For personal use. Only reproduce with permission from The Lancet Publishing Group. MECHANISMS OF DISEASE THE LANCET • Vol 359 • February 9, 2002 • www.thelancet.com 487 Summary Background BCR-ABL, a constitutively activated tyrosine kinase, is the oncogene that causes Philadelphia- chromosome-positive (Ph+) leukaemia. STI571, a competitive inhibitor at the ATP-binding site of BCR-ABL, has been shown to have high activity in this type of leukaemia. However, most patients with advanced disease relapse despite continued treatment with STI571. We aimed to find out whether point mutations in BCR-ABL cause resistance to STI571. Methods We analysed clinical samples from eight patients resistant to STI571—who had advanced-stage Ph+ leukaemia—for mutations within the ATP-binding site and activation loop of BCR-ABL. Analysis was done before treatment with STI571 and at time of relapse. Findings We identified five distinct point mutations in the BCR-ABL kinase domain in seven patients. All point mutations arose at positions that have proved to be important for drug binding and have conferred resistance to STI571 in vitro. All patients with mutations had lymphoid leukaemia. Interpretation Different mutations within the kinase domain of BCR-ABL can be responsible for refractoriness of Ph+ leukaemia to STI571. Mutation in the BCR-ABL kinase domain might be a frequent mechanism of STI571 resistance in lymphoid disease. Lancet 2002; 359: 487–91 See Commentary page 458 Introduction BCR-ABL is a constitutively activated tyrosine kinase that arises from formation of the PHILADELPHIA CHROMOSOME. 1 This enzyme is the characteristic molecular abnormality present in almost all cases of chronic myeloid leukaemia and in up to 20% of cases of adult acute lymphoblastic leukaemia. 2,3 BCR-ABL can cause chronic myeloid leukaemia in mice, 4 and the ability of this enzyme to cause leukaemia is dependent on tyrosine kinase activity. 5 STI571 (imatinib; Glivec; Novartis, Basel, Switzerland), which is a competitive inhibitor of BCR-ABL at the ATP-BINDING SITE, rapidly reverses clinical and haematological abnormalities of Philadelphia- chromosome-positive (Ph+) leukaemias. 6,7 Whereas almost all patients with chronic-phase chronic myeloid leukaemia respond well to treatment with STI571, remissions in patients with chronic myeloid leukaemia blast crisis and Ph+ acute lymphoblastic leukaemia are transient, and most patients relapse despite continued treatment. 7 In-vitro analysis of STI571-resistant Ph+ cell lines showed amplification of the BCR-ABL gene, high expression of the BCR-ABL protein, and increased expression of the multidrug-resistance protein MDR1. 8–10 Gorre and colleagues 11 reported a point mutation in the region coding for the ATP-binding site of the ABL kinase domain in six of nine people with advanced-stage leukaemia, who were Ph+ and STI571-refractory. This mutation resulted in one aminoacid substitution at position 315, from threonine to isoleucine (T315I). Threonine 315 forms an important hydrogen bond with the secondary amino group of STI571, 12 suggesting that this mutation could be a major cause of resistance to STI571. We analysed clinical samples from eight patients with advanced-stage leukaemia, who were Ph+ and resistant to STI571 treatment to find out whether point mutations in BCR-ABL caused resistance to STI571. Methods Procedures We took clinical samples routinely before treatment. All patients who relapsed and who were able to come to the hospital again were included in the study. There was no selection of patients. We thus obtained peripheral blood samples, bone marrow samples, or both before STI571 treatment and at relapse from eight patients with either chronic myeloid leukaemia or acute lymphoblastic leukaemia who were Ph+. Patients had been given 600 mg per day STI571 as part of a randomised trial and we recorded their haematological and cytogenetic responses to this treatment. We obtained written informed consent from all patients, and the study protocol was approved by the local ethics committee. Accelerated-phase leukaemia was diagnosed when the percentage of blasts in bone marrow was 16–29%, and blast crisis was defined when this percentage was 30% or greater. Complete haematological remission was based on all these criteria: blast count of less than 5% in bone BCR-ABL gene mutations in relation to clinical resistance of Philadelphia-chromosome-positive leukaemia to STI571: a prospective study Nikolas von Bubnoff, Folker Schneller, Christian Peschel, Justus Duyster Department of Internal Medicine III, Laboratory of Leukemogenesis, Technical University of Munich, Ismaningerstrasse 22, 81675 Munich, Germany (N von Bubnoff MD, F Schneller MD, Prof C Peschel MD, J Duyster MD) Correspondence to: Dr Justus Duyster (e-mail: justus.duyster@lrz.tum.de)