IMATINIB — 100 mg / 400 mg tablets (and capsules)

Imatinib mesylate is a first‑generation tyrosine kinase inhibitor (TKI) that revolutionized the treatment of chronic myeloid leukemia (CML) and gastrointestinal stromal tumors (GIST). Approved by the FDA in May 2001, it was the first targeted cancer therapy that specifically inhibits the BCR‑ABL tyrosine kinase, the constitutive oncoprotein resulting from the Philadelphia chromosome translocation. Its development is considered a landmark achievement in molecular oncology, shifting CML from a fatal disease to a manageable chronic condition for most patients.

Mechanism of action: Imatinib competitively inhibits the ATP‑binding site of several tyrosine kinases, including BCR‑ABL (the hallmark of CML and Ph+ ALL), c‑KIT (CD117, overexpressed in GIST), and platelet‑derived growth factor receptors (PDGFR‑α and PDGFR‑β). By blocking kinase activity, imatinib prevents phosphorylation of downstream signaling proteins, leading to inhibition of cellular proliferation and induction of apoptosis in cells dependent on these pathways. This targeted mechanism spares most normal cells, resulting in a more favorable adverse effect profile compared to conventional chemotherapy.

Clinical efficacy: The pivotal International Randomized Interferon versus STI571 (IRIS) trial demonstrated the superiority of imatinib over previous standard therapy (interferon‑α plus cytarabine) in newly diagnosed chronic‑phase CML. After 19 months, 96.7% of imatinib‑treated patients achieved a complete hematologic response, and 87.1% achieved a major cytogenetic response. Long‑term follow-up (10 years) from the IRIS trial showed that more than 80% of patients who continued imatinib maintained a complete cytogenetic response, with overall survival rates exceeding 80%. For GIST, the phase III trial demonstrated that imatinib 400 mg daily produced disease control in approximately 80% of patients with metastatic/unresectable tumors, with median survival prolonged from 9‑12 months to nearly 5 years.

Pharmacokinetics: Imatinib is well absorbed after oral administration (absolute bioavailability 98%), with peak plasma concentrations reached in 2‑4 hours. It is extensively metabolized in the liver, primarily by CYP3A4, with the main metabolite (N‑desmethyl imatinib) also possessing pharmacological activity. The elimination half‑life is approximately 18 hours for imatinib and 40 hours for the active metabolite, allowing once‑daily dosing. Excretion is predominantly via the feces. Imatinib is a substrate of CYP3A4 and P‑glycoprotein, making it susceptible to numerous drug interactions.

• None. Imatinib has no absolute contraindications listed in the prescribing information. However, use is not recommended in patients with hypersensitivity to imatinib or any component of the formulation.

• Strong CYP3A4 inhibitors (ketoconazole, itraconazole, voriconazole, ritonavir, clarithromycin, grapefruit juice): May increase imatinib concentrations, increasing risk of toxicity. Avoid concomitant use if possible. If unavoidable, consider reducing imatinib dose and monitor for toxicity.
• Strong CYP3A4 inducers (rifampin, carbamazepine, phenytoin, phenobarbital, St. John’s wort, dexamethasone): May decrease imatinib concentrations, potentially reducing efficacy. Dose increase may be necessary. Avoid concomitant use if possible.
• CYP3A4 substrates (simvastatin, cyclosporine, pimozide, triazolam, dihydropyridine calcium channel blockers): Imatinib may increase concentrations of these drugs. Monitor for toxicity.
• Acetaminophen: Caution with high‑dose or chronic acetaminophen use; case reports of hepatotoxicity when combined with imatinib.
• Warfarin: Imatinib may increase warfarin levels via CYP3A4 and CYP2C9 inhibition. Use low molecular weight heparin or monitor INR closely if warfarin is required.
• Levothyroxine: Imatinib may increase levothyroxine metabolism; monitor TSH in hypothyroid patients.

• Pregnancy: FDA Category D. Can cause fetal harm based on animal studies. Women of reproductive potential should use effective contraception during treatment and for at least 14 days after the final dose.
• Lactation: Imatinib and its active metabolite are excreted in human milk. Advise women not to breastfeed during treatment and for at least 1 month after the final dose.
• Pediatric use: Approved for pediatric patients with Ph+ CML (chronic phase) and Ph+ ALL. Dosing based on body surface area (BSA). Monitor growth and development.
• Geriatric use: No overall differences in safety or efficacy observed between elderly and younger patients, but greater sensitivity cannot be ruled out. Fluid retention (peripheral edema, pleural effusion) may be more common.
• Hepatic impairment: Mild‑moderate: no dose adjustment required, but monitor LFTs. Severe: use with caution; start at lower dose (400 mg) and monitor closely.
• Renal impairment: Mild‑moderate: no dose adjustment. Severe: use with caution (limited data); consider starting at lower dose (400 mg) and monitor.

• Chronic phase CML: 400 mg orally once daily.
• Accelerated phase CML, blast crisis CML, Ph+ ALL: 600 mg orally once daily.
• GIST (unresectable or metastatic): 400 mg orally once daily. May increase to 600‑800 mg daily in patients with disease progression.
• Adjuvant GIST (after resection): 400 mg orally once daily for 3 years.
• Other indications (MDS/MPD, HES/CEL, DFSP): 400 mg orally once daily (DFSP 800 mg daily).
• Dose adjustment: Dose reductions (to 300‑400 mg) may be required for adverse reactions. Dose increases (to 600‑800 mg) may be considered for inadequate response.
• Administration: Take with a meal and a large glass of water. Tablets can be dispersed in a glass of water or apple juice (50 mL for 100 mg, 200 mL for 400 mg). Stir until dissolved and drink immediately.

• Symptoms: Increased risk of severe myelosuppression (neutropenia, thrombocytopenia), hepatotoxicity, fluid retention, and gastrointestinal symptoms.
• Management: Hospitalization, supportive care, monitoring of CBCs and LFTs. No specific antidote. Discontinue imatinib and provide supportive measures including transfusions and antibiotics as needed.