Serial Monitoring of BCR-ABL Transcripts in Chronic Myelogenous Leukemia (CML) Treated with Imatinib Mesylate

Survival among chronic myelogenous leukemia (CML) patients can be linked to the reduction in leukemic cell burden. Treatment with imatinib mesylate results in a high frequency of complete cytogenetic response, which can be further stratified using quantitative reverse-transcriptase polymerase chain reaction (qRT-PCR). We have serially monitored peripheral blood and bone marrow BCR-ABL transcripts using qRT-PCR in CML patients commencing imatinib therapy, and compared the results with bone marrow cytogenetics. Seventeen patients (aged 25-74 yr) with Philadelphia chromosome positive CML in first chronic phase were treated with imatinib targeting a dose of 400 mg/d. The median follow up is 30 mo (range 9-33 mo). Every third month the product of the BCR-ABL fusion gene was evaluated in both blood and bone marrow specimens by real-time RT-PCR using the TaqMan probe system. In 113 simultaneously obtained blood and bone marrow samples, the BCR-ABL transcript values agreed well with cytogenetic data. Blood and bone marrow specimens gave comparable values for BCR-ABL transcripts. Before start of imatinib therapy there was a considerable variation in BCR-ABL transcripts among the patients, ranging approximately one log (base 10). Similarly, patients with a complete cytogenetic response following imatinib therapy had variable BCR-ABL transcript levels, ranging at least three logs (base 10). The major decline in BCR-ABL transcripts occurred within 6 mo after start of imatinib therapy. The decline in BCR-ABL transcripts, following imatinib therapy, appears to level off at 12-15 mo. Two late responders were identified with a still decreasing level in BCR-ABL transcripts after 24 mo of treatment. It is concluded that BCR-ABL mRNA quantification in peripheral blood is suitable for routine monitoring of the response to treatment and long-term disease status in CML, especially in patients who have achieved a complete cytogenetic response. A plateau in BCR-ABL transcripts seems to have been reached after 12-15 mo of imatinib treatment; however, some "late responders" are seen.