Imatinib mesylate (STI571), a specific Bcr-Abl inhibitor, has shown a potent 
antileukemic activity in clinical studies of chronic myeloid leukemia (CML) 
patients. Early prediction of response to imatinib cannot be anticipated. We 
used a standardized quantitative reverse-transcriptase polymerase chain reaction 
(QRT-PCR) for BCR-ABL transcripts on 191 out of 200 late-chronic phase CML 
patients enrolled in a phase II clinical trial with imatinib 400 mg/day. Bone 
marrow samples were collected before treatment, after 12, 20 and at the end of 
study treatment (52 weeks) while peripheral blood samples were obtained after 2, 
3, 6, 10, 14, 20 and 52 weeks of therapy. The amount of BCR-ABL transcript was 
expressed as the ratio of BCR-ABL to beta2-microglobulin (beta2M). We show that, 
following initiation of imatinib, the early BCR-ABL level trends in both bone 
marrow and peripheral blood samples made it possible to predict the subsequent 
cytogenetic outcome and response. We propose this method as the method of choice 
for monitoring patients on imatinib therapy. QRT-PCR studies may be able to 
identify degrees of molecular response that predict both complete cytogenetic 
response and long term stability, as well as patterns of response that provide 
an early indication of relapse and imatinib resistance.