Focal adhesion kinase (FAK) is a member of a family of nonreceptor protein-tyrosine kinases that regulates integrin and growth factor signaling pathways involved in cell migrationThe ability of cells to respond appropriately to environmental cues is critical to maintaining cellular, tissue, and organism homeostasis. One such environmental cue is derived from cellular adhesion to the extracellular matrix. The loss of adhesiondependent cellular regulation can lead to increased cellular proliferation, decreased cell death, changes in cellular differentiation status, and altered cellular migratory capacity, all of which are critical components of cell carcinogenesis and metastatic progression.The FAK 4 family kinases (which include FAK and Pyk2) regulate cell adhesion, migration, and proliferation in a variety of cell types (for review see Refs. 1-3). Adhesion of cells to the extracellular matrix is mediated by heterodimeric transmembrane integrin receptors located within sites of close opposition to the underlying matrix called focal adhesions. Integrin engagement and clustering stimulates FAK phosphorylation on Tyr 397 , creating a high affinity binding site for Src and Src family kinases. The FAK⅐Src complex phosphorylates many components of the focal adhesion, resulting in changes in adhesion dynamics and the initiation of signaling cascades. In addition to FAK catalytic activity, FAK also functions as a scaffold to organize structural and signaling proteins within focal adhesions.The importance of FAK as a regulator of normal cellular function is underscored by the number of cancers reported to have alterations in FAK expression and/or activity, including colon, breast, thyroid, prostate, cervical, ovarian, head and neck, oral, liver, stomach, sarcoma, glioblastoma, and melanoma (4, 5). Additionally, alterations in FAK expression and/or activity have been associated with tumorigenesis and increased metastatic potential (4, 5). Currently, it is unclear how the catalytic and/or scaffolding function of FAK contributes to tumor progression. To date studies of FAK function have relied on the expression of dominant interfering mutants or elimination of FAK expression by genetic knock-out, antisense oligonucleotide expression, or small interfering RNA.Herein, we report the biochemical and cellular characterization of a novel small molecule inhibitor, PF-573,228 (here after referred to as PF-228), that targets FAK catalytic activity. The inhibitor interacts with FAK in the ATP-binding pocket and effectively blocks the catalytic activity of recombinant FAK protein or endogenous FAK expressed in a variety of normal and cancer cell lines. Treatment of cells with PF-228 blocked FAK phosphorylation on Tyr 397 and concomitantly reduced the tyrosine phosphorylation of paxillin, a recognized downstream effector of FAK signaling. Drug treatment of normal and cancer cells resulted in decreased cell migration and inhibited adhesion turnover, biological activities previously ascribed to FAK. Interestingly, inhibition of FAK activi...