The management of CML was revolutionized by the development and ultimate regulatory approval of imatinib mesylate in 2001. Imatinib was the first small-molecule cancer therapy developed and approved. It acts by binding to the adenosine triphosphate (ATP) binding site in the catalytic domain of BCR-ABL, thus inhibiting the oncoprotein's tyrosine kinase activity.1
The International Randomized Study of Interferon versus STI571 (IRIS) trial was a randomized phase 3 study that compared imatinib 400 mg daily to interferon α (IFNα) plus cytarabine. More than 1000 CP-CML patients were randomly assigned 1:1 to either imatinib or IFNα plus cytarabine and were assessed for event-free survival, hematologic and cytogenetic responses, freedom from progression to accelerated phase (AP) or blast phase (BP), and toxicity. Imatinib was superior to the prior standard of care for all these outcomes.2 The long-term follow up of the IRIS trial reported an 83% estimated 10-year overall survival (OS) and 79% estimated event-free survival for patients on the imatinib arm of this study.3 The cumulative rate of complete cytogenetic response (CCyR) was 82.8%. Of the 204 imatinib-treated patients who could undergo a molecular response evaluation at 10 years, 93.1% had a major molecular response (MMR) and 63.2% had a molecular response 4.5 (MR4.5), suggesting durable, deep molecular responses for many patients (see Chronic Myeloid Leukemia: Evaluation and Diagnosis for discussion of the hematologic parameters, cytogenetic results, and molecular responses ussed in monitoring response to TKI therapy). The estimated 10-year rate of freedom from progression to AP or BP was 92.1%.
Higher doses of imatinib (600-800 mg daily) have been studied in an attempt to overcome resistance and improve cytogenetic and molecular response rates. The Tyrosine Kinase Inhibitor Optimization and Selectivity (TOPS) trial was a randomized phase 3 study that compared imatinib 800 mg daily to imatinib 400 mg daily. Although the 6-month assessments found increased rates of CCyR and a MMR in the higher-dose imatinib arm, these differences were no longer present at the 12-month assessment. Furthermore, the higher dose of imatinib led to a significantly higher incidence of grade 3/4 hematologic adverse events, and approximately 50% of patients on imatinib 800 mg daily required a dose reduction to less than 600 mg daily because of toxicity.4
The Therapeutic Intensification in De Novo Leukaemia (TIDEL) -II study used plasma trough levels of imatinib on day 22 of treatment with imatinib 600 mg daily to determine if patients should escalate the imatinib dose to 800 mg daily. In patients who did not meet molecular milestones at 3, 6, or 12 months, cohort 1 was dose escalated to imatinib 800 mg daily and subsequently switched to nilotinib 400 mg twice daily for failing the same target 3 months later, and cohort 2 was switched to nilotinib. At 2 years, 73% of patients achieved MMR and 34% achieved MR4.5, suggesting that initial treatment with higher-dose imatinib subsequently followed by a switch to nilotinib in those failing to achieve desired milestones could be an effective strategy for managing newly diagnosed CP-CML.5
Imatinib 400 mg is considered the standard starting dose in CP-CML patients. The safety profile of imatinib has been very well established. In the IRIS trial, the most common adverse events (all grades in decreasing order of frequency) were peripheral and periorbital edema (60%), nausea (50%), muscle cramps (49%), musculoskeletal pain (47%), diarrhea (45%), rash (40%), fatigue (39%), abdominal pain (37%), headache (37%), and joint pain (31%). Grade 3/4 liver enzyme elevation can occur in 5% of patients.6 In the event of severe liver toxicity or fluid retention, imatinib should be held until the event resolves. At that time, imatinib can be restarted if deemed appropriate, but this is dependent on the severity of the inciting event. Fluid retention can be managed by the use of supportive care, diuretics, imatinib dose reduction, dose interruption, or imatinib discontinuation if the fluid retention is severe. Muscle cramps can be managed by the use of a calcium supplements or tonic water. Management of rash can include topical or systemic steroids, or in some cases imatinib dose reduction, interruption, or discontinuation.7
Grade 3/4 imatinib-induced hematologic toxicity is not uncommon, with 17% of patients experiencing neutropenia, 9% thrombocytopenia, and 4% anemia. These adverse events occurred most commonly during the first year of therapy, and the frequency decreased over time.3,6 Depending on the degree of cytopenias, imatinib dosing should be interrupted until recovery of the absolute neutrophil count or platelet count, and can often be resumed at 400 mg daily. However, if cytopenias recur, imatinib should be held and subsequently restarted at 300 mg daily.7