The closely related members of the Rho family, Rac and Cdc42, have been extensively studied due to their pivotal roles in actin cytoskeleton BYL719 organization, migration/invasion and metastasis, epithelial to mesenchymal transition, transcription, cell proliferation, cell cycle progression, apoptosis, vesicle trafficking, angiogenesis, and cell adhesions [3], [4] and [5]. Indeed, studies from us and others have implicated hyperactive Rac1 and Rac3 with increased survival, proliferation, and invasion of many cancer types [6], [7], [8], [9] and [10]. In addition

to promoting cancer malignancy, Rac and Cdc42 have also been shown to be essential for Ras and other oncogene-mediated transformation [11] and [12]. Racs [1], [2] and [3] are activated by a myriad of cell surface receptors that include: integrins, G protein coupled receptors, growth factor receptors, and cytokine receptors. These cell surface receptors regulate cancer promoting signal cascades that have been implicated with Rac and its direct downstream effector p21-activated kinase (PAK) activity [13].

These pathways include: phosphoinositide 3-kinase (PI3-K)/Akt/mammalian target of Rapamycin (mTOR); signal transducer and activator of transcription (STATs); and the mitogen activated protein kinases (MAPKs): extracellular regulated kinase (ERK), jun kinase (JNK), and p38 MAPK [14], [15], [16], [17] and [18]. Activated Rac has also been shown to affect cell proliferation via Wnt activity signaling to the oncogenes c-Myc and Cyclin D [19]. Therefore, Rac GTPases play

a pivotal role in regulation of cancer malignancy, and targeting Racs appear to be a viable strategy to impede cancer metastasis [8], [15], [20] and [21]. Unlike Ras, Rho GTPases are not mutated in disease but activated via the deregulation of expression and/or activity of their upstream regulators, guanine nucleotide exchange factors (GEFs) [22]. Accordingly, although ~ 9% of melanomas were recently found to contain an activating Rac mutation [23], and the hyperactive splice variant Rac1b is frequently overexpressed in cancer [24], a majority of the Rac proteins in human cancer are activated due to up-regulated GEFs [21], [25] and [26]. So far, over 70 potential Rac GEFs are known; and many members of the largest family Alanine-glyoxylate transaminase of Rac GEFs, the Dbl family, have been identified as oncogenes [22], [27], [28] and [29]. Of the Rac GEFs, T-cell invasion and metastasis gene product (Tiam-1), Trio, Vav (1/2/3), and PIP3-dependent Rac exchanger (p-Rex1/2) have been implicated in the progression of metastatic breast and other cancers [30], [31], [32], [33], [34] and [35]. Therefore, the binding of GEFs to Rac and Cdc42 has been targeted as a rational strategy to inhibit their activity; and thus, metastasis. The Rac inhibitor NSC23766 was identified as a small molecule compound that inhibits the interaction of Rac with the GEFs Trio and Tiam1 [36], [37] and [38].