Abstract 2068: A highly selective dual Haspin-kinase and PIM-inhibitor overcomes RAF/MEK-inhibitor resistance in melanoma

Intrinsic or acquired resistance to targeted therapies, such as RAF/MEK inhibitors in melanoma, represents a significant clinical challenge. We performed a small molecule screen in isogenic sensitive (S) and RAF/MEK-inhibitor-resistant (RMR) melanoma cell lines and identified a nominal PIM kinase inhibitor (Comp-1) with high activity against A375-S and A375-RMR, with an EC50of ~100 nM. While the phosphorylation of some PIM targets, including 4EBP1 and MYC, were significantly inhibited in a dose-dependent fashion, phosphorylation of BAD was not reduced as one would expect. To determine the on-target activity of this compound, we generated A375 cell lines overexpressing WT PIM isoforms 1-3 or their kinase-dead (KD) forms, followed by treatment with PIMi-1. We found only modest shifts in EC50and hypothesized that the beneficial effects of this compounds were due to inhibition of an alternative target. A screen of Comp-1, along with structurally related compounds, against 468 human kinases revealed exquisite affinity for Haspin kinase (HK). We determined the IC50of Comp-1 against HK to be 14.9 nM. The only described role of HK is phosphorylation of Histone H3 at Thr 3 (H3T3) and thereby regulation of mitotic progression. Quantitative single-cell immunofluorescence (IF) analysis of Comp-1 treated A375 confirmed complete abrogation of p-H3T3, indicating on-target activity in vitro. To understand the temporal effects of HK inhibition with Comp-1, we performed live-cell imaging and single-cell analysis of A375 transduced with a mitotic phase-specific reporter. Cells experienced mitotic stress or mitotic slippage resulting in DNA-damage and an increase gamma-H2AX foci per nuclei. In line with these observation, we observed increased phosphorylation p53 and CHECK2. Furthermore, a portion of cells developed micronuclei in a dose-dependent fashion providing a substrate for the cytosolic DNA-sensing machinery, including the cGAS-STING pathway, and rationale for combining Comp-1 with immunotherapies. Ongoing experiments in tumor-bearing BALB/c mice indicate safety and potentially enhanced efficacy of Comp-1 when combined with either anti-CTLA-4 or anti-PD-1 therapy. Ex vivotreatment of patient-derived tumor-infiltrating lymphocytes (TILs) with Comp-1 revealed no toxic effects. A pan-cancer RNA-seq analysis revealed significant upregulation of HK in 34 of 37 tumor types supporting its role as a potential therapeutic target. Overall, our work identified a highly selective Haspin Kinase inhibitor with activity against RAF/MEK inhibitor resistant melanoma, insights into the underlying mechanisms, and potential in vivo benefit when combined with immune checkpoint inhibitors. Citation Format: Johannes C. Melms, Sreeram Vallabhaneni, Caitlin E. Mills, Peter K. Sorger, Kai Wucherpfennig, Benjamin Izar. A highly selective dual Haspin-kinase and PIM-inhibitor overcomes RAF/MEK-inhibitor resistance in melanoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2068.