Poly(ADP-Ribose) polymerase inhibition synergizes with 5-fluorodeoxyuridine but not 5-fluorouracil in ovarian cancer cells

REV3 promotes cellular tolerance to 5-fluorodeoxyuridine by activating translesion DNA synthesis and intra-S checkpoint

The drug floxuridine (5-fluorodeoxyuridine, FUdR) is an active metabolite of 5-Fluorouracil (5-FU). It converts to 5-fluorodeoxyuridine monophosphate (FdUMP) and 5-fluorodeoxyuridine triphosphate (FdUTP), which on incorporation into the genome inhibits DNA replication. Additionally, it inhibits thymidylate synthase, causing dTMP shortage while increasing dUMP availability, which induces uracil incorporation into the genome. However, the mechanisms underlying cellular tolerance to FUdR are yet to be fully elucidated. In this study, we explored the mechanisms underlying cellular resistance to FUdR by screening for FUdR hypersensitive mutants from a collection of DT40 mutants deficient in each genomic maintenance system. We identified REV3, which is involved in translesion DNA synthesis (TLS), to be a critical factor in FUdR tolerance. Replication using a FUdR-damaged template was attenuated in REV3-/- cells, indicating that the TLS function of REV3 is required to maintain replication on the FUdR-damaged template. Notably, FUdR-exposed REV3-/- cells exhibited defective cell cycle arrest in the early S phase, suggesting that REV3 is involved in intra-S checkpoint activation. Furthermore, REV3-/- cells showed defects in Chk1 phosphorylation, which is required for checkpoint activation, but the survival of FUdR-exposed REV3-/- cells was further reduced by the inhibition of Chk1 or ATR. These data indicate that REV3 mediates DNA checkpoint activation at least through Chk1 phosphorylation, but this signal acts in parallel with ATR-Chk1 DNA damage checkpoint pathway. Collectively, we reveal a previously unappreciated role of REV3 in FUdR tolerance.

Novel Therapeutic Strategies for Refractory Ovarian Cancers: Clear Cell and Mucinous Carcinomas

Ovarian cancer has the worst prognosis among gynecological cancers. In particular, clear cell and mucinous carcinomas are less sensitive to chemotherapy. The establishment of new therapies is necessary to improve the treatment outcomes for these carcinomas. In previous clinical studies, chemotherapy with cytotoxic anticancer drugs has failed to demonstrate better treatment outcomes than paclitaxel + carboplatin therapy. In recent years, attention has been focused on treatment with molecular target drugs and immune checkpoint inhibitors that target newly identified biomarkers. The issues that need to be addressed include the most appropriate combination of therapies, identifying patients who may benefit from each therapy, and how results should be incorporated into the standard of care for ovarian clear cell and mucinous carcinomas. In this article, we have reviewed the most promising therapies for ovarian clear cell and mucinous carcinomas, which are regarded as intractable, with an emphasis on therapies currently being investigated in clinical studies.

Antimicrobial compounds from an FDA-approved drug library with activity against Streptococcus suis

AIM

Antimicrobial resistance (AMR) has become a global concern. Developing novel antimicrobials is one of the most effective approaches in tackling AMR. Considering its relatively low cost and risk, drug repurposing has been proposed as a valuable approach for novel antimicrobial discovery. The aim of this study was to screen for antimicrobial compounds against Streptococcus suis, an important zoonotic bacterial pathogen, from an Food and Drug Administration (FDA)-approved drug library.

METHODS AND RESULTS

In this study, we tested the antimicrobial activity of 1815 FDA-approved drugs against S. suis. Sixty-seven hits were obtained that showed a growth inhibition of more than 98%. After excluding already known antibiotics and antiseptics, 12 compounds were subjected to minimal inhibition concentration (MIC) assessment against S. suis. This showed that pralatrexate, daunorubicin (hydrochloride), teniposide, aclacinomycin A hydrochloride and floxuridine gave a relatively low MIC, ranging from 0.85 to 5.25 μg/ml. Apart from pralatrexate, the remaining four drugs could also inhibit the growth of antimicrobial-resistant S. suis. It was also demonstrated that these four drugs had better efficacy against Gram-positive bacteria than Gram-negative bacteria. Cytotoxicity assays showed that floxuridine and teniposide had a relatively high 50% cytotoxic concentration (CC₅₀ ). The in vivo efficacy of floxuridine was analysed using a Galleria mellonella larvae infection model, and the results showed that floxuridine was effective in treating S. suis infection in vivo.

CONCLUSIONS

Five compounds from the FDA-approved drug library showed high antimicrobial activity against S. suis, among which floxuridine displayed potent in vivo efficacy that is worth further development.

SIGNIFICANCE AND IMPACT OF STUDY

Our study identified several antimicrobial compounds that are effective against S. suis, which provides a valuable starting point for further antimicrobial development.

Maximum Tolerated Dose and Anti-Tumor Activity of Intraperitoneal Cantrixil (TRX-E-002-1) in Patients with Persistent or Recurrent Ovarian Cancer, Fallopian Tube Cancer, or Primary Peritoneal Cancer: Phase I Study Results

Survival outcomes in ovarian cancer are poor. The aims of this Phase I progressive design study (NCT02903771) were to evaluate the maximum tolerated dose (MTD), tolerability, and antitumor activity of Cantrixil-a novel third-generation benzopyran molecule-in patients (n = 25) with advanced, recurrent/persistent epithelial ovarian, primary peritoneal, or fallopian tube cancer. All had completed ≥ 2 prior regimens; 3 (12%) had platinum-refractory disease, and 17 (68%) had platinum-resistant disease. Following intraperitoneal (IP) port placement, patients received weekly IP Cantrixil in 3-week cycles as monotherapy (Cycles 1-2), and then in combination with intravenous (IV) chemotherapy (Cycles 3-8). Part A (dose escalation) enrolled 11 patients in 6 dose-level cohorts. An MTD of 5 mg/kg was established with dose-limiting toxicity of ileus. Most treatment-related adverse events were gastrointestinal. Across Parts A and B (dose expansion), 16 (64%) patients received ≥ 1 3-week Cantrixil cycle, and had ≥ 1 post-baseline efficacy measurement available. The results show promising anti-tumor activity in monotherapy (stable disease rate of 56%) and in combination with IV chemotherapy (objective response rate of 19%, disease control rate of 56%, and median progression-free survival of 13.1 weeks). The molecular target and mechanism of action of Cantrixil are yet to be confirmed. Preliminary analysis of stem cell markers suggests that IP Cantrixil might induce ovarian cancer stem cell death and sensitize cells to standard chemotherapy, warranting further evaluation.

Role of PARP1-mediated autophagy in EGFR-TKI resistance in non-small cell lung cancer

Resistance to epidermal growth factor receptor-tyrosine kinase inhibitors (EGFR-TKIs) has become the main clinical challenge of advanced lung cancer. This research aimed to explore the role of PARP1-mediated autophagy in the progression of TKI therapy. PARP1-mediated autophagy was evaluated in vitro by CCK-8 assay, clonogenic assay, immunofluorescence, and western blot in the HCC-827, H1975, and H1299 cells treated with icotinib (Ico), rapamycin, and AZD2281 (olaparib) alone or in combination. Our results and GEO dataset analysis confirmed that PARP1 is expressed at lower levels in TKI-sensitive cells than in TKI-resistant cells. Low PARP1 expression and high p62 expression were associated with good outcomes among patients with NSCLC after TKI therapy. AZD2281 and a lysosomal inhibitor reversed resistance to Ico by decreasing PARP1 and LC3 in cells, but an mTOR inhibitor did not decrease Ico resistance. The combination of AZD2281 and Ico exerted a markedly enhanced antitumor effect by reducing PARP1 expression and autophagy in vivo. Knockdown of PARP1 expression reversed the resistance to TKI by the mTOR/Akt/autophagy pathway in HCC-827IR, H1975, and H1299 cells. PARP1-mediated autophagy is a key pathway for TKI resistance in NSCLC cells that participates in the resistance to TKIs. Olaparib may serve as a novel method to overcome the resistance to TKIs.

Targeting BER enzymes in cancer therapy

Base excision repair (BER) repairs mutagenic or genotoxic DNA base lesions, thought to be important for both the etiology and treatment of cancer. Cancer phenotypic stress induces oxidative lesions, and deamination products are responsible for one of the most prevalent mutational signatures in cancer. Chemotherapeutic agents induce genotoxic DNA base damage that are substrates for BER, while synthetic lethal approaches targeting BER-related factors are making their way into the clinic. Thus, there are three strategies by which BER is envisioned to be relevant in cancer chemotherapy: (i) to maintain cellular growth in the presence of endogenous DNA damage in stressed cancer cells, (ii) to maintain viability after exogenous DNA damage is introduced by therapeutic intervention, or (iii) to confer synthetic lethality in cancer cells that have lost one or more additional DNA repair pathways. Here, we discuss the potential treatment strategies, and briefly summarize the progress that has been made in developing inhibitors to core BER-proteins and related factors.

A Phase I Clinical Trial of the Poly(ADP-ribose) Polymerase Inhibitor Veliparib and Weekly Topotecan in Patients with Solid Tumors

Purpose: To determine the dose limiting toxicities (DLT), maximum tolerated dose (MTD), and recommended phase II dose (RP2D) of veliparib in combination with weekly topotecan in patients with solid tumors. Correlative studies were included to assess the impact of topotecan and veliparib on poly(ADP-ribose) levels in peripheral blood mononuclear cells, serum pharmacokinetics of both agents, and potential association of germline repair gene mutations with outcome.Experimental Design: Eligible patients had metastatic nonhematologic malignancies with measurable disease. Using a 3 + 3 design, patients were treated with veliparib orally twice daily on days 1-3, 8-10, and 15-17 and topotecan intravenously on days 2, 9, and 16 every 28 days. Tumor responses were assessed by RECIST.Results: Of 58 patients enrolled, 51 were evaluable for the primary endpoint. The MTD and RP2D was veliparib 300 mg twice daily on days 1-3, 8-10, and 15-17 along with topotecan 3 mg/m2 on days 2, 9, and 16 of a 28-day cycle. DLTs were grade 4 neutropenia lasting >5 days. The median number of cycles was 2 (1-26). The objective response rate was 10%, with 1 complete and 4 partial responses. Twenty-two patients (42%) had stable disease ranging from 4 to 26 cycles. Patients with germline BRCA1, BRCA2, or RAD51D mutations remained on study longer than those without homologous recombination repair (HRR) gene mutations (median 4 vs. 2 cycles).Conclusions: Weekly topotecan in combination with veliparib has a manageable safety profile and appears to warrant further investigation. Clin Cancer Res; 24(4); 744-52. ©2017 AACR.

Human Thymidylate Synthase Inhibitors Halting Ovarian Cancer Growth

Human thymidylate synthase (hTS) has an important role in DNA biosynthesis, thus it is essential for cell survival. TS is involved in the folate pathways, specifically in the de novo pyrimidine biosynthesis. Structure and functions are intimately correlated, account for cellular activity and, in a broader view, with in vivo mechanisms. hTS is a target for anticancer agents, some of which are clinical drugs. The understanding of the detailed mechanism of TS inhibition by currently used drugs and of the interaction with the mechanism of action of other anticancer agents can suggest new perspective of TS inhibition able to improve the anticancer effect and to overcome drug resistance. TS-targeting drugs in therapy today are inhibitors that bind at the active site and that mostly resemble the substrates. Nonsubstrate analogs offer an opportunity for allosteric binding and novel mode of inhibition in the cancer cells. This chapter illustrates the relationship among the large number of hTS actions at molecular and clinical levels, its role as a target for ovarian cancer therapy, in particular in cases of overexpression of hTS and other folate proteins such as those induced by platinum drug treatments, and address the potential combination of TS inhibitors with other suitable anticancer agents.

Loss of Uracil DNA Glycosylase Selectively Resensitizes p53-Mutant and -Deficient Cells to 5-FdU

Thymidylate synthase (TS) inhibitors including fluoropyrimidines [e.g., 5-Fluorouracil (5-FU) and 5-Fluorodeoxyuridine (5-FdU, floxuridine)] and antifolates (e.g., pemetrexed) are widely used against solid tumors. Previously, we reported that shRNA-mediated knockdown (KD) of uracil DNA glycosylase (UDG) sensitized cancer cells to 5-FdU. Because p53 has also been shown as a critical determinant of the sensitivity to TS inhibitors, we further interrogated 5-FdU cytotoxicity after UDG depletion with regard to p53 status. By analyzing a panel of human cancer cells with known p53 status, it was determined that p53-mutated or -deficient cells are highly resistant to 5-FdU. UDG depletion resensitizes 5-FdU in p53-mutant and -deficient cells, whereas p53 wild-type (WT) cells are not affected under similar conditions. Utilizing paired HCT116 p53 WT and p53 knockout (KO) cells, it was shown that loss of p53 improves cell survival after 5-FdU, and UDG depletion only significantly sensitizes p53 KO cells. This sensitization can also be recapitulated by UDG depletion in cells with p53 KD by shRNAs. In addition, sensitization is also observed with pemetrexed in p53 KO cells, but not with 5-FU, most likely due to RNA incorporation. Importantly, in p53 WT cells, the apoptosis pathway induced by 5-FdU is activated independent of UDG status. However, in p53 KO cells, apoptosis is compromised in UDG-expressing cells, but dramatically elevated in UDG-depleted cells. Collectively, these results provide evidence that loss of UDG catalyzes significant cell death signals only in cancer cells mutant or deficient in p53.Implications: This study reveals that UDG depletion restores sensitivity to TS inhibitors and has chemotherapeutic potential in the context of mutant or deficient p53. Mol Cancer Res; 16(2); 212-21. ©2017 AACR.

Fission yeast strains with circular chromosomes require the 9-1-1 checkpoint complex for the viability in response to the anti-cancer drug 5-fluorodeoxyuridine

Thymidine kinase converts 5-fluorodeoxyuridine to 5-fluorodeoxyuridine monophosphate, which causes disruption of deoxynucleotide triphosphate ratios. The fission yeast Schizosaccharomyces pombe does not express endogenous thymidine kinase but 5-fluorodeoxyuridine inhibits growth when exogenous thymidine kinase is expressed. Unexpectedly, we found that 5-fluorodeoxyuridine causes S phase arrest even without thymidine kinase expression. DNA damage checkpoint proteins such as the 9-1-1 complex were required for viability in the presence of 5-fluorodeoxyuridine. We also found that strains with circular chromosomes, due to loss of pot1+, which have higher levels of replication stress, were more sensitive to loss of the 9-1-1 complex in the presence of 5-fluorodeoxyuridine. Thus, our results suggest that strains carrying circular chromosomes exhibit a greater dependence on DNA damage checkpoints to ensure viability in the presence of 5-fluorodeoxyuridine compared to stains that have linear chromosomes.

The Inhibitory Effects of HYDAMTIQ, a Novel PARP Inhibitor, on Growth in Human Tumor Cell Lines With Defective DNA Damage Response Pathways

The poly(ADP-ribose) polymerase (PARP) enzymes play a key role in the regulation of cellular processes (e.g., DNA damage repair, genomic stability). It has been shown that PARP inhibitors (PARPIs) are selectively cytotoxic against cells having dysfunctions in genes involved in DNA repair mechanisms (synthetic lethality). Drug-induced PARP inhibition potentiates the activity of anticancer drugs such as 5-fluorouracil in enhancing DNA damage, whose repair involves PARP-1 activity. The aim of this study was to evaluate the inhibitory effects of a novel PARPI, HYDAMTIQ, on growth in human tumor cell lines characterized by different features with regard to DNA damage response pathways (BRCA mutational status, microsatellite status, and ATM expression level) and degree of sensitivity/resistance to 5-fluorouracil. HYDAMTIQ showed a more potent inhibitory effect on cell growth in a BRCA2 mutant cell line (CAPAN-1) compared with wild-type cells (C2-6, C2-12, and C2-14 CAPAN-1 clones, and MCF-7). No statistically significant difference was observed after HYDAMTIQ exposure between cells having a different MS status or a different MRE11 mutational status. HYDAMTIQ induced greater antiproliferative effects in SW620 cells expressing a low level of ATM than in H630 cells expressing a high level of ATM. Finally, the combination of HYDAMTIQ and 5-fluorouracil exerted a synergistic effect on the inhibition of SW620 cell growth and an antagonistic effect on that of H630 cell growth. Our results show that the novel PARP inhibitor HYDAMTIQ potently inhibits the growth of human tumor cells with defective DNA damage response pathways and exerts synergistic cytotoxicity in combination with 5-fluorouracil. These data provide relevant examples of synthetic lethality and evidence for further development of this novel PARPI.

XRCC1 mediated the development of cervival cancer through a novel Sp1/Krox-20 swich

Cervical cancer is the second leading cause of mortality among women. Impairment of the base excision repair (BER) pathway is one of the major causes of the initiation and progression of cervical cancer. However, whether the polymorphisms of the BER pathway components (i.e., HOGG1, XRCC1, ADPRT, and APE1) can affect the risk of cervical cancer remains unknown. Herein, we applied a hospital-based case-control study covering two independent cohorts and a subsequent functional assay to determine the roles of the single nucleotide polymorphisms (SNPs) of the BER pathway genes in cervical cancer. Results indicated that the XRCC1 rs3213245 (-77TC) TT genotype was associated with an increased risk of cervical cancer. The immunohistochemistry assay showed that XRCC1 protein expression levels were upregulated in cervical cancer patients with the XRCC1 rs3213245 CC genotype compared with the CT or TT genotypes. Further, results from ChIP assay showed that Sp1 could bind to the −77 site and that the rs3213245 C genotype promoted the binding of Sp1 to the XRCC1 promoter. Moreover, ChIP/Re-ChIP assays revealed that transcription factor Krox-20 was recruited to the XRCC1 rs3213245 mutation region and regulated the transcription of the XRCC1 gene by interacting with Sp1, ultimately mediated cervical cancer development. In summary, the findings indicated that the functional XRCC1 SNP rs3213245 was associated with the risk of cervical cancer based on the Sp1/Krox-20 switch.

Glycogen Synthase Kinase 3 (GSK-3)-mediated Phosphorylation of Uracil N-Glycosylase 2 (UNG2) Facilitates the Repair of Floxuridine-induced DNA Lesions and Promotes Cell Survival

Uracil N-glycosylase 2 (UNG2), the nuclear isoform of UNG, catalyzes the removal of uracil or 5-fluorouracil lesions that accumulate in DNA following treatment with the anticancer agents 5-fluorouracil and 5-fluorodeoxyuridine (floxuridine), a 5-fluorouracil metabolite. By repairing these DNA lesions before they can cause cell death, UNG2 promotes cancer cell survival and is therefore critically involved in tumor resistance to these agents. However, the mechanisms by which UNG2 is regulated remain unclear. Several phosphorylation sites within the N-terminal regulatory domain of UNG2 have been identified, although the effects of these modifications on UNG2 function have not been fully explored, nor have the identities of the kinases involved been determined. Here we show that glycogen synthase kinase 3 (GSK-3) interacts with and phosphorylates UNG2 at Thr⁶⁰ and that Thr⁶⁰ phosphorylation requires a Ser⁶⁴ priming phosphorylation event. We also show that mutating Thr⁶⁰ or Ser⁶⁴ to Ala increases the half-life of UNG2, reduces the rate of in vitro uracil excision, and slows UNG2 dissociation from chromatin after DNA replication. Using an UNG2-deficient ovarian cancer cell line that is hypersensitive to floxuridine, we show that GSK-3 phosphorylation facilitates UNG2-dependent repair of floxuridine-induced DNA lesions and promotes tumor cell survival following exposure to this agent. These data suggest that GSK-3 regulates UNG2 and promotes DNA damage repair.

Preclinical evaluation of olaparib and metformin combination in BRCA1 wildtype ovarian cancer

OBJECTIVES

BRCA mutated ovarian cancers show increased responsiveness to PARP inhibitors. PARP inhibitors target DNA repair and provide a second hit to BRCA mutated tumors, resulting in "synthetic lethality". We investigated a combination of metformin and olaparib to provide "synthetic lethality" in BRCA intact ovarian cancer cells.

METHODS

Ovarian cancer cell lines (UWB1.289, UWB1.289.BRCA, SKOV3, OVCAR5, A2780 and C200) were treated with a combination of metformin and olaparib. Cell viability was assessed by MTT and colony formation assays. Flow cytometry was used to detect cell cycle events. In vivo studies were performed in SKOV3 or A2780 xenografts in nude mice. Animals were treated with single agent, metformin or olaparib or combination. Molecular downstream effects were examined by immunohistochemistry.

RESULTS

Compared to single drug treatment, combination of olaparib and metformin resulted in significant reduction of cell proliferation and colony formation (p<0.001) in ovarian cancer cells. This treatment was associated with a significant S-phase cell cycle arrest (p<0.05). Combination of olaparib and metformin significantly inhibited SKOV3 and A2780 ovarian tumor xenografts which were accompanied with decreased Ki-index (p<0.001). Metformin did not affect DNA damage signaling, while olaparib induced adenosine monophosphate activated kinase activation; that was further potentiated with metformin combination in vivo.

CONCLUSION

Combining PARP inhibitors with metformin enhances its anti-proliferative activity in BRCA mutant ovarian cancer cells. Furthermore, the combination showed significant activity in BRCA intact cancer cells in vitro and in vivo. This is a promising treatment regimen for women with epithelial ovarian cancer irrespective of BRCA status.

Evaluation of rucaparib and companion diagnostics in the PARP inhibitor landscape for recurrent ovarian cancer therapy

Rucaparib camsylate (CO-338; 8-fluoro-2-{4-[(methylamino)methyl]phenyl}-1,3,4,5-tetrahydro-6H-azepino[5,4,3-cd]indol-6-one ((1S,4R)-7,7-dimethyl-2-oxobicyclo[2.2.1]hept-1-yl)methanesulfonic acid salt) is a PARP1, 2 and 3 inhibitor. Phase I studies identified a recommended Phase II dose of 600 mg orally twice daily. ARIEL2 Part 1 established a tumor genomic profiling test for homologous recombination loss of heterozygosity quantification using a next-generation sequencing companion diagnostic (CDx). Rucaparib received US FDA Breakthrough Therapy designation for treatment of platinum-sensitive BRCA-mutated advanced ovarian cancer patients who received greater than two lines of platinum-based therapy. Comparable to rucaparib development, other PARP inhibitors, such as olaparib, niraparib, veliparib and talazoparib, are developing CDx tests for targeted therapy. PARP inhibitor clinical trials and CDx assays are discussed in this review, as are potential PARP inhibitor combination therapies and likely resistance mechanisms.

Effects of Anticancer Drugs on Chromosome Instability and New Clinical Implications for Tumor-Suppressing Therapies

Whole chromosomal instability (CIN), manifested as unequal chromosome distribution during cell division, is a distinguishing feature of most cancer types. CIN is generally considered to drive tumorigenesis, but a threshold level exists whereby further increases in CIN frequency in fact hinder tumor growth. While this attribute is appealing for therapeutic exploitation, drugs that increase CIN beyond this therapeutic threshold are currently limited. In our previous work, we developed a quantitative assay for measuring CIN based on the use of a nonessential human artificial chromosome (HAC) carrying a constitutively expressed EGFP transgene. Here, we used this assay to rank 62 different anticancer drugs with respect to their effects on chromosome transmission fidelity. Drugs with various mechanisms of action, such as antimicrotubule activity, histone deacetylase inhibition, mitotic checkpoint inhibition, and targeting of DNA replication and damage responses, were included in the analysis. Ranking of the drugs based on their ability to induce HAC loss revealed that paclitaxel, gemcitabine, dactylolide, LMP400, talazoparib, olaparib, peloruside A, GW843682, VX-680, and cisplatin were the top 10 drugs demonstrating HAC loss at a high frequency. Therefore, identification of currently used compounds that greatly increase chromosome mis-segregation rates should expedite the development of new therapeutic strategies to target and leverage the CIN phenotype in cancer cells.

Genomically Incorporated 5-Fluorouracil that Escapes UNG-Initiated Base Excision Repair Blocks DNA Replication and Activates Homologous Recombination

5-Fluorouracil (5-FU) and its metabolite 5-fluorodeoxyuridine (FdUrd, floxuridine) are chemotherapy agents that are converted to 5-fluorodeoxyuridine monophosphate (FdUMP) and 5-fluorodeoxyuridine triphosphate (FdUTP). FdUMP inhibits thymidylate synthase and causes the accumulation of uracil in the genome, whereas FdUTP is incorporated by DNA polymerases as 5-FU in the genome; however, it remains unclear how either genomically incorporated U or 5-FU contributes to killing. We show that depletion of the uracil DNA glycosylase (UNG) sensitizes tumor cells to FdUrd. Furthermore, we show that UNG depletion does not sensitize cells to the thymidylate synthase inhibitor (raltitrexed), which induces uracil but not 5-FU accumulation, thus indicating that genomically incorporated 5-FU plays a major role in the antineoplastic effects of FdUrd. We also show that 5-FU metabolites do not block the first round of DNA synthesis but instead arrest cells at the G1/S border when cells again attempt replication and activate homologous recombination (HR). This arrest is not due to 5-FU lesions blocking DNA polymerase δ but instead depends, in part, on the thymine DNA glycosylase. Consistent with the activation of HR repair, disruption of HR sensitized cells to FdUrd, especially when UNG was disabled. These results show that 5-FU lesions that escape UNG repair activate HR, which promotes cell survival.

Hyperthermia combined with 5-fluorouracil promoted apoptosis and enhanced thermotolerance in human gastric cancer cell line SGC-7901

This study was designed to investigate the proliferation inhibition and apoptosis-promoting effect under hyperthermia and chemotherapy treatment, at cellular level. Human gastric cancer cell line SGC-7901 was cultivated with 5-fluorouracil at different temperatures. Cell proliferation and apoptosis were determined, and expression of Bcl-2 and HSP70 was measured at different treatments. Cell survival rates and inhibition rates in chemotherapy group, thermotherapy group, and thermo-chemotherapy group were drastically lower than the control group (P<0.05). For tumor cells in the thermo-chemotherapy group, survival rates and inhibition rates at three different temperatures were all significantly lower than those in chemotherapy group and thermotherapy group (P<0.05). 5-Fluorouracil induced apoptosis of SGC-7901 cells with a strong temperature dependence, which increased gradually with increase in temperature. At 37°C and 43°C there were significant differences between the thermotherapy group and chemotherapy group and between the thermo-chemotherapy group and thermotherapy group (P<0.01). The expression of Bcl-2 was downregulated and HSP70 was upregulated, with increase in temperature in all groups. Cell apoptosis was not significant at 46°C (P>0.05), which was probably due to thermotolerance caused by HSP70 accumulation. These results suggested that hyperthermia combined with 5-fluorouracil had a synergistic effect in promoting apoptosis and enhancing thermotolerance in gastric cancer cell line SGC-7901.

The Quest for an Effective Treatment for an Intractable Cancer: Established and Novel Therapies for Pancreatic Adenocarcinoma

With therapies that date back to the 1950s, and few newly approved treatments in the last 20 years, pancreatic cancer remains a significant challenge for the development of novel therapeutics. Current regimens have successfully extended patient survival, although they still lead to prognoses measured in months rather than years. The genetic diversity inherent in pancreatic tumors forms the roadblocks that must be overcome in future therapeutics. Recent insight into the genetic patterns found in tumor cells may provide clues leading to better understanding of the challenges hindering the development of treatments. Here, we review currently used drugs and established combination therapies that comprise the standard of care for a highly recalcitrant disease. Novel approaches can improve upon current therapies in a variety of ways. Enhancing specificity, such that growth inhibition and cytotoxic effects act preferentially on tumor cells, is one approach to advance treatments. This can be accomplished through the targeting of extracellular markers specific to cancer cells. Additionally, enlisting natural defenses and overcoming tumor-driven immune suppression could prove to be a useful tactic. Recent studies utilizing these approaches have yielded promising results and could contribute to an ongoing effort battling a particularly difficult cancer.

Poly (ADP-ribose) polymerase inhibitors: recent advances and future development

Poly (ADP-ribose) polymerase (PARP) inhibitors have shown promising activity in epithelial ovarian cancers, especially relapsed platinum-sensitive high-grade serous disease. Consistent with preclinical studies, ovarian cancers and a number of other solid tumor types occurring in patients with deleterious germline mutations in BRCA1 or BRCA2 seem to be particularly sensitive. However, it is also becoming clear that germline BRCA1/2 mutations are neither necessary nor sufficient for patients to derive benefit from PARP inhibitors. We provide an update on PARP inhibitor clinical development, describe recent advances in our understanding of PARP inhibitor mechanism of action, and discuss current issues in the development of these agents.

Poly(ADP-ribose) polymerase inhibitors sensitize cancer cells to death receptor-mediated apoptosis by enhancing death receptor expression

Recombinant human tumor necrosis factor-α-related apoptosis inducing ligand (TRAIL), agonistic monoclonal antibodies to TRAIL receptors, and small molecule TRAIL receptor agonists are in various stages of preclinical and early phase clinical testing as potential anticancer drugs. Accordingly, there is substantial interest in understanding factors that affect sensitivity to these agents. In the present study we observed that the poly(ADP-ribose) polymerase (PARP) inhibitors olaparib and veliparib sensitize the myeloid leukemia cell lines ML-1 and K562, the ovarian cancer line PEO1, non-small cell lung cancer line A549, and a majority of clinical AML isolates, but not normal marrow, to TRAIL. Further analysis demonstrated that PARP inhibitor treatment results in activation of the FAS and TNFRSF10B (death receptor 5 (DR5)) promoters, increased Fas and DR5 mRNA, and elevated cell surface expression of these receptors in sensitized cells. Chromatin immunoprecipitation demonstrated enhanced binding of the transcription factor Sp1 to the TNFRSF10B promoter in the presence of PARP inhibitor. Knockdown of PARP1 or PARP2 (but not PARP3 and PARP4) not only increased expression of Fas and DR5 at the mRNA and protein level, but also recapitulated the sensitizing effects of the PARP inhibition. Conversely, Sp1 knockdown diminished the PARP inhibitor effects. In view of the fact that TRAIL is part of the armamentarium of natural killer cells, these observations identify a new facet of PARP inhibitor action while simultaneously providing the mechanistic underpinnings of a novel therapeutic combination that warrants further investigation.

Standing the test of time: targeting thymidylate biosynthesis in cancer therapy

Over the past 60 years, chemotherapeutic agents that target thymidylate biosynthesis and the enzyme thymidylate synthase (TS) have remained among the most-successful drugs used in the treatment of cancer. Fluoropyrimidines, such as 5-fluorouracil and capecitabine, and antifolates, such as methotrexate and pemetrexed, induce a state of thymidylate deficiency and imbalances in the nucleotide pool that impair DNA replication and repair. TS-targeted agents are used to treat numerous solid and haematological malignancies, either alone or as foundational therapeutics in combination treatment regimens. We overview the pivotal discoveries that led to the rational development of thymidylate biosynthesis as a chemotherapeutic target, and highlight the crucial contribution of these advances to driving and accelerating drug development in the earliest era of cancer chemotherapy. The function of TS as well as the mechanisms and consequences of inhibition of this enzyme by structurally diverse classes of drugs with distinct mechanisms of action are also discussed. In addition, breakthroughs relating to TS-targeted therapies that transformed the clinical landscape in some of the most-difficult-to-treat cancers, such as pancreatic, colorectal and non-small-cell lung cancer, are highlighted. Finally, new therapeutic agents and novel mechanism-based strategies that promise to further exploit the vulnerabilities and target resistance mechanisms within the thymidylate biosynthesis pathway are reviewed.

Evaluation of poly (ADP-ribose) polymerase inhibitor ABT-888 combined with radiotherapy and temozolomide in glioblastoma

BACKGROUND

The cytotoxicity of radiotherapy and chemotherapy can be enhanced by modulating DNA repair. PARP is a family of enzymes required for an efficient base-excision repair of DNA single-strand breaks and inhibition of PARP can prevent the repair of these lesions. The current study investigates the trimodal combination of ABT-888, a potent inhibitor of PARP1-2, ionizing radiation and temozolomide(TMZ)-based chemotherapy in glioblastoma (GBM) cells.

METHODS

Four human GBM cell lines were treated for 5 h with 5 μM ABT-888 before being exposed to X-rays concurrently with TMZ at doses of 5 or 10 μM for 2 h. ABT-888's PARP inhibition was measured using immunodetection of poly(ADP-ribose) (pADPr). Cell survival and the different cell death pathways were examined via clonogenic assay and morphological characterization of the cell and cell nucleus.

RESULTS

Combining ABT-888 with radiation yielded enhanced cell killing in all four cell lines, as demonstrated by a sensitizer enhancement ratio at 50% survival (SER50) ranging between 1.12 and 1.37. Radio- and chemo-sensitization was further enhanced when ABT-888 was combined with both X-rays and TMZ in the O6-methylguanine-DNA-methyltransferase (MGMT)-methylated cell lines with a SER50 up to 1.44. This effect was also measured in one of the MGMT-unmethylated cell lines with a SER50 value of 1.30. Apoptosis induction by ABT-888, TMZ and X-rays was also considered and the effect of ABT-888 on the number of apoptotic cells was noticeable at later time points. In addition, this work showed that ABT-888 mediated sensitization is replication dependent, thus demonstrating that this effect might be more pronounced in tumour cells in which endogenous replication lesions are present in a larger proportion than in normal cells.

CONCLUSIONS

This study suggests that ABT-888 has the clinical potential to enhance the current standard treatment for GBM, in combination with conventional chemo-radiotherapy. Interestingly, our results suggest that the use of PARP inhibitors might be clinically significant in those patients whose tumour is MGMT-unmethylated and currently derive less benefit from TMZ.

MicroRNA-18a attenuates DNA damage repair through suppressing the expression of ataxia telangiectasia mutated in colorectal cancer

Background

miR-18a is one of the most up-regulated miRNAs in colorectal cancers (CRC) based on miRNA profiling. In this study, we examined the functional significance of miR-18a in CRC.

Methods

Expression of miR-18a was investigated in 45 CRC patients. Potential target genes of miR-18a were predicted by in silico search and confirmed by luciferase activity assay and Western blot. DNA damage was measured by comet assay. Gene function was measured by cell viability, colony formation and apoptosis assays.

Results

The up-regulation of miR-18a was validated and confirmed in 45 primary CRC tumors compared with adjacent normal tissues (p<0.0001). Through in silico search, the 3′UTR of Ataxia telangiectasia mutated (ATM) contains a conserved miR-18a binding site. Expression of ATM was down-regulated in CRC tumors (p<0.0001) and inversely correlated with miR-18a expression (r = -0.4562, p<0.01). Over-expression of miR-18a in colon cancer cells significantly reduced the luciferase activity of the construct with wild-type ATM 3′UTR but not that with mutant ATM 3′UTR, inferring a direct interaction of miR-18a with ATM 3′UTR. This was further confirmed by the down-regulation of ATM protein by miR-18a. As ATM is a key enzyme in DNA damage repair, we evaluated the effect of miR-18a on DNA double-strand breaks. Ectopic expression of miR-18a significantly inhibited the repair of DNA damage induced by etoposide (p<0.001), leading to accumulation of DNA damage, increase in cell apoptosis and poor clonogenic survival.

Conclusion

miR-18a attenuates cellular repair of DNA double-strand breaks by directly suppressing ATM, a key enzyme in DNA damage repair.

Poly(ADP-ribose) polymerase 1 inhibition protects against low shear stress induced inflammation

BACKGROUND

Atherosclerosis begins as local inflammation of vessels at sites of disturbed flow, where low shear stress (LSS) leads to mechanical irritation and plaque development and progression. Nuclear enzyme poly(ADP-ribose) polymerase 1 (PARP-1) is associated with the inflammation response during atherosclerosis. We investigated the role and underlying mechanism of PARP-1 in LSS-induced inflammation in human umbilical vein endothelial cells (HUVECs).

METHODS AND RESULTS

HUVECs were simulated by LSS (0.4Pa). PARP-1 expression was inhibited by ABT888 or siRNA. The inducible nitric oxide synthase (iNOS) and intercellular adhesion molecular-1 (ICAM-1) expression was regulated by LSS in a time dependent manner. LSS could increase superoxide production and 3-nitrotyrosine formation. LSS induced DNA damage as assessed by H2A.X phosphorylation and comet assay. Compared with cells under static, LSS increased PARP-1 expression and PAR formation via MEK/ERK signaling pathway. PARP-1 inhibition increased Sirt1 activity through an increased intracellular nicotinamide adenine dinucleotide (NAD(+)) level. Moreover, PARP-1 inhibition attenuated LSS-induced iNOS and ICAM-1 upregulation by inhibiting nuclear factor kappa B (NF-κB) nuclear translocation and activity, with a reduced NF-κB phosphorylation.

CONCLUSIONS

LSS induced oxidative damage and PARP-1 activation via MEK/ERK pathway. PARP-1 inhibition restored Sirt1 activity by increasing NAD(+) level and decreased iNOS and ICAM-1 expression by inhibiting NF-κB nuclear translocation and activity as well as NF-κB phosphorylation. PARP-1 played a fundamental role in LSS induced inflammation. Inhibition of PARP-1 might be a mechanism for treatment of inflammation response during atherosclerosis.

Identification of DNA repair pathways that affect the survival of ovarian cancer cells treated with a poly(ADP-ribose) polymerase inhibitor in a novel drug combination

Floxuridine (5-fluorodeoxyuridine, FdUrd), a U.S. Food and Drug Administration-approved drug and metabolite of 5-fluorouracil, causes DNA damage that is repaired by base excision repair (BER). Thus, poly(ADP-ribose) polymerase (PARP) inhibitors, which disrupt BER, markedly sensitize ovarian cancer cells to FdUrd, suggesting that this combination may have activity in this disease. It remains unclear, however, which DNA repair and checkpoint signaling pathways affect killing by these agents individually and in combination. Here we show that depleting ATR, BRCA1, BRCA2, or RAD51 sensitized to ABT-888 (veliparib) alone, FdUrd alone, and FdUrd + ABT-888 (F+A), suggesting that homologous recombination (HR) repair protects cells exposed to these agents. In contrast, disabling the mismatch, nucleotide excision, Fanconi anemia, nonhomologous end joining, or translesion synthesis repair pathways did not sensitize to these agents alone (including ABT-888) or in combination. Further studies demonstrated that in BRCA1-depleted cells, F+A was more effective than other chemotherapy+ABT-888 combinations. Taken together, these studies 1) identify DNA repair and checkpoint pathways that are important in ovarian cancer cells treated with FdUrd, ABT-888, and F+A, 2) show that disabling HR at the level of ATR, BRCA1, BRCA2, or RAD51, but not Chk1, ATM, PTEN, or FANCD2, sensitizes cells to ABT-888, and 3) demonstrate that even though ABT-888 sensitizes ovarian tumor cells with functional HR to FdUrd, the effects of this drug combination are more profound in tumors with HR defects, even compared with other chemotherapy + ABT-888 combinations, including cisplatin + ABT-888.

Association between polymorphisms of XRCC1 and ADPRT genes and ovarian cancer survival with platinum-based chemotherapy in Chinese population

The role of DNA repair gene polymorphisms in cancer development, progression, and response to treatment has received increased attention. We conducted a prospective study to determine whether associations exist between two polymorphisms in XRCC1 and ADPRT and the outcomes of Chinese ovarian cancer patients treated with platinum-based chemotherapy. A total of 335 new cases of ovarian cancer were consecutively collected between May 2005 and May 2007. Follow-up lasted for 4 years, and the outcome measure was survival time. Individuals carrying XRCC1 194Trp/Trp had a longer survival time than did those with the Arg/Arg genotype. Similarly, those carrying XRCC1 399 Gln/Gln genotypes had 0.44-fold the risk of death than those with the Arg/Arg genotype. The combination of XRCC1 194 Trp allele and 399 Gln allele could decrease the death risk of ovarian cancer. In summary, this study is the first to evaluate the associations between polymorphisms in DNA repair gene polymorphism and the risk of ovarian cancer in Chinese population. Our study found a significant association between XRCC1 Arg399Gln and XRCC1 Arg194Trp polymorphism and the clinical outcome of ovarian cancer. Furthermore, studies with larger sample sizes are still needed to confirm these associations in Chinese population.

HER2 overexpression renders human breast cancers sensitive to PARP inhibition independently of any defect in homologous recombination DNA repair

HER2 overexpression in breast cancer confers increased tumor aggressiveness. Although anti-HER2 therapies have improved patient outcome, resistance ultimately occurs. PARP inhibitors target homologous recombination (HR)-deficient tumors, such as the BRCA-associated breast and ovarian cancers. In this study, we show that HER2+ breast cancers are susceptible to PARP inhibition independent of an HR deficiency. HER2 overexpression in HER2 negative breast cancer cells was sufficient to render cells susceptible to the PARP inhibitors ABT-888 and AZD-2281 both in vitro and in vivo, which was abrogated by HER2 reduction. In addition, ABT-888 significantly inhibited NF-κB (p65/RelA) transcriptional activity in HER2+ but not HER2 negative breast cancer cells. This corresponded with a reduction in phosphorylated p65 and total IKKα levels, with a concomitant increase in IκBα. Overexpression of p65 abrogated cellular sensitivity to ABT-888, whereas IκBα overexpression reduced cell viability to a similar extent as ABT-888. Therefore, susceptibility of HER2+ breast cancer cells to PARP inhibition may be because of inhibition of NF-κB signaling driven by HER2. Our findings indicate that PARP inhibitors may be a novel therapeutic strategy for sporadic HER2+ breast cancer patients.

Failure of iniparib to inhibit poly(ADP-Ribose) polymerase in vitro

PURPOSE

Poly(ADP-ribose) polymerase (PARP) inhibitors are undergoing extensive clinical testing for their single-agent activity in homologous recombination (HR)-deficient tumors and ability to enhance the action of certain DNA-damaging agents. Compared with other PARP inhibitors in development, iniparib (4-iodo-3-nitrobenzamide) is notable for its simple structure and the reported ability of its intracellular metabolite 4-iodo-3-nitrosobenzamide to covalently inhibit PARP1 under cell-free conditions. The present preclinical studies were conducted to compare the actions iniparib with the more extensively characterized PARP inhibitors olaparib and veliparib.

EXPERIMENTAL DESIGN

The abilities of iniparib, olaparib, and veliparib to (i) selectively induce apoptosis or inhibit colony formation in HR-deficient cell lines, (ii) selectively sensitize HR-proficient cells to topoisomerase I poisons, and (iii) inhibit formation of poly(ADP-ribose) polymer (pADPr) in intact cells were compared.

RESULTS

Consistent with earlier reports, olaparib and veliparib selectively induced apoptosis and inhibited colony formation in cells lacking BRCA2 or ATM. Moreover, like earlier generation PARP inhibitors, olaparib and veliparib sensitized cells to the topoisomerase I poisons camptothecin and topotecan. Finally, olaparib and veliparib inhibited formation of pADPr in intact cells. In contrast, iniparib exhibited little or no ability to selectively kill HR-deficient cells, sensitize cells to topoisomerase I poisons, or inhibit pADPr formation in situ. In further experiments, iniparib also failed to sensitize cells to cisplatin, gemcitabine, or paclitaxel.

CONCLUSIONS

While iniparib kills normal and neoplastic cells at high (>40 μmol/L) concentrations, its effects are unlikely to reflect PARP inhibition and should not be used to guide decisions about other PARP inhibitors.

Role of PARP inhibitors in cancer biology and therapy

Deeper understanding of DNA repair mechanisms and their potential value as therapeutic targets in oncology heralded the clinical development of poly(ADP-ribose) polymerase (PARP) inhibitors. Although initially developed to exploit synthetic lethality in models of cancer associated with defective DNA repair, our burgeoning knowledge of PARP biology has resulted in these agents being exploited both in cancer with select chemotherapeutic agents and in non-malignant diseases. In this review article, we briefly review the mechanisms of DNA repair and pre-clinical development of PARP inhibitors before discussing the clinical development of the various PARP inhibitors in depth.

Checkpoint signaling, base excision repair, and PARP promote survival of colon cancer cells treated with 5-fluorodeoxyuridine but not 5-fluorouracil

The fluoropyrimidines 5-fluorouracil (5-FU) and FdUrd (5-fluorodeoxyuridine; floxuridine) are the backbone of chemotherapy regimens for colon cancer and other tumors. Despite their widespread use, it remains unclear how these agents kill tumor cells. Here, we have analyzed the checkpoint and DNA repair pathways that affect colon tumor responses to 5-FU and FdUrd. These studies demonstrate that both FdUrd and 5-FU activate the ATR and ATM checkpoint signaling pathways, indicating that they cause genotoxic damage. Notably, however, depletion of ATM or ATR does not sensitize colon cancer cells to 5-FU, whereas these checkpoint pathways promote the survival of cells treated with FdUrd, suggesting that FdUrd exerts cytotoxicity by disrupting DNA replication and/or inducing DNA damage, whereas 5-FU does not. We also found that disabling the base excision (BER) repair pathway by depleting XRCC1 or APE1 sensitized colon cancer cells to FdUrd but not 5-FU. Consistent with a role for the BER pathway, we show that small molecule poly(ADP-ribose) polymerase 1/2 (PARP) inhibitors, AZD2281 and ABT-888, remarkably sensitized both mismatch repair (MMR)-proficient and -deficient colon cancer cell lines to FdUrd but not to 5-FU. Taken together, these studies demonstrate that the roles of genotoxin-induced checkpoint signaling and DNA repair differ significantly for these agents and also suggest a novel approach to colon cancer therapy in which FdUrd is combined with a small molecule PARP inhibitor.