Pulmonary Pharmacology

Correct Answer

B — Theophylline

Rationale

Theophylline is classified as a methylxanthine, a drug class that includes caffeine and theobromine. Methylxanthines act primarily by inhibiting phosphodiesterase enzymes and antagonizing adenosine receptors. Montelukast is a leukotriene receptor antagonist. Zileuton is a 5-lipoxygenase inhibitor. Cromolyn sodium is a mast cell stabilizer.

Correct Answer

D — A theophylline-ethylenediamine salt that serves as the intravenous prodrug form of theophylline

Rationale

Aminophylline is classified as the intravenous prodrug form of theophylline — specifically a theophylline-ethylenediamine salt that is approximately 80% theophylline by weight. Doses are calculated on this basis to achieve target theophylline concentrations. Aminophylline is not an inhaled agent and not a phosphodiesterase 4 selective inhibitor; theophylline inhibits both phosphodiesterase 3 and phosphodiesterase 4 non-selectively. Leukotriene receptor antagonists such as montelukast are oral agents, not intravenous.

Correct Answer

A — Montelukast

Rationale

Montelukast is classified as a selective competitive antagonist at the cysteinyl leukotriene type 1 (CysLT1) receptor, blocking the bronchoconstrictor and pro-inflammatory effects of leukotriene C4, D4, and E4. Zafirlukast shares this classification. Zileuton is classified differently — as a 5-lipoxygenase inhibitor that prevents leukotriene synthesis rather than blocking the receptor. Theophylline is a methylxanthine. Cromolyn sodium is a mast cell stabilizer.

Correct Answer

C — Zileuton

Rationale

Zileuton is classified as a 5-lipoxygenase inhibitor. It prevents leukotriene synthesis by blocking the enzyme that converts arachidonic acid to leukotriene A4, the first committed step in the leukotriene biosynthetic pathway. Montelukast and zafirlukast are leukotriene receptor antagonists — they block the CysLT1 receptor rather than inhibiting synthesis. This mechanistic distinction places zileuton in a separate pharmacological class from the leukotriene receptor antagonists. Cromolyn sodium is a mast cell stabilizer with no direct effect on the lipoxygenase pathway.

Correct Answer

B — Zafirlukast

Rationale

Zafirlukast is classified as a leukotriene receptor antagonist with significant cytochrome P450 2C9 inhibitory activity, which distinguishes it from montelukast within the same drug class. This interaction profile means zafirlukast can raise plasma levels of CYP2C9 substrates such as warfarin, requiring dose monitoring. Montelukast has a cleaner drug interaction profile and is generally preferred in clinical practice, particularly in pediatric patients. Zileuton is a 5-lipoxygenase inhibitor — not a leukotriene receptor antagonist — and its primary interaction concern is CYP1A2 inhibition rather than CYP2C9. Cromolyn sodium is a mast cell stabilizer without significant cytochrome P450 interactions.

Correct Answer

A — Cromolyn sodium

Rationale

Cromolyn sodium is classified as a mast cell stabilizer. It inhibits degranulation of sensitized mast cells in response to allergen challenge, preventing release of histamine, leukotrienes, and other inflammatory mediators. Nedocromil shares this classification. Cromolyn is used prophylactically rather than for acute relief and has largely been supplanted by inhaled corticosteroids for controller therapy, though it retains a role in exercise-induced bronchoconstriction prevention. Zafirlukast is a leukotriene receptor antagonist. Theophylline is a methylxanthine. Zileuton is a 5-lipoxygenase inhibitor.

Correct Answer

C — 10 to 20 mcg/mL; below 10 mcg/mL produces limited bronchodilatory efficacy, above 20 mcg/mL toxicity becomes likely

Rationale

The theophylline therapeutic window is 10 to 20 micrograms per milliliter. Below 10 micrograms per milliliter, bronchodilatory efficacy is limited. Above 20 micrograms per milliliter, toxicity becomes likely, with early manifestations including nausea and vomiting, progressing to seizures and cardiac arrhythmias at severely elevated concentrations. The margin between therapeutic and toxic concentrations is narrow enough that therapeutic drug monitoring is mandatory for any patient receiving scheduled theophylline therapy. Small changes in clearance — from drug interactions, illness, or smoking status — can shift concentrations from therapeutic to toxic without a dose change.

Correct Answer

A — Theophylline follows Michaelis-Menten kinetics; at therapeutic concentrations the metabolizing enzymes are near saturation, so small dose increases produce disproportionately large concentration increases

Rationale

Theophylline exhibits Michaelis-Menten (saturable) kinetics at therapeutic plasma concentrations. When the hepatic enzymes responsible for theophylline metabolism are operating near their maximum capacity, the relationship between dose and plasma concentration becomes nonlinear: a proportionally modest dose increase produces a disproportionately large rise in plasma concentration, because the additional drug cannot be metabolized at a proportionally faster rate. This nonlinear pharmacokinetics is the fundamental pharmacokinetic basis for theophylline's danger at therapeutic concentrations and explains why a dose that is safe one day can become toxic with a seemingly small adjustment or a change in clearance factors.

Correct Answer

D — Polycyclic aromatic hydrocarbons in cigarette smoke induce CYP1A2, accelerating hepatic theophylline metabolism and lowering plasma levels

Rationale

Polycyclic aromatic hydrocarbons in cigarette smoke are potent inducers of the hepatic enzyme CYP1A2, which is the primary enzyme responsible for theophylline metabolism. Induction of CYP1A2 increases the rate of theophylline clearance, lowering steady-state plasma concentrations and requiring higher doses to maintain therapeutic levels in smokers. When a patient on theophylline quits smoking, CYP1A2 induction reverses over days to weeks, clearance falls, and theophylline levels rise — sometimes into the toxic range — without any change in dose. This is one of the most clinically consequential and commonly tested drug-environment interactions for theophylline, and prompt re-measurement of theophylline levels is required whenever a patient's smoking status changes.

Correct Answer

B — Theophylline seizures are notoriously refractory to standard anticonvulsant therapy, and chronic toxicity can produce seizures at lower concentrations than acute overdose

Rationale

Two features distinguish theophylline-induced seizures from most other toxic seizures. First, they are notoriously refractory to standard anticonvulsant therapy — benzodiazepines may provide partial control but seizures often persist, carrying high morbidity. Severe cases may require barbiturate coma or hemodialysis. Second, patients with chronic theophylline toxicity from slow accumulation (for example, due to a drug interaction reducing clearance) may develop seizures at plasma concentrations that would not cause seizures in acute overdose, because prolonged central nervous system exposure leads to sensitization. This means a patient whose level is 25 mcg/mL from chronic accumulation may be at greater seizure risk than an acute overdose patient at the same measured level.

Correct Answer

C — Leukotriene D4, acting at the CysLT1 receptor on airway smooth muscle

Rationale

Leukotriene D4 is the most potent bronchoconstrictor among the cysteinyl leukotrienes, acting at CysLT1 receptors on airway smooth muscle to produce bronchoconstriction approximately 100 to 1000 times more potently than histamine on a molar basis. The cysteinyl leukotrienes — C4, D4, and E4 — are produced by sequential peptide cleavage from leukotriene C4, which is itself formed by conjugation of leukotriene A4 with glutathione. Leukotriene A4 is the unstable epoxide intermediate produced directly by 5-lipoxygenase; it has no direct bronchoconstrictor activity at CysLT1 receptors. Leukotriene E4 is the least potent of the three cysteinyl leukotrienes. The BLT1 receptor is a leukotriene B4 receptor involved in neutrophil chemotaxis, not bronchoconstriction.

Correct Answer

A — Serious neuropsychiatric effects including agitation, depression, and suicidal ideation, of particular concern in pediatric patients

Rationale

Montelukast carries a black box warning for serious neuropsychiatric adverse effects including agitation, anxiety, depression, suicidal ideation, and completed suicides, added by the FDA in 2020 based on post-marketing surveillance data. This warning applies to all age groups but is of particular concern in pediatric patients, where montelukast is commonly prescribed for allergic rhinitis and mild asthma. Prescribers are advised to weigh the benefits against these risks and to counsel patients and caregivers about neuropsychiatric symptoms requiring prompt discontinuation. Hepatotoxicity requiring monitoring is the primary concern for zileuton, not montelukast. Anaphylaxis with supervised administration applies to omalizumab. Cardiac arrhythmias are not a class effect of leukotriene receptor antagonists.

Correct Answer

D — Cyclooxygenase inhibition diverts arachidonic acid away from prostaglandin synthesis toward the 5-lipoxygenase pathway, producing excess cysteinyl leukotrienes that cause bronchospasm

Rationale

Arachidonic acid is metabolized through two competing pathways: the cyclooxygenase pathway produces prostaglandins and thromboxanes, while the 5-lipoxygenase pathway produces leukotrienes. In patients with aspirin-exacerbated respiratory disease, cyclooxygenase inhibition by aspirin or non-steroidal anti-inflammatory drugs blocks the prostaglandin pathway, shunting accumulated arachidonic acid toward 5-lipoxygenase. This produces a surge of cysteinyl leukotrienes — particularly leukotriene D4, the most potent bronchoconstrictor in the class — triggering bronchospasm. Because the mechanism is leukotriene overproduction rather than IgE-mediated allergy, leukotriene receptor antagonists and 5-lipoxygenase inhibitors are the pharmacological mainstay of management in aspirin-exacerbated respiratory disease.

Correct Answer

B — Hepatotoxicity requiring liver function monitoring, and CYP1A2 inhibition raising plasma levels of theophylline and warfarin

Rationale

Zileuton's two most important safety considerations are hepatotoxicity and cytochrome P450 drug interactions. Elevations in liver enzymes occur with zileuton therapy, and liver function testing is recommended before starting treatment and periodically thereafter. Zileuton is also a CYP1A2 inhibitor, which raises plasma concentrations of CYP1A2 substrates — theophylline and warfarin chief among them — potentially causing theophylline toxicity or excessive anticoagulation. Neuropsychiatric effects and the black box warning are associated with montelukast, not zileuton. CYP2C9 inhibition is the concern with zafirlukast. Anaphylaxis with supervised administration applies to omalizumab. Adenosine receptor antagonism and CYP1A2 induction are properties of theophylline, not zileuton.

Correct Answer

C — Ciprofloxacin inhibits CYP1A2, reducing theophylline metabolism and causing plasma levels to rise into the toxic range

Rationale

Ciprofloxacin and other fluoroquinolones are CYP1A2 inhibitors. Because CYP1A2 is the principal enzyme responsible for theophylline metabolism, ciprofloxacin reduces theophylline clearance, causing plasma concentrations to rise. Given theophylline's narrow therapeutic window and nonlinear Michaelis-Menten kinetics, even a modest reduction in clearance can shift levels from therapeutic into the toxic range. The resulting nausea, vomiting, and tremor are early manifestations of theophylline toxicity. This interaction is predictable and well-documented; theophylline levels should be checked promptly when any CYP1A2 inhibitor is added to a patient's regimen.

Correct Answer

A — Leukotriene receptor antagonists, which block the CysLT1 receptor activated by the excess cysteinyl leukotrienes produced when cyclooxygenase is inhibited

Rationale

Aspirin-exacerbated respiratory disease is driven by excess cysteinyl leukotriene production: cyclooxygenase inhibition diverts arachidonic acid toward the 5-lipoxygenase pathway, generating a surge of leukotriene C4, D4, and E4 that activates CysLT1 receptors on airway smooth muscle and produces bronchospasm. Leukotriene receptor antagonists — primarily montelukast — are a mainstay of ongoing management because they block CysLT1 receptors, directly interrupting the downstream effector mechanism. The reaction is not IgE-mediated, so antihistamines and mast cell stabilizers do not address the primary mechanism. Anticholinergics reduce bronchoconstriction through a separate pathway but do not target the excess leukotriene production that characterizes this condition.

Correct Answer

D — Loss of CYP1A2 induction from cigarette smoke reduced theophylline clearance, causing plasma levels to accumulate to toxic concentrations

Rationale

Polycyclic aromatic hydrocarbons in cigarette smoke induce CYP1A2, the principal enzyme for theophylline metabolism. Active smokers therefore clear theophylline faster and require higher doses to maintain therapeutic levels. When a patient quits smoking, CYP1A2 induction gradually reverses over days to weeks as the inducing stimulus is removed. As enzyme activity falls back to baseline, theophylline clearance decreases and plasma levels rise — sometimes into the toxic range — without any change in dose. This is a predictable and common precipitant of theophylline toxicity, and plasma levels should be measured and the dose reduced proactively when a patient on theophylline stops smoking.

Correct Answer

B — These symptoms are consistent with the FDA black box warning for montelukast, which covers serious neuropsychiatric effects including agitation, sleep disturbances, and behavioral changes

Rationale

Montelukast carries an FDA black box warning for serious neuropsychiatric adverse effects, added in 2020 based on post-marketing data. The warning encompasses agitation, aggression, depression, sleep disturbances including nightmares, suicidal ideation, and completed suicides. It applies to all ages but is of particular concern in pediatric patients, where montelukast is commonly prescribed for mild asthma and allergic rhinitis. The temporal relationship between starting montelukast two weeks prior and the onset of behavioral symptoms in an otherwise well-controlled child is the key clinical signal. Discontinuing montelukast is appropriate when neuropsychiatric symptoms emerge, and alternative controller or antihistamine strategies should be considered. Hepatotoxicity is an adverse effect of zileuton, not montelukast.