oa Identification and characterization of inhibitors of G protein-coupled receptor kinase 6 (GRK6) as potential therapeutics for multiple myeloma

Multiple myeloma (MM) is one of the most common hematological malignancies, but current therapy options are limited to high-dose chemotherapy or high-risk stem-cell transplantation. In a recent kinome-wide RNAi study by Tiedemann and colleagues (2010), the G-protein coupled receptor kinase 6 (GRK6) was identified as a critical kinase required for survival of MM cells. This study also suggests that MM cells, but not other cell types, are dependent on GRK6; and that gene silencing by shRNA or siRNA of GRK6, but not other GRKs, results in decreased survival. At present, the G protein-coupled receptor (GPCR) signaling mediated by GRK6 in MM cells is not well understood. Through gene silencing techniques and expression of either the wild-type or kinase-dead form of GRK6 protein, we determined that a functional GRK6 kinase domain is required for survival of MM cells. These findings helped validate that the GRK6 kinase domain is a potential target for MM, and have spurred the investigation of small molecule kinase inhibitors of GRK6. By screening a cassette of compounds that are known as kinase inhibitors, compounds with moderate potency in preliminary biochemical assays were identified and further evaluated. As part of this effort we identified CX-4945, which is a known potent and selective ATP-competitive small molecule protein kinase CK2 inhibitor (IC₅₀ of 1 nM for recombinant human CK2α) as a moderately potent inhibitor of GRK6 (IC₅₀ on GRK6 = 300-500 nM). Herein we describe the design and synthesis of novel analogs of CX-4945 and key structure activity relationships (SAR) of this chemical series against GRK6 that serve as a platform for further optimization.