The cell cycle checkpoint inhibitors in the treatment of leukemias
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The inhibition of the DNA damage response (DDR) pathway in the treatment of cancers has recently reached an exciting stage with several cell cycle checkpoint inhibitors that are now being tested in several clinical trials in cancer patients. Although the great amount of pre-clinical and clinical data are from the solid tumor experience, only few studies have been done on leukemias using specific cell cycle checkpoint inhibitors. This review aims to summarize the most recent data found on the biological mechanisms of the response to DNA damages highlighting the role of the different elements of the DDR pathway in normal and cancer cells and focusing on the main genetic alteration or aberrant gene expression that has been found on acute and chronic leukemias. This review, for the first time, outlines the most important pre-clinical and clinical data available on the efficacy of cell cycle checkpoint inhibitors in single agent and in combination with different agents normally used for the treatment of acute and chronic leukemias.
KeywordsDNA damage response Checkpoint kinase inhibitor Acute lymphoblastic leukemia Acute myeloid leukemia Chronic myeloid leukemia Chronic lymphocytic leukemia
Acute lymphoblastic leukemia
Acute myeloid leukemia
ATM and Rad3 related
Cell division cycle 25
Cyclin-dependent kinase inhibitor 1A
Checkpoint kinase 1
Checkpoint kinase 2
Chronic lymphocytic leukemia
Chronic myeloid leukemia
DNA damage response
DNA-dependent protein kinase
Membrane-associated tyrosine/threonine 1
Reactive oxygen species
Replication protein A
Topoisomerase II binding protein 1
The DNA damage response (DDR) pathway
DNA damage sensors and mediators
DNA damage effectors
Different effectors are substrates of ATM and ATR kinases, and most of them are involved in cell cycle regulation (cell cycle checkpoint kinases) and in the mechanisms of DNA repair. Here, we will focus on the cell cycle regulation ATM/ATR-mediated. The most important substrates of ATR and ATM are the checkpoint kinase 1 (CHK1) and 2 (CHK2), respectively. CHK1 kinase is activated by ATR through the phosphorylation on serine 317 (ser317) and on serine 345 (ser345). Rapidly, CHK1 auto-phosphorylates on serine 296 stabilizing its structure and creating a binding site for the interaction with its direct substrates, the phosphatases CDC25 (CDC25A/B/C). The activation of CHK2 is enhanced by ATM through the phosphorylation on threonine 68 (thr68) and followed by several auto-phosphorylation events. CHK2 shares the substrate homology with CHK1 and inhibits CDC25A/B/C phosphatases in a similar way . In eukaryotic cells, the cell cycle is finely regulated by the oscillation in the activity of different cyclin-dependent kinases, CDKs, which are positively regulated by proteins, called cyclins, and negatively regulated by CDK inhibitors (CDKI) and by a mechanisms of inhibitory phosphorylation [24, 25]. The transition from a phase of the cell cycle to another is regulated by different cell cycle checkpoints and in particular by the G1/S (transition through the G1 phase to the S phase), the intra-S and the G2/M checkpoints (transition to the G2 phase and entry in the mitosis). The activation of the G1/S checkpoint is mainly regulated through the activity of the tumor suppressor p53 which has been showed to be one of the direct substrate of ATM/ATR activation via the phosphorylation on serine 15 (ser15). Different sequential phosphorylations contribute to p53 stabilization and prevent the ubiquitination and consequently degradation enhanced by the negative regulator of p53, MDM2 . The regulation of p53 in the G1/S checkpoint is also related to the activation of two direct substrates of both ATM and ATR, respectively, the checkpoint kinase 2 (CHK2) and the checkpoint kinase 1 (CHK1), that promote the activatory phosphorylation of p53 on serine 20 (ser20) [27, 28]. Once fully activated, p53 promotes the transcription of different genes involved in cell cycle regulation, like CDKN1A (cyclin-dependent kinase inhibitor 1A (P21, Cip1)), and induction of apoptosis, like BAX/PUMA/NOXA proteins . The transition through the S phase is mainly regulated by a specific phosphatase, CDC25A . This protein is necessary to remove the inhibitory phosphorylation on tyrosine 15 (tyr15) and threonine 14 (thr14) on cyclin-dependent kinase 2 (CDK2, CDC1). During normal replication, CDC25A activates CDK2 promoting the formation of the complex CDK2-cyclin E/cyclin A necessary for the entry into the S phase and for the DNA synthesis. In the presence of DNA damages, both CHK1 and CHK2 phosphorylate CDC25A on serine 136 (ser136) promoting its ubiquitination by SCF/TrCP ubiquitin ligase complex and following proteasomal degradation. The inhibition of CDC25A causes an S phase delay. Similar to the regulation of the S phase, also, the transition from the G2 to the M phase is strictly dependent on the activation of specific phosphates and in particular on the activation of both CDC25B and CDC25C. During the checkpoint activation, CDC25B is phosphorylated at serine 323 (ser323), by CHK1 and bound by 14-3-3 that blocks its catalytic activity [31, 32]. Both CHK1 and CHK2 negatively regulate CDC25C via phosphorylation of a serine residue (ser216); this event creates a site for the binding to 14-3-3 protein resulting in its cytoplasmic sequestration and G2/M checkpoint activation. The events that follow CDC25C sequestration are similar to those follow CDC25A degradation. The segregation of this phosphatase in the cytoplasm prevents its accumulation into the nucleus and consequently the inactivation of a protein complex crucial for the transition through the G2/M phase, the CDK1 (CDC2)-cyclin B complex. This complex is finely regulated not only by CHK1 or CHK2 but also by two proteins of the WEE1 family, WEE1, and MYT1. While both kinases can inhibit CDK1 through the phosphorylation on tyrosine 15 (Tyr15), MYT1 can also phosphorylate on threonine 14 (thr14), which has been shown to negatively regulate CDK1 as well. Thus, after the activation of the G2/M checkpoint, CHK1, CHK2, and WEE1 cooperate to negatively regulate CDK1 to prevent the formation of the complex with the cyclin B . Although the regulation of WEE1 during normal cell cycle has been established , the mechanisms by which WEE1/MYT1 are activated in response to DNA damage in human is still not fully understood . During normal cell division polo kinase 1 (PLK1) phosphorylates WEE1 promoting its degradation and, consequently, the beginning of mitosis. After DDR activation, both ATM and ATR promote the inhibitory phosphorylation of PLK1, leading to the nuclear accumulation of WEE1 .
Cell cycle checkpoint-related proteins alteration in acute and chronic leukemias
Although in normal cells the cell cycle checkpoint kinases as well as other elements of the DDR pathway act as tumor suppressor and are crucial for the maintenance of genetic stability, in cancer, they have been found to protect tumor cells from different stresses and, consequently, to promote tumor progression . Indeed, in normal cells, DNA errors are fixed by the repair mechanisms and if not, cell proliferation is arrested and cell death often ensues. The following section summarizes the main genetic alterations (mutations, copy number alteration, and gene expression alterations) that, although rare, have been reported in leukemias.
Mutations and copy number alterations in key cell cycle checkpoint genes in leukemias
The loss of function of ATM leads to the genetic disorder ataxia-telangiectasia (A-T), characterized by cerebellar degeneration, immunodeficiency, radiation sensitivity, chromosomal instability, and cancer pre-disposition [38, 39]. Mutations in ATM pre-dispose A-T patients to the development of lymphoid neoplasms, with a risk for leukemia approximately 70 times higher than the normal population . Inactivating mutations and copy number alterations have been reported in both acute and chronic leukemia subtypes. In acute leukemia, Haidar and colleagues reported a high frequency of ATM deletions (10 out of 36; 28%), including 7 (19.4%) cases with loss of heterozygosity (LOH) and 3 (8.4%) cases with homozygous deletions in adult acute lymphoblastic leukemia (ALL) patients. Interestingly, in the ALL subgroup, the ATM protein deficiency (due to LOH or homozygous deletions) correlates with a favorable prognosis . Copy number gains of ATM in 3 out of 191 (1.6%) adult patients with de novo acute myeloid leukemia (AML) have been reported by the Cancer Genome Atlas Research Network . In chronic myeloid leukemia (CML), ATM was investigated as a potential candidate gene for the increased genetic instability following the evolution from chronic phase to blasts crisis (BC). Initial mutational analysis of 57 CML cases in BC highlighted no deleterious nucleotide changes in ATM and lack of correlation with BC progression . However, the correlation between the loss of ATM and the acceleration of BC has been recently reported in CML mouse models . LOH events involving the ATM locus and ATM protein deficiency occur in 14% and 34%, respectively, of patients with chronic lymphocytic leukemia (CLL) and have been found to correlate with aggressive disease and worse outcome . Recent studies in large cohorts of CLL primary samples revealed a high frequency of missense/truncating mutation of ATM and deletion of ATM (associated with 11q22.3-23.2 deletion) [46, 47, 48]. ATR mutations, as well as copy number alterations, are rare in tumor cells due to the fundamental biological role of this kinase. Currently, no mutations affecting ATR have been annotated in acute and chronic leukemia patients, and only one case of single-nucleotide variant (SNV) out of 50 samples has been described in AML patients . The downstream target of ATM, CHK2, has been found instead mutated in low rate in several kinds of cancer and in particular in hereditary cancers (CHK2 1100delC protein-truncating mutation confers a twofold increased risk of breast cancer) [50, 51]. In both acute (AML) and chronic (CLL) leukemias, only few studies reported mutations or copy number alterations of CHK2 and with a very low percentage [52, 53, 54]. Similarly to ATR, no mutations have been reported in CHK1 in acute and chronic leukemias.
Gene expression alteration of key cell cycle checkpoint genes in leukemia
Cell cycle checkpoint kinase inhibitors against leukemias
Due to the central role in the DNA damage response, different cell cycle checkpoint inhibitors (ATM/ATR/CHK1/CHK2/WEE1 inhibitors) have been developed to specifically inhibit the mechanisms by which tumor cells respond to DNA damaging agents. Initially, this class of compounds has been developed for the treatment of p53 mutated tumors because of their impaired G1/S checkpoint, and then, their applicability has been extended also to p53 wild-type tumors [42, 43]. These compounds have been developed to potentiate the efficacy of different chemotherapeutic compounds especially for the treatment of solid tumors . The following section summarizes the main studies that have been performed to explore the efficacy of different cell cycle checkpoint inhibitors in acute (ALL and AML) and chronic (CLL and CML) leukemias.
ATM/ATR inhibitors against leukemias
KU-55933 was the first developed potent ATM inhibitor (KuDOS Pharmaceuticals, AstraZeneca). Hickinson and colleagues showed that KU-55933 confers marked sensitization to ionizing radiation and DNA DSB-inducing chemotherapeutics, such as the topoisomerase II inhibitors (etoposide and doxorubicin), in cancer cells . The efficacy of KU-55933 against leukemic cells was evaluated in Jurkat cells with or without etoposide. The combination between the DSBs inducer and the ATM inhibitor deeply affected the cell viability of the leukemic cells . Although KU-55933 showed strong efficacy in vitro, its high lipophilicity limited the use in in vivo studies.
KU-59403 is a novel ATM inhibitor with improved potency, solubility, and bioavailability over the KU-55933. Batey and colleagues demonstrated a good tissue distribution and a good efficacy in mice . In acute leukemia, Grosjean-Raillard and colleagues demonstrated that treatment with KU-59403 represses the antiapoptotic transcription factor nuclear factor-κB (NF-κB) pathway, which has been found to be constitutively activated in CD34+ myeloblasts of high-risk myelodysplastic syndrome (MDS) and AML patients and consequently, it induces cell death via apoptosis . Despite none clinical trials have been yet performed using ATM inhibitors, the results of several in vitro studies carried out that the pharmacological inhibition of this protein has great potential as a cancer therapy in combination with radiotherapy or certain chemotherapeutic drugs (like topoisomerase inhibitors).
Schisandrin B was the first ATR-selective small molecule inhibitor that has been evaluated in vitro. Nishida and colleagues reported that schisandrin B was able to abrogate UV-induced intra-S phase and G2/M cell cycle checkpoints and increase the cytotoxicity of UV radiation in human lung cancer cells . Then, Vertex Pharmaceuticals using a large high-throughput screening led to the discovery of the first series of both potent and selective ATR kinase inhibitors . The first selective ATR inhibitor, VE-821, had >100-fold selectivity for ATR versus ATM, PI3K, DNA-PK, and mTOR and sensitized leukemic cell lines to radiotherapy [72, 73].
VE-822 (VX-970), a further analogs of VE-821, has been improved with increased solubility, potency, selectivity, and pharmacodynamic properties . Several pre-clinical studies have shown that VX-970 robustly sensitizes multiple tumor cell lines to cisplatin, ionizing radiation, gemcitabine, PARP inhibitors, topoisomerase I inhibitors, etoposide, and oxaliplatin in vitro [75, 76, 77, 78, 79, 80]. In vivo studies using both VE-821 and VX-970 showed robust results. Indeed, these two ATR inhibitors synergized with radiotherapy and gemcitabine in pancreatic cancer xenograft models [76, 77] and with irinotecan in a colorectal cancer model . Nowadays, different clinical trials are ongoing against solid tumors to assess the safety, tolerability, and pharmacokinetics of VX-970 in combination with cytotoxic chemotherapy (NCT02157792, NCT02595931, NCT02567422, and NCT02595892).
AZD6738 is the second ATR inhibitor currently in clinical development that possesses significantly improved solubility bioavailability and pharmacokinetic properties compared to other ATR inhibitors and is suitable for oral dosing . Treatments with AZD6738 inhibit the phosphorylation of CHK1 while increasing phosphorylation of γH2AX in vitro. In in vivo models, combinatorial studies with carboplatin or ionizing radiation (IR) demonstrated significantly reduction of tumor progression in comparison with the effects of the single treatments [81, 82]. In hematological malignances, AZD6738 showed activity as monotherapy in mantle cell lymphoma xenograft mouse models with ATM and p53 deficiencies  and in primary CLL patient-derived xenografts with 11q deletion (ATM deficient) and 17p deletion (p53 deficient) . Finally, preliminary data highlighted that AZD6738 synergizes with carboplatin, bendamustine, and cyclophosphamide in an ATM-deficient diffuse large B cell lymphoma model. Currently, no data have been published using ATR inhibitors in acute leukemia.
CHK1/CHK2 inhibitors against leukemias
MK-8776 (SCH900776) is a potent and selective CHK1 inhibitor in clinical development. It rapidly, less than 2 h, induced γH2AX accumulation and suppressed CHK1 functionality (shown by the reduction of the auto-phosphorylation site of serine 296). The efficacy of this inhibitor was assessed not only in single agent but also in combination with different genotoxic compounds showing chemotherapy sensitization by increasing the level of DSBs. Many other studies confirmed the great efficacy of this compound for the treatment of different kinds of tumor, and today, MK-8776 entered in phase II clinical trials in combination with chemotherapy [86, 90, 91, 92]. The efficacy of the compound was also evaluated in hematological malignances. Day and colleagues demonstrated that MK-8776 synergistically potentiated the histone deacetylase (HDAC) inhibitor (HDACI) vorinostat in both AML cell lines and primary cells . Moreover, they showed that efficacy of the combination was independent on the mutational status of p53 and that the synergistic interactions were associated with inhibition of CHK1 activity, interference with the intra-S phase checkpoint, disruption of DNA replication, and down-regulation of proteins involved in DNA replication and repair . Zemanova J. and colleagues reported that SCH900776 enhanced the cytotoxicity of different nucleoside analogs (fludarabine, cytarabine, and gemcitabine) on the p53-deficient CLL cell line MEC1 and primary cells isolated from CLL patients .
AZD7762 is an ATP competitive CHK1/CHK2 inhibitor. This compound was evaluated in different trials as a chemo-sensitizer agent for conventional chemotherapy. It has been described that lung cancer cells expressing high levels of CHK1 were hyper-sensitive to AZD7762. This suggests a correlation between CHK1 inhibitor-mediated sensitivity and elevated amounts of CHK1. Different further studies were performed to investigate the efficacy of ASD7762 in combination with different compounds. Indeed, it has been reported that combination of AZD7762 with gemcitabine and ionizing radiation deeply sensitized pancreatic cells to radiation . The efficacy of the compound was evaluated also in hematologic malignances, e.g., in different myeloma multiple (MM) cell lines. The combination of AZD7762 with alkylating agents (melphalan) promoted apoptosis and mitotic catastrophe of p53-mutated MM cells . Moreover, Didier et al. showed that AZD7762 enhances genotoxic treatment efficacy in immature KG1 AML cell line and in AML primary leukemic cells . In this study, they also found a correlation between the sensitivity to the checkpoint kinase inhibitors and a complex karyotype, usually a poor prognostic marker to conventional chemotherapy. Thus, the basal level of DNA damage (γH2AX, CHK1, and phosphorylated ATM/ATR substrates) could be a useful marker to select AML patients susceptible to receive this type of combination therapy .
PF-0477736 is a selective and competitive inhibitor for the CHK1 ATP site. Its specificity is 100 times stronger for CHK1 than that for CHK2. The efficacy of this compound has been well established against different kinds of tumor. In ovarian cancer, it has been shown that tumor cells strongly respond to treatment with PF-0477736 but they generate metastasis and chemo-resistant clones . The efficacy of PF-0477736 has been evaluated also in leukemia. Sarmento et al.  demonstrated that the T-ALL primary samples express higher level of CHK1 kinase in comparison to normal thymocytes. The treatment with PF-0477736 promoted apoptotic cell death and CHK1 inhibition and consequently impaired replication and abrogation of G2/M checkpoint in T-ALL cells. Interestingly, in vitro treatment did not significantly affect the viability of normal thymocyte cells . Similar results have been shown by our group. The inhibition of CHK1/CHK2 by PF-0477736 as single agent deeply reduced the cell viability of ALL primary cells and leukemia cell lines. The results from the in vitro/ex vivo studies were further confirmed using an in vivo model . Recently, Nguyen T. and colleagues reported the in vitro/in vivo synergic efficacy of PF-00477736 in combination with the Src/ABL inhibitor bosutinib (SKI-606) in BCR-ABL1-positive CML or ALL cells, focusing on highly imatinib-resistant models with ABL kinase mutations. The authors speculated that the combination acts through a BCR-ABL1-independent process that may involve multiple mechanisms, including inactivation of ERK1/2 and Src, up-regulation of BIM, down-regulation of MCL-1 (BCL-2-like protein), activation of CDK1, and induction of DNA damage .
LY2603618, a potent and selective inhibitor of CHK1, is the first second-generation checkpoint kinase inhibitor that has been evaluated in a clinical trial . King and colleagues  reported that the treatment with LY2603618 produced a cellular phenotype similar to that reported for depletion of CHK1 by RNA interference (RNAi). Moreover, they reported that the inhibition of CHK1 caused impaired DNA synthesis, elevated H2AX phosphorylation, and pre-mature entry into mitosis. Finally, they showed that LY2603618 was able to override the G2/M checkpoint activated after the exposure to doxorubicin, resulting in cells entering into metaphase with poorly condensed chromosomes . In several studies, LY2603618 potentiated the effect of DNA damage compounds like pemetrexed and cisplatin in vitro. This result was confirmed in vivo using a tumor xenograft model and placed the bases for a phase I clinical trial evaluating the effectiveness of LY2603618 in combination with pemetrexed and cisplatin in patients with advanced cancer . Zhao J. and colleagues have recently reported the efficacy of LY2603618 in combination with the BCL-2 inhibitor ABT-199 in AML cell lines and primary cells (n = 26). The authors demonstrated that the treatment with LY2606368 reduced the total amount of MCL-1 and, consequently, enhanced the efficacy of ABT-199 in terms of induction of apoptosis .
LY2606368 (prexasertib) is a novel CHK1/CHK2 inhibitor which has been reported to cause as a single agent DBSs while simultaneously removing the protection of the DNA damage checkpoints. King and colleagues reported that LY2606368 increases extensive DNA damage in the cell population in S phase highlighting the possible mechanism of death through replication catastrophe . In a recent study from our group in acute lymphoblastic leukemia, the efficacy of LY2606368 was evaluated both as single agent and in combination with different compounds currently used in clinical practice. This study showed that LY2606368 deeply sensitized both primary and leukemic cells to the antimetabolite, clofarabine, and to tyrosine kinase inhibitors (imatinib and dasatinib) .
WEE1 inhibitors against leukemias
MK-1775 (AZD1775) is the mostly studied WEE1 inhibitor. Several studies have shown that this inhibitor selectively sensitizes p53-deficient cancer cells to the toxic effect of gemcitabine, carboplatin, 5-fluorouracil, and cisplatin [106, 107, 108, 109]. The sensitizing activity of MK-1775 selectively on p53-deficient cells has been shown also after gamma ray irradiation. Although all the above mentioned studies, recent findings highlighted that the effectiveness of this compound in different types of tumor is independent on the functional status of p53 . In hematological malignancies, recent studies mainly on acute myeloid leukemia have shown the efficacy of this compound, not only as single agent  but also in combination with different compounds like HDAC (vorinostat), CDK inhibitor (roscovitine), LY2603618, or cytarabine [111, 112, 113, 114, 115, 116]. The combination of cytarabine and MK-1775 enhanced chemotherapy cytotoxicity by abrogating the mechanisms of DNA repair and by the inhibition of the S phase arrest induced by cytarabine [110, 117]. Similar results were recently found in T-ALL cell lines and in in vivo models . Tibes and colleagues showed the efficacy of MK-1775 in both CML primary cell and cell lines as single agent and in combination with cytarabine. In this study, they showed that the inhibition of WEE1 significantly sensitizes leukemic cells to cytarabine in terms of reduction of cell viability and induction of apoptosis . These data support the development of clinical trials including AZD1775 in combination with conventional chemotherapeutic compounds for leukemias.
Data available from clinical trials in solid tumors
A phase I dose-escalation study aiming to examine the safety and tolerability of LY2603618 in combination with pemetrexed 500 mg/m2 every 21 days in patients with cancer defined a maximum tolerated dose (MTD) of 150 mg/m2. A following phase I study using LY2603618 in combination with gemcitabine in patients with solid tumors showed that among the 50 patients enrolled, frequent adverse events, possibly related to study drug treatment, included fatigue (44%), decreased platelets (42%), decreased neutrophils (32%), nausea (26%), and decreased hemoglobin (20%). Systemic exposure of LY2603618 increased dose dependently, while clearance was relatively dose independent. The mean LY2603618 half-life varied. However, the durations were still suitable for maintaining human exposures while minimizing accumulation. LY2603618 pharmacokinetic (PK) was not altered by gemcitabine administration. Plasma exposures that correlate with the maximal pharmacodynamic effect in non-clinical models were achieved for all doses. One patient with non-small cell lung cancer carcinoma achieved a partial response; 22 patients had stable disease. They conclude that the MTD of LY2603618 combined with gemcitabine was 200 mg/m2, but a fixed LY2603618 dose of 230 mg combined with gemcitabine was selected as the recommended phase II dose . A consequent phase II study evaluating the effect of LY2603618 in combination with pemetrexed in patients with advanced or metastatic non-small cell lung cancer highlighted no significant improvement of pemetrexed efficacy as single agent in non-small cell lung cancer . Until now, only a phase I study has been done using LY2606368 as single agent in advanced solid tumors. Forty-five patients were treated with two different dose-escalation schedule: from 10 to 50 mg/m2 on schedule 1 (days 1 to 3 every 14 days) or from 40 to 130 mg/m2 on schedule 2 (day 1 every 14 days); seven experienced dose-limiting toxicities (all hematologic). The MTDs were 40 mg/m2 (schedule 1) and 105 mg/m2 (schedule 2). The most common related grade 3 or 4 treatment-emergent adverse events were neutropenia, leukopenia, anemia, thrombocytopenia, and fatigue. Grade 4 neutropenia occurred in 73.3% of patients and it was transient (typically <5 days). Febrile neutropenia incidence was low (7%). The LY2606368 exposure over the first 72 h (area under the curve from 0 to 72 h) at the MTD for each schedule coincided with the exposure in mouse xenografts that resulted in maximal tumor responses. Minor intra- and intercycle accumulation of LY2606368 was observed at the MTDs for both schedules. Two patients (4.4%) had a partial response. Fifteen patients (33.3%) had a best overall response of stable disease (range, 1.2 to 6.7 months), six of whom had squamous cell carcinoma. An LY2606368 dose of 105 mg/m2 once every 14 days is being evaluated as the recommended phase II dose in dose-expansion cohorts for patients with squamous cell carcinoma. A phase I study of single-agent AZD-1775 involving 25 patients with refractory solid tumors showed that the MTD was established as 225 mg twice per day orally over 2.5 days per week for 2 weeks per 21-day cycle. Confirmed partial responses were observed in two patients carrying BRCA mutations: one with head and neck cancer and one with ovarian cancer. Common toxicities were myelosuppression and diarrhea. The on-target efficacy of the compound was assessed looking at the levels of phosphorylated Tyr15-Cdk (pY15-Cdk) and γH2AX in paired tumor biopsies obtained at the MTD . A second phase I study demonstrated target inhibition (Tyr15-Cdk) at MTD in combination with carboplatin adult patients with advanced solid tumors (NCT00648648). Patients with p53 mutated ovarian cancer refractory or resistant (<3 months) to standard first line therapy (carboplatin plus paclitaxel) were re-exposed to carboplatin (AUC 5), plus five bi-daily doses of 225 mg AZD-1775 in a 21-day cycle (MTD). Bone marrow toxicity, fatigue, diarrhea, nausea, and vomiting were the most common adverse events. Out of 24 patients enrolled, 22 patients were evaluable for study endpoints. As best response (RECIST 1.0), six patients (27%) showed confirmed partial response (PR) with a median progression-free survival (PFS) of 10.9 months. Nine patients (41%) had stable disease and seven patients (32%) had progressive disease as best response, with a median PFS of 5.3 and 1.3 months, respectively (NCT01164995).
Nowadays, the amount of pre-clinical data has confirmed the efficacy of different cell cycle checkpoint inhibitors against different kinds of hematologic as well as solid tumors, as single agent, or in combination with a wide number of drugs. The efficacy as well as the safety of different combinations is now being established also in several phase I/II clinical trials. Most of the studies were based on the use of cell cycle checkpoint inhibitors in combination with standard chemotherapy in order to enhance its effectiveness. Although the good successes that have been achieved have many questions needed to be answered regarding the safety and the effectiveness of this class of compounds. Some acute leukemia subtypes are characterized by high genetic instability that should make this kind of tumor very sensitive to cell cycle checkpoint inhibitors. However, few clones can take advantage from the inhibition of DNA repair, acquire novel invasive features, and start to proliferate. Long-period safety of cell cycle checkpoint inhibitors should be addressed also in normal tissues in order to exclude tumor transformation of healthy cells. A second crucial question that should be answered, at least in hematological malignances, is the ability of cell cycle checkpoint inhibitors to eradicate leukemic stem cells in the contest of bone marrow niche. Indeed, until today, very few studies have addressed, for example, the efficacy of the checkpoint inhibitors under hypoxic condition or more general in experimental settings that mime the niche micro-environment. Finally, prognostic markers should be evaluated to stratify patients that could be more sensitive to checkpoint kinase inhibitors. One predictive marker could be the evaluation of basal expression of elements involved in the DDR and the level of genetic instability (γH2AX expression). In our opinion, based on the results from the clinical trials, a last important question should be answered: can we substitute DNA damaging agents (chemotherapy) with DDR inhibitors in standard therapeutic regimens in which a specific inhibitor, for example, BCR-ABL1 inhibitors, is associated with conventional chemotherapy? Several studies have been done to evaluate the chemotherapy-induced genetic instability in various types of cancers [121, 122, 123]. It is generally believed that DNA damaging compounds can positively select tumor cells that harbor particular mutations or can increase genetic instability leading to the generation of novel clones with more aggressive phenotypes. These two scenarios are the biological explanations for the failure of standard chemotherapy and for tumor relapses. For the abovementioned reasons, we speculate that a winning strategy to avoid relapse may be to substitute chemotherapy with cell cycle checkpoint inhibitors in the treatment of hematological malignances that can be treated with specific targeted inhibitors.
The study was funded by the University of Bologna and by the Italian Association for Cancer Research (AIRC).
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Data sharing is not applicable to this article as no datasets were generated or analyzed during the current study.
AGLDR drafted the first version of the manuscript and created the figures. II and GM equally contributed to the final version of the manuscript and gave the final approval. All authors read and approved the final manuscript.
GM has competing interests with Novartis, BMS, Roche, Pfizer, ARIAD, MSD.
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