Pre-clinical and clinical evaluation of PARP inhibitors as tumour-specific radiosensitisers

Cancer Treatment Reviews 36 (2010) 566–575

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Cancer Treatment Reviews journal homepage: www.elsevierhealth.com/journals/ctrv

New Drugs

Pre-clinical and clinical evaluation of PARP inhibitors as tumour-specific radiosensitisers C. Powell a, C. Mikropoulos a, S.B. Kaye a,b, C.M. Nutting a,b, S.A. Bhide a,b, K. Newbold a, K.J. Harrington a,b,* a b

The Royal Marsden Hospital, 203 Fulham Road, London SW3 6JJ, UK The Institute of Cancer Research, 237 Fulham Road, London SW3 6JB, UK

a r t i c l e

i n f o

Article history: Received 2 February 2010 Received in revised form 8 March 2010 Accepted 11 March 2010

Keywords: PARP inhibitor Radiation Radiosensitiser Tumour

s u m m a r y Approximately two million fractions of radiotherapy are administered in the UK every year, as part of adjuvant, radical or palliative cancer treatment. For many tumour types, radiotherapy is routinely combined with concomitant chemotherapy as part of adjuvant or radical treatment. In addition, new agents have been developed in recent years and tested in phase 1, 2 and 3 trials concomitantly with radiotherapy or chemoradiotherapy. One such class of drugs, the poly(ADP-ribose) polymerase (PARP) inhibitors, has shown activity in conjunction with radiotherapy in several cancer cell lines. Pre-clinical data suggest that PARP inhibitors may potentiate the effects of radiotherapy in several tumour types, namely lung, colorectal, head and neck, glioma, cervix and prostate cancers. In vitro, PARP inhibitors are radiosensitisers in various cell lines with enhancement ratios of up to 1.7. In vivo, non-toxic doses of PARP inhibitors have been shown to increase radiation-induced growth delay of xenograft tumours in mice. Clinical trials to assess the toxicity and potential benefit of combining radiotherapy with PARP inhibition are now needed. Ó 2010 Elsevier Ltd. All rights reserved.

Introduction In order to improve the efficacy of radiation treatment, several cytotoxic agents, such as 5-fluorouracil, cisplatin, carboplatin and mitomycin C have been used. For tumour types such as head and neck and cervical cancer, there are now robust data from metaanalyses to confirm the benefit of combining platinum-based chemotherapy with radiation.1,2 However, these combination regimens are also more toxic, both in terms of acute radiation reactions and long-term treatment-related morbidity and these effects limit the scope for combining additional agents (e.g. taxanes) with standard chemoradiotherapy protocols. There has been a significant upsurge in interest in new targeted treatments as radiosensitisers – not least because they offer the prospect of augmenting or even replacing the activity of radiosensitising chemotherapy agents without causing overlapping doselimiting toxicities. Proof-of-concept data for this approach have been obtained for a chimeric monoclonal antibody (cetuximab, Erbitux) that binds the epidermal growth factor receptor. In a randomized phase 3 trial of radiotherapy alone versus radiotherapy plus cetuximab in patients with locally advanced head and neck cancer, the targeted drug was shown to yield highly significant

* Corresponding author at: The Institute of Cancer Research, Targeted Therapy Team, Section of Cell and Molecular Biology, 237 Fulham Road, London SW3 6JB, UK. Tel.: +44 20 7153 5157; fax: +44 20 7808 2235. E-mail address: [email protected] (K.J. Harrington). 0305-7372/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ctrv.2010.03.003

improvements in disease-free and overall survival.3,4 A small molecule inhibitor of both EGFR and c-erbB2 (Lapatinib, Tykerb) has been assessed in a phase 1 trial with chemoradiotherapy in head and neck cancer and is currently undergoing randomized phase 2 and 3 evaluation in this indication.5 Selection of appropriate targeted agents to combine with chemoradiotherapy represents a challenge. Novel targeted agents for evaluation should be selected on the basis of their effects on known predictors of tumour cell radiosensitivity and, as there are a large number in pre-clinical and early-phase clinical trials, attempting to test all of them in combination with chemoradiotherapy would be highly impractical. One such group of agents includes those that are able to impair the repair of radiation-induced DNA damage. In this review, we present data that demonstrate the activity of pharmacological inhibitors of poly(ADP-ribose) polymerase (PARP) as potent radiosensitisers and suggest that these drugs should be the subject of careful clinical evaluation in combination regimens with radiotherapy or chemoradiotherapy. Factors affecting cancer cell radiosensitivity When tumours are treated with radiotherapy, the probability of tumour control is governed by a number of factors: the ability to repair DNA damage; the number of clonogenic cells and their rate of repopulation; their redistribution in the cell cycle over time; their intrinsic radiosensitivity; and the presence of tissue hypoxia. Such factors are often described as the 5 Rs of radiobiology and

C. Powell et al. / Cancer Treatment Reviews 36 (2010) 566–575

there is a growing appreciation of their connectivity with the underlying molecular biological processes that drive cancer causation, progression and treatment outcome.6 In clinical practice, reliably predicting radiosensitivity has proved challenging as accurate molecular analysis is often not feasible due to tumour heterogeneity and difficulties with acquiring fresh, pre-treatment tumour samples. Previously, the surviving fraction at 2 Gy (SF2) provided a global estimation of radiosensitivity, but this in vitro assay failed to model potentially important three-dimensional effects that operate in the in vivo situation and has not become established in clinical practice.7 TP53 gene has been evaluated in many studies regarding its potential as a predictive factor of tumour sensitivity but with varying results.8 In many clinically relevant situations, the basis of radiosensitisation is an alteration in some aspect of DNA repair. Intrinsic radiosensitivity is related to the ability of the cell to detect DNA damage and facilitate its repair. At least five distinct pathways have been identified by which the cell can detect and repair this damage and, thus, protect the integrity of the genome: direct repair; mismatch repair (MMR); base excision repair (BER); nucleotide excision repair (NER); and double-strand break repair, which includes both non-homologous end joining (NHEJ) and homologous recombinational repair (HR). The cancer cell’s response to irradiation is consequently based on the integrity of the different pathways implicated in DNA repair. Well-known tumour suppressor genes (e.g. ataxia telangiectasia, TP53, BRCA-1) are involved in these pathways, and mutations in these genes have been associated with a predisposition to cancer. In addition, genetic polymorphisms of some of these genes have been correlated to late toxicity of normal tissues.9 Targeting DNA repair in order to enhance radiosensitivity, however, has been viewed with extreme caution due to the clinical manifestations of naturally occurring syndromes in which DNA repair mechanisms are defective (e.g. ataxia telangiectasia, Fanconi anemia). More recently, the realization that cancers may be more susceptible than normal cells to pharmacological modulation of DNA repair pathways has opened up the prospect of exploiting this so-called ‘‘synthetic lethality”.10 Such studies have highlighted PARP inhibition as an extremely attractive target for future development.

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(PARG) hydrolyses the polymers present on PARP which is subsequently recycled.15 This ‘‘PARP cycling” allows access of DNA repair enzymes to the DNA strand breaks in order to facilitate their repair as discussed below (Fig. 2).16 PARP-1 and DNA repair The type of DNA break is significant for PARP stimulation and its activation is linearly related to the number of DNA strand breaks.14,17 The most well-characterised role for PARP-1 is in sensing single-strand DNA breaks (SSB) to which it binds via the two zinc fingers located at its NH2-terminus.17,18 This binding facilitates dimerization of PARP-1 which in turn leads to polymerization of linear or branched chains of poly(ADP-ribose) onto its targets, enabling it to perform a pivotal role in DNA damage repair. The nuclear recipients of poly(ADP-ribosylation) include substrates involved in various aspects of nucleic acid metabolism including; modulation of chromatin structure (histones), DNA synthesis (DNA polymerases), DNA repair (DNA ligases), transcription (RNA polymerases), cell cycle progression (p53) and PARP-1 itself (automodification).13 PARP-1 activity stimulated by DNA damage depletes cells of NAD+ and subsequently ATP levels fall. The consequent slowing of cellular metabolism and proliferation, together with PARP’s suggested interaction with p53 and inhibition of cyclin-dependent kinase (cdk) activity is thought to culminate in preventing cells with unrepaired DNA damage from entering mitosis.19–21 By the mechanisms described above, mild genotoxic stimuli lead to activation of PARP-1, DNA repair, and cell survival. In the presence of severe DNA damage which cannot be repaired, however, PARP-1 plays an important role in Fas-mediated apoptosis (Fig. 3). Experiments in both human and murine cell lines have suggested that there is a transient surge in poly(ADP-ribosyl)ation of nuclear proteins followed by caspase-3-mediated cleavage of PARP in cells undergoing apoptosis.22 This degradation of PARP prevents depletion of cellular levels of NAD and ATP which are required for the execution of apoptosis, and therefore also prevents necrotic cell death. Normal single-strand break repair

PARP-1 PARP-1 is a 113 kDa nuclear metalloenzyme and was the first identified member of the PARP superfamily which currently contains 18 proteins.11 The signature b–a–loop-b–a NAD+ fold is variably conserved among the PARP members which play important roles in DNA repair, transcription, mitotic segregation, telomere homeostasis and cell death.12 The molecular structure shows PARP-1 to be a multifunctional enzyme characterised by three domains; a DNA binding domain (DBD), an automodification domain and a catalytic domain (Fig. 1).13 PARP-1 binds to DNA via zinc fingers contained within the DBD to catalyze the polymerization of APD-ribose from NAD+ on target proteins, a process known as poly(ADP-ribosyl)ation. Once PARP-1 is hyper(ADP-ribosyl)ated it is highly negatively charged causing it to be repulsed by DNA and hence inactivated. The polymer is rapidly degraded with a half-life less than 10 min14 and poly(ADP-ribose) glycohydrolase

Fig. 1. Schematic diagram of the molecular structure of PARP (DBD DNA binding domain, AD automodification domain, CD catalytic domain) represents zinc finger.

Single-strand breaks (SSB) are interruptions in the backbone of one strand of DNA double helix and occur endogenously as a result of interactions with reactive oxygen species (ROS) and due to the intrinsic instability of DNA.23 Thousands of SSB occur each hour and if left unrepaired can develop into potentially lethal doublestrand breaks (DSB) due to collapse of DNA replication forks during S phase of the cell cycle. Exogenous causes of SSB include UV light, cytotoxic drugs and ionizing radiation, which also causes DSB directly. SSB can result from direct damage to the DNA backbone or indirectly from repair of damaged bases by base excision repair (BER). Most SSB are repaired by a rapid and efficient global SSB repair process which involves four steps; SSB detection, DNA end processing, DNA gap filling and DNA ligation.24 The first step in SSB repair (detection) involves binding of PARP1 to damaged DNA and the resultant poly(ADP-ribosyl)ation that is described above. In addition, PARP-1 is thought to recruit repair proteins such as the molecular scaffold protein, X-ray repair cross complementing group 1 (XRCC1) which is involved in BER.25 The next step is to remove the damaged termini of the DNA backbone in order to facilitate restoration of 30 -hydroxy and 50 -phosphate end groups which can act as primers for DNA polymerases. This process is predominantly performed by DNA polymerase-b and polynucleotide kinase which is stimulated by XRCC1.26 Other enzymes may be involved in this process depending on the type of damaged termini that arise.27 Usually a single nucleotide is lost

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Fig. 2. Simplified diagram of PARP (poly(ADP-ribose) polymerase) cycling by PARG (poly(ADP-ribose) glycohydrolase).

at the site of SSB and this is replaced by gap filling by DNA polymerase-b28 and the process is know as short patch repair (SPR). The final step in repairing SSB is performed by DNA ligase III which seals the gap in the DNA strand and this too is bound and stabilised by XRCC1 thus providing a physical link with DNA polymerase-b.29 A smaller proportion of SSB repair involves insertion of multiple nucleotides (long patch repair). Although DNA polymerase-b and hence PARP-1 have been implicated in this process, other enzymes such as DNA polymerase-d and DNA polymerase-e have also been shown to be involved in inhibition studies.30 Long patch repair is also dependent on proliferating cell nuclear antigen (PCNA)31 and it is thought this process may occur during S phase of the cell cycle as many of the required enzymes are core components of the replication machinery.27 PARP inhibition as a therapeutic strategy The recent upsurge in interest in PARP inhibitors has stemmed from the discovery that breast cancer associated (BRCA) protein 1 and 2 deficient cells are extremely sensitive to PARP inhibition.10,32 BRCA-1 and -2 are breast cancer susceptibility proteins which are involved in the repair of double-strand DNA breaks (DSB) by

homologous recombination (HR). Cells which lack BRCA proteins are forced to repair defects by more error prone pathways which in turn lead to genomic instability.32 PARP inhibition impairs the repair of SSB which, as a result, are converted to DSB during replication and this, in turn, increases the burden for repair by HR. In BRCA-deficient cells, which are defective in HR, the damage cannot be repaired and consequently cell cycle arrest, chromosome instability and cell death results.10 The most well-characterised of the familial breast and ovarian cancers are caused by an inherited defect in one of the BRCA-1 or -2 alleles.33 Tumours develop when cells acquire a mutation that inactivates the remaining normal allele, rendering the cells deficient in HR of DNA damage. The cells in normal tissues retain one normal copy of the gene and express adequate levels of functional protein. This difference between the tumour and normal tissue cells is open to exploitation with PARP inhibitors to which, in principle, only the BRCA-deficient tumour cells are sensitive.34 Proof-of-principle for this therapeutic action of PARP inhibitors has recently been demonstrated with phase 1 trial data showing PARP inhibition with olaparib (a novel orally active PARP inhibitor) which was well-tolerated and showed activity in BRCA-1 and -2 carriers with cancer.35 This has been confirmed in the phase 2 set-

C. Powell et al. / Cancer Treatment Reviews 36 (2010) 566–575

Fig. 3. Simplified diagram of the role of PARP (poly(ADP-ribose) polymerase) in repair of DNA single-strand breaks (SSB). Cdk cyclin-dependent kinase, cross complementing group 1), DNA polymerase-b, DNA ligase III.

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XRCC1 (X-ray repair

ting of heavily pre-treated patients with BRCA-1 and -2-associated advanced ovarian cancer in which objective response rates of 33% and clinical benefit rates (including P50% decline in CA125) of 57.6% were seen with doses of 400 mg bd.36 Although PARP/ mice are viable and fertile37 they demonstrate increased sensitivity to radiation and alkylating agents and PARP-1 and -2 double knockout mice suffer embryonic lethality.38 Additionally, the accumulation of foci of cH2AX (phosphorylated form of histone H2A histone family member X) which is a known marker of DNA double-strand breaks was seen in plucked eyebrow-hair follicles in the phase 1 trial of olaparib.35 This raises concerns that long-term PARP inhibition may lead to an accumulation of DNA damage in normal tissues and the outcome of larger clinical trials are awaited to fully evaluate their safety alone, and in combination with anti-cancer agents. PARP inhibitors may also be of benefit in tumour cells deficient in other components of homologous recombination, other than BRCA1 and -2 proteins. It has been shown that deficiencies of RAD51, RAD54, DSS1 (deleted in split hand/split foot 1), RPA1 (replication protein A1), NBS1 (Nijmegen breakage syndrome 1), ATR (ataxiatelangiectasia-related), ATM (ataxia telangiectasia mutated), CHK1 (checkpoint kinase 1), CHK2, FANCD2 (Fanconi anemia protein D2), FANCA (Fanconi anemia complementation group A) and FANCC (Fanconi anemia complementation group C) all render cells sensitive to PARP inhibition through defective DNA damage repair.39

Despite the newer generation of PARP inhibitors being more specific for the PARP family, it is likely that they inhibit both PARP-1 and PARP-2 due to the similarity in the structure of their catalytic domains. In laboratory experiments in HR competent cell lines, INO-1001 (a novel potent PARP inhibitor) at concentrations as low as 1 lM fully inhibited PARP activity, with concentrations up to 100 lM remaining non-toxic as measured by clonogenic assay.41 These observations are in keeping with the mechanism of action of PARP in which activation is initiated by DNA damage. It also means that the therapeutic effects of PARP inhibitors are likely to be limited to cells that are repairing DNA damage induced by anti-cancer agents, such as radiotherapy, and will not be manifest in non-irradiated tissues. In light of the role of PARP-1 discussed above, it is theorized that ionizing radiation would induce SSB that would go unrepaired in cells in which PARP is inhibited and subsequently result in potentially lethal DSB at replication. As PARP-1 has been specifically shown to bind DNA strand breaks formed by ionizing radiation to facilitate repair, it as an attractive target for radiation enhancement (Fig. 4). Models to test these hypotheses include in vitro cell line and in vivo xenograft models, the data for which will be discussed below. PARP activity in these models can be abolished either by the addition of chemical PARP inhibitors or through gene silencing.

PARP inhibitors as radiosensitisers

In vitro experimental data

Several PARP inhibitors have been identified and refined over time to become more specific and effective. The majority of PARP inhibitors is derived from the benzamide structure and act by competitively inhibiting the catalytic domain of the PARP enzyme.40

There are now good data from several cells lines that PARP inhibitors are radiosensitising agents in vitro. Brock et al. exposed three cell lines (Chinese Hamster Ovary (CHO), human fibroblast and murine sarcoma tumour line (SaNH)) to ionizing radiation with

Agents in pre-clinical and clinical studies

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Fig. 4. Simplified diagram of how PARP (poly(ADP-ribose) polymerase) inhibitors could act as radiosensitising agents. (A) The left-hand side of the diagram represents the normal situation where single-strand breaks (SSB) in DNA resulting from damage by ionizing radiation are sensed by PARP which recruits DNA repair proteins facilitating repair of the SSB and enabling the cell replicate to normally. (B) The right hand side represents the situation when PARP inhibitors prevent the automodification of PARP thereby preventing its release from damaged DNA and denying access of the SSB to the DNA repair proteins. PARP inhibition also prevents recruitment of some DNA repair proteins such as XRCC1 (X-ray repair cross complementing group 1). Collision of unrepaired SSB with replication forks in S phase result in lethal double-strand breaks (DSB) thereby enhancing the radiation effect.

or without 10 lM INO-1001 30 min prior to and 3 h following irradiation. They demonstrated enhancement ratios for all cell lines ranging from 1.4 to 1.7 at 10% survival.41 Additionally, when cultures were treated with split doses of 4 Gy of irradiation, the recovery ratio was reduced from 5.3 to 2.2 by the addition of INO-1001 suggesting that PARP inhibition may block a significant proportion of DNA repair that occurs between fractions. Other groups have also demonstrated radiosensitisation by PARP inhibitors when cultured with various cell lines as summarized in Table 1.41–61 The phase of the cell cycle at the time of exposure to ionizing radiation seems to be an important determinant of PARP inhibitor-associated radiosensitisation in human cell lines. Several groups have shown radiosensitisation with the addition of PARP inhibitors to rodent cell lines but this has been less evident in human cell lines42–47,49–52 (Table 1). Noel et al. investigated the PARP inhibitor 4-amino-1,8-naphthalimide (ANI) as a potential radiosensitiser in five rodent and five human cell lines in order to elucidate this differential radiosensitisation. They showed that the radiosensitisation depended primarily on the proportion of cells undergoing DNA replication and that in synchronised HeLa human cervix carcinoma cells ANI-induced radiosensitisation occurred specifically during S phase of the cell cycle.53 Furthermore, irradiation of HeLa cells in the presence of ANI caused an accumulation of DSB many hours after irradiation. Therefore, it is hypothesized that the action of ANI in cells irradiated in S phase is as a result of collision of unrepaired SSB with replication forks. The explanation offered by the authors for the differential radiosensitisation between rodent and human cells lines relates to the increased

length of G1 transit in human cells which allows time for the SSB to be repaired by PARP-independent mechanisms and reduces the chance that asynchronously growing cells are irradiated during S phase.53 This enhanced radiosensitivity during S phase could provide a mechanistic basis for the use of PARP inhibitors as radiosensitisers in rapidly dividing tumours, such as squamous cell carcinomas of the head and neck or cervix. This cell cycle phase dependency of PARP inhibitor-mediated radiosensitisation was also shown by Chalmers et al. The novel third generation PARP inhibitor PJ34 (N-(6-oxo-5,6-dihydrophenanthridin-2-yl) N,N-dimethylacetamide, hydrochloride) modified clonogenic cell survival of T98G but not U373-MG human glioma cell lines when exposed to low-dose irradiation55 (Table 1). When the cell populations had been confluent for 24 h and were confirmed to be arrested in G1, the radiosensitisation of PJ34 on the T98G cell line was lost. Additionally, PARP-1/ knockout mouse cells showed sensitivity only to radiation doses P1.5 Gy in contrast to cells cultured with PARP inhibitors which are not specific for PARP-1 (as discussed previously). The results from preliminary immunofluorescence data support the hypothesis that PARP-2 can compensate for a lack of PARP-1 after low-dose radiation but not after higher radiation doses where greater levels of DNA damage are present.55 Additional work has confirmed radiation sensitivity by KU-0059436 (an oral inhibitor of PARP-1 and -2, now known as olaparib) by a replication-dependent mechanism involving generation of DSB and this is augmented by radiation fractionation.58 One of the most important contributors to radioresistance is tumour hypoxia. It is, therefore, advantageous that any new radio-

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C. Powell et al. / Cancer Treatment Reviews 36 (2010) 566–575 Table 1 Published data showing the radiosensitisation effect of PARP inhibitors in vitro in rodent and human cell lines. Author

PARP inhibitor

Cell line

Enhancement ratio

Brocket al.

INO-1001

Nduka et al.42 Thraves et al.43

5-Methyl-nicotinamide 3-Aminobenzamide Benzamide

Kelland et al.44 Leith45 Little et al.46

3-Aminobenzamide 3-Aminobenzamide 3-Aminobenzamide

Utsumi and Elkind47 Arundel-Suto et al.48 Weltin et al.49

3-Aminobenzamide

Chinese Hamster Ovary Human fibroblast Murine sarcoma tumour Mouse leukaemic lymphoblasts Ewings sarcoma Normal human fibroblasts Lung adenocarcinoma Osteosarcoma Human germ cell tumour Chinese hamster cells Chinese Hamster Ovary Human fibroblast Chinese hamster cells

1.6 1.4 1.7 ‘Small’ but NS Present Present Absent Absent Present 1.4 Present Absent Present

PD128763

Chinese hamster cells (V79)

Present

6(5H)-phenanthridinone 3-Aminobenzamide NU1025 (8-hydroxy-2-methyl-quinazolin-4-[3H]one)

Murine lymphoma cells

1.6 Present 1.4

NU1025 4-Amino-1,8-naphthalimide

41

Bowman et al.50 Boulton et al.51 Schlicker et al.52

a b c

L1210

Russo et al.59

E7016

Calabrese et al.60 Thomas et al.61

AG14361

Chinese Hamster Ovary-K1 Hamster lung fibroblasts Rat prostate carcinoma Human prostate carcinoma Rodent asynchronous cells Human asynchronous cells HeLa cells in S, G2 and G1 phases HeLa cells in S phase and asynchronous Hamster fibroblasts (V79 and CHO) Human glioma cells (T98G) (exponentially growing) Human glioma cells (U373) T98G (confluent) PARP-1 knockout murine cells H460 lung cancer cells Human prostate cancer(DU-145) Human prostate cancer (22RV1) Non-small cell lung cancer (H1299) Four human glioma cell lines Exponential phase Synchronised S phase Single dose/fractionated Human glioma cell line (U251) Pancreatic carcinoma (MiaPaCa2) Prostate carcinoma (DU-145) LoVo colorectal cancer cells

10 PARP inhibitor compounds (more potent than AG14361)

LoVo colorectal cancer cells

Noel et al.53

4-Amino-1,8-naphthalimide

Godon et al.54

PARP-1 silencing (PARP-1KD)a

Chalmers et al.55

N-(6-oxo-5,6-dihydrophenanthridin-2-yl) N,Ndimethylacetamide, hydrochloride (PJ34)

Albert et al.56 Liu et al.57

ABT-888 ABT-888

Dungey et al.58

KU-0059436 (AZD2281)

1.7 1.3 1.5 1.3 Present Absent Present in S phase only 2.5-fold increase in radiosensitivity Present Present Absent Absent Absent 1.27 SF2b; from 0.44 to 0.36 SF2b; from 0.27 to 0.2 SF2b; from 0.54 to 0.3, SERc 1.38 1.17–1.38 1.6 1.27/1.55 1.6 1.4 1.7 Recovery of cells inhibited by 73% at 24 h Potentially lethal damage recovery inhibited by 54–91%

PARP-1KD PARP-1 knock down cells (PARP-1 silenced). SF2 Survival fraction at 2 Gy. SER sensitizer enhancement ratio.

sensitiser should be active in hypoxic as well as euoxic conditions. Liu et al. investigated the ability of the PARP inhibitor ABT-888 to radiosensitise human prostate (DU-145 and 22RV1) and non-small cell lung cancer (H1299) cells under conditions of acute hypoxia. Although radioresistance was induced in all three cell lines, as expected, with the hypoxic experimental conditions, the ability of ABT-888 to radiosensitise cells was not impaired.57 Sensitizer enhancement ratios (SER) of 1.38 were obtained under both euoxic and hypoxic conditions for H1299 cells (Table 1). In vivo experimental data In vivo pre-clinical testing of PARP inhibitors as enhancers of anti-cancer agents has been hampered by a lack of potency and specificity of first and second generation compounds and most initial studies assessed the efficacy of chemotherapy in combination

with PARP inhibitors.62 The published data relating to PARP inhibitors in combination with radiation are summarized in Table 2.56,59,60,63–65 The tricyclic benzimidazole AG14361 has been developed as an extremely potent PARP inhibitor (>1000-fold more potent than benzamides) and tested for its activity in cell culture and tumour xenograft-bearing mice in combination with ionizing radiation, temozolomide and topoisomerase I inhibitors.60 AG14361 or saline was administered intra-peritoneally (IP) to CD-1 nude mice bearing subcutaneous LoVo or SW620 (colorectal cancer cell lines) xenografts daily for 5 days, 30 min before delivering 2 Gy of X-irradiation locally to the tumours. Tumour volumes were determined by twodimensional calliper measurements and tumour growth delay was defined as the time to RTV4 (tumour volume four times the volume on the initial day of treatment) in irradiated mice with AG14361 compared with control. IP administration of AG14361 resulted in

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Table 2 Published data showing the radiosensitisation effect of PARP inhibitors in vivo in mouse xenograft models. Author

PARP inhibitor

Xenograft

Efficacy with radiotherapy

Kelland and Tonkin63

3-Aminobenzamide

Human cervix carcinoma

Enhancement ratio

Calabrese et al.60 Albert et al.56

AG14361

70 cG/min 5 cGy/min Colorectal cancer (LoVo and SW620)

ABT-888

Lung cancer (H460)

Khan et al.64

GPI-15427 (10-(4-Methyl-piperazin-1-ylmethyl)-2H-7-oxa-1,2-diazabenzo[de]-anthracen-3-one) E7016

Head and neck squamous cell carcinoma

1.5–2.4 1.02–1.37 Tumour growth delay increased by 18 days (2-fold) Tumour growth delay increased by 6.5 days (2-fold) Reduced tumour volume

Russo et al.59 Donawho et al.65

ABT-888

rapid absorption into the bloodstream and distribution to the tumour and liver with good retention in tumour tissue over time achieving tumour concentrations in excess of that required to inhibit PARP-1 activity in vitro. Doses of 5 and 15 mg/kg alone did not result in toxicity or affect tumour growth. However, when AG14361 was administered at 5 mg/kg prior to irradiation in mice with LoVo xenografts, tumour growth was delayed by 37 days (95% confidence interval (CI) 35–39 days) compared with a delay of 19 days (95% CI 16–22 days) with local tumour irradiation alone (P = 0.008). Importantly, no local toxicity was observed in either group. AG14361 administration also enhanced temozolomide and irinotecan-induced growth delay of LoVo xenografts by 2- to 3-fold and the combination of AG14361 and temozolomide caused complete regression of SW620 xenograft tumours.60 Higher doses of temozolomide and AG14361 did result in significant body weight loss and there was a toxic death, although irinotecan was better tolerated with minimal weight loss alone or in combination with AG14361 and no diarrhoea was observed. Interesting data are emerging for a role for PARP inhibitors in protecting against irinotecan-induced diarrhoea (a clinical dose-limiting toxicity). In a study combining the PARP inhibitor GPI-15427 with irinotecan in a rat model, the histological changes of inflammatory infiltrates and intestinal mucosal damage seen with irinotecan were abolished with the addition of GPI-15427.66 Additionally, jejunal sections were stained with antibodies against poly(ADP-ribosyl)ated proteins demonstrating that the intestinal damage was associated with PARP-1 activation. This dual role for PARP inhibitors to both reduce irinotecan-associated toxicity and potentiate its anti-tumour activity is possibly due to the fact that PARP acts as a survival factor promoting DNA repair in the presence of moderate DNA damage, however, in the presence of extensive damage that stimulates PARP over-activation it acts as a death mediator. This multifaceted activity has potential clinical benefit to enhance the therapeutic ratio of combining anti-cancer agents with PARP inhibitors. In the xenograft lung cancer model developed by Albert et al., a 2-fold increase in tumour growth delay with the addition of the PARP inhibitor ABT-888 to irradiation was reported.56 For a 5-fold increase in tumour volume, ABT-888 and radiotherapy combination treatment induced a tumour growth delay of 13.5 days, compared with 7 days for radiation alone (P = 0.045 for ABT-888/ radiation versus radiation alone). This was well-tolerated, being associated with 15% body weight loss for radiation alone but only 10% weight loss for the combination treatment. Additionally they showed that the combination of ABT-888 and radiation resulted in reduced tumour vascular density in histological sections compared with ABT-888 or radiation alone or control (1.6 vs. 7.3 vs. 4 vs. 11 vessels per microscopic field respectively, P = 0.02 for ABT-888/radiation versus radiation alone).

Glioblastoma (U251) (in combination with temozolomide) Colon cancer (HCT-116)

Tumour growth delay 10.8 days (1.5fold) Median survival time increased by 13 days (1.5-fold)

Clinical data PARP inhibitors are currently undergoing evaluation in a variety of clinical settings. There has been successful transition from preclinical to clinical use of PARP inhibitors as single agents in phase 1 and 2 trials for BRCA-1 and -2-deficient patients as already discussed. Following encouraging pre-clinical data, there is emerging clinical evidence for the use of PARP inhibitors in combination with chemotherapy. Proof-of-principle of the mechanism of action, tolerability and evaluation of the PARP inhibitory dose (PID) of the PARP inhibitor AG014699 in combination with temozolomide in patients with metastatic melanoma was reported by Plummer et al.67 In the phase 2 trial of full-dose temozolomide and a PID of AG014699, enhanced temozolomide-induced myelosuppression was seen with one toxic death necessitating a 25% dose reduction of temozolomide, however, modest increases in response rate and median time to progression in comparison to historical controls were observed.68 This enhanced myelosuppression was confirmed when the PARP inhibitor INO-101 was combined with full-dose temozolomide (given daily for 5 days in a 4-weekly regimen).69 Further data are emerging for combining PARP inhibitors with other cytotoxic agents, including gemcitabine and cisplatin70 and topotecan,71 all of which result in a dose-limiting toxicity of myelosuppression. In contrast, however, the potent PARP inhibitor BSI201 has shown promising phase 1 data and preliminary analysis of the randomized phase 2 study of gemcitabine/carboplatin chemotherapy with or without bi-weekly BSI-201 in triple negative breast cancer patients was reported at ASCO in 2009. Interestingly BSI-201 was well-tolerated with no difference in adverse events between the two arms. However, a marked increase in efficacy was seen with the addition of the PARP inhibitor with significantly improved clinical benefit rate, progression-free survival and overall-survival.72 The result of a phase 3 trial to confirm these findings is awaited. There are no published data from clinical trials relating to the combination of PARP inhibitors and radiotherapy at the time of writing. In light of the toxicity experienced when pre-clinical data were translated into clinical trials by combining PARP inhibitors with chemotherapy (as discussed above), careful design of trials using PARP inhibitors as radiosensitisers will be essential.

Discussion Ionizing radiation plays a major role in cancer treatment but, unfortunately, in many circumstances cure rates remain suboptimal. Therapeutic advances are, therefore, needed to enhance the efficacy of radiation therapy. The challenge in combining radiation

C. Powell et al. / Cancer Treatment Reviews 36 (2010) 566–575

with novel agents is in augmenting its ability to kill tumour cells without worsening normal tissue toxicity. In this way, the combination can be used to improve the therapeutic ratio. PARP-1 is a ubiquitous intracellular enzyme which plays a pivotal role in the cell’s response to DNA damage and, in particular, SSB repair. As ionizing radiation exerts its cytotoxic effects by inducing DNA damage and PARP has been shown to bind DNA strand breaks formed by radiation, it represents an interesting potential target for pharmacological modulation of the radiation response. PARP inhibitors have been tested in the laboratory for several years and the recent development of third generation compounds with improved specificity and efficacy has improved their potential as anti-cancer agents, either as single agents in tumours deficient in homologous recombination repair of DNA damage (such as BRCA-1 or -2 proteins) or in combination with chemotherapy or radiotherapy. In a number of in vitro studies with different cell lines and several PARP inhibitor compounds, radiosensitising effects have been demonstrated with a median enhancement ratio of 1.4 (range 1.17–1.7) from published data (Table 1). This effect is seen under both hypoxic and euoxic conditions which is important if one of the resistance mechanisms of radiotherapy (hypoxia) is to be overcome. The in vitro data also suggest that chemical PARP inhibitors are most effective in S phase of the cell cycle in human cancer cell lines and therefore rapidly dividing tumour cells may be more susceptible than non-dividing normal tissues – thus enhancing the therapeutic index. The caution to this, however, is the potential for normal tissues within the radiation field, which may be rapidly dividing in order to repair radiation-induced damage, to be equally susceptible to radiation sensitization. Therefore, careful clinical evaluation is required to ensure that the radiation reaction is not exacerbated in tissues such as the skin or oral and gut mucosae. Interesting data have also been published which support a dual role for PARP in acting as a survival factor promoting DNA repair in the presence of moderate DNA damage and as a death mediator in the presence of extensive damage that stimulates PARP over-activation. In animal models, protection of rats against the dose-limiting toxicity of irinotecan-induced diarrhoea has been seen when the cytotoxic drug is combined with a PARP inhibitor. Further work is needed to evaluate the effects on normal tissues within the radiation field. One of the difficulties in extrapolating from data derived from xenograft models is that radiation doses used in the experimental situation may not be comparable with those used in clinical situations. PARP inhibitors offer great promise as radiosensitising agents and carefully designed clinical trials are now required to evaluate their safety and efficacy in this setting. Although, successful proof-of-principle clinical trials have been undertaken in patients with BRCA-1 and -2-deficient advanced ovarian cancers, the enhanced myelosuppression seen when PARP inhibitors have been combined with temozolomide in early-phase clinical trials confirms the need for careful evaluation of each potential role for PARP inhibitors. It is unclear what the relative expectations for PARP inhibitors as radiosensitisers will be in homologous-recombination-deficient cancers (e.g. BRCA breast cancer) compared to other fast growing cancers without HR deficiency. Future studies will help to define which population of patients will derive the most benefit from PARP inhibitors and their careful design will help minimize the risk of excess toxicity. Phase 1 trials establishing the minimum inhibitory dose (MID) (optimum biological dose rather than the conventional maximum tolerated dose) of PARP inhibitors together with safety and tolerability endpoints in combination with fractionated radiotherapy should now be designed. As discussed previously, tumour samples are not always easy to obtain and are often heterogeneous, making MID challenging to determine. Surrogate pharmacodynamic endpoints, such as PARP

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inhibition in circulating peripheral blood mononuclear cells, have been evaluated, although the level of inhibition does not always correlate with that seen in histopathological tumour samples.67 The scheduling of the PARP inhibitor in relation to radiotherapy will also need to be determined as it is possible that the differential toxicity of combining temozolomide and PARP inhibitors delivered daily for 5 days or bi-weekly may be related to the differential pharmacokinetics. A further challenge in combining novel agents with radical radiotherapy with the aim of cure is that it is not ethical to dose escalate the radiation dose from a lower than conventional dose in combination with the new agent in order to avoid exacerbation of toxicity. One option therefore is to combine PARP inhibitors with radiotherapy initially in the palliative setting where repeated lower doses are practicable to assess safety and any possible potentiation. The relative heterogeneity of treatment indications in this group may make toxicity assessment very difficult. For example, it would be difficult to make definitive statements about dose-limiting toxicity from a cohort of three patients who received treatment to three different anatomical sub-sites (e.g. pelvis, abdomen or thorax). Therefore, phase 1 studies will have to be conducted in homogeneous groups of patients with a similar indication for palliative radiation – for example, cutaneous deposits of malignant melanoma. As these studies will be carried out in patients with superficial disease lying within a radiation portal, access to tumours for histological confirmation of enzyme inhibition should be facilitated. Secondary objectives of such a trial would be to enable a preliminary assessment of the anti-tumour activity of PARP inhibitors and radiotherapy and to determine and recommend phase 2 doses utilizing this schedule for subsequent chemoradiation studies. One tumour model, in particular, has been proposed to benefit from PARP inhibition. The primary brain tumour glioblastoma multiforme is currently treated by surgical resection followed by radiotherapy combined with concomitant and adjuvant temozolomide in those who are fit enough.62 This is an attractive model for testing PARP inhibition for several reasons: first, the outcome from the current treatment paradigm is poor; second, PARP inhibitors have been shown to potentiate the efficacy of temozolomide in vitro, in vivo and in patients with metastatic melanoma59,68; and, third, the rapidly dividing tumour cells are surrounded by non-dividing brain parenchymal cells, in theory limiting toxicity in surrounding normal tissues. Other potential targets for trials combining radiotherapy and PARP inhibitors are rapidly dividing tumours such as squamous cell carcinomas or tumours in which low-dose irradiation is used as PARP inhibitors have been shown to potentiate the effects of low-dose as well as high dose ionizing radiation.55,73 The need to monitor patients during a 4–6 week course of radiotherapy and the subsequent recovery period from acute effects (probably a further 4–6 weeks) means that such studies will, however, be relatively slow with prolonged intervals between opening sequential dose escalation cohorts. This is a very exciting time in clinical oncology with the development of new targeted agents that have the potential to enhance the effects of radiotherapy in a variety of solid cancers. PARP inhibitors represent a novel group of agents whose mechanisms of action have direct relevance to combination regimens with radiation. The current challenge lies in designing and conducting phase 1 clinical trials to assess their safety as a prelude to studies of their potential efficacy. References 1. Pignon JP, Bourhis J, Domenge C, Designe L. Chemotherapy added to locoregional treatment for head and neck squamous-cell carcinoma: three

574

2.

3.

4.

5.

6.

7. 8. 9. 10.

11. 12.

13. 14.

15.

16.

17.

18. 19.

20.

21.

22.

23. 24. 25.

26.

27. 28.

29.

30.

C. Powell et al. / Cancer Treatment Reviews 36 (2010) 566–575 meta-analyses of updated individual data. MACH-NC Collaborative Group. Meta-analysis of chemotherapy on head and neck cancer. Lancet 2000;355(9208):949–55. Green JA, Kirwan JM, Tierney JF, Symonds P, Fresco L, Collingwood M, et al. Survival and recurrence after concomitant chemotherapy and radiotherapy for cancer of the uterine cervix: a systematic review and meta-analysis. Lancet 2001;358(9284):781–6. Bonner JA, Harari PM, Giralt J, Azarnia N, Shin DM, Cohen RB, et al. Radiotherapy plus cetuximab for squamous-cell carcinoma of the head and neck. N Engl J Med 2006;354(6):567–78. Bonner JA, Harari PM, Giralt J, Cohen RB, Jones CU, Sur RK, et al. Radiotherapy plus cetuximab for locoregionally advanced head and neck cancer: 5-year survival data from a phase 3 randomised trial, and relation between cetuximabinduced rash and survival. Lancet Oncol. Harrington KJ, El-Hariry IA, Holford CS, Lusinchi A, Nutting CM, Rosine D, et al. Phase I study of lapatinib in combination with chemoradiation in patients with locally advanced squamous cell carcinoma of the head and neck. J Clin Oncol 2009;27(7):1100–7. Harrington K, Jankowska P, Hingorani M. Molecular biology for the radiation oncologist: the 5Rs of radiobiology meet the hallmarks of cancer. Clin Oncol (R Coll Radiol) 2007;19(8):561–71. Bentzen SM. Potential clinical impact of normal-tissue intrinsic radiosensitivity testing. Radiother Oncol 1997;43(2):121–31. Vazquez A, Bond EE, Levine AJ, Bond GL. The genetics of the p53 pathway, apoptosis and cancer therapy. Nat Rev Drug Discov 2008;7(12):979–87. Fernet M, Hall J. Genetic biomarkers of therapeutic radiation sensitivity. DNA Repair (Amst) 2004;3(8–9):1237–43. Farmer H, McCabe N, Lord CJ, Tutt AN, Johnson DA, Richardson TB, et al. Targeting the DNA repair defect in BRCA mutant cells as a therapeutic strategy. Nature 2005;434(7035):917–21. Schreiber V, Dantzer F, Ame JC, de Murcia G. Poly(ADP-ribose): novel functions for an old molecule. Nat Rev Mol Cell Biol 2006;7(7):517–28. Mortusewicz O, Ame JC, Schreiber V, Leonhardt H. Feedback-regulated poly(ADP-ribosyl)ation by PARP-1 is required for rapid response to DNA damage in living cells. Nucleic Acids Res 2007;35(22):7665–75. D’Amours D, Desnoyers S, D’Silva I, Poirier GG. Poly(ADP-ribosyl)ation reactions in the regulation of nuclear functions. Biochem J 1999;342(Pt 2):249–68. Benjamin RC, Gill DM. ADP-ribosylation in mammalian cell ghosts. Dependence of poly(ADP-ribose) synthesis on strand breakage in DNA. J Biol Chem 1980;255(21):10493–501. Davidovic L, Vodenicharov M, Affar EB, Poirier GG. Importance of poly(ADPribose) glycohydrolase in the control of poly(ADP-ribose) metabolism. Exp Cell Res 2001;268(1):7–13. Smulson M, Istock N, Ding R, Cherney B. Deletion mutants of poly(ADP-ribose) polymerase support a model of cyclic association and dissociation of enzyme from DNA ends during DNA repair. Biochemistry 1994;33(20):6186–91. Benjamin RC, Gill DM. Poly(ADP-ribose) synthesis in vitro programmed by damaged DNA. A comparison of DNA molecules containing different types of strand breaks. J Biol Chem 1980;255(21):10502–8. de Murcia G, Menissier de Murcia J. Poly(ADP-ribose) polymerase: a molecular nick-sensor. Trends Biochem Sci 1994;19(4):172–6. Masutani M, Nozaki T, Nishiyama E, Shimokawa T, Tachi Y, Suzuki H, et al. Function of poly(ADP-ribose) polymerase in response to DNA damage: genedisruption study in mice. Mol Cell Biochem 1999;193(1–2):149–52. Wang X, Ohnishi K, Takahashi A, Ohnishi T. Poly(ADP-ribosyl)ation is required for p53-dependent signal transduction induced by radiation. Oncogene 1998;17(22):2819–25. Soldatenkov VA, Smulson M. Poly(ADP-ribose) polymerase in DNA damageresponse pathway: implications for radiation oncology. Int J Cancer 2000;90(2):59–67. Simbulan-Rosenthal CM, Rosenthal DS, Iyer S, Boulares AH, Smulson ME. Transient poly(ADP-ribosyl)ation of nuclear proteins and role of poly(ADPribose) polymerase in the early stages of apoptosis. J Biol Chem 1998;273(22):13703–12. Lindahl T. Instability and decay of the primary structure of DNA. Nature 1993;362(6422):709–15. Caldecott KW. Protein–protein interactions during mammalian DNA singlestrand break repair. Biochem Soc Trans 2003;31(Pt 1):247–51. Pleschke JM, Kleczkowska HE, Strohm M, Althaus FR. Poly(ADP-ribose) binds to specific domains in DNA damage checkpoint proteins. J Biol Chem 2000;275(52):40974–80. Whitehouse CJ, Taylor RM, Thistlethwaite A, Zhang H, Karimi-Busheri F, Lasko DD, et al. XRCC1 stimulates human polynucleotide kinase activity at damaged DNA termini and accelerates DNA single-strand break repair. Cell 2001;104(1):107–17. Caldecott KW. Single-strand break repair and genetic disease. Nat Rev Genet 2008;9(8):619–31. Dianov G, Price A, Lindahl T. Generation of single-nucleotide repair patches following excision of uracil residues from DNA. Mol Cell Biol 1992;12(4):1605–12. Caldecott KW, McKeown CK, Tucker JD, Ljungquist S, Thompson LH. An interaction between the mammalian DNA repair protein XRCC1 and DNA ligase III. Mol Cell Biol 1994;14(1):68–76. DiGiuseppe JA, Dresler SL. Bleomycin-induced DNA repair synthesis in permeable human fibroblasts: mediation of long-patch and short-patch repair by distinct DNA polymerases. Biochemistry 1989;28(24):9515–20.

31. Matsumoto Y, Kim K, Hurwitz J, Gary R, Levin DS, Tomkinson AE, et al. Reconstitution of proliferating cell nuclear antigen-dependent repair of apurinic/apyrimidinic sites with purified human proteins. J Biol Chem 1999;274(47):33703–8. 32. Tutt A, Bertwistle D, Valentine J, Gabriel A, Swift S, Ross G, et al. Mutation in Brca2 stimulates error-prone homology-directed repair of DNA double-strand breaks occurring between repeated sequences. EMBO J 2001;20(17):4704–16. 33. Venkitaraman AR. Cancer susceptibility and the functions of BRCA1 and BRCA2. Cell 2002;108(2):171–82. 34. Bryant HE, Schultz N, Thomas HD, Parker KM, Flower D, Lopez E, et al. Specific killing of BRCA2-deficient tumours with inhibitors of poly(ADP-ribose) polymerase. Nature 2005;434(7035):913–7. 35. Fong PC, Boss DS, Yap TA, Tutt A, Wu P, Mergui-Roelvink M, et al. Inhibition of poly(ADP-ribose) polymerase in tumors from BRCA mutation carriers. N Engl J Med 2009;361(2):123–34. 36. Audeh MW, Penson RT, Friedlander M, Powell B, Bell-McGuinn KM, Scott C, et al. Phase II trial of the oral PARP inhibitor olaparib (AZD2281) in BRCAdeficient advanced ovarian cancer. J Clin Oncol 2009;27:15s. [Suppl.; abstr 5500]. 37. de Murcia JM, Niedergang C, Trucco C, Ricoul M, Dutrillaux B, Mark M, et al. Requirement of poly(ADP-ribose) polymerase in recovery from DNA damage in mice and in cells. Proc Natl Acad Sci USA 1997;94(14):7303–7. 38. Menissier de Murcia J, Ricoul M, Tartier L, Niedergang C, Huber A, Dantzer F, et al. Functional interaction between PARP-1 and PARP-2 in chromosome stability and embryonic development in mouse. EMBO J 2003;22(9):2255–63. 39. McCabe N, Turner NC, Lord CJ, Kluzek K, Bialkowska A, Swift S, et al. Deficiency in the repair of DNA damage by homologous recombination and sensitivity to poly(ADP-ribose) polymerase inhibition. Cancer Res 2006;66(16):8109–15. 40. Woon EC, Threadgill MD. Poly(ADP-ribose)polymerase inhibition – where now? Curr Med Chem 2005;12(20):2373–92. 41. Brock WA, Milas L, Bergh S, Lo R, Szabo C, Mason KA. Radiosensitization of human and rodent cell lines by INO-1001, a novel inhibitor of poly(ADP-ribose) polymerase. Cancer Lett 2004;205(2):155–60. 42. Nduka N, Skidmore CJ, Shall S. The enhancement of cytotoxicity of N-methyl-Nnitrosourea and of gamma-radiation by inhibitors of poly(ADP-ribose) polymerase. Eur J Biochem 1980;105(3):525–30. 43. Thraves PJ, Mossman KL, Brennan T, Dritschilo A. Differential radiosensitization of human tumour cells by 3-aminobenzamide and benzamide: inhibitors of poly(ADP-ribosylation). Int J Radiat Biol Relat Stud Phys Chem Med 1986;50(6):961–72. 44. Kelland LR, Burgess L, Steel GG. Radiation damage repair capacity of a human germ-cell tumour cell line: inhibition by 3-aminobenzamide. Int J Radiat Biol Relat Stud Phys Chem Med 1987;51(2):227–41. 45. Leith JT. Effects of sodium butyrate and 3-aminobenzamide on survival of Chinese hamster HA-1 cells after X irradiation. Radiat Res 1988;114(1):186–91. 46. Little JB, Ueno AM, Dahlberg WK. Differential response of human and rodent cell lines to chemical inhibition of the repair of potentially lethal damage. Radiat Environ Biophys 1989;28(3):193–202. 47. Utsumi H, Elkind MM. Inhibitors of poly (ADP-ribose) synthesis inhibit the two types of repair of potentially lethal damage. Int J Radiat Oncol Biol Phys 1994;29(3):577–8. 48. Arundel-Suto CM, Scavone SV, Turner WR, Suto MJ, Sebolt-Leopold JS. Effect of PD 128763, a new potent inhibitor of poly(ADP-ribose) polymerase, on X-rayinduced cellular recovery processes in Chinese hamster V79 cells. Radiat Res 1991;126(3):367–71. 49. Weltin D, Holl V, Hyun JW, Dufour P, Marchal J, Bischoff P. Effect of 6(5H)phenanthridinone, a poly (ADP-ribose)polymerase inhibitor, and ionizing radiation on the growth of cultured lymphoma cells. Int J Radiat Biol 1997;72(6):685–92. 50. Bowman KJ, White A, Golding BT, Griffin RJ, Curtin NJ. Potentiation of anticancer agent cytotoxicity by the potent poly(ADP-ribose) polymerase inhibitors NU1025 and NU1064. Br J Cancer 1998;78(10):1269–77. 51. Boulton S, Kyle S, Durkacz BW. Interactive effects of inhibitors of poly(ADPribose) polymerase and DNA-dependent protein kinase on cellular responses to DNA damage. Carcinogenesis 1999;20(2):199–203. 52. Schlicker A, Peschke P, Burkle A, Hahn EW, Kim JH. 4-Amino-1,8naphthalimide: a novel inhibitor of poly(ADP-ribose) polymerase and radiation sensitizer. Int J Radiat Biol 1999;75(1):91–100. 53. Noel G, Godon C, Fernet M, Giocanti N, Megnin-Chanet F, Favaudon V. Radiosensitization by the poly(ADP-ribose) polymerase inhibitor 4-amino1,8-naphthalimide is specific of the S phase of the cell cycle and involves arrest of DNA synthesis. Mol Cancer Ther 2006;5(3):564–74. 54. Godon C, Cordelieres FP, Biard D, Giocanti N, Megnin-Chanet F, Hall J, et al. PARP inhibition versus PARP-1 silencing: different outcomes in terms of singlestrand break repair and radiation susceptibility. Nucleic Acids Res 2008;36(13):4454–64. 55. Chalmers A, Johnston P, Woodcock M, Joiner M, Marples B. PARP-1, PARP-2, and the cellular response to low doses of ionizing radiation. Int J Radiat Oncol Biol Phys 2004;58(2):410–9. 56. Albert JM, Cao C, Kim KW, Willey CD, Geng L, Xiao D, et al. Inhibition of poly(ADP-ribose) polymerase enhances cell death and improves tumor growth delay in irradiated lung cancer models. Clin Cancer Res 2007;13(10):3033–42. 57. Liu SK, Coackley C, Krause M, Jalali F, Chan N, Bristow RG. A novel poly(ADPribose) polymerase inhibitor, ABT-888, radiosensitizes malignant human cell lines under hypoxia. Radiother Oncol 2008;88(2):258–68.

C. Powell et al. / Cancer Treatment Reviews 36 (2010) 566–575 58. Dungey FA, Loser DA, Chalmers AJ. Replication-dependent radiosensitization of human glioma cells by inhibition of poly(ADP-Ribose) polymerase: mechanisms and therapeutic potential. Int J Radiat Oncol Biol Phys 2008;72(4):1188–97. 59. Russo AL, Kwon HC, Burgan WE, Carter D, Beam K, Weizheng X, et al. In vitro and in vivo radiosensitization of glioblastoma cells by the poly (ADP-ribose) polymerase inhibitor E7016. Clin Cancer Res 2009;15(2):607–12. 60. Calabrese CR, Almassy R, Barton S, Batey MA, Calvert AH, Canan-Koch S, et al. Anticancer chemosensitization and radiosensitization by the novel poly(ADP-ribose) polymerase-1 inhibitor AG14361. J Natl Cancer Inst 2004;96(1):56–67. 61. Thomas HD, Calabrese CR, Batey MA, Canan S, Hostomsky Z, Kyle S, et al. Preclinical selection of a novel poly(ADP-ribose) polymerase inhibitor for clinical trial. Mol Cancer Ther 2007;6(3):945–56. 62. Chalmers AJ. The potential role and application of PARP inhibitors in cancer treatment. Br Med Bull 2009;89:23–40. 63. Kelland LR, Tonkin KS. The effect of 3-aminobenzamide in the radiation response of three human cervix carcinoma xenografts. Radiother Oncol 1989;15(4):363–9. 64. Khan K, Araki K, Wang D, Li G, Li X, Zhang J, et al. Head and neck cancer radiosensitization by the novel poly(ADP-ribose) polymerase inhibitor GPI15427. Head Neck. 65. Donawho CK, Luo Y, Penning TD, Bauch JL, Bouska JJ, Bontcheva-Diaz VD, et al. ABT-888, an orally active poly(ADP-ribose) polymerase inhibitor that potentiates DNA-damaging agents in preclinical tumor models. Clin Cancer Res 2007;13(9):2728–37. 66. Tentori L, Leonetti C, Scarsella M, Muzi A, Mazzon E, Vergati M, et al. Inhibition of poly(ADP-ribose) polymerase prevents irinotecan-induced intestinal damage

67.

68.

69.

70.

71.

72.

73.

575

and enhances irinotecan/temozolomide efficacy against colon carcinoma. FASEB J 2006;20(10):1709–11. Plummer R, Jones C, Middleton M, Wilson R, Evans J, Olsen A, et al. Phase I study of the poly(ADP-ribose) polymerase inhibitor, AG014699, in combination with temozolomide in patients with advanced solid tumors. Clin Cancer Res 2008;14(23):7917–23. Plummer R, lorigan P, Evans J, Steven N, Middleton M, Wilson R, et al. First and final report of a phase II study of the poly(ADP-ribose) polymerase (PARP) inhibitor, AG014699, in combination with temozolomide (TMZ) in patients with metastatic malignant melanoma (MM). J Clin Oncol:24. [abstract 8013]. Bedikian AY, Papadopoulos NE, Kim KB, Hwu WJ, Homsi J, Glass MR, et al. A phase IB trial of intravenous INO-1001 plus oral temozolomide in subjects with unresectable stage-III or IV melanoma. Cancer Invest 2009;27(7):756–63. Rajan A, Gutierrez M, Kummar S, Yancey M, Ji J, Zhang Y, et al. A phase I combination study of AZD2281 and cisplatin plus gemcitabine in adults with solid tumors. Ann Oncol 2009;20(Suppl. 3):iii42–=0?>iii44. [abstract G04]. Kummar S, Ji J, Zhang H, Simmons D, Kinders R, Gutierrez M, et al. A phase I combination study of ABT-888 and topotecan hydrochloride in adults with refractory solid tumours and lymphomas. Ann Oncol 2009;20(Suppl. ):iii42–=0?>iii43. [abstract G03]. O’Shaughnessy J, Osborne C, Pippen J, Yoffe M, Patt D, Monaghan G, et al. Efficacy of BSI-201, a poly (ADP-ribose) polymerase-1 (PARP1) inhibitor, in combination with gemcitabine/carboplatin (G/C) in patients with metastatic triple-negative breast cancer (TNBC): results of a randomized phase II trial. J Clin Oncol:27. [18s abstr 3]. Williams JR, Zhang Y, Zhou H, Gridley DS, Koch CJ, Slater JM, et al. Overview of radiosensitivity of human tumor cells to low-dose-rate irradiation. Int J Radiat Oncol Biol Phys 2008;72(3):909–17.