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

Traditional chemotherapeutics that induce DNA damage are the most widely used class of anticancer drugs today and will continue to be so into the foreseeable future (1). Lacking the strict control of cell division inherent in normal cells, cancerous cell masses are in general more sensitive to agents targeting DNA integrity due to an increased fraction of actively replicating cells. Normal replicating cells are somewhat protected from DNA damage resulting from chemotherapy due to functional cell-cycle checkpoints and DNA repair processes. Paradoxically, although a rapidly growing tumor mass may be sensitive to DNA-damaging chemotherapy, individual cancer cells are relatively tolerant of genomic damage because of resistance to cell senescence and apoptosis (2). Depending upon the functionality of each of the DNA repair pathways in a tumor, certain types of DNA damage are less well tolerated than others. Many types of DNA-damaging therapeutics have been developed that utilize widely differing mechanisms of action (3). These differences in mechanism make it possible to find a therapy that is effective against cancers that are resistant to other types of DNA-damaging agents. Chemotherapeutic drugs are classified by mechanism, chemical structure, or similarity to other agents. On the basis of these criteria, there are four main groups of DNA-damaging chemotherapeutics: alkylating agents, antimetabolites, topoisomerase inhibitors, and antitumor antibiotics. Most chemotherapeutics also have nonspecific cytotoxic effects that can contribute to patient toxicity (3).

A recent strategy to improve the action of DNA-damaging drugs in cancer treatment is to prevent the activation of the DNA damage checkpoints in tumor cells during treatment with chemotherapy (4). Upon sensing DNA damage, normal cells stop progression through the cell cycle at specific points in the G₁, S, and G₂ phases. The purpose of these checkpoints is to provide the cell with time to repair breaks and chemical damage to DNA and to determine the final fate of the cell. Mutated in the majority of cancers, the p53 protein is the primary regulator of the G₁ and G₂–M checkpoints and controls the decision tree leading to cell survival, death, or senescence (5). Although loss of p53 function occurs in most human tumors, sufficient pathway redundancy exists to maintain functional intra-S and G₂–M checkpoints. One key regulator of these two checkpoints is the checkpoint kinase 1 (CHK1; ref. 6). In the presence of DNA damage, CHK1 is activated through phosphorylation by the ataxia-telangiectasia and RAD3-related protein (ATR) leading to the phosphorylation of the CDC25 phosphatases leading to the degradation of CDC25A and the nuclear exclusion of CDC25B and CDC25C. CDC25A is a key regulator of CDK2 and DNA replication. CHK1-mediated loss of CDC25A activity suspends CDK2 in an inactive phosphorylated state blocking initiation of DNA replication origins. Cytosolic sequestration of CDC25B and CDC25C prevents the activation of CDK1 resulting in cell-cycle arrest at the G₂–M boundary. A number of CHK1 inhibitors have been developed and entered the clinic as checkpoint inhibitors to increase the efficacy of chemotherapy in patients with p53-mutant cancer (7, 8).

One unexpected consequence of CHK1 inhibition is the generation of double-stranded DNA breaks (DSB; ref. 9). Without CHK1 to regulate CDC25A during a normal cell cycle, CDK2 activity is increased leading to unscheduled DNA replication initiating at both normal and dormant replication origins (10). Simultaneous activation of such an excess of replication origins results in the slowing and stalling of replication forks, apparently due to an insufficient number of replication proteins (11, 12). In a cascade effect, loss of CHK1 activity may also lead to fork instability and collapse. A recent study using inhibitors of ATR and CHK1 reported that the abundance of ssDNA during replication stress exhausts the available pool of RPA increasing the likelihood that unprotected ssDNA will be cleaved by endonucleases (13). Stalled forks are believed to be repaired primarily, but not exclusively, by homologous recombination (14, 15). The resulting DNA structure intermediates generated during homologous recombination repair (HRR; i.e., Holliday junctions) are cleaved by endonucleases such as MUS81/EME1 yielding a double-stranded break. However, inhibition of CHK1 function also prevents localization of RAD51 to the invading repair strand during HRR, maintaining the accumulated breaks in the collapsed forks (16). Disaster continues to escalate for the CHK1 inhibited cell as the loss of the intra-S checkpoint permits the cell to continue up to the G₂–M checkpoint with broken DNA (7). Ultimately, the cell enters mitosis with fragmented chromosomes resulting in cell death. Cell death caused by failure of the ATR/CHK1 axis during replication stress has been described as replication catastrophe (13).

Although a number of chemotherapeutics yield DSB, CHK1 inhibitors are unique in that not only do they cause DNA damage, but also abrogate critical DNA damage checkpoints and hamper HRR. One such inhibitor is LY2606368. It is currently in clinical development as a single agent and in combination with both cytotoxic and targeted agents. In this report, we describe the biochemical and biologic properties of LY2606368. LY2606368 causes replication catastrophe in vitro and in vivo and is unique in its mechanism of action from all of the major classes of DNA-damaging agents.

HeLa cervical cancer cells (lot 2619582 obtained in 2003), NCI-H460 non–small cell lung cancer cells (lot 1613811 obtained in 2001), PANC-1 pancreas cancer cells (lot 1077384 obtained in 2000), HT-29 colon cancer cells (lot 2463682 obtained in 2003), and HCT 116 colon cancer cells (lot 1562770 obtained in 2002) were from ATCC. Calu-6 non–small cell lung cancer cells and U-2 OS osteosarcoma cells were obtained from ATCC before 2003. Upon receipt from ATCC, each cell line was revived, expanded (two to eight passages) and frozen working stocks prepared. Cell lines were maintained as recommended by ATCC and passaged no more than ten times following revival from working stock. The frozen working stock of each cell line was authenticated in December 2014 by IDEXX-Radil using STR-based DNA profiling and multiplex PCR. The genetic profiles for the samples were identical to the genetic profiles reported for these cell lines.

The following antibodies against corresponding proteins and phosphoproteins were purchased and used according to the manufacturer's instructions. Abbreviations used are “p” for phospho protein followed by the amino acid letter code and position of phosphorylation. Antibodies against CDK2 (#05-596), pH2AX (S139) (#05-636), and pH3 (S10) (#06-570) were from Millipore. pCHK1 (S296) (#2349) and pCHK1 (S345) (#2341) antibodies were from Cell Signaling Technology. Other antibodies were to CHK1 (Stressgen, #KAM-CC111), GAPDH (Fitzgerald, #10R-G109a), and RPA32/RPA2 (Abcam, #61184), pRPA32 (S4/S8; Bethyl Labs, #A300-245A). CDC25A (#SC-7389) and donkey anti-goat HRP (SC-2020) were from Santa Cruz Technology. Goat anti-mouse-Alexa-555 (#A21422), donkey anti-rabbit-Alexa-555 (#A31572), and goat anti-mouse-Alexa-488 (#A11001) were from Invitrogen. Goat anti-mouse-Dylight 550 (#84540) was from Thermo Scientific, donkey anti-rabbit HRP (NA934V) and sheep anti-mouse HRP (NA9310V) were from Amersham.

Cell proliferation assays were performed as previously described using a Cell Titer-96 AQ kit (Promega; ref. 17). Absolute IC₅₀ values were calculated in Microsoft EXCEL using an XLFit software add-in (ID Business Solutions).

Doxorubicin (Sigma) was prepared as a 10 mmol/L stock in H₂O. Phorbol 12-myristate 13-acetate (Sigma) was prepared as a 10 mmol/L stock in ethanol. SN38 (Tocris Bioscience), Z-VAD-FMK (R&D Systems), BI-D1870 (Symansis), U0126 (Alexis), and staurosporine (Eli Lilly & Co.) were prepared as separate 10 mmol/L stocks in DMSO. Hydroxyurea (HU; Calbiochem) was prepared as a 1 mol/L stock in H₂O. LY2606368 (Eli Lilly & Co.) was prepared as a 10 mmol/L stock in DMSO for in vitro use and in 20% Captisol (CyDex Inc), pH4, for in vivo use.

High content cell imaging and analysis were performed using the Thermo Scientific Cell Insight NXT platform and software suite as described (18). Cells (2,500–5,000 per well) were plated in 96-well poly D-lysine–coated black clear bottom plates (BD Biocoat). Following appropriate experiment time, the cells were formaldehyde fixed, permeabilized, then blocked with BSA, and stained according to figure legends. Hoechst 33342 was purchased from Molecular Probes. TUNEL assay was performed using the in situ cell death detection kit purchased from Roche Diagnostics.

HeLa cells were plated onto T25 flasks and allowed to recover for 24 hours. LY2606368 was then added to give final concentrations of 33 or 100 nmol/L. In some experiments, 20μmol/L Z-VAD-FMK was included during the drug treatment. Cells were treated for 12 hours, and during the last 2 hours, colchicine was added to 1 μg/mL. Fixation of nuclei for metaphase spreads was done following the method of Bayani and Squire (19). Chromosome spreads were done with a modification of the method of Deng and colleagues (20). A 12-μL volume of cell suspension in 3:1 methanol/acetic acid fixative was dropped from a height of 3 cm onto dry glass slides or coverslips. The slides were then heated for 45 seconds on a 43°C metal block, before being removed to allow drying to complete at room temperature. Coverslips were mounted on slides with Vectashield Hard Set mounting medium with DAPI (Vector laboratories, Inc.). Slides were examined with a Leica DMR fluorescence microscope and images were captured using a SPOT RT3 Slider camera.

Transfection of U-2 OS cells with siRNAs followed the Invitrogen Lipofectamine RNAiMAX (cat. 13778-075) reverse-transfection protocol. Cells were plated with the transfection mixtures and treated with LY2606368 or DMSO 48 hours later. The siRNA used as a control was the ON-TARGETplus non-targeting pool from Thermo Scientific (cat. D-001810-10-20), whereas the CDK2 (cat. L-003236-00-0005); and CDC25A (cat. SC-29254) targeted siRNAs were obtained from Dharmacon/Thermo Scientific and Santa Cruz Biotechnology, respectively. The final concentration of siRNA used for each transfection was 20 nmol/L.

Cells were harvested and then fixed in ice-cold 70% ethanol and stored at −20°C. The fixed cells were recovered by centrifugation and then washed in PBS. The final cell pellets were resuspended in 500 μL of 0.1% Triton X-100/propidium iodide solution and were incubated for 30 minutes at room temperature. The samples were analyzed on a Beckman Coulter FC500 flow cytometer and cell-cycle profiles were generated using ModFit LT software (Verity Software).

The cells were lysed either in RIPA or Cell Extraction Buffer (Invitrogen) supplemented with phosphatase inhibitors (Sigma) and protease inhibitors (Roche Diagnostics) using ice-cold sonication. The protein concentration of each lysate was first determined using the Pierce BCA Protein Assay Kit (Thermo Scientific) followed by dilution with 4 × Laemmli Sample Buffer (21) and heating at 95°C. Proteins were separated using SDS-PAGE Criterion gels (Biorad), transferred onto Immobilon-P (Millipore) membranes. Primary antibodies were incubated with the membranes overnight at 4°C and secondary antibodies coupled to HRP were incubated with the membranes for 2 hours at room temperature. Membrane-bound antibody complexes were detected with Pierce Supersignal West Pico or Femto Chemiluminescent Substrate (Thermo Scientific). Immunoblot band intensity was determined using a LAS-4000 imaging system (FUJIFILM Corp) and quantified using TotalLab gel analysis software (Nonlinear Dynamics LTD). Only the brightness and contrast of each image were adjusted for optimal printing.

Female CD-1 nu-/nu- mice (26–28 g) from Charles River Labs were used for this study. Tumor growth was initiated by subcutaneous injection of 1 × 10⁶ Calu-6 cells in a 1:1 mixture of serum-free growth medium and Matrigel (BD Bioscience) in the rear flank of each subject animal. When tumor volumes reached approximately 150 mm³ in size, the animals were randomized by tumor size and body weight, and placed into their respective treatment groups. Vehicle consisting of 20% Captisol (CyDex Inc) pH4 or LY2606368 was administered by subcutaneous injection in a volume of 200 μL. Four, eight, 12, 24, and 48 hours after drug administration, blood for plasma drug exposure was extracted via cardiac puncture and assayed on a Sciex API 4000 LC/MS-MS system. The xenograft tissue was promptly removed and prepared as previously described (17). Lysates were analyzed by immunoblot analysis for protein phosphorylation levels. Group means, SEs and P values were calculated using Kronos (22).

To measure xenograft tumor growth inhibition, tumors were implanted, established, and the animals randomized as above. Eight animals were used in each treatment group. Vehicle alone or LY2606368 was administered BIDx3, followed by 4 days of rest and repeated for an additional two cycles. Tumor size and body weight were recorded biweekly and compared between vehicle- and drug-treated groups (23).

The properties of LY2606368 have been previously reported at scientific conferences but not yet in a peer-reviewed journal. The chemical structure of LY2606368 consists of a cyanopyrazine group linked to by an amine to a pyrazole core as shown (Fig. 1). The pyrazole core is further substituted with a 2,6 dialkoxy phenyl group where one of the alkoxy groups bears a pendant amine. The general characteristics of the compound are briefly summarized here in Table 1 with supporting data to be found in the supplementary data section. LY2606368 is an ATP-competitive protein kinase inhibitor with a K_i of 0.9 nmol/L against purified CHK1 (Supplementary Fig. S1). In cell assays measuring CHK1 activity through autophosphorylation of serine 296 induced by doxorubicin or gemcitabine, LY2606368 had an EC₅₀ of <1 nmol/L (Supplementary Fig. S2A). In a functional assay, LY2606368 potently abrogated the G₂–M checkpoint activated by doxorubicin in p53-deficient HeLa cells with an EC₅₀ of 9 nmol/L (Supplementary Fig. S2B). To gauge the selectivity of the compound, a broad panel of 224 protein kinases was tested for sensitivity to LY2606368 (Supplementary Table S1). Only six additional kinases were found to have an IC₅₀ of less than 100 nmol/L for the compound (Supplementary Table S2). Of those, only CHK2 and the RSK family kinases had IC₅₀s of less than 10 nmol/L. In cell assays, LY2606368 did inhibit DNA damage-induced CHK2 autophosphorylation with an IC₅₀ of less than 31 nmol/L (Supplementary Fig. S3). However, 100 nmol/L LY2606368 did not inhibit PMA-stimulated RSK but instead weakly stimulated phosphorylation of S6 on serines 235/236 (Supplementary Fig. S4). LY2606368 was broadly antiproliferative with IC₅₀ values typically <50 nmol/L in the most sensitive cell lines with a minority of cell lines showing considerable resistance with IC₅₀'s >1,000 nmol/L (Table 1).

HeLa cells were selected as a representative model due to a well-characterized progression through the cell cycle, inactivated p53, and sensitivity to LY2606368 (IC₅₀ = 37 nmol/L; Table 1). Cells were treated with LY2606368 for 7 hours and stained for evidence of DNA DSB using both TUNEL and an antibody for H2AX phosphorylated on serine 139. Fluorescent microscopy revealed that by 7 hours, a subpopulation of cells stained strongly for DSB by both TUNEL and pH2AX (S139; Fig. 2A).

High-content image analysis revealed that the pH2AX (S139) and TUNEL-positive staining cells increased predominantly in S-phase cells (Fig. 2B). A similar result was obtained with LY2606368-treated Calu-6 cells stained for pH2AX (S139) and by TUNEL (Supplementary Fig. S5). The majority of Calu-6 cells staining for pH2AX (S139) were in early to mid-S-phase and also stained positive for DNA strand breaks by TUNEL. This is consistent with other reports that inhibition of CHK1 results in arrested replication origins and DNA strand breakage. Time-lapse photo microscopy confirmed that cells required S-phase entry/transit in the presence of LY2606368 in order to enter terminal mitosis and that non-replicating cells are visually unaffected (Supplementary Fig. S6). DSB marker staining also increased in the diminished G₂–M population. Additional high-content analysis indicated that the G₂–M population had received DNA damage during S-phase but continued to progress through the cell cycle into an early mitosis. This is likely due to the absence of p53 and CHK1 activity at the S and G₂–M DNA damage checkpoints.

TUNEL staining and H2AX phosphorylation are also used as markers for apoptotic cell death. One possibility for the appearance of DSB in cells treated with LY2606368 is that of apoptotic DNA fragmentation. To examine this possibility, HeLa cells were treated with LY2606368 plus Z-VAD-FMK, a widely used pan-caspase inhibitor. Although not effective against activation of the mitochondrial endonuclease ENDO-G, Z-VAD-FMK does prevent activation of the caspase-dependent DNA fragmentation factor CAD/ICAD (24). As seen in Fig. 2B, including Z-VAD-FMK with 100 nmol/L LY2606368 did not significantly alter the number of DNA breaks or the degree of DSB marker staining when compared with LY2606368 treatment alone. As a control, cells were also treated with 500 nmol/L Staurosporine, an effective inducer of apoptosis and DNA fragmentation in HeLa cells. At the 7-hour time point, Staurosporine had induced strong TUNEL staining, which was dramatically reduced by the inclusion of Z-VAD-FMK (Supplementary Fig. S7A and S7B and Supplementary Table S3). Staurosporine treatment alone failed to induce any pH2AX (S139) staining.

The high level of DNA damage associated with the loss of CHK1 is dependent upon CDC25A and CDK2 to promote unscheduled DNA replication. To determine whether the same mechanisms were in play for LY2606368 activity, CDC25A and CDK2 transcripts were knocked down by siRNA in U-2 OS cells. Knockdown cell pools were then treated with LY2606368 and examined for pH2AX-marked DNA damage and changes in DNA content by flow cytometry (Fig. 3A–D). Twenty-four hour treatment of U-2 OS cells with 4 nmol/L LY2606368 resulted in a large shift in cell-cycle populations from G₁ and G₂–M to S-phase with an accompanied induction of H2AX phosphorylation. siRNA knockdown of CDC25A and CDK2 alone caused only minor changes in the cell-cycle profile of U-2 OS cells and did not induce H2AX phosphorylation. When cells depleted of CDC25A or CDK2 were treated for 6 hours with 4 nmol/L LY2606368, H2AX phosphorylation was greatly reduced relative to LY2606368 alone, indicating reduced double strand DNA breakage. Similarly, the large S-phase population induced by LY2606368 after 24 hours was reduced in the absence of CDC25A and CDK2 protein. These results demonstrate that the DNA damage and S-phase defects induced by LY2606368 are in large part dependent upon the CDC25A/CDK2 axis, consistent with an increase in replication initiation leading to double stand breaks at stalled replication forks.

Toledo and colleagues recently reported that the ATR/CHK1 signaling axis maintains the integrity of stalled replication forks in part by regulating the available pool of RPA to coat exposed ssDNA (13). Under conditions of replication stress, inhibition of ATR and CHK1 resulted in depletion of the nuclear RPA pool, breakage of stalled forks, and fragmentation of chromosomes. Given the potency of LY2606368 and its ability to cause DNA damage during S-phase, a reasonable hypothesis for the compound's mechanism of action is that it induces replication catastrophe as a single agent. To test this hypothesis, we examined mitotic chromosomes from treated HeLa cells for signs of breakage. Prometaphase chromosome spreads were prepared from HeLa cells treated for a total of 12 hours with vehicle or LY2606368 (Fig. 4). Although 90% of the mitotic nuclei of the vehicle-treated cells had intact and paired chromosomes, 97% of the mitotic nuclei from LY2606368-treated cells had broken chromosomes. In some cases, LY2606368 nuclei were a mixture of free chromosome fragments mixed with smaller, paired fragments of sister chromatids. In the majority of cases, the chromosomes from the LY2606368-treated cells were shredded into hundreds to thousands of single chromosome fragments. Cotreatment of cells with LY2606368 and with the apoptosis inhibitor Z-VAD-FMK had no effect upon the number of mitotic nuclei with fragmented chromosomes (Supplementary Table S4).

Another hallmark of replication catastrophe caused by loss of ATR/CHK1 origin control is depletion of the pool of RPA protein available to protect ssDNA at replication forks (13). This is due to the excess of open replication forks with exposed ssDNA and can be measured by high-content image analysis of the coaccumulation of RPA2 protein and phosphorylated H2AX (S139) on the chromatin of each cell. When RPA2 becomes limited during replication catastrophe, H2AX (S139) phosphorylation continues to associate with new DSBs. This is characterized by a time-dependent increase in pH2AX (S139) staining intensity in cells without a corresponding increase in chromatin-associated RPA2 staining. Graphing the RPA2 and pH2AX (S139) staining intensity of each individual nucleus results in a diagnostic upturn in pH2AX (S139) intensity as a function of time when replication catastrophe is occurring. In order to get a confirmation of whether or not LY2606368 was causing DNA fragmentation through replication catastrophe, HeLa cells were treated with either vehicle or LY2606368 over a time span of 0.5 hours to 9 hours. Cells were fixed at the various time points, stained with antibodies specific for RPA2 and pH2AX (S139) and analyzed by high-content imagining. RPA2 chromatin localization in S-phase nuclei was detected by one hour and increased to a maximum intensity by 3 hours, indicating replication stress in the LY2606368-treated cells. H2AX phosphorylation was detected by 3 hours (Fig. 5A). From 6 hours on, deflection in each subsequent plot shows how individual cells continued to phosphorylate H2AX (S139) without an increase in the colocalization of additional RPA2 (red boxes). While not as pronounced as that reported for an ATR inhibitor plus hydroxyurea (HU), the upturned shape of the plot of the nuclear ratio of RPA2 to pH2AX (S139) following LY2606368 treatment is a characteristic fingerprint of cells undergoing replication catastrophe. The time lag between RPA2 loading and H2AX (S139) phosphorylation suggests that ssDNA breakage is not immediate as has been reported previously for ATR inhibition during replication stress (13). Cells were also treated with HU, a DNA-damaging agent that causes extensive stalling of replication forks and endonuclease-dependent DSB (25, 26). HU-treated cells were similar to LY2606368 in that replication stress was first noted by RPA2 association with chromatin by one hour and was maximal by 3 hours (Fig. 5B). However, HU was less efficient than LY2606368 at causing replication catastrophe and DSBs based on lower levels of pH2AX (S139) phosphorylation (red boxes). The greatest degree of replication catastrophe was observed when cells were treated together with HU and LY2606368 (Fig. 5C).

To determine whether LY2606368 also caused DNA damage in in vivo models of cancer, mice bearing Calu-6 tumor xenografts were dosed with either vehicle or a single subcutaneous dose of 15 mg/kg LY2606368. Tumors were removed 4 hours after dosing and at increasing intervals of time out to 48 hours. Lysates were prepared from the tumors, which were then probed by immunoblotting with antibodies specific for CHK1 autophosphorylation and DNA damage (Fig. 6A). Basal CHK1 (S296) autophosphorylation was reduced 3- to 3.5-fold between 4 hours and 12 hours following dosing with activity restored by 24 hours. CHK1 inhibition tracked with drug exposure in the blood, with measured plasma exposures of 7 ng/mL at 12 hours and 3 ng/mL by 24 hours (Fig. 6B). This was in close agreement with a previous experiment providing an EC₅₀ exposure of 8 ng/mL for in vivo CHK1 inhibition by LY2606368. Phosphorylation of both H2AX(S139) and RPA2(S4/S8) was also detectable at 4 hours after dosing of LY2606368, showing the rapid occurrence of DNA damage (Fig. 6A). The intensity of phosphorylation of these two DDR markers gradually increased out to 24 hours after dosing even while CHK1 activity returned. DNA damage was still detectable by 48 hours. The persistence of these markers even 24 hours after treatment indicates that the type of damage induced by LY2606368 is not readily repaired and is consistent with observations in tissue culture models.

The accumulation of DNA damage in LY2606368-treated tumors was also demonstrated to result in clear antitumor activity. Animals bearing Calu-6 xenograft tumors were dosed twice daily (BID) for 3 days with 1, 3.3, or 10 mg/kg of LY2606368 followed by 4 days of rest for three cycles. Tumor growth inhibition was determined by tumor volume measurement performed twice a week until the end of the study on day 64. As shown in Fig. 6C, all three doses of LY2606368 caused statistically significant tumor growth inhibition (up to 72.3%). LY2606368 was well tolerated in this experiment with animal weight loss not exceeding 3% in any of the treatment groups. Furthermore, tumor regrowth of the highest dose group was slow during the 28-day recovery period, indicating a durable response to LY2606368.

There is a growing realization that a potent inhibitor of CHK1 might not only potentiate traditional drugs, but also act as a stand-alone antitumor agent (7, 27, 28). It is well documented that chemically induced DNA damage activates CHK1-dependent S and G₂–M checkpoints to aid DNA repair. Abrogation of these checkpoints by a CHK1 inhibitor dramatically increases the sensitivity of cancer cells to traditional DNA-damaging agents (8). The key role that CHK1 plays in the rapid activation of the S and G₂–M checkpoints is due to the tight integration of CHK1 into normal DNA replication and DNA damage repair. During replication stress, CHK1 suppresses initiation of new replication origins through downregulation of CDK2 and association with the Treslin protein at origins (29). Loss or chemical inhibition of CHK1 kinase activity during an unperturbed replication cycle results in significantly increased replication origin activity and increased exposure of ssDNA to cleavage by endonucleases (30).

Recent work with an ATR inhibitor has provided evidence that RPA protein is the rate-limiting step for fork stabilization during replication stress (13). Inhibition of ATR during severe replication stress results in massive replication fork firing, exposing ssDNA, which absorbs the limited pool of available RPA, leaving many replication forks unprotected and subject to endonucleolytic cleavage. Similar results were also obtained with UCN-01, a moderately selective CHK1 inhibitor (13). Without the ATR/CHK1-dependent DNA damage checkpoints, the cell will continue to progress through the cell-cycle program despite DNA breaks. The persistence of DSB may be a consequence of the inhibition of CHK1 phosphorylation of RAD51 and a resulting loss of HRR. Cell death brought about by this combination of replication failure, DSB generation, and loss of DNA damage checkpoints has been called replication catastrophe (13). A number of CHK1 inhibitors have entered clinic trials, primarily in combination with DNA-damaging agents (7, 8). LY2606368 was developed as a second-generation CHK1 inhibitor to have increased potency in vivo and is being assessed clinically both as a single agent (NCT01115790) as well as in combination with cytotoxic and targeted agents (NCT02124148). Not simply a DNA damage checkpoint drug, LY2606368 appears to cause replication catastrophe in the absence of exogenous replication stress, through the inhibition of both the replication control and the DNA damage response activities of CHK1.

In this study, we have confirmed that LY2606368 kills cancer cells through a CHK1-dependent mechanism. The potency of LY2606368 is sufficiently high that complete inhibition of CHK1 is achieved in cells at low compound concentrations. Treatment with LY2606368 phenocopies genetic models of CHK1 loss and acts in part by removing canonical CHK1 suppression of replication origin activation and DNA damage checkpoint arrest in S and G₂–M-phases. This activity results in rapid saturation of RPA binding to chromatin, presumably regions of ssDNA, followed by the appearance of DSBs and chromosome fragments. Dysregulation of CHK1-dependent replication and checkpoint control by LY2606368 is an extremely efficient method of shredding chromosomes in cancer cells.

Although many clinical agents cause DNA DSB, they differ significantly in how the breaks are generated. Differences in the cell-cycle timing of the break as well as the structure of the free ends of the break determine which pathway(s) will be used to repair the damage (31). This is one reason that different tumor types show tendencies to respond better to one particular class of DNA-damaging agent. Tumors generally possess inactivating mutations in genes involved in one or more DNA repair pathways resulting in resistance to some classes of agents and sensitivity to others (32–34). Because of the lack of deep understanding of predicting drug sensitivity, the discovery of novel agents helps to increase the number of responsive indications. The mechanism of action of LY2606368 is uniquely different from traditional DNA-damaging agents. Not only did CHK1 Inhibition by LY2606368 induce DSBs by replication catastrophe, it also effectively abrogated the S and G₂–M DNA damage checkpoints, reduced HRR, and led to cell death in mitosis. Although LY2606368 is also a potent inhibitor of CHK2, we have no evidence that inhibition of CHK2 contributes to the known biologic effects of LY2606368. CHK2 is activated by DSBs through ATM and has been implicated in the p53-dependent G₁ DNA damage checkpoint, chromosome stability, and surveillance of DNA damage in female meiosis and mitotic exit (35, 36). Mutations in the CHEK2 gene increase susceptibility to cancer, likely due to impairment of the G₁ checkpoint (35). Comparison of selective CHK1 and dual CHK1/CHK2 inhibitors in vitro shows no differences in ability to cause DNA damage and kill cells (17).

Finally, CHK1 is required for RAD51 localization to DSBs to promote HRR. The lack of a strong HRR response may be the reason that LY2606368 induces DNA damage lasting many hours. Persistent levels of DNA damage were observed in tumors treated with efficacy doses of LY2606368 linking the antitumor activity of the molecule with replication catastrophe as a result of CHK1 inhibition. LY2606368 efficiently caused the hallmarks of replication catastrophe in vitro; depletion of RPA2 pools followed by massive DSB and chromosome fragmentation (13). Significantly, this occurred in the absence of additional chemically induced replication stress. In the absence of the CHK1-dependent DNA damage checkpoints, cells entered into mitosis with highly fragmented chromosomes followed by cell death. LY2606368 is a potent CHK1 inhibitor that helps to define a new class of drug capable of killing cancerous cells through RPA-dependent replication catastrophe.

No potential conflicts of interest were disclosed.

Conception and design: C. King, S. McNeely, R. Beckmann, D. Barda, M.S. Marshall

Development of methodology: C. King, H.B. Diaz, S. McNeely, D. Barnard, W. Blosser, M.S. Marshall

Acquisition of data (provided animals, acquired and managed patients, provided facilities, etc.): C. King, H.B. Diaz, S. McNeely, D. Barnard, J. Dempsey, W. Blosser, M.S. Marshall

Analysis and interpretation of data (e.g., statistical analysis, biostatistics, computational analysis): C. King, H.B. Diaz, S. McNeely, D. Barnard, W. Blosser, D. Barda, M.S. Marshall

Writing, review, and/or revision of the manuscript: C. King, H.B. Diaz, D. Barnard, R. Beckmann, D. Barda, M.S. Marshall

Administrative, technical, or material support (i.e., reporting or organizing data, constructing databases): C. King, D. Barnard, M.S. Marshall

Study supervision: R. Beckmann, M.S. Marshall

The authors acknowledge the essential contributions of the CHK1 biology and medicinal chemistry team members formerly located at Icos Pharmaceuticals: Phyllis Goldman, Erik Christenson, Darcey Clark, Jeff Dantzler, Frank Diaz, Heather Douanpanya, Francine Farouz, Ryan Holcomb, Angela Judkins, Adam Kashishian, Ed Kesicki, Kim McCaw, Harch Ooi, Vanessa Rada, Fuqiang Ruan, Alex Rudolf, Frank Stappenbeck, Janelle Taylor, Gene Thorsett, Jen Treiberg, Margaret Weidner, and Steve White. They also thank Michele Dowless, Karen Cox, Lisa Kays, and Bonita Jones for superb technical support; and Eric Westin, Aimee Bence Lin, and Robert Ilaria for insightful intellectual input.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.