Ruth Thompson

The cancer therapeutic potential of Chk1 inhibitors: how mechanistic studies impact on clinical trial design

Many anticancer agents damage DNA and activate cell cycle checkpoints that permit time for the cells to repair their DNA and recover. These checkpoints have undergone intense investigation as potential therapeutic targets and Chk1 inhibitors have emerged as promising novel therapeutic agents. Chk1 was initially recognized as a regulator of the G2/M checkpoint, but has since been demonstrated to have additional roles in replication fork stability, replication origin firing and homologous recombination. Inhibition of these pathways can dramatically sensitize cells to some antimetabolites. Current clinical trials with Chk1 inhibitors are primarily focusing on their combination with gemcitabine. Here, we discuss the mechanisms of, and emerging uses for Chk1 inhibitors as single agents and in combination with antimetabolites. We also discuss the pharmacodynamic issues that need to be addressed in attaining maximum efficacy in vivo. Following administration of gemcitabine to mice and humans, tumour cells accumulate in S phase for at least 24 h before recovering. In addition, stalled replication forks evolve over time to become more Chk1 dependent. We emphasize the need to assess cell cycle perturbation and Chk1 dependence of tumours in patients administered gemcitabine. These assessments will define the optimum dose and schedule for administration of these drug combinations.

Sensitization of human cancer cells to gemcitabine by the Chk1 inhibitor MK-8776: cell cycle perturbation and impact of administration schedule in vitro and in vivo.

BackgroundChk1 inhibitors have emerged as promising anticancer therapeutic agents particularly when combined with antimetabolites such as gemcitabine, cytarabine or hydroxyurea. Here, we address the importance of appropriate drug scheduling when gemcitabine is combined with the Chk1 inhibitor MK-8776, and the mechanisms involved in the schedule dependence.MethodsGrowth inhibition induced by gemcitabine plus MK-8776 was assessed across multiple cancer cell lines. Experiments used clinically relevant “bolus” administration of both drugs rather than continuous drug exposures. We assessed the effect of different treatment schedules on cell cycle perturbation and tumor cell growth in vitro and in xenograft tumor models.ResultsMK-8776 induced an average 7-fold sensitization to gemcitabine in 16 cancer cell lines. The time of MK-8776 administration significantly affected the response of tumor cells to gemcitabine. Although gemcitabine induced rapid cell cycle arrest, the stalled replication forks were not initially dependent on Chk1 for stability. By 18 h, RAD51 was loaded onto DNA indicative of homologous recombination. Inhibition of Chk1 at 18 h rapidly dissociated RAD51 leading to the collapse of replication forks and cell death. Addition of MK-8776 from 18–24 h after a 6-h incubation with gemcitabine induced much greater sensitization than if the two drugs were incubated concurrently for 6 h. The ability of this short incubation with MK-8776 to sensitize cells is critical because of the short half-life of MK-8776 in patients’ plasma. Cell cycle perturbation was also assessed in human pancreas tumor xenografts in mice. There was a dramatic accumulation of cells in S/G2 phase 18 h after gemcitabine administration, but cells had started to recover by 42 h. Administration of MK-8776 18 h after gemcitabine caused significantly delayed tumor growth compared to either drug alone, or when the two drugs were administered with only a 30 min interval.ConclusionsThere are two reasons why delayed addition of MK-8776 enhances sensitivity to gemcitabine: first, there is an increased number of cells arrested in S phase; and second, the arrested cells have adequate time to initiate recombination and thereby become Chk1 dependent. These results have important implications for the design of clinical trials using this drug combination.

The Mre11 Nuclease Is Critical for the Sensitivity of Cells to Chk1 Inhibition

The Chk1 kinase is required for the arrest of cell cycle progression when DNA is damaged, and for stabilizing stalled replication forks. As a consequence, many Chk1 inhibitors have been developed and tested for their potential to enhance DNA damage-induced tumor cell killing. However, inhibition of Chk1 alone, without any additional exogenous agent, can be cytotoxic. Understanding the underlying mechanisms of this sensitivity is critical for defining which patients might respond best to therapy with Chk1 inhibitors. We have investigated the mechanism of sensitivity in U2OS osteosarcoma cells. Upon incubation with the Chk1 inhibitor MK-8776, single-stranded DNA regions (ssDNA) and double-strand breaks (DSB) begin to appear within 6 h. These DSB have been attributed to the structure-specific DNA endonuclease, Mus81. The Mre11/Rad50/Nbs1 complex is known to be responsible for the resection of DSB to ssDNA. However, we show that inhibition of the Mre11 nuclease activity leads, not only to a decrease in the amount of ssDNA following Chk1 inhibition, but also inhibits the formation of DSB, suggesting that DSB are a consequence of ssDNA formation. These findings were corroborated by the discovery that Mre11-deficient ATLD1 cells are highly resistant to MK-8776 and form neither ssDNA nor DSB following treatment. However, once complimented with exogenous Mre11, the cells accumulate both ssDNA and DSB when incubated with MK-8776. Our findings suggest that Mre11 provides the link between aberrant activation of Cdc25A/Cdk2 and Mus81. The results highlight a novel role for Mre11 in the production of DSB and may help define which tumors are more sensitive to MK-8776 alone or in combination with DNA damaging agents.

A subset of cancer cell lines is acutely sensitive to the Chk1 inhibitor MK-8776 as monotherapy due to CDK2 activation in S phase

DNA damage activates Checkpoint kinase 1 (Chk1) to halt cell cycle progression thereby preventing further DNA replication and mitosis until the damage has been repaired. Consequently, Chk1 inhibitors have emerged as promising anticancer therapeutics in combination with DNA damaging drugs, but their single agent activity also provides a novel approach that may be particularly effective in a subset of patients. From analysis of a large panel of cell lines, we demonstrate that 15% are very sensitive to the Chk1 inhibitor MK-8776. Upon inhibition of Chk1, sensitive cells rapidly accumulate DNA double-strand breaks in S phase in a CDK2- and cyclin A-dependent manner. In contrast, resistant cells can continue to grow for at least 7 days despite continued inhibition of Chk1. Resistance can be circumvented by inhibiting Wee1 kinase and thereby directly activating CDK2. Hence, sensitivity to Chk1 inhibition is regulated upstream of CDK2 and correlates with accumulation of CDC25A. We conclude that cells poorly tolerate CDK2 activity in S phase and that a major function of Chk1 is to ensure it remains inactive. Indeed, inhibitors of CDK1 and CDK2 arrest cells in G1 or G2, respectively, but do not prevent progression through S phase demonstrating that neither kinase is required for S phase progression. Inappropriate activation of CDK2 in S phase underlies the sensitivity of a subset of cell lines to Chk1 inhibitors, and this may provide a novel therapeutic opportunity for appropriately stratified patients.

Treatment with the Chk1 inhibitor Gö6976 enhances cisplatin cytotoxicity in SCLC cells

Acquired chemoresistance is a major obstacle in successful treatment of small cell lung cancer (SCLC). DNA damage responses can potentially contribute to resistance by halting the cell cycle following exposure to therapeutic agents, thereby facilitating repair of drug-induced lesions and protecting tumour cells from death. The Chk1 protein kinase is a key regulator in this response. We analysed the status of cell cycle checkpoint proteins and the effects of the Chk1 inhibitor Gö6976 on cisplatin toxicity in SCLC cell lines. IC50s for cisplatin were determined using the MTT assay in six SCLC cell lines. Effects on cell cycle distribution and apoptosis were determined by flow cytometry and caspase 3 activation in the presence or absence of the Chk1 inhibitor Gö6976. The activation of checkpoint proteins was determined by Western blotting. Cell lines were divided into chemosensitive and chemoresistant groups on the basis of our results. While checkpoint responses were detected in these cell lines through Western blotting, some of these responses were delayed or weaker than those seen in other cell types in response to DNA damage and replication stress. Gö6976 significantly (p<0.05) enhanced the levels of apoptosis seen in response to a clinically relevant dose of cisplatin (<6 µM) and decreased drug-induced G2 arrest in chemosensitive cells. Our data suggest a role for Chk1 in chemoresistance of SCLC cells and a potential approach to improve initial response of SCLC to cisplatin therapy.

NPM1 directs PIDDosome-dependent caspase-2 activation in the nucleolus

The PIDDosome (PIDD–RAIDD–caspase-2 complex) is considered to be the primary signaling platform for caspase-2 activation in response to genotoxic stress. Yet studies of PIDD-deficient mice show that caspase-2 activation can proceed in the absence of PIDD. Here we show that DNA damage induces the assembly of at least two distinct activation platforms for caspase-2: a cytoplasmic platform that is RAIDD dependent but PIDD independent, and a nucleolar platform that requires both PIDD and RAIDD. Furthermore, the nucleolar phosphoprotein nucleophosmin (NPM1) acts as a scaffold for PIDD and is essential for PIDDosome assembly in the nucleolus after DNA damage. Inhibition of NPM1 impairs caspase-2 processing, apoptosis, and caspase-2–dependent inhibition of cell growth, demonstrating that the NPM1-dependent nucleolar PIDDosome is a key initiator of the caspase-2 activation cascade. Thus we have identified the nucleolus as a novel site for caspase-2 activation and function.

A mitosis-sensing caspase activation platform? New insights into the PIDDosome

ABSTRACT In contrast to the apoptosome and death-inducing signaling complex, the PIDDosome remains an orphan caspase activation platform unassigned to a specific apoptotic pathway. We found that DNA damage-induced PIDDosome formation is blocked by the mitotic checkpoint factor BUBR1 (budding uninhibited by benzimidazole-related 1), via a direct interaction that disrupts the PIDDosome core scaffold. This inhibition occurs at the kinetochore, thus physically connecting the mitotic and apoptotic machineries.

Abstract 2040: The Mre11 nuclease is critical for sensitivity of cells to Chk1 inhibition

The Chk1 kinase is required for the arrest of cell cycle progression when DNA is damaged, thereby providing time for cells to repair their DNA and recover. Chk1 also stabilizes stalled replication forks. As a consequence, many Chk1 inhibitors have been developed and tested for their potential to enhance DNA damage-induced tumor cell killing. However, inhibition of Chk1 alone, without any additional exogenous agent, can be cytotoxic. Using the novel Chk1 inhibitor MK-8776 (previously known as SCH900776), we screened a panel of cell lines and found that some are killed rapidly at low concentrations while other cell lines continue to grow in the presence of much higher concentrations. Understanding the underlying mechanisms of this sensitivity is critical for defining which patients might respond best to therapy with Chk1 inhibitors. We have investigated the mechanism of sensitivity in U2OS osteosarcoma cells. Upon incubation with MK-8776, single-stranded DNA regions (ssDNA) and double-strand breaks (DSB) begin to appear within 6 h. These DSB have been attributed to the structure-specific DNA endonuclease, Mus81. The Mre11/Rad50/Nbs1 (MRN) complex is known to be responsible for the resection of DSB to ssDNA. However, we show that inhibition of the Mre11 nuclease activity leads to a decrease not only in the amount of ssDNA following Chk1 inhibition, but also in the formation of DSB, suggesting that DSB arise as a consequence of ssDNA formation. These findings were corroborated by the discovery that Mre11-deficient ATLD1 cells are highly resistant to MK-8776 and form neither ssDNA nor DSB following treatment, but once complimented with exogenous Mre11, the cells accumulate both ssDNA and DSB when incubated with MK-8776. Inhibition of Chk1 also leads to aberrant activation of Cdc25A and Cdk2, and our findings suggest that Mre11 provides the link between Cdk2 activation and Mus81. We propose that the variable sensitivity of cell lines to Chk1 inhibition reflects defects in this pathway. These data highlight a novel role for Mre11 in the production of DSB and may help define which tumors are more sensitive to MK-8776 alone or in combination with DNA damaging agents. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2040. doi:1538-7445.AM2012-2040