Acquired Extramedullary Resistance to Dasatinib Due to Selection of Philadelphia-Positive Lymphoblast Clone Harboring a T315I BCR-ABL Gene Mutation: Reversal by Dose Escalation and Hydroxyurea.

Most patients with advanced Philadelphia-positive (Ph+) hematologic malignancies develop resistance to imatinib. Acquired resistance to imatinib is commonly a result of selection for subclones bearing point-mutations in the catalytic kinase domain of BCR-ABL. Dasatinib (BMS-354825), a dual-specific SRC/ABL kinase inhibitor, has shown activity in imatinib-resistant Ph+ diseases both in vitro and in vivo. Preliminary data also indicate efficacy in patients. Based on laboratory evidence, dasatinib appears to inhibit all known BCR-ABL mutant clones, with the exception of T315I, a gatekeeper mutation conferring resistance to several kinase inhibitors. Here we describe a Ph+ ALL patient, who initially developed imatinib resistance (hematologic) possibly due to BCR-ABL amplification (FISH). His disease relapsed as extensive extramedullary tumors bearing wild-type BCR-ABL. He received dasatinib 70 mg BID as part of the BMS CA180–015 study and achieved a very good partial remission. After 5 months of therapy, the disease relapsed as a solitary axillary tumor and several small palmar skin lesions. He also had blasts in the CSF indicative of neuroleukemia. Bone marrow remained in cytogenetic remission. FISH analysis of the tumor revealed 2–3 copies of BCR-ABL as previously. A highly sensitive, quantitative, mutation-specific PCR (Gruber F, ASH 2004) showed the presence of the T315I mutation, which was confirmed by sequencing. A very low level of T315I transcript was also detected in the blood. Dasatinib dose was escalated to 100 mg BID, and low-dose hydroxyurea 500 mg BID was initiated to putatively enhance the access of dasatinib in the CSF sanctuary. He also received two doses of i.t. therapy (methotrexate, cytarabine). Patient’s symptoms (confusion, headache) related to neuroleukemia resolved rapidly, skin lesions disappeared and axillary tumor decreased in size. He is currently symptom-free and has no signs of active ALL. The favorable response to dasatinib dose escalation and low-dose hydroxyurea was unexpected. Preclinical data on T315I mutant cell lines would argue against a significant concentration dependence in kinase inhibition by dasatinib. Putatively, targets other than BCR-ABL may be of importance in particular in Ph+ ALL (e.g. Src, Lyn), and this effect may account for the response. Similar off-target activity of hydroxyurea is utilized in clinical trials to overcome resistance to multidrug HIV therapy - a setting resembling current treatment of Ph+ malignancies with kinase inhibitors.