Significant Activity of Dasatinib in CLL Demonstrated by Ex-Vivo Assay.

Introduction: Chronic lymphocytic leukemia (CLL) is a lymphoproliferative disorder characterized by the accumulation of mature monoclonal B-lymphocytes. Combination therapies produce high response rates, including complete responses, but all patients will eventually relapse. Novel therapies are needed. Ex-vivo chemosensitivity assays offer two distinct advantages for identifying new CLL therapies. First, agents can be screened without exposing patients to unnecessary risk. Second, no CLL clonogenic cell lines exist, requiring that patient samples be analyzed in an ex-vivo manner. Dasatinib is a potent inhibitor of Src, Abl, and several other tyrosine kinases. B-CLL has been characterized as having anomalous signaling through Lyn kinase, a member of the Src family. We sought to determine the single agent and combination ex-vivo activity of dasatinib in CLL samples.

Methods: We used an ATP-luminescent chemosensitivity assay (ViaLight, Lonza, Basel, Switzerland) to assess the activity of single agent dasatinib (0.01 nM to 1 μM), and dasatinib (1, 10, 100 nM) in combination with fludarabine (0.06 to 64 μg/ml), chlorambucil (0.2 to 102.4 μg/ml), or methylprednisolone (0.02 to 2,000 μg/ml) in CLL patient peripheral blood samples. Patients had a diagnosis of B-CLL, WBC count greater than 20 x 10⁹/L, no chemotherapy within 14 days, and no antibody therapy within 28 days.

Results: Five patient samples were obtained; 4 from untreated patients and 1 from a patient being treated with fludarabine. Rai stages ranged from 0 to IV. Single agent dasatinib induced apoptosis in 3 of 5 CLL samples tested. The two samples without apoptosis were from untreated patients. IC50 values were >1 (no apoptosis), >1 (no apoptosis), 0.2, 0.2, and 0.04 (fludarabine treated patient) μM. The IC50 values of single agent fludarabine were 1.3, 0.18, 1, 0.22, and 3 (fludarabine treated patient) μg/ml, respectively. The IC50 values of fludarabine in combination with 100 nM dasatinib were 0.41, <0.06, 0.45, <0.06, and <0.06 (fludarabine treated patient) μg/ml, respectively.

Conclusions: In this ex-vivo CLL model, dasatinib has significant single agent activity and may be synergistic in combination regimens. All responses were achieved with clinically attainable concentrations of dasatinib. These ATP-luminescent chemosensitivity results provide the pre-clinical rationale for clinical studies of dasatinib in patients with CLL. Currently, two single agent dasatinib phase II studies are accruing patients and plans exist for a combination dasatinib study in CLL patients next year.