of CF pathogenesis.

The Basic Defect

Cystic Fibrosis is characterised by a number of seemingly unrelated features; abnormal airway ion transport, abnormal mucus rheology and an abnormal inflammatory response, to name but three. At the 2001 NACFC, one had the impression of these fields coming together to give an inclusive picture of how mutation of the CFTR gene results in CF pathophysiology.

More evidence was shown to support the hypothesis of Boucher and colleagues at the University of North Carolina at Chapel Hill, that the primary defect in CF relates to depletion and dehydration of airway surface liquid (ASL). For example, studies on CF mice, utilising microdialysis techniques, demonstrated the ion content of ASL to be nearly isotonic, consistent with periciliary fluid depletion (Grubb, abstract#146).

This year, the situation was taken forward by linking this hypothesis with the abnormal mucus that characterises CF. Depletion of the periciliary ASL layer causes a profound disruption of mucociliary transport in vitro and in CF airways is associated with continued secretion of mucus from goblet cells and submucosal glands leading to the tenacious and abnormal mucus plugs that characterise this condition (Ballard #98; Parsons #73).

The complex matrix that constitutes mucus is being better characterised. Mucus is formed from many different components but is dependent on mucins for its unique structure. A number of genes have been recognised that code for mucin proteins, but MUC5AC and MUC5B have a predominant role in the airway. Studies on a porcine model have characterised the secretory processes of the submucosal gland. These processes rely on CFTR-mediated Cl- and HCO3- secretion. In CF these processes are absent and plugging of the submucosal gland with mucus occurs (Trout #160; Ballard #98).

An overall picture is therefore emerging that gives a clear explanation of CF pathology and reconciles a number of different pathological observations.

Genetics

A number of themes were expanded on in Orlando, including some further characterisation of splice mutations such as the 5T variant, which can result in a mild phenotype. The modifier gene that has been tracked to chromosome 19 remains elusive despite attempts to characterise it (Zielenski #133). One study reported a relationship between disease severity and polymorphisms of the detoxifying gene, glutathione-S transferase (Henrion #120, McKone #128). Another study suggested a phenotype relationship to the expression of IL10 and TNFa (mild disease associated with high levels of IL10). Patients expressing a high TNFa polymorphism in conjunction with a low IL10 have worse disease severity, although the rate of deterioration in FEV1 was not different between the groups (Marsick #121).

Microbiology

Characterisation of the full Pseudomonas aeruginosa (Pa) genome is beginning to have a significant impact on our understanding of Pa biology. In particular there were a number of presentations and posters addressing the issue of "quorum sensing" (the situation in which bacteria in colonies have different behaviour, i.e. produce a biofilm, compared to isolated pseudomonas). The genes that regulate these processes are beginning to be recognised and the influence of environment (i.e., the hypoxic conditions inside a mucus plug in the airways) on these genes. Iglewski gave a clear overview of the importance of bacterial growth pattern on "quorum sensing". She provided a clear summary supporting the hypothesis that these genes are predominantly active when the bacteria are in the stationary phase (i.e., the situation that likely corresponds to the airway infection). Standard recognition of antibiotic resistance in the laboratory is during log growth of bacteria on agar plates. This suggests we need to reconsider orthodox views on antibiotic sensitivity. In addition, this work may lead to new anti pseudomonal therapies; for example, Pa is resistant to the antibiotic azithromycin during log growth but has significantly increased sensitivity during the stationary phase. Azithromycin appears to cause a decrease in biofilm production, albeit temporary, and this may relate to downregulation of "quorum sensing" genes, however these mechanisms are still not clarified.

New Therapies

The "UNC" tissue culture model of the airway epithelium has been a very powerful tool in increasing understanding of CF pathology. With this model workers have demonstrated that with isotonic fluid on the airway surface Na+ absorption is the predominant ion transport process. However, at Orlando, they provided data demonstrating that in the "steady state", chloride secretory processes (mediated through CFTR) become as important. It is difficult to know whether such a "steady state" equates to the in vivo situation, however the UNC group proposed that this evidence supported the use of a chloride secretagogue as a novel therapy for CF. A phase 1 RCT is currently recruiting patients to examine the effect of a new chloride secretory compound, INSPIRE37217 (with a similar action to UTP but with a longer duration of action), on pragmatic and explanatory outcomes.

With respect to gene therapy the conference was understated, however there was good work presented regarding further development of vectors (such as the lentivirus, Feline IV), examining the mechanism for viral and DNA uptake by airway cells and increasing transgene expression. A lot of progress has been made over the past few years in increasing understanding of these mechanisms but no vectors are currently being assessed in clinical trials.

Another potential therapy that received attention at previous conferences has been phenylbutyrate, an agent that has been shown in vitro to overcome the trafficking defect associated with the DF508 mutation and result in normal CFTR function at the cell membrane. An RCT examined the safety and efficacy of phenylbutyrate taken orally (Zeitlin #242). A dose of 40 g three times a day resulted in "unpleasant" side effects. Using the nasal PD measure of chloride secretion (change in PD following perfusion with a chloride free solution and isoprenaline) as a surrogate or explanatory outcome measure, the study demonstrated a significant increase in chloride secretion in the 6 patients that received 20 g and in six that received 30, compared to 4 controls.

Two years ago, in Seattle, there was much interest in the results of studies on CF mice that demonstrated histopathological correction of CF lesions in the gut following supplementation with the essential fatty acid, docosahexaenoic acid (DHA). An RCT of DHA supplementation was presented in Orlando. 19 patients completed the study. Nine received 50 mg/kg/day of algal DHA and had significant rises in their red blood cell DHA levels with no apparent adverse events (Lloyd-Still, #262).

Clinical

Possibly the most important announcement with respect to clinical trials was the early termination of the TOBI study of eradication of Pseudomonas. The data monitoring committee observed a marked discrepancy between the two treatment groups and the study code was broken. They found that 8/8 children treated with TOBI had successful eradication of Pseudomonas in contrast to 1/13 that received placebo. This result highlights the importance of data monitoring in CF clinical trials and the urgent need for further evaluation of TOBI for Pseudomonas eradication compared with, for example, nebulised colistin.

The Brompton group announced the preliminary results of their RCT examining long term Azithromycin in children with CF. 40 children completed the study and, although the crossover design can be criticised, the study did show a significant increase in FEV1 during treatment periods. Of more importance was the lack of adverse events particularly hearing and liver impairment. This study will be a valuable addition to the body of evidence on macrolides (Equi #396).

Another crossover RCT examining whether delivery of Dornase alfa before physio is more effective than delivery after, found no difference between the two regimes with significant improvement with both (Fitzgerald #404). Another Australian study examined this practical issue and concluded that some patients may respond better to Dornase given pre physio and some to it given post. The authors suggest that in non-responders the alternative regime should be employed (Middleton #405).

One of the most disturbing papers came from the Melbourne group reporting their six year study of bronchoalveolar lavage studies on infants (Armstrong #363). The incidence of Pa isolated increased dramatically from 5% at three years of age to 21% at five. The authors suggested this might have been due to a single mucoid clonal strain that has been recognised in their clinic population. 6/7 infants with the non-mucoid strain had successful eradication of Pa compared to 0/8 with the mucoid strain. Five infants with the mucoid strain had died compared to none with a non-mucoid strain. The clinic has now established a segregation policy.