Incidence of Imatinib Resistance

Primary and Acquired Resistance in Chronic Phase CML

Primary resistance to imatinib in chronic phase CML, defined as failure to achieve a complete haematologic response within 3 to 6 months of starting imatinib, occurs in only 2%–4% of patients.¹³ Of those with an initial response, up to 25% eventually stop responding, having acquired resistance to imatinib.^4,5

A 5-year follow-up of patients in the IRIS study showed that 11% discontinued imatinib due to suboptimal response

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Another study* with a 6-year median follow-up showed that 29% of patients discontinued imatinib due to suboptimal response

• *Phase II, single arm, open-label, multicentre, non-randomised study that investigated the efficacy and safety of imatinib 400mg/day* in 454 "late Chronic Phase" patients with CP CML who received prior IFN-based therapy.
• †Imatinib dose escalation to 600 mg/d & then 800 mg/d was permitted in patients who did not achieve CHR after 3 months of treatment, whose disease relapsed within 3 months after CHR achievement, or in whom major cytogenic response (MCyR) had not been achieved after 12 months.
• ‡"Unsatisfactory therapeutic effect" was counted as progression. Any discontinuation due to "Unsatisfactory therapeutic effect" was considered progression although this reason was also used in cases where loss or lack of response were the reasons, which did not necessarily indicate a progression to a more advanced phase of CML.

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Initial response to imatinib is not a reliable predictor of subsequential efficacy

Hammersmith Study

A single institution study of 204 consecutive newly diagnosed adult patients with chronic phase CML treated with imatinib 400 mg/d first line evaluated cytogenetic and molecular responses, PFS, and OS.

Median follow-up: 38 months (range: 12 to 85 months)

• 78% (159/204) achieved CCyR
  • 15% (24/159) who had initially achieved CCyR lost CCyR, lost CHR, or progressed to AP/BC [acquired resistance]
• 22% (45/204) did not achieve CCyR [initial response]

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