In vivo therapy of neoplastic meningitis with methotrexate and 5

Auger electron therapy of glioblastoma using [125I]5-iodo-2'-deoxyuridine and concomitant chemotherapy - Evaluation of a potential treatment strategy

INTRODUCTION

Treatment of glioblastomas (GBM) using the Auger electron emitting compound [¹²⁵I]5-Iodo-2'-deoxyuridine ([¹²⁵I]I-UdR), combined with the thymidylate synthase inhibitor methotrexate (MTX) and concomitant chemotherapy with temozolomide (TMZ) has recently shown very promising therapeutic effects in vitro and in vivo in animals. The aim of the current study was to investigate if the therapeutic effects of this multimodal treatment strategy could be further increased by the thymidylate synthase inhibitor, 5-fluoro-2'-deoxyuridine (F-UdR), in comparison to MTX, and if the co-treatment should be given in a neoadjuvant or adjuvant setting.

METHODS

A patient-derived GBM cancer stem cell (CSC)-enriched cell line, grown as neurospheres, was employed to evaluate DNA-incorporation of [¹²⁵I]I-UdR, determined by a DNA precipitation assay, using either pre-treatment or co-treatment with MTX or F-UdR. The therapeutic effects in the CSC-enriched cell line after exposure to various combinations of MTX, F-UdR, TMZ and [¹²⁵I]I-UdR were also investigated by a CellTiter-Blue assay.

RESULTS

The highest general increase in [¹²⁵I]I-UdR incorporation was observed with F-UdR co-treatment, which resulted in approx. 2.5-fold increase in the DNA-associated activity. Also the cell viability was significantly decreased when F-UdR was combined with [¹²⁵I]I-UdR compared to [¹²⁵I]I-UdR alone at all activity concentrations tested. MTX was redundant when combined with 400 and 500 Bq/ml [¹²⁵I]I-UdR. TMZ was effective in combination with either [¹²⁵I]I-UdR alone or with both thymidylate synthase inhibitors combined with 50-100 Bq/ml [¹²⁵I]I-UdR.

CONCLUSIONS

Overall, our study revealed a higher incorporation and therapeutic effect of [¹²⁵I]I-UdR when GBM cells were co-treated with F-UdR compared to MTX. The therapeutic effects were further increased when TMZ was combined with [¹²⁵I]I-UdR in combination with the thymidylate synthase inhibitors.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

Auger electron therapy in combination with thymidylate synthase inhibition and concomitant chemotherapy has the potential to become a future therapeutic treatment option for patients with glioblastoma.

The Exploitation of Low-Energy Electrons in Cancer Treatment

Given the distinct characteristics of low-energy electrons (LEEs), particularly at energies less than 30 eV, they can be applied to a wide range of therapeutic modalities to improve cancer treatment. LEEs have been shown to efficiently produce complex molecular damage resulting in substantial cellular toxicities. Since LEEs are produced in copious amounts from high-energy radiation beam, including photons, protons and ions; the control of LEE distribution can potentially enhance the therapeutic radio of such beams. LEEs can play a substantial role in the synergistic effect between radiation and chemotherapy, particularly halogenated and platinum-based anticancer drugs. Radiosensitizing entities containing atoms of high atomic number such as gold nanoparticles can be a source of LEE production if high-energy radiation interacts with them. This can provide a high local density of LEEs in a cell and produce cellular toxicity. Auger-electron-emitting radionuclides also create a high number of LEEs in each decay, which can induce lethal damage in a cell. Exploitation of LEEs in cancer treatment, however, faces a few challenges, such as dosimetry of LEEs and selective delivery of radiosensitizing and chemotherapeutic molecules close to cellular targets. This review first discusses the rationale for utilizing LEEs in cancer treatment by explaining their mechanism of action, describes theoretical and experimental studies at the molecular and cellular levels, then discusses strategies for achieving modification of the distribution and effectiveness of LEEs in cancerous tissue and their associated clinical benefit.

Highly Effective Auger-Electron Therapy in an Orthotopic Glioblastoma Xenograft Model using Convection-Enhanced Delivery

Glioblastoma, the most common and malignant primary brain tumor, always recurs after standard treatment. Therefore, promising new therapeutic approaches are needed. Short-range Auger-electron-emitters carry the ability of causing highly damaging radiation effects in cells. The aim of this study was to test the effect of [¹²⁵I]5-Iodo-2'-deoxyuridine (¹²⁵I-UdR, a radioactive Auger-electron-emitting thymidine analogue) Auger-therapy on immature glioblastoma spheroid cultures and orthotopic xenografted glioblastoma-bearing rats, the latter by means of convection-enhanced delivery (CED). Moreover, we aimed to determine if the therapeutic effect could be enhanced when combining ¹²⁵I-UdR therapy with the currently used first-line chemotherapeutic agent temozolomide. ¹²⁵I-UdR significantly decreased glioblastoma cell viability and migration in vitro and the cell viability was further decreased by co-treatment with methotrexate and/or temozolomide. Intratumoral CED of methotrexate and ¹²⁵I-UdR with and without concomitant systemic temozolomide chemotherapy significantly reduced the tumor burden in orthotopically xenografted glioblastoma-bearing nude rats. Thus, 100% (8/8) of the animals survived the entire observation period of 180 days when subjected to the combined Auger-chemotherapy while 57% (4/7) survived after the Auger-therapy alone. No animals (0/8) treated with temozolomide alone survived longer than 50 days. Blood samples and post-mortem histology showed no signs of dose-limiting adverse effects. In conclusion, the multidrug approach consisting of CED of methotrexate and ¹²⁵I-UdR with concomitant systemic temozolomide was safe and very effective leading to 100% survival in an orthotopic xenograft glioblastoma model. Therefore, this therapeutic strategy may be a promising option for future glioblastoma therapy.

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.

ErbB-2 blockade and prenyltransferase inhibition alter epidermal growth factor and epidermal growth factor receptor trafficking and enhance (111)In-DTPA-hEGF Auger electron radiation therapy

The intracellular distribution of Auger electron-emitting radiopharmaceuticals is a determinant of cytotoxicity. However, the mechanisms by which these agents are routed through the cell are ill understood. The aim of this study was to investigate how trafficking of (111)In-labeled human epidermal growth factor ((111)In-DTPA-hEGF) relates to that of the EGF receptor (EGFR) and whether coadministration of agents that modulate EGFR signaling alters the efficacy of (111)In-DTPA-hEGF.

METHODS

The spatiotemporal interaction between AlexaFluor488-EGF (AF488-EGF) and Cy3-conjugated anti-EGFR antibody (Cy3-anti-EGFR) was studied in the breast cancer cell line MDA-MB-468 using fluorescence resonance energy transfer and 2-photon fluorescence lifetime imaging. (111)In internalization and nuclear fractionation assays were performed to investigate the effect of the ErbB-2-blocking antibody trastuzumab and a prenyltransferase inhibitor, L-778,123, on the subcellular localization of (111)In-DTPA-hEGF in MDA-MB-468 (1.3 × 10(6) EGFR per cell; ErbB-2 negative) and 231-H2N (0.2 × 10(6) EGFR per cell; 0.4 × 10(5) ErbB-2 per cell) cell lines. The cytotoxicity of (111)In-DTPA-hEGF (0-64 nM) plus trastuzumab (0-50 μg/mL) or L-778,123 (0-22.5 μM) was measured using clonogenic assays in a panel of breast cancer cell lines that express different levels of EGFR and ErB-2. Clonogenic survival data were used to calculate combination indices. Tumor growth inhibition was measured in vivo in 231-H2N xenograft-bearing mice treated with (111)In-DTPA-hEGF plus trastuzumab or L-788,123.

RESULTS

Using fluorescence resonance energy transfer, we showed that EGF interacts with EGFR in the cytoplasm and nucleus after internalization of the ligand-receptor complex in MDA-MB-468 cells. Nuclear localization of (111)In-DTPA-hEGF is enhanced by trastuzumab and L-788,123. Trastuzumab and L-788,123 sensitized 231-H2N cells to (111)In-DTPA-hEGF. Nuclear localization and cytotoxicity of (111)In-DTPA-hEGF were significantly increased in 231-H2N xenografts by cotreatment with L-788,123 (P < 0.0001).

CONCLUSION

The therapeutic efficacy of (111)In-DTPA-hEGF is increased through the coadministration of selected molecularly targeted drugs that modulate EGFR signaling and trafficking.

Molecular and cellular radiobiological effects of Auger emitting radionuclides

Although the general radiobiologic principles underlying external beam therapy and radionuclide therapy are similar, significant differences in the biophysical and radiobiologic effects from the two types of radiation continue to accumulate. Here, I will address the unique features that distinguish the molecular and cellular radiobiological effects of Auger electron-emitting radionuclides consequent to (1) the physical characteristics of the decaying atom and its subcellular localisation, (2) DNA topology and (3) the bystander effect. Based on these experimental findings, I postulate that the ability of track structural simulations as primary tools in modelling DNA damage and cellular survival at the molecular level would be greatly enhanced when these contributions are factored in.

Anti-tumor effect of 125I-UdR in combination with Egr-1 promoter-based IFNγ gene therapy in vivo

Although (125)I-UdR treatment of malignant tumors in animal models and patients has achieved a certain effect, the short half-life of (125)I-UdR in vivo and its cellular uptake only in S phase of the cell cycle are limiting factors with regard to tumor eradication, and therefore its combination with other applications is a promising strategy in cancer therapy. In this study, we show that (125)I-UdR radionuclide therapy in combination with Egr-1 promoter-based IFNγ gene therapy is more effective than (125)I-UdR therapy alone in suppressing tumor growth and extending survival duration in mice bearing H22 hepatomas. Combined therapy could significantly inhibit cell proliferation and tumor angiogenesis, induce apoptosis and enhance cytotoxic activities of splenic CTL of the mice. Our results suggest that (125)I-UdR in combination with Egr-1 promoter-based IFNγ gene therapy may provide novel approaches for cancer treatment.

First human treatment of resistant neoplastic meningitis by intrathecal administration of MTX plus (125)IUdR

PURPOSE

Neoplastic meningitis is often the final outcome of disseminated cancer and is rapidly lethal. Its limited treatment relies on systemic or intrathecal chemotherapy with methotrexate (MTX) or thiotepa. When 5-iodo-2'-deoxyuridine labeled with (125)I ((125)IUdR) is incorporated into the DNA of mitotic tumor cells, the Auger electrons emitted during iodine decay are highly cytotoxic. The radiotherapeutic efficacy of (125)IUdR administered intrathecally has also been established in animals bearing spinal cord tumors, and MTX is known to potentiate the response. This approach has not been tested in the clinic.

METHODS

A 44-year-old woman, with locally advanced pancreatic cancer, was treated for three years with complete systemic remission, but then relapsed with cytologically proven neoplastic meningitis. The patient was given four successive intrathecal injections of MTX (10 mg) every 12 h and, with the fourth dose, 1850 MBq (125)IUdR, followed by four additional MTX doses. The response was monitored by cytology and CA19.9 (carbohydrate antigen 19.9) levels in the cerebrospinal fluid (CSF) as well as by clinical status of the patient.

RESULTS

The follow-up of cytology and CA19.9 levels in the CSF showed dramatic improvement within 26 days followed by a biological relapse on Day +36. There was no evidence of local central nervous system toxicity. Three months later, neoplastic meningitis recurred and meningeal tumor infiltration was observed on magnetic resonance imaging. Six months after MTX-(125)IUdR treatment, the patient died.

CONCLUSION

(125)IUdR treatment proved to be feasible without acute neurological toxicity and seemed to have produced a biological response. This attempt provides the basis for designing prospective clinical trials.

Preferential tumor targeting and selective tumor cell cytotoxicity of 5-[131/125I]iodo-4'-thio-2'-deoxyuridine

PURPOSE

Auger electron emitting radiopharmaceuticals are attractive for targeted nanoirradiation therapy, provided that DNA of malignant cells is selectively addressed. Here, we examine 5-[123/125/131I]iodo-4'-thio-2'-deoxyuridine (ITdU) for targeting DNA in tumor cells in a HL60 xenograft severe combined immunodeficient mouse model.

EXPERIMENTAL DESIGN

Thymidine kinase and phosphorylase assays were done to determine phosphorylation and glycosidic bond cleavage of ITdU, respectively. The biodistribution and DNA incorporation of ITdU were determined in severe combined immunodeficient mice bearing HL60 xenografts receiving pretreatment with 5-fluoro-2'-deoxyuridine (FdUrd). Organ tissues were dissected 0.5, 4, and 24 h after radioinjection and uptake of [131I]ITdU (%ID/g tissue) was determined. Cellular distribution of [125I]ITdU was imaged by microautoradiography. Apoptosis and expression of the proliferation marker Ki-67 were determined by immunohistologic staining using corresponding paraffin tissue sections.

RESULTS

ITdU is phosphorylated by thymidine kinase 1 and stable toward thymidylate phosphatase-mediated glycosidic bond cleavage. Thymidylate synthase-mediated deiodination of [123/125/131I]ITdU was inhibited with FdUrd. Pretreatment with FdUrd increased preferentially tumor uptake of ITdU resulting in favorable tumor-to-normal tissue ratios and tumor selectivity. ITdU was exclusively localized within the nucleus and incorporated into DNA. In FdUrd-pretreated animals, we found in more than 90% of tumor cells apoptosis induction 24 h postinjection of ITdU, indicating a highly radiotoxic effect in tumor cells but not in cells of major proliferating tissues.

CONCLUSION

ITdU preferentially targets DNA in proliferating tumor cells and leads to apoptosis provided that the thymidylate synthase is inhibited.

Effects of DNA-targeted ionizing radiation produced by 5-[125I]iodo-2'-deoxyuridine on global gene expression in primary human cells

Background

This study assesses the whole-genome gene expression changes in a panel of primary human cell lines in response to DNA damage mediated by decay of DNA-incorporated radioiodinated thymidine analog 5-[¹²⁵I]iodo-2'-deoxyuridine (¹²⁵I-IUdR). Three normal human cell lines of different origin, namely, gingival fibroblasts AG09319, fetal skin fibroblasts GM05388 and neonatal foreskin epidermal keratinocytes (NHFK) were used in this study. DNA molecules were radiolabeled by incubation of cells in culture in a medium supplemented with either 3.7 kBq/ml or 18.5 kBq/ml of ¹²⁵I-IUdR for 24 h followed by incubation in IUdR-free medium for additional 24 hours. Each experiment was carried out in quadruplicate. ¹²⁵I-IUdR uptake was monitored by measuring DNA-associated radioactivity. The whole-genome gene expression changes were evaluated using Agilent Human Whole Genome oligo microarrays containing 44,290 elements representing all known and predicted human genes. DNA microarray dataset was independently partially validated with quantitative real-time PCR (RT-PCR).

Results

AG09319 gingival cells in culture responded to ¹²⁵I-IUdR treatment by changing the expression level of 335 genes in total, whereas under the same conditions GM05388 and NHFK cells differentially expressed 49 genes and 27 genes, respectively. However, for GM05388 cells the number of differentially expressed genes increases with the rise of ¹²⁵I-IUdR concentrations in cell culture media. The key up-regulated biological processes in a chosen panel of cell lines concern the regulation of protein kinase activities and/or cell death. Genes repressed in response to ¹²⁵I-IUdR treatment are involved in cytokinesis, M phase of the cell cycle, chromosome architecture and organization, DNA metabolism, DNA packaging, DNA repair and response to DNA damage. Despite the disparate nature of the gene patterns elicited by ¹²⁵I-induced DNA damage among the different cell lines, the differentially expressed transcripts reveal strikingly non-random chromosomal distribution in all the cell lines we used.

Conclusion

Our data suggest that DNA-targeted ionizing radiation produced by ¹²⁵I-IUdR results in changes in expression of only a limited subset of genes in primary human cells. The responsive genes are distributed non-randomly among the chromosomes; and a significant fraction of them is p53-dependent in the transcriptional regulation.

Radiotargeting agents for cancer therapy

Although the general radiobiological principles underlying external beam therapy and radionuclide therapy are the same, there are significant differences in the radiobiological effects observed in mammalian cells. In external beam therapy and brachytherapy, emissions are composed of photons, whereas in radionuclide therapy, the radiations of interest are particulate. This article will explore the special features that characterise the biological effects consequent to the traversal of charged particles through mammalian cells. In addition, it will highlight what has been learned when these radionuclides and targeting radiopharmaceuticals are used to treat cancers.

The amazing world of auger electrons

Over the past 40 years, a small and highly committed group of scientists has pursued various investigations focused on understanding the physical phenomena underlying the emission of Auger electrons, the dosimetric implications of their submicroscopic deposition of energy, their radiobiological effects at the molecular and cellular levels, and their therapeutic potential in tumor-bearing animals and patients with cancer. Herein, I present an overview--historic vignette--of the exciting findings reported in this field and outline the unique opportunities given to the fortunate few who have, mostly through serendipity, been working within the fascinating world of Auger electron emitters.

Therapeutic potential of 5-[125I]iodo-2'-deoxyuridine and methotrexate in the treatment of advanced neoplastic meningitis

PURPOSE

To assess the therapeutic potential of methotrexate (MTX) and 5-[1251]liodo-2'-deoxyuridine (125IdUrd) administered sequentially in rats bearing advanced (ten-day-old) intrathecal (i.t.) TE671 human rhabdomyosarcoma tumours.

MATERIALS AND METHODS

Nude rats were injected with TE671 cells through an i.t. placed catheter. Ten days later, the animals were injected i.t. over a 12-day period with (i) saline daily, (ii) MTX every other day, (iii) 125IdUrd every other day, or (iv) MTX and 125IdUrd on alternating days. Onset of paralysis was determined as a function of time, and the medians for onset (M), percentage of cells killed (% kill), and log cell kill were calculated.

RESULTS

The data show that (i) injection of MTX leads to a moderate delay in the onset of paralysis (M(MTX) = 29 d versus Msaline = 20 d), (ii) administration of 125IdUrd is more effective (M(IdUrd) = 36 d), and (iii) sequential administration of MTX- 125IdUrd further increases the therapeutic efficacy of 125IdUrd (M(MTX)-IdUrd = 47 d).

CONCLUSIONS

Intrathecal injection of MTX-(125)IdUrd is efficacious in the therapy of advanced intrathecal tumours.

Improving the outcome of patients with leptomeningeal cancer: new clinical trials and experimental therapies

Current therapy for leptomeningeal metastases is predominantly palliative. In an effort to improve disease control and patient outcome, new strategies are being developed to target the cerebrospinal space. These include new intrathecal formulations of systemic chemotherapy as well as the development of radiolabeled immunoconjugates and antitumor antibodies. Furthermore, there is debate as to the optimal strategy of drug delivery for leptomeningeal tumor.

Boron-Containing Nucleosides as Potential Delivery Agents for Neutron Capture Therapy of Brain Tumors

The purpose of the present study was to evaluate both in vitro and in vivo a series of boron-containing nucleosides that potentially could be used as delivery agents for neutron capture therapy. The rationale for their synthesis was based on the fact that proliferating neoplastic cells have increased requirements for nucleic acid precursors, and, therefore, they should preferentially localize in the tumor. A series of 3-carboranlyalkyl thymidine analogs has been synthesized and a subset, designated N4, N5, and N7, and the corresponding 3-dihydroxypropyl derivatives, designated N4-2OH, N5-2OH, and N7-2OH, have been selected for evaluation. Using these compounds as substrates for recombinant human thymidine kinase-1 and the mitochondrial isoenzyme thymidine kinase-2, the highest phosphorylation levels relative to thymidine were seen with N5 and the corresponding dihydroxypropyl analog N5-2OH. In contrast, N4, N4-OH, N7, and N7-OH had substantially lower phosphorylation levels. To compare compounds with high and low thymidine kinase-1 substrate activity, N5 and N7 and the corresponding dihydroxypropyl derivatives were selected for evaluation of their cellular toxicity, uptake and retention by the F98 rat glioma, human MRA melanoma, and murine L929 cell lines, all of which are thymidine kinase-1(+), and a mutant L929 cell line that is thymidine kinase-1(-). N5-2OH was the least toxic (IC50, 43-70 microm), and N7 and N7-2OH were the most toxic (IC50, 18-49 microm). The highest boron uptake was seen with N7-2OH by the MRA 27 melanoma and L929 wild-type (wt) cell lines. The highest retention was seen with L929 (wt) cells, and this ranged from 29% for N5-2OH to 46% for N7. Based on the in vitro toxicity and uptake data, N5-2OH was selected for in vivo biodistribution studies either in rats bearing intracerebral implants of the F98 glioma or in mice bearing either s.c. or intracerebral implants of L929 (wt) tumors. At 2.5 hours after convection-enhanced delivery, the boron values for the F98 glioma and normal brain were 16.2 +/- 2.3 and 2.2 microg/g, respectively, and the tumor to brain ratio was 8.5. Boron values at 4 hours after convection-enhanced delivery of N5-2OH to mice bearing intracerebral implants of L929 (wt) or L929 thymidine kinase-1(-) tumors were 39.8 +/- 10.8 and 12.4 +/- 1.6 microg/g, respectively, and the corresponding normal brain values were 4.4 and 1.6 microg/g, thereby indicating that there was selective retention by the thymidine kinase-1(+) tumors. Based on these favorable in vitro and in vivo data, neutron capture therapy studies will be initiated using N5-2OH in combination with two non-cell cycle dependent boron delivery agents, boronophenylalanine and sodium borocaptate.

Highly efficient DNA incorporation of intratumourally injected [125I]iododeoxyuridine under thymidine synthesis blocking in human glioblastoma xenografts

Intratumoural (i.t.) injection of radio-iododeoxyuridine (IdUrd), a thymidine (dThd) analogue, is envisaged for targeted Auger electron- or beta-radiation therapy of glioblastoma. Here, biodistribution of [(125)I]IdUrd was evaluated 5 hr after i.t. injection in subcutaneous human glioblastoma xenografts LN229 after different intravenous (i.v.) pretreatments with fluorodeoxyuridine (FdUrd). FdUrd is known to block de novo dThd synthesis, thus favouring DNA incorporation of radio-IdUrd. Results showed that pretreatment with 2 mg/kg FdUrd i.v. in 2 fractions 0.5 hr and 1 hr before injection of radio-IdUrd resulted in a mean tumour uptake of 19.8% of injected dose (% ID), representing 65.3% ID/g for tumours of approx. 0.35 g. Tumour uptake of radio-IdUrd in non-pretreated mice was only 4.1% ID. Very low uptake was observed in normal nondividing and dividing tissues with a maximum concentration of 2.9% ID/g measured in spleen. Pretreatment with a higher dose of FdUrd of 10 mg/kg prolonged the increased tumour uptake of radio-IdUrd up to 5 hr. A competition experiment was performed in FdUrd pretreated mice using i.t. co-injection of excess dThd that resulted in very low tumour retention of [(125)I]IdUrd. DNA isolation experiments showed that in the mean >95% of tumour (125)I activity was incorporated in DNA. In conclusion, these results show that close to 20% ID of radio-IdUrd injected i.t. was incorporated in tumour DNA after i.v. pretreatment with clinically relevant doses of FdUrd and that this approach may be further exploited for diffusion and therapy studies with Auger electron- and/or beta-radiation-emitting radio-IdUrd.

Treatment of intracranial rat glioma model with implant of radiosensitizer and biomodulator drug combined with external beam radiotherapy

PURPOSE

To evaluate an intracranial polymer implant containing bromodeoxyuridine (BrdUrd) and N-(phosphonacetyl)-L-aspartic acid (PALA) in combination with external beam radiotherapy (EBRT) in the treatment of a rat glioma.

METHODS AND MATERIALS

Combinations of the biomodulators 5-fluorouracil, methotrexate, or PALA with BrdUrd were evaluated as radiosensitizers in vitro by clonogenic assay. In in vivo experiments, BrdUrd and PALA were incorporated into a polyanhydride-based polymer, bis(p-carboxyphenoxy)propane sebacic acid, and implanted in the C6 rat glioma growing intracranially. The effectiveness of treatment was evaluated on the basis of survival. EBRT was given as 10-MV X-rays.

RESULTS

In tissue culture experiments, C6 cells were refractory to radiosensitization by BrdUrd even when the thymidine analog was combined with a biomodulator intended to reduce de novo thymidine synthesis. The most effective compound in vitro was PALA. When PALA and BrdUrd in a polymer formulation were implanted intracranially and combined with 10-Gy EBRT, the treatment was highly effective, with 83% of treated rats surviving 180 days.

CONCLUSION

Although the in vitro results were not encouraging, the combination of intratumoral BrdUrd and PAL with 10-Gy EBRT was highly effective in treating a rat glioma. These results indicate the clinical potential of combined and mixed modality treatments involving intratumoral sustained-release drug delivery.

Radionuclide Therapy with Iodine-125 and Other Auger–Electron-Emitting Radionuclides: Experimental Models and Clinical Applications

Auger-electron emitters represent an attractive alternative to beta-particle emitters for cancer therapy if they can be placed intracellularly, especially in close proximity to (or within) nuclear DNA. Based on investigations in animal tumor models, including those for ovarian cancer, bladder cancer, and brain and spinal cord tumors, in which the thymidine analog 5-radioiodo-2'-deoxyuridine (*IUdR) has been shown to be therapeutically efficacious, it is hypothesized that iodine-125 and other Auger-electron-emitting radionuclides might be valuable in the treatment of certain malignant diseases, assuming that uptake of the radiopharmaceutical by tumor cells exceeds that by normal dividing cells. Preliminary patient studies have shown that this requirement can be met partially by the locoregional administration of the radiopharmaceutical and metabolic modulation of its uptake by tumor cells. Investigators continue to seek molecules that can carry Auger-electron emitters to nuclear DNA, especially those radionuclides with higher Auger-electron yields and varying half-lives.

A balanced deoxyribonucleoside mixture increased the rate of DNA incorporation of 5-[125I]Iodo-2'-deoxyuridine in glioblastoma cells

Administration of nucleotide mixtures has been shown to restore and sustain the proliferation of leukocytes and enterocytes. Since it has been suggested that cancer cells use exogenous nucleotides more efficiently than normal cells, we hypothesized that administration of nucleotide mixtures would also stimulate the proliferation of cancer cells, thereby increasing the number of cells targeted by the thymidine analog 5-[(125)I]iodo-2'-deoxyuridine ([(125)I]IUdR). We first evaluated the influence of different deoxyribonucleoside mixtures on the DNA incorporation of [(125)I]IUdR in 3 human glioblastoma cell lines. Results showed that a 4-h coincubation with a mixture of identical concentration (10 microM) of deoxyadenosine, deoxyuridine, deoxyguanosine and deoxycytidine (AUGC) increased by 8.5-, 6.2-, and 2.0-fold the rate of DNA incorporation of [(125)I]IUdR in exponentially growing LN229, U87 and U251 cells, respectively. Replacing deoxyuridine by thymidine (ATGC) reversed the effect of the mixture, whereas removing deoxyuridine allowed a mixture of 10 microM AGC to increase by 2.2-fold the rate of DNA incorporation of [(125)I]IUdR in LN229 cells. Furthermore, the rate of DNA incorporation of [(125)I]IUdR in LN229 and U87 cells was increased up to 19.9- and 9.4-fold, respectively, by extending the coincubation time with 10 microM AUGC to 9 h, and up to 40.9- and 26.8-fold by incubating confluent cells for 4 h with 10 microM AUGC. Flow cytometry analysis showed that exposure of confluent cells to AUGC increased the percentage of cells in S phase of the cell cycle. Thus, co-administration of a balanced deoxyribonucleoside mixture may improve the use of radiolabeled nucleotide analogs, such as [(125)I]IUdR, for the targeting of cancer cells.

Lack of cell death enhancement after irradiation with monochromatic synchrotron X rays at the K-shell edge of platinum incorporated in living SQ20B human cells as cis-diamminedichloroplatinum (II)

In this paper we describe the results of experiments using synchrotron radiation to trigger the Auger effect in living human cancer cells treated with a widely used chemotherapy drug: cis-diamminedichloroplatinum (II) (cisplatin). The experiments were carried out at the ID17 beamline of the European Synchrotron Radiation Facility, which produces a high-fluence monochromatic beam that is adjustable from 20 to 80 keV. Cisplatin was chosen as the carrier of platinum atoms in the cells because of its alkylating-like activity and the irradiation was done with monochromatic beams above and below the platinum K-shell edge (78.39 keV). Cell survival curves were comparable with those obtained for the same cells under conventional irradiation conditions. At a low dose of cisplatin (0.1 microM, 48 h), no difference was seen in survival when the cells were irradiated above and below the K-shell edge of platinum. Higher cisplatin concentrations were investigated to enhance the cellular platinum content. The results with 1 microM cisplatin for 12 h showed no difference when the cells were irradiated with beams above or below the platinum K-shell edge with the exception of the higher cell death resulting from drug toxicity. The intracellular content of platinum was significant, as measured macroscopically by inductively coupled plasma mass spectrometry. Its subcellular localization and particularly its presence in the cell nucleus were verified by microscopic synchrotron X-ray fluorescence. This was the first known attempt at K-shell edge photon activation of stable platinum in living cells with a platinum complex used for chemotherapy. Its evident toxicity in these cells leads us to put forth the hypothesis that cisplatin toxicity can mask the enhancement of cell death induced by the irradiation above the K-shell edge. However, K-shell edge photon activation of stable elements provides a powerful technique for the understanding of the biological effects of Auger processes. Further avenues of development are discussed.

Bystander effect produced by radiolabeled tumor cells in vivo

The bystander effect, originating from cells irradiated in vitro, describes the biologic response(s) of surrounding cells not directly targeted by a radiation insult. To overcome the limitations of in vitro tissue culture models and determine whether a bystander effect that is initiated by the in vivo decay of a radionuclide can be demonstrated in an animal, the ability of 5-[(125)I]iodo-2'-deoxyuridine ((125)IUdR)-labeled tumor cells to exert a damaging effect on neighboring unlabeled tumor cells growing s.c. in nude mice has been investigated. When mice are injected with a mixture of human colon LS174T adenocarcinoma cells and LS174T cells prelabeled with lethal doses of DNA-incorporated (125)I, a distinct inhibitory effect on the growth of s.c. tumor (derived from unlabeled cells) is observed. Because (i) the (125)I present within the cells is DNA-bound, (ii) approximately 99% of the electrons emitted by the decaying (125)I atoms have a subcellular range (<0.5 microm), and (iii) the overall radiation dose deposited by radiolabeled cells in the unlabeled cells within the growing tumor is <10 cGy, we conclude that the results obtained are a consequence of a bystander effect that is generated in vivo by factor(s) present within and/or released from the (125)IUdR-labeled cells. These in vivo findings significantly impact the current dogma for assessing the therapeutic potential of internally administered radionuclides. They also call for reevaluation of the approaches currently used for estimating the risks to individuals and populations inadvertently exposed internally to radioactivity as well as to patients undergoing routine diagnostic nuclear medical procedures.