Various Mechanisms Underlie Cytogenetic Refractoriness to Imatinib.

Background: Imatinib is the first line drug therapy for patients with chronic myeloid leukemia (CML). Drug resistance, after an initial response, is frequently due to reactivation of the BCR-ABL kinase as a result of point mutations within the kinase domain (KD) or overexpression of the protein. In contrast, mechanisms underlying disease persistence (refractoriness) at the cytogenetic level are unknown.

Patients and Methods: We investigated potential causes for cytogenetic refractoriness in 28 patients with CML who achieved a complete hematological response (CHR) but remained at least 65% Philadelphia chromosome positive (Ph+) during the first 10 months of imatinib therapy. At the time of starting imatinib, 5 patients were in accelerated phase (AP), of which 3 had only cytogenetic clonal evolution as the defining criteria for AP; the remaining 23 were in first chronic phase (CP). All patients initiated therapy with at least 400mg of imatinib daily, with 3 also receiving imatinib in combination with low dose cytarabine. Median time from diagnosis to starting imatinib was 43.3 months (range, 9 to 103.4) and median time to CHR was 41.5 days (range, 19 to 220). Patients were analyzed for evidence of clonal evolution (conventional cytogenetics), mutations in the KD (D-HPLC and direct sequencing of PCR products generated with BCR-ABL-specific primers), expression of BCR-ABL, and MDR1 mRNA (quantitative RT-PCR). Samples prior to starting imatinib were available in 16 patients. Median follow up was 28.3 months (range, 10 to 58.3).

Results: Between 9 and 15 months from starting imatinib, 4 of 28 patients had a single KD mutation detected (M351T, G250E, Y253H, Y253F). Three out of 16 patients demonstrated a >2 fold increase in BCR-ABL expression (range, 2.68 to 15.14), and 4 out of 16 patients showed a >2 fold increased expression of MDR1 (range, 2.72 to 47.57), compared to pre-imatinib levels. One patient demonstrated an increase in both BCR-ABL and MDR1 expression. (None of the patients with increased BCR-ABL or MDR1 expression had a KD mutation detected.) Overall, 14 patients progressed, including all patients with a KD mutation. The majority of patients with increased BCR-ABL (2/3) or MDR1 (3/4) expression, including the patient with dual increased expression, did not progress during follow up but remained at least 95% Ph+.

Conclusions: Twenty eight patients with cytogenetic refractoriness to imatinib were examined and KD mutations were detected in 4. Of the patients with samples available prior to starting imatinib, a >2 fold increase in expression of BCR-ABL and the drug resistance gene MDR1 were seen in 3/16 and 4/16 patients respectively. Overall, these data suggest that various mechanisms, including increased BCR-ABL expression and induction of drug resistance gene expression, underlie cytogenetic refractoriness in our patient population. Studies are ongoing to further examine the role of drug resistance genes in patients with cytogenetic refractoriness, as compared to patients who do achieve a major cytogenetic response within the first year of imatinib therapy.