Fluorodeoxyuridine-mediated cytotoxicity and radiosensitization require S phase progression

Fibrin glue mediated direct delivery of radiation sensitizers results in enhanced efficacy of radiation treatment

PURPOSE

Radiation therapy (RT) plays an important role in the treatment of glioblastoma multiforme (GBM). However, inherent intrinsic resistance of tumors to radiation, coupled with the need to consider the tolerance of normal tissues and the potential effects on neurocognitive function, impose constraints on the amount of RT that can be safely delivered. A strategy for augmenting the effectiveness of RT involves the utilization of radiation sensitizers (RS). Directly implanting RS-loaded fibrin glue (FG) into the tumor resection cavity would by-pass the blood brain barrier, potentially enhancing the impact of RT on tumor recurrence. This study investigated the ability of FG to incorporate and release, in non-degraded form, the radiation sensitizers 5-Fluorouracil (5FU) and Motexafin gadolinium (MGd).

METHODS

FG layers were created in a 24-well plate by combining thrombin, fibrinogen, and 5FU or MGd. Supernatants from these layers were collected at various intervals and added to F98 glioma spheroid cultures in 96-well plates. Radiation was applied either before or after RS application as single or fractionated dosages. Spheroid growth was monitored for 14 days.

RESULTS

Combined treatment of FG-released 5FU and RT significantly inhibited spheroid growth compared to RS or RT as a single treatment. As a free drug, MGd demonstrated its efficacy in reducing spheroid volume, but had diminished potency as a released RS. Fractionated radiation was more effective than single dose radiation.

CONCLUSION

Non-degraded RS was released from the FG for up to 72 h. FG-released 5FU greatly increased the efficacy of radiation therapy.

Interventional Oncology Service Development

Interventional oncology is rapidly expanding its suite of oncologic therapies, providing unique proven therapeutic benefits. To grow a practice alongside other oncology specialties, knowledge of cancer fundamentals is required. Areas of interest include methods to assess disease stage, treatment toxicity, and response. Additionally, techniques to leverage opportunities and resources available at one's institution toward practice development and efficiency will be reviewed.

Chemoradiotherapy and concurrent radiofrequency thermal therapy to treat primary rectal cancer and prediction of treatment responses

The present study aimed to evaluate a previously reported predictive formula of output-limiting symptoms induced by radiofrequency (RF) to determine the efficacy of this neoadjuvant chemoradiation (NACR) and concurrent RF thermal therapy. The present study included 81 consecutive patients with confirmed diagnoses of rectal adenocarcinoma that was localized in the mid-low rectum (up to 12 cm from the anal verge) who received NACR [intensity-modulated radiotherapy (IMRT), 50 Gy/25 fractions, capecitabine 1,700 mg/m2/day for 5 days/week)] with concurrent thermal therapy (Thermotron-RF8, once a week for 5 weeks with 50 min irradiation). Patients with progressive disease (PD) did not receive RF outputs higher than the predicted value. Some patients who were predicted to receive more output in fact received more than the predicted output. In patients who were predicted to receive moderately higher outputs, 37.5% of the patients experienced pathological complete responses, which was the highest rate, while in those who did not receive more than the predicted output, 66.7% of the patients experienced PD, which was the highest rate in the present study. We speculate that RF thermal therapy may offset the chemoradiation effects in some patients. Adding thermal therapy as a multimodality therapy to NACR potentially affects patients with lower predicted outputs and actual observed outputs slightly higher than the predictive value. Our predictive equation for initial energy output, in which output‑limiting symptoms can be used to predict treatment efficacy, consequently, can be used to decide whether to continue this treatment modality.

Thymineless death in F10-treated AML cells occurs via lipid raft depletion and Fas/FasL co-localization in the plasma membrane with activation of the extrinsic apoptotic pathway

The polymeric fluoropyrimidine F10 displays excellent anti-leukemia activity in pre-clinical models of acute myelogenous leukemia (AML) through dual targeting of thymidylate synthase and DNA topoisomerase 1. Here we report that F10 activates the extrinsic apoptotic pathway in AML cells by enhancing localization of Fas and Fas ligand (FasL) at the plasma membrane and while reducing overall lipid raft levels promotes Fas/FasL co-localization in remaining lipid rafts. The HMG-CoA synthase inhibitor simvastatin was synergistic with F10 and induced cell death via similar apoptotic processes. Our results are consistent with diverse processes activating a common apoptotic pathway characterized by reduced overall levels of lipid rafts and Fas/FasL co-localization in the plasma membrane, including in remaining lipid rafts which may play a role in both cell-survival and cell death signaling.

Altering the response to radiation: sensitizers and protectors

A number of agents are used clinically to enhance the efficacy of radiotherapy today, many of which are cytotoxic chemotherapies. Agents that enhance radiation induced tumor cell killing or protect normal tissues from the deleterious effects of ionizing radiation are collectively termed radiation modifiers. A significant effort in radiobiological research is geared towards describing and testing radiation modifiers with the intent of enhancing the therapeutic effects of radiation while minimizing normal tissue toxicity. In this review, we discuss the characteristics of these agents, the testing required to translate these agents into clinical trials, and highlight some challenges in these efforts.

Low dose rate radiosensitization of hepatocellular carcinoma in vitro and in patients

Transarterial radioembolization (TARE) with ⁹⁰Y microspheres delivers low dose rate radiation (LDR) to intrahepatic tumors. In the current study, we examined clonogenic survival, DNA damage, and cell cycle distribution in hepatocellular carcinoma (HCC) cell lines treated with LDR in combination with varying doses and schedules of 5-fluorouracil (5-FU), gemcitabine, and sorafenib. Radiosensitization was seen with 1 to 3 μM 5-FU (enhancement ratio 2.2–13.9) and 30 to 100 nM gemcitabine (enhancement ratio 1.9–2.9) administered 24 hours before LDR (0.26 Gy/h to 4.2 Gy). Sorafenib radiosensitized only at high concentrations (3–10 μM) when administered after LDR. For a given radiation dose, greater enhancement was seen with LDR compared to standard dose rate therapy. Summarizing our clinical experience with low dose rate radiosensitization, 13 patients (5 with HCC, 8 with liver metastases) were treated a total of 16 times with TARE and concurrent gemcitabine. Six partial responses and one complete response were observed with a median time to local failure of 7.1 months for all patients and 9.9 months for patients with HCC. In summary, HCC is sensitized to LDR with clinically achievable concentrations of gemcitabine and 5-FU in vitro. Encouraging responses were seen in a small cohort of patients treated with TARE and concurrent gemcitabine. Future studies are needed to validate the safety and efficacy of this approach.

The status of targeted agents in the setting of neoadjuvant radiation therapy in locally advanced rectal cancers

Radiotherapy has a longstanding and well-defined role in the treatment of resectable rectal cancer to reduce the historically high risk of local recurrence. In more advanced borderline or unresectable cases, where the circumferential resection margin (CRM) is breached or threatened according to magnetic resonance imaging (MRI), despite optimized local multimodality treatment and the gains achieved by modern high quality total mesorectal excision (TME), at least half the patients fail to achieve sufficient downstaging with current schedules. Many do not achieve an R0 resection. In less locally advanced cases, even if local control is achieved, this confers only a small impact on distant metastases and a significant proportion of patients (30-40%) still subsequently develop metastatic disease. In fact, distant metastases have now become the predominant cause of failure in rectal cancer. Therefore, increasing the intensity and efficacy of chemotherapy and chemoradiotherapy by integrating additional cytotoxics and biologically targetted agents seems an appealing strategy to explore-with the aim of enhancing curative resection rates and improving distant control and survival. However, to date, we lack validated biomarkers for these biological agents apart from wild-type KRAS. For cetuximab, the appearance of an acneiform rash is associated with response, but low levels of magnesium appear more controversial. There are no molecular biomarkers for bevacizumab. Although some less invasive clinical markers have been proposed for bevacizumab, such as circulating endothelial cells (CECS), circulating levels of VEGF and the development of overt hypertension, these biomarkers have not been validated and are observed to emerge only after a trial of the agent. We also lack a simple method of ongoing monitoring of 'on target' effects of these biological agents, which could determine and pre-empt the development of resistance, prior to radiological and clinical assessessments or even molecular imaging. These shortcomings probably explain our current relative lack of success in the arena of combining these agents with chemoradiation.

Cancer concepts and principles: primer for the interventional oncologist-part II

This is the second of a two-part overview of the fundamentals of oncology for interventional radiologists. The first part focused on clinical trials, basic statistics, assessment of response, and overall concepts in oncology. This second part aims to review the methods of tumor characterization; principles of the oncology specialties, including medical, surgical, radiation, and interventional oncology; and current treatment paradigms for the most common cancers encountered in interventional oncology, along with the levels of evidence that guide these treatments.

Re-evaluation of cellular radiosensitization by 5-fluorouracil: high-dose, pulsed administration is effective and preferable to conventional low-dose, chronic administration

PURPOSE

It is widely believed that the anticancer drug 5- fluorouracil (5-FU) must be administered chronically and in low doses to maximize radiosensitization during chemoradiotherapy. The rationale is based upon cell experiments that assumed identical mechanisms of 5-FU action between low-dose chronic (LDC) and high-dose pulsed (HDP) exposures. Here we challenge the paradigm and demonstrate the effectiveness of HDP 5-FU as a radiosensitizer and the wide range of dose/time schedules that can be used to synergize with radiation as compared to the relatively restrictive protocols prescribed for current LDC administrations.

MATERIALS AND METHODS

Clonogenic survival of human glioblastoma and colon cancer cell lines, U87MG-VIII and HCT-116, respectively, was used to assess temporal and dose effects of 5-FU on radiosensitivity and in split-dose experiments to characterize changes in sublethal damage repair.

RESULTS

We show that HDP 5-FU administration does indeed radiosensitize both the highly radioresistant U87MG-VIII and HCT-116. Additionally, we show that this radiosensitization lasts for at least 24 h if cells are pre-irradiated with 2 Gy immediately after HDP 5-FU exposure as a result of a decrease in sublethal damage repair capacity for subsequent irradiations, suggesting the ideal combination of 5-FU bolus injection with fractionation radiotherapy schemes.

CONCLUSIONS

5-FU bolus administration protocols combined with radiation would not only help improve treatment outcomes and reduce development of 5-FU resistance, but it would greatly benefit patients by shortening clinical stays and lowering overall therapeutic costs.

Current status of intensified neo-adjuvant systemic therapy in locally advanced rectal cancer

The addition of 5-fluorouracil (5-FU) or its prodrug capecitabine to radiotherapy (RT) is a standard approach in the neo-adjuvant treatment of patients with rectal tumors extending beyond the muscularis propria (stage II) and/or with clinical evidence of regional lymph node metastases (stage III). According to European randomized trials, the combined treatment modality resulted in favorable local control rates as compared with radiotherapy (RT) alone, but no improvement was found regarding the occurrence of distant metastases or overall survival. In an effort to further enhance the response rates and to decrease the high incidence of distant metastases in locally advanced rectal cancer patients, the addition of other chemotherapeutical drugs and biologic agents as radiation sensitizers to neo-adjuvant 5-FU based chemoradiotherapy (CRT) has been recently investigated. The role of those agents is however questionable as first results from phase III data do not show improvement on pathologic complete remission and circumferential resection margin negative resection rates as compared to 5-FU based CRT, nevertheless an increased toxicity.

18F-FLT and 125I-IdUrd uptake increase in human tumour cell lines induced by the thymidylate synthase inhibitor FdUrd

AIM

5-fluoro-2'-deoxyuridine (FdUrd) depletes the endogenous 5'-deoxythymidine triphosphate (dTTP) pool. We hypothesized whether uptake of exogenous dThd analogues could be favoured through a feedback enhanced salvage pathway and studied the FdUrd effect on cellular uptake of 3'-deoxy-3'-18F-fluorothymidine (18F-FLT) and 5-125I-iodo-2'-deoxyuridine (125I-IdUrd) in different cancer cell lines in parallel.

METHODS

Cell uptake of 18F-FLT and 125I-IdUrd was studied in 2 human breast, 2 colon cancer and 2 glioblastoma lines. Cells were incubated with/without 1 µmol/l FdUrd for 1 h and, after washing, with 1.2 MBq 18F-FLT or 125I-IdUrd for 0.3 to 2 h. Cell bound 18F-FLT and 125I-IdUrd was counted and expressed in % incubated activity (%IA). Kinetics of 18F-FLT cell uptake and release were studied with/without FdUrd modulation. 2'-3H-methyl-fluorothymidine (2'-3H-FLT) uptake with/without FdUrd pretreatment was tested on U87 spheroids and monolayer cells.

RESULTS

Basal uptake at 2 h of 18F-FLT and 125I-IdUrd was in the range of 0.8-1.0 and 0.4-0.6 Bq/cell, respectively. FdUrd pretreatment enhanced 18F-FLT and 125I-IdUrd uptake 1.2-2.1 and 1.7-4.4 fold, respectively, while co-incubation with excess thymidine abrogated all 18F-FLT uptake. FdUrd enhanced 18F-FLT cellular inflow in 2 breast cancer lines by factors of 1.8 and 1.6, respectively, while outflow persisted at a slightly lower rate. 2'-3H-FLT basal uptake was very low while uptake increase after FdUrd was similar in U87 monolayer cells and spheroids.

CONCLUSIONS

Basal uptake of 18F-FLT was frequently higher than that of 125I-IdUrd but FdUrd induced uptake enhancement was stronger for 125I-IdUrd in five of six cell lines. 18F-FLT outflow from cells might be an explanation for the observed difference with 125I-IdUrd.

Targeted endoradiotherapy using nucleotides

Increased cellular proliferation is an integral part of the cancer phenotype. Hence, the sustained and continued demand on supply of DNA building blocks during the DNA replication presents a potential target for therapeutic intervention. For this propose, the α and Auger electron emitting nucleotides analogs are attractive for targeted endoradiotherapy, given that DNA of malignant cells is selectively addressed. This review summarizes development and preclinical and clinical studies of endoradiotherapeutic acting nucleoside analogs with a special focus on thymidine analogs.

Radiosensitization of human breast cancer cells to ultraviolet light by 5-fluorouracil

Ultraviolet light B (UVB) phototherapy is widely used to treat dermatological diseases and therefore may be a potential optional strategy in the treatment of a skin lesion infiltrated by a malignant tumor. Currently, little is known regarding the effect of UVB phototherapy on human breast cancer cells. The present study aimed to investigate the effect of UVB phototherapy, as well as the potential effect of 5-fluorouracil (5-FU), the first-line anticancer drug for breast cancer, on radiosensitizing MCF-7 human breast cancer cells, in an attempt to develop new therapeutic strategies for the treatment of locoregional recurrence of breast cancer. MCF-7 cells were incubated in the presence of 5-FU for 48 h, and UVB irradiation at 750 mJ/cm(2) was administered in the midterm of 5-FU treatment. The viability of MCF-7 cells was analyzed by the trypan blue staining method. Apoptosis was quantified by flow cytometry and Hoechst 33258 staining. The cell cycle was evaluated by flow cytometry after the staining of cells with propidium iodide. The combination treatment of 5-FU and UVB resulted in a strong potentiation of the inhibitory effect of MCF-7 cell growth, dependent on the intra-S phase cell cycle arrest and induction of apoptosis, when compared to treatment with 5-FU or UVB alone. In conclusion, 5-FU sensitized human breast cancer cells to UVB phototherapy, and this combination therapy is an effective and promising strategy for the treatment of breast cancer, particularly for locoregional recurrence.

Cetuximab and chemoradiation for rectal cancer--is the water getting muddy?

The epidermal growth factor receptor (EGFR) inhibitor cetuximab has been successfully combined with radical radiotherapy in head and neck cancer. In colorectal cancer, increased response rates are achieved by cetuximab and panitumumab within standard chemotherapy schedules, but not in chemoradiation regimens. This review examines the clinical evidence and potential mechanisms for an interaction when EGFR inhibitors are added to fluoropyrimidine-based chemoradiation in rectal adenocarcinoma.

METHODS

This review was compiled by searching PubMed and Medline for English language articles published until 2009 with established search strategies, supplemented by hand searching of abstracts from the proceedings of relevant international meetings. The primary outcome measure was pathological complete response (pCR).

RESULTS

Only 13 publications and three presentations in abstract of 13 phase I/II trials of preoperative chemoradiation with cetuximab in rectal cancer were identified. A total of 316 patients were identified who received cetuximab in combination with radiotherapy and 5-fluorouracil or capecitabine preoperatively. One hundred and thirty eight of these patients received either additional irinotecan or oxaliplatin. One study with panitumumab with safety but no efficacy results was identified, and two studies with gefinitib. The pCR rate ranged from 0-20%. The overall pooled pCR for cetuximab based chemoradiation was 9.1% (29/316). The rate of G3/G4 gastrointestinal toxicity, in terms of diarrhoea, varied from 5-30%, with an overall pooled rate of 47/313 (15%).

DISCUSSION

Potential reasons for the disappointing results of EGFR inhibition with fluoropyrimidine-based preoperative chemoradiation include a less critical role of repopulation in rectal adenocarcinoma using a non-curative radiation dose; or antagonistic effects on 5FU-based chemoradiation and oxaliplatin, if some cells arrest in G1 or G2-M and fail to pass through S phase.

CONCLUSION

Cetuximab combined with fluoropyrimidine-based chemoradiation is not currently recommended. A better understanding of the mechanisms involved in combinations of chemotherapy and radiotherapy might allow more effective future scheduling of biological and chemical agents in combination with radiation.

Recent advances in combined modality therapy

Combined modality therapy emerged from preclinical data showing that carefully chosen drugs could enhance the sensitivity of tumor cells to radiation while having nonoverlapping toxicities. Recent advances in molecular biology involving the identification of cellular receptors, enzymes, and pathways involved in tumor growth and immortality have resulted in the development of biologically targeted drugs. This review highlights the recent clinical data in support of newer generation cytotoxic chemotherapies and systemic targeted agents in combination with radiation therapy.

MLH1 deficiency enhances radiosensitization with 5-fluorodeoxyuridine by increasing DNA mismatches

The antitumor drug 5-fluoro-2'-deoxyuridine (FdUrd) also sensitizes tumor cells to ionizing radiation in vitro and in vivo. Although radiosensitization with FdUrd requires dTTP depletion and S-phase arrest, the exact mechanism by which these events produce radiosensitization remains unknown. We hypothesized that the depletion of dTTP produces DNA mismatches that, if not repaired before irradiation, would result in radiosensitization. We evaluated this hypothesis in mismatch repair (MMR)-deficient HCT116 0-1 cells that lack the expression of the required MMR protein MLH1 (inactive MLH1), and in MMR-proficient (wild-type MLH1) HCT116 1-2 cells. Although HCT116 0-1 cells were less sensitive to FdUrd (IC(50) = 3.5 microM) versus HCT116 1-2 cells (IC(50) = 0.75 microM), when irradiation followed FdUrd (IC(50)) the MLH1-inactivated cells exhibited greater radiosensitization compared with MMR-wild-type cells [radiation enhancement ratio (RER) = 1.8 +/- 0.28 versus 1.1 +/- 0.1, respectively] and an increase (> or =8-fold) in nucleotide misincorporations. In SW620 cells and HCT116 1-2 MLH1-wild-type cells, FdUrd (IC(50)) did not produce radiosensitization nor did it increase the mutation frequency, but after short hairpin RNA-directed suppression of MLH1 this concentration produced excellent radiosensitization (RER = 1.6 +/- 0.10 and 1.5 +/- 0.06, respectively) and an increase in nucleotide misincorporations (8-fold and 6-fold, respectively). Incubation with higher concentrations of FdUrd (IC(90)) after suppression of MLH1 produced a further increase in ionizing radiation sensitivity in both SW620 and HCT116 1-2 cells (RER = 1.8 +/- 0.03 and 1.7 +/- 0.13, respectively) and nucleotide misincorporations (>10-fold in both cell lines). These results demonstrate an important role for MLH1 and implicate mismatches in radiosensitization by FdUrd.

Chemoradiotherapy for localized esophageal cancer: regimen selection and molecular mechanisms of radiosensitization

Concurrent chemoradiotherapy administered either before surgery or as definitive treatment has a central role in the multimodality treatment of locally advanced esophageal cancer. Initial studies of this combined-modality regimen were based on models of squamous-cell cancers from other primary sites; this approach progressed from use of bleomycin or fluorouracil plus cisplatin concurrent with radiation in early trials, to the integration of taxanes, camptothecins and platinum analogs in recent trials. These trials demonstrated the tumoricidal effect of concurrent chemotherapy and radiotherapy and showed the survival advantages of this approach. Preoperative concurrent chemoradiation is used to downstage the tumor, ideally to a pathological complete response status in which there is no residual tumor in the resected primary and nodal tissues. A pathological complete response is associated with long-term survival but occurs in a minority (30%) of patients. While clinical trials have demonstrated an improvement in survival with concurrent chemoradiotherapy this effect is limited, as indicated by the plateau in survival beyond 5 years of approximately 30% or less. The recent clinical development of biologic, targeted therapies provides a new avenue for the study of chemoradiotherapy and an opportunity to increase long-term survival.

Base excision repair of both uracil and oxidatively damaged bases contribute to thymidine deprivation-induced radiosensitization

PURPOSE

Increased cellular sensitivity to ionizing radiation due to thymidine depletion is the basis of radiosensitization with fluoropyrimidine and methotrexate. The mechanism responsible for cytotoxicity has not been fully elucidated but appears to involve both the introduction of uracil into, and its removal from, DNA. The role of base excision repair of uracil and oxidatively damaged bases in creating the increased radiosensitization during thymidine depletion is examined.

METHODS AND MATERIALS

Isogenic strains of S. cerevisiae differing only at loci involved in DNA repair functions were exposed to aminopterin and sulfanilamide to induce thymidine deprivation. Cultures were irradiated and survival determined by clonogenic survival assay.

RESULTS

Strains lacking uracil base excision repair (BER) activities demonstrated less radiosensitization than the parental strain. Mutant strains continued to show partial radiosensitization with aminopterin treatment. Mutants deficient in BER of both uracil and oxidatively damaged bases did not demonstrate radiosensitization. A recombination deficient rad52 mutant strain was markedly sensitive to radiation; addition of aminopterin increased radiosensitivity only slightly. Radiosensitization observed in rad52 mutants was also abolished by deletion of the APN1, NTG1, and NTG2 genes.

CONCLUSION

These data suggest radiosensitization during thymidine depletion is the result of BER activities directed at both uracil and oxidatively damaged bases.

5-fluoro-2'-deoxyuridine-induced cdc25A accumulation correlates with premature mitotic entry and clonogenic death in human colon cancer cells

The ability to inappropriately progress through S phase during drug treatment is a key determinant of tumor cell sensitivity to thymidylate synthase inhibitors such as 5-fluoro-2'-deoxyuridine (FdUrd). Previous studies suggest that SW620 cells, which are relatively resistant to FdUrd, have an intact early S-phase checkpoint that protects against FdUrd-induced DNA damage and cytotoxicity and that this checkpoint is defective in the relatively sensitive HT29 cells, which continue to progress through S phase during drug treatment. To test this hypothesis, we examined the expression and activation of known S-phase checkpoint mediators in FdUrd-treated SW620 and HT29 cells. FdUrd induced degradation of cdc25A in SW620, but not HT29 cells, in a manner that correlated with the previously described drug-induced S-phase arrest. This difference, however, could not be attributed to differences in either chk1 activation, which was similar in both cell lines, or chk2 activation, which only occurred in HT29 cells and correlated with uracil misincorporation/misrepair-induced DNA double-stranded breaks. These observations suggest that although FdUrd-induced S-phase arrest and associated cdc25A degradation are impaired in HT29 cells, signaling by ATM/ATR is intact upstream of chk1 and chk2. Finally, FdUrd induced premature mitotic entry, a phenomenon associated with deregulated cdc25A expression, in HT29 but not SW620 cells. Blocking cdc25A expression in HT29 cells with small interfering RNA attenuated FdUrd-induced premature mitotic entry, suggesting that progression of HT29 cells through S phase during drug treatment results in part from the inability of these cells to degrade cdc25A in response to FdUrd-induced DNA damage.

The mechanism of action of radiosensitization of conventional chemotherapeutic agents

It is not an exaggeration to state that most of the advances in curing cancer in the last decade have come from successful combinations of conventional chemotherapeutic agents with radiation therapy. Further improvements in therapy will depend on understanding the mechanisms by which chemotherapy improves the effectiveness of radiation in model systems and in patients. In this review, we discuss the mechanisms of action of the fluoropyrimidines, gemcitabine, and the platinums. The fluoropyrimidines (5-fluorouracil and fluorodeoxyuridine) increase the effectiveness of radiation chiefly when given before and during radiation. Increased radiation sensitivity occurs in cells that progress inappropriately into S phase in the presence of drug, suggesting a key role for dysregulation of S-phase checkpoints. Gemcitabine may radiosensitize by a similar mechanism, although the relative roles of specific DNA repair pathways (such as homologous end rejoining) and of apoptosis remain to be determined. For both of these categories of drugs, sensitization probably results when cells that are progressing inappropriately through S phase misrepair DNA damage inflicted by radiation. Thus, loss of the S-phase checkpoint in cancer cells may provide the molecular basis for selective killing of tumors compared with normal tissues. Cisplatin has multiple effects on cells, such as adduct formation and DNA damage repair inhibition, but the mechanism for selectivity against cancer cells compared with normal cells is not yet determined. The identification of the enzymatic targets for these drugs offers the potential to develop predictive assays for response and to develop methods of imaging the progress of therapy.

Interaction of pemetrexed disodium (ALIMTA, multitargeted antifolate) and irradiation in vitro

PURPOSE

Pemetrexed disodium (Alimta, multitargeted antifolate, LY231514; Eli Lilly and Co., Indianapolis, Indiana) ("pemetrexed") is a new folate antimetabolite with significant antitumor activity. Different from classic antifolates, pemetrexed inhibits several key enzymes of thymidylate and purine synthesis, but a radiosensitizing potential may also be presumed. Therefore, the interaction of pemetrexed and ionizing radiation was studied for in vitro clonogenic survival of different human tumor cell lines.

METHODS AND MATERIALS

Human colon (Widr), breast (MCF-7), cervix (Hela), and lung (LXI) carcinoma cells from log-phase cultures were exposed to pemetrexed (2 h) in combination with different radiation doses given 1 h before pemetrexed washout (all cell lines) or at different points of time before or after pemetrexed addition (Widr). Survival curves were analyzed according to the linear-quadratic (LQ) model, and mean inactivation doses (MID) and radiation enhancement ratios were calculated from the survival curve parameters. Cell-cycle progression of serum-stimulated and pemetrexed- or mock-treated Widr cells was monitored by flow cytometry.

RESULTS

Radiosensitization was found for all cell lines at moderately toxic pemetrexed exposures (0.05-0.3 microg/ml [106-636 nM]), but this was cell-type dependent and was most pronounced at roughly isotoxic concentrations, for the least pemetrexed-sensitive Widr cells. Enhancement ratios ranged from about 1.2 (MCF-7 and Hela) to 1.8 (Widr), with a tendency to increase with pemetrexed concentration. Little, if any, change of radiosensitization was observed (Widr) when the time of irradiation was varied from 4 h before to 10 h after the beginning of pemetrexed treatment. Cell-cycle progression of serum-stimulated Widr cells was only marginally affected by pemetrexed.

CONCLUSIONS

Pemetrexed enhances radiation-induced cell inactivation at moderately toxic exposures and over many hours after drug removal. This effect is not due to disturbed cell-cycle progression, but likely involves an interaction of pemetrexed with long-lived (>4 h) cellular radiation damage and needs to be considered when introducing a combined clinical application.

Lack of effect of TP53 status on fluorodeoxyuridine-mediated radiosensitization

Substantial evidence suggests that TP53 (also known as p53) status can influence the response of cells to chemotherapy and radiation. We wished to determine if TP53 function affected the response of cells to fluoropyrimidines and radiation, a combination used for tens of thousands of patients each year. To assess the role of TP53 in fluoropyrimidine-mediated radiosensitization, we carried out experiments using RKO parental cells (wild-type TP53) and RKO cells overexpressing mutant TP53 (which blocks TP53 function) or expressing E6 (which degrades TP53). We found that TP53 function had no effect on the ability of fluorodeoxyuridine to increase radiation sensitivity. These findings are consistent with the hypothesis that the late G(1)-phase checkpoint, which is mediated by TP53, is not crucial to radiosensitization. Rather, the ability of cells to progress in to S phase in the presence of the drug, which is independent of TP53, is more closely associated with increased radiation sensitivity.

HPV16-E7 expression causes fluorodeoxyuridine-mediated radiosensitization in SW620 human colon cancer cells

We have reported that HT29 colon cancer cells, which are radiosensitized by fluorodeoxyuridine (FdUrd), exhibit a greater increase in cyclin E-dependent kinase activity and progress further into S phase in the presence of FdUrd than do SW620 colon cancer cells, which are only minimally sensitized by this drug (Cancer Res 56: 3203, 1996). Although these findings suggested that the ability to progress into S phase in the presence of FdUrd permits cells to be radiosensitized, we wished to test this hypothesis by attempting to drive SW620 human colon cells into S phase by transducing them with the HPV16-E7 gene. Two-parameter flow cytometry showed that E7-transduced cells progressed through S phase after radiation and FdUrd treatment more rapidly than SW620 parental cells. We found that E7-transduced SW620 cells were significantly radiosensitized by FdUrd (100 nmol/L, 14 hours) with an enhancement ratio for 2 clones of 1.47 +/- 0.03 and 1.51 +/- 0.14, compared with 1.24 +/- 0.04 in SW620 parental cells. These data strongly support the hypothesis that dysregulation of S-phase progression is an important factor in FdUrd-mediated radiosensitization.

Clonogenic inactivation of colon cancer-derived cells treated with 5-fluorouracil and indomethacin in hybrid spheroids

The clonogenic hybrid spheroid assay has been used to determine the toxicity of 5-fluorouracil (5-FU), alone or in combination with indomethacin, in LoVo cells (a human colon adenocarcinoma line). The principal finding was that 5-FU toxicity, determined as loss of colony-forming ability, increased as a function of dose (concentration x duration of exposure), and that indomethacin causes a generalized alleviation of 5-FU toxicity, but only if given concurrently with 5-FU. The implications of these findings in the control of cancer cells by 5-FU are discussed.

The potentiation of radiation response in human colon carcinoma cells in vitro and murine lymphoma in vivo by AG337 (Thymitaq), a novel thymidylate synthase inhibitor

PURPOSE

To determine whether the administration of Thymitaq (AG337), a selective inhibitor of thymidylate synthase (TS), enhances radiation-induced cytotoxicity in vitro and increases tumor control rate in vivo.

METHODS AND MATERIALS

In vitro studies were carried out with HT-29 human colon carcinoma cells. In vivo studies were carried out using L5178Y(TK-) murine lymphoma implanted in DBA/2 mice.

RESULTS

Pretreatment of HT-29 cells to nontoxic concentration of AG337 (<10 microM) for a short period of time (< 24 h) significantly enhanced the radiation induced cell lethality. The radiosensitizing enhancement ratio was 1.7. In contrast, there was no increased cell killing when the drug was exposed immediately after irradiation. In studies using L5178Y(TK-) tumors, the drug alone (50 mg/kg, i.p. x 5) had a minimal tumor growth delay, while a single dose of radiation (17 Gy) resulted in < 10% tumor control at day 30. When radiation and drug (17 Gy + AG337, 50 mg/kg, i.p. x 5) were combined, the tumor control rate reached 90% at Day 30. Using the local tumor control assay (TCD50), the radiation dose modification factor after a single dose of radiation was 2.6.

CONCLUSION

The concentration of drug shown to be of radiosensitizing value in the in vivo studies is achievable in humans. The results of the present study further supports the potential utility of AG337 in the treatment of human tumors by radiotherapy.

The effect of activation of wild-type p53 function on fluoropyrimidine-mediated radiosensitization

PURPOSE

We have hypothesized that fluoropyrimidine-mediated (FdUrd) radiosensitization occurs in cells that inappropriately enter S phase in the presence of drug, resulting in defective repair of radiation-induced DNA damage (14). This model would predict that prevention of entry into S phase would abrogate sensitization produced by FdUrd. We wished to test this prediction by blocking S phase entry of HT29 human colon cancer cells.

METHODS

We used HT29 cells that had been transduced with a murine temperature-sensitive p53 (ts29G) and, as a control, HT29 cells transduced with a neomycin plasmid (HT29neo). The murine temperature-sensitive p53 demonstrates wild-type p53 function when cells are incubated at the permissive temperature (32 degrees C) and mutant p53 function at the nonpermissive temperature (38 degrees C). We determined the effect of wt p53 function on FdUrd-mediated radiosensitization and cell cycle progression.

RESULTS

Incubation of ts29G cells at the permissive temperature (32 degrees C) activated p21 and blocked entry of cells into S phase. Whereas FdUrd greatly increased the radiosensitivity of HT29neo cells and ts29G cells incubated at 38 degrees C, FdUrd had no effect on the radiation sensitivity of ts29G cells incubated at the permissive temperature (32 degrees C).

CONCLUSIONS

These findings are consistent with our hypothesis that FdUrd-mediated radiosensitization requires progression into S phase. It is possible that the heterogeneity of clinical responses seen after combined therapy with fluoropyrimidines and radiation is explained, in part, by differences among tumor cells in the control of S phase progression.