Pharmacokinetics and Biotransformation

Correct Answer

B — Rifampin

Rationale

Rifampin is classified as one of the most potent cytochrome P450 enzyme inducers in clinical use, particularly inducing cytochrome P450 3A4 and cytochrome P450 2C9. This induction can reduce plasma concentrations of sensitive substrates by 80 to 90 percent, leading to treatment failure for co-administered drugs including cyclosporine, warfarin, and oral contraceptives. Fluconazole and voriconazole are azole antifungals classified as cytochrome P450 inhibitors. Erythromycin is a macrolide antibiotic classified as a cytochrome P450 3A4 inhibitor.

Correct Answer

D — Fluconazole

Rationale

Fluconazole is classified as a potent inhibitor of cytochrome P450 2C9 and also inhibits cytochrome P450 3A4. Its inhibition of cytochrome P450 2C9 reduces metabolism of the active S-enantiomer of warfarin, substantially raising warfarin plasma concentrations and international normalized ratio. Terbinafine inhibits cytochrome P450 2D6 but has minimal effect on cytochrome P450 2C9. Amphotericin B and nystatin act via membrane disruption and do not inhibit cytochrome P450 enzymes in a clinically relevant way.

Correct Answer

A — Codeine

Rationale

Codeine is classified as a prodrug whose analgesic effect depends on cytochrome P450 2D6-mediated conversion to morphine. Patients who are poor metabolizers of cytochrome P450 2D6 receive no meaningful analgesia from codeine because little morphine is generated. Ultra-rapid metabolizers convert codeine to morphine at an accelerated rate, risking morphine toxicity. Morphine, oxycodone, and hydromorphone are pharmacologically active as administered and do not require cytochrome P450 2D6 activation for their primary analgesic effect.

Correct Answer

C — Isoniazid

Rationale

Isoniazid is classified as an N-acetyltransferase 2 substrate whose pharmacokinetics differ substantially between slow and rapid acetylator phenotypes. Slow acetylators accumulate higher isoniazid concentrations, increasing the risk of peripheral neuropathy and hepatotoxicity. Rapid acetylators clear isoniazid faster and may have reduced drug exposure with intermittent dosing regimens. Warfarin is metabolized primarily by cytochrome P450 2C9. Phenytoin is metabolized by cytochrome P450 2C9 and cytochrome P450 2C19. Metformin is not metabolized by hepatic enzymes and is excreted unchanged by the kidneys.

Correct Answer

B — Grapefruit juice

Rationale

Grapefruit juice contains furanocoumarins that are classified as mechanism-based (suicide) inhibitors of cytochrome P450 3A4 in intestinal enterocytes. The inhibitor is metabolized by the enzyme to a reactive intermediate that covalently inactivates the enzyme irreversibly. Because the enzyme is destroyed rather than merely blocked, recovery requires synthesis of new enzyme protein, so the inhibitory effect persists for 24 to 72 hours after a single exposure to grapefruit juice. Cimetidine is a reversible inhibitor of multiple cytochrome P450 isoforms. Omeprazole inhibits cytochrome P450 2C19 by competitive mechanisms. Amiodarone is a reversible inhibitor of cytochrome P450 2C9 and cytochrome P450 3A4.

Correct Answer

D — Clopidogrel

Rationale

Clopidogrel is classified as a prodrug requiring a two-step hepatic activation, with the second and rate-limiting step catalyzed by cytochrome P450 2C19. The active thienopyridine metabolite irreversibly blocks the platelet adenosine diphosphate receptor, preventing platelet aggregation. Poor metabolizers of cytochrome P450 2C19 generate inadequate amounts of active metabolite and have higher rates of cardiovascular events after coronary stent placement. Aspirin acts directly as administered. Ticagrelor is an active drug that does not require metabolic activation. Prasugrel is also a prodrug but is activated primarily by cytochrome P450 3A4 and cytochrome P450 2B6, not cytochrome P450 2C19, and is less affected by cytochrome P450 2C19 polymorphisms.

Correct Answer

A — N-acetyl-para-benzoquinone imine, formed by cytochrome P450 2E1

Rationale

At therapeutic doses, acetaminophen is metabolized primarily by glucuronidation and sulfation to non-toxic conjugates. A small fraction is oxidized by cytochrome P450 2E1 (and to a lesser extent cytochrome P450 3A4 and 1A2) to N-acetyl-para-benzoquinone imine, a highly reactive electrophile. Under normal conditions, N-acetyl-para-benzoquinone imine is immediately conjugated with hepatic glutathione and excreted harmlessly. In overdose, glutathione stores are exhausted and N-acetyl-para-benzoquinone imine accumulates, covalently binding hepatocyte proteins and causing centrilobular necrosis. The antidote N-acetylcysteine replenishes glutathione precursors, allowing continued conjugation of N-acetyl-para-benzoquinone imine. Glucuronide and sulfate conjugates are non-toxic elimination products, not toxic intermediates.

Correct Answer

C — Higher plasma concentrations, because portosystemic shunting bypasses hepatic first-pass metabolism

Rationale

Propranolol is a high-extraction drug whose clearance is flow-limited — the liver is so efficient at metabolizing propranolol that nearly all drug delivered to it is removed on the first pass. In cirrhosis with significant portosystemic shunting, blood from the portal circulation bypasses the liver and enters the systemic circulation directly through collateral vessels. Drug absorbed from the gastrointestinal tract therefore reaches the systemic circulation without undergoing hepatic first-pass metabolism, dramatically increasing oral bioavailability. The same oral dose that produces a modest plasma concentration in a patient with normal liver function can produce a much higher and potentially toxic concentration in a patient with advanced cirrhosis and portosystemic shunting. Dose reduction is required for high-extraction drugs in this setting.

Correct Answer

B — Warfarin effect decreases gradually over one to three weeks as new cytochrome P450 enzyme protein is synthesized

Rationale

Rifampin induces cytochrome P450 2C9 and cytochrome P450 3A4 by increasing transcription of the corresponding genes, requiring de novo synthesis of new enzyme protein. Because this is a transcriptional mechanism, the full inductive effect develops gradually over one to three weeks of continuous rifampin exposure. The warfarin dose may need to be increased substantially — sometimes two to fivefold — to maintain therapeutic anticoagulation during rifampin therapy. Equally important, when rifampin is stopped, the extra enzyme protein is degraded gradually over one to three weeks, and warfarin clearance returns to baseline. Failing to reduce the warfarin dose after rifampin discontinuation can cause serious bleeding. Enzyme induction is not immediate and rifampin does not inhibit cytochrome P450 enzymes or displace warfarin from albumin.

Correct Answer

D — Poor metabolizers cannot convert codeine to morphine, so the active analgesic form is not generated

Rationale

Codeine is a prodrug with minimal intrinsic analgesic activity. Its opioid effect depends almost entirely on cytochrome P450 2D6-mediated conversion to morphine. In poor metabolizers of cytochrome P450 2D6 — who have two non-functional alleles and no functional enzyme activity — this conversion does not occur. The patient receives codeine's side effects, such as constipation and nausea, without meaningful analgesia. This is why poor metabolizers of cytochrome P450 2D6 are sometimes labeled as apparent opioid non-responders when given codeine. The appropriate response is to switch to an opioid that does not require cytochrome P450 2D6 activation, such as morphine or oxycodone. Poor metabolizer status does not affect opioid receptor sensitivity or codeine clearance rate.

Correct Answer

A — Glucuronidation by uridine diphosphate glucuronosyltransferase

Rationale

Chloramphenicol is eliminated primarily by hepatic glucuronidation. Uridine diphosphate glucuronosyltransferase enzyme expression is markedly reduced at birth and does not reach adult levels for several weeks to months. When chloramphenicol was administered to neonates at doses scaled by body weight from adult regimens, the immature glucuronidation system could not eliminate the drug adequately. Chloramphenicol accumulated to toxic concentrations, causing cardiovascular collapse, abdominal distension, cyanosis, and ashen skin — the gray baby syndrome. Cytochrome P450 3A4 is a phase one oxidative enzyme that is also immature at birth but is not the pathway responsible for chloramphenicol elimination. Acetylation and sulfation play minor roles in chloramphenicol metabolism and were not the cause of this toxicity.

Correct Answer

C — For low-extraction drugs, enzyme activity is the rate-limiting step, so reducing enzyme activity directly and proportionally reduces clearance

Rationale

For low-extraction (capacity-limited) drugs, the rate of elimination is determined by enzyme activity and the concentration of free drug — not by hepatic blood flow. The enzymes are operating far below their maximum capacity, so they are the bottleneck. When an enzyme inhibitor reduces enzyme activity by 50 percent, clearance of a low-extraction drug also falls by approximately 50 percent, and steady-state plasma concentration approximately doubles. For high-extraction (flow-limited) drugs, by contrast, enzyme activity is already far in excess of what is needed to metabolize the drug presented to it; reducing enzyme activity modestly has little effect because the system has a large reserve capacity. This is why enzyme inhibitor interactions are clinically most dangerous for low-extraction, narrow-therapeutic-index drugs such as warfarin, phenytoin, and cyclosporine.

Correct Answer

B — Reduced platelet inhibition and increased risk of stent thrombosis

Rationale

Clopidogrel requires cytochrome P450 2C19-mediated hepatic activation to generate its active thienopyridine metabolite, which irreversibly inhibits platelet adenosine diphosphate receptors. Poor metabolizers of cytochrome P450 2C19 generate inadequate amounts of the active metabolite, resulting in substantially less platelet inhibition than extensive metabolizers receiving the same clopidogrel dose. Clinical studies have shown that cytochrome P450 2C19 poor metabolizers have higher rates of major adverse cardiovascular events, including stent thrombosis, after percutaneous coronary intervention. The United States Food and Drug Administration issued a black box warning for clopidogrel noting reduced effectiveness in poor metabolizers. Alternative antiplatelet agents not requiring cytochrome P450 2C19 activation — such as prasugrel or ticagrelor — should be considered in high-risk patients.

Correct Answer

D — Fluoxetine inhibits cytochrome P450 2D6, reducing formation of endoxifen, the most potent active metabolite of tamoxifen

Rationale

Tamoxifen is metabolized to endoxifen, which has approximately 100-fold greater potency at the estrogen receptor than tamoxifen itself and is responsible for the majority of tamoxifen's clinical efficacy in breast cancer. Endoxifen formation requires cytochrome P450 2D6 activity. Fluoxetine is a potent inhibitor of cytochrome P450 2D6, and co-administration effectively converts an extensive metabolizer to a functionally poor metabolizer phenotype. This substantially reduces endoxifen plasma concentrations and may compromise the effectiveness of adjuvant tamoxifen therapy. Antidepressants with weaker cytochrome P450 2D6 inhibitory activity — such as venlafaxine, desvenlafaxine, or sertraline — are preferred when antidepressant therapy is needed in patients on tamoxifen.

Correct Answer

C — Rifampin induces cytochrome P450 3A4, dramatically accelerating cyclosporine metabolism and reducing its plasma concentration

Rationale

Cyclosporine is a cytochrome P450 3A4 substrate with a narrow therapeutic index. Rifampin is the most potent cytochrome P450 3A4 inducer in clinical use, capable of reducing plasma concentrations of sensitive substrates by 80 to 90 percent through increased enzyme synthesis. The three-week time course reflects the gradual buildup of new cytochrome P450 3A4 enzyme protein — consistent with enzyme induction rather than inhibition, which would act within hours. The resulting subtherapeutic cyclosporine concentration allowed immune-mediated rejection to develop. This combination is generally contraindicated in transplant recipients; if rifampin must be used, cyclosporine doses may need to increase several-fold with intensive monitoring, then be reduced again after rifampin is stopped as enzyme levels normalize over weeks.

Correct Answer

A — The mother converted codeine to morphine at an accelerated rate, producing high morphine concentrations in breast milk that the infant ingested

Rationale

Ultra-rapid metabolizers of cytochrome P450 2D6 carry gene duplications that produce multiple copies of functional enzyme, converting codeine to morphine at a rate far exceeding that of extensive metabolizers. The mother's plasma and breast milk morphine concentrations were therefore much higher than expected from a standard codeine dose. The breastfeeding infant ingested morphine through breast milk, developing life-threatening opioid toxicity. This mechanism led to documented infant deaths and prompted the United States Food and Drug Administration to issue a black box warning against codeine use by breastfeeding mothers. The warning applies regardless of maternal cytochrome P450 2D6 genotype, since genotype is often unknown. Alternative analgesics such as acetaminophen or ibuprofen should be used instead.

Correct Answer

B — Fluconazole inhibits cytochrome P450 2C9, reducing metabolism of the active S-enantiomer of warfarin and raising its plasma concentration

Rationale

The active anticoagulant form of warfarin is the S-enantiomer, which is metabolized primarily by cytochrome P450 2C9. Fluconazole is a potent inhibitor of cytochrome P450 2C9 and also inhibits cytochrome P450 3A4. By reducing cytochrome P450 2C9 activity, fluconazole substantially decreases the clearance of S-warfarin, raising its plasma concentration and producing a marked increase in anticoagulant effect within days of starting treatment. This is one of the most clinically important and well-documented drug interactions involving warfarin. When fluconazole is prescribed to patients on warfarin, the international normalized ratio should be monitored closely and the warfarin dose reduced preemptively. The interaction resolves as fluconazole is eliminated after the course is completed.

Correct Answer

D — Pantoprazole, because it has weaker cytochrome P450 2C19 inhibitory activity and less potential to reduce clopidogrel activation

Rationale

Clopidogrel requires cytochrome P450 2C19 for hepatic activation. Omeprazole and esomeprazole are moderate inhibitors of cytochrome P450 2C19 and can reduce the formation of clopidogrel's active metabolite, potentially diminishing antiplatelet efficacy. Pantoprazole has substantially less inhibitory effect on cytochrome P450 2C19 compared with omeprazole and esomeprazole, making it the preferred proton pump inhibitor when co-administration with clopidogrel is necessary. Although the clinical magnitude of the omeprazole-clopidogrel interaction remains debated, regulatory guidance and clinical practice guidelines recommend avoiding omeprazole and esomeprazole with clopidogrel when possible and choosing pantoprazole instead. All proton pump inhibitors provide effective acid suppression; the selection here is driven by the pharmacokinetic interaction profile, not by comparative acid suppression potency.