Over the past decade, therapeutics that target subsets of the 518 human protein kinases have played a vital role in the fight against cancer. Protein kinases are typically targeted at the adenosine triphosphate (ATP) binding cleft by type I and II inhibitors, however, the high sequence and structural homology shared by protein kinases, especially at the ATP binding site, inherently leads to polypharmacology. In order to discover or design truly selective protein kinase inhibitors as both pharmacological reagents and safer therapeutic leads, new efforts are needed to target kinases outside the ATP cleft. Recent advances include the serendipitous discovery of type III inhibitors that bind a site proximal to the ATP pocket as well as the truly allosteric type IV inhibitors that target protein kinases distal to the substrate binding pocket. These new classes of inhibitors are often selective but usually display moderate affinities. In this review we will discuss the different classes of inhibitors with an emphasis on bisubstrate and bivalent inhibitors (type V) that combine different inhibitor classes. These inhibitors have the potential to couple the high affinity and potency of traditional active site targeted small molecule inhibitors with the selectivity of inhibitors that target the protein kinase surface outside ATP cleft.
"Two current approaches to generate selective small molecule kinase inhibitors are the allosteric approach, which involves induction or prevention of enzyme conformational changes via targeting sites outside the catalytic region, and the active site approach –. The allosteric approach is an area of active investigation that has yet to be generally validated or reduced to standard practice. "
[Show abstract][Hide abstract] ABSTRACT: Serine-threonine protein kinases are critical to CNS function, yet there is a dearth of highly selective, CNS-active kinase inhibitors for in vivo investigations. Further, prevailing assumptions raise concerns about whether single kinase inhibitors can show in vivo efficacy for CNS pathologies, and debates over viable approaches to the development of safe and efficacious kinase inhibitors are unsettled. It is critical, therefore, that these scientific challenges be addressed in order to test hypotheses about protein kinases in neuropathology progression and the potential for in vivo modulation of their catalytic activity. Identification of molecular targets whose in vivo modulation can attenuate synaptic dysfunction would provide a foundation for future disease-modifying therapeutic development as well as insight into cellular mechanisms. Clinical and preclinical studies suggest a critical link between synaptic dysfunction in neurodegenerative disorders and the activation of p38αMAPK mediated signaling cascades. Activation in both neurons and glia also offers the unusual potential to generate enhanced responses through targeting a single kinase in two distinct cell types involved in pathology progression. However, target validation has been limited by lack of highly selective inhibitors amenable to in vivo use in the CNS. Therefore, we employed high-resolution co-crystallography and pharmacoinformatics to design and develop a novel synthetic, active site targeted, CNS-active, p38αMAPK inhibitor (MW108). Selectivity was demonstrated by large-scale kinome screens, functional GPCR agonist and antagonist analyses of off-target potential, and evaluation of cellular target engagement. In vitro and in vivo assays demonstrated that MW108 ameliorates beta-amyloid induced synaptic and cognitive dysfunction. A serendipitous discovery during co-crystallographic analyses revised prevailing models about active site targeting of inhibitors, providing insights that will facilitate future kinase inhibitor design. Overall, our studies deliver highly selective in vivo probes appropriate for CNS investigations and demonstrate that modulation of p38αMAPK activity can attenuate synaptic dysfunction.
PLoS ONE 06/2013; 8(6):e66226. DOI:10.1371/journal.pone.0066226 · 3.23 Impact Factor
"Alternatively, one can expect that stage-, tissue-, and HDAC-specific inhibitors will be developed. A similar approach is now being implemented with respect to protein kinase inhibitors, aimed at treating various diseases, especially cancer (Lamba and Ghosh, 2012). Certainly, use of epigenetic drugs can have multiple and complex effects on different systems and tissues. "
"Furthermore, since catalytic kinase inhibitors in current clinical use are ATP competitors, they need to be used at relatively high and potentially toxic concentrations in order to effectively compete with ATP, whose intracellular concentration is ∼1 mM. As a result, there has recently been considerable interest and progress in developing allosteric kinase inhibitors, which bind to sites other than the catalytic site in kinases and, thus, are likely to be much more selective and less toxic (Lamba and Ghosh, 2012). Our recent study (Kong et al., 2011) demonstrates a new potential approach for attenuating PKCθ-dependent functions utilizing allosteric compounds based on the critical PR motif in the V3 domain of PKCθ that will block its Lckmediated association with CD28 and recruitment to the IS, which is obligatory for its downstream signaling functions. "
[Show abstract][Hide abstract] ABSTRACT: Protein kinase C-theta (PKCθ) is a key enzyme in T lymphocytes, where it plays an important role in signal transduction downstream of the activated TCR and the CD28 costimulatory receptor. TCR/CD28 engagement triggers the translocation of the cytosolic PKCθ to the plasma membrane, where it localizes at the center of the immunological synapse (IS), which forms at the contact site between an antigen-specific T cell and antigen-presenting cells (APC). The cellular redistribution of PKCθ in resting versus activated T cells has been thoroughly investigated, but the mechanisms governing its translocation to the center of the IS, and how this unique localization relates to the biological activity of PKCθ have remained unclear until recently. A very recent study has shown that the unique V3 (hinge) domain of PKCθ is essential and sufficient for its localization at the IS, where it is anchored to the cytoplasmic tail of CD28 via an indirect mechanism, involving the Lck as an intermediate. Furthermore, the PKCθ-CD28 complex, which forms upon antigen stimulation, is localized at a newly recognized, TCRlow subregion of the central IS, where it forms an outer ring around the very center, TCRhigh subregion. Importantly, the association of PKCθ with CD28 is also essential for PKCθ-mediated activation of downstream signaling pathways, including the transcription factors NF-κB and NF-AT, which are sine qua non for the productive activation of T lymphocytes. Indeed, the use of V3-altered PKCθ mutants or the isolated V3 domain as a negative dominant mutant demonstrated that strategies, which disrupt the interaction between PKCθ and CD28, block T cell activation, proliferation and differentiation into pathogenic Th2 and Th17 (but not Th1) effector helper T cells. The recent progress made in understanding of the mechanism of recruitment and regulation of PKCθ activity at the IS is likely to facilitate the development of PKCθ-based therapeutic modalities for T cell-mediated diseases.
Frontiers in Immunology 08/2012; 3:273. DOI:10.3389/fimmu.2012.00273
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