Imatinib

Imatinib is a 2-phenylaminopyrimidine derivative neoplastic agent that belongs to the class of tyrosine kinase inhibitors. Although imatinib inhibits a number of tyrosine kinases, it is quite selective toward the BCR-ABL fusion protein created by the Philadelphia chromosome translocation. The BCR-ABL pathway controls many downstream cascades that are heavily implicated in neoplastic growth — Ras/MAPK (cellular proliferation), Src/Pax/Fak/Rac (cellular motility), and PI3K/AKT/BCL-2 (apoptosis regulation). While normal cells also depend on these pathways, they typically have redundant tyrosine kinases that allow continued function despite ABL inhibition, whereas BCR-ABL-dependent cancer cells are disproportionately impacted.

Mechanism of Action

Imatinib mesylate inhibits the BCR-ABL tyrosine kinase by binding to the ATP-binding pocket in the active site, preventing downstream phosphorylation of target proteins. ABL activation is overexpressed in various tumors and is heavily implicated in cancer cell growth and survival.

Imatinib also inhibits the receptor tyrosine kinases for platelet-derived growth factor (PDGFR-α and -β) and stem cell factor / c-KIT, blocking PDGF- and SCF-mediated cellular events. In vitro, imatinib inhibits proliferation and induces apoptosis in GIST cells expressing activating c-KIT mutations.

Additional confirmed targets (per DrugBank target profile):

• BCR-ABL fusion kinase — primary target in CML and Ph+ ALL.
• c-KIT (CD117) — driver in GIST and mast cell disorders.
• PDGFR-α and -β — drivers in DFSP, hypereosinophilic syndrome, MDS/MPN with PDGFR rearrangements.
• RET, CSF1R (FMS), DDR1, DDR2 — additional kinases inhibited at clinical concentrations, contributing to broader activity (e.g., osteoclast suppression via CSF1R, fibrosis-related signaling via DDR1/2).

Pharmacokinetics

• Absorption: well absorbed orally; mean absolute bioavailability 98%. Cmax achieved within 2–4 hours post-dose. AUC increases proportionally with dose across 25–1000 mg. Steady-state accumulation 1.5–2.5-fold with once-daily dosing.
• Volume of distribution: steady-state Vd 295 ± 62.5 L in adult CML patients; 167 ± 84 L in pediatric patients (at 340 mg/m²).
• Protein binding: ~95% to plasma proteins (mainly albumin and α₁-acid glycoprotein) at clinically relevant concentrations.
• Metabolism: predominantly CYP3A4; minor contributions from CYP1A2, CYP2C9, CYP2C19, and CYP2D6. Main active metabolite is the N-demethylated piperazine derivative (CGP74588), with in vitro potency similar to parent. Imatinib is also a mechanism-based inhibitor of CYP3A4, which underlies several of its clinically significant drug interactions.
• Half-life: ~18 hours (parent); ~40 hours (CGP74588 active metabolite).
• Clearance: ~8 L/h (50 kg, age 50) to ~14 L/h (100 kg, age 50); inter-patient variability ~40%.
• Excretion: primarily fecal — ~81% of dose recovered in 7 days (68% feces, 13% urine); 25% as unchanged drug (5% urine, 20% feces), remainder as metabolites.
• Transporters: substrate of OCT1 (SLC22A1) for cellular uptake — low OCT1 activity in CML CD34+ cells is a documented mechanism of imatinib resistance. Also a substrate/inhibitor of P-glycoprotein (ABCB1) and BCRP (ABCG2).

• Chronic myeloid leukemia: complete cytogenetic response in 70–85% of newly diagnosed chronic-phase CML. 10-year overall survival is approximately 80–90% (compared with <20% in the pre-imatinib era). Many patients achieve deep molecular responses.
• Gastrointestinal stromal tumors (GIST): response rate 50–70% in advanced disease; disease control rate >80%; dramatically improved survival.

Chronic myeloid leukemia:

• Newly diagnosed Philadelphia chromosome-positive (Ph+) CML in chronic phase.
• CML in chronic phase, accelerated phase, or blast crisis after interferon-alpha failure.

Gastrointestinal stromal tumors:

• KIT (CD117)-positive unresectable or metastatic GIST.
• Adjuvant treatment after surgical resection of KIT-positive GIST.

Other indications:

• Philadelphia chromosome-positive acute lymphoblastic leukemia (Ph+ ALL).
• Myelodysplastic/myeloproliferative diseases associated with PDGFR gene rearrangements.
• Aggressive systemic mastocytosis without the D816V c-KIT mutation.
• Hypereosinophilic syndrome and chronic eosinophilic leukemia.
• Dermatofibrosarcoma protuberans.

• Hypersensitivity to imatinib or any of the excipients.
• Pregnancy (causes fetal harm).

Very Common (>10%): edema (periorbital, lower limbs), nausea, vomiting, diarrhea, muscle cramps, musculoskeletal pain, rash, fatigue, headache.

Common (1–10%): myelosuppression (anemia, neutropenia, thrombocytopenia), abdominal pain, dyspepsia, weight gain, insomnia.

Serious (less common):

• Severe fluid retention (pleural effusion, pericardial effusion, ascites, pulmonary edema).
• Severe skin reactions (Stevens-Johnson syndrome — rare).
• Hepatotoxicity (elevated transaminases, rarely severe hepatitis).
• Cardiotoxicity (heart failure).
• Tumor lysis syndrome.
• Gastrointestinal perforation or bleeding.

• Strong CYP3A4 inhibitors (ketoconazole, clarithromycin, ritonavir): increase imatinib exposure.
• Strong CYP3A4 inducers (rifampin, phenytoin, St. John’s wort): decrease imatinib exposure — avoid.
• CYP3A4 substrates: imatinib is a mechanism-based CYP3A4 inhibitor and can increase exposure of co-administered CYP3A4 substrates (e.g., simvastatin, certain calcium channel blockers, immunosuppressants).
• Warfarin: imatinib affects warfarin metabolism via CYP2C9 inhibition; consider low-molecular-weight heparin instead, or monitor INR closely.
• Acetaminophen (paracetamol): imatinib may increase exposure.

Food Interactions

• Avoid grapefruit products: grapefruit inhibits CYP3A4 metabolism and can increase serum imatinib levels.
• St. John’s Wort: induces CYP3A4 — may reduce imatinib serum concentration; avoid.
• Take with food and a full glass of water: reduces gastric irritation.

Standard Dosing

• CML, chronic phase: 400 mg orally once daily.
• CML, accelerated phase or blast crisis: 600 mg once daily.
• GIST: 400 mg once daily; may be increased to 600–800 mg in case of disease progression.

Take with a meal and a large glass of water. Tablets should be swallowed whole. If the patient cannot swallow, tablets may be dispersed in water or apple juice.

Monitoring

• Complete blood count: weekly initially, then monthly.
• Liver function tests: monthly; more frequently with abnormal baseline.
• Weight and edema: assess at each visit.
• Pregnancy test: before starting in women of childbearing potential.

Pregnancy

Imatinib is highly teratogenic. Effective contraception is required during treatment and the drug should be discontinued at least 15 days before a planned pregnancy.

Management of Common Side Effects

• Edema: diuretics; dose reduction if severe.
• Nausea: take with food and a full glass of water; antiemetics if needed.
• Muscle cramps: quinine; calcium and magnesium supplementation.
• Rash: antihistamines, topical corticosteroids; systemic corticosteroids for severe reactions.

Store at room temperature, below 30°C. Protect from moisture. Keep out of reach of children.

3 years. Do not use after the expiry date.

Prescription required. Specialist oncology or hematology prescribing is typically required. Note: the centralized EMA marketing authorization for the originator brand was withdrawn in October 2023; imatinib remains widely available as generic and through national authorizations across Europe and globally.