Tightly controlled by the action of protein kinases, protein phosphorylation regulates most aspects of cellular life. Abnormalities in protein phosphorylation are either the cause or consequence of most diseases. Therefore, protein kinase inhibitors are becoming attractive agents to assess the physiological roles of individual kinases in cell signaling networks and potential tools for disease treatment. The most exploited avenue in protein kinase inhibition is the design of ATP competitive inhibitors, but this strategy brings the challenge of selectivity due to the high similarity of the ATP binding sites among different kinases. To address this challenge, our lab focuses on the development of organometallic compounds as protein kinase inhibitors. We aim to design organometallic compounds where the overall three-dimensional structure of the compounds, not reactivity of the metal, is responsible for their biological activity which can then easily be modified by shuffling the organic ligands around the metal center. In this work, first the concept of creating metal complexes with specific biological activities where the metal possesses a purely structural role is verified. It is demonstrated that exchanging ruthenium for its heavier homologue osmium in a bioactive half-sandwich scaffold does not alter its biological properties such as kinase binding, activation of a signaling pathway and anticancer activity. This is a unique example in which the replacement of a metal in an anticancer scaffold by its heavier homolog does not significantly alter the biological activity. Next, it was investigated if switching the selectivity between kinases could be achieved by varying the ligands around the metal center. By combining organoruthenium chemistry, small molecule screening and structure based inhibitor design, a highly potent and selective GSK-3 and PIM-1 half-sandwich complex NP309 was successfully converted into the octahedral PAK1 inhibitors Λ-FL172 and Λ-FL411. These compounds were shown to inhibit the PAK1 kinase with nanomolar potency and show significant selectivity when tested against a panel of kinases. In addition, these compounds were shown to be cell permeable and able to evoke cellular responses associated with PAK1 inhibition. Finally, the binding mode of an organoruthenium inhibitor to the kinase PAK4 was examined using X-ray crystallography. The main goal of this particular study was to demonstrate that PAK1 and PAK4 have significantly different inhibitor selectivity profiles and to lay the groundwork to develop potent and selective organometallic PAK4 inhibitors
