1. A 26-year-old woman with cystic fibrosis (CF) homozygous for F508del is maintained on elexacaftor-tezacaftor-ivacaftor (ETI) with stable forced expiratory volume in one second as percent predicted (FEV1% predicted) of 72%. She develops allergic bronchopulmonary aspergillosis (ABPA) confirmed by elevated total immunoglobulin E (IgE), Aspergillus-specific IgE, and CT findings of central bronchiectasis. Her pulmonologist initiates voriconazole. Which of the following is the correct management of her ETI regimen during voriconazole therapy?
A) Discontinue ETI for the entire duration of voriconazole therapy and restart at full dose 2 weeks after voriconazole completion, because no dose adjustment protocol exists for co-administration of ETI with strong cytochrome P450 isoform CYP3A4 (CYP3A4) inhibitors.
B) Continue ETI at the standard daily dose without adjustment and add weekly liver function test (LFT) monitoring, because the primary concern with voriconazole co-administration is additive hepatotoxicity rather than a pharmacokinetic interaction affecting ETI plasma concentrations.
C) Reduce ETI dosing to every other day for the duration of voriconazole therapy, because voriconazole is a strong CYP3A4 inhibitor that substantially increases ivacaftor plasma concentrations at standard daily dosing, and the ETI prescribing label specifies this adjustment for all strong CYP3A4 inhibitor co-administration.
D) Switch voriconazole to fluconazole, which is a weak CYP3A4 inhibitor and therefore does not require any ETI dose adjustment, making it a safer antifungal choice for CF patients on modulator therapy.
E) Increase the evening ivacaftor dose from 150 mg to 300 mg to compensate for the expected reduction in ivacaftor bioavailability caused by voriconazole's induction of intestinal efflux transporters.
ANSWER: C
Rationale:
Voriconazole is a strong inhibitor of cytochrome P450 isoform CYP3A4 (CYP3A4), the primary hepatic enzyme responsible for ivacaftor metabolism. When voriconazole is co-administered with any ivacaftor-containing regimen including ETI, CYP3A4-mediated ivacaftor metabolism is substantially blocked, causing ivacaftor plasma concentrations to rise well above the therapeutic range at the standard daily dose. The ETI prescribing label specifies that when a strong CYP3A4 inhibitor is co-prescribed, the ETI regimen should be reduced to every-other-day dosing for the duration of co-administration; this applies to all strong CYP3A4 inhibitors including the azole antifungals itraconazole, voriconazole, posaconazole, ketoconazole, and fluconazole at doses used for systemic infection. This interaction is clinically recurring in CF because Aspergillus colonization and ABPA are recognized pulmonary complications requiring repeated antifungal courses, and the dose adjustment must be documented and re-applied at every encounter where an azole is initiated.
Option A: Option A is incorrect because a defined dose-adjustment protocol does exist in the ETI label; complete discontinuation is not the appropriate response to co-prescription of a strong CYP3A4 inhibitor.
Option B: Option B is incorrect because the primary pharmacokinetic interaction is CYP3A4 inhibition raising ivacaftor concentrations, not a shared hepatotoxicity risk; failing to adjust the ETI dose exposes the patient to supratherapeutic ivacaftor levels, and monitoring alone does not address this.
Option D: Option D is incorrect because fluconazole is also a strong CYP3A4 inhibitor at doses used for systemic fungal infections and carries the same dose-adjustment requirement; it is not a safe alternative that avoids the interaction.
Option E: Option E is incorrect because voriconazole inhibits, not induces, CYP3A4; the effect is to increase, not decrease, ivacaftor bioavailability, so increasing the evening dose would compound ivacaftor accumulation.
2. A 33-year-old man with cystic fibrosis (CF) carries one F508del allele and one G542X allele and has been on elexacaftor-tezacaftor-ivacaftor (ETI) for 18 months with FEV1% predicted stable at 58%. Sputum cultures grow Mycobacterium avium complex (MAC), and his infectious disease team proposes a three-drug regimen including rifampin, azithromycin, and ethambutol. Which of the following best describes the pharmacokinetic consequence of adding rifampin and the correct management approach?
A) Rifampin is a potent inducer of cytochrome P450 isoform CYP3A4 (CYP3A4) and would markedly reduce ivacaftor plasma concentrations to sub-therapeutic levels, potentially eliminating ETI efficacy; rifampin-containing regimens should generally be avoided with any ivacaftor-containing modulator, and the MAC antibiotic regimen should be reformulated to exclude rifampin, with rifabutin or other rifampin-free alternatives considered under specialist guidance.
B) Rifampin interacts only with lumacaftor-ivacaftor because lumacaftor's own CYP3A4 induction compounds rifampin's induction; ETI does not share this restriction because tezacaftor and elexacaftor are themselves CYP3A4 inducers and are therefore pharmacokinetically insensitive to rifampin co-administration.
C) The correct management is to reduce the ETI morning dose to every other day during rifampin co-administration using the same adjustment protocol applied for strong CYP3A4 inhibitors such as azole antifungals, which normalizes the net CYP3A4 activity and maintains therapeutic ivacaftor concentrations.
D) Rifampin can be safely co-administered with ETI provided that ivacaftor plasma trough concentrations are monitored monthly; if trough levels fall below the lower limit of the therapeutic range, the ETI dose can be doubled to compensate for the induction-driven increase in clearance.
E) Azithromycin, not rifampin, is the component of the MAC regimen that poses the primary pharmacokinetic interaction risk with ETI because azithromycin is a moderate CYP3A4 inhibitor that raises ivacaftor concentrations; rifampin has no clinically significant CYP3A4 interaction with ivacaftor-containing regimens.
ANSWER: A
Rationale:
Rifampin is one of the most potent inducers of cytochrome P450 isoform CYP3A4 (CYP3A4) available in clinical practice, and its co-administration with any ivacaftor-containing CFTR modulator regimen — including ETI — markedly accelerates ivacaftor hepatic metabolism, reducing ivacaftor plasma concentrations to levels likely below therapeutic threshold. The ETI prescribing label and ivacaftor labeling categorically classify strong CYP3A4 inducers including rifampin, carbamazepine, phenytoin, and St. John's wort as agents to be avoided with any ivacaftor-containing regimen. Unlike the CYP3A4 inhibitor scenario (where dose reduction to every-other-day compensates for reduced clearance), there is no established dose-escalation protocol that reliably compensates for rifampin's potent induction; the preferred strategy is to avoid the combination entirely. This creates a clinically important challenge in CF because nontuberculous mycobacteria (NTM) lung disease — including MAC infection — is an increasingly recognized CF complication, and rifampin is a standard component of multidrug MAC regimens. Antibiotic selection must therefore prioritize rifampin-free alternatives (such as rifabutin in some cases, or rifampin-free MAC regimens) and should be made in consultation with both infectious disease and CF specialist teams.
Option B: Option B is incorrect because the rifampin-ivacaftor interaction applies to all ivacaftor-containing regimens regardless of the corrector component; tezacaftor and elexacaftor are not CYP3A4 inducers, and their presence does not protect ivacaftor from rifampin-driven clearance acceleration.
Option C: Option C is incorrect because the every-other-day adjustment applies to CYP3A4 inhibitors (which increase ivacaftor concentrations and require reduced dosing frequency), not to CYP3A4 inducers (which decrease ivacaftor concentrations and cannot be compensated by simple frequency reduction).
Option D: Option D is incorrect because no validated therapeutic drug monitoring protocol exists for ivacaftor dose escalation in the setting of rifampin co-administration; doubling the dose is not an established or labeled management strategy.
Option E: Option E is incorrect because azithromycin is not a moderate CYP3A4 inhibitor in the clinical context of CF MAC treatment; the primary pharmacokinetic concern with rifampin is its potent CYP3A4 induction reducing ivacaftor concentrations, not azithromycin raising them.
3. A 19-year-old man with cystic fibrosis (CF) homozygous for F508del has been on elexacaftor-tezacaftor-ivacaftor (ETI) for 5 months. His 3-month liver function tests (LFTs) showed alanine aminotransferase (ALT) at 1.8 times the upper limit of normal (ULN). Repeat LFTs obtained because of the borderline result now show ALT at 4.2 times the ULN. He is entirely asymptomatic with no jaundice, right upper quadrant pain, or nausea. Which of the following is the correct next step in management?
A) Continue ETI at the current dose, recheck ALT in 4 weeks, and intervene only if ALT exceeds 10 times the ULN or if symptoms of hepatotoxicity develop, because transient transaminase elevations up to 5 times the ULN are expected and acceptable in asymptomatic patients during the first year of ETI therapy.
B) Reduce the ETI morning dose by half (one tablet instead of two) while maintaining the evening ivacaftor dose, then recheck ALT in 2 weeks; dose reduction rather than full interruption is the labeled first-line response to asymptomatic transaminase elevation between 3 and 5 times the ULN.
C) Add ursodeoxycholic acid (UDCA) at 15 mg/kg/day to the current ETI regimen to provide hepatoprotection while continuing ETI at full dose, because UDCA co-administration is the standard of care for managing ETI-associated transaminase elevations in patients with CF-related liver disease.
D) Interrupt ETI and recheck liver function tests, because the ETI prescribing label specifies that ALT or aspartate aminotransferase (AST) exceeding 5 times the ULN without symptoms — or 3 times the ULN with symptoms — warrants interruption; at 4.2 times the ULN the patient is asymptomatic, but the trajectory warrants close reassessment and interruption per the labeled threshold approach.
E) Permanently discontinue ETI and transition the patient to tezacaftor-ivacaftor dual therapy, which carries a substantially lower hepatotoxicity risk than the triple combination and can maintain adequate CFTR rescue in F508del homozygous patients without the hepatic adverse effect burden of elexacaftor.
ANSWER: D
Rationale:
The ETI prescribing label specifies that alanine aminotransferase (ALT) or aspartate aminotransferase (AST) elevations exceeding five times the upper limit of normal (ULN) without hepatotoxicity symptoms, or exceeding three times the ULN with symptoms (jaundice, right upper quadrant pain, nausea), warrant ETI interruption and liver function reassessment. This patient is asymptomatic with ALT at 4.2 times the ULN — below the 5× ULN threshold for mandatory interruption but on a rising trajectory (1.8× at 3 months, now 4.2×) that places the patient in close proximity to the labeled interrupt threshold. The clinically appropriate action is to interrupt ETI now and recheck LFTs, rather than waiting for the threshold to be crossed, particularly given the rising trajectory. If LFTs normalize after interruption, restarting ETI with more frequent monitoring may be feasible under specialist guidance; permanent discontinuation is premature at this stage. This question illustrates that the labeled threshold is 5× ULN without symptoms, but clinical judgment about a rising trajectory may prompt earlier action.
Option A: Option A is incorrect because 10 times the ULN is not the labeled interrupt threshold; the labeled threshold is 5 times the ULN without symptoms, and this patient at 4.2 times the ULN on a rising trajectory requires closer action than continued observation.
Option B: Option B is incorrect because half-dose reduction of the morning ETI tablet is not a labeled dose-adjustment strategy for managing transaminase elevations; the label specifies interruption, not partial dose reduction.
Option C: Option C is incorrect because ursodeoxycholic acid co-administration is not a labeled or guideline-recommended management strategy for ETI-associated transaminase elevations; it does not substitute for ETI interruption when LFTs are rising toward the labeled threshold.
Option E: Option E is incorrect because permanent ETI discontinuation is premature without first interrupting the drug, allowing LFTs to normalize, and attempting rechallenge under specialist monitoring; tezacaftor-ivacaftor is not clearly lower in hepatotoxicity risk than ETI, and the patient would lose the superior CFTR rescue efficacy of the triple combination without a justified clinical basis.
4. A 29-year-old man with cystic fibrosis (CF) carrying one G551D allele has been on ivacaftor monotherapy for 3 years with excellent clinical response, FEV1% predicted stable at 81%, and sweat chloride reduced to 38 mmol/L. He develops a pulmonary exacerbation complicated by Aspergillus fumigatus growth on bronchoalveolar lavage, and his CF team prescribes itraconazole. Which of the following correctly describes the required management of his ivacaftor regimen during itraconazole therapy?
A) Discontinue ivacaftor for the entire duration of itraconazole therapy because no safe dose adjustment allows concurrent use of ivacaftor with a strong cytochrome P450 isoform CYP3A4 (CYP3A4) inhibitor; ivacaftor can be restarted at full dose 2 weeks after itraconazole completion.
B) Reduce ivacaftor dosing frequency to every other day for the duration of itraconazole co-administration, per ivacaftor prescribing labeling, because itraconazole is a strong CYP3A4 inhibitor that substantially increases ivacaftor plasma concentrations at the standard twice-daily dose.
C) Continue ivacaftor at the standard twice-daily dose and add daily LFT monitoring for hepatotoxicity, because the itraconazole-ivacaftor interaction is primarily a shared hepatic adverse effect rather than a pharmacokinetic interaction requiring dose modification.
D) Halve the ivacaftor dose at each administration (from 150 mg to 75 mg twice daily) rather than reducing dosing frequency, because the pharmacokinetic modeling for itraconazole co-administration supports dose reduction at each administration rather than extended dosing intervals.
E) Switch from itraconazole to caspofungin for Aspergillus treatment, because caspofungin is an echinocandin that is not a CYP3A4 inhibitor and can be co-administered with ivacaftor at full dose without pharmacokinetic interaction or dose adjustment.
ANSWER: B
Rationale:
Ivacaftor is metabolized primarily by cytochrome P450 isoform CYP3A4 (CYP3A4), and itraconazole is a potent CYP3A4 inhibitor. The ivacaftor prescribing label specifies that when a strong CYP3A4 inhibitor is co-prescribed, ivacaftor dosing frequency should be reduced to every other day for the duration of the inhibitor course; this labeled adjustment applies to all strong CYP3A4 inhibitors including itraconazole, voriconazole, posaconazole, ketoconazole, and fluconazole at systemic doses. This interaction is clinically recurrent in CF because Aspergillus colonization and Aspergillus-related pulmonary disease require repeated azole antifungal courses throughout the patient's lifetime, and the dose adjustment must be re-applied and documented at each encounter where an azole is initiated. For this patient on ivacaftor monotherapy — unlike patients on the fixed ETI combination — the adjustment applies to ivacaftor alone, reducing from twice-daily to every-other-day administration.
Option A: Option A is incorrect because a defined dose-adjustment protocol exists in the ivacaftor label; complete discontinuation is unnecessary and would deprive the patient of CFTR potentiation during a pulmonary exacerbation when maintaining modulator efficacy is particularly important.
Option C: Option C is incorrect because the pharmacokinetic interaction is the primary clinical concern — CYP3A4 inhibition by itraconazole substantially raises ivacaftor concentrations — and LFT monitoring alone does not address the risk of supratherapeutic ivacaftor exposure; the dose must be adjusted per labeling.
Option D: Option D is incorrect because the labeled management is reduced dosing frequency (every other day), not dose reduction at each administration; 75 mg twice daily is not a labeled adjustment strategy and has not been pharmacokinetically validated for strong inhibitor co-administration.
Option E: Option E is incorrect because while caspofungin does not inhibit CYP3A4, it has limited efficacy for the full spectrum of Aspergillus-related pulmonary disease in CF, particularly allergic forms; the clinical decision to switch antifungals must be based on the indication, not solely on avoiding the ivacaftor interaction, and the labeled dose adjustment for itraconazole is straightforward and well-characterized.
5. A 3-year-old girl is newly diagnosed with cystic fibrosis (CF) after newborn screening and confirmatory sweat chloride testing. Genotyping reveals she is homozygous for F508del. Her CF team is reviewing CFTR modulator options. Which of the following correctly identifies the approved modulator regimen for this patient and the rationale for early initiation?
A) Ivacaftor monotherapy is the appropriate choice for this patient because it is approved for the broadest age range of any single modulator agent and is effective in F508del homozygous patients when started before significant lung injury has occurred; elexacaftor-tezacaftor-ivacaftor (ETI) is reserved for patients aged 6 and older.
B) No CFTR modulator is approved for patients under age 5; treatment at this age consists exclusively of airway clearance therapy, inhaled dornase alfa, pancreatic enzyme replacement, and nutritional optimization until the patient reaches the minimum age threshold for modulator eligibility.
C) Lumacaftor-ivacaftor (Orkambi) is the appropriate choice because it was the first corrector-potentiator combination approved specifically for F508del homozygous patients and remains the only regimen with a labeled indication extending to age 2 and younger in this mutation group.
D) Tezacaftor-ivacaftor is the preferred regimen for this patient because it carries a better tolerability profile than elexacaftor-tezacaftor-ivacaftor (ETI) in young children and is specifically indicated for F508del homozygous patients aged 2 and older, whereas ETI approval in this age group remains investigational pending completion of pediatric safety trials.
E) Elexacaftor-tezacaftor-ivacaftor (ETI) is the appropriate choice; it is approved in the United States for patients aged 2 years and older who carry at least one F508del allele, making this patient eligible, and early initiation is specifically recommended to prevent the cumulative airway damage — including bronchiectasis and chronic infection — that accrues from years of CFTR dysfunction before modulator therapy is started.
ANSWER: E
Rationale:
Elexacaftor-tezacaftor-ivacaftor (ETI) is approved in the United States for CF patients aged 2 years and older who carry at least one F508del CFTR allele, and this 3-year-old F508del homozygous patient meets the age and genotype eligibility criteria. The labeled age threshold has progressively lowered as pediatric clinical trial data have become available in younger age groups, reflecting the fundamental rationale that CFTR modulator therapy initiated as early as possible prevents the cumulative airway injury — progressive bronchiectasis, chronic Pseudomonas aeruginosa colonization, and obstructive lung disease — that results from years of CFTR dysfunction before treatment begins. Because structural lung damage such as bronchiectasis does not reverse with ETI initiation, prevention of that damage through early treatment is substantially more beneficial than rescue after injury has occurred. ETI is the preferred regimen for this patient because it provides the highest degree of F508del CFTR rescue of any currently approved regimen and because its age 2 approval covers her.
Option A: Option A is incorrect because ivacaftor monotherapy is not effective in F508del patients; F508del CFTR is retained in the endoplasmic reticulum and does not reach the apical membrane in meaningful amounts without a corrector, so a potentiator alone has no surface-expressed CFTR to act upon; ETI is not restricted to age 6 and older.
Option B: Option B is incorrect because ETI is approved for patients aged 2 and older, making a 3-year-old F508del homozygous patient eligible; withholding modulator therapy until age 5 would delay CFTR rescue and allow preventable airway injury to accumulate.
Option C: Option C is incorrect because lumacaftor-ivacaftor is approved for F508del homozygotes aged 2 and older but is a less effective first-generation corrector regimen that also carries the CYP3A4 induction problem with lumacaftor; ETI is the preferred regimen when available and indicated, and lumacaftor-ivacaftor is not the first-choice agent.
Option D: Option D is incorrect because tezacaftor-ivacaftor approval in young children does not supersede ETI eligibility for this patient; ETI is approved for age 2 and older in F508del homozygotes, its pediatric approval is not investigational, and ETI provides substantially greater CFTR rescue than tezacaftor-ivacaftor.
6. A 31-year-old man with cystic fibrosis (CF) underwent liver transplantation 2 years ago for CF-related end-stage liver disease and is maintained on tacrolimus for immunosuppression with stable trough levels of 8 ng/mL. He is F508del homozygous and his CF team is considering initiating a CFTR corrector-potentiator regimen now that his post-transplant status has stabilized. Lumacaftor-ivacaftor is proposed. Which of the following best describes the anticipated pharmacokinetic interaction and the correct management approach if lumacaftor-ivacaftor is initiated?
A) Lumacaftor inhibits the intestinal P-glycoprotein efflux pump responsible for tacrolimus excretion, increasing tacrolimus systemic exposure and raising trough levels; the tacrolimus dose should be empirically reduced by 30 to 50 percent before lumacaftor-ivacaftor initiation to prevent calcineurin inhibitor toxicity.
B) Ivacaftor is a moderate CYP3A4 inhibitor that accumulates to inhibitory concentrations with twice-daily dosing, raising tacrolimus plasma concentrations by blocking its hepatic and intestinal metabolism; tacrolimus trough levels should be monitored weekly for the first month after lumacaftor-ivacaftor initiation and the dose adjusted accordingly.
C) Lumacaftor is a strong CYP3A4 inducer that substantially accelerates the metabolism of tacrolimus — a narrow therapeutic index CYP3A4 substrate — through both hepatic and intestinal CYP3A4; tacrolimus trough concentrations are expected to fall significantly after lumacaftor-ivacaftor initiation, requiring close monitoring and likely substantial tacrolimus dose increases to maintain therapeutic immunosuppression and prevent allograft rejection.
D) Lumacaftor-ivacaftor has no clinically significant interaction with tacrolimus because tacrolimus is primarily metabolized by CYP3A5 rather than CYP3A4, and lumacaftor's induction effect is specific to CYP3A4; standard tacrolimus monitoring intervals (monthly) are sufficient without any anticipatory dose adjustment.
E) The combination of lumacaftor-ivacaftor with tacrolimus is absolutely contraindicated by the transplant immunosuppression labeling, and this patient should be transitioned to tezacaftor-ivacaftor before any corrector-potentiator therapy is initiated; tezacaftor does not interact with tacrolimus through any mechanism.
ANSWER: C
Rationale:
Lumacaftor is a potent inducer of cytochrome P450 isoform CYP3A4 (CYP3A4), and tacrolimus is a narrow therapeutic index immunosuppressant that is primarily metabolized by CYP3A4 (and to a lesser extent CYP3A5) in both hepatic and intestinal tissues. When lumacaftor-ivacaftor is initiated in a patient maintained on tacrolimus, lumacaftor's CYP3A4 induction substantially accelerates tacrolimus metabolism, reducing tacrolimus trough concentrations — potentially to sub-therapeutic levels that risk acute allograft rejection. For a post-liver-transplant patient on stable immunosuppression, a sudden drop in tacrolimus trough concentrations below the therapeutic range represents a high-stakes adverse consequence; close monitoring of tacrolimus levels is mandatory after lumacaftor-ivacaftor initiation, and substantial tacrolimus dose increases are frequently required to maintain therapeutic troughs. In practice, this interaction — along with the CYP3A4 induction effect on hormonal contraceptives and other substrates — is one of the reasons tezacaftor-ivacaftor or ETI are preferred over lumacaftor-ivacaftor when available, as tezacaftor and elexacaftor do not induce CYP3A4.
Option A: Option A is incorrect because the direction of the interaction is opposite: lumacaftor induces CYP3A4 and reduces, not increases, tacrolimus levels; the concern is sub-therapeutic tacrolimus concentrations, not toxicity from elevated levels.
Option B: Option B is incorrect because ivacaftor is not a clinically significant CYP3A4 inhibitor; the CYP3A4-interacting component is lumacaftor, and the interaction is induction reducing tacrolimus concentrations, not inhibition raising them.
Option D: Option D is incorrect because tacrolimus is substantially metabolized by CYP3A4 in addition to CYP3A5, and lumacaftor's CYP3A4 induction does produce a clinically meaningful reduction in tacrolimus exposure; standard monthly monitoring intervals are inadequate in the first weeks after lumacaftor-ivacaftor initiation.
Option E: Option E is incorrect because while tezacaftor-ivacaftor is indeed preferred over lumacaftor-ivacaftor in this patient for exactly this reason, lumacaftor-ivacaftor is not absolutely contraindicated by transplant immunosuppression labeling; it carries a significant interaction requiring vigilant management, but the clinical decision requires specialist judgment rather than a categorical prohibition.
7. A 24-year-old woman with cystic fibrosis (CF) on elexacaftor-tezacaftor-ivacaftor (ETI) is referred for evaluation of possible CF-related diabetes (CFRD) after her primary care physician found a fasting plasma glucose of 118 mg/dL on routine labs. Her hemoglobin A1c (HbA1c) is 5.4%, which the primary care physician interprets as normal and inconsistent with a diabetes diagnosis. Her CF nutritionist notes she has lost 3 kg over the past 6 months and has worsening postprandial fatigue. Which of the following best describes the correct diagnostic approach and explains why the HbA1c result should not be used to exclude CFRD?
A) Oral glucose tolerance testing (OGTT) with a 75-gram oral glucose load and 2-hour plasma glucose measurement is the recommended diagnostic test for CFRD, and a normal HbA1c does not reliably exclude CFRD because increased red blood cell turnover in CF patients — from chronic inflammation, repeated infections, and nutritional factors — shortens erythrocyte lifespan and reduces the time available for hemoglobin glycosylation, producing falsely low HbA1c values that underestimate chronic glycemic exposure.
B) Continuous glucose monitoring (CGM) worn for 14 days is the current diagnostic standard for CFRD endorsed by CF clinical guidelines, replacing the oral glucose tolerance test (OGTT) because CGM captures the predominantly postprandial glucose excursions of early CFRD that a single 2-hour OGTT measurement misses; HbA1c is unreliable in CF because of increased hemoglobin F (fetal hemoglobin) production.
C) A fasting plasma glucose of 118 mg/dL on two separate occasions is diagnostic of CFRD and no further testing is required; the HbA1c is unreliable in this population because CF patients are routinely anemic from chronic inflammation, and anemia itself falsely lowers HbA1c by reducing the total available hemoglobin for glycosylation.
D) HbA1c at the standard 6.5% threshold remains the most practical diagnostic tool for CFRD and the result of 5.4% effectively excludes diabetes in this patient; the OGTT is reserved for patients with HbA1c between 5.7% and 6.4% in the prediabetic range, consistent with American Diabetes Association (ADA) diagnostic guidelines applied uniformly across patient populations.
E) The correct diagnostic next step is a 72-hour inpatient fast with serial plasma glucose and C-peptide measurements to characterize the degree of insulin secretory reserve, because CFRD is caused by progressive pancreatic beta-cell destruction and C-peptide depletion is the definitive marker that distinguishes CFRD from incipient type 1 diabetes mellitus in young CF patients.
ANSWER: A
Rationale:
Oral glucose tolerance testing (OGTT) — measuring plasma glucose at fasting and 2 hours after a 75-gram oral glucose load — is the recommended diagnostic test for CF-related diabetes (CFRD) according to established CF clinical guidelines. CFRD characteristically presents with postprandial hyperglycemia before fasting hyperglycemia develops, because the pancreatic islet destruction in CF tends to produce relative insulin deficiency with preserved fasting glucose regulation until the disease is more advanced; a single fasting glucose of 118 mg/dL is elevated but below the diagnostic threshold of 126 mg/dL, and OGTT captures the 2-hour postprandial excursion that may already be diagnostic. Critically, HbA1c is unreliable in CF patients as a diagnostic or monitoring tool because CF is associated with increased red blood cell (RBC) turnover resulting from chronic systemic inflammation, repeated pulmonary infections, nutritional deficiencies, and hemolytic contributions; shorter erythrocyte lifespan reduces the time available for hemoglobin glycosylation, producing HbA1c values that are systematically lower than the true glycemic exposure over the preceding 2 to 3 months. An HbA1c of 5.4% in a CF patient does not reliably exclude CFRD.
Option B: Option B is incorrect because continuous glucose monitoring (CGM), while a useful adjunctive clinical tool in CFRD management, is not the current recommended diagnostic standard; OGTT remains the preferred diagnostic test, and increased fetal hemoglobin production is not the mechanism behind HbA1c unreliability in CF.
Option C: Option C is incorrect because a single fasting glucose of 118 mg/dL does not meet the diagnostic criteria for diabetes mellitus (which requires fasting glucose ≥126 mg/dL on two occasions); further diagnostic testing with OGTT is required, and while anemia can affect HbA1c, the primary mechanism of HbA1c unreliability in CF is increased RBC turnover from multiple causes, not anemia per se.
Option D: Option D is incorrect because standard American Diabetes Association (ADA) diagnostic criteria using HbA1c thresholds should not be applied uniformly to CF patients, for whom OGTT is specifically recommended because HbA1c is an unreliable marker; a 5.4% HbA1c does not exclude CFRD in this population.
Option E: Option E is incorrect because a 72-hour inpatient fasting protocol with C-peptide measurement is not the recommended or standard diagnostic approach for CFRD; OGTT is the clinically validated and guideline-recommended test, and C-peptide measurement while useful for characterizing residual beta-cell function is not the diagnostic standard.
8. A 38-year-old woman with cystic fibrosis (CF) homozygous for F508del has been on elexacaftor-tezacaftor-ivacaftor (ETI) for 14 months. Her FEV1% predicted has improved from 44% to 57%, sweat chloride has normalized to 28 mmol/L, and her pulmonary exacerbation rate has fallen from four to one per year. A follow-up high-resolution CT chest still shows bilateral cylindrical bronchiectasis with mucus plugging in the upper lobes. She asks her pulmonologist whether continued ETI will eventually reverse the bronchiectatic changes visible on imaging. Which of the following is the most accurate and clinically appropriate response?
A) The bronchiectatic changes visible on CT will resolve completely within 3 to 5 years of continuous ETI therapy as the restored CFTR function eliminates mucus stasis, reduces chronic infection and inflammation, and allows the permanently dilated airway walls to remodel back toward normal caliber with sustained mucociliary clearance.
B) Bronchiectasis in CF is a purely inflammatory condition that resolves once the underlying CFTR dysfunction is corrected; the persistent CT findings at 14 months reflect the slow pace of airway wall inflammatory resolution rather than permanent structural damage, and imaging at 3 years is expected to show substantial improvement.
C) ETI reverses bronchiectasis only in patients who initiate treatment before age 18; in adults with established disease, the structural changes are permanent, and the primary benefit of ETI is slowing of further deterioration rather than any improvement in either lung function or exacerbation frequency.
D) Current evidence indicates that established bronchiectasis does not regress significantly with ETI therapy; the airway wall dilation, fibrosis, and structural remodeling that have already occurred represent irreversible tissue changes that CFTR functional restoration cannot undo, even though ETI stabilizes lung function, reduces exacerbation frequency, and prevents further damage accumulation — findings that underscore the benefit of the earliest possible ETI initiation before structural injury accrues.
E) Bronchiectatic changes on CT are expected to persist because they reflect recurrent mucus plugging rather than permanent airway wall structural change; the CT appearance will remain abnormal regardless of treatment, but bronchiectasis in CF is not truly irreversible, and the term is applied loosely to what is actually mucus-impaction-related airway dilation that resolves when airway clearance is optimized.
ANSWER: D
Rationale:
Established bronchiectasis — the permanent dilation of airways resulting from recurrent cycles of bacterial infection, neutrophilic inflammation, and airway wall destruction with loss of elastic and muscular components — does not regress with ETI therapy. Real-world registry data and extension studies from the ETI clinical trials consistently demonstrate that structural lung abnormalities present before ETI initiation persist on serial CT imaging despite dramatic improvements in FEV1% predicted, sweat chloride normalization, and reduced exacerbation rates. The mechanisms are straightforward: ETI restores CFTR function and thereby prevents further mucociliary dysfunction-driven airway damage, but cannot repair the fibrosis, cartilage destruction, and airway wall architectural changes that have already occurred over years of disease before treatment was initiated. This irreversibility is the clinical basis for emphasizing the earliest possible ETI initiation — at age 2 under current US approval — before cumulative structural airway damage accrues. This patient's dramatic functional improvements (FEV1 +13 percentage points, normalized sweat chloride, reduced exacerbations) are real and clinically significant, even though the CT bronchiectatic changes persist.
Option A: Option A is incorrect because current evidence does not support reversal of established bronchiectasis over any time frame with ETI; CT studies from ETI trials show stability of structural abnormalities, not regression, even over multiple years.
Option B: Option B is incorrect because bronchiectasis is not a purely inflammatory condition; it represents permanent structural remodeling of the airway wall including dilation, loss of elasticity, cartilaginous destruction, and fibrosis — changes that do not resolve when CFTR is restored.
Option C: Option C is incorrect because ETI produces meaningful improvements in FEV1% predicted, exacerbation frequency, and quality of life in adults with established disease as demonstrated in this patient; the statement that adult-onset ETI provides only slowing of deterioration without functional improvement mischaracterizes the trial data.
Option E: Option E is incorrect because bronchiectasis in CF is not simply reversible mucus-impaction-related airway dilation; the term in CF denotes true permanent structural airway wall dilation confirmed by established radiological criteria, and the persistence of CT findings after ETI is not merely mucus-related.
9. A 22-year-old woman with cystic fibrosis (CF) homozygous for F508del uses a combined oral contraceptive pill for menstrual management and contraception. She is being considered for initiation of a CFTR corrector-potentiator regimen. Elexacaftor-tezacaftor-ivacaftor (ETI) is not available in her setting and the choice is between lumacaftor-ivacaftor and tezacaftor-ivacaftor. Which of the following correctly identifies the preferred regimen for this patient and the pharmacokinetic rationale?
A) Lumacaftor-ivacaftor is preferred because lumacaftor's CYP3A4 induction effect actually increases ethinyl estradiol plasma concentrations by displacing it from protein binding sites, providing a pharmacokinetic advantage by maintaining more free active hormone despite the induction; hormonal contraception efficacy is therefore better maintained on lumacaftor-ivacaftor than on tezacaftor-ivacaftor.
B) Tezacaftor-ivacaftor is preferred because tezacaftor, unlike lumacaftor, does not induce cytochrome P450 isoform CYP3A4 (CYP3A4); lumacaftor's strong CYP3A4 induction would substantially reduce plasma concentrations of the estrogen and progestin components of the combined oral contraceptive, potentially impairing contraceptive efficacy and requiring a switch to non-hormonal contraception during lumacaftor-ivacaftor therapy.
C) The two regimens are pharmacokinetically equivalent with respect to hormonal contraceptive interactions; the choice between them should be based solely on clinical trial FEV1% predicted outcomes, where lumacaftor-ivacaftor demonstrated superiority for F508del homozygotes in the TRAFFIC and TRANSPORT trials relative to tezacaftor-ivacaftor's EVOLENT trial results.
D) Lumacaftor-ivacaftor is preferred in this patient because combined oral contraceptives are metabolized exclusively by CYP1A2 and UGT2B7 enzymes, rendering them pharmacokinetically insensitive to lumacaftor's CYP3A4 induction; the relevant interaction concern is only with narrow therapeutic index drugs such as cyclosporine or tacrolimus, not with hormonal contraceptives.
E) Neither regimen is appropriate for this patient; CF patients on combined oral contraceptives should receive ivacaftor monotherapy to avoid all CYP3A4 drug interaction risks, and corrector therapy should be deferred until hormonal contraception is discontinued or replaced with a non-hormonal method.
ANSWER: B
Rationale:
Tezacaftor-ivacaftor is the preferred choice for this patient. Lumacaftor is a strong inducer of cytochrome P450 isoform CYP3A4 (CYP3A4), which substantially accelerates the hepatic and intestinal metabolism of CYP3A4 substrates including the estrogen (ethinyl estradiol) and progestin components of combined oral contraceptives, reducing their plasma concentrations to potentially sub-therapeutic levels and impairing contraceptive efficacy. The lumacaftor-ivacaftor prescribing label specifically states that hormonal contraceptive efficacy may be reduced and recommends non-hormonal contraception during therapy. This same CYP3A4 induction also partially attenuates ivacaftor concentrations within the combination product itself, contributing to the more modest clinical efficacy of lumacaftor-ivacaftor relative to newer regimens. Tezacaftor, by contrast, does not induce CYP3A4; when tezacaftor-ivacaftor is used, the oral contraceptive pharmacokinetics are not meaningfully affected, and no change in contraceptive method is required. This CYP3A4 interaction difference between lumacaftor and tezacaftor is one of the key clinical reasons tezacaftor-ivacaftor was developed and preferred over lumacaftor-ivacaftor in patients on CYP3A4-sensitive medications.
Option A: Option A is incorrect because lumacaftor induces CYP3A4, which increases estrogen and progestin metabolism and reduces, not increases, their plasma concentrations; protein binding displacement is not the mechanism and does not increase free hormone levels in the relevant clinical context.
Option C: Option C is incorrect because the two regimens are not pharmacokinetically equivalent with respect to hormonal contraceptive interactions; lumacaftor's CYP3A4 induction is the defining clinical difference, and lumacaftor-ivacaftor did not demonstrate superiority over tezacaftor-ivacaftor in F508del homozygotes — the EVOLENT trial showed tezacaftor-ivacaftor produced comparable or greater FEV1 improvements with better tolerability.
Option D: Option D is incorrect because combined oral contraceptives are substantially metabolized by CYP3A4 and are clinically sensitive to lumacaftor's CYP3A4 induction; the labeling explicitly addresses this interaction.
Option E: Option E is incorrect because ivacaftor monotherapy is not effective in F508del patients (who lack meaningful surface-expressed CFTR for potentiation without a corrector), and deferring corrector therapy solely to preserve a hormonal contraceptive regimen is clinically inappropriate when tezacaftor-ivacaftor provides an equivalent or superior corrector-potentiator option without the contraceptive interaction.
10. A 27-year-old man is newly referred to a CF center after delayed diagnosis. He carries two class I CFTR mutations: W1282X on one allele and G542X on the other. His FEV1% predicted is 41%, he has chronic Pseudomonas aeruginosa airway colonization, moderate bronchiectasis on CT, and pancreatic exocrine insufficiency. He has read about elexacaftor-tezacaftor-ivacaftor (ETI) and asks whether it is appropriate for him. Which of the following best describes the correct modulator selection and the rationale for that decision?
A) Elexacaftor-tezacaftor-ivacaftor (ETI) is the appropriate choice because both W1282X and G542X are nonsense mutations that produce truncated CFTR proteins with residual gating activity; the corrector components of ETI stabilize these truncated proteins in the endoplasmic reticulum and the potentiator component opens the truncated channel gates, making this patient fully eligible under the current FDA approval.
B) Ivacaftor monotherapy is appropriate because both W1282X and G542X produce truncated CFTR proteins that retain partial gating function at the apical membrane; ivacaftor increases the open probability of these surface-expressed truncated channels, providing meaningful CFTR functional restoration without requiring corrector therapy.
C) Lumacaftor-ivacaftor is appropriate because both W1282X and G542X produce misfolded CFTR proteins that are retained in the endoplasmic reticulum; lumacaftor's corrector action rescues this ER-retained truncated protein in the same manner as it rescues F508del CFTR, and ivacaftor then potentiates the corrected channels.
D) Tezacaftor-ivacaftor is appropriate because tezacaftor is approved for patients with any class I mutation at one allele provided the other allele carries a residual function mutation; W1282X is classified as a residual function mutation in patients who retain partial nonsense suppression activity, making this patient eligible under a recent FDA label expansion.
E) No currently approved CFTR modulator is indicated for this patient; both W1282X and G542X are class I nonsense mutations that trigger nonsense-mediated mRNA decay (NMD), resulting in absent or severely truncated CFTR protein with no functional channel at the apical membrane — the pharmacological target required by all approved correctors and potentiators; management relies on intensive airway clearance, inhaled mucoactive agents, aggressive antibiotic treatment of exacerbations, nutritional support with pancreatic enzyme replacement, and active discussion of clinical trial enrollment.
ANSWER: E
Rationale:
Both W1282X and G542X are class I CFTR mutations — nonsense mutations that introduce premature stop codons into the CFTR mRNA. These premature stop codons trigger nonsense-mediated mRNA decay (NMD), a cellular surveillance mechanism that degrades aberrant mRNAs before they can be translated, resulting in absent or severely truncated CFTR protein with no functional channel present at the apical epithelial membrane. This is the fundamental pharmacological obstacle: CFTR correctors (lumacaftor, tezacaftor, elexacaftor) require the presence of a misfolded but translatable CFTR protein in the endoplasmic reticulum (ER) to stabilize — there is no such protein when NMD has eliminated the mRNA. CFTR potentiators (ivacaftor) require surface-expressed CFTR channels to increase the open probability of — without functional channel at the membrane, potentiators have no pharmacological target. No currently approved CFTR modulator addresses the class I mutation problem. Management for this patient is best supportive care: aggressive airway clearance therapy, inhaled dornase alfa to reduce mucus viscosity, inhaled hypertonic saline to hydrate airway surface liquid, prompt antibiotic treatment of pulmonary exacerbations, pancreatic enzyme replacement for exocrine insufficiency, fat-soluble vitamin supplementation, and nutritional optimization. With FEV1% predicted of 41%, lung transplant evaluation should be in the clinical horizon, and enrollment in clinical trials of investigational class I therapies (read-through agents, RNA-targeted approaches) should be actively discussed.
Option A: Option A is incorrect because W1282X and G542X do not produce truncated proteins with residual gating activity available for rescue; NMD eliminates the mRNA in both cases, and the CFTR protein targets required by ETI's corrector and potentiator components are absent.
Option B: Option B is incorrect because the premise that truncated proteins from nonsense mutations retain gating activity at the apical membrane is false; NMD degrades the mRNA before meaningful protein production occurs.
Option C: Option C is incorrect because lumacaftor's corrector mechanism requires misfolded F508del or similar class II CFTR protein in the ER; it cannot rescue the absent/truncated protein of class I mutations.
Option D: Option D is incorrect because tezacaftor-ivacaftor requires at least one F508del allele or a residual function mutation; W1282X is not classified as a residual function mutation under current labeling, and no such FDA label expansion for this combination in class I homozygotes exists.
11. A 34-year-old man with cystic fibrosis (CF) homozygous for F508del has been on elexacaftor-tezacaftor-ivacaftor (ETI) for 2 years with FEV1% predicted improved from 39% to 52%. He continues to produce thick sputum daily and requires twice-daily chest physiotherapy. He asks his CF team whether he can discontinue inhaled dornase alfa and hypertonic saline now that his underlying CFTR function is substantially restored by ETI. Which of the following best describes the rationale for continuing both adjunctive therapies?
A) Dornase alfa and hypertonic saline should be discontinued because their mechanisms of action — DNA degradation and osmotic airway hydration, respectively — are redundant with ETI's restoration of CFTR-mediated chloride secretion; continuing them adds cost and treatment burden without providing additional mucociliary clearance benefit once CFTR function is pharmacologically restored.
B) Dornase alfa should be discontinued because neutrophil-derived extracellular DNA is no longer produced in significant quantities once chronic airway infection is suppressed by ETI; hypertonic saline should be continued because it provides additive osmotic airway hydration independent of CFTR function.
C) Both agents remain clinically relevant for this patient; dornase alfa degrades extracellular DNA from neutrophils in chronically infected airways and hypertonic saline osmotically hydrates the airway surface liquid — mechanisms that address the ongoing consequences of established bronchiectasis and chronic infection that persist despite ETI, and patients with significant sputum burden and established structural disease benefit from continued mucoactive therapy even after CFTR function is partially restored.
D) Hypertonic saline should be discontinued because ETI's restoration of CFTR-mediated chloride secretion normalizes airway surface liquid hydration, making additional osmotic hydration by hypertonic saline unnecessary and potentially over-hydrating the airway lumen; dornase alfa should be continued because bacterial DNA from chronic Pseudomonas aeruginosa colonization persists regardless of CFTR functional status.
E) Both agents should be discontinued and replaced by N-acetylcysteine (NAC) nebulization, which is now the preferred mucolytic agent for ETI-treated patients because it cleaves mucin disulfide bonds without the redundancy issues of dornase alfa and hypertonic saline in the setting of partially restored CFTR function.
ANSWER: C
Rationale:
Despite the dramatic CFTR functional improvements achieved with elexacaftor-tezacaftor-ivacaftor (ETI), adjunctive airway clearance therapies including inhaled dornase alfa and inhaled hypertonic saline retain clinical relevance for patients with established bronchiectasis and ongoing airway secretion burden — exactly as seen in this patient who continues to produce thick sputum daily. Dornase alfa (recombinant human DNase I) degrades high-molecular-weight extracellular deoxyribonucleic acid (DNA) released by neutrophils in chronically infected CF airways; this neutrophil-derived DNA is a major contributor to sputum viscosity and its persistence reflects chronic airway infection and neutrophilic inflammation that does not fully resolve after ETI initiation in patients with longstanding disease and established bronchiectasis. Inhaled hypertonic saline improves mucociliary clearance by drawing water osmotically onto the airway surface, restoring hydration of the periciliary liquid layer; while ETI restores some degree of CFTR-mediated chloride secretion, patients with established structural disease and thick secretions may benefit from additional osmotic augmentation. The degree of ongoing benefit from these agents after ETI initiation requires individualized reassessment, but clinical guidelines support continuing them in patients with significant secretion burden.
Option A: Option A is incorrect because dornase alfa and hypertonic saline address the ongoing consequences of chronic airway infection and structural disease that persist after ETI initiation; their mechanisms are not made redundant by CFTR functional restoration in patients with established bronchiectasis and sputum production.
Option B: Option B is incorrect because neutrophil-derived extracellular DNA continues to be produced as long as chronic airway infection and neutrophilic inflammation persist, which is the case for most patients with established bronchiectasis even after ETI initiation; discontinuing dornase alfa is not supported on the basis that infection is eliminated.
Option D: Option D is incorrect because hypertonic saline's osmotic hydration mechanism provides complementary benefit to CFTR-mediated chloride secretion rather than replacing it; complete normalization of airway surface liquid hydration is not achieved in all ETI-treated patients with established disease, and hypertonic saline over-hydration is not a clinical concern at standard inhaled doses.
Option E: Option E is incorrect because N-acetylcysteine (NAC) nebulization is not recommended as a replacement for dornase alfa and hypertonic saline in ETI-treated patients; NAC has a different mechanism (mucin disulfide bond cleavage) and has not demonstrated superiority over or equivalence to dornase alfa in CF clinical trials.
12. A 24-year-old woman with cystic fibrosis (CF) carries one G551D allele and has a baseline FEV1% predicted of 48%. She is being initiated on ivacaftor monotherapy. Based on the clinical trial data from the STRIVE trial, which of the following best describes the expected magnitude and nature of her lung function response to ivacaftor and the mechanistic basis for why this response is achievable in G551D patients?
A) The STRIVE trial demonstrated a mean improvement in FEV1% predicted of approximately 10.6 percentage points compared with placebo in G551D patients over 48 weeks; for this patient with a baseline FEV1% predicted of 48%, an improvement of this magnitude would bring her to approximately 58 to 59 percent predicted, a clinically meaningful gain, achievable because G551D CFTR is correctly processed and surface-expressed at the apical membrane in normal amounts but fails to open due to a gating defect that ivacaftor directly corrects by increasing channel open probability.
B) The STRIVE trial demonstrated a mean improvement in FEV1% predicted of approximately 4.0 percentage points compared with placebo in G551D patients over 48 weeks, similar to the benefit seen with lumacaftor-ivacaftor in F508del homozygotes, suggesting a class-level ceiling for all CFTR modulator therapy regardless of mutation class; this patient should expect her FEV1% predicted to reach approximately 52 percent.
C) The STRIVE trial demonstrated stabilization of FEV1% predicted with no decline over 48 weeks compared with progressive decline in the placebo group, but no active improvement above baseline; for this patient, the expected benefit is prevention of further deterioration rather than an absolute gain in lung function from her current 48% baseline.
D) The STRIVE trial demonstrated a mean improvement in FEV1% predicted of approximately 10.6 percentage points in G551D patients, but this improvement is confined to patients with baseline FEV1% predicted above 60%; patients with FEV1% predicted below 50%, such as this patient, showed no statistically significant improvement in the STRIVE trial and are unlikely to benefit from ivacaftor monotherapy at this stage of disease.
E) The STRIVE trial demonstrated a mean improvement in FEV1% predicted of approximately 10.6 percentage points in G551D patients, but this benefit requires 12 to 18 months to become measurable on spirometry; for the first year of ivacaftor therapy, sweat chloride normalization is the only reliable early biomarker of response, and lung function improvement should not be expected until after the first annual review.
ANSWER: A
Rationale:
The STRIVE trial enrolled 161 patients aged 12 and older with at least one G551D CFTR allele and demonstrated a mean improvement in forced expiratory volume in one second as percent predicted (FEV1% predicted) of 10.6 percentage points compared with placebo over 48 weeks, accompanied by a reduction in sweat chloride concentration of approximately 48 mmol/L. For this patient with a baseline FEV1% predicted of 48%, a 10 to 11 percentage-point improvement would bring her to approximately 58 to 59% predicted — a clinically meaningful gain that crosses from severe toward moderate obstruction and would be expected to improve exercise capacity, reduce exacerbation risk, and improve quality of life. The mechanistic basis for this robust response in G551D patients is that G551D CFTR folds normally and is processed through the endoplasmic reticulum and Golgi apparatus to the apical epithelial cell membrane in normal or near-normal amounts; the dysfunction is exclusively a gating defect — the G551D substitution impairs ATP-binding and channel opening, so the CFTR sits at the membrane but remains predominantly closed. Ivacaftor binds to surface-expressed CFTR and increases the open probability of the channel gate, directly addressing the sole defect in G551D CFTR without requiring any corrector-mediated trafficking rescue.
Option B: Option B is incorrect because the STRIVE trial result was 10.6 percentage points, not 4.0; the 4.0 percentage-point result describes lumacaftor-ivacaftor in F508del homozygotes from the TRAFFIC/TRANSPORT trials, and the two results are not equivalent; there is no class-level ceiling at 4 percentage points for CFTR modulation.
Option C: Option C is incorrect because the STRIVE trial demonstrated active improvement in FEV1% predicted above baseline, not merely stabilization or slowing of decline; this distinction is important because stabilization alone would be inadequate to describe the STRIVE primary outcome.
Option D: Option D is incorrect because the STRIVE trial did not demonstrate that the FEV1 response to ivacaftor in G551D patients is restricted to those with baseline FEV1% predicted above 60%; subgroup analyses across baseline FEV1 categories consistently showed benefit, and patients with more severe baseline obstruction who remain on ivacaftor show meaningful improvements.
Option E: Option E is incorrect because lung function improvements with ivacaftor in G551D patients are typically measurable within weeks of initiation, not requiring 12 to 18 months; the STRIVE trial showed statistically significant FEV1 improvements at the first assessment visit, and sweat chloride normalization, while an early biomarker, does not precede lung function improvement by this duration.
13. A 21-year-old woman with cystic fibrosis (CF) carries one F508del allele and one G551D allele. She has never been on a CFTR modulator regimen. Her CF team is selecting the most appropriate modulator therapy. Which of the following correctly identifies the preferred regimen and explains the pharmacological rationale for addressing both of her CFTR alleles?
A) Ivacaftor monotherapy is the preferred regimen because it is sufficient to address both alleles simultaneously: it potentiates the G551D gating defect on one allele and also addresses the residual gating defect of the small amount of F508del CFTR that reaches the membrane without corrector assistance, making corrector therapy unnecessary.
B) Lumacaftor-ivacaftor is the preferred regimen because lumacaftor corrects both F508del and G551D CFTR protein misfolding in the endoplasmic reticulum, and ivacaftor then potentiates both corrected proteins at the apical membrane; the combination addresses the full pharmacological defect at both alleles simultaneously.
C) No approved modulator regimen is appropriate for this patient because compound heterozygous genotypes with one F508del and one gating mutation allele are not covered by any current FDA approval; she should be enrolled in a clinical trial for compound heterozygotes with mixed mutation class genotypes.
D) Elexacaftor-tezacaftor-ivacaftor (ETI) is the preferred regimen; it is approved for any patient with at least one F508del allele including F508del/G551D compound heterozygotes, with the corrector components elexacaftor and tezacaftor rescuing F508del CFTR from endoplasmic reticulum retention and ivacaftor potentiating the gating of both the corrected F508del CFTR and the surface-expressed G551D CFTR that was already reaching the membrane before treatment.
E) Tezacaftor-ivacaftor should be selected over elexacaftor-tezacaftor-ivacaftor (ETI) for this patient because tezacaftor specifically targets compound heterozygotes with one residual function allele such as G551D, whereas ETI was approved only for F508del homozygotes and F508del/minimal-function heterozygotes and does not cover the F508del/G551D genotype.
ANSWER: D
Rationale:
Elexacaftor-tezacaftor-ivacaftor (ETI) is approved for patients aged 2 years and older who carry at least one F508del allele, and this patient with F508del/G551D is fully eligible. ETI addresses both of this patient's CFTR alleles through complementary mechanisms: the corrector components elexacaftor and tezacaftor bind to the misfolded F508del CFTR protein in the endoplasmic reticulum (ER), stabilize its conformation at two distinct binding sites, reduce ER-associated degradation (ERAD), and allow substantially more F508del CFTR to complete folding and traffic to the apical membrane. Once F508del CFTR reaches the membrane, and for the G551D CFTR that was already trafficking normally to the surface before ETI, ivacaftor increases the open probability of the CFTR channel gate, addressing the gating defects present in both F508del CFTR (residual gating defect after correction) and G551D CFTR (primary gating defect). The triple combination thus provides simultaneous corrector rescue of the F508del allele and potentiator benefit for both alleles. ETI is preferred over tezacaftor-ivacaftor for this genotype because ETI provides substantially greater F508del CFTR rescue through the dual-corrector synergy of elexacaftor and tezacaftor.
Option A: Option A is incorrect because ivacaftor monotherapy is insufficient for this patient's F508del allele: F508del CFTR undergoes extensive ER-associated degradation and reaches the apical membrane in minimal amounts without corrector assistance; potentiating the negligible surface-expressed F508del CFTR without corrector rescue provides very limited benefit compared with corrector-potentiator combination therapy.
Option B: Option B is incorrect because lumacaftor does not correct G551D CFTR — G551D is a class III gating mutation, not a class II processing mutation; G551D CFTR already traffics normally to the membrane and does not require a corrector; furthermore, lumacaftor-ivacaftor approval is restricted to F508del homozygotes, not compound heterozygotes.
Option C: Option C is incorrect because compound heterozygotes with one F508del allele and one gating mutation allele such as G551D are covered by current FDA approvals for both tezacaftor-ivacaftor and ETI; no clinical trial is required for a patient in this genotypic category.
Option E: Option E is incorrect because ETI approval explicitly includes patients with at least one F508del allele regardless of the second allele genotype, which covers F508del/G551D; ETI is not restricted to homozygotes or minimal-function heterozygotes, and it is generally preferred over tezacaftor-ivacaftor because of its superior CFTR rescue efficacy.
14. A 29-year-old man with cystic fibrosis (CF) homozygous for F508del has been on elexacaftor-tezacaftor-ivacaftor (ETI) for 20 months. His FEV1% predicted has improved from 51% to 67%, his sweat chloride is now 31 mmol/L, and he has had no pulmonary exacerbations in 18 months. He feels significantly better and asks his CF team whether he can discontinue his twice-daily high-frequency chest wall oscillation (HFCWO) vest therapy, which he finds burdensome. He argues that since his lung function is now nearly normal, the vest is no longer necessary. Which of the following is the most appropriate clinical response?
A) Airway clearance therapy can be discontinued immediately because FEV1% predicted of 67% with sweat chloride normalization confirms that CFTR function has been restored to near-normal levels, making HFCWO physiologically redundant; the patient's subjective sense of improved health accurately reflects the degree of disease reversal achieved with ETI.
B) Airway clearance therapy should generally be continued in patients with established bronchiectasis and prior significant airway disease, because the structural bronchiectatic changes and chronic airway infection patterns that predated ETI initiation do not fully reverse with modulator therapy; the patient's functional improvement reflects CFTR rescue and reduced exacerbation risk but does not indicate that the underlying structural disease has resolved, and individualized reassessment with the CF team is appropriate rather than unilateral discontinuation.
C) Airway clearance therapy can be reduced to once daily based on the patient's symptomatic improvement, but complete discontinuation requires demonstration of FEV1% predicted above 80% on two consecutive spirometry measurements at least 6 months apart, which is the validated threshold for airway clearance discontinuation in ETI-treated adults.
D) The vest can be discontinued and replaced with voluntary coughing and deep breathing exercises, which are sufficient for airway clearance maintenance in ETI-treated patients with FEV1% predicted above 60%, as residual mucus production at this level of lung function is adequately cleared by physiological respiratory mechanics without assisted clearance devices.
E) Airway clearance discontinuation should be considered only after the patient completes a 2-week washout trial of the vest under close monitoring with daily peak flow measurements; if peak flow does not decline by more than 10% during washout, the vest can be permanently discontinued, because peak flow stability confirms adequate mucociliary clearance by restored CFTR function alone.
ANSWER: B
Rationale:
Airway clearance therapy — including high-frequency chest wall oscillation (HFCWO) vest therapy — should generally be continued in patients with established bronchiectasis and prior significant airway disease even after marked functional improvement with ETI, because the structural consequences of years of CFTR dysfunction do not reverse with modulator therapy. This patient has established bronchiectasis documented before ETI initiation; the bronchiectatic airway architecture, chronic airway colonization patterns, and impaired local defense mechanisms that characterize bronchiectatic airways persist despite FEV1 normalization and sweat chloride reduction. The functional improvements he has achieved — FEV1 +16 percentage points, no exacerbations in 18 months, symptom improvement — reflect meaningful CFTR rescue but do not indicate structural disease reversal. Airway clearance therapy provides mechanical mucus mobilization from bronchiectatic airways that cannot be fully replaced by restored CFTR-mediated mucociliary function alone when structural remodeling has already occurred. The appropriate response is individualized reassessment with the full CF team rather than abrupt discontinuation; some patients with ETI-associated dramatic improvements may be appropriate candidates for reduced airway clearance frequency under specialist monitoring, but this is an individualized decision based on ongoing sputum production, symptom assessment, and imaging findings — not a categorical policy based on FEV1 threshold alone.
Option A: Option A is incorrect because FEV1 improvement and sweat chloride normalization do not indicate reversal of bronchiectasis or elimination of structural airway disease; the patient's subjective sense of improved health does not reflect complete disease reversal, and unilateral discontinuation without specialist guidance is inappropriate.
Option C: Option C is incorrect because no validated threshold of FEV1% predicted above 80% with a specific 6-month spirometry criterion for airway clearance discontinuation in ETI-treated adults exists in clinical guidelines; this precise threshold is not an established standard.
Option D: Option D is incorrect because voluntary coughing and deep breathing are insufficient substitutes for mechanical airway clearance in patients with established bronchiectasis; there is no validated threshold FEV1 above which physiological respiratory mechanics alone adequately substitute for assisted clearance.
Option E: Option E is incorrect because a 2-week washout with daily peak flow monitoring is not an established protocol for guiding airway clearance discontinuation in ETI-treated patients; peak flow stability during a brief washout does not reliably predict long-term adequacy of mucociliary clearance in bronchiectatic airways.
15. A 25-year-old woman with cystic fibrosis (CF) homozygous for F508del on elexacaftor-tezacaftor-ivacaftor (ETI) is diagnosed with chronic pulmonary aspergillosis (CPA) confirmed by positive Aspergillus fumigatus precipitins, a cavitary lesion on CT chest, and two consecutive positive cultures. Her pulmonologist initiates posaconazole as antifungal therapy, expected to continue for at least 12 months. Which of the following correctly describes the required ETI dose adjustment and the duration over which that adjustment must be maintained?
A) Posaconazole is a moderate CYP3A4 inhibitor that raises ivacaftor concentrations by approximately 25%; the ETI dose should be reduced to three-quarters of standard (one and a half morning tablets and three-quarters of the evening tablet) for the duration of posaconazole therapy, based on pharmacokinetic modeling of moderate inhibitor interactions.
B) No ETI dose adjustment is required for posaconazole co-administration because posaconazole is administered as a delayed-release tablet that is absorbed in the small intestine, bypassing hepatic first-pass metabolism and therefore not interacting with the hepatic CYP3A4 pathway responsible for ivacaftor metabolism.
C) ETI should be discontinued for the entire 12-month posaconazole course and restarted at full dose after posaconazole completion, because the cumulative ivacaftor overexposure from 12 months of CYP3A4 inhibition at any dose of ETI carries unacceptable hepatotoxicity risk that outweighs the CFTR rescue benefit during long-term antifungal therapy.
D) The ETI morning dose should be maintained at standard (two tablets) while the evening ivacaftor tablet is held for the duration of posaconazole therapy, because the evening ivacaftor dose is the component most affected by CYP3A4 inhibition and selectively holding it normalizes the 24-hour ivacaftor exposure without disrupting corrector continuity.
E) ETI dosing should be reduced to every other day for the entire duration of posaconazole therapy per prescribing labeling, because posaconazole is a strong CYP3A4 inhibitor that substantially increases ivacaftor plasma concentrations at the standard daily dose; the every-other-day schedule must be maintained continuously throughout posaconazole use and reverted to the standard daily schedule only after posaconazole has been discontinued.
ANSWER: E
Rationale:
Posaconazole is a strong inhibitor of cytochrome P450 isoform CYP3A4 (CYP3A4). Like other azole antifungals used in CF pulmonary disease (itraconazole, voriconazole, ketoconazole), posaconazole substantially blocks the hepatic CYP3A4-mediated metabolism of ivacaftor, raising ivacaftor plasma concentrations well above the therapeutic range at the standard daily ETI dosing schedule. The ETI prescribing label specifies that when any strong CYP3A4 inhibitor is co-prescribed, the ETI regimen should be reduced to every-other-day dosing for the entire duration of the inhibitor use. For a patient on a 12-month posaconazole course for chronic pulmonary aspergillosis, the every-other-day ETI schedule must be maintained continuously throughout the entire antifungal course — not just for an initial period — and reverted to the standard daily schedule only after posaconazole has been discontinued. This requires careful longitudinal documentation and communication between the CF team and prescribing pulmonologist at every follow-up encounter.
Option A: Option A is incorrect because posaconazole is a strong, not moderate, CYP3A4 inhibitor, and the labeled adjustment is every-other-day dosing, not a fractional tablet reduction; no pharmacokinetically validated three-quarters dose reduction exists in the label.
Option B: Option B is incorrect because posaconazole's delayed-release formulation affects its gastrointestinal absorption characteristics but does not prevent its systemic CYP3A4 inhibition; once absorbed, posaconazole inhibits hepatic CYP3A4 regardless of its formulation, and this inhibition is clinically significant for ivacaftor metabolism.
Option C: Option C is incorrect because complete ETI discontinuation for a 12-month antifungal course is not appropriate or necessary; the labeled every-other-day dose adjustment is specifically designed to allow continued CFTR modulator therapy during strong CYP3A4 inhibitor co-administration at a dose that avoids supratherapeutic ivacaftor exposure.
Option D: Option D is incorrect because selectively holding the evening ivacaftor dose while maintaining the standard morning dual-corrector and ivacaftor morning tablet is not the labeled management strategy; the every-other-day adjustment applies to the full ETI regimen, not selectively to the evening component alone.
16. A 23-year-old woman with cystic fibrosis (CF) homozygous for F508del is initiated on lumacaftor-ivacaftor. Within the first week of therapy she develops worsening chest tightness, increased dyspnea, and a 4% absolute decrease in FEV1% predicted from her pre-treatment baseline. She has no fever and her sputum culture shows no new pathogens. Which of the following best identifies the likely cause of this presentation and the correct management approach?
A) The worsening dyspnea and FEV1 decline represent a pulmonary exacerbation triggered by immune reconstitution — restoration of CFTR function releases previously sequestered inflammatory mediators from airway mucus, producing a transient exacerbation-like syndrome; the correct management is a 2-week course of intravenous antibiotics while continuing lumacaftor-ivacaftor at full dose.
B) The chest tightness and FEV1 decline indicate that lumacaftor-ivacaftor is ineffective for this patient because her residual CFTR function is too low for the corrector to produce meaningful protein rescue; the treatment should be discontinued and tezacaftor-ivacaftor initiated instead, which has a lower incidence of worsening respiratory symptoms.
C) Chest tightness and worsening dyspnea with acute FEV1 decline are a recognized respiratory adverse effect of lumacaftor-ivacaftor, occurring particularly in patients with more severe baseline lung disease (FEV1% predicted below 40%); the appropriate management is to temporarily hold lumacaftor-ivacaftor, allow respiratory symptoms to resolve, and consider either rechallenge at a lower escalating dose under specialist monitoring or transition to tezacaftor-ivacaftor or ETI, which do not share this lumacaftor-specific respiratory adverse effect.
D) The presentation is consistent with lumacaftor-induced bronchospasm mediated by ivacaftor's paradoxical inhibition of CFTR in airway smooth muscle cells; the correct management is to add an inhaled short-acting beta-2 agonist (SABA) 30 minutes before each lumacaftor-ivacaftor dose and continue the regimen at full dose, as this pre-treatment strategy reliably prevents further bronchospasm episodes.
E) The worsening chest tightness and FEV1 decline are expected and self-limiting during the first 2 to 4 weeks of lumacaftor-ivacaftor therapy in all patients, representing transient airway inflammation as CFTR is restored; no management change is required and the patient should be reassured that these symptoms invariably resolve by week 4 without any dose modification.
ANSWER: C
Rationale:
Chest tightness, worsening dyspnea, and acute FEV1 decline are a recognized respiratory adverse effect associated specifically with lumacaftor-ivacaftor (Orkambi), occurring particularly in patients with more severe baseline lung disease — those with FEV1% predicted below 40% at the time of initiation. The mechanism is not fully characterized but is thought to involve lumacaftor-related airway inflammatory responses or bronchospasm. This adverse effect is not seen with the same frequency or severity with tezacaftor-ivacaftor or elexacaftor-tezacaftor-ivacaftor (ETI), which is one of the clinical reasons to prefer newer regimens over lumacaftor-ivacaftor when they are available and indicated. The appropriate management when respiratory symptoms occur acutely after lumacaftor-ivacaftor initiation is to temporarily hold the drug, allow symptoms to resolve, and then discuss with the patient and CF specialist whether to attempt rechallenge with a gradual dose escalation protocol under close monitoring or to transition to an alternative regimen (tezacaftor-ivacaftor or ETI) that does not carry this lumacaftor-specific respiratory effect. Pre-treatment with bronchodilators before each dose has been used empirically in some patients during initiation to attenuate the acute respiratory symptoms but is not a universally validated protocol.
Option A: Option A is incorrect because immune reconstitution inflammatory syndrome driving an exacerbation-like syndrome is not an established mechanism of lumacaftor-ivacaftor respiratory adverse effects; treating with intravenous antibiotics when there is no infectious trigger and continuing the causative drug at full dose is not appropriate management.
Option B: Option B is incorrect because the respiratory adverse effect of lumacaftor-ivacaftor is not an indicator of drug inefficacy due to insufficient residual CFTR function; it is a tolerability problem specific to lumacaftor that occurs despite adequate CFTR correction activity.
Option D: Option D is incorrect because ivacaftor does not paradoxically inhibit CFTR in airway smooth muscle; the respiratory adverse effect is attributed to lumacaftor's properties, and while pre-treatment bronchodilator use has been employed empirically, it is not an established protocol that reliably prevents all further episodes, and simply adding a SABA and continuing at full dose without specialist reassessment is insufficient management.
Option E: Option E is incorrect because the respiratory adverse effects of lumacaftor-ivacaftor are not universal, self-limiting, or invariably resolving by week 4 in all patients; in some patients the symptoms are severe enough to require drug interruption or discontinuation, and reassurance alone without management action is inappropriate for a patient with measurable FEV1 decline.
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