Efficacy of Focal Adhesion Kinase Inhibition in Combination with Dasatinib in BCR-ABL1 Acute Lymphoblastic Leukemia

Introduction: BCR-ABL1⁺ B-progenitor acute lymphoblastic leukemia (Ph⁺ B-ALL) is a highly aggressive disease that is often refractory to currently available therapies. Our previous genomic profiling studies have identified loss-of-function or dominant negative mutations in IKZF1, encoding the lymphoid transcription factor Ikaros, in over 80% of Ph+ ALL. In addition, deletion of CDKN2A, which encodes the INK4A and ARF tumor suppressors, is observed in approximately half of all cases (Mullighan et al., 2008). Alterations of IKZF1 are associated with poor outcome despite the use of tyrosine kinase inhibitors (TKIs). Ikzf1 alterations, including Ikaros isoform 6 (IK6), result in the acquisition of stem cell-like features, enhanced self-renewal, expression of adhesion molecules, and transcriptional upregulation of focal adhesion kinase (FAK), resulting in increased adhesion in vitro and in vivo, and decreased sensitivity to TKIs (Churchman, Cancer Cell, in press). VS-4718 is a potent, selective, and orally bioavailable FAK inhibitor currently under evaluation in a phase 1 clinical trial in subjects with various solid tumors, however in vivo efficacy in hematological malignancies had not been evaluated.  Targeting FAK with VS-4718 is an attractive approach to abrogate the adhesive phenotype of IKZF1-altered leukemic cells potentially enhancing the effects of dasatinib in the treatment of high-risk BCR-ABL1 B-ALL.

Methods: We examined the efficacy and mechanisms of FAK inhibition using VS-4718 as a single agent and in combination with dasatinib in vitro and in vivo in a range of xenograft and genetically engineered mouse models of BCR-ABL1 ALL. Each model had concomitant deletion of Arf which is observed in approximately 50% of human cases. 

Results: A pre-clinical in vivo trial of dasatinib and VS-4718 combination therapy in a murine C57Bl/6 Arf^-/- BCR-ABL1 pre-B cell model resulted in a marked increase in survival in both IK6-expressing and non-IK6 cohorts of mice, and one complete long-term remission in the IK6-expressing group.  Further, we showed increased efficacy of VS-4718 and dasatinib, compared to either agent alone, against two highly aggressive human Ph+ IK6-expressing B-ALL xenografts in vivo, with decreased infiltration of leukemic cells in bone marrow and spleens demonstrating  a  synergistic effect of the VS-4718/dasatinib combination. In vitro cell viability was reduced with induction of apoptosis at increasing concentrations of VS-4718 as a single agent, and further potentiated the effects of dasatinib in cytotoxicity assays using human xenografted and murine leukemic cells. 

VS-4718 profoundly diminished the ability of BCR-ABL1-expressing cells to form cell-matrix adhesions in vitro, as evident by the reduced adherence to fibronectin monolayers and bone marrow stromal cells. VS-4718 almost completely abolished the colony-forming potential of BCR-ABL1-expressing murine pre-B cells with and without Ikzf1 alterations at drug concentrations that do not affect cell viability suggestive of a reduction in self-renewal. 

Calvarial imaging of mice transplanted with Ikzf1-altered BCR-ABL1 leukemic cells and treated with VS-4718 alone in vivo revealed a discernible reduction in adhesion in the intact bone marrow niche of Prrx1-Cre; LSL-tdTomato recipient mice. VS-4718 treated leukemic cells localized to Prrx1-expressing perivascular endothelial cells and exhibited round morphology in contrast to the typical spindle-like appearance of Ikzf1-altered pre-B cells adhering to the bone marrow stroma, suggesting that VS-4718 treatment abolished the aberrant leukemic cell-stromal adhesion induced by Ikaros alterations in vivo.

Conclusions: Direct inhibition of FAK with VS-4718 attenuates the adhesive, stem-like properties of IKZF1-altered BCR-ABL1 leukemic cells that contribute to the poor prognosis of patients treated with currently available therapies. Targeted FAK inhibition is thus a promising avenue for improving the response of BCR-ABL1 ALL to dasatinib, particularly in refractory cases harboring IKZF1 alterations. These data support the clinical development of VS-4718 in combination with dasatinib in Ph+ B-ALL.