Many anticancer agents damage DNA and arrest cell cycle progression primarily in S or G2 phase of the cell cycle. Previous studies with the topoisomerase I inhibitor SN38 have demonstrated the efficacy of the Chk1 inhibitor UCN-01 to overcome this arrest and induce mitotic catastrophe. UCN-01 was limited in clinical trials by unfavorable pharmacokinetics. SCH900776 is a novel and more selective Chk1 inhibitor that potently inhibits Chk1 and abrogates cell cycle arrest induced by SN38. Like UCN-01, abrogation of SN38-induced arrest enhances the rate of cell death but does not increase overall cell death. In contrast, SCH900776 reduced the growth-inhibitory concentration of hydroxyurea by 20-70 fold. A similar magnitude of sensitization was observed with cytarabine, a 5-10 fold sensitization occurred with gemcitabine, but no sensitization occurred with cisplatin, 5-fluorouracil or 6-thioguanine. Sensitization occurred at hydroxyurea concentrations that marginally slowed DNA replication without apparent activation of Chk1, but this led to dependence on Chk1 that increased with time. For example, when added 18 h after hydroxyurea, SCH900776 induced DNA double-strand breaks consistent with rapid collapse of replication forks. In addition, some cell lines were highly sensitive to SCH900776 alone, and these cells required lower concentrations of SCH900776 to sensitize them to hydroxyurea. We conclude that some tumors may be very sensitive to the combination of SCH900776 and hydroxyurea. Delayed administration of SCH900776 may be more effective than concurrent treatment. SCH900776 is currently in Phase I clinical trials, and these results provide the rationale and schedule for future clinical trials.

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.

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 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.

Gemcitabine irreversibly inhibits ribonucleotide reductase and induces S phase arrest but whether this occurs in tumors in mice or patients has not been established. Tumor cells in culture were incubated with gemcitabine for 6 h to approximate the administration schedule in a patient. Concentrations that induced persistent S phase arrest thereafter correlated with cell killing. Administration of gemcitabine to mice also demonstrated a persistent S phase arrest in their tumor. The minimum dose that induced almost complete S phase arrest after 24 h (40 mg/kg) was well below the maximum tolerated dose in mice. S phase arrest was also observed in tumors of bladder cancer patients receiving gemcitabine. The Chk1 inhibitor MK-8776 sensitized cells to gemcitabine with the greatest cell killing when added 18 h after gemcitabine. In mice, the administration of MK-8776 18 h after gemcitabine elicited positivity for the DNA damage marker γH2AX; this also occurred at relatively low dose (40 mg/kg) gemcitabine. Hence, in both cell culture and xenografts, MK-8776 can markedly enhance cell killing of cells reversibly arrested in S phase by gemcitabine. Some cell lines are hypersensitive to MK-8776 as monotherapy, but this was not observed in xenograft models. Effective monotherapy requires a higher dose of Chk1 inhibitor, and target inhibition over a longer time period as compared to its use in combination. These results have important implications for combining Chk1 inhibitors with gemcitabine and suggest that Chk1 inhibitors with increased bioavailability may have improved efficacy both in combination and as monotherapy.

Hypoxia is a critical hallmark of glioma, and significantly compromises treatment efficacy. Unfortunately, techniques for monitoring glioma pO2 to facilitate translational research are lacking. Furthermore, poor prognoses of patients with malignant glioma, in particular glioblastoma multiforme, warrant effective strategies that can inhibit hypoxia and improve treatment outcome. EPR oximetry using implantable resonators was implemented for monitoring pO2 in normal cerebral tissue and U251 glioma in mice. Breathing carbogen (95% O2 + 5% CO2) was tested for hyperoxia in the normal brain and glioma xenografts. A new strategy to inhibit glioma growth by rationally combining gemcitabine and MK-8776, a cell cycle checkpoint inhibitor, was also investigated. The mean pO2 of left and right hemisphere were approximately 56 – 69 mmHg in the normal cerebral tissue of mice. The mean baseline pO2 of U251 glioma on the first and fifth day of measurement was 21.9 ± 3.7 and 14.1 ± 2.4 mmHg, respectively. The mean brain pO2 including glioma increased by at least 100% on carbogen inhalation, although the response varied between the animals over days. Treatment with gemcitabine + MK-8776 significantly increased pO2 and inhibited glioma growth assessed by MRI. In conclusion, EPR oximetry with implantable resonators can be used to monitor the efficacy of carbogen inhalation and chemotherapy on orthotopic glioma in mice. The increase in glioma pO2 of mice breathing carbogen can be used to improve treatment outcome. The treatment with gemcitabine + MK-8776 is a promising strategy that warrants further investigation.

Many anticancer agents damage DNA and arrest cell cycle progression primarily in S or G2 phase of the cell cycle depending on drug concentration. Previous studies with the topoisomerase I inhibitor SN38 have demonstrated the efficacy of the Chk1 inhibitor UCN-01 to overcome arrest forcing cells through S, G2 and lethal mitosis. UCN-01 was limited in clinical trials by high plasma protein binding; as a consequence many alternate Chk1 inhibitors have now been developed. SCH900776 is a novel Chk1 inhibitor entering Phase II clinical trials. SCH900776 inhibits Chk1 and overcomes SN38-mediated S and G2 arrest at 300 nM in several carcinoma cell lines which is comparable to UCN-01 at 50 nM. While both Chk1 inhibitors accelerated the rate of cell killing induced by SN38, neither enhanced the overall cell killing. Similarly, no overall sensitization to cisplatin was observed. In contrast, both Chk1 inhibitors induced dramatic sensitization of cells to antimetabolite-induced damage. For example, inhibition of Chk1 sensitized MDA-MB-231 cells to hydroxyurea with the 50% growth inhibitory concentration decreasing from 3 mM to 30 µM, a 100 fold sensitization. Importantly, concentrations of hydroxyurea that exhibited no activation of a checkpoint and no growth inhibition alone were sensitized by inhibition of Chk1. SCH900776 also induced a 50-fold sensitization to cytarabine, 10 fold sensitization to gemcitabine, but no sensitization to 5-fluorouracil. Hence, this sensitization does not extend to all antimetabolites. The 100-fold sensitization to hydroxyurea appeared to require complete inhibition of Chk1 (2 µM 776), although 10-fold sensitization to hydroxyurea was still observed at 200 nM SCH900776. In an MDA-MB-231 derivative expressing shRNA that resulted in almost undetectable levels of Chk1, the full level of sensitization to hydroxyurea could be achieved at 200 nM SCH900776 demonstrating that the response of tumors may vary with the endogenous level of Chk1. We are currently comparing the differential response of a panel of carcinoma cell lines and find some that are sensitized to hydroxyurea up to 100-fold at 200 nM SCH900776, although there appear to be determinants other than the Chk1 levels. These results can be extrapolated to suggest that marked differences in efficacy of SCH900776 can be expected in patients. It will be important to define the parameters of response as these may facilitate selection of patients most likely to respond to combination therapy with Chk1 inhibitors. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2510.

<div>Abstract<p>Many anticancer agents damage DNA and arrest cell-cycle progression primarily in S or G₂ phase of the cell cycle. Previous studies with the topoisomerase I inhibitor SN38 have shown the efficacy of the Chk1 inhibitor UCN-01 to overcome this arrest and induce mitotic catastrophe. UCN-01 was limited in clinical trials by unfavorable pharmacokinetics. SCH900776 is a novel and more selective Chk1 inhibitor that potently inhibits Chk1 and abrogates cell-cycle arrest induced by SN38. Like UCN-01, abrogation of SN38-induced arrest enhances the rate of cell death but does not increase overall cell death. In contrast, SCH900776 reduced the growth-inhibitory concentration of hydroxyurea by 20- to 70-fold. A similar magnitude of sensitization was observed with cytarabine. A 5- to 10-fold sensitization occurred with gemcitabine, but no sensitization occurred with cisplatin, 5-fluorouracil, or 6-thioguanine. Sensitization occurred at hydroxyurea concentrations that marginally slowed DNA replication without apparent activation of Chk1, but this led to dependence on Chk1 that increased with time. For example, when added 18 hours after hydroxyurea, SCH900776 induced DNA double-strand breaks consistent with rapid collapse of replication forks. In addition, some cell lines were highly sensitive to SCH900776 alone, and these cells required lower concentrations of SCH900776 to sensitize them to hydroxyurea. We conclude that <b>s</b>ome tumors may be very sensitive to the combination of SCH900776 and hydroxyurea. Delayed administration of SCH900776 may be more effective than concurrent treatment. SCH900776 is currently in phase I clinical trials, and these results provide the rationale and schedule for future clinical trials. <i>Mol Cancer Ther; 11(2); 427–38. ©2011 AACR</i>.</p></div>