Radiosensitization of rat glioma with bromodeoxycytidine and adenovirus expressing herpes simplex virus-thymidine kinase delivered by slow, rate-controlled positive pressure infusion

Electrophilic 5-Substituted Uracils as Potential Radiosensitizers: A Density Functional Theory Study

Although 5-bromo-2'-deoxyuridine (5BrdU) possesses significant radiosensitizing power in vitro, clinical studies do not confirm any advantages of radiotherapy employing 5BrdU. This situation calls for a continuous search for efficient radiosensitizers. Using the proposed mechanism of radiosensitization by 5BrdU, we propose a series of 5-substituted uracils, XYU, that should undergo efficient dissociative electron attachment. The DFT-calculated thermodynamic and kinetic data concerning the XYU degradations induced by electron addition suggests that some of the scrutinized derivatives have much better characteristics than 5BrdU itself. Synthesis of these promising candidates for radiosensitizers, followed by studies of their radiosensitizing properties in DNA context, and ultimately in cancer cells, are further steps to confirm their potential applicability in anticancer treatment.

The radiosensitivity of 5- and 6-bromocytidine derivatives--electron induced DNA degradation

Halogenated nucleotides belong to the group of radiosensitizers that sensitize solid tumors when incorporated into genomic DNA. Here, we consider the propensity of two isomeric bromocytidine derivatives, 3',5'-diphosphates of 5-bromo-2'-deoxycytidine (5BrdCDP) and 6-bromo-2'-deoxycytidine (6BrdCDP), to be damaged by electrons - one of the most abundant products formed during radiotherapy. An intranucleotide degradation mechanism leading to phosphodiester bond breakage (a model of single strand breakage in labeled DNA) and a ketone derivative formation was found for 6BrdCDP, while for 5BrdCDP a similar mechanism is sterically hindered. 5BrdCDP is, therefore, suggested to undergo electron induced degradation involving hydrogen transfer from a neighboring nucleotide or environment.

Photoinduced single strand breaks and intrastrand cross-links in an oligonucleotide labeled with 5-bromouracil

5-Bromouracil (BrU) is photoreactive toward near UVB photons and can be introduced into genomic DNA during its biosynthesis in cells. However, PCR seems to be a simpler approach, which can be used to obtain labeled DNA similar to that synthesized within the cell. In the current work, PCR has been employed and optimized in order to substitute all thymines (besides those present in starters) with BrU in the dsDNA fragment of 80 base pairs (bp) in length. The modified oligonucleotide was irradiated with 300 nm photons in a buffered aqueous solution (pH = 7) and digested with a cocktail of enzymes specific to the phosphodiester bond cleavage. Initially, the extent of damage in the intact photolyte was measured with DHPLC. Then, the digested reaction mixture was subjected to HPLC and MS analyses and, in addition to the formation of 5-bromo-2'-deoxuyridine, which proves the occurrence of single strand breaks (SSBs) due to irradiation, U∧U and U∧C dimers were found, whose molecular structure was confirmed by MS/MS analysis. Although the abundance of such tandem lesions is lower than that of the SSB type, they pose a potent threat to genome integrity. Thus, our findings shed new light on the photosensitizing properties of BrU toward DNA.

Electron induced single strand break and cyclization: a DFT study on the radiosensitization mechanism of the nucleotide of 8-bromoguanine

Cleavage of the O-P bond in 8-bromo-2'-deoxyguanosine-3',5'-diphosphate (BrdGDP), considered as a model of single strand break (SSB) in labelled double-stranded DNA (ds DNA), is investigated at the B3LYP/6-31++G(d,p) level. The thermodynamic and kinetic characteristics of the formation of SSB are compared to those related to the 5',8-cycloguanosine lesion. The first reaction step, common to both damage types, which is the formation of the reactive guanyl radical, proceeds with a barrier-free or low-barrier release of the bromide anion. The guanyl radical is then stabilized by hydrogen atom transfer from the C3' or C5' sites of the 2'-deoxyribose moiety to its C8 center. The C3' path, via the O-P bond cleavage, leads to a ketone derivative (the SSB model), while the C5' path is more likely to yield 5',8-cycloguanosine.

Fundamental mechanisms of DNA radiosensitization: damage induced by low-energy electrons in brominated oligonucleotide trimers

The replacement of nucleobases with brominated analogs enhances DNA radiosensitivity. We examine the chemistry of low-energy electrons (LEEs) in this sensitization process by experiments with thin films of the oligonucleotide trimers TBrXT, where BrX = 5-BrU (5-bromouracil), 5-BrC (5-bromocytosine), 8-BrA (8-bromoadenine), or 8-BrG (8-bromoguanine). The products induced from irradiation of thin (∼ 2.5 nm) oligonucleotide films, with 10 eV electrons, under ultrahigh vacuum (UHV) are analyzed by HPLC-UV. The number of damaged brominated trimers ranges from about 12 to 15 × 10(-3) molecules per incident electron, whereas under the identical conditions, these numbers drop to 4-7 × 10(-3) for the same, but nonbrominated oligonucleotides. The results of HPLC analysis show that the main degradation pathway of trinucleotides containing brominated bases involve debromination (i.e., loss of the bromine atom and its replacement with a hydrogen atom). The electron-induced sum of products upon bromination increases by factors of 2.1 for the pyrimidines and 3.2 for the purines. Thus, substitution of any native nucleobase with a brominated one in simple models of DNA increases LEE-induced damage to DNA and hence its radiosensitivity. Furthermore, besides the brominated pyrimidines that have already been tested in clinical trials, brominated purines not only appear to be promising sensitizers for radiotherapy, but could provide a higher degree of radiosensitization.

Electron-induced elimination of the bromide anion from brominated nucleobases. A computational study

The enhancement of radiodamage to DNA labeled with halonucleobases is attributed to the reactive radical produced from a halonucleobase by the attachment of an electron. We examined at the B3LYP/6-31++G** level electron capture by four brominated nucleobases (BrNBs): 8-bromo-9-methyladenine, 8-bromo-9-methylguanine, 5-bromo-1-methylcytosine, and 5-bromo-1-methyluracil followed by the release of the bromide anion and a nucleobase radical. We demonstrate that neutral BrNBs in both gas and aqueous phases are better electron acceptors than unsubstituted NBs and that resulting anion radicals, BrNBs(•-), can easily transform into the product complex of the bromide anion and the nucleobase radical ([Br(-)···NB(•)]). The overall thermodynamic stimulus for the process starting with the neutral BrNB and ending with the isolated bromide anion and the NB(•) radical is similar in the case of all four BrNBs studied, which suggests their comparable radiosensitizing capabilities.

Electron stimulated desorption of anions from native and brominated single stranded oligonucleotide trimers

We measured the low energy electron stimulated desorption (ESD) of anions from thin films of native (TXT) and bromine monosubstituted (TBrXT) oligonucleotide trimers deposited on a gold surface (T = thymidine, X = T, deoxycytidine (C), deoxyadenosine (A) or deoxyguanosine (G), Br = bromine). The desorption of H(-), CH(3)(-)/NH(-), O(-)/NH(2)(-), OH(-), CN(-), and Br(-) was induced by 0 to 20 eV electrons. Dissociative electron attachment, below 12 eV, and dipolar dissociation, above 12 eV, are responsible for the formation of these anions. The comparison of the results obtained for the native and brominated trimers suggests that the main pathways of TBrXT degradation correspond to the release of the hydride and bromide anions. Significantly, the presence of bromine in oligonucleotide trimers blocks the electron-induced degradation of nuclobases as evidenced by a dramatic decrease in CN(-) desorption. An increase in the yields of OH(-) is also observed. The debromination yield of particular oligonucleotides diminishes in the following order: BrdU > BrdA > BrdG > BrdC. Based on these results, 5-bromo-2(')-deoxyuridine appears to be the best radiosensitizer among the studied bromonucleosides.

An evaluation of site-specific immune responses directed against first-generation adenoviral vectors administered by convection-enhanced delivery

BACKGROUND

Direct adenoviral vector injections into the brain have been used in clinical trials to treat patients with high-grade gliomas. However, a recent phase 3 trial using first-generation vectors failed to demonstrate significant survival benefits. Malignant gliomas infiltrate extensively through the white matter, making them difficult to treat, and chemotherapy is at best partially effective. Convection enhanced delivery (CED) represents a rationale approach for achieving widespread targeting of infiltrating tumour cells. Previous studies have demonstrated that infusions of particle numbers above a threshold level [10(8) plaque-forming units (pfu)] are associated with a pronounced inflammatory response in rat grey matter, although no such comparisons have been made with CED infusions into the white matter.

METHODS

In the present study, we investigated the distribution and immune response after the administration of 10(7) and 10(9) pfu of a first-generation adenoviral vector (Ad.CMV.EGFP) by CED in both small and large animal models.

RESULTS

We show that Ad.CMV.EGFP can be efficiently distributed by CED over large volumes of brain. A threshold vector dose of between 10(7) and 10(9) pfu was seen in both rat striatum and white matter, above which transgene expression was lost at 30 days. Furthermore, all adenoviral infusions were associated with evidence of significant tissue damage, as demonstrated by loss of neurones and astrocytes or the presence of extensive astrocytosis.

CONCLUSIONS

These results indicate that CED is capable of mediating widespread adenoviral vector distribution, although these vectors are associated with significant tissue toxicity that may render their safe application in clinical trials unfeasible.

External irradiation models for intracranial 9L glioma studies

PURPOSE

Radiotherapy has been shown to be an effective for the treatment human glioma and consists of 30 fractions of 2 Gy each for 6-7 weeks in the tumor volume with margins. However. in preclinical studies, many different radiation schedules are used. The main purpose of this work was to review the relevant literature and to propose an external whole-brain irradiation (WBI) protocol for a rat 9L glioma model.

MATERIALS AND METHODS

9L cells were implanted in the striatum of twenty 344-Fisher rats to induce a brain tumor. On day 8, animals were randomized in two groups: an untreated group and an irradiated group with three fractions of 6 Gy at day 8, 11 and 14. Survival and toxicity were assessed.

RESULTS

Irradiated rats had significantly a longer survival (p = 0.01). No deaths occurred due to the treatment. Toxicities of reduced weight and alopecia were increased during the radiation period but no serious morbidity or mortality was observed. Moreover, abnormalities disappeared the week following the end of the therapeutic schedule.

CONCLUSIONS

Delivering 18 Gy in 3 fractions of 6 Gy every 3 days, with mild anaesthesia, is safe, easy to reproduce and allows for standardisation in preclinical studies of different treatment regimens glioma rat model.

Anti‐erbB receptor strategy as a gene therapeutic intervention to improve radiotherapy in malignant human tumours

PURPOSE

We explored and quantified the therapeutic potential of using dominant-negative EGFR transduction with replication-incompetent adenovirus (Ad-EGFR-CD533 or Ad-CD533) as a genetic approach for radiosensitization in different carcinoma and malignant glioma cell lines in vitro and in established tumour xenografts in vivo.

MATERIAL AND METHODS

The cell lines MDA-MB-231, A-431, U-373 MG, U-87 MG and T47D were used. The ErbB expression profiles were quantified by Western blotting. MAPK immune complex assay measured MAPK activity with or without EGFR-CD533 expression after ionizing radiation. Radiosensitization was determined and quantified in vitro by colony-formation assays, in vivo by use of an ex vivo-in vitro colony-formation assay after intratumoral infusion of the adenoviral vectors expressing EGFR-CD533 or the control LacZ.

RESULTS

Western blotting demonstrated widely varied expression levels of the ErbB receptors in the tested cell lines. Expression of EGFR-CD533 effectively blocked the radiation-induced activation of MAPK, leading to significant radiosensitization in vitro and in vivo.

CONCLUSIONS

The radiation-induced ErbB activation can be effectively modulated by a gene therapeutic approach of over-expressing EGFR-CD533 leading to tumour cell radiosensitization after single and repeated radiation exposures both in vitro and in vivo.

Transient exposure of carcinoma cells to RAS/MEK inhibitors and UCN-01 causes cell death in vitro and in vivo

The present studies were initiated to determine in greater molecular detail how MEK1/2 inhibitors [PD184352 and AZD6244 (ARRY-142886)] interact with UCN-01 (7-hydroxystaurosporine) to kill mammary carcinoma cells in vitro and radiosensitize mammary tumors in vitro and in vivo and whether farnesyl transferase inhibitors interact with UCN-01 to kill mammary carcinoma cells in vitro and in vivo. Expression of constitutively activated MEK1 EE or molecular suppression of JNK and p38 pathway signaling blocked MEK1/2 inhibitor and UCN-01 lethality, effects dependent on the expression of BAX, BAK, and, to a lesser extent, BIM and BID. In vitro colony formation studies showed that UCN-01 interacted synergistically with the MEK1/2 inhibitors PD184352 or AZD6244 and the farnesyl transferase inhibitors FTI277 and R115,777 to kill human mammary carcinoma cells. Athymic mice carrying approximately 100 mm(3) MDA-MB-231 cell tumors were subjected to a 2-day exposure of either vehicle, R115,777 (100 mg/kg), the MEK1/2 inhibitor PD184352 (25 mg/kg), UCN-01 (0.2 mg/kg), or either of the drugs in combination with UCN-01. Transient exposure of tumors to R115,777, PD184352, or UCN-01 did not significantly alter tumor growth rate or the mean tumor volume in vivo approximately 15 to 30 days after drug administration. In contrast, combined treatment with R115,777 and UCN-01 or with PD184352 and UCN-01 significantly reduced tumor growth. Tumor cells isolated after combined drug exposure exhibited a significantly greater reduction in plating efficiency using ex vivo colony formation assays than tumor cells that were exposed to either drug individually. Irradiation of mammary tumors after drug treatment, but not before or during treatment, significantly enhanced the lethal effects of UCN-01 and MEK1/2 inhibitor treatment. These findings argue that UCN-01 and multiple inhibitors of the RAS-MEK pathway have the potential to suppress mammary tumor growth, and to interact with radiation, in vitro and in vivo.

Sequence-dependent formation of intrastrand crosslink products from the UVB irradiation of duplex DNA containing a 5-bromo-2'-deoxyuridine or 5-bromo-2'-deoxycytidine

The replacement of thymidine with 5-bromo-2′-deoxyuridine (^BrdU) is well-known to sensitize cells to ionizing radiation and photoirradiation. We reported here the sequence-dependent formation of intrastrand crosslink products from the UVB irradiation of duplex oligodeoxynucleotides harboring a ^BrdU or its closely related 5-bromo-2′-deoxycytidine (^BrdC). Our results showed that two types of crosslink products could be induced from d(^BrCG), d(^BrUG), d(G^BrU), or d(A^BrU); the C(5) of cytosine or uracil could be covalently bonded to the N(2) or C(8) of its neighboring guanine, and the C(5) of uracil could couple with the C(2) or C(8) of its neighboring adenine. By using those crosslink product-bearing dinucleoside monophosphates as standards, we demonstrated, by using liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS), that all the crosslink products described above except d(G[N(2)-5]U) and d(G[N(2)-5]C) could form in duplex DNA. In addition, LC-MS/MS quantification results revealed that both the nature of the halogenated pyrimidine base and its 5′ flanking nucleoside affected markedly the generation of intrastrand crosslink products. The yields of crosslink products were much higher while the 5′ neighboring nucleoside was a dG than while it was a dA, and ^BrdC induced the formation of crosslink products much more efficiently than ^BrdU. The formation of intrastrand crosslink products from these halopyrimidines in duplex DNA may account for the photosensitizing effects of these nucleosides.

Introduction to the background, principles, and state of the art in suicide gene therapy

Gene therapy is defined as a technology that aims to modify the genetic component of cells to gain therapeutic benefits. Suicide gene therapy (or gene-directed enzyme prodrug therapy [GDEPT]) is a two-step treatment for cancer (especially, solid tumors). In the first step, a gene for a foreign enzyme is delivered to the tumor by a vector. Following the expression of the foreign enzyme, a prodrug is administered during the second step, which is selectively activated in the tumor. This article discusses the principles and the theorectical background of GDEPT. A special emphasis is put on enzyme/prodrug systems developed for GDEPT, the design of prodrugs and the kinetic of their activation, the types and the mechanisms of bystander effect and its immunological implications. The possible strategies to improve GDEPT are also discussed.

Inhibition of the type III epidermal growth factor receptor variant mutant receptor by dominant-negative EGFR-CD533 enhances malignant glioma cell radiosensitivity

PURPOSE

The commonly expressed variant epidermal growth factor receptor (EGFR), the type III EGFR variant (EGFRvIII), functions as an oncoprotein promoting neoplastic transformation and tumorigenicity. The role of EGFRvIII in cellular responses to genotoxic stress, such as ionizing radiation, is only minimally defined. Thus, we have investigated EGFRvIII as a potential modulator of cellular radiation responses and explored the feasibility of adenovirus (Ad)-mediated expression of dominant-negative EGFR-CD533 as a gene therapeutic approach for inhibiting EGFRvIII function in vitro and in vivo.

EXPERIMENTAL DESIGN AND RESULTS

EGFR-CD533 and EGFRvIII were expressed in vitro and in vivo in malignant U-373 MG glioma cells through transduction with an Ad vector, Ad-EGFR-CD533 and Ad-EGFRvIII, respectively. In vivo studies defined the importance of EGFRvIII as a modulator of radiation responses, demonstrating a 2.6-fold activation of EGFRvIII in U-373 malignant glioma tumors. Concomitant expression of EGFR-CD533 inhibited the radiation-induced activation of EGFRvIII in vitro and completely abolished the enhanced clonogenic survival conferred by EGFRvIII. The ability of EGFR-CD533 to inhibit EGFRvIII function was further confirmed in vivo through complete inhibition of EGFRvIII-mediated increased tumorigenicity and radiation-induced activation of EGFRvIII. Growth delay assays with U-373 xenograft tumors demonstrated that the expression of EGFR-CD533 significantly enhanced radiosensitivity of tumor cells under conditions of intrinsic and Ad-mediated EGFRvIII expression.

CONCLUSIONS

We conclude that EGFRvIII confers significant radioresistance to tumor cells through enhanced cytoprotective responses, and we have demonstrated that dominant-negative EGFR-CD533 effectively inhibits EGFRvIII function. These data affirm the broad potential of EGFR-CD533 to radiosensitize human malignant glioma cells.

Radiation-induced activation of a common variant of EGFR confers enhanced radioresistance

BACKGROUND AND PURPOSE

The type-III EGFR variant (EGFRvIII) is known to promote enhanced tumorigenicity. We have previously defined the importance of EGFRvIII in cellular radiation responses using Chinese hamster ovary cells (CHO). In the current study, we have extended our investigations of EGFRvIII to human tumor cells in vitro and in vivo and further verified the important role of EGFRvIII in modulating radiosensitivity.

MATERIAL AND METHODS

The cell lines MDA-MB-231, U-87 MG, A-431 and U-373 MG were used. Adenoviral (Ad) vectors were produced to overexpress EGFRvIII in vitro or in xenograft tumors in vivo. The EGFR, EGFRvIII expression and tyrosine phosphorylation (Tyr-P) levels were quantified by Western blotting. The relative radiosensitivities were assessed in vitro by standard colony formation and in vivo by tumor growth delay assays.

RESULTS

The presence of EGFRvIII was verified in all xenograft tumors tested with no detectable expression in the corresponding cells under in vitro culture conditions. MDA-MB-231 xenograft tumors demonstrated EGFRvIII expression levels, which were 1.9-fold higher relative to EGFRwt compared to a 14.5-fold higher Tyr-P. Ionizing radiation of these tumors at 4 Gy induced an average 3.2-fold increase in EGFRvIII Tyr-P. EGFRvIII expression in U-373 MG cells significantly enhanced survival after 4Gy, which was completely abolished by dominant-negative EGFR-CD533. Finally, the ability of EGFRvIII to accelerate tumor growth during irradiation was confirmed in vivo.

CONCLUSION

EGFRvIII is frequently expressed in a variety of different tumor types and can confer significant radioresistance, thus further providing evidence for EGFRvIII as an additional important target in our approaches to radiosensitize malignant solid tumors.

Halogenated 7-deazapurine nucleosides: stereoselective synthesis and conformation of 2'-deoxy-2'-fluoro-beta-D-arabinonucleosides

The stereoselective syntheses of 5-halogenated 7-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)-7H-pyrrolo[2,3-d]pyrimidine nucleosides 3b-d, 4a-c as well as 7-deaza-2'-deoxyisoguanosine are described. Nucleobase anion glycosylation of 2-amino-4-chloro-7H-pyrrolo[2,3-d]pyrimidine (5) with 3,5-di-O-benzoyl-2-deoxy-2-fluoro-alpha-D-arabinofuranosyl bromide (6) exclusively gave the beta-D-anomer, which was deblocked (--> 8), aminated at C4 (--> 3a) and selectively deaminated at C2 to yield 2'-deoxy-2'-fluoro-beta-D-arabinofuranosyl 7-deazaisoguanine (2). Condensation of the 5-halogenated 4-chloro-2-pivaloylamino-7H-pyrrolo[2,3-d]pyrimidines 9a-c with 6 furnished the N7-nucleosides 10a-c together with N2,N7-bisglycosylated compounds 11a-c. The former was converted to the corresponding 2,4-diamino-compounds 3b-d, and the latter was deblocked by NaOMe/MeOH to yield the 4-methoxy-nucleosides 4a-c. Conformational analysis of the sugar moiety of the nucleosides 2 and 3a-d was performed on the basis of vicinal [1H,1H] coupling constants. The fluorine atom in the sugar moiety shifts the sugar conformation from S towards N by about 10%, while the halogen substituents in the base moiety increase the hydrophobicity and polarizability of the nucleobases.

Biocompatibility of adenoviral vectors in poly(vinyl chloride) tubing catheters with presence or absence of plasticizer di-2-ethylhexyl phthalate

Adenoviral (Ad) vectors feature attractive characteristics for gene therapy of a wide variety of diseases. In many cases, the Ad vector must be administered using catheters and other plastic medical devices. Although poly(vinyl chloride) is one of the most frequently used catheter materials, it is relatively rigid and requires the addition of a plasticizer such as di-2-ethylhexyl phthalate (DEHP) to increase its flexibility. In this study, we demonstrated that exposure to a DEHP-containing catheter decreased the infectivity of Ad vectors but not the total particle number of the vector. Loss of Ad vector infectivity was directly related to the time of exposure to the DEHP-containing catheter, but it was not due to simple leaching of the chemical from the plastic. The loss of Ad vector infectivity could be prevented by preflushing the tube with albumin. Careful consideration of the compatibility between gene therapy vectors and medical delivery devices will be critical to the success of human gene therapy applications.

Radiosensitization of human glioma cells in vitro and in vivo with acyclovir and mutant HSV-TK75 expressed from adenovirus

PURPOSE

We recently reported that an adenovirus-expressing mutant HSV-TK75 (AdCMV-TK75) radiosensitized rat syngeneic gliomas in combination with low concentrations of acyclovir (ACV) much more effectively than a virus expressing wild-type herpes simplex virus thymidine kinase (HSV-TK). In this report we have examined whether similar radiosensitizing effects are also seen with human glioma cells in vitro and in vivo.

METHODS AND MATERIALS

Human U87 MG glioma cells were transduced with AdCMV-TK75 and exposed to ACV followed by single-dose irradiation and colony-forming survival assays. Similarly, U87 MG xenografts were infused with AdCMV-TK75 or Adbetagal control virus, followed by ACV administration and fractionated irradiation. Therapeutic efficacy was monitored by tumor growth.

RESULTS

U87 MG cells transduced with AdCMV-TK75 were significantly more sensitive to ACV (3 microM) than cells transduced with either wild-type HSV-TK or control virus. To determine whether human cells also demonstrate improved radiosensitization similar to that seen with rat glioma cells and tumors, we transduced U87 MG cells with either AdCMV-TK75, AdCMV-TK, expressing wild-type HSV-TK, or Adbetagal and then treated the cells with 3 microM of ACV. Cells transduced with AdCMV-TK75 were significantly more radiosensitive (dose enhancement ratio [D(37)]: 2.6) by colony-forming survival assay than cells transduced with either AdCMV-TK or Adbetagal. Furthermore, we found that U87 MG xenografts infused with AdCMV-TK75 by slow positive pressure infusion were more radiosensitive after administration of ACV than tumors infused with Adbetagal. A more dramatic result was achieved when fractionated irradiation was carried out concurrently with ACV administration, in which case AdCMV-TK75-treated tumors did not grow at all.

CONCLUSIONS

These results demonstrate that transduction of human glioma cells in vitro and infusion of xenografts in vivo with AdCMV-TK75 and treatment with concentrations of ACV that can be achieved in vivo produce similar radiosensitization, as previously reported with rat glioma cells and intracerebral syngeneic tumors. In addition, concurrent treatment with ACV and radiation therapy is more efficient than when ACV is administered before radiation therapy.

Adenovirus-mediated overexpression of dominant negative epidermal growth factor receptor-CD533 as a gene therapeutic approach radiosensitizes human carcinoma and malignant glioma cells

PURPOSE

Epidermal growth factor receptor (EGFR) and other members of the ErbB family of receptor tyrosine kinases (RTK) mediate autocrine growth regulation in a wide spectrum of human tumor cells. We have previously demonstrated that in stably transfected mammary carcinoma cells a dominant negative (DN) mutant of EGFR, EGFR-CD533 is a potent inhibitor of EGFR and its cytoprotective signaling after exposure to ionizing radiation. In the present study, we further investigate the capacity of a genetic approach, using replication-incompetent adenovirus (Ad)-mediated transfer of EGFR-CD533 (Ad-EGFR-CD533), to enhance the radiosensitivity in vitro of four cell lines representative of three major cancer phenotypes.

METHODS AND MATERIALS

The cell lines MDA-MB-231 and T-47D mammary carcinoma, A-431 squamous carcinoma, and U-373 MG malignant glioma cells were used. The ErbB expression profiles and the EGFR tyrosine phosphorylation (Tyr-P) levels following irradiation were quantified by Western blotting. The relative radiosensitivities of tumor cells were assessed by standard colony formation assays after infection with control vector (Ad-LacZ) or Ad-EGFR-CD533.

RESULTS

The expression profiles demonstrated varying levels of EGFR, ErbB2, ErbB3, and ErbB4 expression. The overexpression of EGFR-CD533 after infection with Ad-EGFR-CD533 completely inhibited the radiation-induced stimulation of EGFR Tyr-P relative to the immediate 2.4- to 3.1-fold increases in EGFR Tyr-P in control infected cells (Ad-LacZ). Ad-EGFR-CD533-infected cells demonstrated significant (p < 0.001) radiosensitization over a range of radiation doses (1-8 Gy), yielding dose-enhancement ratios (DER) between 1.4 and 1.7. This radiosensitization was maintained under conditions of repeated radiation exposures, using 3 x 2 Gy, yielding DERs of 1.6 and 1.7 for MDA-MB-231 and U-373 cells, respectively.

CONCLUSIONS

Overexpression of EGFR-CD533 significantly sensitizes human carcinoma and glioma cells to single and repeated radiation exposures irrespective of their ErbB expression levels. Therefore, transduction of human tumor cells with EGFR-CD533 holds promise as a gene therapeutic approach for the radiosensitization of neoplastic cells that are growth-regulated by EGFR or other ErbB receptors.

Epidermal growth factor receptor as a genetic therapy target for carcinoma cell radiosensitization

BACKGROUND

Exposure of human cancer cells to ionizing radiation activates the epidermal growth factor receptor (EGFR), which, in turn, mediates a cytoprotective response that reduces the cells' sensitivity to ionizing radiation. Overexpression of a dominant-negative EGFR mutant, EGFR-CD533, disrupts the cytoprotective response by preventing radiation-induced activation of the receptor and its downstream effectors. To investigate whether gene therapy with EGFR-CD533 has the potential to increase tumor cell radiosensitivity, we introduced an adenoviral vector containing EGFR-CD533 into xenograft tumors in nude mice and evaluated the tumor response to ionizing radiation.

METHODS

Xenograft tumors established from the human mammary carcinoma cell line MDA-MB-231 were transduced via infusion with the adenoviral vector Ad-EGFR-CD533 or a control vector containing the beta-galactosidase gene, Ad-LacZ. The transduced tumors were then exposed to radiation in the therapeutic dose range, and radiation-induced EGFR activation was assessed by examining the tyrosine phosphorylation of immunoprecipitated EGFR. Radiosensitization was determined in vitro by colony-formation assays. All statistical tests were two-sided.

RESULTS

The transduction efficiency of MDA-MB-231 tumors by Ad-LacZ was 44%. Expression of EGFR-CD533 in tumors reduced radiation-induced EGFR activation by 2.94-fold (95% confidence interval [CI] = 2.23 to 4.14). The radiosensitivity of Ad-EGFR-CD533-transduced tumors was statistically significantly higher (46%; P<.001) than that of Ad-LacZ-transduced tumors, yielding a dose-enhancement ratio of 1.85 (95% CI = 1.54 to 2.51).

CONCLUSIONS

Transduction of MDA-MB-231 xenograft tumors with Ad-EGFR-CD533 conferred a dominant-negative EGFR phenotype and induced tumor radiosensitization. Therefore, disruption of EGFR function through overexpression of EGFR-CD533 may hold promise as a gene therapeutic approach to enhance the sensitivity of tumor cells to ionizing radiation.