Dr. Tillie-Louise HackettDepartment of Anesthesiology, Pharmacology and Therapeutics
University of British Columbia
Associate Head, Centre of Heart Lung Innovation, St Paul’s [email protected]
Respiratory Pharmacology:
Treatment of Cystic Fibrosis
Aims of Lecture
Define the disease pathology of Cystic Fibrosis, specifically
the role of CFTR in normal lung function and disease.
Explain the different classes of CFTR mutations.
Describe the maintenance therapies used in CF compared to
treatment for specific mutations.
Cystic Fibrosis: Epidemiology
Name refers to scarring (fibrosis) and cyst
formation in pancreas -1930s
It is the most common life-threatening
disease inherited in the Caucasian
population
Autosomal recessive (must have mutations
in both alleles
4% of people of European descent are
carries for one allele for CF
Incidence: 1 in 2000-3000
Asia and Africa: 1 in 90,000
Hereditary, chronic disorder affecting the exocrine glands that causes
severe damage to the lungs and digestive system
CFTR Defect in a gene, called the cystic fibrosis transmembrane
conductance regulator (CFTR)
CFTR is the main chloride channel in epithelia of various tissues
-ENaC - epithelial
sodium channel
-CFTR – chloride
Channel
-AQP - aquaporin
(water channel)
Water “follows the salt (NaCL) gradient” by osmosis
CF pathophysiology Fluid / volume secretion problem
In CF, the defect in CFTR causes decreased Cl- ions to apical
surface
Inhibits H20 transcellular flow, water influx and dehydration of mucus
Low fluid volume
CFTR is the main chloride channel in
epithelia of various tissues
Epithelial perform diverse functions:
1) water or volume-absorbing (airways and intestinal tract)
2) Salt-absorbing (sweat ducts, lung)
3) water or volume-secreting (pancreas, lung)
all processes involve chloride ion transport; disruption of
this transport in cystic fibrosis leads to multiple effects
Cystic Fibrosis: Clinical Features
Affects epithelial in multiple organs
Chronic lung infections and inflammatory destruction of the lungs
Nutritional abnormalities due to gastrointestinal obstruction and lack of fluid secretion by intestinal cells
Pancreatic insuffciency
Fertility
Males: Can affect development of vas deferens, or blockage by thick mucus
Females: Thick mucus may affect fertilization of the egg
Excessive salt loss from the sweat glands leading to heat exhaustion and dehydration in hot weather and fever
Thick
viscous
mucus
Lung Defenses
Human airway contains two distinct aqueous layers
1) Mucus layer – traps inhaled bacteria and foreign particles
2) Airway surface liquid (ASL)
provides microenvironment for beating cilia to clear mucus layer with assistance of coughing
ASL is also rich in proteases, antibiotics & oxidants
Defensins: small peptide molecules (12-50 aa) containing positively charged and hydrophobic residues.
Kill microbes via membrane disruption
Their non-specific action makes it difficult for bacteria to acquire resistance
Problem: High salt inactivates defensins
Problem: Thick mucus inactivates mucociliary clearance
Biofilms: Sticky mucus traps bacteria, difficult to remove by cilia
of the lung epithelium
CF Progressive lung Disease Lungs are plagued by persistent
bacterial infections
Biodiversity of bacteria decreases
Main culprits: Pseudomonas
aeruginosa, B. cepacia, S. aureus
Once Colonization occurs impossible
to eradicate (prevention measures)
Lung tissue eventually destroyed by
onslaught of immunes cells
(Neutrophils) that respond to the
infections
Respiratory failure due to obstructive
lung disease
CFTR Gene
250 kb, 27 exons, located on chromosome 7
Over 2000 different mutations identified in the CFTR gene
2 membrane spanning
domains (MSD): anchor
protein to the plasma
membrane and form the
ion channel
2 nucleotide-binding
domains (NBD-1, NBD-2):
hydrolyze ATP and control
ion channel gating
(opening and closing)
1 Regulatory domain:
controls activation of CFTRPositive charge of Arg = electrostatic barrier
Cl- to Na+ permeability ratio = 150 (without Arg352 = 15)
Classes of CFTR mutations Class I: Defective protein production
(premature stop condon). No or little CFTR protein is produced.
Class II: Defective trafficking (DF508) results in misfolding of protein and degradation in endoplasmic reticulum. Affects 75% of CF patients.
Class III: Mutation (G551D) in regulatoryregions, protein reaches surface but remains closed. Affects 4-5% of CF patients.
Class IV: Mutation (Arg 352) in the membrane-spanning regions, unable to move CL- ions efficiently
Class V: Reduced synthesis of functional CFTR, unstable mRNA
Class VI: Accelerated turnover of membrane bound protein
Cystic Fibrosis TreatmentAirway Clearance Technique
Bronchodilators
Mucous thinners – mucolytics Pulmozyme (Dnase) removes DNA from white blood cells
Airway clearance – percussion, coughing & huffing
Antibiotics – inhaled via nebulizer (Tobramycin)
Hypertonic saline
• Salt-water inhaled twice per day. Helps draw water into the mucus.
Side effects
• Airway irritation – cough, chest tightness
Cystic Fibrosis Treatment: Pancreas
One of the main functions of the pancreas is the secretion of digestive enzymes
Low fluid secretion leads to thick mucus which prevents enzymesfrom reaching the intestines:
Enzymes are retained in the pancreas and eventually destroy all pancreatic tissue
Lack of digestive enzymes lead to poor nutrient absorption, weight loss etc
Treatments Pancreatic enzyme replacement – Pancrelipase replaces natural
enzymes (Amylase, Lipase, proteases) so that fat and proteins can be absorbed properly
Adequate nutrients to maintain BMI
Ivacaftor – First FDA approved drug for CF - 2012
G551D mutation (class IV)
Mutation causes glycine to aspartic acid switch
Present in cell membrane but defective in CL- transport
Affects 4-5% of patients
Mechanisms of action
Oral bio-available CFTR potentiator
Binds to the channel inducing a non-conventional mode of gating, increasing the probability that the channel remains open
Treatment
150 mg dose (half life 12 hours) given twice a day to children >2 years and adults
$300,000 per year
Cystic Fibrosis: Targeted Treatment I
Lumacaftor/Ivacaftor –FDA approved drug for CF – July 2015
dF508 mutation (class II)
deletion causes loss of the amino acid phenylalanine 508
defective trafficking of CFTR to cell membrane
Affects 75% of patients
47% dF508 homozygous – severe phenotype
40% dF508 heterozygous – not treated
Mechanisms of action
Oral bio-available CFTR corrector
Promotes correct folding of CFTR in endoplasmic reticulum and trafficking to plasma
membrane
Treatment
Lumacaftor 200 mg/ Ivacaftor 125 mg every 12 hours in adults and children >12 years
$259,000 per year
Side effects
Shortness of breath, diarrhea, respiratory infections, rash
Cystic Fibrosis: Targeted Treatment II
Questions
