1. A 34-year-old man with cystic fibrosis (CF) homozygous for F508del has been on elexacaftor-tezacaftor-ivacaftor (ETI) for 26 months with FEV1% predicted stable at 63%. Bronchoalveolar lavage cultures grow Mycobacterium abscessus. His infectious disease consultant proposes a multidrug regimen including rifampin, azithromycin, and amikacin. The CF pharmacist flags rifampin as contraindicated with ETI and suggests rifabutin as a potential alternative rifamycin. Which of the following best describes the pharmacokinetic basis for avoiding rifampin with ETI, whether rifabutin represents a safe substitute, and what monitoring is required if rifabutin is used?
A) Rifampin is contraindicated with ETI because it is a strong inhibitor of cytochrome P450 isoform CYP3A4 (CYP3A4) that raises ivacaftor concentrations to supratherapeutic levels; rifabutin shares this inhibitory profile and is equally contraindicated; the only safe rifamycin for use with ETI is rifapentine, which has no CYP3A4 interaction.
B) Rifampin is contraindicated because it inhibits the P-glycoprotein efflux transporter responsible for intestinal ETI absorption, reducing ETI bioavailability by more than 80%; rifabutin does not inhibit P-glycoprotein and can be used as a direct substitute at standard dosing without pharmacokinetic interaction or additional monitoring.
C) Both rifampin and rifabutin are equally potent CYP3A4 inducers and are equally contraindicated with ETI; no rifamycin is safe to use with any ivacaftor-containing regimen, and the nontuberculous mycobacteria (NTM) regimen must be constructed entirely from non-rifamycin antibiotics including azithromycin, amikacin, and imipenem.
D) Rifampin is a potent CYP3A4 inducer that markedly reduces ivacaftor plasma concentrations to sub-therapeutic levels and is contraindicated with any ivacaftor-containing regimen; rifabutin is a weaker CYP3A4 inducer than rifampin and may be considered as an alternative in some NTM regimens, but it still produces a measurable degree of CYP3A4 induction, so ivacaftor concentrations should be monitored and ETI efficacy assessed clinically if rifabutin is used — the substitution is not interaction-free and requires specialist pharmacokinetic oversight.
E) Rifampin is avoided with ETI because rifampin competitively inhibits the hepatic organic anion transporting polypeptide (OATP1B1) responsible for elexacaftor and tezacaftor uptake into hepatocytes for biliary excretion, causing corrector accumulation and hepatotoxicity; rifabutin does not inhibit OATP1B1 and is a safe direct substitute at standard NTM dosing.
ANSWER: D
Rationale:
Rifampin is one of the most potent inducers of cytochrome P450 isoform CYP3A4 (CYP3A4) in clinical use and is classified in the ETI and ivacaftor prescribing labels as a strong CYP3A4 inducer that should be avoided with any ivacaftor-containing regimen. By markedly accelerating ivacaftor hepatic metabolism, rifampin reduces ivacaftor plasma concentrations to levels likely below therapeutic threshold, compromising CFTR potentiation and potentially eliminating the clinical benefit of the modulator regimen. This creates a significant clinical challenge for CF patients with nontuberculous mycobacteria (NTM) lung disease — particularly Mycobacterium abscessus, the most common and treatment-refractory NTM pathogen in CF — because rifamycins are important components of multidrug NTM regimens. Rifabutin is a rifamycin derivative that is a weaker CYP3A4 inducer than rifampin; while it does not carry the same degree of induction potency, it does still induce CYP3A4 to a measurable extent, meaning it is not completely interaction-free when co-administered with ivacaftor. The use of rifabutin in place of rifampin may be considered by CF and infectious disease specialists in carefully selected cases, but requires monitoring of ivacaftor pharmacokinetics (ideally plasma concentration measurement), clinical assessment of ETI efficacy (sweat chloride, FEV1 trajectory), and specialist oversight. It is not a simple direct substitute that eliminates the interaction.
Option A: Option A is incorrect because rifampin is a CYP3A4 inducer, not an inhibitor; its interaction with ivacaftor is via induction reducing ivacaftor concentrations, not inhibition raising them; rifabutin is not equally contraindicated, and rifapentine is also a CYP3A4 inducer.
Option B: Option B is incorrect because the rifampin-ivacaftor interaction is mediated by hepatic CYP3A4 enzyme induction, not by P-glycoprotein inhibition affecting intestinal ETI absorption; rifabutin also has CYP3A4 induction activity and cannot be used as an interaction-free substitute.
Option C: Option C is incorrect because while rifampin is contraindicated, rifabutin — being a weaker CYP3A4 inducer — is not equally contraindicated; with appropriate monitoring it may be considered in specialist-guided NTM therapy, and the statement that no rifamycin can ever be used with ETI overstates the contraindication.
Option E: Option E is incorrect because the rifampin-ETI interaction is mediated by CYP3A4 enzyme induction, not by OATP1B1 transporter inhibition causing corrector accumulation; the described mechanism is pharmacologically incorrect.
2. A 22-year-old woman with cystic fibrosis (CF) homozygous for F508del has a baseline FEV1% predicted of 37% and uses a combined oral contraceptive pill for contraception. Elexacaftor-tezacaftor-ivacaftor (ETI) is not available at her center and lumacaftor-ivacaftor is initiated. Within the first week she develops worsening chest tightness and a 5% absolute FEV1 decline. At her 1-month follow-up her CF team also realizes her contraceptive has likely become ineffective. Which of the following best identifies both adverse consequences of lumacaftor-ivacaftor in this patient and the correct integrated management response?
A) Lumacaftor-ivacaftor produces two distinct pharmacological problems in this patient: a respiratory adverse effect — chest tightness and acute FEV1 decline — occurring with heightened frequency in patients with FEV1% predicted below 40%, attributable to lumacaftor-specific airway effects; and impaired hormonal contraceptive efficacy due to lumacaftor's strong CYP3A4 induction accelerating estrogen and progestin metabolism; the correct integrated management is to temporarily hold lumacaftor-ivacaftor while the respiratory symptoms are evaluated, counsel the patient that her oral contraceptive is likely unreliable and that non-hormonal contraception should be used immediately, and reassess whether to rechallenge or transition to tezacaftor-ivacaftor or ETI when available — neither regimen shares either of lumacaftor's adverse effects.
B) Both adverse effects are attributable to ivacaftor rather than lumacaftor: ivacaftor causes bronchospasm through paradoxical CFTR activation in airway smooth muscle cells and also inhibits CYP3A4 in a dose-dependent fashion at the twice-daily doses used in combination products, reducing the hepatic metabolism of oral contraceptive hormones to dangerously low levels; the solution is to switch from the combination product to ivacaftor monotherapy, which eliminates the bronchospasm trigger while maintaining acceptable oral contraceptive hormone levels.
C) The respiratory adverse effect is caused by lumacaftor and the contraceptive failure is caused by ivacaftor; because the two adverse effects have independent pharmacological origins, they can be managed independently — holding lumacaftor while continuing ivacaftor monotherapy resolves the respiratory problem while ivacaftor's CYP3A4 inhibition raises estrogen concentrations above therapeutic levels, actually improving contraceptive efficacy during the lumacaftor-free interval.
D) Both adverse effects are rare idiosyncratic reactions unrelated to lumacaftor-ivacaftor's known pharmacological properties; the respiratory worsening represents coincident viral bronchitis and the contraceptive failure reflects missed doses; the correct management is to treat the respiratory exacerbation with antibiotics, continue lumacaftor-ivacaftor at full dose, and reinforce contraceptive adherence without changing the regimen.
E) The respiratory adverse effect is caused by the rapid increase in airway chloride secretion overwhelming mucus clearance mechanisms, and the contraceptive failure is caused by lumacaftor-induced upregulation of hepatic sex hormone-binding globulin (SHBG), which binds and inactivates free estrogen and progestin; both effects are expected and self-limiting within 4 to 6 weeks as the airway adapts to restored CFTR function and SHBG levels normalize.
ANSWER: A
Rationale:
This patient experiences two simultaneous and mechanistically distinct pharmacological consequences of lumacaftor-ivacaftor. The first is the recognized respiratory adverse effect of lumacaftor-containing regimens: chest tightness and acute FEV1 decline occurring particularly in patients with more advanced baseline disease (FEV1% predicted below 40%), where the respiratory adverse effect is most frequent and clinically significant. The mechanism is incompletely understood but is specific to lumacaftor; neither tezacaftor nor elexacaftor produces this adverse effect. The second consequence is impaired hormonal contraceptive efficacy: lumacaftor is a potent inducer of cytochrome P450 isoform CYP3A4 (CYP3A4), which substantially accelerates the hepatic metabolism of estrogen and progestin — the active hormonal components of combined oral contraceptives — reducing their plasma concentrations to potentially sub-therapeutic levels. The lumacaftor-ivacaftor prescribing label specifically addresses both: patients should be warned about contraceptive failure and advised to use non-hormonal methods while on lumacaftor-ivacaftor. The integrated correct management addresses both problems simultaneously: hold lumacaftor-ivacaftor to allow respiratory symptoms to resolve, immediately counsel the patient about contraceptive unreliability and initiate non-hormonal contraception, and plan transition to tezacaftor-ivacaftor or ETI when available — regimens that do not induce CYP3A4 (eliminating the contraceptive interaction) and do not produce the lumacaftor-specific respiratory adverse effect.
Option B: Option B is incorrect because both adverse effects are attributable to lumacaftor, not ivacaftor; ivacaftor is not a clinically significant CYP3A4 inhibitor and does not cause bronchospasm through CFTR activation in smooth muscle; switching to ivacaftor monotherapy would be ineffective for F508del CFTR, which requires a corrector to reach the membrane.
Option C: Option C is incorrect because the contraceptive failure is caused by lumacaftor's CYP3A4 induction (not ivacaftor), and ivacaftor does not inhibit CYP3A4; holding lumacaftor while continuing ivacaftor monotherapy would not rescue contraceptive efficacy and would also be ineffective for F508del.
Option D: Option D is incorrect because both adverse effects are well-characterized pharmacological consequences of lumacaftor-ivacaftor, not rare idiosyncratic coincidences; dismissing them as viral illness and missed doses fails to address two labeled safety concerns requiring active management.
Option E: Option E is incorrect because the respiratory adverse effect is not caused by overwhelming mucus mobilization, and lumacaftor does not upregulate SHBG; the contraceptive failure is caused by CYP3A4 induction accelerating hormone metabolism, and neither effect is reliably self-limiting within 4 to 6 weeks without management action.
3. A 23-year-old woman with cystic fibrosis (CF) carries one G551D allele and one F508del allele. Her pulmonologist is selecting a CFTR modulator regimen. A colleague suggests that since ivacaftor is highly effective for G551D — with the STRIVE trial demonstrating a 10.6 percentage-point FEV1% predicted improvement — ivacaftor monotherapy should be sufficient and the patient does not need a more complex triple combination. Which of the following most completely explains why elexacaftor-tezacaftor-ivacaftor (ETI) is the preferred regimen over ivacaftor monotherapy for this patient?
A) Ivacaftor monotherapy is actually the superior choice for this patient because the STRIVE trial data apply directly to her G551D allele, and adding elexacaftor and tezacaftor introduces corrector-associated adverse effects including hepatotoxicity and the lumacaftor-type respiratory adverse effect without providing additional clinical benefit, since the G551D allele already responds maximally to potentiation alone.
B) ETI is preferred because elexacaftor and tezacaftor are potentiators that enhance ivacaftor's gating effect at the G551D allele through allosteric synergy, producing channel open probabilities substantially higher than ivacaftor alone can achieve; the corrector components provide no benefit to the F508del allele in the absence of corrector-specific ER-binding sites on this patient's F508del CFTR.
C) ETI is preferred because ivacaftor monotherapy, while highly effective for the G551D allele whose CFTR is surface-expressed in normal amounts, provides essentially no benefit for the F508del allele, which undergoes extensive ER-associated degradation and reaches the apical membrane in negligible quantities without corrector rescue; by adding elexacaftor and tezacaftor, ETI rescues F508del CFTR trafficking to the membrane where ivacaftor can then potentiate both the corrected F508del CFTR and the constitutively surface-expressed G551D CFTR, providing greater total chloride transport than potentiation of the G551D allele alone.
D) ETI is preferred over ivacaftor monotherapy solely because of a regulatory requirement: FDA labeling mandates that all patients with at least one F508del allele must use a corrector-potentiator combination rather than potentiator monotherapy regardless of the second allele's mutation class, and the labeling explicitly prohibits ivacaftor monotherapy for F508del heterozygotes.
E) Ivacaftor monotherapy and ETI produce equivalent clinical outcomes in G551D/F508del compound heterozygotes because the F508del allele contributes negligible CFTR function regardless of corrector therapy, making the corrector components pharmacologically inert in this genotype; ETI's apparent superiority in trials reflects its use in F508del homozygotes, not in G551D/F508del compound heterozygotes where ivacaftor monotherapy is sufficient.
ANSWER: C
Rationale:
The pharmacological argument for ETI over ivacaftor monotherapy in this G551D/F508del compound heterozygote rests on what each allele contributes to the patient's CFTR dysfunction and how each modulator component addresses it. The G551D allele produces CFTR that traffics normally to the apical membrane in near-normal amounts but cannot open its channel gate appropriately — this gating defect is the sole dysfunction. Ivacaftor addresses this precisely by increasing channel open probability, and the STRIVE trial demonstrated the resulting 10.6 percentage-point FEV1% predicted improvement in G551D patients. However, the F508del allele produces CFTR that undergoes extensive ER-associated degradation (ERAD) through the Hsp70/Hsp90 quality control machinery and reaches the apical membrane in negligible amounts in the absence of a corrector; ivacaftor applied to the F508del allele without corrector rescue has essentially no surface-expressed CFTR to potentiate, and the pharmacological benefit from this allele is minimal. When elexacaftor and tezacaftor are added to the regimen, they stabilize F508del CFTR in the ER, reduce ERAD, and allow substantially more F508del CFTR to traffic to the apical membrane; ivacaftor then potentiates the gating of this corrected F508del CFTR as well as the already-surface-expressed G551D CFTR, generating greater total chloride transport than potentiation of the G551D allele alone provides. ETI therefore extracts therapeutic benefit from both alleles simultaneously.
Option A: Option A is incorrect because elexacaftor and tezacaftor are correctors, not agents that introduce lumacaftor-type respiratory adverse effects; the respiratory adverse effect is specific to lumacaftor, and ETI (containing tezacaftor and elexacaftor) does not share this toxicity; the premise of equivalent potentiation and net adverse harm from ETI is pharmacologically incorrect.
Option B: Option B is incorrect because elexacaftor and tezacaftor are CFTR correctors that act at the ER to rescue F508del CFTR trafficking — they are not potentiators that enhance ivacaftor's gating effect at G551D through allosteric synergy at the channel gate; their mechanism is entirely upstream at the protein folding level.
Option D: Option D is incorrect because the preference for ETI over ivacaftor monotherapy in F508del heterozygotes is based on pharmacological benefit from rescuing the F508del allele, not on a regulatory mandate prohibiting ivacaftor monotherapy; ivacaftor monotherapy is labeled for patients with gating or residual function mutations, and the FDA does not explicitly prohibit its use in F508del heterozygotes as a regulatory matter.
Option E: Option E is incorrect because corrector therapy does provide meaningful rescue of the F508del allele in compound heterozygotes; the ETI approval in F508del/residual-function heterozygotes is based on clinical trial data demonstrating meaningful CFTR rescue, and the claim that the F508del allele contributes negligible function regardless of correctors is contradicted by tezacaftor-ivacaftor and ETI trial data in this exact genotypic category.
4. A 29-year-old woman with cystic fibrosis (CF) homozygous for F508del on elexacaftor-tezacaftor-ivacaftor (ETI) is referred to a CF dietitian because of a 5 kg weight loss over 8 months and postprandial fatigue. Her primary care physician recently checked a hemoglobin A1c (HbA1c) of 5.8%, labeled it "prediabetes," and noted it was "reassuringly below the 6.5% diagnostic threshold for diabetes." Her CF physician orders an oral glucose tolerance test (OGTT) that shows a fasting plasma glucose of 112 mg/dL and a 2-hour plasma glucose of 198 mg/dL. Which of the following best integrates the diagnostic findings and explains why the primary care physician's interpretation of the HbA1c is incorrect in this clinical context?
A) The primary care physician's interpretation is correct: the OGTT result of 198 mg/dL at 2 hours is in the prediabetes range (140–199 mg/dL per ADA criteria) and does not meet the diagnostic threshold of 200 mg/dL required for diabetes mellitus; the HbA1c of 5.8% is consistent with prediabetes and both tests agree, so no change in management beyond dietary modification is warranted.
B) The OGTT 2-hour plasma glucose of 198 mg/dL does not meet the standard (≥200 mg/dL) diagnostic threshold for diabetes mellitus, but it falls in the CF impaired glucose tolerance (CF-IGT) range of 140–199 mg/dL; CF clinical guidelines recognize that a 2-hour value in this range in a symptomatic patient (weight loss, postprandial fatigue) constitutes early glucose dysregulation warranting insulin therapy consideration rather than simple dietary reassurance; critically, the HbA1c of 5.8% substantially underestimates her glycemic burden because increased red blood cell (RBC) turnover in CF — from chronic inflammation, repeated infections, and nutritional compromise — shortens erythrocyte lifespan, reducing the time available for hemoglobin glycosylation and producing a falsely low HbA1c that misrepresents true chronic glucose exposure; relying on HbA1c to guide CFRD diagnosis or management in this patient is unreliable and the OGTT is the validated diagnostic standard.
C) The OGTT result confirms type 1 diabetes mellitus because the 2-hour glucose of 198 mg/dL combined with symptomatic weight loss and fatigue in a young patient indicates autoimmune beta-cell destruction; HbA1c is unreliable in CF because fetal hemoglobin (HbF) persists into adulthood in CF patients due to chronic hypoxia-driven erythropoiesis, producing a hemoglobin species that cannot be glycosylated and falsely lowering the measured HbA1c.
D) The HbA1c of 5.8% is the more accurate diagnostic test in this patient because it reflects average glucose over 90 days and is therefore more sensitive to early glucose dysregulation than a single 2-hour OGTT measurement; the OGTT result of 198 mg/dL likely reflects normal postprandial variation in a patient with gastroparesis-related delayed gastric emptying, and further diagnostic workup with a 3-hour gastric emptying study should precede any diabetes diagnosis.
E) Both the OGTT and HbA1c results are unreliable in CF patients on ETI because elexacaftor-tezacaftor-ivacaftor alters pancreatic ductal function sufficiently to change the glucose kinetics of a standard 75-gram oral load; an intravenous glucose tolerance test (IVGTT) should replace the OGTT as the standard diagnostic test in ETI-treated CF patients, and HbA1c should be interpreted with a correction factor of +1.5% to account for ETI-related changes in erythrocyte membrane permeability.
ANSWER: B
Rationale:
This question requires integrating two distinct clinical pharmacology concepts: the correct diagnostic standard for CFRD and the specific mechanism by which HbA1c systematically underestimates glycemic burden in CF patients. The oral glucose tolerance test (OGTT) is the recommended diagnostic test for CFRD per CF clinical guidelines because CFRD characteristically presents with postprandial hyperglycemia before fasting hyperglycemia develops; a 2-hour plasma glucose of 140–199 mg/dL represents impaired glucose tolerance in standard diabetes criteria, but in the symptomatic CF patient (weight loss, postprandial fatigue) this level warrants clinical attention and may constitute early CFRD requiring management — with CF-specific guidelines recommending consideration of insulin therapy for impaired glucose tolerance with clinical symptoms rather than waiting for the 200 mg/dL threshold. The HbA1c is systematically unreliable in CF patients because CF is associated with increased red blood cell (RBC) turnover: chronic systemic inflammation from recurrent pulmonary infections, hemolytic contributions from oxidative stress, and nutritional deficiencies all shorten erythrocyte lifespan below the normal approximately 120-day cycle. Because HbA1c reflects the cumulative glycosylation of hemoglobin over the lifetime of the red blood cell, shorter erythrocyte survival means less time for glycosylation to accumulate, producing HbA1c values that are systematically lower than the true average glucose exposure over the preceding 2 to 3 months. A "reassuringly normal" HbA1c of 5.8% in a CF patient may conceal substantial postprandial hyperglycemia that OGTT reveals. Relying on the primary care physician's HbA1c-based reassurance risks undertreating CFRD with consequences including accelerated lung function decline (hyperglycemia worsens pulmonary infection and inflammation), malnutrition, and increased exacerbation frequency.
Option A: Option A is incorrect because the standard ADA threshold of 200 mg/dL applies to general populations, but CF clinical guidelines specifically recognize that impaired glucose tolerance in symptomatic CF patients warrants management; the primary care physician's interpretation also incorrectly trusts the HbA1c as reliable in this population.
Option C: Option C is incorrect because CFRD is caused by pancreatic exocrine fibrosis destroying islet cells, not by autoimmune beta-cell destruction as in type 1 diabetes mellitus; the mechanism of HbA1c unreliability in CF is increased RBC turnover from chronic inflammation and nutritional factors, not fetal hemoglobin persistence driven by hypoxia-induced erythropoiesis.
Option D: Option D is incorrect because HbA1c is specifically less reliable than OGTT in CF for the reasons described; the OGTT does not reflect gastroparesis-related variation in the manner suggested, and delaying diabetes diagnosis pending a gastric emptying study in a symptomatic patient with abnormal OGTT is clinically inappropriate.
Option E: Option E is incorrect because ETI does not alter the pharmacokinetics or diagnostics of glucose tolerance testing in a way that requires IVGTT substitution; no ETI-specific OGTT replacement protocol or HbA1c correction factor exists in CF clinical practice.
5. A 41-year-old man with cystic fibrosis (CF) homozygous for F508del has been on elexacaftor-tezacaftor-ivacaftor (ETI) for 3 years. His FEV1% predicted has improved from 46% to 60%, his sweat chloride is 26 mmol/L, and he has had no pulmonary exacerbations in 2 years. He feels better than he has in decades. His annual high-resolution CT chest, however, reveals a new 8 mm thin-walled cyst in the right upper lobe not present on his prior scan. He asks his pulmonologist whether ETI's excellent results mean this finding can be ignored. Which of the following best explains why ongoing CT surveillance is warranted despite his outstanding modulator response and what the new cystic lesion may represent?
A) The new pulmonary cyst is a direct consequence of ETI therapy: as CFTR function is restored, air trapping that previously kept overdistended alveoli patent is suddenly released, causing alveolar rupture and cyst formation; this is an expected and self-limiting radiographic adverse effect of ETI seen in approximately 15% of treated patients and does not require further evaluation.
B) The new cyst is almost certainly a resolved mucus plug that has left a thin-walled cavity; because ETI restores mucociliary clearance, previously impacted mucus is mobilized and the residual airspace fills with air, producing a transient radiographic cyst that will disappear on the following year's CT; no further evaluation is needed.
C) Because the patient's sweat chloride has normalized to 26 mmol/L, confirming near-complete CFTR functional restoration, no structural pulmonary complications including malignancy can develop or progress; the cyst is a radiographic artifact from improved aeration of previously collapsed lung segments and does not warrant workup beyond routine annual CT.
D) The new cyst reflects progressive emphysematous destruction of lung parenchyma caused by ETI's stimulation of excessive chloride secretion into alveolar spaces, which secondarily activates alveolar macrophage-mediated matrix metalloproteinase (MMP) release; annual CT surveillance should be replaced by 6-monthly CT to monitor for further emphysematous change.
E) Despite ETI's exceptional clinical and functional outcomes, it does not eliminate the risk of structural pulmonary complications or malignancy; CF patients carry an elevated risk of pulmonary malignancy relative to the general population, and new pulmonary lesions identified on surveillance CT require evaluation regardless of the degree of CFTR functional restoration — a new 8 mm cystic lesion warrants follow-up imaging at a shortened interval and potentially further characterization, as ETI-mediated CFTR rescue does not confer protection against developing lung cancer or other structural complications that are independent of CFTR function.
ANSWER: E
Rationale:
Elexacaftor-tezacaftor-ivacaftor (ETI) produces profound improvements in CFTR function, lung function, and disease burden in CF patients, but it does not eliminate the background risk of structural pulmonary complications including malignancy. Patients with CF have an elevated lifetime risk of pulmonary malignancy relative to the general population — a risk attributable to decades of chronic airway inflammation, recurrent infection, oxidative stress, and the mutagenic consequences of neutrophilic airway disease that precede modulator therapy by years or decades in adult patients. The fact that a patient's sweat chloride has normalized and FEV1 has dramatically improved confirms that CFTR is functioning well pharmacologically, but CFTR restoration does not reverse accumulated genomic damage or eliminate the tumor microenvironment established by years of prior disease. A new 8 mm thin-walled cystic pulmonary lesion in a 41-year-old CF patient who has had decades of airway disease requires evaluation — new cystic lesions on CT must be assessed in the context of the patient's overall risk profile, prior imaging, and clinical picture, and a shortened follow-up CT interval or further characterization (e.g., PET-CT or bronchoscopy depending on characteristics) is warranted. The patient's outstanding ETI response does not justify ignoring a new radiographic finding. This vignette addresses a clinically important emerging issue as CF patients live longer with ETI and the population of older CF survivors grows.
Option A: Option A is incorrect because ETI does not cause pulmonary cyst formation through alveolar rupture from restored CFTR function; no such mechanism exists, and ETI-associated cyst formation at 15% prevalence is not an established adverse effect.
Option B: Option B is incorrect because while mucus plug mobilization can produce transient radiographic changes after ETI initiation, a new 8 mm cyst appearing 3 years into stable ETI therapy in a well-treated patient is not appropriately attributed to late mucus plug resolution and dismissed without evaluation.
Option C: Option C is incorrect because sweat chloride normalization reflects CFTR functional restoration in sweat duct epithelium and does not confer immunity from pulmonary malignancy or structural complications; this reasoning conflates CFTR channel function with cancer protection, which is mechanistically unfounded.
Option D: Option D is incorrect because ETI does not cause emphysematous parenchymal destruction through alveolar chloride secretion activating MMP release; this mechanism is fabricated, and there is no indication to increase CT surveillance frequency to every 6 months based on this rationale.
6. A 38-year-old man with cystic fibrosis (CF) homozygous for F508del underwent renal transplantation 4 years ago for CF-related renal insufficiency and is maintained on cyclosporine with stable trough levels of 120 ng/mL. His CF team plans to initiate elexacaftor-tezacaftor-ivacaftor (ETI). The transplant nephrologist asks whether ETI is safe with cyclosporine and what monitoring is required. Which of the following best characterizes the pharmacokinetic interaction between ETI and cyclosporine and the appropriate monitoring strategy?
A) ETI is contraindicated with cyclosporine because elexacaftor is a potent inhibitor of the calcineurin pathway that directly antagonizes cyclosporine's mechanism of immunosuppression at the T-cell level, reducing its immunosuppressive efficacy and risking acute rejection; tacrolimus should be substituted for cyclosporine before ETI is initiated.
B) ETI is safe with cyclosporine without any pharmacokinetic interaction because cyclosporine is metabolized exclusively by CYP2C9, which is unaffected by any component of ETI; cyclosporine trough monitoring can remain at the standard quarterly interval without adjustment.
C) ETI must be avoided in all transplant patients because the elexacaftor component is a substrate of calcineurin and competes with cyclosporine for calcineurin binding, reducing the availability of calcineurin for cyclosporine-mediated inhibition and causing a paradoxical increase in T-cell activation that risks acute cellular rejection.
D) Neither elexacaftor nor tezacaftor induces CYP3A4, so ETI does not carry the clinically dangerous cyclosporine-lowering interaction that would be seen with lumacaftor-ivacaftor; ETI can be initiated with cyclosporine, but cyclosporine trough concentrations should be monitored more frequently after ETI initiation because cyclosporine is a CYP3A4 substrate and ivacaftor's modest pharmacokinetic effects — while not as dramatic as lumacaftor's induction — warrant confirmation that cyclosporine levels remain in the therapeutic range during the transition period.
E) ETI raises cyclosporine concentrations to potentially nephrotoxic levels because tezacaftor is a strong inhibitor of the renal organic anion transporter (OAT3) responsible for cyclosporine tubular excretion, causing cyclosporine accumulation; cyclosporine trough levels should be measured daily for the first month after ETI initiation and the dose reduced by 40% empirically before starting ETI.
ANSWER: D
Rationale:
The key pharmacological distinction that makes ETI safe to co-administer with cyclosporine — in contrast to lumacaftor-ivacaftor — is that neither elexacaftor nor tezacaftor induces cytochrome P450 isoform CYP3A4 (CYP3A4). Cyclosporine is a narrow therapeutic index calcineurin inhibitor that is substantially metabolized by CYP3A4; when lumacaftor-ivacaftor is used, lumacaftor's potent CYP3A4 induction accelerates cyclosporine metabolism, reducing trough concentrations and risking allograft rejection — an interaction that makes lumacaftor-ivacaftor dangerous in transplant patients on cyclosporine or tacrolimus. Because the corrector components of ETI (elexacaftor and tezacaftor) do not induce CYP3A4, this dangerous reduction in cyclosporine concentrations does not occur with ETI, and the regimen can be initiated. However, cyclosporine is also a substrate of P-glycoprotein and other transporters, and the pharmacokinetic effects of the full ETI combination on cyclosporine concentrations — while not dominated by a dramatic CYP3A4 induction — have not been exhaustively characterized for all possible interactions at the transporter level. Ivacaftor, while not a potent CYP3A4 inducer or inhibitor, may have modest effects on CYP3A4 activity. Clinically, the transplant team should monitor cyclosporine trough concentrations at increased frequency after ETI initiation (e.g., weekly for the first month rather than quarterly) to confirm stability of immunosuppression during the transition, even though a dramatic interaction is not expected.
Option A: Option A is incorrect because elexacaftor does not interact with the calcineurin pathway or antagonize cyclosporine's immunosuppressive mechanism at the T-cell level; elexacaftor is a CFTR protein corrector and has no pharmacodynamic interaction with calcineurin or T-cell signaling.
Option B: Option B is incorrect because cyclosporine is substantially metabolized by CYP3A4 (not exclusively CYP2C9), and while ETI's CYP3A4 interaction is less dramatic than lumacaftor's, continuing quarterly cyclosporine monitoring without any increase in frequency after initiating a new interacting drug regimen is suboptimal clinical practice for a narrow therapeutic index immunosuppressant.
Option C: Option C is incorrect because elexacaftor is not a substrate of calcineurin and does not compete with cyclosporine at the calcineurin binding site; this pharmacodynamic interaction is fabricated.
Option E: Option E is incorrect because tezacaftor is not a strong inhibitor of renal OAT3, and cyclosporine is not primarily renally excreted by tubular secretion in a way that makes OAT3 inhibition clinically relevant; the described mechanism and the recommendation for daily monitoring with empirical 40% dose reduction are not supported by established pharmacokinetic data.
7. A 36-year-old man with cystic fibrosis (CF) homozygous for F508del on elexacaftor-tezacaftor-ivacaftor (ETI) has two concurrent pulmonary complications: allergic bronchopulmonary aspergillosis (ABPA) requiring itraconazole, and Mycobacterium avium complex (MAC) lung disease for which his infectious disease team proposes a regimen including rifampin, azithromycin, and ethambutol. The CF pharmacist identifies that these two antibiotics have opposing pharmacokinetic interactions with ETI. Which of the following best resolves this conflict and identifies the correct management approach for both pulmonary complications simultaneously?
A) The two drugs have directly opposing effects on ivacaftor concentrations: itraconazole is a strong CYP3A4 inhibitor that substantially raises ivacaftor levels (requiring every-other-day ETI dosing per label), while rifampin is a potent CYP3A4 inducer that markedly reduces ivacaftor levels (and is contraindicated with ETI); co-administration of both agents simultaneously creates an unpredictable net pharmacokinetic effect on ivacaftor and cannot be managed with a simple dose adjustment; the correct approach is to avoid rifampin entirely and construct a rifampin-free MAC regimen (using azithromycin, ethambutol, and amikacin), then apply the standard every-other-day ETI adjustment for itraconazole co-administration.
B) The conflict can be resolved by administering itraconazole and rifampin at 12-hour intervals — itraconazole in the morning and rifampin in the evening — so that their opposing CYP3A4 effects occur at different times of day and partially cancel each other out, allowing ETI to be continued at full standard daily dosing without any adjustment.
C) Both interactions are manageable simultaneously by doubling the ETI dose: rifampin's CYP3A4 induction reduces ivacaftor concentrations while itraconazole's CYP3A4 inhibition raises them, and at double the ETI dose the net pharmacokinetic effect approximates the standard therapeutic range; this approach avoids the need to modify either antibiotic regimen.
D) The itraconazole-rifampin conflict takes clinical priority over the ETI interaction; because rifampin induces the metabolism of itraconazole itself through CYP3A4, the two antibiotics cannot be co-administered and the itraconazole must be replaced with voriconazole for ABPA treatment; once itraconazole is replaced, only rifampin's CYP3A4 induction interaction with ETI requires management, which is addressed by doubling the ETI morning dose.
E) Both interactions are minor and can be managed conservatively: itraconazole raises ivacaftor concentrations by only 15 to 20%, which is within the acceptable therapeutic window at standard dosing, and rifampin reduces ivacaftor concentrations by only 20 to 25%; the net effect of co-administering both agents results in essentially no change to average ivacaftor concentrations, and ETI can be continued at standard dosing with monthly sweat chloride monitoring to confirm ongoing CFTR potentiation.
ANSWER: A
Rationale:
This question requires integrating two opposing drug-drug interactions with ivacaftor and recognizing that they cannot simply cancel each other out or be managed with a dose adjustment. Itraconazole is a potent inhibitor of cytochrome P450 isoform CYP3A4 (CYP3A4) that substantially blocks ivacaftor hepatic metabolism, raising ivacaftor plasma concentrations well above the therapeutic range at standard daily dosing; the ETI prescribing label addresses this by specifying every-other-day dosing during co-administration of any strong CYP3A4 inhibitor. Rifampin is a potent CYP3A4 inducer that markedly accelerates ivacaftor metabolism, reducing plasma concentrations to potentially sub-therapeutic levels; rifampin is explicitly classified in the ETI and ivacaftor labeling as an agent to be avoided entirely with any ivacaftor-containing regimen. Co-administering both itraconazole and rifampin with ETI simultaneously creates an unpredictable, bidirectionally competing pharmacokinetic situation — the degree of net inhibition or induction depends on the relative potency, timing, and plasma concentrations of both interactors and cannot be reliably managed by a simple numerical dose adjustment. The correct resolution eliminates the contraindicated component (rifampin) from the regimen rather than attempting to balance two opposing interactions. A rifampin-free MAC regimen — typically built around azithromycin, ethambutol, and amikacin, with specialist guidance for Mycobacterium avium complex — avoids the ETI-rifampin contraindication. Once rifampin is removed, only the itraconazole interaction remains, and the standard every-other-day ETI dose adjustment is applied for the duration of itraconazole therapy.
Option B: Option B is incorrect because pharmacokinetic enzyme interactions — particularly CYP3A4 induction and inhibition — operate at the level of hepatic enzyme expression and activity, not moment-to-moment drug concentration competition; staggering itraconazole and rifampin administration by 12 hours does not prevent their opposing CYP3A4 effects from occurring simultaneously through the same induced enzyme pool, and this strategy has no pharmacological validity.
Option C: Option C is incorrect because doubling the ETI dose in the presence of opposing CYP3A4 induction and inhibition creates an unpredictable and unvalidated pharmacokinetic situation; a doubled dose in the context of net CYP3A4 inhibition (itraconazole dominating) would produce supratherapeutic ivacaftor concentrations with hepatotoxicity risk.
Option D: Option D is incorrect because while rifampin does induce the metabolism of itraconazole, this does not resolve the ETI-rifampin contraindication by switching to voriconazole (which is also a strong CYP3A4 inhibitor carrying the same every-other-day ETI adjustment requirement); and doubling the ETI morning dose is not a validated or labeled strategy for managing the rifampin-ETI interaction.
Option E: Option E is incorrect because both the itraconazole and rifampin interactions with ivacaftor are clinically significant — itraconazole raises ivacaftor concentrations by substantially more than 15 to 20%, and rifampin reduces them by far more than 25%; the characterization of both interactions as minor and mutually offsetting dramatically understates their pharmacological magnitude.
8. A CF nurse educator is counseling a 20-year-old man with cystic fibrosis (CF) homozygous for F508del on elexacaftor-tezacaftor-ivacaftor (ETI) who has been prescribed inhaled dornase alfa once daily and twice-daily high-frequency chest wall oscillation (HFCWO) vest therapy. He asks whether the order in which he performs these treatments matters. Which of the following best explains the mechanistic rationale for the recommended sequencing of dornase alfa relative to chest physiotherapy?
A) Dornase alfa should be administered immediately after HFCWO vest therapy because mechanical airway clearance mobilizes mucus from peripheral airways into central airways where it is more accessible to inhaled dornase alfa; administering dornase alfa before physiotherapy wastes the drug on mucus in small airways from which it cannot be effectively cleared by subsequent vest therapy.
B) The sequence does not matter pharmacologically because dornase alfa acts systemically after absorption through the alveolar-capillary membrane and reaches its DNA substrate through the bloodstream rather than by direct airway contact; vest therapy timing is therefore pharmacokinetically independent of dornase alfa inhalation timing.
C) Dornase alfa should be inhaled before HFCWO vest therapy; by cleaving the high-molecular-weight extracellular DNA that cross-links within the mucus gel matrix and markedly increases sputum viscosity and stiffness, dornase alfa reduces the cohesive and adhesive properties of mucus in the time period between inhalation and physiotherapy — typically 30 to 60 minutes — so that when mechanical airway clearance is applied, the less viscous mucus is more easily mobilized from airway walls and transported toward the central airways for expectoration.
D) Dornase alfa should be administered after HFCWO vest therapy and after a 2-hour rest period; vest therapy first loosens mucus from airway walls and concentrates it in central airways, and dornase alfa then degrades the DNA within this centrally pooled mucus; if dornase alfa is given before vest therapy, the drug is absorbed into the airway epithelium before it can act on the mucus DNA, reducing its mucolytic efficacy.
E) The sequence of dornase alfa relative to vest therapy is irrelevant in ETI-treated patients because ETI restores CFTR-mediated chloride secretion, which normalizes airway surface liquid hydration and eliminates the pathological DNA-mucin cross-linking that dornase alfa targets; in ETI-treated patients dornase alfa is pharmacologically inert regardless of when it is administered relative to mechanical clearance.
ANSWER: C
Rationale:
The mechanistic rationale for administering dornase alfa before airway clearance physiotherapy rests on the sequence of events needed to maximize mucus mobilization. Dornase alfa is a recombinant human deoxyribonuclease I (DNase I) that cleaves the phosphodiester backbone of extracellular high-molecular-weight DNA released by lysed neutrophils in chronically infected CF airways. This neutrophil-derived extracellular DNA forms a densely cross-linked polymer network within the CF mucus gel matrix, dramatically increasing mucus viscosity, elasticity, and adhesion to airway walls — properties that impair mechanical clearance by any physical method. When dornase alfa is inhaled before physiotherapy, it penetrates the mucus layer and begins cleaving DNA polymer chains during the interval between inhalation and the start of mechanical clearance (optimally 30 to 60 minutes later); this enzymatic degradation reduces mucus viscosity and the tenacious adhesion of mucus to airway walls. When HFCWO vest therapy is then applied, the mechanically oscillating airway pressure waves work against mucus that has been pharmacologically pre-thinned by dornase alfa-mediated DNA degradation — the combination is synergistic in mobilizing mucus from peripheral airways toward the central airways and pharynx for expectoration. If dornase alfa is administered after physiotherapy, the drug is applied to mucus that has already been mechanically cleared as well as possible without pharmacological pre-treatment; any residual mucus is then exposed to dornase alfa but cannot benefit from the combined sequence.
Option A: Option A is incorrect because dornase alfa does not act more effectively on centrally pooled mucus than on peripherally distributed mucus; its mechanism of DNA cleavage operates on contact with the sputum wherever it is distributed, and pre-treatment before physiotherapy is the mechanistically sound sequence.
Option B: Option B is incorrect because dornase alfa acts locally on airway mucus by direct enzymatic contact with extracellular DNA; it does not act systemically after alveolar absorption, and its mechanism is entirely topical within the airway lumen.
Option D: Option D is incorrect because dornase alfa is not absorbed into airway epithelium in a way that prevents it from acting on mucus DNA; it is a large protein that does not cross the epithelial barrier and remains in the airway lumen where it acts on sputum DNA; the 2-hour post-therapy delay recommendation is not established physiotherapy sequencing guidance.
Option E: Option E is incorrect because ETI does not eliminate the pathological DNA-mucin network in patients with established bronchiectasis and ongoing neutrophilic airway infection; the DNA-driven viscosity problem reflects chronic airway infection that persists after ETI initiation in patients with longstanding structural disease, and dornase alfa retains efficacy in these patients regardless of CFTR functional restoration.
9. A 2-year-old boy is diagnosed with cystic fibrosis (CF) after newborn screening. Genotyping reveals F508del on one allele and G542X (a class I nonsense mutation) on the other. His parents ask whether elexacaftor-tezacaftor-ivacaftor (ETI) is appropriate, what the drug will actually do for each of their son's alleles, and what biomarker response to expect after initiation. Which of the following most accurately answers all three questions?
A) ETI is not appropriate for this child because he carries a class I mutation (G542X) on one allele; ETI approval requires that both alleles carry F508del or another amenable mutation, and a class I nonsense mutation at the second allele disqualifies a patient from modulator therapy regardless of F508del at the first allele.
B) ETI is appropriate because the child carries at least one F508del allele and is aged 2 years and older, meeting the US FDA approval criteria; the corrector components elexacaftor and tezacaftor rescue F508del CFTR from ER-associated degradation on the F508del allele, and ivacaftor potentiates the gating of the corrected F508del CFTR that reaches the apical membrane; the G542X allele, which produces absent CFTR protein through nonsense-mediated mRNA decay, provides no pharmacological target for the corrector or potentiator and receives no benefit from ETI; the expected biomarker response includes a significant reduction in sweat chloride concentration reflecting improved F508del CFTR function and potentially a smaller absolute sweat chloride reduction than seen in F508del homozygotes, because only one allele contributes rescuable CFTR.
C) ETI is appropriate and will rescue both alleles: the corrector components stabilize the F508del CFTR misfolding defect, and the potentiator ivacaftor promotes read-through of the G542X premature stop codon at a frequency sufficient to produce partial-length CFTR protein that then reaches the apical membrane; the expected biomarker response includes normalization of sweat chloride and complete restoration of CFTR function at both alleles.
D) ETI is appropriate for this child, and both alleles will respond equally to the corrector components because G542X produces a truncated CFTR protein with a misfolding defect similar to F508del that is rescued by elexacaftor and tezacaftor; ivacaftor then potentiates the gating of the corrected truncated G542X protein at the apical membrane; sweat chloride is expected to normalize fully as both alleles contribute corrected CFTR to the apical membrane.
E) ETI is not appropriate at age 2 because the US FDA approval for ETI in compound heterozygotes with one F508del and one class I allele applies only to patients aged 6 and older, pending completion of pediatric safety studies in this specific genotypic subgroup; the child should receive best supportive care until age 6 when ETI eligibility can be reassessed.
ANSWER: B
Rationale:
ETI is approved in the United States for patients aged 2 years and older who carry at least one F508del CFTR allele; this child's genotype (F508del/G542X) meets both criteria — he has one F508del allele and is aged 2. The pharmacological activity of ETI acts exclusively through the F508del allele in this compound heterozygote. The corrector components elexacaftor and tezacaftor bind to the misfolded F508del CFTR protein in the endoplasmic reticulum (ER), stabilizing its conformation at two distinct binding sites, reducing ER-associated degradation (ERAD), and allowing substantially more F508del CFTR to traffic to the apical epithelial membrane. Ivacaftor then potentiates the gating of the corrected F508del CFTR that reaches the surface. The G542X allele, by contrast, is a class I nonsense mutation that introduces a premature stop codon into the CFTR mRNA; this triggers nonsense-mediated mRNA decay (NMD), resulting in absent or severely truncated CFTR protein with no functional channel at the apical membrane. Neither the corrector components (which require misfolded but translatable ER protein) nor the potentiator (which requires surface-expressed CFTR channels) has a pharmacological target on the G542X allele; it receives no benefit from ETI. The expected biomarker response — sweat chloride reduction reflecting the F508del CFTR that has been rescued — will likely be measurable and clinically significant, though the absolute reduction may be smaller than in F508del homozygotes because only one allele contributes rescuable CFTR to the total channel pool.
Option A: Option A is incorrect because ETI approval requires at least one F508del allele, not that both alleles carry F508del or another amenable mutation; compound heterozygotes with one F508del and one class I minimal-function allele are explicitly covered by the approval, as demonstrated by the VX-445-102 trial in this exact population.
Option C: Option C is incorrect because ivacaftor does not promote nonsense-mediated read-through of the G542X premature stop codon; read-through of premature stop codons is an entirely distinct pharmacological mechanism from CFTR potentiation, and ivacaftor has no established activity as a read-through agent.
Option D: Option D is incorrect because G542X produces absent or severely truncated CFTR protein through NMD — it is not a misfolding mutation with a processing defect similar to F508del that responds to corrector therapy; the truncated protein (if any survives NMD) does not present at the ER in a correctable form.
Option E: Option E is incorrect because the ETI approval for patients aged 2 and older covers compound heterozygotes with F508del and a minimal-function allele; there is no genotype-specific age restriction delaying approval for F508del/class-I compound heterozygotes to age 6.
10. A 33-year-old woman with cystic fibrosis (CF) homozygous for F508del on elexacaftor-tezacaftor-ivacaftor (ETI) has established CF-related diabetes (CFRD) managed with basal-bolus insulin therapy. She is admitted for a severe pulmonary exacerbation requiring intravenous antibiotics. Her team also initiates a short course of systemic prednisone 40 mg daily for airway inflammation. On day 2 of prednisone, her point-of-care glucose readings are consistently in the range of 280–340 mg/dL despite her usual insulin doses. Which of the following best explains the mechanism of the hyperglycemia worsening and the correct management approach?
A) Prednisone activates transcription of the CFTR gene through glucocorticoid response elements in the CFTR promoter region, paradoxically improving CFTR-mediated chloride secretion in pancreatic ductal cells but simultaneously inducing massive insulin hypersecretion that overwhelms peripheral glucose uptake and causes severe postprandial hyperglycemia refractory to basal insulin.
B) The hyperglycemia worsening is caused by intravenous antibiotic-induced disruption of the gut microbiome, which produces short-chain fatty acids that directly inhibit pancreatic beta-cell glucose sensing; the correct management is to switch from intravenous to oral antibiotics and add a glucagon-like peptide-1 (GLP-1) receptor agonist to enhance endogenous insulin secretion during the hospitalization.
C) The glucose elevations reflect ETI-induced pancreatitis: as CFTR function is restored in pancreatic ductal epithelium, inspissated secretions are suddenly mobilized, causing transient obstructive pancreatitis with secondary beta-cell destruction and acute insulin deficiency; prednisone worsens the inflammation but is not the primary cause; ETI should be held until amylase and lipase normalize.
D) Systemic corticosteroids lower blood glucose in CFRD patients by increasing peripheral insulin sensitivity through glucocorticoid receptor-mediated upregulation of GLUT4 transporters in muscle and adipose tissue; the glucose readings of 280–340 mg/dL are spuriously elevated due to the point-of-care glucometer's cross-reactivity with prednisone metabolites and should be confirmed by laboratory serum glucose before adjusting insulin.
E) Systemic corticosteroids — particularly prednisone — produce dose-dependent insulin resistance and impair hepatic glucose output suppression, causing steroid-induced hyperglycemia that is superimposed on the underlying insulin deficiency of CFRD; this worsening is expected and well-recognized in CFRD during corticosteroid courses, and the correct management is to increase insulin doses — typically by increasing both basal and prandial components and adding correction doses — while monitoring glucose closely throughout the steroid course and tapering insulin adjustments as prednisone is weaned.
ANSWER: E
Rationale:
Systemic corticosteroids, including prednisone, produce dose-dependent adverse effects on glucose metabolism through two primary mechanisms: they induce peripheral insulin resistance in skeletal muscle and adipose tissue by impairing post-receptor insulin signaling and reducing GLUT4 transporter translocation, and they stimulate hepatic glucose output (gluconeogenesis) while impairing the ability of insulin to suppress this output. These effects are superimposed on the underlying pathophysiology of CF-related diabetes (CFRD), which already involves relative insulin deficiency from progressive pancreatic islet cell destruction by exocrine fibrosis; the combination of impaired insulin secretory capacity and corticosteroid-driven insulin resistance produces particularly severe and treatment-refractory hyperglycemia. This pattern — steroid-induced hyperglycemia worsening CFRD glucose control — is a well-recognized clinical scenario in CF inpatient management. The correct management is to increase insulin doses to match the degree of corticosteroid-driven glucose elevation: for prednisone-induced hyperglycemia, increases in prandial (rapid-acting) insulin are often most effective since corticosteroids produce particularly pronounced postprandial peaks, while basal insulin also needs upward adjustment; correction (supplemental) doses should be available for hyperglycemic values. Glucose monitoring frequency should be increased during the steroid course (typically four times daily or more), and insulin doses should be tapered back as prednisone is weaned to avoid hypoglycemia.
Option A: Option A is incorrect because prednisone does not activate CFTR gene transcription through glucocorticoid response elements in a clinically meaningful way, and the mechanism of steroid-induced hyperglycemia is peripheral insulin resistance and hepatic glucose output enhancement, not hypersecretion of insulin causing paradoxical hyperglycemia.
Option B: Option B is incorrect because antibiotic-induced gut microbiome disruption producing short-chain fatty acid-mediated beta-cell inhibition is not the established mechanism of glucose worsening in this scenario; the corticosteroid-induced insulin resistance is the primary driver, and GLP-1 receptor agonists are not an appropriate inpatient management strategy for acute CFRD hyperglycemia during systemic steroid therapy.
Option C: Option C is incorrect because steroid-initiated pancreatitis from ETI-mediated CFTR rescue is not an established complication of ETI, and the hyperglycemia in this patient is clearly temporally related to prednisone initiation, not ETI; holding ETI during a pulmonary exacerbation in a patient who needs CFTR function maximized is clinically counterproductive.
Option D: Option D is incorrect because systemic corticosteroids produce hyperglycemia (not hypoglycemia) through insulin resistance and increased hepatic glucose output; glucocorticoids do not upregulate GLUT4 in muscle; and point-of-care glucometers do not cross-react with prednisone metabolites to produce spuriously elevated readings.
11. A 45-year-old woman with cystic fibrosis (CF) homozygous for F508del has been on lumacaftor-ivacaftor for 14 months. She also takes warfarin for a history of deep vein thrombosis (DVT) related to a prior central venous catheter. Her anticoagulation clinic pharmacist notes that her international normalized ratio (INR) has been consistently below the therapeutic range of 2.0 to 3.0 over the past year and that her warfarin dose has required progressive upward titration — from 5 mg/day at lumacaftor-ivacaftor initiation to 9 mg/day currently — to maintain a sub-therapeutic INR of 1.7. The pharmacist suspects a drug interaction. Which of the following best identifies the pharmacokinetic mechanism responsible for warfarin's apparent loss of efficacy and the correct management approach?
A) The warfarin dose requirement increase is caused by ivacaftor's competitive inhibition of the vitamin K-dependent carboxylase enzyme responsible for activating clotting factors II, VII, IX, and X; because ivacaftor blocks warfarin's downstream pharmacodynamic target rather than altering warfarin's plasma concentrations, increasing the warfarin dose cannot restore anticoagulation and the patient should be switched to a direct oral anticoagulant (DOAC) not affected by ivacaftor pharmacodynamics.
B) Warfarin's reduced efficacy reflects lumacaftor-ivacaftor-induced upregulation of vitamin K synthesis in intestinal epithelial cells through CFTR-mediated bicarbonate secretion; as CFTR function is partially restored, intraluminal bicarbonate activates vitamin K-dependent gamma-carboxylase at a higher rate, pharmacodynamically opposing warfarin's mechanism and requiring progressive dose escalation; switching to a DOAC is recommended.
C) The interaction is caused by ivacaftor's inhibition of cytochrome P450 isoform CYP2C9 (CYP2C9), the primary enzyme responsible for S-warfarin metabolism; paradoxically, CYP2C9 inhibition reduces warfarin metabolism and should raise, not lower, the INR; the sub-therapeutic INR suggests a non-pharmacokinetic cause such as dietary vitamin K excess and the warfarin dose escalation should be reversed while dietary assessment is performed.
D) Lumacaftor is a potent inducer of cytochrome P450 isoform CYP3A4 (CYP3A4) and also induces CYP2C9, the primary isoform responsible for the metabolism of the more pharmacologically active S-enantiomer of warfarin; lumacaftor-driven CYP2C9 induction accelerates S-warfarin metabolism and reduces its plasma concentrations, explaining the progressive increase in warfarin dose requirement to maintain anticoagulation; if lumacaftor-ivacaftor is continued, close INR monitoring with ongoing warfarin dose adjustment is required; transitioning to ETI or tezacaftor-ivacaftor would eliminate the interaction because tezacaftor and elexacaftor do not induce CYP2C9 or CYP3A4.
E) The sub-therapeutic INR despite escalating warfarin doses is caused by lumacaftor-ivacaftor improving CF-related fat malabsorption, increasing intestinal absorption of dietary vitamin K from fat-soluble food sources; the higher vitamin K intake pharmacodynamically reduces warfarin's anticoagulant effect by providing more substrate for clotting factor gamma-carboxylation; dietary vitamin K restriction is the correct management rather than continued warfarin dose escalation.
ANSWER: D
Rationale:
Warfarin's apparent loss of efficacy in this patient — manifesting as a progressively rising warfarin dose requirement to maintain a sub-therapeutic INR — is explained by lumacaftor's broad cytochrome P450 enzyme induction profile. Lumacaftor is established as a strong inducer of CYP3A4 and also induces CYP2C9, the principal isoform responsible for metabolism of S-warfarin — the pharmacologically active enantiomer that accounts for the majority of warfarin's anticoagulant effect. CYP2C9 induction by lumacaftor accelerates the hepatic metabolism of S-warfarin, reducing its plasma concentrations and thereby reducing its anticoagulant effect, which manifests as a falling INR requiring progressive warfarin dose escalation to maintain therapeutic anticoagulation. This interaction is not limited to drugs metabolized by CYP3A4 alone; lumacaftor's induction extends to CYP2C9 and potentially other cytochrome P450 isoforms, creating a broad drug interaction burden for patients on multiple medications. Clinical management requires close INR monitoring and continued warfarin dose titration for as long as lumacaftor-ivacaftor is continued. Importantly, transitioning this patient to tezacaftor-ivacaftor or ETI would eliminate the interaction, since neither tezacaftor nor elexacaftor induces CYP2C9 or CYP3A4, allowing the warfarin dose to be titrated back down to its original pre-lumacaftor requirement.
Option A: Option A is incorrect because ivacaftor does not competitively inhibit vitamin K-dependent carboxylase; ivacaftor is a CFTR potentiator and has no established pharmacodynamic interaction with the coagulation factor activation pathway; the option confuses the mechanism entirely.
Option B: Option B is incorrect because lumacaftor-ivacaftor does not restore CFTR-mediated bicarbonate secretion in a way that upregulates intestinal vitamin K synthesis by gamma-carboxylase activation; vitamin K gamma-carboxylase activity is not regulated by intestinal luminal bicarbonate concentrations in the described manner.
Option C: Option C is incorrect because ivacaftor does not inhibit CYP2C9; the pharmacokinetic interaction is driven by lumacaftor's induction of CYP2C9 (which reduces warfarin levels), not ivacaftor's inhibition (which would raise them and increase INR); the option also reverses the direction of the interaction.
Option E: Option E is incorrect because while improved fat absorption from enhanced CFTR function could theoretically increase vitamin K bioavailability, this is not the established primary mechanism for the degree of warfarin resistance observed in clinical practice with lumacaftor-ivacaftor; the dominant pharmacokinetic driver is lumacaftor-induced CYP2C9 induction, and dietary restriction alone would not adequately manage the pharmacokinetic interaction.
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