Selective targeting of homologous DNA recombination repair by gemcitabine

New and emerging radiosensitizers and radioprotectors

The combination of chemotherapy and radiation has led to clinical breakthroughs in several disease sites, and current work continues to define optimum combinations of proven chemotherapy as well as more recently available, noncytotoxic agents. Administration of systemic therapies allows modulation of radiation response to improve tumor control (radiosensitization) or to prevent normal tissue toxicity (radioprotection). Substantial progress has been made in identifying the targets of standard chemotherapeutic radiation sensitizers and protectors as well as in the introduction of a new generation of molecularly targeted therapies in combination with radiation. We have reviewed the most recent, predominantly early phase clinical trials combining systemic agents with radiation. Although the proof of an improved schedule ultimately needs to come from well-run Phase III trials, the search among schedules could be shortened by the use of surrogate endpoints such as presence of active drug metabolites in the tumor. This has been accomplished only in a few cases and needs to become a more standard part of radiation sensitizer and protector trials.

The relation between deoxycytidine kinase activity and the radiosensitising effect of gemcitabine in eight different human tumour cell lines

Background

Gemcitabine (dFdC) is an active antitumour agent with radiosensitising properties, shown both in preclinical and clinical studies. In the present study, the relation between deoxycytidine kinase (dCK) activity and the radiosensitising effect of gemcitabine was investigated in eight different human tumour cell lines.

Methods

Tumour cells were treated with dFdC (0–100 nM) for 24 h prior to radiotherapy (RT) (γ-Co⁶⁰, 0–6 Gy, room temperature). Cell survival was determined 7, 8, or 9 days after RT by the sulforhodamine B test. dCK activity of the cells was determined by an enzyme activity assay.

Results

A clear concentration-dependent radiosensitising effect of dFdC was observed in all cell lines. The degree of radiosensitisation was also cell line dependent and seemed to correlate with the sensitivity of the cell line to the cytotoxic effect of dFdC. The dCK activity of our cell lines varied considerably and differed up to three fold from 5 to 15 pmol/h/mg protein between the tested cell lines. In this range dCK activity was only weakly related to radiosensitisation (correlation coefficient 0.62, p = 0.11).

Conclusion

Gemcitabine needs to be metabolised to the active nucleotide in order to radiosensitise the cells. Since dFdCTP accumulation and incorporation into DNA are concentration dependent, the degree of radiosensitisation seems to be related to the extent of dFdCTP incorporated into DNA required to inhibit DNA repair. The activity of dCK does not seem to be the most important factor, but is clearly a major factor. Other partners of the intracellular metabolism of gemcitabine in relation to the cell cycle effects and DNA repair could be more responsible for the radiosensitising effect than dCK activity.

Single nucleotide polymorphisms of RecQ1, RAD54L, and ATM genes are associated with reduced survival of pancreatic cancer

PURPOSE

Our goal was to determine whether single nucleotide polymorphisms (SNPs) in DNA repair genes influence the clinical outcome of pancreatic cancer.

PATIENTS AND METHODS

We evaluated 13 SNPs of eight DNA damage response and repair genes in 92 patients with potentially resectable pancreatic adenocarcinoma. All patients were treated with neoadjuvant concurrent gemcitabine and radiotherapy with or without a component of induction gemcitabine/cisplatin at The University of Texas M.D. Anderson Cancer Center (Houston, TX) from February 1999 to August 2004 and observed through August 2005. Response to the pretreatment was assessed by evaluating time to tumor progression and overall survival. Kaplan-Meier plot, log-rank test, and Cox regression were used to compare survival of patients according to genotype.

RESULTS

The RecQ1 A159C, RAD54L C157T, XRCC1 R194W, and ATM T77C genotypes had a significant effect on the overall survival with log-rank P values of .001, .004, .001, and .02, respectively. A strong combined effect of the four genotypes was observed. Patients with none of the adverse genotypes had a mean survival time of 62.1 months, and those with one, two, or three or more at-risk alleles had median survival times of 27.5, 14.4, and 9.9 months, respectively (log-rank P < .001). There is a significant interaction between the RecQ1 gene and other genotypes. All four genes except XRCC1 remained as independent predictors of survival in multivariate Cox regression models adjusted for other clinical predictors.

CONCLUSION

These observations support the hypothesis that polymorphic variants of DNA repair genes affect clinical prognosis of patients with pancreatic cancer.

Tumor radiosensitizers--current status of development of various approaches: report of an International Atomic Energy Agency meeting

PURPOSE

The International Atomic Energy Agency (IAEA) held a Technical Meeting of Consultants to (1) discuss a selection of relatively new agents, not those well-established in clinical practice, that operated through a variety of mechanisms to sensitize tumors to radiation and (2) to compare and contrast their tumor efficacy, normal tissue toxicity, and status of development regarding clinical application. The aim was to advise the IAEA as to which developing agent or class of agents would be worth promoting further, by supporting additional laboratory research or clinical trials, with the eventual goal of improving cancer control rates using radiotherapy, in developing countries in particular.

RESULTS

The agents under discussion included a wide, but not complete, range of different types of drugs, and antibodies that interfered with molecules in cell signaling pathways. These were contrasted with new molecular antisense and gene therapy strategies. All the drugs discussed have previously been shown to act as tumor cell radiosensitizers or to kill hypoxic cells present in tumors.

CONCLUSION

Specific recommendations were made for more preclinical studies with certain of the agents and for clinical trials that would be suitable for industrialized countries, as well as trials that were considered more appropriate for developing countries.

Significant effect of homologous recombination DNA repair gene polymorphisms on pancreatic cancer survival

Genetic variation in DNA repair may affect the clinical response to cytotoxic therapies. We investigated the effect of six single nucleotide polymorphisms of the RecQ1, RAD54L, XRCC2, and XRCC3 genes on overall survival of 378 patients with pancreatic adenocarcinoma who were treated at University of Texas M.D. Anderson Cancer Center during February 1999 to October 2004 and were followed up to October 2005. Genotypes were determined using the MassCode method. Survival was determined from pathologic diagnosis to death. Patients who were alive at the last follow-up evaluation were censored at that time. Kaplan-Meier plot, log-rank test, and Cox regression were used to compare overall survival by genotypes. A significant effect on survival of all patients was observed for RecQ1 and RAD54L genes. The median survival time was 19.2, 14.7, and 13.2 months for the RecQ1 159 AA, AC, and CC genotypes, and 16.4, 13.3, and 10.3 months for RAD54L 157 CC, CT, and TT genotypes, respectively. A significantly reduced survival was associated with the variant alleles of XRCC2 R188H and XRCC3 A17893G in subgroup analysis. When the four genes were analyzed in combination, an increasing number of adverse alleles were associated with a significantly decreased survival. Subgroup analyses have shown that the genotype effect on survival was present among patients without metastatic disease or among patients who receive radiotherapy. These observations suggest that polymorphisms of genes involved in the repair of DNA double-strand breaks significantly affect the clinical outcome of patients with pancreatic cancer.

ERCC1, hRad51, and BRCA1 protein expression in relation to tumour response and survival of stage III/IV NSCLC patients treated with chemotherapy

Aim of this explorative study was to determine the prognostic value of protein expression of the DNA damage repair enzymes ERCC1, hRad51, and BRCA1 for tumour response and survival of non-small-cell lung cancer patients treated with chemotherapy. Patients with either a short or long overall survival were selected from a randomized phase III trial comparing cisplatin-gemcitabine and epirubicin-gemcitabine. Tumour biopsies were assessed for differences in immunohistochemical staining using antibodies against ERCC1, hRad51, and BRCA1. A total of 33 patients were included. A positive nuclear staining for ERCC1, hRad51, and BRCA1 was observed in 44, 12, and 90% of biopsies, respectively. In large cell carcinoma nuclear hRad51 staining was absent. In five biopsies stained for hRad51 an unexpected membrane-like staining was observed; these biopsies showed no nuclear staining. DNA damage repair protein expressions were not significantly different in responders versus non-responders, or in patients with a short or long overall survival. In conclusion, immunohistochemical staining of ERCC1, hRad51, and BRCA1, in tumour biopsies from non-small-cell lung cancer patients was not predictive for tumour response and survival after chemotherapy.

Mismatch repair proficiency is not required for radioenhancement by gemcitabine

PURPOSE

Mismatch repair (MMR) proficiency has been reported to either increase or decrease radioenhancement by 24-h incubations with gemcitabine. This study aimed to establish the importance of MMR for radioenhancement by gemcitabine after short-exposure, high-dose treatment and long-exposure, low-dose treatment.

METHODS AND MATERIALS

Survival of MMR-deficient HCT116 and MMR-proficient HCT116 + 3 cells was analyzed by clonogenic assays. Mild, equitoxic gemcitabine treatments (4 h, 0.1 microM vs. 24 h, 6 nM) were combined with gamma-irradiation to determine the radioenhancement with or without recovery. Gemcitabine metabolism and cell-cycle effects were evaluated by high-performance liquid chromatography analysis and bivariate flow cytometry.

RESULTS

Radioenhancement after 4 h of 0.1 microM of gemcitabine was similar in both cell lines, but the radioenhancement after 24 h of 6 nM of gemcitabine was reduced in MMR-proficient cells. No significant differences between both cell lines were observed in the gemcitabine metabolism or cell-cycle effects after these treatments. Gemcitabine radioenhancement after recovery was also lower in MMR-proficient cells than in MMR-deficient cells.

CONCLUSION

Mismatch repair proficiency decreases radioenhancement by long incubations of gemcitabine but does not affect radioenhancement by short exposures to a clinically relevant gemcitabine dose. Our data suggest that MMR contributes to the recovery from gemcitabine treatment.

Preoperative Concomitant Hyperfractionated Radiotherapy and Gemcitabine for Locally Advanced Rectal Cancers

PURPOSE

The purpose of this study was to determine the maximum tolerated dose of gemcitabine when it was administered concomitantly with hyperfractionated radiotherapy before surgery in patients with locally advanced rectal cancers and to investigate the midterm efficacy of such a regimen.

PATIENTS AND METHODS

Thirty-seven patients with stage II-III tumors as assessed by computed tomography/echoendoscopy were enrolled. Radiotherapy consisted of 50 Gy given in two daily fractions of 1.25 Gy over 4 weeks. The starting dose of gemcitabine was 10 mg/m(2)/day (in a 30-minute i.v. perfusion) twice weekly with planned escalation steps of 5 mg/m(2)/day. Surgery was planned at 6 weeks after the end of radiotherapy. Main end-points of the study were complete pathological tumor response, the rate of clear margin resection, and actuarial locoregional control and disease-free survival. The median follow-up for all patients was 32 months (range: 10-51 months).

RESULTS

At the level of 45 mg/m(2), two of four patients presented with dose-limiting rectal toxicities (severe acute proctitis requiring hospitalization in the immediate postradiotherapy period). Thus, the gemcitabine biweekly dose of 40 mg/m(2) was considered to be the maximum tolerated dose. Among the 36 patients who underwent surgery, 17 (47%) had a marked pathological response, including six patients (17%) with a microscopically complete response and 11 (30%) with only microscopically residual carcinoma of less than 1 cm. All of them had clear surgical margins. At 3 years, actuarial overall survival rate was 85%, locoregional control was 94.5%, and disease-free survival was 67%.

DISCUSSION

The present study determined the recommended dose of gemcitabine to be 40 mg/m(2) when administered concurrently twice a week with 50 Gy hyperfractionated radiotherapy for the preoperative treatment of locally advanced rectal cancers. The encouraging pathological response rate and the very low locoregional recurrence rate suggest that this innovative approach merits further investigation.

Combined Modality Therapy of Gemcitabine and Radiation

The combination of gemcitabine and radiotherapy is a promising combined modality therapy. However, the clinical application of this combination has to be implemented carefully because of an increased toxicity to normal tissues. A body of experimental evidence shows that gemcitabine is a potent radiosensitizer in vitro and in vivo. The observations so far indicate that various mechanisms are responsible for the radiosensitizing effect. Although it is often difficult to transfer experimental data to the clinic, these studies offer the possibility to develop an improved schedule of administration for patient treatment, based on rational evidence in tumor biology. In the current review, the preclinical data that support the use of gemcitabine as a radiosensitizing agent and the clinical trials that have been conducted to date are summarized.

Radiosensitization by hyperthermia in the chicken B-lymphocyte cell line DT40 and its derivatives lacking nonhomologous end joining and/or homologous recombination pathways of DNA double-strand break repair

Hyperthermia has a radiosensitizing effect, which is one of the most important biological bases for its use in cancer therapy with radiation. Although the mechanism of this effect has not been clarified in molecular terms, possible involvement of either one or both of two major DNA double-strand break (DSB) repair pathways, i.e. nonhomologous end joining (NHEJ) and homologous recombination (HR), has been speculated. To test this possibility, we examined cells of the chicken B-lymphocyte cell line DT40 and its derivatives lacking NHEJ and/or HR: KU70(-/-), DNA-PKcs(-/-/-), RAD54(-/-) and KU70(-/-)/RAD54(-/-). Radiosensitization by hyperthermia could be seen in all of the mutants, including KU70(-/-)/RAD54(-/-), which lacked both NHEJ and HR. Therefore, radiosensitization by hyperthermia cannot be explained simply by its inhibitory effects, if any, on NHEJ and/or HR alone. However, in NHEJ-defective KU70(-/-) and DNA-PKcs(-/-/-), consisting of two subpopulations with distinct radiosensitivity, the radiosensitive subpopulation, which is considered to be cells in G(1) and early S, was not sensitized. Substantial sensitization was seen only in the radioresistant subpopulation, which is considered to be cells in late S and G(2), capable of repairing DSBs through HR. This observation did not exclude possible involvement of NHEJ in G(1) and early S phase and also suggested inhibitory effects of hyperthermia on HR. Thus partial contribution of NHEJ and HR in radiosensitization by hyperthermia, especially that depending on the cell cycle stage, remains to be considered.

Mechanism of radiosensitization by hyperthermia (> or = 43 degrees C) as derived from studies with DNA repair defective mutant cell lines

All biochemical and cytogenetic data on radiosensitization by heat treatment at and above 43 degrees C indicate that inhibition of DNA repair plays a central role. There are several DNA repair pathways involved in restoration of damage after ionising irradiation and the kinetics of all of them are affected by heat shock. This, however, does not imply that the inhibition of each of these pathways is relevant to the effect of heat on cellular radiosensitivity. The current review evaluates the available data on heat radiosensitization in mutant or knockout cell lines defective in various DNA repair proteins and/or pathways. The data show that thermal inhibition of the non-homologous end-joining pathway (NHEJ) plays no role in heat radiosensitization. Furthermore, limited data suggest that the homologous recombination pathway may also not be a major heat target. By deduction, it is suggested that inhibition of base damage repair (BER) could be the crucial step in radiosensitization by heat. While a lack of mutant cell lines and redundancy of the BER pathway have hampered efforts toward a conclusive study, biochemical and correlative evidence support this hypothesis.