The role of the bystander effect in suicide gene therapy

Synergistic targeting strategies for prostate cancer

Prostate cancer is the second most commonly diagnosed cancer and the fifth leading cause of death among men worldwide. Androgen deprivation therapy is a common prostate cancer treatment, but its efficacy is often hindered by the development of resistance, which results in reducing survival benefits. Immunotherapy showed great promise in treating solid tumours; however, clinically significant improvements have not been demonstrated for patients with prostate cancer, highlighting specific drawbacks of this therapeutic modality. Hence, exploring novel strategies to synergistically enhance the efficacy of prostate cancer immunotherapy is imperative. Clinical investigations have focused on the combined use of targeted or gene therapy and immunotherapy for prostate cancer. Notably, tumour-specific antigens and inflammatory mediators are released from tumour cells after targeted or gene therapy, and the recruitment and infiltration of immune cells, including CD8+ T cells and natural killer cells activated by immunotherapy, are further augmented, markedly improving the efficacy and prognosis of prostate cancer. Thus, immunotherapy, targeted therapy and gene therapy could have reciprocal synergistic effects in prostate cancer in combination, resulting in a proposed synergistic model encompassing these three therapeutic modalities, presenting novel potential treatment strategies for prostate cancer.

Serum-Resistant Ternary DNA Polyplexes for Suicide Gene Therapy of Uterine Leiomyoma

Uterine leiomyoma (UL) is a prevalent benign tumor in women that frequently gives rise to a multitude of reproductive complications. The use of suicide gene therapy has been proposed as a highly promising method for treating UL. To achieve successful gene therapy, it is essential to develop carriers that can efficiently transport nucleic acids into targeted cells and tissues. The instability of polyplexes in blood and other biological fluids is a crucial factor to consider when using non-viral carriers. In this study, we present serum-resistant and cRGD-modified DNA complexes for targeted delivery genes to UL cells. Ternary polyplexes were formed by incorporating cystine-cross-linked polyglutamic acid modified with histidine residues. We employed two techniques in the production of cross-linked polyanionic coating: matrix polymerization and oxidative polycondensation. In this study, we investigated the physicochemical properties of ternary DNA complexes, including the size and zeta-potential of the nanoparticles. Additionally, we evaluated cellular uptake, toxicity levels, transfection efficiency and specificity in vitro. The study involved introducing the HSV-TK gene into primary UL cells as a form of suicide gene therapy modeling. We have effectively employed ternary peptide-based complexes for gene delivery into the UL organtypic model. By implementing in situ suicide gene therapy, the increase in apoptosis genes expression was detected, providing conclusive evidence of apoptosis occurring in the transfected UL tissues. The results of the study strongly suggest that the developed ternary polyplexes show potential as a valuable tool in the implementation of suicide gene therapy for UL.

Radiogenic Therapy: Novel Approaches for Enhancing Tumor Radiosensitivity

Radiotherapy (RT) is a well established modality for treating many forms of cancer. However, despite many improvements in treatment planning and delivery, the total radiation dose is often too low for tumor cure, because of the risk of normal tissue damage. Gene therapy provides a new adjunctive strategy to enhance the effectiveness of RT, offering the potential for preferential killing of cancer cells and sparing of normal tissues. This specificity can be achieved at several levels including restricted vector delivery, transcriptional targeting and specificity of the transgene product. This review will focus on those gene therapy strategies that are currently being evaluated in combination with RT, including the use of radiation sensitive promoters to control the timing and location of gene expression specifically within tumors. Therapeutic transgenes chosen for their radiosensitizing properties will also be reviewed, these include:

gene correction therapy, in which normal copies of genes responsible for radiation-induced apoptosis are transfected to compensate for the deletions or mutated variants in tumor cells (p53 is the most widely studied example). enzymes that synergize the radiation effect, by generation of a toxic species from endogenous precursors ( e.g., inducible nitric oxide synthase) or by activation of non toxic prodrugs to toxic species ( e.g., herpes simplex virus thymidine kinase/ganciclovir) within the target tissue. conditionally replicating oncolytic adenoviruses that synergize the radiation effect. membrane transport proteins ( e.g., sodium iodide symporter) to facilitate uptake of cytotoxic radionuclides.

The evidence indicates that many of these approaches are successful for augmenting radiation induced tumor cell killing with clinical trials currently underway.

Intracavitary therapeutics for pleural malignancies

Pleural malignancies are ideal for novel therapeutic approaches because they are invariably fatal. Intrapleural (IP) chemotherapy has only marginal benefit in pleural malignancies, but may prove efficacious with hyperthermic chemotherapy administered in combination with maximal tumor debulking. IP immunotherapies may be most effective in those patients with early-stage pleural malignancy, and may prove superior to standard pleurodesis methods in control of effusion and prolongation of survival. Immunogene therapy may be unable to successfully treat bulky tumors on its own, but success may be achieved with combination approaches that combine debulking surgery and chemotherapy with IP genetic immunotherapy.

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.

Fusion enzymes containing HSV-1 thymidine kinase mutants and guanylate kinase enhance prodrug sensitivity in vitro and in vivo

Herpes simplex virus thymidine kinase (HSVTK) with ganciclovir (GCV) is currently the most widely used suicide gene/prodrug system in cancer gene therapy. A major limitation in this therapy is the inefficient activation of GCV by HSVTK to its active antimetabolites. We described earlier two strategies to overcome this limitation: (1) generation of HSVTK mutants with improved GCV activation potential and (2) construction of a fusion protein encoding HSVTK and mouse guanylate kinase (MGMK), the second enzyme in the GCV activation pathway. As a means to further enhance GCV activation, two MGMK/HSVTK constructs containing the HSVTK mutants, mutant 30 and SR39, were generated and evaluated for their tumor and bystander killing effects in vitro and in vivo. One fusion mutant, MGMK/30, shows significant reduction in IC(50) values of approximately 12 500-fold, 100-fold, and 125-fold compared with HSVTK, mutant 30 or MGMK/HSVTK, respectively. In vitro bystander analyses show that 5% of MGMK/30-expressing cells are sufficient to induce 75% of tumor cell killing. In an xenograft tumor model, MGMK/30 displays the greatest inhibition of tumor growth at a GCV concentration (1 mg kg(-1)) that has no effect on wild-type HSVTK-, MGMK/HSVTK-, or mutant 30-transfected cells. Another fusion construct, MGMK/SR39, sensitizes rat C6 glioma cells to GCV by 2500-fold or 25-fold compared with HSVTK or MGMK/HSVTK, respectively. In vitro analyses show similar IC(50) values between cells harboring SR39 and MGMK/SR39, although MGMK/SR39 seems to elicit stronger bystander killing effects in which 1% of MGMK/SR39-transfected cells result in 60% cell death. In a xenograft tumor model, despite observable tumor growth inhibition, no statistical significance in tumor volume was detected between mice harboring SR39- and MGMK/SR39-transfected cells when dosed with 1 mg kg(-1) GCV. However, at a lower dose of GCV (0.1 mg kg(-1)), MGMK/SR39 seems to have slightly greater tumor growth inhibition properties compared with SR39 (P< or =0.05). In vivo studies indicate that both mutant fusion proteins display substantial improvements in bystander killing in the presence of 1 mg kg(-1) GCV, even when only 5% of the tumor cells are transfected. Such fusion mutants with exceptional prodrug converting properties will allow administration of lower and non-myelosuppressive doses of GCV concomitant with improved tumor killing and as such are promising candidates for translational gene therapy studies.

Synthesis and in vitro evaluation of 5-[(18)f]fluoroalkyl pyrimidine nucleosides for molecular imaging of herpes simplex virus type 1 thymidine kinase reporter gene expression

Two novel series of 5-fluoroalkyl-2'-deoxyuridines (FPrDU, FBuDU, FPeDU) and 2'-fluoro-2'-deoxy-5-fluoroalkylarabinouridines (FFPrAU, FFBuAU, FFPeAU) that have three, four, or five methylene units (propyl, butyl, or pentyl) at C-5 were prepared and tested as reporter probes for imaging herpes simplex virus type 1 thymidine kinase (HSV1- tk) gene expression. The Negishi coupling methodology was employed in efficiently synthesizing the radiolabeling precursors. All six 5-[(18)F]fluoroalkyl pyrimidines were readily prepared from 3-N-benzoyl-3',5'-di-O-benzoyl-protected 5-O-mesylate precursors in 17-35% radiochemical yield (decay-corrected). In vitro studies highlighted that all six [(18)F]-labeled nucleosides selectively accumulated in cells expressing the HSV1-TK protein and there was negligible uptake in control cells. [(18)F]FPrDU, [(18)F]FBuDU, [(18)F]FPeDU, and [(18)F]FFBuAU had the best uptake profiles. Despite their selective accumulation in HSV1- tk-expressing cells, all 5-fluoroalkyl pyrimidine nucleosides had low-to-negligible cytotoxic activity (CC50 > 1000-1209 microM). Ultimately, the results demonstrated that 5-[(18)F]fluoropropyl, [(18)F]fluorobutyl, and [(18)F]fluoropentyl pyrimidine nucleosides have the potential to be in vivo HSV1-TK PET reporter probes over a dynamic range of reporter gene expression levels.

Inhibition of NF-kappaB in fusogenic membrane glycoprotein causing HL-60 cell death: implications for acute myeloid leukemia

Viral fusogenic membrane glycoproteins (FMGs) are new therapeutic genes for the control of tumor growth, the cellular mechanisms mediating cell death is non-apoptotic. However, the precise molecular mechanism remains to be elucidated. Here, we showed overexpression of HSP70 in HL-60 cells mediated by Gibbon Ape leukemia virus hyperfusogenic envelope protein (GALV-FMG) inhibited the nuclear translocation of p65, the transcriptive activity of NF-kappaB and prevented the degradation of IkappaB. NF-kappaB may negatively regulate HSP70 expression, which made a positive feed back loop for expression of HSP70. FMG expression in HL-60 cells leaded to the formation of multinucleated syncytia and cell death, the main death mode of cells is necrosis. This form of cell death should be effective in vivo, gene therapy basing on FMG deserve further study for the treatment of AML.

Cellular and molecular events associated with the antitumor response induced by the cytosine deaminase/5-fluorocytosine suicide gene therapy system in a rat liver metastasis model

The bacterial cytosine deaminase (CD) gene converts the non-toxic prodrug 5-fluorocytosine (5-FC) into 5-fluorouracil. We have previously shown, in a rat liver metastasis model from colon carcinoma, that intratumoral injection of a CD-expressing plasmid into the animals followed by 5-FC treatment results in the regression of the treated tumor as well as distant uninjected tumors. The aim of this study was to further analyze the mechanisms associated with tumor regression induced upon application of suicide CD/5-FC strategy. Tumor regression was associated with an increased apoptosis, the recruitment of natural killer cells, CD4- and CD8 T lymphocytes within the tumors and an increased expression of several cytokines/chemokines mRNAs. These data indicate that the CD/5-FC suicide strategy is associated with the triggering of cellular and molecular events leading to an efficient antitumor immune response involving both innate and acquired immunity.

Transfection of oral cancer cells mediated by transferrin-associated lipoplexes: Mechanisms of cell death induced by herpes simplex virus thymidine kinase/ganciclovir therapy

The Herpes Simplex Virus thymidine kinase (HSV-tk) suicide gene/ganciclovir (GCV) approach has been used for the treatment of a variety of cancers. The purpose of the present study was to evaluate the cytotoxic effect of ganciclovir in oral squamous cancer cells, previously transfected with HSV-tk gene delivered by transferrin-associated complexes (Tf-lipoplexes), as well as to investigate the mechanisms involved in the bystander effect and in the process of cell death. The delivery of HSV-tk gene to the oral cancer cells, HSC-3 and SCC-7, mediated by Tf-lipoplexes followed by ganciclovir treatment resulted in essentially 100% cytotoxicity, the observed toxic effect being dependent both on GCV dose and incubation time. Cell death was shown to occur mainly by an apoptotic process. Different experimental approaches demonstrated that the observed cytotoxicity was mainly due to diffusion of the toxic agent into neighbouring, non-transfected cells, via gap junctions. Preliminary in vivo studies in a murine model for oral squamous cell carcinoma have shown a significant inhibition of tumor growth upon injection of Tf-lipoplexes carrying HSV-tk followed by intraperitonal injection of GCV, as compared to controls.

Gene therapy of uterine leiomyoma: adenovirus-mediated herpes simplex virus thymidine kinase/ganciclovir treatment inhibits growth of human and rat leiomyoma cells in vitro and in a nude mouse model

Uterine leiomyomas (LM) affect a high percentage of reproductive-age women. They develop as discrete, well-defined tumors that are easily accessible with imaging techniques--making this disease ideal for localized gene therapy approaches. In this study, we determined the efficacy of adenovirus-mediated herpes simplex virus thymidine kinase gene transfer in combination with ganciclovir (Ad-TK/GCV) as a potential therapy for LM. Rat ELT-3 LM cells and human LM cells were transfected with different multiplicity of infections (10-100 plaque forming units [PFU]/cell) of Ad-TK and treated with GCV (5, 10, or 20 microg/ml) for 5 days. To test the bystander effect, Ad-TK-transfected ELT-3 cells (100 PFU/cell) or LM cells (10 PFU/cell) were cocultured with corresponding nontransfected cells at increasing percentages and treated with GCV followed by cell counting. In ELT-3 cells transfected with Ad-TK/GCV (10, 20, 50, or 100 PFU/cell), the cell count was reduced by 24, 42, 77, and 87%, respectively, compared with the control cells (transfected with Ad-Lac Z/GCV). Similarly, in LM cells transfected with Ad-TK/GCV (10, 50, or 100 PFU/cell), the cell count was reduced by 31, 62, and 82%, respectively, compared with the control. A strong bystander effect was noted in both ELT-3 and LM cells with significant killing (p = 0.001) at a ratio of infected:uninfected cells of only 1:99 and maximal killing at 1:4. This study demonstrates the potential efficacy of the Ad-TK/GCV gene therapy approach as a viable nonsurgical alternative treatment for uterine LM.

Sensitivity of 5-fluorouracil-resistant cancer cells to adenovirus suicide gene therapy

A promising approach for cancer gene therapy is the combination of adenovirus vectors (AdV) with the suicide gene cytosine deaminase and uracil phosphoribosyl transferase (CDColon, two colonsUPRT). While such vectors have been tested in tumor cell lines and xenograft models, it is not clear how these therapeutic vectors would perform in primary human tumors. We, thus, examined the effect of the combination of a recombinant adenovirus expressing the CDColon, two colonsUPRT (AdCU) with 5-fluorocytosine (5-FC) on primary cancer cells isolated from the ascites or pleural fluids of patients with metastatic cancers. In such models, we have found a direct correlation between the patients' response to 5-FU and the response shown by the cancer cells in vitro, confirming the clinical relevance of this methodology. Our findings demonstrated that this combination was able to kill primary tumor cells, including those that had developed resistance to 5-FU. Furthermore, while proliferating cells were more susceptible to 5-FU, the combination was effective in both rapid and slow proliferating samples. Our study demonstrated that this gene therapy approach could provide an effective therapeutic option for cancers and is not affected by acquired 5-FU resistance. Also of importance is the effectiveness of this gene therapy approach on slower proliferating cells that is typical of the majority of cancers in vivo. This suggests a greater likelihood that it will be effective in a clinical setting.

Hydroxyurea induces bystander cytotoxicity in cocultures of herpes simplex virus thymidine kinase-expressing and nonexpressing HeLa cells incubated with ganciclovir

Suicide gene therapy with the herpes simplex virus thymidine kinase (HSV-TK) cDNA and ganciclovir can elicit cytotoxicity to transgene-expressing and nonexpressing bystander cells via transfer of ganciclovir phosphates through gap junctions. HeLa cells do not exhibit bystander cytotoxicity, although we showed recently that they transfer low levels of ganciclovir phosphates to bystander cells. Here, we attempted to induce bystander cytotoxicity using hydroxyurea, an inhibitor of ribonucleotide reductase, to decrease the endogenous dGTP pool, which should lessen competition with ganciclovir triphosphate for DNA incorporation. Addition of hydroxyurea to cocultures of HSV-TK-expressing and bystander cells synergistically increased ganciclovir-mediated cytotoxicity to both cell populations while producing primarily an additive effect in cultures of 100% HSV-TK-expressing cells. Whereas HSV-TK-expressing cells in coculture were approximately 50-fold less sensitive to ganciclovir compared with cultures of 100% HSV-TK-expressing cells, addition of hydroxyurea restored ganciclovir sensitivity. Quantification of deoxynucleoside triphosphate pools showed that hydroxyurea decreased dGTP pools without significantly affecting ganciclovir triphosphate levels. Although hydroxyurea significantly increased the ganciclovir triphosphate:dGTP value for 12 to 24 hours in HSV-TK-expressing and bystander cells from coculture (1.4- to 4.9-fold), this value was increased for <12 hours (2.5-fold) in 100% HSV-TK-expressing cells. These data suggest that the prolonged increase in the ganciclovir triphosphate:dGTP value in cells in coculture resulted in synergistic cytotoxicity. Compared with enhancement of bystander cytotoxicity through modulation of gap junction intercellular communication, this strategy is superior because it increased cytotoxicity to both HSV-TK-expressing and bystander cells in coculture. This approach may improve clinical efficacy.

Long-term follow-up of patients with malignant pleural mesothelioma receiving high-dose adenovirus herpes simplex thymidine kinase/ganciclovir suicide gene therapy

PURPOSE

Delineation of the long-term follow-up data on a series of patients with malignant mesothelioma, who received a single intrapleural dose of a nonreplicative adenoviral (Ad) vector encoding the herpes simplex virus thymidine kinase "suicide gene" (Ad.HSVtk) in combination with systemic ganciclovir.

EXPERIMENTAL DESIGN

This report focuses on the 21 patients receiving "high-dose" therapy, defined by an intrapleural dose of vector (> or =1.6 x 10(13) viral particles), where transgene-encoded tk protein was reliably identified on immunohistochemical staining. In 13 patients, the vector was deleted in the E1 and E3 regions of the Ad; in the other eight patients, the vector had deletions in the Ad genes E1 and E4. Safety, immunologic responses, transgene expression, and clinical responses were evaluated.

RESULTS

Both the E1/E3-deleted vector and the E1/E4-deleted vector were well tolerated and safe, although production of the E1/E4 vector was more difficult. Posttreatment antibody responses against the tumors were consistently seen. Interestingly, we observed a number of clinical responses in our patients, including two long-term (>6.5 year) survivors, both of whom were treated with the E1/E4-deleted vector.

CONCLUSIONS

Intrapleural Ad.HSVtk/ganciclovir is safe and well tolerated in mesothelioma patients and resulted in long-term durable responses in two patients. Given the limited amount of gene transfer observed, we postulate that Ad.HSVtk may have been effective due to induction of antitumor immune responses. We hypothesize that approaches aiming to augment the immune effects of Ad gene transfer (i.e., with the use of cytokines) may lead to increased numbers of therapeutic responses in otherwise untreatable pleural malignancies.

Gene therapy for malignant mesothelioma: beyond the infant years

Mesothelioma may be particularly well suited for gene therapy treatment owing to its accessibility, allowing both intrapleural and intratumoral gene delivery. At least four gene therapy trials have been carried out in mesothelioma patients, using different vector systems (adenovirus, vaccinia virus, irradiated tumor cells), and different transgenes (herpes simplex virus thymidine kinase (HSVtk) combined with ganciclovir, IL-2, IFN-beta). Although small in scale, these trials have given an inkling of hope for therapeutic efficacy. However, it is clear that gene therapy protocols need to be optimized further. This paper will review progress made in (i) vector development, (ii) defining optimal transgenes, and (iii) gene delivery. Adenoviruses are the most commonly used vectors for gene therapy, and are continuously being improved. With respect to the nature of the transgenes, five categories can be distinguished: (i) 'suicide' or sensitivity genes (e.g., HSVtk), (ii) cytokines and other immune modulators, (iii) replacements for mutant tumor suppressor genes (e.g., p53), (iv) antiangiogenic proteins and (v) tumor antigens. It seems clear that expression of a single transgene is unlikely to be sufficient to eradicate a tumor, such as mesothelioma, that is diagnosed late in disease progression. Hence, multimodality therapy, including conventional therapy (chemo- and radiotherapy, surgery) with one or more transgenes has a higher chance of success.

The role of a HSV thymidine kinase stimulating substance, scopadulciol, in improving the efficacy of cancer gene therapy

BACKGROUND

The most extensively investigated strategy of suicide gene therapy for treatment of cancer is the transfer of the herpes simplex virus thymidine kinase (HSV-TK) gene followed by administration of antiviral prodrugs such as acyclovir (ACV) and ganciclovir (GCV). The choice of the agent that can stimulate HSV-TK enzymatic activity is one of the determinants of the usefulness of this strategy. Previously, we found that a diterpenoid, scopadulciol (SDC), produced a significant increase in the active metabolite of ACV. This suggests that SDC may play a role in the HSV-TK/prodrug administration system.

METHODS

The anticancer effect of SDC was evaluated in HSV-TK-expressing (TK+) cancer cells and nude mice bearing TK+ tumors. In vitro and in vivo enzyme assays were performed using TK+ cells and tumors. The phosphorylation of ACV monophosphate (ACV-MP) was measured in TK- cell lysates. The pharmacokinetics of prodrugs was evaluated by calculating area-under-the-concentration-time-curve values.

RESULTS

SDC stimulated HSV-TK activity in TK+ cells and tumors, and increased GCV-TP levels, while no effect of SDC was observed on the phosphorylation of ACV-MP to ACV-TP by cellular kinases. The SDC/prodrug combination altered the pharmacokinetics of the prodrugs. In accord with these findings, SDC enhanced significantly the cell-killing activity of prodrugs. The bystander effect was also significantly augmented by the combined treatment of ACV/GCV and SDC.

CONCLUSIONS

SDC was shown to be effective in the HSV-TK/prodrug administration system and improved the efficiency of the bystander effect of ACV and GCV. The findings will be considerably valuable with respect to the use of GCV in lower doses and less toxic ACV. This novel strategy of drug combination could provide benefit to HSV-TK/prodrug gene therapy.

Gene Therapy for Malignant Pleural Mesothelioma

Gene therapy for mesothelioma is currently in its adolescence. The expansion of knowledge regarding molecular aspects of mesothelioma carcinogenesis has facilitated the development of promising gene therapy modalities that target specific oncoproteins and mutant tumor suppressor genes. Although implementation of any of these gene therapy approaches as part of standard medical care for patients who have mesothelioma remains years in the future, the field is finally progressing toward more definitive phase II/III efficacy studies. Unfortunately, the marginal benefits garnered from standard anticancer treatments in mesothelioma argue strongly for continued participation in clinical studies of various experimental approaches, particularly gene therapy. These trials serve multiple purposes: to establish safety, determine proper dosing, evaluate for efficacy, and, in an iterative fashion, guide future avenues of laboratory investigation.

Nonviral Jet-Injection Gene Transfer for Efficient in Vivo Cytosine Deaminase Suicide Gene Therapy of Colon Carcinoma

Jet-injection technology has developed into an efficient gene delivery system for nonviral in vivo gene transfer. In this study the jet-injector system was used for the intratumoral gene transfer of small volumes of naked DNA encoding the Escherichia coli cytosine deaminase (CD) suicide gene. In our in vivo studies human colon carcinoma (patient-derived tumor model Colo5734 and SW480 colon carcinoma)-bearing NMRI-nu/nu male mice received four jet injections (10 microl per injection) of the CD-gene-carrying plasmid, representing 40 microg plasmid DNA per animal. Forty-eight hours after jet-injection, treatment of tumors with 5-fluorocytosine (5-FC; 500 mg/kg ip) was started and during treatment tumor volumes were measured. Starting from day 5 of 5-FC treatment inhibition of tumor growth was seen in the CD-gene-transduced tumors compared to the respective control groups, which lasted for the entire observation time. Expression analysis at the mRNA and protein levels revealed efficient expression of the CD gene in the jet-injected tumors. Therefore, in this in vivo study jet-injection gene transfer of 40 microg CD-expressing naked plasmid DNA leads to a significant tumor growth inhibition. This study demonstrates the applicability of the jet-injection technology for in vivo gene transfer into tumors to achieve efficient tumor gene therapy.

Gene therapy of mesothelioma

Despite improved diagnostic skills and new chemotherapeutic regimens, malignant mesothelioma (MM) remains a pathological disease with survival expectations after diagnosis remaining < 1 year. As the incidence of this disease has yet to peak, there is a pressing need for new therapeutic approaches. One such approach is gene therapy, which inserts 'therapeutic' genes into (generally) tumour cells seeking to induce tumour regression via a number of different theoretical mechanisms. This approach may be particularly relevent for mesothelioma as it is localised to body cavities and is readily accessible for biopsy sampling or for gene delivery. Furthermore, as MM patients rarely die from distant metastases, treating the primary tumour site may result in significant symptomatic and survival benefit. Herein, the paper discusses past, present and future views on gene therapy in the treatment of MM.

GCV phosphates are transferred between HeLa cells despite lack of bystander cytotoxicity

The role of gap junctional intercellular communication (GJIC) in bystander killing with herpes simplex virus thymidine kinase (HSV-TK) and ganciclovir (GCV) was evaluated in U251 cells expressing a dominant-negative connexin 43 cDNA (DN14), and in HeLa cells, reportedly devoid of connexin protein. These cell lines both exhibited 0% GJIC when assayed by Lucifer Yellow fluorescent dye microinjection. Bystander cytotoxicity was still apparent in 50:50 cocultures of DN14 and HSV-TK-expressing U251 cells, but not in 50:50 cocultures of HeLa cells. However, the sensitivity of HeLa HSV-TK-expressing cells to GCV decreased nearly 100-fold (IC90=109 microM) when cocultured with bystander cells compared to results in 100% cultures of HSV-TK-expressing cells (IC90=1.2 microM). A more sensitive flow cytometry technique to measure GJIC over 24 h revealed that the DN14 and HeLa cells exhibited detectable levels of communication (29 and 23%, respectively). Transfer of phosphorylated GCV to HeLa bystander cells occurred within 4 h after drug addition, and GCV triphosphate (GCVTP) accumulated to 213+/-84 pmol/10(6) cells after 24 h. In addition, GCVTP levels were decreased in HSV-TK-expressing cells in coculture (867+/-33 pmol/10(6) cells) compared to 100% cultures of HSV-TK-expressing cells (1773+/-188 pmol/10(6) cells). The half-life of GCVTP in the HSV-TK-expressing cells was approximately four times that measured in the bystander cells (12.3 and 3.1 h, respectively). These data suggest that the lack of bystander cytotoxicity in HeLa cocultures is due to low transfer of phosphorylated GCV and a rapid half-life of GCVTP in the bystander cells. Thus, GCV phosphate transfer to non-HSV-TK-expressing bystander cells may mediate either bystander cell killing or sparing of HSV-TK-positive cells, depending upon the cell specific drug metabolism.

Characterisation of systemic dissemination of nonreplicating adenoviral vectors from tumours in local gene delivery

Systemic virus dissemination is a potential problem during local gene delivery in solid tumours. However, the kinetics and pathways of the dissemination have not been well characterised during the first 24 h after the infusion is started. To this end, we infused adenoviral vectors for luciferase or enhanced green fluorescence protein into three different tumour models in mice. During and/or after the infusion, we determined the amount of adenoviruses in the tumour, blood, and liver, and examined the transgene expression in the liver, lung, blood, and tumour. In addition, we intravenously injected tumour cells expressing luciferase and examined the biodistribution of these cells in the body. We observed transgene expression in the liver and tumour at 24 h after the infusion, but could not detect transgene expression in the blood and lung. The peak concentration of viral vectors in the plasma occurred during the intratumoral infusion. At 10 min after the infusion, few viral vectors remained in the blood and the ratio of copy numbers of adenoviruses between liver and tumour was >2 in 80% and ⩾10 in 40% of the mice. Most tumour cells injected intravenously accumulated in the lung within the first 24 h. Taken together, these data indicated that systemic virus dissemination occurred mainly during the first 10 min after the intratumoral infusion was started, and that the dissemination was due to infusion-induced convective transport of viral vectors into leaky tumour microvessels.

Herpes simplex virus thymidine kinase/ganciclovir system in multicellular tumor spheroids

We have developed multicellular spheroids (MCS) established from LM05e and LM3 spontaneous Balb/c-murine mammary adenocarcinoma and B16 C57-murine melanoma derived cell lines as an in vitro model to study the efficacy of the herpes simplex virus thymidine kinase/ganciclovir (HSVtk/GCV) suicide system. We demonstrated for the first time that HSVtk-expressing cells assembled as MCS manifested a GCV resistance phenotype compared to the same cells grown as sparse monolayers. HSVtk-expressing LM05e, LM3 and B16 spheroids were 16-, three- and nine-fold less sensitive to GCV than their respective monolayers, even though they could express transgenes 10-, eight- and five-fold more efficiently. Mixed populations of HSVtk- and their respective beta gal-expressing cells displayed a cell-type specific bystander effect that was higher in monolayers than in MCS. However, HSVtk-expressing cells in two- or three-dimensional cultures were always significantly more sensitive to GCV than the beta gal-expressing counterparts, supporting the feasibility of this suicide approach in vivo. We present evidence showing that HSVtk-expressing tumor cells, when transferred from monolayers to MCS, displayed: (i) lower GCV cytotoxic activity and bystander effect; (ii) higher and efficient expression of genes transferred as lipoplexes; (iii) lower cell proliferation rates; and (iv) changes in intracellular Bax/Bcl-xL rheostat of mitochondria-mediated apoptosis.

The cyclin-dependent kinase inhibitor p21cip1/waf1 enhances the cytotoxicity of ganciclovir in HSV-tk transfected ovarian carcinoma cells

Suicide gene therapy could be an attractive addition to the treatment of ovarian carcinomas, for which acquired chemoresistance frequently results in treatment failure. Here we show that transfection of the HSV-tk gene, followed by incubation with up to 1 mM ganciclovir fails to induce cell death in SKOV3 chemoresistant human ovarian carcinoma cells. However, co-transfection of HSV-tk with Cip1/Waf1 encoding the p21(cip1/waf1) inhibitor of cdks, allows 100 microM ganciclovir to eradicate the population of tumor cells. Potentiation of a drug by co-transfer of HSV-tk with Cip1/Waf1could thus represent another therapeutic approach for tumours that are resistant to conventional therapy.

Lung cancer vaccines and gene therapy

A number of cancer vaccine and gene therapy approaches are being evaluated in patients with lung cancer. Cancer vaccine strategies include GM-CSF gene-modified cancer cells, liposomal MUC1 peptide, anti-idiotype antibody targeting GD3, Mage-3 peptide, and mutant p53 pulsed dendritic cells among others. Preliminary human trials have demonstrated immune responses as well as tumor regression in late stage disease. The largest human gene therapy experience in lung cancer is with intratumoral gene replacement therapy, predominantly with p53, but such approaches are limited to locoregional disease control. Earlier stage gene therapy programs targeting the immune system or tumor vasculature hold promise as systemic therapies for treatment of advanced, disseminated disease.

Sustained P450 expression and prodrug activation in bolus cyclophosphamide-treated cultured tumor cells. Impact of prodrug schedule on P450 gene-directed enzyme prodrug therapy

Cytochrome P450-based gene therapy can substantially increase the sensitivity of tumor cells to P450-activated cancer chemotherapeutic prodrugs such as cyclophosphamide (CPA) without increasing host toxicity. While the role of 4-OH-CPA, the primary active metabolite of CPA, in eliciting tumor cell death is well established, the effect of 4-OH-CPA exposure on the capacity of P450-expressing tumor cells for continued metabolism and activation of CPA has not been investigated. The present study addresses this question and characterizes the impact of CPA dose and treatment schedule on the ability of P450-expressing tumor cells to sustain prodrug activation over time. 9L gliosarcoma cells expressing human P450 2B6 and treated with CPA in a continuous manner exhibited a time- and CPA dose-dependent decrease in P450-catalyzed CPA 4-hydroxylase activity. This decrease reflects a selective, 4-OH-CPA-induced loss of cellular P450 protein content. By contrast, when the P450-expressing tumor cells were treated with CPA as a single 8 hours exposure, cellular CPA 4-hydroxylase activity and P450 protein expression were substantially prolonged when compared to continuous prodrug treatment. This schedule-dependent effect of CPA was influenced by the level of P450 protein expressed in the tumor cells. At high P450 protein and activity levels, which could be achieved by culturing the tumor cells at high cell density, net production and release of 4-OH-CPA into the culture media was increased substantially. This increase fully offset the decline in CPA 4-hydroxylase activity as the tumor cells underwent CPA-induced apoptotic death. These findings demonstrate the impact of CPA dose and treatment schedule on the efficacy of P450 gene-directed enzyme prodrug therapy, with bolus CPA treatment being compatible with sustained expression of P450 protein and maintenance of P450-dependent prodrug activation by the target tumor tissue.

Heat-directed tumor cell fusion

In previous studies we demonstrated that a modified human HSP70b promoter (HSE.70b) directs high levels of gene expression to tumor cells after mild hyperthermia treatment in the range of 41.5-44 degrees C. This transcriptional targeting system exhibits low basal activity at 37 degrees C, is highly induced (950-fold) after mild heat treatment (43 degrees C/30 min), and returns to basal activity levels within 12-24 hours of activation. Here we describe heat-directed targeting of an activated form of the Gibbon ape leukemia virus env protein (GALV FMG) to tumor cells. GALV FMG mediates cell-cell fusion, and when expressed in tumor cells can produce bystander effects of up to 1:200. Transient transfection of a HSE70b.GALV FMG minigene caused extensive syncytia formation in HeLa and HT-1080 cells following mild heat treatment (44 degrees C/30 min). Stable transfection into HT-1080 cells produced a cell line (HG5) that exhibits massive syncytia formation and a 60% reduction in viability relative to a vector-only control (CI1) following heat treatment in vitro. Mild hyperthermia also resulted in syncytia formation, necrosis, and complete macroscopic regression of HG5 xenograft tumors grown in the footpads of mice with severe combined immunodeficiency disorders (SCID). Median survival increased from 12.5 (in heated CI1 controls) to 52 days after a single heat treatment. Heat-directed tumor cell fusion may prove to be a highly beneficial adjunct to existing cancer treatment strategies that take advantage of the synergistic interaction between mild hyperthermia and radiation or chemotherapeutic drugs.

Prodrugs for Gene-Directed Enzyme-Prodrug Therapy (Suicide Gene Therapy)

This review focuses on the prodrugs used in suicide gene therapy. These prodrugs need to satisfy a number of criteria. They must be efficient and selective substrates for the activating enzyme, and be metabolized to potent cytotoxins preferably able to kill cells at all stages of the cell cycle. Both prodrugs and their activated species should have good distributive properties, so that the resulting bystander effects can maximize the effectiveness of the therapy, since gene transduction efficiencies are generally low. A total of 42 prodrugs explored for use in suicide gene therapy with 12 different enzymes are discussed, particularly in terms of their physiocochemical properties. An important parameter in determining bystander effects generated by passive diffusion is the lipophilicity of the activated form, a property conveniently compared by diffusion coefficients (log P for nonionizable compounds and log D(7) for compounds containing an ionizable centre). Many of the early antimetabolite-based prodrugs provide very polar activated forms that have limited abilities to diffuse across cell membranes, and rely on gap junctions between cells for their bystander effects. Several later studies have shown that more lipophilic, neutral compounds have superior diffusion-based bystander effects. Prodrugs of DNA alkylating agents, that are less cell cycle-specific than antimetabolites and more effective against noncycling tumor cells, appear in general to be more active prodrugs, requiring less prolonged dosing schedules to be effective. It is expected that continued studies to optimize the bystander effects and other properties of prodrugs and the activated species they generate will contribute to improvements in the effectiveness of suicide gene therapy.

Identification of novel enzyme-prodrug combinations for use in cytochrome P450-based gene therapy for cancer

Gene-directed enzyme prodrug therapy can be used to increase the therapeutic activity of anti-cancer prodrugs that undergo liver cytochrome P450 (CYP)-catalyzed prodrug to active drug conversion. The present report describes a cell-culture-based assay to identify CYP gene-CYP prodrug combinations that generate bystander cytotoxic metabolites and that may potentially be useful for CYP-based gene therapy for cancer. A panel of rat liver microsomes, comprising distinct subsets of drug-inducible hepatic CYPs, was evaluated for prodrug activation in a four-day 9L gliosarcoma cell growth inhibition assay. A strong NADPH- and liver microsome-dependent increase in 9L cytotoxicity was observed for the CYP prodrugs cyclophosphamide, ifosfamide, and methoxymorpholinyl doxorubicin (MMDX) but not with three other CYP prodrugs, procarbazine, dacarbazine, and tamoxifen. MMDX activation was potentiated approximately 250-fold by liver microsomes from dexamethasone-induced rats (IC(50) (MMDX) approximately 0.1nM), suggesting that dexamethasone-inducible CYP3A enzymes contribute to activation of this novel anthracycline anti-tumor agent. This CYP3A dependence was verified in studies using liver microsomes from uninduced male and female rats and by using the CYP3A-selective inhibitors troleandomycin and ketoconazole. These findings highlight the advantages of using cell culture assays to identify novel CYP prodrug-CYP gene combinations that are characterized by production of cell-permeable, cytotoxic metabolites and that may potentially be incorporated into CYP-based gene therapies for cancer treatment.

Cyanide bystander effect of the linamarase/linamarin killer-suicide gene therapy system

BACKGROUND

The killer-suicide system linamarase/linamarin (lis/lin) uses the plant gene linamarase (beta-glucosidase) to convert the cyanogenic glucoside substrate, linamarin, into glucose and cyanide. We have studied the bystander effect associated with this new system mediated by the production of the cyanide ion that diffuses freely across membranes.

METHODS

Immunofluorescent staining of cells treated with an anti-linamarase antibody allowed us to localize the enzyme within the cells. Flow cytometry was used to determine the sensitivity of different mixtures of cells, C6lis and C6gfp (green), to linamarin as a percentage of cell survival.

RESULTS

We demonstrate here that rat glioblastoma C6 cells carrying the linamarase gene (lis), mixed with naive C6 cells and exposed to linamarin, induce generalized cell death. Cells expressing lis efficiently export linamarase, whereas linamarin enters cells poorly by endocytosis; as a result most of the cyanide is produced outside the cells. The study was facilitated by the presence of the green fluorescent protein (gfp) gene in the bystander population. As few as 10% C6lis-positive cells are sufficient to eliminate the entire cell culture in 96 h.

CONCLUSIONS

This bystander mechanism does not preferentially kill toxic metabolite producer cells compared with bystander cells, thus allowing production of sufficient cyanide to cause tumor regression. In this report we confirm the potential of the lis/lin gene therapy system as a powerful tool to eliminate tumors in vivo.

Multi-log cytotoxicity of carbocyclic 2'-deoxyguanosine in HSV-TK-expressing human tumor cells

Ganciclovir (GCV) is widely used as a prodrug for selective activation in tumor cells expressing herpes simplex virus thymidine kinase (HSV-TK) because of its ability to induce multi-log cytotoxicity to HSV-TK-expressing as well as nonexpressing bystander cells. We now report that another substrate for HSV-TK, D-carbocyclic 2'-deoxyguanosine (CdG), induces multi-log cytotoxicity in HSV-TK-expressing and bystander cells at concentrations <or=3 microM. We have compared the cytotoxicity and cell cycle effects of CdG to that observed with GCV in two human tumor cell lines. The results demonstrated that cytotoxicity of CdG was similar to that of GCV in both U251 glioblastoma and SW620 colon carcinoma cells that stably expressed HSV-TK. In addition, CdG induced a potent bystander effect in both cell types in co-cultures consisting of HSV-TK-expressing and nonexpressing bystander (lacZ-expressing) cells at ratios of 50:50 or 10:90. Selectivity for HSV-TK-expressing compared to lacZ-expressing cells was similar for CdG and GCV in the U251 cells, however CdG was less selective than GCV in the SW620 cell lines. Despite their ability to induce multi-log cytotoxicity at similar concentrations, CdG and GCV exhibited differential effects on cell cycle progression. Cells incubated with 1 microM CdG for 24 hr accumulated in S-phase and G(2)/M after drug washout, and the majority of cells died prior to cell division. This contrasts with the delayed effects of 1 microM GCV that were not evident until after cell division when cells attempted S-phase for the second time. Thus, CdG is a potent cytotoxic agent that merits further investigation to determine whether it will be therapeutically effective in enzyme-prodrug therapy with HSV-TK.

Gene therapy for lung neoplasms

The field of gene therapy is still in its infancy, but significant accomplishments have been achieved. The ability to transfer genes safely and successfully into animals and patients clearly has been established. It is highly likely that in the near future, gene therapy will be shown to have clear therapeutic efficacy in diseases such as the treatment of hemophilia (using adeno-associated virus vectors) and the stimulation of angiogenesis in peripheral vascular disease and myocardial ischemia. Although only Phase 1 cancer gene therapy trials for thoracic malignancy have been conducted (usually in patients with large tumor burdens and at submaximal doses), there are some hints of efficacy at higher doses of vector in trials for localized malignancy. The studies reviewed in this article demonstrate the first attempts to use gene therapy vectors for lung cancer and mesothelioma. Although none of the diseases studied was "cured," valuable lessons have been learned from these trials, especially in defining the challenges of relatively inefficient and transient delivery of transgene in vivo. Using this knowledge, the second phase of gene therapy research has begun, with a strong focus on developing improved vector technology. Given the progress so far, there is little doubt that gene therapy will become a key approach for the treatment of thoracic malignancies in the near future.

Molecular approaches to chemo-radiotherapy

Although radiotherapy is used to treat many solid tumours, normal tissue tolerance and inherent tumour radioresistance can hinder successful outcome. Cancer gene therapy is one approach being developed to address this problem. However, the potential of many strategies are not realised owing to poor gene delivery and a lack of tumour specificity. The use of treatment-, condition- or tumour-specific promoters to control gene-directed enzyme prodrug therapy (GDEPT) is one such method for targeting gene expression to the tumour. Here, we describe two systems that make use of GDEPT, regulated by radiation or hypoxic-responsive promoters. To ensure that the radiation-responsive promoter is be activated by clinically relevant doses of radiation, we have designed synthetic promoters based on radiation responsive CArG elements derived from the Early Growth Response 1 (Egr1) gene. Use of these promoters in several tumour cell lines resulted in a 2-3-fold activation after a single dose of 3 Gy. Furthermore, use of these CArG promoters to control the expression of the herpes simplex virus (HSV) thymidine kinase (tk) gene in combination with the prodrug ganciclovir (GCV) resulted in substantially more cytotoxicity than seen with radiation or GCV treatment alone. Effectiveness was further improved by incorporating the GDEPT strategy into a novel molecular switch system using the Cre/loxP recombinase system of bacteriophage P1. The level of GDEPT bystander cell killing was notably increased by the use of a fusion protein of the HSVtk enzyme and the HSV intercellular transport protein vp22. Since hypoxia is also a common feature of many tumours, promoters containing hypoxic-responsive elements (HREs) for use with GDEPT are described. The development of such strategies that achieve tumour targeted expression of genes via selective promoters will enable improved specificity and targeting thereby addressing one of the major limitations of cancer gene therapy.

Modulation of cyclophosphamide-based cytochrome P450 gene therapy using liver P450 inhibitors

The sensitivity of tumors to cyclophosphamide (CPA) and other anticancer prodrugs can be substantially enhanced by transduction of tumors with a prodrug-activating mammalian cytochrome P450 (CYP) enzyme in combination with the flavoenzyme P450 reductase. This gene therapy strategy provides for intratumoral prodrug activation, but is also associated with a high level of hepatic prodrug activation, which reduces the extent of intratumoral prodrug activation and contributes to systemic drug toxicity. To address this issue, five P450 inhibitors were tested for their ability to block liver CYP2C-catalyzed CPA activation selectively, i.e., without inhibiting the corresponding intratumoral activation of CPA catalyzed by a transduced CYP2B enzyme. In vitro studies revealed that the P450 inhibitors 1-aminobenzotriazole and DDEP were preferentially inhibitory toward CYP2C-dependent liver microsomal CPA activation, whereas the P450 inhibitor SKF-525A inhibited CYP2C- and CYP2B-dependent CPA activation without P450 form selectivity. By contrast, the P450 inhibitors chloramphenicol and metyrapone preferentially inhibited CYP2B-dependent CPA activation. Rat pharmacokinetic studies confirmed the inhibitory action of these compounds in vivo, with up to a 4-fold decrease in C(max) and a 7-fold increase in apparent half-life of the activated CPA metabolite, 4-hydroxy-CPA, seen in the case of 1-aminobenzotriazole. Although the rate of hepatic CPA activation could thus be decreased substantially by P450 inhibitor treatment, the net extent of hepatic CPA activation was only modestly decreased, as judged by plasma area-under-the-curve values for 4-hydroxy-CPA. Moreover, P450 inhibitor treatment did not decrease CPA's host toxicity and did not enhance the tumor growth delay response to CPA in rats bearing CYP2B1-transduced gliosarcomas. These findings are discussed in the context of P450-based gene therapy strategies and ongoing efforts to enhance anticancer drug activity by increasing the exposure of P450-expressing tumors to the P450-activated prodrug CPA.

A hypoxia-regulated adeno-associated virus vector for cancer-specific gene therapy

The presence of hypoxic cells in human brain tumors is an important factor leading to resistance to radiation therapy. However, this physiological difference between normal tissues and tumors also provides the potential for designing cancer-specific gene therapy. We compared the increase of gene expression under anoxia (<0.01% oxygen) produced by 3, 6, and 9 copies of hypoxia-responsive elements (HRE) from the erythropoietin gene (Epo), which are activated through the transcriptional complex hypoxia-inducible factor 1 (HIF-1). Under anoxic conditions, nine copies of HRE (9XHRE) yielded 27- to 37-fold of increased gene expression in U-251 MG and U-87 MG human brain tumor cell lines. Under the less hypoxic conditions of 0.3% and 1% oxygen, gene activation by 9XHRE increased expression 11- to 18-fold in these cell lines. To generate a recombinant adeno-associated virus (rAAV) in which the transgene can be regulated by hypoxia, we inserted the DNA fragment containing 9XHRE and the LacZ reporter gene into an AAV vector. Under anoxic conditions, this vector produced 79- to 110-fold increase in gene expression. We believe this hypoxia-regulated rAAV vector will provide a useful delivery vehicle for cancer-specific gene therapy.

Non-viral vectors in cancer gene therapy: principles and progress

This review focuses on the use of synthetic (non-viral) delivery systems for cancer gene therapy. Therapeutic strategies such as gene replacement/mutation correction, immune modulation and molecular therapy/'suicide' gene therapy type approaches potentially offer unique and novel ways of fighting cancer, some of which have already shown promise in early clinical trials. However, the specific and efficient delivery of the genetic material to remote tumors/metastases remains a challenge, which is being addressed using a variety of viral and non-viral systems. Each of these disparate systems has distinct advantages and disadvantages, which need to be taken into account when a specific therapeutic gene is being used. The review concentrates on particulate gene delivery systems, which are formed through non-covalent complexation of cationic carrier molecules (e.g. lipids or polymers) and the negatively charged plasmid DNA. Such systems tend to be comparatively less efficient than viral systems, but have the inherent advantage of flexibility and safety. The DNA-carrier complex acts as a protective package, and needs to be inert and stable while in circulation. Once the remote site has been reached the complex needs to efficiently transfect the targeted (tumor) cells. In order to improve overall transfection specificity and efficiency it is necessary to optimize intracellular trafficking of the DNA complex as well as the performance after systemic administration. Common principles and specific advantages or disadvantages of the individual synthetic gene delivery systems are discussed, and their interaction with tumor-specific and generic biological barriers are examined in order to identify potential strategies to overcome them.

Gap junctional intercellular communication is not a major mediator in the bystander effect in photodynamic treatment of MDCK II cells

Photodynamic treatment (PDT) of confluent MDCK II cells resulted in a noticeable clustering of dead cells, consistent with a significant bystander effect. Likewise, PDT of cells in microcolonies resulted in an overabundance of microcolonies that had responded to the treatment as a single unit, that is, in which either all or no cells were dead. Confluent MDCK II cells appeared to communicate via gap junction channels, while cells in microcolonies did not. Monte Carlo simulation models were fitted to the distributions of dead cells in confluent monolayers and in microcolonies. The simulations showed that the degree of the bystander effect was higher in microcolonies than in confluent cells, suggesting that gap junction communication may be involved in the bystander effect. However, when the gap junction hypothesis was tested by treatment of microcolonies with 30 microM dieldrin, an inhibitor of gap junctional intercellular communication, there was no reduction of the bystander effect, indicating that this effect was not mediated by gap junctional intercellular communication. PDT influenced phosphorylation of tyrosine residues in several proteins in the cells. Protein phosphorylation is important in cellular signaling pathways and may be involved in the bystander effect, for example by influencing the mode of cell death.

The bystander effect in photodynamic inactivation of cells

Treatment of MDCK II cells with the lipophilic photosensitizer tetra(3-hydroxyphenyl)porphyrin and light was found to induce a rapid apoptotic response in a large fraction of the cells. Furthermore, the distribution of apoptotic cells in microcolonies of eight cells was found to be different from the binomial distribution, indicating that the cells are not inactivated independently, but that a bystander effect is involved in cell killing by photodynamic treatment. The observation of a bystander effect disagrees with the common view that cells are inactivated only by direct damage and indicates that communication between cells in a colony plays a role in photosensitized induction of apoptosis. The degree of bystander effect was higher for cells dying by necrosis than for cell dying by apoptosis.

Gene therapy for lung disease: hype or hope?

Gene therapy, the treatment of any disorder or pathophysiologic state on the basis of the transfer of genetic information, was a high-priority goal in the 1990s. The lung is a major target of gene therapy for genetic disorders, such as cystic fibrosis and alpha1-antitrypsin deficiency, and for other diseases, including lung cancer, malignant mesothelioma, pulmonary inflammation, surfactant deficiency, and pulmonary hypertension. This paper examines general concepts in gene therapy, summarizes the results of published clinical trials, and highlights areas of research aimed at overcoming challenges in the field. Although progress has been slower than anticipated, gene transfer has been safely achieved in patients with lung diseases. Recent advancements in understanding of the molecular basis of lung disease and the development of improved vector systems make it likely that gene therapy will be an important tool for the 21st-century clinician.

Boron neutron capture therapy of brain tumors: enhanced survival and cure following blood-brain barrier disruption and intracarotid injection of sodium borocaptate and boronophenylalanine

PURPOSE

Boronophenylalanine (BPA) and sodium borocaptate (Na(2)B(12)H(11)SH or BSH) have been used clinically for boron neutron capture therapy (BNCT) of high-grade gliomas. These drugs appear to concentrate in tumors by different mechanisms and may target different subpopulations of glioma cells. The purpose of the present study was to determine if the efficacy of BNCT could be further improved in F98-glioma-bearing rats by administering both boron compounds together and by improving their delivery by means of intracarotid (i.c.) injection with or without blood-brain barrier disruption (BBB-D).

METHODS AND MATERIALS

For biodistribution studies, 10(5) F98 glioma cells were implanted stereotactically into the brains of syngeneic Fischer rats. Eleven to 13 days later animals were injected intravenously (i.v.) with BPA at doses of either 250 or 500 mg/kg body weight (b.w.) in combination with BSH at doses of either 30 or 60 mg/kg b.w. or i.c. with or without BBB-D, which was accomplished by i.c. infusion of a hyperosmotic (25%) solution of mannitol. For BNCT studies, 10(3) F98 glioma cells were implanted intracerebrally, and 14 days later animals were transported to the Brookhaven National Laboratory (BNL). They received BPA (250 mg/kg b.w.) in combination with BSH (30 mg/kg b.w. ) by i.v. or i.c. injection with or without BBB-D, and 2.5 hours later they were irradiated with a collimated beam of thermal neutrons at the BNL Medical Research Reactor.

RESULTS

The mean tumor boron concentration +/- standard deviation (SD) at 2.5 hours after i. c. injection of BPA (250 mg/kg b.w.) and BSH (30 mg/kg b.w.) was 56. 3 +/- 37.8 microgram/g with BBB-D compared to 20.8 +/- 3.9 microgram/g without BBB-D and 11.2 +/- 1.8 microgram/g after i.v. injection. Doubling the dose of BPA and BSH produced a twofold increase in tumor boron concentrations, but also concomitant increases in normal brain and blood levels, which could have adverse effects. For this reason, the lower boron dose was selected for BNCT studies. The median survival time was 25 days for untreated control rats, 29 days for irradiated controls, 42 days for rats that received BPA and BSH i.v., 53 days following i.c. injection, and 72 days following i.c. injection + BBB-D with subsets of long-term survivors and/or cured animals in the latter two groups. No histopathologic evidence of residual tumor was seen in the brains of cured animals.

CONCLUSIONS

The combination of BPA and BSH, administered i.c. with BBB-D, yielded a 25% cure rate for the heretofore incurable F98 rat glioma with minimal late radiation-induced brain damage. These results demonstrate that using a combination of boron agents and optimizing their delivery can dramatically improve the efficacy of BNCT in glioma-bearing rats.

Gene therapy for lung cancer

The advances that have been made over the past decade in the field of gene transfer as well as in the fields of immunology and the molecular biology of tumorigenesis have brought to reality the possibility of using gene transfer as an anti-cancer treatment modality. The published results of clinical trials using this approach to date have been very limited, and a considerable amount of work still needs to be done in order to make this an effective treatment modality. However the developments that have occurred in the past several years indicate that this modality will become efficacious in the foreseeable future.

Effective suicide gene therapy in vivo by EBV-based plasmid vector coupled with polyamidoamine dendrimer

This study demonstrates in vivo effectiveness of a nonviral vector system, Epstein-Barr virus (EBV)-based plasmid vector coupled with polyamidoamine (PAMAM) dendrimer (EBV/polyplex), in suicide gene therapy of cancer. The EBV-based vector is a plasmid vector containing EBV nuclear antigen 1 (EBNA1) gene and oriP from EBV genome. HSV-1 tk gene was transferred into Ewing's sarcoma cell lines, A4573 and KP-EWS-YI, by using an EBV-based plasmid vector, pSES.Tk, or a conventional plasmid vector, pS.Tk. Cells transfected with pSES.Tk/dendrimer showed approximately 100 times lower ID50 to ganciclovir (GCV) compared with those transfected with pS. Tk/dendrimer. Intratumoral injection of pSES.Tk/dendrimer but not pS. Tk/dendrimer drastically suppressed the growth of tumors which had generated from A4573 or Huh7 hepatocellular carcinoma (HCC) cells inoculated into severe combined immunodeficiency (SCID) mice. The treatment with pSES.Tk/dendrimer also resulted in significant prolongation of survival of the mice implanted with A4573. These results suggest that the EBV/polyplex system could be useful for in vivo suicide gene therapy of cancer. Gene Therapy (2000) 7, 53-60.

Therapeutic genes for cancer gene therapy

Cancer still represents a disease of high incidence and is therefore one major target for gene therapy approaches. Gene therapy for cancer implies that ideally selective tumor cell killing or inhibition of tumor cell growth can be achieved using nucleic acids (DNA and RNA) as the therapeutic agent. Therefore, the majority of cancer gene therapy strategies introduce foreign genes into tumor cells which aim at the immunological recognition and destruction, the direct killing of the target cells or the interference with tumor growth. To achieve this goal for gene therapy of cancer, a broad variety of therapeutic genes are currently under investigation in preclinical and in clinical studies. These genes are of very different origin and of different mechanisms of action, such as human cytokine genes, genes coding for immunostimulatory molecules/antigens, genes encoding bacterial or viral prodrug-activating enzymes (suicide genes), tumor suppressor genes, or multidrug resistance genes.

Killing of brain tumor cells by hypoxia-responsive element mediated expression of BAX

The presence of radioresistant hypoxic cells in human brain tumors limits the overall effectiveness of conventional fractionated radiation therapy. Tumor-specific therapies that target hypoxic cells are clearly needed. We have investigated the expression of suicide genes under hypoxia by a hypoxia-responsive element (HRE), which can be activated through hypoxia-inducible factor-1 (HIF-1). We transfected plasmids containing multiple copies of HRE into U-87 MG and U-251 MG-NCI human brain tumor cells and tested their ability to induce LacZ gene expression under anoxia. Gene expression under anoxia versus oxia was increased about 12-fold for U-87 MG cells and about fourfold for U-251 MG-NCI cells. At intermediate hypoxic conditions, increased LacZ gene expression in U-87 MG cells was induced by the plasmid that contained three HREs, but not by the plasmid with two HREs. Lastly, when we placed a suicide gene BAX under the control of HREs, cells transfected with the BAX plasmids were preferentially killed through apoptosis under anoxia. Our studies demonstrate that HRE-regulated gene expression is active in brain tumor cells, and that the amount of increased gene expression obtained is dependent on the cell line, the HRE copy number, and the degree of hypoxia.

Advances in the treatment of malignant pleural mesothelioma

Malignant pleural mesothelioma is a neoplasm that is commonly fatal and for which there are no widely accepted curative approaches. Mesothelioma is unresponsive to most chemotherapy and radiotherapy regimens, and it typically recurs even after the most aggressive attempts at surgical resection. Multimodality approaches have been of some benefit in prolonging survival of very highly selected subgroups of patients, but they have had a relatively small impact on the majority of the patients diagnosed with this disease. As the incidence of pleural mesothelioma peaks in the United States and Europe over the next 10 to 20 years, new therapeutic measures will be necessary. This review will discuss the roles of chemotherapy, radiotherapy, surgery, and combined modality approaches in the treatment of pleural mesothelioma, as well as scientific advances made in the past decade that have led to the development of experimental techniques, such as photodynamic therapy, immunotherapy, and gene therapy, that are currently undergoing human clinical trials. These promising new avenues may modify the therapeutic nihilism that is rampant among clinicians dealing with mesothelioma.

Cytochrome P450-based cancer gene therapy: recent advances and future prospects

Cytochrome P450-based cancer gene therapy is a novel prodrug activation strategy for cancer treatment that has substantial potential for improving the safety and efficacy of cancer chemotherapeutics. The primary goal of this strategy is to selectively increase tumor cell exposure to cytotoxic drug metabolites generated locally by a prodrug-activating P450 enzyme. This strategy has been exemplified for the alkylating agents cyclophosphamide and ifosfamide, which are bioactivated by select P450 enzymes whose expression is generally high in liver and deficient in tumor cells. Transduction of tumors with a prodrug-activating P450 gene, followed by prodrug treatment, greatly increases intratumoral formation of activated drug metabolites. This leads to more efficient killing of the transduced tumor cells without a significant increase in host toxicity. P450 gene therapy is accompanied by substantial bystander cytotoxicity which greatly enhances the therapeutic effect by extending it to nearby tumor cells not transduced with the therapeutic P450 gene. Although endogenous P450 reductase is not expected to be a limiting factor in prodrug activation in tumor cells that express moderate levels of an exogenous P450 gene, P450 reductase transduction has recently been found to substantially enhance intratumoral prodrug activation and its associated therapeutic effects. Using this gene combination, an overall 50- to 100-fold increase in tumor cell kill in vivo over that provided by hepatic drug activation alone has been observed. Striking improvements in therapeutic effects can thus be achieved using an established anticancer drug in an intratumoral prodrug activation strategy based on the combination of a cytochrome P450 gene with the gene encoding NADPH-P450 reductase. This strategy is readily extendable to several other widely used P450-activated cancer chemotherapeutic prodrugs, as well as to prodrugs that undergo P450 reductase-dependent bioreductive activation and which may exhibit synergy when combined with P450-activated prodrugs in a P450/P450 reductase-based cancer gene therapeutic regimen.