Jack Dempsey

Mitotic requirement for aurora A kinase is bypassed in the absence of aurora B kinase

We investigated why treatment of cells with dual aurora A and B kinase inhibitors produces phenotypes identical to inactivation of aurora B. We found that dual aurora kinase inhibitors in fact potently inhibit cellular activities of both kinases, indicating that inactivation of aurora B bypasses aurora A in mitosis. RNAi experiments further established that inactivation of aurora B indeed bypasses the requirement for aurora A and leads to polyploidy. Inactivation of aurora A activates checkpoint kinase BubR1 in an aurora B‐dependent manner. Our results thus show that aurora B is responsible for mitotic arrest in the absence of aurora A.

Novel, potent and selective cyclin D1/CDK4 inhibitors: indolo[6,7-a]pyrrolo[3,4-c]carbazoles.

The synthesis and CDK inhibitory properties of a series of indolo[6,7-a]pyrrolo[3,4-c]carbazoles is reported. In addition to their potent CDK activity, the compounds display antiproliferative activity against two human cancer cell lines. These inhibitors also effect strong G1 arrest in these cell lines and inhibit Rb phosphorylation at Ser780 consistent with inhibition of cyclin D1/CDK4.

Aryl[a]pyrrolo[3,4-c]carbazoles as selective cyclin D1-CDK4 inhibitors

The synthesis of new analogues of Arcyriaflavin A in which one indole ring is replaced by an aryl or heteroaryl ring is described. These new series of aryl[a]pyrrolo[3,4-c]carbazoles were evaluated as inhibitors of Cyclin D1-CDK4. A potent and selective D1-CDK4 inhibitor, 7a (D1-CDK4 IC(50)=45 nM), has been identified. The potency, selectivity profile against other kinases, and structure-activity relationship (SAR) trends of this class of compounds are discussed.

Synthesis of quinolinyl/isoquinolinyl[a]pyrrolo [3,4-c] carbazoles as cyclin D1/CDK4 inhibitors.

A novel series of pyrrolo[3,4-c] carbazoles fused with a quinolinyl/isoquinolinyl moiety were synthesized and their D1/CDK4 inhibitory and antiproliferative activity were evaluated. Compound 8H, 14H-isoquinolinyl[6,5-a]-pyrrolo[3,4-c]carbazole-7,9-dione (1d) was found to be a highly potent D1/CDK4 inhibitor with an IC(50) of 69 nM. Compound 1d also inhibited tumor cell growth, arrested tumor cells in G1 phase and inhibited pRb phosphorylation.

Studies on cyclin-dependent kinase inhibitors: indolo-[2,3-a]pyrrolo[3,4-c]carbazoles versus bis-indolylmaleimides

A series of indolo[2,3-a]pyrrolo[3,4-c]carbazoles and their bis-indolylmaleimides precursors have been prepared in order to compare their activity as D1-CDK4 inhibitors. Both enzymatic and antiproliferative assays have shown that the structurally more constrained indolo[2,3-a]pyrrolo[3,4-c]carbazoles are consistently more active (8-42-fold) in head-to-head comparison with their bis-indolylmaleimides counterparts. Cell-cycle analysis using flow cytometry have also shown that the indolocarbazoles are selective G1 blockers while the bis-indolylmaleimides arrest cells in the G2/M phase.

LY2606368 Causes Replication Catastrophe and Antitumor Effects through CHK1-Dependent Mechanisms

CHK1 is a multifunctional protein kinase integral to both the cellular response to DNA damage and control of the number of active replication forks. CHK1 inhibitors are currently under investigation as chemopotentiating agents due to CHK1s role in establishing DNA damage checkpoints in the cell cycle. Here, we describe the characterization of a novel CHK1 inhibitor, LY2606368, which as a single agent causes double-stranded DNA breakage while simultaneously removing the protection of the DNA damage checkpoints. The action of LY2606368 is dependent upon inhibition of CHK1 and the corresponding increase in CDC25A activation of CDK2, which increases the number of replication forks while reducing their stability. Treatment of cells with LY2606368 results in the rapid appearance of TUNEL and pH2AX-positive double-stranded DNA breaks in the S-phase cell population. Loss of the CHK1-dependent DNA damage checkpoints permits cells with damaged DNA to proceed into early mitosis and die. The majority of treated mitotic nuclei consist of extensively fragmented chromosomes. Inhibition of apoptosis by the caspase inhibitor Z-VAD-FMK had no effect on chromosome fragmentation, indicating that LY2606368 causes replication catastrophe. Changes in the ratio of RPA2 to phosphorylated H2AX following LY2606368 treatment further support replication catastrophe as the mechanism of DNA damage. LY2606368 shows similar activity in xenograft tumor models, which results in significant tumor growth inhibition. LY2606368 is a potent representative of a novel class of drugs for the treatment of cancer that acts through replication catastrophe. Mol Cancer Ther; 14(9); 2004–13. ©2015 AACR.

Abstract LB-122: LY2835219, a selective inhibitor of CDK4 and CDK6, inhibits growth in preclinical models of human cancer.

Cell growth is regulated by the activity of certain cyclin-dependent kinases (CDKs), including CDK4 and CDK6 (hereafter CDK4/6), which specifically regulate cell cycle progression through the G1 restriction point. These CDKs act with the D-type cyclins to phosphorylate and inactivate the retinoblastoma (Rb) tumor suppressor protein, which in turn enables cell cycle progression from G1 to S phase. LY2835219 is a selective inhibitor of CDK4/6 with IC50 of 2 and 10 nM for CDK4 and CDK6, respectively. The current study evaluates the activity of LY2835219 in preclinical models of human cancer including non-small cell lung cancer (NSCLC), melanoma, mantle cell lymphoma (MCL), and ovarian cancer. A secondary focus of this study is to identify potential predictive biomarkers for response to LY2835219 in NSCLC, melanoma, and other cancers. For NSCLC, increased sensitivity to LY2835219 is associated both in vitro and in vivo with KRAS mutations. For established melanoma cell lines, similar levels of in vitro sensitivity are observed in different genetic subtypes. Accordingly, LY2835219 shows single-agent activity in the A375 melanoma xenograft model with daily oral dosing at 45 or 90 mg/kg. For MCL, durable growth inhibition of subcutaneously implanted xenografts is achieved with daily oral dosing at 50mg/kg. Finally, in an intra-peritoneal model that recapitulates clinical features of human ovarian cancer, LY2835219 not only inhibits tumor growth but also increases survival. These studies suggest that LY2835219 may have potential therapeutic application in the treatment of human cancers with specific molecular alterations. Citation Format: Jack A. Dempsey, Edward M. Chan, Teresa F. Burke, Richard P. Beckmann. LY2835219, a selective inhibitor of CDK4 and CDK6, inhibits growth in preclinical models of human cancer. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr LB-122. doi:10.1158/1538-7445.AM2013-LB-122

Abstract 583: The CDK4/6 inhibitor abemaciclib induces synergistic immune activation and antitumor efficacy in combination with PD-L1 blockade

Targeting cyclin dependent kinases 4 and 6 (CDK4/6) with inhibitors such as abemaciclib has shown promise in early and late phase clinical trials in both breast cancer and NSCLC. While there is evidence that patients benefit from single-agent abemaciclib, combination strategies leveraging this compound together with immunotherapy are of interest for the treatment of these and other cancers. Consequently, it is important to understand if and how a cell cycle inhibitor can be combined with immunotherapy. However, because most preclinical studies have been performed using xenograft tumors in immune-compromised mice, the potential immunomodulatory effects of abemaciclib have not been adequately ascertained. To investigate the immune combinatorial potential of abemaciclib, we studied the effects of treatment alone and in combination with checkpoint immunotherapy in a murine syngeneic tumor model sensitive to abemaciclib using immuno-competent mice. Abemaciclib monotherapy of established murine CT26 tumors, which harbor KRAS G12C mutation and CDKN2A deletion, caused a dose-dependent delay in tumor growth. Surprisingly, gene expression analysis showed that treatment was associated with an increase in intra-tumor immune inflammation without major alteration in immune subset frequencies. Testing of various dosing regimens in this preclinical model found that monotherapy abemaciclib pretreatment followed by combination with anti-PD-L1 antibody therapy, induced an enhanced anti-tumor response compared to abemaciclib and anti-PD-L1 monotherapies. Optimal combination therapy exhibited superior anti-tumor efficacy, resulting in complete tumor regression (CR) in 50-60% of mice in a setting where anti-PD-L1 monotherapy showed little or no efficacy (0% CRs). Mice which maintained CRs after cessation of combination therapy were able to resist later CT26 rechallenge, demonstrating that abemaciclib in combination with anti-PD-L1 enabled the generation of an immunologic memory. Examination of intra-tumor gene expression during treatment found that combination therapy further amplified the immune/T cell activation signature compared to both monotherapies. Intra-tumoral suppression of cell cycle genes, which are indicative of inhibition of CDK4/6, was also greater during the combination therapy, suggesting that the effects anti-PD-L1 therapy may augment the cell cycle arrest induced by abemaciclib. Although it was uncertain if agents that inhibit cell proliferation could be combined with immunotherapy, these preclinical results demonstrate that it is possible to combine CDK4/6 inhibition by abemaciclib with checkpoint immunotherapy to improve tumor efficacy. The synergistic responses observed in terms of tumor efficacy, immune activation, and cell cycle control provides support for the clinical investigation of this combination. Citation Format: Jack Dempsey, Lysiane Huber, Amelie Forest, Jennifer R. Stephens, Thompson N. Doman, Jason Manro, Andrew Capen, Robert S. Flack, Gregory P. Donoho, Sean Buchanan, Alfonso De Dios, Kyla Driscoll, Michael Kalos, Ruslan Novosiadly, Richard P. Beckmann, David A. Schaer. The CDK4/6 inhibitor abemaciclib induces synergistic immune activation and antitumor efficacy in combination with PD-L1 blockade [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 583. doi:10.1158/1538-7445.AM2017-583

Abstract A49: Circulating tumor cell (CTC) assay development for detection of H2AX as a clinical pharmacodynamic (PD) marker for Chk1 kinase inhibitors.

Circulating tumor cells are prognostic of survival in metastatic breast, colon, and prostate cancers. Additionally, CTCs are of interest because they may be representative of the phenotype/genotype of the primary and metastatic tumors, and a useful tool (e.g. patient stratification) for drug development. CTCs, as “liquid biopsies” are potentially useful as a pharmacodynamic marker allowing easy repeat sampling before and after drug treatment. We describe the development of a CTC assay measuring H2AX induction, a marker of DNA damage. The CTC assay was developed using the FDA cleared Veridex/CellSearch™ instrument and reagents for enumeration of CTCs from blood. CellSearch defines CTCs as EpCam+/DAPI+/CK 8,18,19+/CD45−. For assay development, cells from tumor cell lines representing major solid tumor types were chosen, cultured, treated with inhibitors of checkpoint kinase 1 (Chk1, LY2603618, LY2606368) or with various standard-of-care (SOC) chemotherapeutics. These treated cells were then spiked into whole blood drawn into CellSave™ (preservative+EDTA). LY2603618 and LY2606368 are inhibitors of Chk1 kinase currently in clinical development. LY2603618 is meant to be used in combination with a DNA damaging agent, while LY2606368 has potent single agent activity. Inhibition of Chk1 in cells with DNA damage allows the cells to progress into mitosis without DNA repair, leading to cell death. In tumor xenograft mouse models, treatment reduced tumor volume with elevation of γH2AX in tumors and anagen hair follicles at drug exposure levels similar to concentrations used for the CTC γH2AX development assay. For initial assay development, mouse whole blood was used and results reproduced subsequently with human whole blood from healthy subjects. The spiked tumor cells in blood samples were recovered using the CellSearch Mouse/Rat CTC kit (for mouse blood) or human CXC kit (for human blood), supplemented with the R-PE conjugated anti-γH2AX antibody. γH2AX in the recovered tumor cells was detected in the open/fourth channel on the CellSearch instrument. Results were confirmed by flow analysis. Without drug treatment, only about 0.2–2% of all cultured tumor cells were γH2AX positive. The magnitude of the induction of γH2AX in the cells after treatment was dose and cell-line dependent. Significant induction of H2AX (>20% cells) was observed after 24 hour treatment with only a few SOC agents (cisplatin, etoposide) in sensitive tumor cell lines; the majority induced moderate to low levels (usually Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr A49.

Abemaciclib is Active in Preclinical Models of Ewing's Sarcoma via Multi-pronged Regulation of Cell Cycle, DNA Methylation, and Interferon Pathway Signaling

Purpose: Ewing sarcoma (ES) is a rare and highly malignant cancer that occurs in the bone and surrounding tissue of children and adolescents. The EWS/ETS fusion transcription factor that drives ES pathobiology was previously demonstrated to modulate cyclin D1 expression. In this study, we evaluated abemaciclib, a small-molecule CDK4 and CDK6 (CDK4 and 6) inhibitor currently under clinical investigation in pediatric solid tumors, in preclinical models of ES. Experimental Design: Using Western blot, high-content imaging, flow cytometry, ELISA, RNA sequencing, and CpG methylation assays, we characterized the in vitro response of ES cell lines to abemaciclib. We then evaluated abemaciclib in vivo in cell line–derived xenograft (CDX) and patient-derived xenograft (PDX) mouse models of ES as either a monotherapy or in combination with chemotherapy. Results: Abemaciclib induced quiescence in ES cell lines via a G1 cell-cycle block, characterized by decreased proliferation and reduction of Ki-67 and FOXM1 expression and retinoblastoma protein (RB) phosphorylation. In addition, abemaciclib reduced DNMT1 expression and promoted an inflammatory immune response as measured by cytokine secretion, antigen presentation, and interferon pathway upregulation. Single-agent abemaciclib reduced ES tumor volume in preclinical mouse models and, when given in combination with doxorubicin or temozolomide plus irinotecan, durable disease control was observed. Conclusions: Collectively, our data demonstrate that the antitumor effects of abemaciclib in preclinical ES models are multifaceted and include cell-cycle inhibition, DNA demethylation, and immunogenic changes.