Pulmonary Pharmacology

Correct Answer

C — Ivacaftor

Rationale

Ivacaftor is classified as a CFTR potentiator. It binds CFTR protein already present at the apical cell membrane and increases the probability that the channel gate will open in response to protein kinase A phosphorylation, enhancing ion transport function. Ivacaftor does not correct protein misfolding or improve trafficking to the cell surface — it acts exclusively on protein that has already reached the membrane. Lumacaftor, tezacaftor, and elexacaftor are all classified as CFTR correctors — agents that assist protein folding and trafficking to increase the amount of CFTR protein reaching the cell surface.

Correct Answer

A — Lumacaftor

Rationale

Lumacaftor is classified as a first-generation CFTR corrector that stabilizes the misfolded F508del protein during synthesis, enabling more of it to escape endoplasmic reticulum quality control and reach the apical membrane. Its most consequential pharmacokinetic property is potent CYP3A4 induction — it dramatically reduces ivacaftor plasma concentrations when the two are combined in lumacaftor/ivacaftor (Orkambi), partially negating the dual-mechanism rationale and requiring higher ivacaftor doses. Tezacaftor is a second-generation corrector that is not a CYP3A4 inducer, eliminating this pharmacokinetic limitation. Elexacaftor is a next-generation corrector. Ivacaftor is a potentiator, not a corrector.

Correct Answer

D — Tezacaftor

Rationale

Tezacaftor is classified as a second-generation CFTR corrector that improves F508del protein folding and trafficking by a different molecular mechanism than lumacaftor. Its defining pharmacokinetic advantage over lumacaftor is the absence of CYP3A4 induction — it does not reduce ivacaftor plasma concentrations when combined, eliminating the pharmacokinetic self-sabotage that limits lumacaftor/ivacaftor efficacy. Tezacaftor/ivacaftor (Symdeko) was approved for F508del homozygous patients and patients with certain residual function mutations, with better tolerability than lumacaftor/ivacaftor though still modest efficacy. Elexacaftor is a next-generation corrector acting at a different binding site than tezacaftor. Lumacaftor is the first-generation corrector with the CYP3A4 induction problem. Ivacaftor is a potentiator.

Correct Answer

B — Elexacaftor

Rationale

Elexacaftor is classified as a next-generation CFTR corrector that binds the CFTR protein at membrane-spanning domain 1, a site distinct from the binding site of tezacaftor. Because the two correctors act at different structural sites, their effects on F508del protein stabilization are additive and cooperative rather than redundant, enabling substantially greater protein rescue than either corrector alone achieves. This dual corrector approach is the mechanistic basis for the triple combination elexacaftor-tezacaftor-ivacaftor (Trikafta). Lumacaftor and tezacaftor are first- and second-generation correctors, respectively. Ivacaftor is a potentiator.

Correct Answer

C — Two correctors acting at distinct binding sites plus one potentiator; approved for patients with at least one F508del allele aged 2 and older

Rationale

Elexacaftor-tezacaftor-ivacaftor (Trikafta) is classified as a triple combination consisting of two correctors — elexacaftor and tezacaftor, acting at distinct binding sites on the CFTR protein — combined with the potentiator ivacaftor. Elexacaftor binds membrane-spanning domain 1; tezacaftor acts at nucleotide-binding domain 1. Together they produce additive F508del protein rescue that is far greater than either corrector alone. Ivacaftor then potentiates the gating function of the rescued protein at the cell surface. The combination is approved for patients with at least one F508del allele aged 2 and older, covering approximately 90% of the cystic fibrosis population. It is not approved for patients with Class I stop codon mutations on both alleles where no F508del is present.

Correct Answer

A — Dornase alfa

Rationale

Dornase alfa is classified as recombinant human deoxyribonuclease. In cystic fibrosis airways, degenerating neutrophils release large quantities of extracellular deoxyribonucleic acid strands that contribute to the abnormal viscosity of airway secretions. Dornase alfa cleaves these extracellular nucleic acid strands, reducing sputum viscosity and improving mucociliary clearance. It remains a standard component of cystic fibrosis airway management even in patients receiving CFTR modulator therapy, because modulator therapy does not reverse established bronchiectasis or fully normalize airway secretion physiology. Hypertonic saline draws water osmotically into the airway lumen to rehydrate the periciliary liquid layer but does not cleave extracellular nucleic acid. Ivacaftor is a CFTR potentiator. N-acetylcysteine breaks disulfide bonds in mucus glycoproteins rather than cleaving extracellular nucleic acid.

Correct Answer

D — Class I: premature stop codon producing absent or truncated protein; Class II: protein misfolding causing endoplasmic reticulum retention; Class III: protein reaches membrane but gates abnormally

Rationale

CFTR mutations are classified by their protein-level defect. Class I mutations produce premature stop codons resulting in absent or severely truncated CFTR protein — the most difficult class to address pharmacologically. Class II mutations cause protein misfolding; the mutant protein is recognized by endoplasmic reticulum quality control machinery and degraded before reaching the cell surface. F508del, the most common mutation, is Class II. Class III mutations produce a protein that reaches the membrane but cannot gate (open and close) normally — the target for potentiators such as ivacaftor. Class Four mutations affect ion conductance through the open channel. Class V mutations reduce the amount of normal protein produced. Class VI mutations reduce protein stability at the cell surface.

Correct Answer

B — F508del protein rarely reaches the cell surface in sufficient quantity without a corrector; ivacaftor can only potentiate protein already present at the membrane and has no effect on folding or trafficking

Rationale

Ivacaftor's mechanism is restricted to CFTR protein already present at the apical cell membrane — it increases the probability that the channel gate will open, but it does not correct protein misfolding or improve endoplasmic reticulum trafficking. F508del is a Class II mutation that causes severe protein misfolding; the mutant protein is almost entirely degraded by endoplasmic reticulum quality control before reaching the cell surface. Without a corrector to rescue folding and enable trafficking, there is insufficient F508del protein at the membrane for ivacaftor to act on, explaining its lack of meaningful monotherapy efficacy in F508del homozygous patients. The STRIVE trial demonstrating ivacaftor's dramatic efficacy enrolled patients with the G551D gating mutation — a Class III defect where the protein reaches the membrane but gates abnormally, providing an adequate substrate for potentiation.

Correct Answer

C — Lumacaftor is a potent CYP3A4 inducer that dramatically reduces ivacaftor plasma concentrations, partially negating the rationale for combining them and requiring higher ivacaftor doses

Rationale

Lumacaftor is a potent inducer of CYP3A4, the primary enzyme responsible for ivacaftor metabolism. When lumacaftor and ivacaftor are co-administered, lumacaftor induces its own pharmacological antidote — it dramatically accelerates ivacaftor metabolism, reducing ivacaftor plasma concentrations and limiting the potentiation that was the rationale for combining the two drugs. This pharmacokinetic self-sabotage means that lumacaftor/ivacaftor produces only modest F508del protein at the cell surface (corrector effect) and then achieves only partial potentiation (because ivacaftor levels are reduced). The combination still provides net benefit, but the benefit is substantially smaller than would be predicted from the individual drug mechanisms in isolation. Tezacaftor was developed as a second-generation corrector specifically because it lacks CYP3A4 induction activity, correcting this limitation.

Correct Answer

A — F508del produces both a Class II folding defect preventing cell surface trafficking and a Class III gating defect in protein that does reach the membrane; restoring meaningful function requires a corrector to increase surface protein and a potentiator to open the channel

Rationale

F508del causes two simultaneous protein-level defects. The primary and dominant defect is Class II: the F508del protein is misfolded and recognized by endoplasmic reticulum quality control machinery, preventing trafficking to the cell surface. Even the small amount of F508del protein that does reach the membrane has a secondary Class III defect: it gates abnormally, opening less frequently than normal CFTR. This dual defect explains the treatment requirement: a corrector is needed to rescue folding and increase the amount of F508del protein reaching the cell surface, and a potentiator is needed to improve the gating function of the rescued protein once it arrives there. Neither intervention alone is sufficient — the corrector without a potentiator leaves gating-defective protein at the membrane, and the potentiator without a corrector has minimal protein to act on.

Correct Answer

D — Elexacaftor binds membrane-spanning domain 1 and tezacaftor acts at nucleotide-binding domain 1 — two distinct structural sites — producing additive, cooperative stabilization of the misfolded F508del protein that is greater than either corrector alone

Rationale

Elexacaftor and tezacaftor achieve cooperative F508del protein rescue because they act at structurally distinct binding sites on the CFTR protein. Elexacaftor binds membrane-spanning domain 1, while tezacaftor acts at nucleotide-binding domain 1. Because these are different structural regions, their stabilizing effects on the misfolded F508del protein are not redundant — each corrector addresses a different aspect of the folding problem, and together they produce protein rescue of a magnitude substantially greater than either corrector alone can achieve. This cooperative stabilization enables far more F508del protein to escape endoplasmic reticulum degradation and reach the cell surface, where ivacaftor can then potentiate its gating function. The result is CFTR function restoration sufficient to transform disease outcomes in patients carrying at least one F508del allele.

Correct Answer

B — Approximately 14 percentage point improvement in forced expiratory volume in 1 second percent predicted, 63% reduction in exacerbations, and near-normalization of sweat chloride concentration

Rationale

The pivotal trial in F508del homozygous patients demonstrated a 14 percentage point improvement in forced expiratory volume in 1 second percent predicted at week 24 — more than three times the improvement seen with tezacaftor/ivacaftor dual therapy — along with a 63% reduction in pulmonary exacerbations and near-normalization of sweat chloride concentration. This sweat chloride near-normalization confirmed that the triple combination was restoring CFTR function at the biochemical level to a degree approaching normal. These results were transformative compared with any prior modulator therapy, establishing elexacaftor-tezacaftor-ivacaftor as the standard of care for eligible patients and transforming the natural history of cystic fibrosis for approximately 90% of patients who carry at least one F508del allele.

Correct Answer

C — Normal sweat chloride is below 30 mmol/L; untreated cystic fibrosis typically produces values above 60 mmol/L; effective modulator therapy — particularly triple combination — produces near-normalization confirming CFTR function rescue

Rationale

Sweat chloride concentration is the gold-standard pharmacodynamic biomarker for CFTR channel function. Normal sweat chloride is below 30 mmol/L. In untreated cystic fibrosis, impaired CFTR-mediated chloride reabsorption in the sweat duct produces values typically above 60 mmol/L — the diagnostic threshold. Effective CFTR modulator therapy, particularly elexacaftor-tezacaftor-ivacaftor triple combination, produces substantial reductions in sweat chloride, with many patients achieving near-normalization below 30 mmol/L. This biochemical improvement confirms that CFTR function has been rescued at the protein level, providing a direct pharmacodynamic marker of drug effect independent of lung function measurements. Sweat chloride testing is used both to confirm modulator eligibility and to verify treatment response.

Correct Answer

A — Modulator therapy restores CFTR function and prevents further damage but does not reverse established bronchiectasis, eliminate chronic bacterial colonization already present before treatment, or fully normalize airway secretion physiology — airway clearance addresses these residual problems

Rationale

CFTR modulator therapy rescues CFTR protein function at the molecular level and prevents the progressive structural lung damage that would otherwise accumulate — but it does not repair damage that has already occurred. Patients who begin triple combination therapy with established bronchiectasis retain that structural abnormality. Those who are already chronically colonized with Pseudomonas aeruginosa or Staphylococcus aureus before starting treatment remain colonized, because modulator therapy does not eradicate established infection. Additionally, despite near-normalization of sweat chloride, airway secretion physiology may not be fully restored to normal, particularly in patients with advanced disease. Airway clearance techniques, dornase alfa, hypertonic saline, and inhaled antibiotics therefore continue to address the residual burden of disease that modulator therapy cannot reverse.

Correct Answer

B — G551D is a Class III gating mutation producing a protein that reaches the cell surface but gates abnormally; ivacaftor potentiates the gating function of the cell-surface protein without requiring corrector assistance

Rationale

The G551D mutation is a Class III gating mutation — the CFTR protein is normally folded, correctly trafficked to the apical cell membrane, but cannot open and close the channel normally. Because the protein reaches the cell surface without folding or trafficking assistance, a corrector is not needed. Ivacaftor, a potentiator, directly addresses the Class III defect by binding the cell-surface CFTR protein and increasing the probability of channel opening. This mechanism explains both the dramatic efficacy of ivacaftor monotherapy in the STRIVE trial — a 10 percentage point improvement in forced expiratory volume in 1 second percent predicted and near-normalization of sweat chloride — and why the drug has no meaningful effect as monotherapy in F508del homozygous patients, whose Class II folding defect prevents sufficient protein from reaching the membrane in the first place.

Correct Answer

D — Elexacaftor binds a different structural site on CFTR than tezacaftor — membrane-spanning domain 1 versus nucleotide-binding domain 1 — producing additive protein rescue that delivers far more F508del protein to the cell surface than tezacaftor alone can achieve

Rationale

The rationale for adding elexacaftor to tezacaftor/ivacaftor rests on structural complementarity. Tezacaftor stabilizes F508del protein at nucleotide-binding domain 1, enabling some additional protein to escape endoplasmic reticulum degradation. However, F508del protein misfolding is complex, involving multiple structural domains. Elexacaftor addresses the misfolding at a different structural location — membrane-spanning domain 1 — and the two correctors produce additive, cooperative stabilization of the F508del protein that is substantially greater than either corrector alone. The result is far more F508del protein reaching the cell surface, where ivacaftor can then potentiate its gating function. This explains why the triple combination produces approximately three times the lung function improvement of tezacaftor/ivacaftor dual therapy, despite using the same potentiator. F508del is not a stop codon mutation, and elexacaftor has no CYP3A4 inhibitory activity.

Correct Answer

A — Rifampin is a potent CYP3A4 inducer; elexacaftor and ivacaftor are CYP3A4 substrates and co-administration would reduce their plasma levels, risking loss of CFTR modulator efficacy

Rationale

Elexacaftor-tezacaftor-ivacaftor triple combination is metabolized by CYP3A4, and the component drugs — particularly elexacaftor and ivacaftor — are CYP3A4 substrates. Rifampin is one of the most potent CYP3A4 inducers in clinical use, and co-administration would dramatically reduce plasma concentrations of the modulator components, risking loss of CFTR function rescue and clinical deterioration. For this reason, strong CYP3A4 inducers including rifampin and certain anticonvulsants are contraindicated or should be avoided with elexacaftor-tezacaftor-ivacaftor. The prescribing information requires dose reduction of the triple combination when strong CYP3A4 inhibitors are used, and advises avoiding strong inducers. An alternative antituberculous regimen that does not induce CYP3A4 — such as rifabutin, which is a weaker inducer, or a rifampin-free regimen — should be considered in consultation with infectious disease.

Correct Answer

C — Modulator therapy does not reverse established bronchiectasis or eliminate existing bacterial colonization; airway clearance and dornase alfa address structural and infectious disease burden that persists despite CFTR function rescue

Rationale

Sweat chloride normalization confirms that CFTR function has been rescued at the biochemical level — a genuine therapeutic achievement — but does not indicate that existing structural lung damage has resolved. This patient almost certainly has established bronchiectasis from years of disease before starting modulator therapy at age 20 or 21. Bronchiectatic airways retain abnormal architecture and impaired mucociliary clearance regardless of CFTR function, and remain susceptible to recurrent infection and mucus pooling. Dornase alfa reduces sputum viscosity from neutrophil-derived extracellular nucleic acid that accumulates in bronchiectatic airways. Airway clearance techniques mobilize secretions from structurally damaged airways that ciliary function cannot fully clear. Both interventions address the residual disease burden that CFTR rescue cannot reverse. Discontinuing them based on sweat chloride normalization alone risks clinical deterioration that may not be immediately apparent given the patient's current stability.