by Caitlin Smith
Protein kinases are enzymes with great power. Up to 30% of human proteins may be regulated by kinase activity. In fact, kinases regulate most intracellular pathways, especially signal transduction processes. Kinases effect such powerful changes by merely adding a phosphate group to their targets – removing a phosphate from ATP and transferring it to the target. The result of this chemical event varies depending on the target: a change in enzymatic activity, targeted translocation to a new place in the cytoplasm, or the opportunity to interact with other proteins.
Kinase inhibitors evolving
Assay Designs, an Enzo Life Sciences company, released new inhibitors this year that are actually inhibitors of the heat shock protein Hsp90. The abundant and highly conserved heat shock proteins are chaperone molecules that require the binding and hydrolysis of ATP for proper functioning. Assay Designs’ senior product manager for reagents, Dana Haley-Vicente, says that their new Hsp90 inhibitors “mechanistically work like kinase inhibitors by binding to the ATPase site of the target protein, [such as on] Hsp90.”
Symansis offers a collection of kinase inhibitors that are designed to offer you better-than-average quality, as well as choices. “We have a unique range of PI3 kinase inhibitors that no one else has,” says Leanne Daly, sales, marketing and business development executive at Symansis. For example, their inhibitors are native, tag-free proteins that are expressed in human cells for correct protein folding, and Symansis claims they will show better protein-protein interactions.
More specificity wanted – or not
Kinase inhibitors are famous – or infamous, depending upon your point of view – for not having as much specificity as one would desire to separate out the different targets from one another, posing a formidable obstacle for researchers to overcome. “The biggest challenge remains the unspecific nature of kinase inhibitors,” says Haley-Vicente. “For example, staurosporine inhibits a variety of kinases including PKA, PKG, MLCK, PKC, CaMK, tyrosine kinases and phosphorylase kinases via interaction with the ATP binding site. Scientists are challenged to improve the specificity by using new technologies.” For instance, Haley-Vicente cites two examples of new developments in inhibitors as “methodologies of high throughput technologies, and structure-based design software techniques to improve the specificity of the inhibitor.”
Yet sometimes a lack of selectivity is even more beneficial. In an era where researchers are striving to understand the off target effects of kinase inhibitors, they are also on the lookout for opportunities to exploit them – carefully. “For example,” says Daly, “sorafinib inhibits several kinases important in cancer development.”
Kinase inhibitors as therapeutics
Given the ability of kinase inhibitors to affect the development of cancer, it is not surprising that they are being viewed now as therapeutic agents. “There are a number of inhibitors against important lipid and protein kinases with therapeutic potential,” says Daly. “Several of these are in clinical trials at the moment. These include PI3 kinase, mTOR, RAF, and SRC inhibitors.” In addition, many of the “client proteins” that are chaperoned by Hsp90 participate in oncogenic pathways; some Hsp90 inhibitors show potential cancer-fighting properties based on their ability to steer oncoproteins into ubiquitin-mediated protein degradation pathways.
Daly explains that clinical uses of kinase inhibitors pose specific challenges. “For therapeutic applications, it is important to understand the physiological changes the inhibitors induce,” she says. “Negative feedback loops can limit efficacy of some inhibitors. Many labs are looking at the effects of combinations of different inhibitors, what proteins these affect, how they interact, and how cells respond.”
Looking toward the future of kinase inhibitors, Daly believes that important developments will include the “development of compounds that target protein-protein interactions, and the development of new strategies to inhibit signal transmission, including incorporating transcriptional inhibition such as siRNA.”