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Keywords:

  • Cancer therapy;
  • DNA damage response;
  • DNA repair;
  • PARP inhibitors;
  • synthetic lethality

Abstract

  1. Top of page
  2. Abstract
  3. Mechanism of DNA Damage Response
  4. Aberrations in DNA Damage Responses in Human Cancers
  5. How Can Different DNA Damage Response Pathways be Targeted for Cancer Therapy?
  6. Current Limitations and Future Perspectives
  7. Conclusions
  8. Acknowledgments
  9. Disclosure Statement
  10. References

Cancer chemotherapy and radiotherapy are designed to kill cancer cells mostly by inducing DNA damage. DNA damage is normally recognized and repaired by the intrinsic DNA damage response machinery. If the damaged lesions are successfully repaired, the cells will survive. In order to specifically and effectively kill cancer cells by therapies that induce DNA damage, it is important to take advantage of specific abnormalities in the DNA damage response machinery that are present in cancer cells but not in normal cells. Such properties of cancer cells can provide biomarkers or targets for sensitization. For example, defects or upregulation of the specific pathways that recognize or repair specific types of DNA damage can serve as biomarkers of favorable or poor response to therapies that induce such types of DNA damage. Inhibition of a DNA damage response pathway may enhance the therapeutic effects in combination with the DNA-damaging agents. Moreover, it may also be useful as a monotherapy when it achieves synthetic lethality, in which inhibition of a complementary DNA damage response pathway selectively kills cancer cells that have a defect in a particular DNA repair pathway. The most striking application of this strategy is the treatment of cancers deficient in homologous recombination by poly(ADP-ribose) polymerase inhibitors. In this review, we describe the impact of targeting the cancer-specific aberrations in the DNA damage response by explaining how these treatment strategies are currently being evaluated in preclinical or clinical trials.

The genome DNA is constantly exposed to various genotoxic insults. Among the variety of types of DNA damage, the most deleterious is the DNA double-strand break (DSB).[1] Double-strand breaks can be generated by endogenous sources such as reactive oxygen species produced during cellular metabolic processes and replication-associated errors, as well as by exogenous sources including ionizing radiation and chemotherapeutic agents. Double-strand breaks are also generated in a programmed manner during meiosis and during the V(D)J recombination and class switch recombination required for the development of lymphocytes. If left unrepaired, DSBs can result in cell death. If accurately repaired, DSBs can result in survival of cells with no adverse effects. If insufficiently or inaccurately repaired, DSBs can result in survival of cells showing genomic alterations that may contribute to tumor development.[2] In order to maintain genomic integrity, cells have evolved a well coordinated network of signaling cascades, termed the DNA damage response, to sense and transmit the damage signals to effector proteins, and induce cellular responses including cell cycle arrest, activation of DNA repair pathways, and cell death (Fig. 1).[1]

image

Figure 1. Overview of the diverse spectrum of DNA damage and the DNA damage response. The major repair pathways and key proteins used to process each type of damage are shown. In non-homologous end-joining (NHEJ), the Ku70/Ku80 complex binds to the DNA double-strand break ends and recruits the other indicated components. In base-excision repair (BER), poly(ADP-ribose) polymerase-1 (PARP-1) detects and binds to single-strand breaks and ensures accumulation of other repair factors at the breaks. Single-strand breaks containing modified DNA ends are recognized by damage-specific proteins such as apurinic/apyrimidinic endonuclease (APE1), which subsequently recruits Polβ and XRCC1-DNA ligase IIIα to accomplish the repair. All the molecules indicated here are aberrated in sporadic cancers. The proteins targeted for cancer therapy in the present clinical trials are marked with red asterisks. alt-NHEJ, alternative NHEJ; ATM, ataxia telangiectasia mutated; ATR, ataxia telangiectasia and Rad3-related; FA, Fanconi anemia; HR, homologous recombination; MGMT, O6-methylguanine-DNA methyltransferase; MMR, mismatch repair; MRN, MRE11–RAD50–NBS1; NER, nucleotide excision repair; TLS, translesion synthesis.

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Cancer chemotherapeutic agents and radiotherapy exert their cytotoxic effects by inducing DNA DSBs. As cancer cells often have specific abnormalities in the DNA damage response, therapeutic strategies based on such properties of cancer cells have been developed. Several inhibitors that block specific DNA damage responses or repair proteins have been tried not only as sensitizing agents in combination with DNA-damaging agents but also as single agents against cancers with defects in particular DNA repair pathways. The most prominent example of the latter is the killing effect of poly(ADP-ribose) polymerase (PARP) inhibitors on BRCA1- or BRCA2-defective tumors, which takes advantage of the defects in DNA repair in cancer cells.[3]

In this review, we will first outline the mechanism of the DNA damage response. Next, we will describe the aberrations in DNA damage responses in human cancers. Finally, we will explain how different DNA damage response pathways can be targeted for cancer therapy.

Mechanism of DNA Damage Response

  1. Top of page
  2. Abstract
  3. Mechanism of DNA Damage Response
  4. Aberrations in DNA Damage Responses in Human Cancers
  5. How Can Different DNA Damage Response Pathways be Targeted for Cancer Therapy?
  6. Current Limitations and Future Perspectives
  7. Conclusions
  8. Acknowledgments
  9. Disclosure Statement
  10. References

DNA-damaging agents induce various types of DNA damage including modification of bases, intrastrand crosslinks, interstrand crosslinks (ICL), DNA–protein crosslinks, single-strand breaks (SSBs), and DSBs. Each type of DNA damage is recognized and processed by proteins involved in the DNA damage response (Fig. 1).

In response to DSBs, the MRE11–RAD50–NBS1 (MRN) complex senses and binds to DSB sites, and recruits and activates the ataxia telangiectasia mutated (ATM) kinase through its autophosphorylation.[4, 5] Once activated, ATM phosphorylates a large number of downstream proteins.[6] Phosphorylation of Chk2 induces phosphorylation of the protein phosphatase CDC25A, leading to cell cycle arrest. Phosphorylation of BRCA1 leads to DSB repair as well as cell cycle arrest in the S phase, whereas activation of p53 triggers cell cycle arrest in the G1 phase or cell death. In the initiation of the response to SSBs or DNA replication fork collapse, the ataxia telangiectasia and Rad3-related (ATR) kinase is activated and recruited to the sites of DNA damage.[7] ATR phosphorylates and activates Chk1,[8] which plays a role in the S and G2/M cell checkpoints by regulating the stability of the CDC25 phosphatases. Activation of the 53BP1 protein, a mediator of the DNA damage response, contributes to the choice of the DSB repair pathways by promoting non-homologous end joining (NHEJ).[9]

The DNA repair pathways can either work independently or coordinately to repair different types of DNA damage (Fig. 1). Double-strand breaks are predominantly repaired by either NHEJ or homologous recombination (HR).[10] Non-homologous end joining is an error-prone repair pathway that is mediated by the direct joining of the two broken ends.[10] Factors involved in NHEJ include the Ku70/Ku80 complex, DNA-PK catalytic subunit (DNA-PKcs), the Artemis nuclease, XLF, XRCC4, and DNA ligase IV. Homologous recombination is an error-free repair pathway that requires a non-damaged sister chromatid to serve as a template for repair (Fig. 2).[10] Factors involved in HR include the MRN complex, CtIP, replication protein A (RPA), BRCA1, PALB2, BRCA2, and RAD51. In addition to NHEJ and HR, an alternative form of NHEJ, namely, alt-NHEJ, is also involved in DSB repair.[11] It exhibits a slower process than the classical NHEJ and can catalyze the joining of unrelated DNA molecules, leading to the formation of translocations as well as large deletions and other sequence alterations at the junction. Factors involved in this pathway include PARP-1, XRCC1, DNA ligase IIIα, polynucleotide kinase, and Flap endonuclease 1.

image

Figure 2. Early steps of homologous recombination. First, the DNA double-strand break is sensed by the MRE11–RAD50–NBS1 (MRN) complex, which subsequently recruits and activates the ataxia telangiectasia mutated (ATM) kinase. Then, the DNA ends are resected by the MRN complex and CtIP, resulting in generation of 3′ single-stranded DNA (ssDNA) overhangs on both sides of the break. These overhangs are coated and stabilized by replication protein A (RPA). Next, BRCA2, which forms the BRCA1–PALB2–BRCA2 complex, directly binds RAD51 and recruits it to the double-stranded DNA–ssDNA junction, and promotes the loading of RAD51 onto ssDNA. This step is followed by displacement of RPA from ssDNA ends and assembly of the RAD51–ssDNA filament, which is mediated by BRCA2, leading to strand invasion into an undamaged homologous DNA template. All the molecules indicated here are aberrated in sporadic cancers. None of the proteins indicated here are targeted for cancer therapy in the present clinical trials. P, phosphorylation.

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Single-strand breaks and subtle changes to DNAs are repaired using base-excision repair (BER) proteins,[12] which include PARP-1, XRCC1, DNA ligase IIIα, and apurinic/apyrimidinic endonuclease (APE1). Bulky DNA lesions such as pyrimidine dimers caused by UV irradiation are processed by the nucleotide excision repair (NER) pathway,[13] which requires the excision repair cross-complementing protein 1 (ERCC1). Base mismatches arising as a result of replication errors can be repaired by the mismatch repair pathway.[14]

In the repair of ICL, ubiquitin-mediated activation of the Fanconi anemia (FA) pathway plays a key role.[15] The FA pathway is constituted by at least 15 FA gene products, whose germline defects result in FA, a cancer predisposition syndrome. Activation of the FA core complex, which is comprised of eight FA proteins (FANCA/B/C/E/F/G/L/M) and associated proteins, leads to monoubiquitination of FANCD2 and FANCDI, which subsequently coordinates three critical DNA repair processes, including nucleolytic incision by XPF-ERCC1 and SLX4 endonucleases, translesion DNA synthesis, and HR.

Aberrations in DNA Damage Responses in Human Cancers

  1. Top of page
  2. Abstract
  3. Mechanism of DNA Damage Response
  4. Aberrations in DNA Damage Responses in Human Cancers
  5. How Can Different DNA Damage Response Pathways be Targeted for Cancer Therapy?
  6. Current Limitations and Future Perspectives
  7. Conclusions
  8. Acknowledgments
  9. Disclosure Statement
  10. References

In sporadic cancers, both activation and inactivation of the DNA damage response are found in various cancers,[16-62] as summarized in Table 1.

Table 1. Examples of aberrations in DNA damage responses in human sporadic cancers
MoleculeActivation or inactivationType of aberrationsType(s) of cancerFrequencyPhenotypesReference(s)
  1. Expression has been confirmed at mRNA and/or protein levels. Studies using cultured cancer cells are excluded.

ATMActivationIncreased autophosphorylationBladder, breast cancers30–68%Cancer barrier function [16, 18]
Increased copy numberProstate cancers~2%  [51]
InactivationMutationPancreatic, lung, colon, endometrial, prostate, skin, kidney, breast, central nervous system, ovarian cancers1–7%  [49, 50]
Hematopoietic and lymphoid malignancies~11%  [49]
Loss of heterozygosity, lossPancreatic cancers~5%  [50]
Decreased copy numberProstate cancers~5%  [51]
Decreased expressionBreast, head and neck cancers25–75%  [54, 55]
MRE11InactivationDecreased expressionBreast cancers7–31%  [19, 54, 56]
Colorectal, gastric, pancreatic cancers with microsatellite instability67–100%  [19]
RAD50ActivationIncreased expressionColorectal cancers~24%  [21]
InactivationDecreased expressionBreast cancers3–28%  [19, 54, 56]
Colorectal, gastric cancers with microsatellite instability28–71%  [19]
NBS1ActivationIncreased expressionEsophageal, head and neck, non-small-cell lung cancers, hepatomas40–52%Poor prognosis [19, 20]
InactivationDecreased expressionBreast cancers10–46%  [19, 54, 56]
Chk1ActivationIncreased phosphorylationCervical cancers~25%  [27]
Increased expressionLung, liver, breast, colorectal, ovarian, cervical cancers46–100%Resistance to chemotherapy, poor prognosis [22-27]
InactivationDecreased expressionLung, ovarian cancers, hetapocellular carcinomas9–32%  [22, 23, 26]
Chk2ActivationIncreased phosphorylationBladder, colon, lung cancers, melanomas30–50%Cancer barrier function [16, 17]
Increased expressionOvarian cancers~37%  [26]
InactivationDecreased expressionBreast, non-small cell lung cancers28–47%  [57, 58]
p53InactivationMutationSolid tumors~50%  [47]
Hematopoietic malignancies~10%  [47]
Decreased expressionSolid and hematopoietic tumors~50%Resistance to chemotherapy, poor prognosis [48]
CDC25AActivationIncreased expressionThyroid, breast, ovarian, liver, colorectal, laryngeal, esophageal cancers, non-Hodgkin's lymphomas17–70%  [28]
CDC25BActivationIncreased expressionThyroid, breast, ovarian, liver, gastric, colorectal, laryngeal, esophageal, endometrial, prostate cancers, gliomas, non-Hodgkin's lymphomas20–79%  [28]
CDC25CActivationIncreased expressionColorectal, endometrial cancers, non-Hodgkin's lymphomas13–27%  [28]
DNA-PKcsActivationIncreased expressionGlioblastoma, prostate cancers~49%Poor survival [29, 30]
RAD51ActivationIncreased expressionBreast, head and neck, non-small-cell lung cell, pancreatic cancers, soft tissue sarcomas24–66%Resistance to platinum agents, poor outcome [31-35]
InactivationDecreased expressionBreast, colorectal cancers~30%  [59, 60]
BRCA1ActivationIncreased expressionLung cancers~22%Resistance to chemotherapy [36]
InactivationMutationBreast, ovarian cancers<10%  [52, 53]
Decreased expressionBreast, ovarian, lung cancers9–30%  [60-62]
BRCA2InactivationMutationBreast, ovarian cancers<10%  [52, 53]
Decreased expressionOvarian cancers13%  [61]
ERCC1ActivationIncreased expressionColorectal, ovarian, gastric, head and neck, non-small-cell lung cancers14–70%Resistance to platinum agents [31, 37-43]
InactivationDecreased expressionColorectal, gastric, non-small-cell lung cancers30–77%  [37, 38, 42, 43]
APE1ActivationIncreased expressionBladder, breast, cervical, head and neck, liver, non-small-cell lung cancers, ovarian cancers, medulloblastomas, gliomas, osteosarcomas, germ cell tumors19–99%Resistance to chemotherapy and/or radiation [44]
PARPActivationIncreased expressionBreast cancers, germ cell tumors5–47%  [45, 46]
FANCAInactivationDecreased expression/loss of expressionAcute myelogenous leukemias4–40%  [64, 65]
MutationAcute myelogenous leukemias~7.6%  [64]
FANCCInactivationMutation, loss of heterozygosityPancreatic cancers~9%  [64]
FANCFInactivationDecreased expression/loss of expressionBreast, cervical, head and neck, non-small-cell lung, ovarian cancers, acute myelogenous leukemias, germ cell tumors6.7~30%  [64, 65]
FANCGInactivationLoss of expressionAcute myelogenous leukemias27%  [65]

Regarding activation of the DNA damage response proteins, increased autophosphorylation of ATM and ATM-dependent phosphorylation of Chk2 are reported in early-stage tumors, suggesting that the DNA damage response may serve as a barrier to the malignant progression of tumors.[16, 17] In contrast, a recent study reports that ATM is hyperactive in late-stage breast tumor tissues, suggesting that the ATM-mediated DNA damage response also plays a role in tumor progression and metastasis.[18] Increased expression of NBS1, RAD50, Chk1, Chk2, CDC25A, CDC25B, and CDC25C are also reported.[19-28] DNA-PK catalytic subunit is reported to be upregulated in radiation-resistant tumors or in tumors with poor survival.[29, 30] Overexpression of RAD51, BRCA1, ERCC1, APE1, and PARP1 is also observed in various cancers and is associated with resistance to chemotherapy.[31-46]

However, inactivation of DNA damage response proteins is also observed in various cancers. The p53 gene is one of the most frequently mutated genes in human sporadic cancers. Although the reported frequencies of p53 mutation vary among the types of cancer, it is estimated that more than half of cancers might have inactivated p53 due to mutations, deletion, loss of heterozygosity of the gene, or decreased expression.[47, 48] Although inactivating mutations in ATM, BRCA1, or BRCA2 are less frequent than those in the p53 gene,[49-53] decreased expression of ATM, the MRN complex, Chk2, RAD51, BRCA1, BRCA2, and ERCC1 is frequently observed, suggesting that aberration of the DNA damage response is common in sporadic cancers.[19, 22, 23, 26, 54-62] Promoter hypermethylation of the BRCA1 gene is frequently observed and may be one of the predominant mechanisms for deregulation of the BRCA1 gene.[62] Furthermore, our group reported the functional inactivation of BRCA2 in cancer cells aberrantly expressing SYCP3, a cancer-testis antigen.[63] Disruption of the FA pathway resulting from mutations or decreases or loss of expression due to promoter hypermethylation has been also described in various cancers.[64, 65]

As described above, both activation and inactivation of the DNA damage response are observed in cancers, and are expected to determine important properties of the DNA damage response machinery present in each cancer. The status of BRCA has been adopted as an important condition factor in current clinical trials, however, the status of other DNA damage response proteins have not yet been translated into clinical trials. In the next section, we will introduce various approaches for taking advantage of these cancer-specific properties of the DNA damage response in cancer therapy.

How Can Different DNA Damage Response Pathways be Targeted for Cancer Therapy?

  1. Top of page
  2. Abstract
  3. Mechanism of DNA Damage Response
  4. Aberrations in DNA Damage Responses in Human Cancers
  5. How Can Different DNA Damage Response Pathways be Targeted for Cancer Therapy?
  6. Current Limitations and Future Perspectives
  7. Conclusions
  8. Acknowledgments
  9. Disclosure Statement
  10. References

Because the efficacy of cancer chemotherapy and radiotherapy relies on generation of DNA damage that will be recognized and repaired by intrinsic DNA repair pathways, aberrant expression of a particular DNA damage response protein should be a biomarker of resistance or favorable response to therapies that induce the corresponding types of DNA damage.[66] For example, patients with surgically treated non-small-cell lung cancer whose tumors lacked expression of ERCC1 were shown to benefit from cisplatin-based adjuvant chemotherapy in a clinical study.[38] Another example is the case of RAD51, whose expression can serve as a marker of cisplatin resistance in non-small-cell lung cancer, which is consistent with the role of HR in the repair of ICL.[31]

In contrast, many inhibitors of the DNA damage response have been developed and some of them have been tested for their potential to enhance DNA damage-induced tumor cell killing in preclinical studies and clinical trials (Tables 2 and 3).

Table 2. Examples of DNA damage response inhibitors in preclinical studies
PathwayTarget(s)Name(s)Preclinical evidence
DNA damageMRE11Mirin, telomelysinSensitization to ionizing radiation
sensors and mediatorsATMKU55933, KU60019, CP466722Sensitization to ionizing radiation and topoisomerase inhibitors
ATRSchisandrin BSensitization to UV treatment
NU6027, VE-821Sensitization to ionizing radiation and a variety of chemotherapy
Cell cycle checkpointsChk1SAR-020106Sensitization to irinotecan and gemcitabine
Chk2VRX0466617Sensitization to ionizing radiation
Non-homologous end joiningDNA-PKNU7026, NU7441Sensitization to ionizing radiation and topoisomerase II inhibitors
DNA-PK and PI3KKU-0060648Sensitization to etoposide and doxorubicin
DNA ligase IVSCR7Sensitization to ionizing radiation and etoposide
Alternative non-homologous end joiningDNA ligases I and IIIαL67Sensitization to ionizing radiation and methyl methanesulfonate
Homologous recombination (HR)RAD51B02, A03, A10Identified by high-throughput screenings of RAD51 inhibitors
Table 3. Examples of DNA damage response inhibitors in clinical trials
PathwayTarget(s)NameCombinationType of cancerClinical trial numberStageTrial periods
  1. For current status and information of clinical trials, refer to http://clinicaltrials.gov/, a service of the US National Institutes of Health. NSCLC, non-small-cell lung cancer.

Cell cycle checkpointsChk1UCN-01Combination therapy
CarboplatinAdvanced solid tumorNCT00036777Phase ICompleted
IrinotecanMetastatic or unresectable solid tumor, triple negative breast cancerNCT00031681Phase ICompleted
CytarabineRefractory or relapsed acute myelogenous leukemia, myelodysplastic syndromeNCT00004263Phase ICompleted
PerifosineRelapsed or refractory acute leukemia, chronic myelogenous leukemia, high risk myelodysplastic syndromeNCT00301938Phase ICompleted
GemcitabineUnresectable or metastatic pancreatic cancerNCT00039403Phase ICompleted
TopotecanRelapsed or progressed small-cell lung cancerNCT00098956Phase IICompleted
CisplatinAdvanced malignant solid tumorNCT00012194Phase ITerminated
FluorouracilMetastatic pancreatic cancerNCT00045747Phase IICompleted
PrednisoneRefractory solid tumor, lymphomaNCT00045500Phase ICompleted
IrinotecanAdvanced solid tumorNCT00047242Phase ICompleted
Fluorouracil, leucovorinMetastatic or unresectable solid tumorNCT00042861Phase ICompleted
TopotecanAdvanced ovarian epithelial, primary peritoneal, fallopian tube cancerNCT00072267Phase IICompleted
FludarabineRecurrent or refractory lymphoma or leukemiaNCT00019838Phase ICompleted
FluorouracilAdvanced or refractory solid tumorNCT00004059Phase ICompleted
CisplatinAdvanced or metastatic solid tumorNCT00006464Phase ICompleted
TopotecanRecurrent ovarian epithelial cancer, fallopian tube cancer, primary peritoneal cavity cancerNCT00045175Phase ICompleted
FludarabineChronic lymphocytic leukemia or lymphocytic lymphomaNCT00045513Phase I, IIActive, not recruiting
Monotherapy
 Relapsed or refractory T-cell lymphomaNCT00082017Phase IICompleted
 Metastatic melanomaNCT00072189Phase IICompleted
 Breast cancer, lymphoma, prostatic neoplasmNCT00001444Phase ICompleted
 Leukemia/lymphoma/unspecified adult solid tumorNCT00003289Phase ICompleted
 Advanced or metastatic kidney cancerNCT00030888Phase IIActive, not recruiting
SCH900776Combination therapy
CytarabineRelapsed acute myeloid leukemiaNCT01870596Phase IIUntil January, 2016
CytarabineAcute myelogenous leukemia/acute lymphocytic leukemiaNCT00907517Phase ITerminated
GemcitabineSolid tumor/lymphomaNCT00779584Phase ICompleted
HydroxyureaAdvanced solid tumorsNCT01521299Phase IWithdrawn
LY2603618Combination therapy
Desipramine, pemetrexed, gemcitabineCancerNCT01358968Phase ICompleted
Pemetrexed, gemcitabine Advanced or metastatic solid tumorNCT01296568Phase ICompleted
Pemetrexed,cisplatinNSCLCNCT01139775Phase I, IIUntil March, 2014
GemcitabinePancreatic cancerNCT00839332Phase I, IICompleted
GemcitabineSolid tumorNCT01341457Phase IUntil December, 2014
PemetrexedCancerNCT00415636Phase ICompleted
PemetrexedNSCLCNCT00988858Phase IIUntil April, 2014
Chk1 and Chk2XL844Combination therapy
GemcitabineAdvanced cancer, lymphomaNCT00475917Phase ITerminated
Monotherapy    
Advanced cancer, lymphomaNCT00475917Phase ITerminated
Chronic lymphocytic leukemiaNCT00234481Phase ITerminated
AZD7762Combination therapy
GemcitabineSolid tumorNCT00413686Phase ICompleted
GemcitabineSolid tumorNCT00937664Phase ITerminated
IrinotecanSolid tumorNCT00473616Phase ITerminated
PF-00477736Combination therapy    
GemcitabineAdvanced solid tumorNCT00437203Phase ITerminated
Non-homologous end joiningDNA-PK and mTORCC-115Monotherapy    
Multiple myeloma, non-Hodgkin's lymphoma, glioblastoma, squamous cell carcinoma of head and neck,NCT01353625Phase IUntil April, 2015
prostate cancer, Ewing's osteosarcoma, chronic lymphocytic leukemia   
Base excision repairAPE1TRC102Combination therapy    
PemetrexedNeoplasmNCT00692159Phase ICompleted
TemozolomideLymphoma, solid tumorNCT01851369Phase IUntil February, 2015
FludarabineRelapsed or refractory hematologic malignancyNCT01658319Phase IUntil January, 2015
LucanthoneCombination therapy    
RadiotherapyBrain metastases from NSCLCNCT02014545Phase IIUntil Decemcer, 2017
Temozolomide and radiationGlioblastoma multiformeNCT01587144Phase IITerminated
PARPRucaparib (AG014688)Combination therapy    
CisplatinTriple negative breast cancer or ER/PR+, HER2− breast cancer with known BRCA1/2 mutationsNCT01074970Phase IIUntil May, 2014
CarboplatinAdvanced solid tumorNCT01009190Phase IUntil Dec, 2013
Monotherapy    
Platinum-sensitive, relapsed, high-grade epithelial ovarian, fallopian tube, or primary peritoneal cancerNCT01891344Phase IIUntil December, 2015
Solid tumor (Phase I), ovarian cancer with germline BRCA mutations (Phase II)NCT01482715Phase I, IIUntil March, 2014
Platinum-sensitive, high-grade serous or endometrioid epithelial ovarian, primary peritoneal or fallopian tube cancerNCT01968213Phase IIIUntil November, 2016
BRCA-mutated locally advanced or metastatic breast cancer or advanced ovarian cancerNCT00664781Phase IIUntil September, 2014
Olaparib (AZD2281)Combination therapy    
CediranibRecurrent ovarian, fallopian tube, peritoneal cancer or recurrent triple-negative breast cancerNCT01116648Phase I, IIUntil May, 2014
Abiraterone, prednisone, or prednisoloneMetastatic castration-resistant prostate cancerNCT01972217Phase IIUntil July, 2018
Bkm120Recurrent triple-negative breast cancer or recurrent high-grade serous ovarian cancerNCT01623349Phase IUntil Dec, 2014
RadiotherapyEsophageal cancerNCT01460888Phase IUntil August, 2018
PaclitaxelRecurrent or metastatic gastric cancerNCT01063517Phase IICompleted
Radiotherapy with or without cisplatinLocally advanced NSCLCNCT01562210Phase IUntil March, 2015
Irinotecan, cisplatin, mitomycin CAdvanced pancreatic cancerNCT01296763Phase I, IIUntil January, 2016
TemozolomideRelapsed glioblastomaNCT01390571Phase IUntil September, 2015
PaclitaxelAdvanced gastric cancerNCT01924533Phase IIIUntil December, 2017
Carboplatin and paclitaxelStage III, stage IV relapsed ovarian cancer or uterine cancerNCT01650376Phase I, IIUntil February, 2015
Radiation therapy and cetuximabAdvanced squamous cell carcinoma of the head/neck with heavy smoking historiesNCT01758731Phase IUntil July, 2016
GefitinibEGFR mutation-positive advanced NSCLCNCT01513174Phase I, IIUntil June, 2015
TemozolomideAdvanced Ewing's sarcomaNCT01858168Phase IUntil July, 2017
CarboplatinMixed muellerian cancer, cervical cancer, ovarian cancer, breast cancer, primary peritoneal cancer, fallopian tube cancer,NCT01237067Phase IUntil September, 2014
endometrial cancer, carcinosarcoma   
Carboplatin and paclitaxelAdvanced ovarian cancerNCT01081951Phase IIUntil June, 2013
Cisplatin-based chemoradiotherapyLocally advanced squamous cell caricinoma of the head and neckNCT01491139Phase IWithdrawn
IrinotecanTriple-negative metastatic breast cancer, advanced ovarian cancerNCT00535353Phase IUntil December, 2013
Carboplatin and/or paclitaxelLocally advanced or metastatic colorectal cancerNCT00516724Phase IUntil December, 2014
DacarbazineAdvanced melanomaNCT00516802Phase ICompleted
PaclitaxelMetastatic triple negative breast cancerNCT00707707Phase IUntil December, 2012
Liposomal doxorubicinAdvanced solid tumorNCT00819221Phase IUntil August, 2013
TopotecanAdvanced solid tumorNCT00516438Phase ICompleted
GemcitabinePancreatic cancerNCT00515866Phase ICompleted
BevacizumabAdvanced solid tumorNCT00710268Phase ICompleted
CisplatinAdvanced solid tumorNCT00782574Phase IUntil December, 2014
CarboplatinBreast and ovarian cancer with BRCA mutations or family historiesNCT01445418Phase IRecruiting
Monotherapy    
Advanced solid tumorNCT01900028Phase IUntil February, 2015
Advanced solid tumorNCT01921140Phase IUntil March, 2015
Advanced solid tumorNCT01929603Phase IUntil May, 2015
Advanced solid tumorNCT01851265Phase IUntil July, 2014
Advanced solid tumor with normal or impaired liver functionNCT01894243Phase IUntil December, 2015
Advanced solid tumor normal or impaired kidney functionNCT01894256Phase IUntil December, 2015
Metastatic breast cancer with germline BRCA1/2 mutationsNCT02000622Phase IIIUntil February, 2021
Advanced castration-resistant prostate cancerNCT01682772Phase IIUntil July, 2015
Advanced solid tumorNCT01813474Phase IUntil November, 2014
BRCA-mutated ovarian cancer after a complete or partial response following platinum-based chemotherapyNCT01874353Phase IIIUntil June, 2020
BRCA-mutated advanced cancerNCT01078662Phase IIUntil December, 2013
BRCA-mutated advanced ovarian cancer following first line platinum based chemotherapyNCT01844986Phase IIIUntil January, 2022
Advanced Ewing's sarcomaNCT01583543Phase IIUntil April, 2015
Stage IV colorectal cancer with microsatellite instabilityNCT00912743Phase IICompleted
BRCA-deficient ovarian, peritoneal, fallopian tube cancerNCT01661868Phase IIWithdrawn
Advanced NSCLCNCT01788332Phase IIUntil May, 2015
BRCA-positive advanced breast cancerNCT00494234Phase IIUntil December, 2013
BRCA-positive advanced ovarian cancerNCT00494442Phase IIUntil December, 2013
Platinum-sensitive relapsed serous ovarian cancerNCT00753545Phase IICompleted
Advanced solid tumorNCT00572364Phase ICompleted
Advanced or metastatic solid tumorNCT00633269Phase ICompleted
Ovarian cancerNCT00516373Phase IUntil December, 2014
Advanced solid tumorNCT00777582Phase IUntil March, 2014
High grade ovarian cancer, triple-negative breast cancer, BRCA-mutated breast cancer or ovarian cancerNCT00679783Phase IIUntil December, 2012
BRCA-positive advanced ovarian cancerNCT00628251Phase IIUntil December, 2013
  Veliparib (ABT-888)Combination therapy    
Gemcitabine, cisplatinLocally advanced or metastatic pancreatic cancer with BRCA or PALB2 mutationsNCT01585805Phase IIUntil July, 2017
Temozolomide or combination with carboplatin and paclitaxelLocally recurrent or metastatic breast cancer with BRCA mutationsNCT01506609Phase IIUntil May, 2015
Radiotherapy and temozolomideNewly diagnosed childhood diffuse pontine gliomaNCT01514201Phase I, IIUntil August, 2019
RadiotherapyAdvanced solid malignancies with peritoneal carcinomatosisNCT01264432Phase IUntil April, 2014
Bendamustine, rituximabAdvanced lymphoma, multiple myeloma, or solid tumorsNCT01326702Phase I, IIUntil November, 2015
TopotecanRelapsed epithelial ovarian, primary fallopian tube, or primary peritoneal cancer with negative or unknown BRCA statusNCT01690598Phase I, IIUntil April, 2015
Gemcitabine and radiotherapyLocally advanced, unresectable pancreatic cancerNCT01908478Phase IUntil July, 2019
Dinaciclib with or without carboplatinAdvanced solid tumors with BRCA mutationsNCT01434316Phase IUntil January, 2016
Radiotherapy, carboplatin, paclitaxelStage III NSCLC that cannot be removed by surgeryNCT01386385Phase I, IIUntil December, 2016
Doxorubicin, carboplatin, bevacizumabRecurrent ovarian cancer, primary peritoneal cancer, or fallopian tube cancerNCT01459380Phase IUntil August, 2015
Cisplatin, gemcitabineAdvanced biliary, pancreatic, urothelial, NSCLCNCT01282333Phase ITerminated
Cisplatin, vinorelbineRecurrent and/or metastatic breast cancer with BRCA mutations, triple-negative breast cancerNCT01104259Phase IUntil September, 2014
Mitomycin CMetastatic, unresectable, or recurrent solid tumorNCT01017640Phase IUntil June, 2014
Capecitabine, radiotherapyLocally advanced rectal cancerNCT01589419Phase IUntil June, 2015
CyclophosphamideLocally advanced or metastatic HER2-negative breast cancerNCT01351909Phase I, IIUntil May, 2015
Docetaxel, cisplatin, fluorouracil, radiotherapy, hydroxyurea, paclitaxelStage IV head and neck cancer Solid tumorNCT01193140Phase IICompleted
Temozolomide NCT01711541Phase I, IIUntil October, 2014
Cisplatin, etoposideExtensive stage small-cell lung cancer, metastatic large cell neuroendocrine NSCLC, small-cell carcinoma of unknown primary or extrapulmonary originNCT01642251Phase I, IIUntil January, 2018
Paclitaxel, carboplatinMetastatic, unresectable solid tumor with liver or kidney dysfunctionNCT01366144Phase IUntil July, 2015
Oxaliplatin, capecitabineBRCA-related malignancy, metastatic colorectal cancer, metastatic ovarian cancer,NCT01233505Phase IUntil July, 2014
metastatic gastrointestinal malignancies in which oxaliplatin has shown some activity   
CarboplatinStage III or stage IV breast cancer with BRCA mutationsNCT01149083Phase IIUntil June, 2014
TemozolomideAcute leukemiaNCT01139970Phase IUntil October, 2013
Carboplatin, paclitaxelSolid tumorNCT01617928Phase ICompleted
TopotecanRecurrent ovarian epithelial cancer, primary peritoneal cavity cancer, unspecified solid tumorNCT01012817Phase I, IIUntil June, 2018
Carboplatin, paclitaxelAdvanced NSCLCNCT01560104Phase IIUntil September, 2014
CarboplatinHER2-negative metastatic or locally advanced breast cancerNCT01251874Phase IUntil September, 2013
Paclitaxel, cisplatinAdvanced, persistent, or recurrent cervical cancerNCT01281852Phase I, IIUntil March, 2020
Topotecan with or without carboplatinRelapsed or refractory acute leukemia, high-risk myelodysplasia, or aggressive myeloproliferative disordersNCT00588991Phase IUntil December, 2012
Abiraterone, prednisoneMetastatic hormone-resistant prostate cancerNCT01576172Phase IIUntil February, 2014
Topotecan and filgrastim or pegfilgrastimPersistent or recurrent cervical cancerNCT01266447Phase IIUntil November, 2016
GemcitabineSolid tumorNCT01154426Phase IUntil October, 2013
Modified FOLFOX6Metastatic pancreatic cancerNCT01489865Phase I, IIUntil December, 2014
FOLFIRIAdvanced gastric cancerNCT01123876Phase IUntil December, 2014
TemozolomideRecurrent or refractory childhood central nervous system tumorNCT00946335Phase IUntil October, 2011
TemozolomideHepatocellular carcinomaNCT01205828Phase IIUntil December, 2013
Carboplatin, paclitaxelAdvanced solid tumorNCT01281150Phase IUntil December, 2013
Carboplatin, paclitaxel, doxorubicin, cyclophosphamideStage IIb-IIIc triple-negative breast cancerNCT01818063Phase IIUntil April, 2018
FloxuridineMetastatic epithelial ovarian, primary peritoneal cavity, or fallopian tube cancerNCT01749397Phase IUntil March, 2016
Liposomal doxorubicinRecurrent ovarian cancer, fallopian tube cancer, or primary peritoneal cancer or metastatic triple-negative breast cancerNCT01145430Phase IUntil March, 2014
Bortezomib, dexamethasoneRelapsed refractory multiple myelomaNCT01495351Phase IUntil October, 2013
TemozolomideRecurrent small-cell lung cancerNCT01638546Phase IIUntil June, 2017
Cyclophosphamide, doxorubicinMetastatic or unresectable solid tumor, non-Hodgkin's lymphomaNCT00740805Phase IUntil December, 2013
Whole brain radiationBrain metastases from NSCLCNCT01657799Phase IIUntil November, 2014
TemozolomideRecurrent high grade serous ovarian, fallopian tube, or primary peritoneal cancerNCT01113957Phase IICompleted
TemozolomideMetastatic or locally advanced breast cancer and BRCA1/2-associated breast cancerNCT01009788Phase IIUntil December, 2014
Carboplatin, paclitaxelAdvanced cancer with liver or kidney problemsNCT01419548Phase IWithdrawn
Whole brain radiationCancer with brain metastasesNCT00649207Phase ICompleted
RadiotherapyInflammatory or loco-regionally recurrent breast cancerNCT01477489Phase IUntil December, 2016
Carboplatin, paclitaxel, bevacizumabNewly diagnosed ovarian epithelial cancer, fallopian tube cancer, or primary peritoneal cancerNCT00989651Phase IUntil July, 2014
Carboplatin, paclitaxelAdvanced solid tumor or BRCA1/2-associated advanced solid tumorNCT00535119Phase IUntil October, 2012
TemozolomideColorectal cancerNCT01051596Phase IIUntil December, 2013
CyclophosphamideRefractory BRCA-positive ovarian, primary peritoneal or ovarian high-grade serous carcinoma, fallopian tube cancer, triple-negative breast cancer, and low-grade non-Hodgkin's lymphomaNCT01306032Phase IIUntil November, 2014
IrinotecanMetastatic or unresectable solid tumor, lymphomaNCT00576654Phase IUntil December, 2013
TemozolomideRecurrent or refractory childhood central nervous system tumorNCT00994071Phase ICompleted
CyclophosphamideRefractory solid tumor or lymphomaNCT01445522Phase ICompleted
TemozolomideRecurrent high-grade gliomaNCT01026493Phase I, IIUntil February, 2014
CyclophosphamideSolid tumor or lymphoma that did not respond to previous therapyNCT00810966Phase IActive, not recruiting
Radiotherapy, temozolomideGrade IV astrocytomaNCT00770471Phase I, IICompleted
TemozolomideMetastatic prostate cancerNCT01085422Phase ICompleted
TemozolomideAdvanced non-hematologic tumorNCT00526617Phase ICompleted
TopotecanRefractory solid tumor or lymphomaNCT00553189Phase ICompleted
TemozolomideMetastatic melanomaNCT00804908Phase IIUntil March, 2014
Carboplatin, gemcitabineAdvanced solid tumorNCT01063816Phase IUntil September, 2014
RadiotherapyBreast cancerNCT01618357Phase IUntil April, 2016
    
Monotherapy    
Solid tumorNCT01199224Phase ICompleted
Locally advanced or metastatic pancreatic cancerNCT01585805Phase IIUntil July, 2017
Metastatic, unresectable, or recurrent solid tumorsNCT01017640Phase IUntil June, 2014
Stage III or Stage IV breast cancer with BRCA mutationsNCT01149083Phase IIUntil June, 2014
BRCA-mutated metastatic or unresectable malignancy, high grade serous ovarian, fallopian tube, or peritoneal cancerNCT01853306Phase IUntil January, 2015
BRCA-mutated epithelial ovarian, fallopian tube, or primary peritoneal cancerNCT01540565Phase IIUntil April, 2014
Advanced solid tumorNCT02009631Phase IUntil December, 2014
BRCA-related malignancy, platinum-refractory ovarian, fallopian tube, or primary peritoneal cancer or basal-like breast cancer, advanced solid tumorNCT00892736Phase IUntil December, 2013
Relapsed epithelial ovarian, primary fallopian or primary peritoneal cancer with BRCA mutationsNCT01472783Phase I, IIUntil December, 2015
Chronic lymphocytic leukemia, follicular lymphoma, unspecified solid tumorNCT00387608Phase ICompleted
Invasive breast cancerNCT01042379Phase IIUntil November, 2014
Advanced solid tumorNCT01827384Phase IIUntil March, 2017
INO-1001Combination therapy    
TemozolomideUnresectable melanomaNCT00272415Phase ITerminated
MK4827Combination therapy    
Liposomal doxorubicinAdvanced solid tumor, platinum-resistant high grade serous ovarian cancerNCT01227941Phase ITerminated
TemozolomideAdvanced solid tumor, glioblastoma multiforme, melanomaNCT01294735Phase ICompleted
Carboplatin, paclitaxel, liposomal doxorubicinAdvanced solid tumorNCT01110603Phase ITerminated
Monotherapy    
Advanced solid tumorNCT01226901Phase ITerminated
Mantle cell lymphomaNCT01244009Phase IIWithdrawn
Advanced solid tumors, chronic lymphocytic leukemia, T-cell-prolymphocytic leukemiaNCT00749502Phase ICompleted
Advanced HER2-negative, germline BRCA mutation-positive breast cancerNCT01905592Phase IIIUntil October, 2015
CEP-9722Combination therapy    
Gemcitabine, cisplatinAdvanced solid tumor or mantle cell lymphomaNCT01345357Phase ICompleted
TemozolomideAdvanced solid tumorNCT00920595Phase ICompleted
Monotherapy    
Advanced solid tumorNCT01311713Phase I, IITerminated
Advanced solid tumorNCT00920595Phase ICompleted
E7016Combination therapy    
TemozolomideAdvanced solid tumorNCT01127178Phase ICompleted
TemozolomideWild-type BRAF stage IV melanoma, unresectable stage III melanomaNCT01605162Phase IIUntil March, 2014
BMN673Monotherapy    
Acute myeloid leukemia, myelodysplastic syndrome, chronic lymphocytic leukemia, mantle cell lymphomaNCT01399840Phase IUntil June, 2013
Advanced or recurrent solid tumorNCT01286987Phase IUntil June, 2013
Advanced solid tumor with deleterious BRCA mutationsNCT01989546Phase I, IIUntil August, 2016
Advanced breast cancer with BRCA mutationsNCT01945775Phase IIIUntil June, 2016

Inhibitors of ATM/ATR and the MRN complex

As ATM and the MRN complex play central roles as sensors or mediators in the DNA damage response, these molecules have been considered to be promising targets for radiosensitization or chemosensitization.[67] Several promising ATM inhibitors have been developed (Table 2). KU55933, the first specific inhibitor of ATM, inhibits radiation-induced ATM-dependent phosphorylation events and sensitizes cancer cells to radiation and topoisomerase inhibitors.[67] KU60019, an improved analog of KU55933, inhibits the DNA damage response and effectively radiosensitizes human glioma cells.[68] Mirin is an inhibitor of the MRN complex, which prevents MRN-dependent activation of ATM without affecting ATM protein kinase activity and inhibits MRE11-associated exonuclease activity.[67] Telomelysin is another inhibitor that inhibits the MRN complex through the adenoviral E1B-55 kDa protein.[67] The therapeutic outcomes of these agents remain to be tested in clinical trials. Although the long search for selective inhibitors of ATR has not yet paid off, schisandrin B was recently identified as a moderate selective ATR inhibitor, although it will also affect ATM at high concentrations.[69] Recently, two novel ATR inhibitors, NU6027 and VE-821, were also shown to sensitize cells to a variety of DNA-damaging agents in preclinical studies.[70, 71]

Inhibitors of Chk1/Chk2 and CDC25

As the triggering of cell cycle checkpoints is crucial in the DNA damage response, these checkpoints have also been widely investigated as a potential target for cancer therapy (Table 3).[72] Among the inhibitors for Chk1 and/or Chk2, UCN-01 was the first to enter clinical trials, but it was discontinued due to toxicities such as symptomatic hypotension and neutropenia and a lack of convincing efficacy after phase II trials.[72] Other Chk1/Chk2 inhibitors with improved specificities, including XL844 and AZD7762, also entered clinical trials but failed to achieve a good response.[72] The selective Chk1 inhibitor SCH900776 has been used in phase I trials for acute leukemia in combination with cytarabine and for solid tumors in combination with gemcitabine, and showed some partial responses and stable disease.[72] The Chk1 inhibitor LY2603618 and the dual Chk1/Chk2 inhibitor LY2606368 are also currently being tested in early clinical trials. CDC25 phosphatases, the key factors in cyclin-dependent kinase activation crucial for cell cycle regulation, are also considered to represent promising novel targets in cancer therapy. CDC25 inhibitors have also been developed, and some have entered into clinical trials, although the clinical data is limited.[73]

Inhibition of NHEJ by DNA-PK inhibitors

Regarding NHEJ, inhibitors of DNA-PK, including NU7026 and NU7441, were found to induce extreme sensitivity to ionizing radiation as well as DNA-damaging agents in preclinical studies (Table 2).[74] However, the therapeutic efficacy of DNA-PK inhibitors depends on the expression levels of DNA-PK in cancer cells versus normal cells, and their clinical application is currently restricted because of their toxicity to normal cells. The dual mTOR and DNA-PKcs inhibitor CC-115 is undergoing early clinical evaluation (Table 3). KU-0060648 is a potent dual inhibitor of DNA-PK and PI-3K, which has recently been reported to enhance etoposide and doxorubicin cytotoxicity (Table 2).[75]

Inhibition of NHEJ or alt-NHEJ by DNA ligase inhibitors

DNA ligases are required for both NHEJ and alt-NHEJ pathways as well as other DNA repair pathways such as BER and NER. Small molecule inhibitors of human DNA ligases have been identified and shown to be cytotoxic and also to enhance the cytotoxicity of DNA-damaging agents. SCR7 is an inhibitor of DNA ligase IV, which is involved in the NHEJ pathway. SCR7 reduces cell proliferation in a DNA ligase IV-dependent manner and increases the tumor-inhibitory effects of agents that cause DSBs.[76] L67 is an inhibitor of DNA ligases I and IIIα, which are involved in the alt-NHEJ pathway as well as BER and NER. The levels of the alt-NHEJ proteins such as DNA ligase IIIα and WRN are reported to be elevated in BCR-ABL-positive CML cell lines,[77] so inhibition of alt-NHEJ factors may be an additional therapeutic approach in BCR-ABL-positive CML, which is usually treated by tyrosine kinase inhibitors. Indeed, CML cell lines with increased alt-NHEJ were shown to be hypersensitive to the combination of L67 and PARP inhibitor.[78]

Inhibitors of RAD51 and tyrosine kinases regulating HR

With respect to HR, there are currently few inhibitors that directly target HR proteins. Along with the RAD51 inhibitors that were recently identified (Table 2),[79] the molecules that indirectly regulate HR may also be candidate targets for inhibiting HR. For example, the non-receptor tyrosine kinase c-Abl is activated by ATM in response to DNA damage, and subsequently phosphorylates RAD51.[80] Oncogenic fusion tyrosine kinases, such as BCR-ABL, TEL-ABL, TEL-JAK2, TEL-PDGFβR, and NPM-ALK, enhance the expression levels and/or tyrosine phosphorylation of RAD51.[81, 82] From these findings, inhibitors of oncogenic tyrosine kinases are expected to sensitize cancer cells to DNA-damaging agents. Consistent with this hypothesis, treatments with the tyrosine inhibitor imatinib have been shown to enhance sensitivity to DNA crosslinking agents and ionizing radiation in cancer cells.[81] Furthermore, targeting RAD51 was shown to overcome imatinib resistance in CML cells.[83]

Inhibitors of histone deacetylases, heat shock protein 90, and DSB repair

Histone deacetylases (HDACs) are powerful regulators of the stability of the genome, and many HDAC inhibitors are shown to downregulate multiple components of the DNA damage response and repair, including HR, NHEJ, the MRN complex, and ATM.[84] Thus, the use of HDAC inhibitors in combination with DNA-damaging agents may be an area of great interest with potential clinical utility. The HDAC inhibitor PCI-24781 caused increased apoptosis by inhibiting RAD51-mediated HR when used in combination with the PARP inhibitor PJ34 in a preclinical study.[85] The inhibitor of heat shock protein 90, 17-allylamino-17-demethoxygeldanamycin, radiosensitizes human tumor cell lines by inhibiting RAD51-mediated HR.[86] Curcumin is a natural product that has been tested for its chemosensitizing potential, and sensitizes cancer cells to PARP inhibitors by inhibiting NHEJ, HR, and the DNA damage checkpoint.[87]

Inhibitors of PARP and APE1 in combination with DNA-damaging agents

Inhibitors of PARP, which inhibit the BER and SSB repair pathways, are the most advanced and promising drugs that target DNA repair.[88] A number of clinical trials using PARP inhibitors are currently underway (Table 3). Inhibitors of PARP were first tried in combination with DNA-damaging agents. Some clinical responses were observed in the phase I and II trials of the PARP inhibitor rucaparib in combination with temozolomide.[89, 90] Further clinical trials of PARP inhibitors have been carried out in combination with various DNA-damaging agents and/or ionizing radiation (Table 3). Inhibitors of another BER protein APE1 are also being tested in combination with DNA-damaging agents in clinical trials (Table 3).

Using PARP inhibitors as single agents in BRCA-deficient cancers based on the principle of synthetic lethality

In 2005, PARP inhibitors were shown to selectively inhibit the growth of cells with defects in either the BRCA1 or BRCA2 genes, suggesting a new use of PARP inhibitors as single agents.[91, 92] A possible explanation for this lethality is as follows. The cancer cells with defects in the BRCA gene are defective in HR, as the wild-type BRCA allele is absolutely lost. However, HR is intact in normal cells of the same patients who carry one wild-type BRCA allele and one mutant BRCA allele. Inhibition of PARP1 results in the accumulation of SSBs, which are converted to lethal DSBs that require HR for their repair. Although such lesions would be repaired by HR in normal cells, they are not repaired in BRCA1- or BRCA2-deficient cancer cells because these cells are defective in HR repair, and thus the tumor cells are led to death. This concept is termed synthetic lethality, namely, the process by which defects in two different genes or pathways together result in cell death while defects in one of the two different genes or pathways do not affect viability (Fig. 3).[3] This attractive new therapeutic strategy based on the principle of synthetic lethality relies on the frequent defects in the DNA damage response observed in cancer as summarized in the previous chapter and Table 1, in which alternative DNA damage response pathways may be activated to allow cancer cells to survive in the presence of genotoxic stress. Because this strategy targets the cancer-specific aberrations in the DNA damage response, it will cause few or no toxicities on normal cells. The first report of a clinical trial of a PARP inhibitor as a single agent in patients with BRCA mutations was the phase I study of the oral PARP inhibitor olaparib.[93] It established the safety of olaparib as a single agent, and good responses were observed in patients with BRCA-mutated breast, ovarian, or prostate tumors. In subsequent phase II studies, approximately one-third of the patients with breast or ovarian cancer with germline BRCA mutations showed a favorable response to the drug with no severe toxicities.[94] Several other PARP inhibitors are currently being investigated in patients with germline BRCA mutations as single agents (Table 3). It is likely that PARP inhibitors have significant benefit to at least a subpopulation of cancer patients with defects in BRCA-mediated HR pathways.

image

Figure 3. Principle of synthetic lethality. DNA damage is often processed by multiple DNA repair pathways. In the example shown here, pathways A and B are both intact in normal cells, whereas pathway A is defective in cancer cells. (a) In the absence of the pathway B inhibitor, cancer cells can survive, because the defect in pathway A is compensated by the alternative pathway B. (b) When the cells are treated with the pathway B inhibitor, both pathways will be blocked in cancer cells, which will result in cell death. However, normal cells will not be affected, because inhibition of pathway B will be compensated by pathway A.

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Using PARP inhibitors as single agents in cancers with no BRCA mutations

The potential for PARP inhibitors as single agents has also been tested in clinical trials of cancers with no germline BRCA mutations, such as high-grade serous ovarian cancers and triple-negative breast cancers.[95] Inhibitors of PARP were also effective in a subset of cancers with no germline BRCA mutations, suggesting that there may be a subset of sporadic cancers that show features of “BRCAness,” which may show good response to PARP inhibitors.[96] Indeed, cancer cells expressing the cancer-testis antigen SYCP3, in which BRCA2 is functionally inactivated, as described above, show extreme hypersensitivity to a PARP inhibitor.[63] Defects in other HR-related proteins such as RAD51, RAD54, and RPA also confer selective sensitivity to PARP inhibition.[97] Moreover, defects in the DNA damage response proteins, such as NBS1, MRE11, ATR, ATM, FANCD2, FANCA, FANCC, Chk1, Chk2, and ERCC1, also confer selective sensitivity to PARP inhibition.[97, 98]

Exploitation of other synthetic lethalities by DNA damage response

Taking advantage of the dysregulated DNA damage response in cancer using the synthetic lethality approach may be one of the most promising prospects for the future of cancer treatment. From this point of view, many efforts have been made to identify defects of two different DNA damage response genes or pathways that are synthetically lethal when combined. For example, ATM inhibition is shown to be synthetically lethal with FA pathway deficiency.[99] The suggested explanation for this lethality is as follows. The FA pathway-deficient cancer cells are defective in the repair of DNA replication fork stalling, which is normally repaired by ATR and the FA pathway. In FA pathway-deficient conditions, the stalled fork will collapse and form a DSB that will alternatively activate an ATM-dependent DNA damage response. Inhibition of ATM in such FA pathway-deficient cells will leave no alternative mechanism for repair, leading to cell death. The FA pathway-deficient cells are also hypersensitive to Chk1 silencing,[100] which may be explained by the hyperdependence of the FA pathway-deficient cells on G2/M checkpoint activation mediated by Chk1 for viability. Because defects in the FA pathway are frequently observed in a number of different types of cancer (Table 1),[64, 65] the use of ATM inhibitors or Chk1 inhibitors in FA pathway-deficient tumors will be a promising approach that should be evaluated in clinical trials in the future. In another example, RAD54B deficiency is shown to be synthetically lethal in cells with reduced Flap endonuclease 1 expression, but the mechanisms of this lethality remain to be elucidated.[101] Recently, inhibition of APE1 was shown to be synthetically lethal in BRCA- and ATM-deficient cells, presenting a novel model for APE inhibition as a synthetic lethal strategy in cells deficient in DSB repair.[102] Briefly, APE1 inhibition leads to AP site accumulation and results in indirect generation of SSBs that are eventually converted to toxic DSBs, which cannot be repaired in cells deficient in DSB repair. The APE1 inhibitors are being tested in combination with DNA-damaging agents in current clinical trials, and they may be evaluated further as a synthetic lethal strategy. More recently, inactivation of the HR protein RAD52 was shown to be synthetically lethal with deficiencies in BRCA2, BRCA1, and PALB2.[103, 104] This lethal effect may be due to the loss of RAD51-dependent HR function mediated by the BRCA1–PALB2–BRCA2 complex, because human RAD52 is suggested to function in an independent and alternative repair pathway of RAD51-dependent HR when deficiencies exist in BRCA1, PALB2, or BRCA2. As no inactivating mutations of RAD52 have been documented in human sporadic cancers, inhibition of RAD52 could be an attractive strategy for improving cancer therapy in the BRCA- or PALB2-defective subgroup of cancers. Although no inhibitors of RAD52 have been developed yet, it would be of great interest to assess the effects of inhibition of RAD52 on cancer-specific killing of the cancers with “BRCAness” profiles and compare them with those of PARP inhibitors in future clinical trials. There might be additional synthetic lethalities to be discovered and exploited in future.

Current Limitations and Future Perspectives

  1. Top of page
  2. Abstract
  3. Mechanism of DNA Damage Response
  4. Aberrations in DNA Damage Responses in Human Cancers
  5. How Can Different DNA Damage Response Pathways be Targeted for Cancer Therapy?
  6. Current Limitations and Future Perspectives
  7. Conclusions
  8. Acknowledgments
  9. Disclosure Statement
  10. References

Although the data from clinical trials of the inhibitors of DNA damage response, including PARP inhibitors, seem encouraging, we should note that the use of PARP inhibitors also faces significant limitations.

The first limitation is the evolution of resistance. In the case of using PARP inhibitors in cancer cells carrying mutations in BRCA1 or BRCA2, the drug resistance can be caused by secondary mutations in the BRCA1 or BRCA2 gene that restore the open reading frame of the gene and enable the generation of functional BRCA proteins possessing the ability to repair DNA damage caused by PARP inhibitors.[105-107] Other suggested mechanisms underlying the resistance to PARP inhibitors include the loss of 53BP1 expression in BRCA-deficient cells and the upregulation of genes that encode P-glycoprotein efflux pumps,[108-111] although the importance of these factors in clinical resistance to PARP inhibitors has not been elucidated. In future clinical trials, it would be desirable to periodically monitor the sequences of BRCA1 and BRCA2 and the expression levels of the key proteins such as 53BP1 or P-glycoprotein efflux pumps.

The second limitation is the lack of reliable biomarkers of response or resistance to the inhibitors. There is a pressing need to identify biomarkers to predict the response to the inhibitors. Regarding the sensitivities to PARP inhibitors, elevated levels of PARP and CDK12 deficiency are suggested to be possible biomarkers for favorable responses.[45, 112] We should also keep in mind that many factors might affect the DNA damage response and take into account the complexity of the networks regulating DNA repair. For instance, most cancer cells grow under hypoxia, a condition that activates hypoxia inducible factor-1 (HIF-1). Because HIF-1 contributes to therapy resistance, it is considered an attractive target molecule for cancer therapy. Diverse functional interactions between HIF-1 and the DNA damage response have also been described,[113] so the efficacy of the combination of HIF-1 inhibitors and inhibitors of the DNA damage response proteins should be examined in the future.

Conclusions

  1. Top of page
  2. Abstract
  3. Mechanism of DNA Damage Response
  4. Aberrations in DNA Damage Responses in Human Cancers
  5. How Can Different DNA Damage Response Pathways be Targeted for Cancer Therapy?
  6. Current Limitations and Future Perspectives
  7. Conclusions
  8. Acknowledgments
  9. Disclosure Statement
  10. References

Defects or upregulation of the proteins involved in DNA damage response and repair are common in cancers, and may be induced by both genetic and epigenetic causes. Inhibition of the DNA damage response proteins can be used to enhance chemotherapy and radiotherapy, and also to selectively kill cancer cells showing deficiencies in particular DNA repair pathway(s) based on the principle of synthetic lethality. Inhibition of PARP in BRCA-defective cancers seemed effective in early clinical trials. Better understanding of the basic biology underlying the DNA damage response and the mechanisms responsible for its dysregulation in cancer will provide exciting opportunities for new and efficient cancer therapy targeting the DNA damage response.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Mechanism of DNA Damage Response
  4. Aberrations in DNA Damage Responses in Human Cancers
  5. How Can Different DNA Damage Response Pathways be Targeted for Cancer Therapy?
  6. Current Limitations and Future Perspectives
  7. Conclusions
  8. Acknowledgments
  9. Disclosure Statement
  10. References

This work was supported by the Japan Society for the Promotion of Science (Kakenhi) (grant nos. 23591836 and 25125705, to N. Hosoya) and by grants from the Takeda Science Foundation and from the Naito Foundation (to N. Hosoya).

References

  1. Top of page
  2. Abstract
  3. Mechanism of DNA Damage Response
  4. Aberrations in DNA Damage Responses in Human Cancers
  5. How Can Different DNA Damage Response Pathways be Targeted for Cancer Therapy?
  6. Current Limitations and Future Perspectives
  7. Conclusions
  8. Acknowledgments
  9. Disclosure Statement
  10. References