Targeting Pim Kinases in Hematological Malignancies.

The three Pim kinases represent a small subfamily of serine/threonine kinases known to be involved in a number of signaling pathways as downstream effectors and potent inhibitors of apoptosis. Unlike most other kinases, Pim kinases lack a regulatory domain which means they are controlled largely at the transcriptional level and that the mRNA expression levels of Pim kinases in cells correlate with their activity. While normal expression of Pim-1 kinase is seen in cells of hematopoietic origin, examination of gene expression of Pim-1 in different malignancies using cDNA microarray analysis suggests that Pim-1 is overexpressed in a large percentage of ALL, AML, CML, DLBCL, Prostatic Adenocarcinoma, Bladder, and Oral cancers. Pim-2 is also largely expressed in ALL, AML, Squamous Cell Lung and Adenocarcinoma of the Lung while Pim-3 is restricted to Melanoma, Pancreatic Adenocarcinoma, Gastric cancers. This implicates Pim-1 and Pim-2 in the onset and progression in several of hematological malignancies and therefore they represent interesting potential targets for drug development. To evaluate the potential of Pim-1 and -2 as a drug targets we have identified and synthesized a series of Pim kinase inhibitors using our proprietary CLIMB™ drug discovery process. Through the use of CLIMB™, the published Pim-1 kinase crystal structure was used to build several models that were then used to predict potential small molecule inhibitors of Pim-1 and Pim-2 from a large virtual library. A subset of leads, based on calculated binding energies as well as additional physical chemical properties, were screened using a number of in silico physicochemical and ADMET prediction algorithms to determine which compounds were most likely to be successful in a biological context. Lead candidates were initially screened using biochemical enzyme-based and cell-based assays. Cell-based activity was determined in HEL (acute megakaryocytic leukemia), K562 (chronic myeloid leukemia), MO7e (myeloid leukemia), and other human leukemia and lymphoma cell lines. From several lead candidates a series of analogs were produced with improved inhibitory activity and pharmacokinetic characteristics. In the Pim-1 and Pim-2 in vitro kinase assay and in the cell-based assay a number of leads exhibited inhibitory activity with IC₅₀ concentrations in the low nanomolar range. Here we present the details of the biochemical and cell-based assay results as well as the activity in tumor xenograft models of our Pim kinase inhibitors.