Imatinib

Introduction Overview

Generic Name

imatinib

Brand Name

Gleevec, Imkeldi

Dosage Form

oral tablet (100 mg; 400 mg), oral solution (80 mg/mL)

Drug Class

BCR-ABL tyrosine kinase inhibitors

Pharmacokinetics

Absorption:
Imatinib is rapidly absorbed after oral administration with peak plasma concentrations occurring 2 to 4 hours post-dose. The absolute bioavailability of imatinib tablets is approximately 98%, indicating almost complete absorption. Food does not significantly affect the extent of absorption but may delay the time to peak concentration (Tmax). Patients are advised to take imatinib with meals and a large glass of water to minimize gastrointestinal irritation. The oral solution form (Imkeldi) offers an alternative for patients who have difficulty swallowing tablets. Variability in absorption has been observed with different formulations, but overall systemic exposure remains consistent.

Distribution:
Imatinib has a large volume of distribution, estimated at approximately 500 liters, indicating extensive tissue distribution. It is approximately 95% bound to plasma proteins, primarily albumin and alpha-1 acid glycoprotein, which can affect free drug concentrations. Imatinib and its primary metabolite (N-desmethyl imatinib) penetrate tissues, including sites of disease such as bone marrow and leukemic cells. Distribution into the central nervous system is limited due to the blood-brain barrier, and the drug is a substrate of P-glycoprotein transporters, which limit CNS exposure.

Metabolism:
Imatinib is extensively metabolized in the liver primarily by cytochrome P450 enzymes CYP3A4 and to a lesser extent CYP1A2, CYP2D6, CYP2C9, and CYP2C19 isoforms. The main metabolite is N-desmethyl imatinib, which retains approximately 20% to 25% of the pharmacologic activity of the parent drug. Metabolic pathways include demethylation and oxidation. First-pass metabolism is significant but does not substantially reduce bioavailability due to high absorption. Co-administration of potent CYP3A4 inhibitors or inducers can alter plasma concentrations of imatinib and its metabolites, necessitating clinical monitoring and dose adjustments.

Elimination:
The elimination half-life of imatinib is approximately 18 hours, supporting once-daily dosing in most indications. The N-desmethyl metabolite has a slightly longer half-life of about 40 hours. Excretion occurs primarily via the feces (approximately 68%) with minimal renal excretion (less than 7%) of unchanged drug. Because of hepatic metabolism and biliary/fecal excretion, patients with liver impairment require cautious monitoring. Plasma clearance can be affected by factors altering hepatic enzyme activity, drug interactions, and patient comorbidities. Steady-state plasma concentrations are typically reached within 4 to 7 days of dosing.

Dosage Forms

Available Dosage Forms:
- Oral tablets available as 100 mg and 400 mg strengths (Gleevec brand and generic imatinib mesylate)
- Oral solution available as 80 mg/mL (Imkeldi brand), supplied in a 140 mL amber PET bottle with strawberry flavor

Dosages for Adults and Adolescents:
- Chronic Myelogenous Leukemia (CML) Chronic Phase: 400 mg orally once daily
- CML Accelerated Phase or Blast Crisis: 600 mg orally once daily
- Dose adjustment to 600 mg once daily or 400 mg twice daily based on disease progression or response
- Acute Lymphoblastic Leukemia (Philadelphia chromosome positive): 600 mg orally once daily
- Myeloproliferative Disorders: 400 mg orally once daily
- Myelodysplastic Disease: 400 mg orally once daily
- Systemic Mastocytosis without D816V c-Kit mutation: 400 mg orally once daily
- Systemic Mastocytosis with FIP1L1-PDGFR alpha fusion kinase: 100 mg orally daily initially; escalate to 400 mg as needed
- Hypereosinophilic Syndrome and Chronic Eosinophilic Leukemia: 400 mg orally once daily; 100 mg daily in fusion kinase positive patients with escalation as needed
- Dermatofibrosarcoma Protuberans: 100 mg orally once daily
- Pediatric CML (aged 1 year and older): 340 mg/m2 orally once daily or 170 mg/m2 twice daily, maximum 600 mg daily

Specific Patient Populations:
- Pediatric dosing is based on body surface area with maximum dose limits, with no data for children under 1 year
- Dose modifications may be required based on tolerability, adverse effects, hepatic or renal impairment
- Patients with liver dysfunction need careful monitoring due to hepatic metabolism and potential accumulation
- Pregnant women must avoid use due to teratogenicity; effective contraception required during and 14 days post-treatment
- Breastfeeding is contraindicated during treatment and for 1 month after the last dose

Detailed administration instructions include taking the drug with meals and large amounts of water, not on empty stomach. Tablets can be dissolved in water or apple juice (using specified volumes) for easier swallowing, but broken tablets should be avoided due to potential local toxicity from contact. Oral solution should be dosed with appropriate milliliter measuring devices; household spoons are not accurate and can cause overdose.

Mechanism Of Action

Imatinib is a selective inhibitor of several tyrosine kinases, primarily targeting the BCR-ABL fusion protein tyrosine kinase produced by the Philadelphia chromosome abnormality commonly seen in chronic myelogenous leukemia (CML) and acute lymphoblastic leukemia (ALL). This fusion protein possesses constitutive kinase activity independent of normal regulatory mechanisms, leading to uncontrolled cell proliferation and survival signaling.

At the molecular level, imatinib binds competitively to the ATP-binding site of the BCR-ABL kinase domain, stabilizing it in an inactive conformation and thereby preventing phosphorylation and subsequent activation of downstream signaling pathways essential for leukemogenesis. This inhibition leads to induction of apoptosis and inhibition of proliferation specifically in BCR-ABL-positive cells.

In addition to BCR-ABL, imatinib inhibits several other receptor tyrosine kinases including c-KIT (CD117), platelet-derived growth factor receptors alpha and beta (PDGFRα/β), and others involved in signal transduction pathways driving tumor growth, angiogenesis, and fibrosis. This broad kinase inhibition underlies its efficacy in treating gastrointestinal stromal tumors (GISTs), systemic mastocytosis, dermatofibrosarcoma protuberans, and diseases associated with PDGFR mutations or fusions.

Clinical implications of imatinib's mechanism include high specificity resulting in targeted cancer therapy with relatively fewer off-target cytotoxic effects compared to traditional chemotherapies. Resistance can develop through mutations in the ATP-binding site reducing imatinib binding or through BCR-ABL amplification, which has led to development of second- and third-generation tyrosine kinase inhibitors. Extensive clinical trials have demonstrated that imatinib significantly improves survival and response rates in CML, making it a paradigm for targeted molecular therapy in oncology.

Pharmacological Actions

Selective inhibition of BCR-ABL tyrosine kinase, preventing aberrant signal transduction in leukemic cells.
Inhibition of c-KIT tyrosine kinase activity, interfering with growth of tumors expressing mutant or overactive c-KIT.
Blockade of platelet-derived growth factor receptors (PDGFR α and β), reducing proliferation in certain myeloproliferative disorders.
Induction of apoptosis and inhibition of cell proliferation in malignant hematopoietic cells.
Suppression of cancer cell growth and metastasis in gastrointestinal stromal tumors.
Modulation of downstream signaling pathways involved in cell cycle regulation.
Reduction of mast cell proliferation in systemic mastocytosis by targeting KIT mutations.
Antiproliferative effects in dermatofibrosarcoma protuberans via PDGFR inhibition.
Altering tumor microenvironment by affecting cytokine and growth factor signaling.
Competitive ATP-binding site occupation leading to kinase inactivation.
Inhibition of fusion kinase activities associated with hypereosinophilic syndrome.
Potential immunomodulatory effects through interference with tyrosine kinase signaling.
Prolongation of survival in cancer patients through targeted molecular action.
Synergistic effects when combined with chemotherapy agents due to disruption of critical oncogenic pathways.

Therapeutic Or Clinical Uses

Treatment of Philadelphia chromosome-positive chronic myelogenous leukemia (Ph+ CML) in chronic, accelerated, and blast phases.
Management of newly diagnosed Ph+ CML patients in chronic phase.
Treatment of Ph+ acute lymphoblastic leukemia (ALL), particularly relapsed or refractory cases.
Therapy for myelodysplastic/myeloproliferative diseases associated with PDGFR gene rearrangements.
Treatment of aggressive systemic mastocytosis without D816V c-Kit mutation.
Management of hypereosinophilic syndrome (HES) and chronic eosinophilic leukemia (CEL), especially cases with FIP1L1-PDGFR alpha fusion kinase.
Treatment of unresectable, recurrent, and/or metastatic dermatofibrosarcoma protuberans (DFSP).
Utilized in select patients with systemic mastocytosis associated with eosinophilia.
Supportive therapy in patients who have failed interferon-alpha treatment for CML.
Off-label or investigative use in certain gastrointestinal stromal tumors (GISTs) expressing c-KIT mutations.
Adjunctive cancer therapy in combination with other treatment modalities aiming to inhibit tyrosine kinase signaling.
Disease stabilization in rare myeloproliferative disorders driven by tyrosine kinase dysregulation.
Prophylactic use in preventing progression of chronic phase CML to more aggressive phases.
Alternative therapy for patients intolerant or resistant to other tyrosine kinase inhibitors.

Side Or Adverse Effects

Fluid retention manifesting as edema, pleural effusion, or weight gain.
Nausea, vomiting, and gastrointestinal disturbances including diarrhea and abdominal pain.
Musculoskeletal pain such as joint or muscle aches.
Skin rash and pruritus varying from mild to severe dermatologic reactions.
Fatigue and general malaise.
Myelosuppression leading to neutropenia, thrombocytopenia, and anemia.
Hepatotoxicity presenting with elevated liver enzymes, jaundice, and rare liver failure.
Signs of hypothyroidism including fatigue, dry skin, hair loss, constipation, and weight gain.
Severe allergic reactions including urticaria, angioedema, and anaphylaxis.
Tumor lysis syndrome characterized by electrolyte imbalances, confusion, and kidney dysfunction.
Cardiac adverse effects such as congestive heart failure and arrhythmias in rare cases.
Signs of gastrointestinal bleeding including hematemesis and melena.
Ocular toxicity with blurred vision or visual disturbances.
Infections due to immunosuppression, increased vulnerability to opportunistic pathogens.
Potential impairment of growth in pediatric patients during long-term therapy.

Precautions

Patients must have regular monitoring of blood counts due to risk of myelosuppression.
Liver function tests are recommended frequently to detect hepatotoxicity early.
Avoid use in patients with known hypersensitivity to imatinib or its components.
Pregnant women should not use imatinib due to teratogenicity; contraception is mandatory during treatment and for 14 days after.
Breastfeeding is contraindicated during therapy and for at least one month post-treatment.
Cautious use in patients with pre-existing liver or kidney impairment; dose adjustment may be needed.
Monitoring for signs of fluid retention and cardiac symptoms is advised.
Patients should avoid grapefruit and grapefruit juice that may increase plasma levels of imatinib.
Use protective measures when handling tablets to avoid contact with broken tablets which can cause local toxicity.
Ensure adequate hydration to minimize renal complications.
Consider evaluation of PDGFR and c-Kit mutation status before initiating therapy for certain indications.
Patients must be informed of potential visual disturbances and cautioned about driving or operating machinery until individual response is known.
Advise patients to avoid activities that increase risk of bleeding or injury due to thrombocytopenia.
Special care is warranted in pediatric patients with monitoring of growth parameters.
Thorough patient education regarding adherence and potential side effects is essential for optimal outcomes.

Warnings

Imatinib can cause severe myelosuppression leading to increased risk of infections and bleeding.
Risk of hepatotoxicity with signs such as jaundice, dark urine, and abdominal pain requires immediate medical attention.
Fluid retention can be severe, causing pulmonary edema, pleural effusion, or congestive heart failure.
Potential for severe skin reactions including Stevens-Johnson syndrome and toxic epidermal necrolysis.
Use cautiously in patients with cardiac disease or history of heart failure due to risk of exacerbation.
Risk of tumor lysis syndrome especially in patients with high tumor burden.
May cause hypothyroidism; thyroid function tests should be routinely monitored.
Avoid during pregnancy due to the risk of fetal harm and teratogenicity.
Avoid breastfeeding due to the possibility of serious adverse reactions in nursing infants.
Patients should be advised to report any signs of unusual bruising, bleeding, or infections immediately.
Do not discontinue therapy abruptly without physician guidance due to potential for disease progression.
Overdosage can lead to severe toxicities requiring emergency medical intervention.
Potential interaction with drugs affecting cytochrome P450 enzymes, altering the safety profile.
Risk of ocular toxicity mandates prompt evaluation if visual symptoms occur.
Patients should be monitored frequently with blood tests to detect adverse effects promptly.

Contraindication

Known hypersensitivity to imatinib or any component of the formulation.
Pregnancy due to risk of teratogenicity and fetal harm.
Breastfeeding during treatment and for at least one month after last dose.
Use of broken or crushed tablets with direct mucous membrane or skin contact due to localized toxicity risk.
Severe hepatic impairment without appropriate dose adjustment or monitoring.
Concomitant use of strong inhibitors or inducers of CYP3A4 without medical supervision.
Patients with active infections or uncontrolled bleeding without proper stabilization.
Children under 1 year of age due to lack of safety and efficacy data.
Known cases of severe nonhematologic toxicity to prior imatinib therapy.
Hypersensitivity reactions to other BCR-ABL tyrosine kinase inhibitors that may predict cross-reactivity.

Drug Interactions

Potent CYP3A4 inhibitors (e.g., ketoconazole, itraconazole) can increase imatinib plasma concentrations, increasing toxicity risk.
CYP3A4 inducers (e.g., rifampin, carbamazepine, phenytoin) reduce imatinib efficacy by decreasing plasma levels.
Co-administration with warfarin enhances anticoagulant effects, necessitating close INR monitoring.
Certain antibiotics, antifungal, and antiviral agents may alter imatinib metabolism and plasma concentrations.
Antiepileptic drugs that induce hepatic enzymes can reduce imatinib exposure.
Iron-containing supplements may interact and affect absorption or efficacy.
Imatinib can increase plasma levels of some drugs metabolized by CYP enzymes leading to toxicity.
Use with other tyrosine kinase inhibitors requires careful evaluation for additive toxicities or interactions.
Concurrent use with immunosuppressive agents may amplify risk of infections.
Herbal products such as St. John's wort can reduce imatinib effectiveness via CYP induction.
Drugs that prolong the QT interval should be used with caution with imatinib due to additive effects.
Co-administration with P-glycoprotein inhibitors may alter CNS distribution and toxicity.
Drugs metabolized by CYP2D6 may be affected due to minor imatinib inhibition.
Monitoring of blood counts is necessary when combined with myelosuppressive agents.
Dose adjustments may be needed for concurrent drugs affecting hepatic metabolism.

Preparations Of The Drug

Gleevec oral tablets: 100 mg imatinib mesylate tablets with coating containing ferric oxides, hydroxypropyl methylcellulose, polyethylene glycol, and talc.
Gleevec oral tablets: 400 mg imatinib mesylate tablets with similar coating components as 100 mg tablets.
Imkeldi oral solution: 80 mg/mL imatinib (as 95.57 mg imatinib mesylate) in a strawberry flavored clear yellow to brownish-yellow liquid vehicle.
Imkeldi excipients include acesulfame potassium, citric acid monohydrate, glycerine, liquid maltitol, purified water, sodium benzoate, artificial strawberry flavor, lactic acid, and triacetin.
Tablets are supplied in blister packs or bottles suitable for oral administration; must be stored at controlled room temperature (20°C to 25°C).
Imkeldi is supplied in amber PET bottles with child-resistant closures and must be discarded 30 days after opening.
Both preparations require storage away from moisture, heat, and direct sunlight to preserve stability.
Inclusion of colloidal silicon dioxide, crospovidone, hydroxypropyl methylcellulose, magnesium stearate, and microcrystalline cellulose as inactive ingredients in tablets.
Special handling instructions must be followed to prevent exposure to broken pellets or crushed tablets due to local toxicity potential.
Packaging for Imkeldi includes oral syringes or measuring devices (not included), and pharmacists are advised to provide these for accurate dosing.
Oral solution formulation facilitates dosing in pediatric patients or individuals with swallowing difficulties.
No generic version of Imkeldi currently available; Gleevec has generic formulations approved since 2015.
Manufactured by Novartis Pharmaceuticals (tablets, Gleevec) and Shorla Oncology Inc. (oral solution, Imkeldi).
Patient leaflets accompany both formulations detailing administration, storage, and disposal instructions.
Both formulations have undergone stability and bioequivalence studies to ensure therapeutic comparability.