Dasatinib in recurrent glioblastoma: failure as a teacher

The Radiation Therapy Oncology Group (RTOG) trial 0627 evaluated the efficacy of dasatinib in recurrent glioblastoma. Why was dasatinib, of unequivocal benefit in Philadelphia chromosome– positive leukemia and so promising in gastrointestinal stromal tumor, of interest in glioblastoma? And why did the drug fail? Dasatinib is a potent oral multitargeted kinase inhibitor that inhibits all members of the Src family of kinases (SFK). At higher concentrations, dasatinib also inhibits BCR-Abl (breakpoint cluster region–Abelson murine leukemia), c-kit, platelet-derived growth factor receptor (PDGFR)–beta, and ephrin receptors. Among these other potential targets of dasatinib, negative clinical trials with imatinib have called into question the utility of attacking c-kit and PDGFR. In contrast, considerable evidence supports a central role of Src and other SFKs in glioblastoma. As the cellular form of the Rous sarcoma virus, Src represents the first discovered human proto-oncogene. Ironically, Src is probably not a causative agent in human tumorigenesis; activating mutations in human tumors are extremely rare, and Src alone is insufficient to transform human cells. Rather, Src is thought to maintain the neoplastic phenotype and to play a role in tumor progression. Src family members are non-receptor tyrosine kinases that function at the interface between extracellular signals and intracellular signaling pathways. Src is normally held in an inactive conformation through intramolecular interaction of the Src homology-2 (SH2) domain with phospho-Y527. Activation of Src occurs in conjunction with dephosphorylation of this residue and binding of the SH2 domain to alternate phospho-tyrosine residues on the cytoplasmic portion of activated receptor tyrosine kinases (including epidermal growth factor receptor [EGFR], PDGFR, insulin-like growth factor receptor, fibroblast growth factor receptor, c-met, and vascular endothelial growth factor receptor [VEGFR]) or other target phosphorylated transmembrane proteins such as integrins or ephrins. Autophosphorylation of Y416 within the kinase domain further contributes to kinase activation. Activated Src signaling has pleiotropic functions within the cell. Src functions in a complex with focal adhesion kinase (FAK) and proline-rich tyrosine kinase 2 to modulate cytoskeletal organization and remodeling accompanying adhesion, motility, invasion, and cell division. By activating the pathways of mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase, Src and Lyn also promote cell proliferation and activate anti-apoptotic or prosurvival pathways. The relationship with receptor tyrosine kinases is bidirectional and synergistic; Src can phosphorylate EGFR and PDGFR, leading to increased MAPK pathway activity and enhanced mitogenesis and transformation. Under hypoxic conditions, Src activation promotes angiogenesis through stimulation of VEGF, matrix metalloproteinases, and interleukin-8 expression. Through phosphorylation of signal transducer and activator of transcription 3, Src activity induces VEGF expression. Downstream from the VEGF receptor, Src signaling is critical for FAK activation and VEGF-mediated disruption of tight junction and adherens junction integrity that contribute to a physical blood–brain barrier (BBB). Thus, SFKs play a central role in signaling downstream from various receptor tyrosine kinases and other signaling pathways that are critical both for normal cell and tumor cell growth, motility, and survival. Preclinical and human tumor studies support a potentially important role of Src in human glioblastoma. Transgenic mice expressing v-Src develop glioblastomas. Bead technology assessing tyrosine kinase phosphorylation demonstrated Src kinase activation in glioblastoma cell lines as well as in primary patient samples. This latter result is consistent with common molecular alternations in glioblastoma multiforme (GBM) that drive Src activation, such as amplification of EGFR and PDGFR and upregulation of integrin receptors such as v3 and v5. The Src family member Lyn kinase activity is very high in human GBM and is thought to promote migration. Based on the role for SFKs in glioma biology, there is a strong rationale to pursue SFK inhibitors such as dasatinib in GBM. Preclinical studies have demonstrated reproducible suppression of various phosphorylation targets. Dasatinib reduces autophosphorylation of Src and downstream signaling to Akt and S6 in GBM cell lines and reduces GBM cell growth and invasion. SFK inhibition by dasatinib also induces autophagic cell death in vitro in GBM. In U87 heterotopic flank tumors, dasatinib therapy was associated with a marked suppression of integrin membrane localization and suppression of tumor growth. Although no direct clinical data were available at the inception of RTOG