1.7.1 Pathogenesis of Cystic Fibrosis
Cystic Fibrosis (CF) is an autosomal recessive genetic disorder that include several medical conditions caused by mutations of the cAMP/PKA-dependent CFTR (Cystic Fibrosis Transmembrane Conductance Regulator) gene protein located on chromosome 7 (7q31.2).35 The estimated prevalence of this genetic disorder is calculated to be 1 in 2500-4500 live births among Caucasians.36 CFTR is a phosphorylation-regulated transmembrane channel for chloride ions exchange, typically present in the majority of the apical membrane of epithelial tissues such as lungs, gut, kidney and uterus.37 The altered chloride membrane conductance in the epithelial cells impairs the physiological osmotic fluid movement thus leading to the presence of greasy mucus in the respiratory and intestinal tract, pancreatic insufficiency, increased sweat salt concentration, infertility in male patients and liver malfunctions (Figure 45).38
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Figure 45. Representation of the mechanism of transport of salts in healthy and CF airways.1
More than 2000 different mutations of CFTR gene have been found to be responsible of the clinical manifestation of the pathology that can therefore be variable in terms of intensity and quality of the symptoms from patient to patient.39,40 In addition, mutations of the gene encoding for alfa-1-antytrypsyn and air pollution have been considered as co-factor in determining symptoms and pathology outcome.41 Mutations of CFTR in heterozygotes Caucasians have been proposed as responsible of the resistance to Tuberculosis infections and this could at least explain why the mutations are maintained over time.42 Anyway, despite the typology of mutation, the chronical obstructive lung disease is a condition that affect the 70% of CFTR homozygotes at the age of seventeen and lung chronic infections are the main causes of death among patients.43
Since Cystic Fibrosis is a genetic disorder, the cure of this pathology would require a genetic therapy that is not available so far. CFTR modulators are drugs able to improve the production, transport and functions of chloride defecting channels. The introduction of these drugs in therapy started in 2011 with the approval of Ivacaftor (Kalydeco®), a CFTR enhancer that was proved to efficiently increase the secretion of chloride ions.44 Apart from Ivacaftor, there are now two combination pharmaceutical forms approved for CF treatment i.e. Lumacaftor/Ivacaftor (Orkambi®) and Tezacaftor/Ivacaftor (Symdeko®). Unfortunately, CFTR modulators are available only for specific mutations and can’t be used a priori. The treatment of Cystic Fibrosis is focused therefore on symptoms relieving and usually requires a therapeutic plan tailored on the needs of the patient. The main target of the treatments available are the lungs and the exocrine pancreas. As for the latter, usually patients are recommended to take pancreatic enzyme supplements and vitamins to increase the amount of essential compounds introduced and absorbed.45 The symptoms in the respiratory tract are often the major factors impacting the quality of the life of CF patients being the loss of pulmonary functionality the main cause of death among this population.
79 1.7.2 Bacterial infections by Pseudomonas Aeruginosa
Due to the lack of fluidity of the mucus, the mucociliary clearance mechanism, responsible for the clearance of bacteria as well as other microorganisms from the respiratory tract, is impaired and altered.46 This is associated with chronical infections by pathogens. Infections of the lower airways by Staphylococcus Aureus and Pseudomonas Aeruginosa (P.
Aeruginosa) are the most occurring in CF patients.47 Chronical infections, associated to sustained and prolonged inflammation, are the main causes of pulmonary function declination. The presence of a thick and viscous mucus can lead to a decrease of the effectiveness of the drug delivery towards the respiratory tract.48 P. Aeruginosa infections are challenging to treat since it is shown to be a resilient and resistant bacterium. In the lungs of CF patients, Pseudomonas is capable of producing a biofilm mucoid strain. The slow growth of the biofilm in CF patients is associated with an enhanced frequency of mutations and an adapting bacteria mechanism to the conditions in the lungs, and to resistance towards antibiotic therapy.49
1.7.3 Antibiotic treatments
The pharmacological treatment usually consists of an association of one or more antibiotics and anti-inflammatory drugs.50 It has been reported that intravenous antibiotic treatments against P. Aeruginosa decreases the concentration of proteinase in secretions, thus improving lung function. For this reason, the administration of intravenous antibiotics should be given at regular intervals in a planned pharmacological regime in order to reduce lung inflammation presumably by reducing the bacterial activity.51 Tobramycin intravenous administration is the only alternative during exacerbations.43 Colistin (administered as colistimethate sodium) is usually used whenever tobramycin is not tolerated or with the tobramycin-resistant Pseudomonas Aeruginosa strains.52 However, since the penetration of tobramycin through the viscous mucus is low following intravenous administration, high doses are necessary to obtain the minimum effective inhibitory concentration towards P.
Aeruginosa. Due to the intravenous high posology, adverse side effects, including ototoxicity and nephrotoxicity, are very frequent.50
Since the concentration of antibiotic at the infection site is of relevant importance, delivering high concentrations of antibiotics directly to the lungs by inhalation is convenient, considering that inhalation therapy has several advantages over the systemic administration.53 First, the side effects of the systemic exposure can be mitigated thus increasing the adhesion to therapy and patient compliance. Moreover, the drug can be
80 released directly to the site of action, i.e. the lung, avoiding the first pass effect. These aspects must be accurately evaluated in CF patients since the chronic colonization of the airways by microorganisms requires a chronical therapy with all the related inconveniences.54 Several antibiotics have been studied for the management of pulmonary infections in CF patients; among these, the most prescribed are the aminoglycosides in combination with a β-lactam or a quinolone.55,56 In fact, clinical trials delivering multiple APIs in combination demonstrated a significant improvement in the efficacy of the treatment of the common respiratory diseases, including asthma and COPD.57
Tobramycin is the aminoglycoside of first choice. It is available for inhalation administration either as solution for nebulization either as dry powder inhaler. Nebulized antibiotics like tobramycin (TOBI®) and aztreonam (Cayston®) have proven to be efficient in the treatment of CF patient’s infections; nevertheless, they are easily cleared away from the lungs.58 What is more, nebulization products generally require an administration time of approximately 20 minutes.59 Apart from tobramycin, other antibiotics such as gentamicin, amikacin and colistimethate sodium (Colobreathe®) are generally used in the management of CF patient’s exacerbations.60
In order to keep sufficiently high levels of antibiotics for antimicrobial efficacy, multiple doses of high concentration of antibiotics are mandatory each day.61 Dry powder inhalers (DPI), since they are activated by patient inspiration flow, are a significant alternative because of rapid dose administration and high local drug concentration.62,63 TOBI® Podhaler™ (Novartis Pharmaceuticals Corporation, East Hanover, NJ, USA) is the dry powder pharmaceutical product of the inhaled antibiotic tobramycin. It is a low resistance device, which allows to generate a high inspiratory flow thus achieving a predictable dose delivery.64
1.7.4 Mucolytic agents in the treatment of cystic fibrosis
Antibiotics represent the mainstay pharmacological treatment in the management of cystic fibrosis patients’ infections. However, the increasing antibiotic resistance is an issue that must be addressed. In the last years, the urgent need for research into new antimicrobial agents able to target the biofilm enhancing the success of antibiotic treatments was enphasised.65 Due to the thickness of the dehydrated mucus and sputum creating an ideal environment for microbial colonization and infections, the co-administration of mucolytic as well as osmotic agents is highly recommended.66 Drug products containing Dornase alfa (Pulmozyme®, Roche), N-acetylcysteine (Mucomyst®) or mannitol (Bronchitol®) have been proved to be effective in reducing the viscosity and elasticity of the mucus thus increasing
81 the effectiveness of the antibiotic treatment. N-acetylcysteine has been previously described in literature as mucolytic agent with antimicrobial properties.67 In fact, it is able to decrease the formation of the biofilm made up by several bacteria promoting at the same time the disruption of the already formed biofilms.