Oncology · Module 1 of 4

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

Generation-by-generation resistance, mutation targets, and class toxicities

Tyrosine kinase inhibitors target constitutively active oncogenic kinases created by chromosomal rearrangements or activating mutations. BCR-ABL (from the Philadelphia chromosome t(9;22) translocation in CML) was the first validated target; imatinib's success inaugurated the precision oncology era. No kinase inhibitor should be started without molecular confirmation of the target — mutation testing is obligatory before treatment selection and at every resistance event.
BCR-ABL inhibitors have evolved through four generations defined by resistance mutation coverage. First-generation imatinib binds the inactive conformation of BCR-ABL and also inhibits KIT and PDGFR. Second-generation agents (dasatinib, nilotinib, bosutinib) overcome most kinase domain mutations but not T315I. Third-generation ponatinib covers T315I but carries a boxed warning for arterial occlusive events. Asciminib (STAMP inhibitor) binds the myristoyl pocket rather than the ATP site, a structurally distinct mechanism that overcomes T315I at high dose.
EGFR inhibitor generations are defined by their resistance profile and selectivity. First- and second-generation agents (erlotinib, gefitinib, afatinib) are overcome by the T790M gatekeeper mutation. Third-generation osimertinib was designed to cover T790M, is now the preferred first-line agent for EGFR-mutant NSCLC (FLAURA trial), has superior CNS penetration, and spares wild-type EGFR — explaining its better tolerability. Acneiform rash, occurring in 50–80% of patients, paradoxically correlates with treatment efficacy.
ALK inhibitor generations reflect improving CNS penetration and resistance mutation coverage. First-generation crizotinib (also active against MET and ROS1) produces high initial response rates but is limited by poor blood-brain barrier penetration and secondary ALK mutations. Second-generation alectinib (preferred first-line, ALEX trial) significantly outperforms crizotinib in progression-free survival and CNS control. Third-generation lorlatinib covers the broadest range of ALK resistance mutations and compound mutations, at the cost of near-universal hyperlipidemia and CNS adverse effects.
Each class carries a distinctive toxicity fingerprint. BCR-ABL agents: imatinib causes edema and muscle cramps; dasatinib causes pleural effusion and pulmonary arterial hypertension; nilotinib causes QTc prolongation and peripheral arterial disease; bosutinib causes diarrhea in the majority of patients. EGFR agents: class-wide acneiform rash and diarrhea; interstitial lung disease is rare but requires immediate drug discontinuation. ALK agents: crizotinib causes visual disturbances (light flashes at light-dark transitions); lorlatinib causes hyperlipidemia and CNS effects in most patients.

ABL = Abelson murine leukemia tyrosine kinase · ALK = anaplastic lymphoma kinase · ATP = adenosine triphosphate · BCR = breakpoint cluster region · CML = chronic myeloid leukemia · CNS = central nervous system · EGFR = epidermal growth factor receptor · EML4 = echinoderm microtubule-associated protein-like 4 · HER = human epidermal growth factor receptor · KIT = stem cell factor receptor · NSCLC = non-small cell lung cancer · PDGFR = platelet-derived growth factor receptor · PgP = P-glycoprotein · ROS1 = ROS proto-oncogene 1 · STAMP = specifically targeting the ABL myristoyl pocket · T315I = threonine-to-isoleucine substitution at position 315 · T790M = threonine-to-methionine substitution at position 790 · TKI = tyrosine kinase inhibitor

BCR-ABL Inhibitor Generations

1st Generation

Imatinib

Binds ATP site; stabilizes inactive (DFG-out) conformation of BCR-ABL
Also inhibits KIT and PDGFR — activity in GIST and hypereosinophilic syndrome
Toxicities: edema, nausea, muscle cramps, hepatotoxicity
First TKI to demonstrate curative potential in CML

2nd Generation

Dasatinib · Nilotinib · Bosutinib

Overcome most P-loop and contact-site mutations that confer imatinib resistance
None active against T315I gatekeeper mutation
Dasatinib: pleural effusion, pulmonary arterial hypertension; avoid proton pump inhibitors
Nilotinib: QTc prolongation, hyperglycemia, peripheral arterial disease; take on empty stomach
Bosutinib: diarrhea in majority of patients; minimal KIT/PDGFR inhibition

3rd Generation

Ponatinib

Pan-BCR-ABL inhibitor: active against T315I and all other common mutations
Boxed warning: arterial occlusive events (myocardial infarction, stroke, peripheral arterial occlusion) — risk increases with dose and cardiovascular comorbidities
Use only when no other TKI option exists or when T315I is confirmed

STAMP Inhibitor

Asciminib

Binds myristoyl pocket (allosteric site) rather than ATP site — distinct from all other BCR-ABL TKIs
Overcomes T315I at high dose (200 mg twice daily)
Less cardiovascular toxicity than ponatinib
Active after 2 or more prior TKIs

EGFR Inhibitor Generations — Non-Small Cell Lung Cancer

Gen	Agent(s)	Mechanism	Key Points
1st	Erlotinib, Gefitinib	Reversible, ATP-competitive EGFR inhibition	Active against classic sensitizing mutations (ex19del, L858R). T790M gatekeeper mutation → acquired resistance. Erlotinib: take on empty stomach (food triples absorption).
2nd	Afatinib, Dacomitinib	Irreversible, pan-HER covalent inhibition (EGFR/HER2/HER4)	More severe diarrhea and mucositis due to broader HER family inhibition. Does not cover T790M. Afatinib: P-glycoprotein substrate — absorption reduced by strong inducers.
3rd	Osimertinib	Irreversible, mutant-selective; covalently binds both sensitizing mutations and T790M; spares wild-type EGFR	First-line preferred for EGFR-mutant NSCLC (FLAURA trial: superior PFS and OS). Best CNS penetration of the class. Acneiform rash and diarrhea present but milder than 2nd-gen. QTc monitoring recommended.

EGFR Class Toxicity & ALK Inhibitor Generations

EGFR Inhibitor Class Toxicities

Skin, GI, and Lung

Acneiform (papulopustular) rash: 50–80% of patients; seborrheic distribution (face, scalp, neck, trunk); onset within 2 weeks; severity correlates with treatment response
Rash management: tetracycline-class antibiotics (doxycycline or minocycline) preferred; does not respond to standard acne treatments
Diarrhea: class-wide; most severe with afatinib and dacomitinib (pan-HER inhibition in intestinal epithelium); treat with loperamide
Paronychia and dry skin: common with prolonged therapy
Interstitial lung disease: rare (~1–3%) but potentially fatal; stop drug immediately and start high-dose corticosteroids

ALK Inhibitor Generations

Crizotinib → Alectinib → Lorlatinib

1st gen — Crizotinib: ALK + MET + ROS1; objective response rate >60%; poor CNS penetration → brain relapse common; visual disturbances (light flashes at light-dark transitions); bradycardia
2nd gen — Alectinib: preferred first-line (ALEX trial: PFS ~34 vs 11 months vs crizotinib); superior CNS penetration; myalgia, elevated creatine kinase; well tolerated
2nd gen — Brigatinib: broader resistance mutation coverage; early-onset pulmonary toxicity (dyspnea/hypoxia within first week in 3–9%); start at reduced dose with escalation after 7 days
3rd gen — Lorlatinib: broadest ALK mutation coverage; active against compound mutations; best CNS penetration; hyperlipidemia (most patients; often requires statin); CNS effects (mood, cognition, speech)

Resistance Rule

T315I (BCR-ABL) and T790M (EGFR) — Gatekeeper Mutations Require Generation Escalation

The T315I mutation in BCR-ABL substitutes isoleucine for the threonine gatekeeper residue, eliminating the hydrogen bond that anchors all 1st- and 2nd-generation TKIs and creating steric clash. Only ponatinib (3rd generation) or high-dose asciminib (STAMP mechanism) retain activity. Molecular testing of the BCR-ABL kinase domain is required at progression — without it, the correct salvage agent cannot be selected.

The T790M mutation in EGFR substitutes methionine for threonine at the gatekeeper position, increasing ATP affinity and sterically blocking 1st- and 2nd-generation reversible inhibitors. Osimertinib was specifically engineered to form a covalent bond with the mutant cysteine adjacent to T790M, restoring potent inhibition. Testing for T790M (by liquid biopsy or repeat tissue biopsy) after progression on 1st- or 2nd-generation EGFR inhibitors should prompt a switch to osimertinib.

References

Author / Source	Title	Publication
Katzung BG (ed)	Basic & Clinical Pharmacology, 15th ed.	McGraw-Hill, 2021
Brunton LL, Knollmann BC (eds)	Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed.	McGraw-Hill, 2023
Rowley JD	A new consistent chromosomal abnormality in chronic myelogenous leukaemia identified by quinacrine fluorescence and Giemsa staining	Nature. 1973;243(5405):290–293
Druker BJ, Talpaz M, Resta DJ, et al	Efficacy and safety of a specific inhibitor of the BCR-ABL tyrosine kinase in chronic myeloid leukemia	N Engl J Med. 2001;344(14):1031–1037
Hochhaus A, Larson RA, Guilhot F, et al	Long-term outcomes of imatinib treatment for chronic myeloid leukemia	N Engl J Med. 2017;376(10):917–927
Shah NP, Tran C, Lee FY, et al	Overriding imatinib resistance with a novel ABL kinase inhibitor	Science. 2004;305(5682):399–401
Cortes JE, Kim DW, Pinilla-Ibarz J, et al	Ponatinib in refractory Philadelphia chromosome-positive leukemias	N Engl J Med. 2013;369(19):1783–1796
Rea D, Mauro MJ, Boquimpani C, et al	A phase 3, open-label, randomized study of asciminib, a STAMP inhibitor, vs bosutinib in CML after 2 or more prior TKIs	Blood. 2021;138(21):2031–2041
Peng B, Lloyd P, Schran H	Clinical pharmacokinetics of imatinib	Clin Pharmacokinet. 2005;44(9):879–894
Soria JC, Ohe Y, Vansteenkiste J, et al	Osimertinib in untreated EGFR-mutated advanced non-small-cell lung cancer	N Engl J Med. 2018;378(2):113–125
Ramalingam SS, Vansteenkiste J, Planchard D, et al	Overall survival with osimertinib in untreated, EGFR-mutated advanced NSCLC	N Engl J Med. 2020;382(1):41–50
Shaw AT, Kim DW, Nakagawa K, et al	Crizotinib versus chemotherapy in advanced ALK-positive lung cancer	N Engl J Med. 2013;368(25):2385–2394
Peters S, Camidge DR, Shaw AT, et al	Alectinib versus crizotinib in untreated ALK-positive non-small-cell lung cancer	N Engl J Med. 2017;377(9):829–838
Kalemkerian GP, Narula N, Kennedy EB, et al	Molecular testing guideline for the selection of patients with lung cancer for treatment with targeted tyrosine kinase inhibitors	J Clin Oncol. 2018;36(9)
Hirsch FR, Varella-Garcia M, Bunn PA Jr, et al	Epidermal growth factor receptor in non-small-cell lung carcinomas: correlation between gene copy number and protein expression and impact on prognosis	J Clin Oncol. 2003;21(20):3798–3807