BCR-ABL, EGFR, and ALK/ROS1 Kinase Inhibitors

Correct Answer

B) Imatinib

Rationale

Imatinib is the prototype BCR-ABL tyrosine kinase inhibitor, the first targeted agent developed for chronic myeloid leukemia. Rituximab is an anti-CD20 monoclonal antibody, erlotinib is an EGFR (epidermal growth factor receptor) tyrosine kinase inhibitor, and bortezomib is a proteasome inhibitor. Class membership is sufficient to answer this question.

Correct Answer

C) Asciminib

Rationale

Asciminib is classified as a STAMP inhibitor, an acronym for specifically targeting the ABL myristoyl pocket. It binds the myristoyl pocket of BCR-ABL rather than the adenosine triphosphate-binding site used by all other BCR-ABL inhibitors. Dasatinib, nilotinib, and ponatinib all bind at the adenosine triphosphate-binding site and are not STAMP inhibitors.

Correct Answer

A) Dasatinib

Rationale

Dasatinib is a second-generation BCR-ABL tyrosine kinase inhibitor, developed after imatinib to overcome imatinib resistance mutations. Imatinib is the first-generation agent. Ponatinib is third-generation, designed to cover the T315I gatekeeper mutation. Asciminib is a STAMP inhibitor, classified separately from generation-based agents.

Correct Answer

D) Osimertinib

Rationale

Osimertinib is a third-generation EGFR (epidermal growth factor receptor) tyrosine kinase inhibitor developed to target the T790M resistance mutation that emerges after first- and second-generation EGFR inhibitors. Gefitinib and erlotinib are first-generation reversible EGFR inhibitors. Afatinib is a second-generation irreversible pan-HER inhibitor.

Correct Answer

B) Crizotinib

Rationale

Crizotinib is the first-generation ALK inhibitor and is classified as an ALK/MET/ROS1 inhibitor, reflecting its activity against all three kinase targets. Alectinib is a second-generation ALK inhibitor with more selective ALK activity. Osimertinib is a third-generation EGFR inhibitor. Lorlatinib is a third-generation ALK inhibitor. Only crizotinib carries the explicit multi-target class label of ALK/MET/ROS1 inhibitor.

Correct Answer

A) Lorlatinib

Rationale

Lorlatinib is the third-generation ALK tyrosine kinase inhibitor, designed to overcome resistance to second-generation agents and to provide the broadest ALK resistance mutation coverage of the class. Crizotinib is the first-generation ALK/MET/ROS1 inhibitor. Brigatinib and ceritinib are second-generation ALK inhibitors.

Correct Answer

C) Competitive inhibition of the adenosine triphosphate-binding site when BCR-ABL is in its inactive conformation

Rationale

Imatinib binds competitively at the adenosine triphosphate-binding site of BCR-ABL and requires the kinase to be in its inactive conformation for binding. This conformational selectivity is central to the imatinib mechanism. Dasatinib, by contrast, binds both active and inactive conformations. Covalent modification of the adenosine triphosphate-binding site describes third-generation covalent inhibitors such as osimertinib. Binding to the myristoyl pocket is the mechanism of asciminib, the STAMP inhibitor. Irreversible binding to both conformations is a property of dasatinib, not imatinib.

Correct Answer

A) The T315I mutation eliminates a hydrogen bond required for drug binding at the adenosine triphosphate site and creates steric hindrance that prevents inhibitor access

Rationale

The T315I mutation substitutes isoleucine for threonine at position 315 in the BCR-ABL kinase domain. This eliminates a hydrogen bond that is critical for imatinib and second-generation inhibitor binding at the adenosine triphosphate site, and the bulkier isoleucine side chain creates steric hindrance that blocks inhibitor access. This structural disruption confers resistance to all first- and second-generation agents. Ponatinib and high-dose asciminib overcome this barrier through different binding geometries. Alternative pathway activation that bypasses BCR-ABL, prevention of proteasomal degradation, and constitutive conformational change are each distinct from the T315I mechanism.

Correct Answer

D) Pleural effusion is a recognized adverse effect of dasatinib, occurring through a mechanism not fully understood; it requires dose interruption in severe cases

Rationale

Pleural effusion is a well-recognized adverse effect of dasatinib occurring in a significant proportion of patients. The precise mechanism is not fully established, though SRC kinase inhibition and effects on vascular permeability have been proposed. Clinically, it requires dose interruption and sometimes corticosteroids in severe cases. PDGFR inhibition causing serosal fluid accumulation describes imatinib's edema mechanism, not dasatinib's specific pleural effusion. QTc prolongation causing cardiogenic effusion describes a nilotinib concern and is not the mechanism of dasatinib pleural effusion. Disrupted lymphatic integrity from SRC inhibition is a proposed hypothesis but not established as the confirmed mechanism, making the factual description in option D the most accurate statement.

Correct Answer

B) Nilotinib prolongs the QTc interval, and a baseline electrocardiogram is required to identify patients with pre-existing QTc prolongation who are at increased risk of life-threatening arrhythmia

Rationale

Nilotinib prolongs the QTc interval, which creates risk for torsades de pointes and other ventricular arrhythmias, particularly in patients who already have a prolonged QTc at baseline or in the presence of electrolyte abnormalities such as hypokalemia or hypomagnesemia. A baseline electrocardiogram is required before starting nilotinib to identify pre-existing QTc prolongation that would constitute a contraindication or require correction before starting therapy. KIT receptor inhibition describes imatinib's mechanism relevant to gastrointestinal stromal tumors. Funny current inhibition describes ivabradine. Na+/K+-ATPase inhibition describes the cardiac glycoside mechanism of digoxin.

Correct Answer

C) EGFR (epidermal growth factor receptor) is expressed in normal skin epithelium and sebaceous glands; inhibition of EGFR in these tissues disrupts normal epidermal homeostasis and produces the rash

Rationale

EGFR is physiologically expressed in normal skin epithelium, sebaceous glands, and hair follicles, where it maintains epidermal homeostasis. Inhibition of EGFR in these tissues by erlotinib (and all EGFR inhibitors) disrupts this normal signaling, producing the characteristic acneiform papulopustular rash. The rash appears in a seborrheic distribution and has a distinctive appearance that differs from acne vulgaris. Immune-mediated mast cell degranulation is the mechanism of allergic/anaphylactic reactions, not this drug's rash. Metabolite deposition in sebaceous glands is not the established mechanism. Activating keratinocyte proliferation by blocking a suppressor is not the mechanism; EGFR inhibition reduces epithelial growth factor signaling, it does not activate it.

Correct Answer

A) Osimertinib is an irreversible inhibitor that forms a covalent bond with mutant T790M-bearing EGFR and is designed with mutant selectivity that preferentially inhibits T790M EGFR over wild-type EGFR

Rationale

Osimertinib is an irreversible, mutant-selective EGFR inhibitor that covalently binds EGFR. Its design incorporates selectivity for T790M-bearing EGFR over wild-type EGFR, which accounts for its improved tolerability relative to earlier generations. The T790M mutation creates a bulkier methionine at position 790 that sterically excludes reversible first-generation inhibitors such as erlotinib but does not prevent osimertinib's covalent binding. Osimertinib is not a reversible competitive inhibitor with higher affinity — it is irreversible and mutant-selective. It inhibits the kinase directly, not downstream signaling proteins. It does not target protein degradation.

Correct Answer

D) Crizotinib is a substrate of P-glycoprotein, an efflux transporter at the blood-brain barrier, whereas alectinib is not a P-glycoprotein substrate and therefore is not pumped back out of the central nervous system

Rationale

Crizotinib is a substrate of P-glycoprotein (P-gp), an adenosine triphosphate-binding cassette efflux transporter expressed at the blood-brain barrier that actively pumps substrate drugs back into the bloodstream, limiting their central nervous system penetration. Alectinib is not a P-glycoprotein substrate, so it is not subject to this efflux mechanism and achieves substantially higher central nervous system drug concentrations. This pharmacokinetic difference explains alectinib's superiority in preventing and treating central nervous system metastases in ALK-positive lung cancer. Half-life differences, dose differences, and lipid solubility do not account for the central nervous system penetration advantage.

Correct Answer

B) Rifampin is a potent inducer of cytochrome P450 3A4, which is the primary enzyme responsible for imatinib metabolism; induction increases imatinib clearance and reduces imatinib plasma concentrations, risking therapeutic failure

Rationale

Imatinib is primarily metabolized by cytochrome P450 3A4. Rifampin is a potent inducer of cytochrome P450 3A4 — it increases the expression of this enzyme, accelerating imatinib metabolism and reducing imatinib plasma concentrations substantially. This interaction can result in subtherapeutic imatinib levels and risk of therapeutic failure. Rifampin is an inducer of cytochrome P450 3A4, not an inhibitor of P-glycoprotein, so absorption is not the mechanism. Rifampin has no direct pharmacodynamic interaction at the BCR-ABL adenosine triphosphate-binding site. Rifampin does not inhibit cytochrome P450 2C9; it induces multiple cytochrome P450 enzymes, and cytochrome P450 3A4 is the dominant pathway for imatinib metabolism.

Correct Answer

C) Ponatinib

Rationale

The T315I gatekeeper mutation confers resistance to all first- and second-generation BCR-ABL tyrosine kinase inhibitors, including imatinib, dasatinib, nilotinib, and bosutinib. Ponatinib is a third-generation pan-BCR-ABL inhibitor specifically designed to overcome the T315I mutation. Its binding geometry accommodates the bulkier isoleucine at position 315, allowing it to retain activity. Asciminib at high dose is the other option active against T315I, but it is not listed here. Nilotinib, bosutinib, and imatinib are all inactive against the T315I mutation.

Correct Answer

A) Osimertinib, an irreversible mutant-selective EGFR inhibitor that retains activity against T790M-bearing EGFR

Rationale

T790M is a gatekeeper resistance mutation that sterically prevents reversible EGFR inhibitors such as gefitinib and erlotinib from binding. Osimertinib is an irreversible, mutant-selective EGFR inhibitor specifically designed to covalently bind and inhibit T790M-bearing EGFR. It is the standard treatment for T790M-positive acquired resistance and has demonstrated superior outcomes compared to platinum-based chemotherapy in this setting. Afatinib is a second-generation irreversible pan-HER inhibitor but does not overcome T790M resistance — it was active before T790M emerged and is no longer effective once T790M is present. Increasing gefitinib dose does not overcome the steric block imposed by the T790M mutation; this is a structural incompatibility, not a concentration problem. Platinum-based chemotherapy forgoes the available targeted option and is reserved for patients without a targetable mutation or after exhaustion of targeted therapies.

Correct Answer

B) Alectinib, because it is not a substrate of P-glycoprotein at the blood-brain barrier, allowing it to achieve higher central nervous system concentrations than crizotinib

Rationale

Alectinib is preferred for central nervous system metastases in ALK-positive lung cancer because it is not a substrate of P-glycoprotein, the adenosine triphosphate-binding cassette efflux transporter expressed at the blood-brain barrier. Crizotinib is a P-glycoprotein substrate and is actively pumped back out of the central nervous system, limiting its brain penetration. Alectinib achieves central nervous system drug concentrations that are effective against brain metastases. The ALEX trial confirmed alectinib's superiority over crizotinib as first-line therapy, with a striking reduction in central nervous system progression. Ceritinib is an early second-generation agent with gastrointestinal tolerability issues and less favorable central nervous system data compared to alectinib. Early-onset pulmonary toxicity with brigatinib is a safety concern, not a rationale for tissue penetration.

Correct Answer

D) Discontinue fluconazole and hold nilotinib until the corrected QT interval returns below 480 milliseconds, then reassess

Rationale

Fluconazole is a potent inhibitor of cytochrome P450 3A4, the primary enzyme responsible for nilotinib metabolism. Inhibiting this pathway raises nilotinib plasma concentrations, increasing blockade of cardiac potassium channels and prolonging the corrected QT interval. A corrected QT interval above 500 milliseconds on nilotinib is a threshold requiring drug interruption — it places the patient at risk for torsades de pointes. The correct response is to discontinue the interacting drug (fluconazole) and hold nilotinib until the corrected QT interval falls below 480 milliseconds, then reassess whether nilotinib can be restarted. Continuing both medications ignores a clinically dangerous corrected QT value. A simple dose reduction does not address the ongoing pharmacokinetic interaction while fluconazole is present. Permanent class change to imatinib is not mandated by corrected QT prolongation that has a reversible pharmacokinetic cause; corrective management and monitoring are the appropriate first steps.