p21-activated kinase 1 (PAK1) has attracted much attention as a potential therapeutic target due to its central role in many oncogenic signaling pathways, its frequent dysregulation in cancers and neurological disorders, and its tractability as a target for small-molecule inhibition. and efficacy. Introduction PAK1 is a founding member of the Pak (p21-activated kinases) Ser/Thr protein kinase family. Initially identified as an interactor of the Rho GTPases RAC1 and CDC42 , PAK1 was later shown to play diverse role in cell signaling by means of its catalytic and scaffolding activities . Signal transduction cascades modulated by PAK1 include proliferation and survival pathways such as MAPK, AKT, Wnt1/-catenin, ER, BAD and NF-B . PAK1 is also critically involved in regulation of cell motility, transmitting variety of signals controlling cytoskeleton dynamics, cell shape and adhesion [2C4]. While PAK1 shares functions with other family members, in particular PAK2 and PAK3 (which are, with PAK1, together referred to as group I Paks) much more is known of the function of PAK1 in terms of human biology and disease than any other isoform. PAK1 expression is dysregulated in several nervous system disorders, including Alzheimer disease and Fragile X syndrome , indicating a role in cognition. Gain-of-function alterations of PAK1 have been observed in a wide range of human malignancies, suggesting that this kinase plays a substantial role in tumor development and progression [2, 6]. Amplification of the gene at 11q13, as well as elevated PAK1 protein levels, are often associated with aggressive tumor phenotypes, chemotherapy resistance, and poor outcome [2, 7C9]. Apart from gene amplification and protein overexpression, PAK1 can be hyperactivated by mutations in upstream regulators such as RAC1 , RAS  and Merlin , linking oncogenic signaling to cancer cell phenotypic changes. For these reasons, targeting PAK1 may represent a promising therapeutic approach in certain disease contexts, and multiple efforts in identification of potent and selective PAK1 inhibitors have been made in the past decade [2, 13]. Here we discuss the suitability of PAK1 as a drug target and recent advances in the HCl salt development of PAK1 inhibitors. PAK1 structure and regulation PAK1 is a 545 amino acid multidomain protein that contains an N-terminal regulatory region and a C-terminal kinase (catalytic) domain (Figure 1) [14, 15]. The PAK1 catalytic domain has the characteristic two-lobe kinase structure with a single phosphorylation site (Thr423) within the activation loop. The amino terminal end of PAK1 harbors several sequence motifs responsible for interacting with partner proteins. Residues 75C90 correspond to the CDC42/RAC1 interactive-binding (CRIB) domain, which partially overlaps the auto-inhibitory domain (AID, aa 83-149). Three Pro-rich N-terminal motifs interact HCl salt with SH3-domain containing adaptor proteins, including GRB2 (aa 12C18), NCK (aa 40C45), and the exchange factor PIX (aa 186C203) . A positively charged basic region adjacent to CRIB domain is critical for PAK1 binding to cell membrane phosphoinositides . Several phosphorylation sites located in the regulatory region play role in enabling and stabilizing the active conformation of PAK1 (Figure 1A) [17C19]. Open in a separate window Figure 1 PAK1 structureOrganization of the PAK1 polypeptide chain highlighting sites of kinase phosphorylation. Numerals indicate residue numbers. PAK1 auto-regulatory region is in magenta, N-lobe of the catalytic domain is HCl salt in green, and C-lobe is in blue. Proline-rich SH3-binding sites are shown as black bars. Phosphoinositide binding region enriched with basic residues is Mouse monoclonal to Influenza A virus Nucleoprotein shown as srossed bar. Diagram of dimeric PAK1 (PDB ID: 1F3M). One PAK1 complex is colored as in (A), Thr 423 is labeled. The other one is presented as surface diagram. Residues 1C77 and 148C248 are omitted. PAK1 activity is regulated by a squamous cell carcinoma mouse model . Another compound of this chemical series, FRAX486 has been studied as a possible treatment of fragile X syndrome (FXS), a genetic disorder caused by inactivation of the fragile X mental retardation 1 (knockout (KO) mice recapitulate human FXS symptoms, including hyperactivity, repetitive behaviors, and seizures, as well as morphological synaptic abnormalities [43, 44]. FRAX486 has excellent PAK1 potency (IC50 = 8.25 nM) and pharmacokinetic properties upon subcutaneous injection, including effective bloodCbrain barrier penetration, allowed its exploitation in an KO model. Strikingly, single administration of FRAX486 was sufficient to ameliorate the FXS phenotype at both cellular and behavioral levels, in line with previous studies on genetic inactivation of Pak in this KO mouse model . An advanced member of this series, FRAX1036 (PDB ID:5DFP), exhibits high PAK1 potency (PAK1 Ki = 23 nM), refined kinome selectivity [42, 46, 47], and represents a useful tool compound for single and.