Parallel synthesis, biological evaluation, and stereochemical control of tris-heteroleptic ruthenium(II) complexes

Transition metals have become increasingly powerful scaffolds for the design of biologically-active agents because the combination of an inert coordination sphere with diverse spectroscopic, magnetic, redox, and catalytic properties makes them attractive models for discovery. In addition, the wealth of geometries and coordination numbers creates numerous opportunities for targeting biological macromolecules that are difficult to probe with standard organic structures. In this vein, the Meggers group has developed potent and selective transition metal-based kinase inhibitors using the organic inhibitor staurosporine as a guide. Despite this success, lead-inspired design (as opposed to combination-driven design) can be inherently biased toward a narrow corner of chemical space. Thus, devising convenient combinatorial methods which are beyond the reach of target-driven design) can be inherently biased toward a narrow corner of chemical space. Thus, devising convenient combinatorial methods which are beyond the reach of target-driven philosophies might reveal more targets and disrupt new biological pathways. Thus, employing this latter approach for the creation of diverse ruthenium(II) complexes constitutes the central goal of this thesis. Chapter 1outlines a rationale for streamlining the discovery of metal-based probes using high-throughput techniques which are ubiquitous in the organic chemistry community, but are lacking for the synthesis of coordination compounds. Chapters 2 and 3 describe initial solutions to this problem in the form of solid and solution phase combinatorial methods. Since the complexes in Chapters 2 and 3 were prepared as mixtures of stereoisomers, a need developed for the synthesis of stereochemically-defined tris-heteroleptic complexes of ruthenium(II). Thus, Chapter 4 discusses a new method toward this goal through the use of a metal-based chiral auxiliary. Finally, with the appropriate synthetic tools in hand, large libraries of complexes were synthesized and screened for biological activity, as described in Chapters 5 and 6. In the former, the solid phase scheme was employed for the successful optimization of a ruthenium(II)-based inhibitor of acetylcholinesterase. Chapter 5 also describes a useful NMR technique for the assignment of the relative stereochemistry of the most active isomer. Finally, more than 560 compounds synthesized according to the solution phase protocol were screened for the ability to effect cell viability in HeLa cancer cells, as outlined in Chapter 6. After exhaustive optimization, a monocationic complex was identified as a nanomolar anticancer agent and was further found to localize in the Golgi apparatus of HeLa cells and promote apoptosis according to a BcI2-dependent mechanism. In summary, the research described in this thesis demonstrates that a wholly target-driven strategy for the design of metal-based probes lacks the diversity afforded by combinatorial techniques to probe unexplored areas of chemical space. Both solid phase and solution phase methods for the synthesis of ruthenium(II) complexes were developed to demonstrate that metal complexes can be synthesized in large numbers in high purity and integrity similarly to organic structures. Successful biological evaluation of these libraries confirmed that diverse collections of ruthenium(II) complexes were a rich source of biologically-active probes, justifying the continued use of combination-driven strategies in metals-based drug discovery