Imatinib 800 mg daily induces deeper molecular responses than imatinib 400 mg daily: results of SWOG S0325, an intergroup randomized PHASE II trial in newly diagnosed chronic phase chronic myeloid leukaemia

The standard dose of imatinib for newly diagnosed patients with chronic phase chronic myeloid leukaemia (CP-CML) is 400mg daily (IM400), but the optimal dose is unknown. This randomized phase II study compared the rates of molecular, haematological and cytogenetic response to IM400 vs. imatinib 400mg twice daily (IM800) in 153 adult patients with CP-CML. Dose adjustments for toxicity were flexible to maximize retention on study. Molecular response (MR) at 12months was deeper in the IM800 arm (4-log reduction of BCR-ABL1 mRNA: 25% vs. 10% of patients, P=0038; 3-log reduction: 53% vs. 35%, P=0049). During the first 12months BCR-ABL1 levels in the IM800 arm were an average 29-fold lower than in the IM400 arm (P=0010). Complete haematological response was similar, but complete cytogenetic response was higher with IM800 (85% vs. 67%, P=0040). Grade 3-4 toxicities were more common for IM800 (58% vs. 31%, P=00007), and were most commonly haematological. Few patients have relapsed, progressed or died, but both progression-free (P=0048) and relapse-free (P=0031) survival were superior for IM800. In newly diagnosed CP-CML patients, IM800 induced deeper MRs than IM400, with a trend for improved progression-free and overall survival, but was associated with more severe toxicity.