Immune-dependent distant bystander effect after adenovirus-mediated suicide gene transfer in a rat model of liver colorectal metastasis

Recent progress in the research of suicide gene therapy for malignant glioma

Malignant glioma, which is characterized by diffuse infiltration into the normal brain parenchyma, is the most aggressive primary brain tumor with dismal prognosis. Over the past 40 years, the median survival has only slightly improved. Therefore, new therapeutic modalities must be developed. In the 1990s, suicide gene therapy began attracting attention for the treatment of malignant glioma. Some clinical trials used a viral vector for suicide gene transduction; however, it was found that viral vectors cannot cover the large invaded area of glioma cells. Interest in this therapy was recently revived because some types of stem cells possess a tumor-tropic migratory capacity, which can be used as cellular delivery vehicles. Immortalized, clonal neural stem cell (NSC) line has been used for patients with recurrent high-grade glioma, which showed safety and efficacy. Embryonic and induced pluripotent stem cells may be considered as sources of NSC because NSC is difficult to harvest, and ethical issues have been raised. Mesenchymal stem cells are alternative candidates for cellular vehicle and are easily harvested from the bone marrow. In addition, a new type of nonlytic, amphotropic retroviral replicating vector encoding suicide gene has shown efficacy in patients with recurrent high-grade glioma in a clinical trial. This replicating viral capacity is another possible candidate as delivery vehicle to tackle gliomas. Herein, we review the concept of suicide gene therapy, as well as recent progress in preclinical and clinical studies in this field.

Adenovirus-mediated herpes simplex virus thymidine kinase gene therapy combined with ganciclovir induces hepatoma cell apoptosis

The present study aimed to examine the apoptotic effects of adenovirus (ADV)-mediated herpes simplex virus thymidine kinase (ADV-TK) combined with ganciclovir (GCV) in tissues obtained from patients with hepatocellular carcinoma in order to provide a theoretical basis for the development of this gene therapy program. Apoptosis detection was conducted using the terminal deoxynucleotidyl-transferase-mediated dUTP nick end labelling assay and the apoptosis index was compared between the experimental; and control groups. Furthermore, the protein expression levels of caspase-3, B-cell lymphoma-2 (Bcl-2), Bcl-2-assoicated protein X (Bax) and nuclear factor (NF)-κB were examined in pathological specimens using immunohistochemical staining. The Bax/Bcl-2 ratio and the release of cytochrome c were examined using western blot analysis. Results indicated that combined ADV-TK and GCV treatment significantly increased the number of apoptotic cells compared with the control group (P<0.05). Immunohistological analysis revealed that ADV-TK and GCV treatment significantly increased the number of caspase-3-positive cells, reduced the Bax/Bcl-2 ratio and NF-κB expression levels and promoted the release of cytochrome c compared with the control group (P<0.01). In conclusion, the present results suggest that combined ADV-TK and GCV treatment exerts its effect through the apoptotic signaling pathway.

Cell-mediated enzyme prodrug cancer therapies

Cell-directed gene therapy is a promising new frontier for the field of targeted cancer therapies. Here we discuss the current pre-clinical and clinical use of cell-mediated enzyme prodrug therapy (EPT) directed against solid tumors and avenues for further development. We also discuss some of the challenges encountered upon translating these therapies to clinical trials. Upon sufficient development, cell-mediated enzyme prodrug therapy has the potential to maximize the distribution of therapeutic enzymes within the tumor environment, localizing conversion of prodrug to active drug at the tumor sites thereby decreasing off-target toxicities. New combinatorial possibilities are also promising. For example, when combined with viral gene-delivery vehicles, this may result in new hybrid vehicles that attain heretofore unmatched levels of therapeutic gene expression within the tumor.

Progress and problems with the use of suicide genes for targeted cancer therapy

Among various gene therapy methods for cancer, suicide gene therapy attracts a special attention because it allows selective conversion of non-toxic compounds into cytotoxic drugs inside cancer cells. As a result, therapeutic index can be increased significantly by introducing high concentrations of cytotoxic molecules to the tumor environment while minimizing impact on normal tissues. Despite significant success at the preclinical level, no cancer suicide gene therapy protocol has delivered the desirable clinical significance yet. This review gives a critical look at the six main enzyme/prodrug systems that are used in suicide gene therapy of cancer and familiarizes readers with the state-of-the-art research and practices in this field. For each enzyme/prodrug system, the mechanisms of action, protein engineering strategies to enhance enzyme stability/affinity and chemical modification techniques to increase prodrug kinetics and potency are discussed. In each category, major clinical trials that have been performed in the past decade with each enzyme/prodrug system are discussed to highlight the progress to date. Finally, shortcomings are underlined and areas that need improvement in order to produce clinical significance are delineated.

Phase II multicenter study of gene-mediated cytotoxic immunotherapy as adjuvant to surgical resection for newly diagnosed malignant glioma

BACKGROUND

Despite aggressive standard of care (SOC) treatment, survival of malignant gliomas remains very poor. This Phase II, prospective, matched controlled, multicenter trial was conducted to assess the safety and efficacy of aglatimagene besadenovec (AdV-tk) plus valacyclovir (gene-mediated cytotoxic immunotherapy [GMCI]) in combination with SOC for newly diagnosed malignant glioma patients.

METHODS

Treatment cohort patients received SOC + GMCI and were enrolled at 4 institutions from 2006 to 2010. The preplanned, matched-control cohort included all concurrent patients meeting protocol criteria and SOC at a fifth institution. AdV-tk was administered at surgery followed by SOC radiation and temozolomide. Subset analyses were preplanned, based on prognostic factors: pathological diagnosis (glioblastoma vs others) and extent of resection.

RESULTS

Forty-eight patients completed SOC + GMCI, and 134 met control cohort criteria. Median overall survival (OS) was 17.1 months for GMCI + SOC versus 13.5 months for SOC alone (P = .0417). Survival at 1, 2, and 3 years was 67%, 35%, and 19% versus 57%, 22%, and 8%, respectively. The greatest benefit was observed in gross total resection patients: median OS of 25 versus 16.9 months (P = .0492); 1, 2, and 3-year survival of 90%, 53%, and 32% versus 64%, 28% and 6%, respectively. There were no dose-limiting toxicities; fever, fatigue, and headache were the most common GMCI-related symptoms.

CONCLUSIONS

GMCI can be safely combined with SOC in newly diagnosed malignant gliomas. Survival outcomes were most notably improved in patients with minimal residual disease after gross total resection. These data should help guide future immunotherapy studies and strongly support further evaluation of GMCI for malignant gliomas.

CLINICAL TRIAL REGISTRY

ClinicalTrials.gov NCT00589875.

Gene-mediated cytotoxic immunotherapy as adjuvant to surgery or chemoradiation for pancreatic adenocarcinoma

BACKGROUND

While surgical resection of pancreatic adenocarcinoma provides the only chance of cure, long-term survival remains poor. Immunotherapy may improve outcomes, especially as adjuvant to local therapies. Gene-mediated cytotoxic immunotherapy (GMCI) generates a systemic anti-tumor response through local delivery of an adenoviral vector expressing the HSV-tk gene (aglatimagene besadenovec, AdV-tk) followed by anti-herpetic prodrug. GMCI has demonstrated synergy with standard of care (SOC) in other tumor types. This is the first application in pancreatic cancer.

METHODS

Four dose levels (3 × 10(10) to 1 × 10(12) vector particles) were evaluated as adjuvant to surgery for resectable disease (Arm A) or to 5-FU chemoradiation for locally advanced disease (Arm B). Each patient received two cycles of AdV-tk + prodrug.

RESULTS

Twenty-four patients completed therapy, 12 per arm, with no dose-limiting toxicities. All Arm A patients were explored, eight were resected, one was locally advanced and three had distant metastases. CD8(+) T cell infiltration increased an average of 22-fold (range sixfold to 75-fold) compared with baseline (p = 0.0021). PD-L1 expression increased in 5/7 samples analyzed. One node-positive resected patient is alive >66 months without recurrence. Arm B RECIST response rate was 25 % with a median OS of 12 months and 1-year survival of 50 %. Patient-reported quality of life showed no evidence of deterioration.

CONCLUSIONS

AdV-tk can be safely combined with pancreatic cancer SOC without added toxicity. Response and survival compare favorably to expected outcomes and immune activity increased. These results support further evaluation of GMCI with more modern chemoradiation and surgery as well as PD-1/PD-L1 inhibitors in pancreatic cancer.

Bortezomib sensitizes non-small cell lung cancer to mesenchymal stromal cell-delivered inducible caspase-9-mediated cytotoxicity

Delivery of suicide genes to solid tumors represents a promising tumor therapy strategy. However, slow or limited killing by suicide genes and ineffective targeting of the tumor has reduced effectiveness. We have adapted a suicide system based on an inducible caspase-9 (iC9) protein that is activated using a specific chemical inducer of dimerization (CID) for adenoviral-based delivery to lung tumors via mesenchymal stromal cells (MSCs). Four independent human non-small cell lung cancer (NSCLC) cell lines were transduced with adenovirus encoding iC9, and all underwent apoptosis when iC9 was activated by adding CID. However, there was a large variation in the percentage of cell killing induced by CID across the different lines. The least responsive cell lines were sensitized to apoptosis by combined inhibition of the proteasome using bortezomib. These results were extended to an in vivo model using human NSCLC xenografts. E1A-expressing MSCs replicated Ad.iC9 and delivered the virus to lung tumors in SCID mice. Treatment with CID resulted in some reduction of tumor growth, but addition of bortezomib led to greater reduction of tumor size. The enhanced apoptosis and anti-tumor effect of combining MSC-delivered Ad.iC9, CID and bortezomib appears to be due to increased stabilization of active caspase-3, as proteasomal inhibition increased the levels of cleaved caspase-9 and caspase-3. Knockdown of X-linked inhibitor of apoptosis protein (XIAP), a caspase inhibitor that targets active caspase-3 to the proteasome, also sensitized iC9-transduced cells to CID, suggesting that blocking the proteasome counteracts XIAP to permit apoptosis. Thus, MSC-based delivery of the iC9 suicide gene to human NSCLC effectively targets lung cancer cells for elimination. Combining this therapy with bortezomib, a drug that is otherwise inactive in this disease, further enhances the anti-tumor activity of this strategy.

Gene therapy for cancer: present status and future perspective

Advancements in human genomics over the last two decades have shown that cancer is mediated by somatic aberration in the host genome. This discovery has incited enthusiasm among cancer researchers; many now use therapeutic approaches in genetic manipulation to improve cancer regression and find a potential cure for the disease. Such gene therapy includes transferring genetic material into a host cell through viral (or bacterial) and non-viral vectors, immunomodulation of tumor cells or the host immune system, and manipulation of the tumor microenvironment, to reduce tumor vasculature or to increase tumor antigenicity for better recognition by the host immune system. Overall, modest success has been achieved with relatively minimal side effects. Previous approaches to cancer treatment, such as retrovirus integration into the host genome with the risk of mutagenesis and second malignancies, immunogenicity against the virus and/or tumor, and resistance to treatment with disease relapse, have markedly decreased with the new generation of viral and non-viral vectors. Several tumor-specific antibodies and genetically modified immune cells and vaccines have been developed, yet few are presently commercially available, while many others are still ongoing in clinical trials. It is anticipated that gene therapy will play an important role in future cancer therapy as part of a multimodality treatment, in combination with, or following other forms of cancer therapy, such as surgery, radiation and chemotherapy. The type and mode of gene therapy will be determined based on an individual's genomic constituents, as well as his or her tumor specifics, genetics, and host immune status, to design a multimodality treatment that is unique to each individual's specific needs.

Gene therapy for cancer: present status and future perspective

Advancements in human genomics over the last two decades have shown that cancer is mediated by somatic aberration in the host genome. This discovery has incited enthusiasm among cancer researchers; many now use therapeutic approaches in genetic manipulation to improve cancer regression and find a potential cure for the disease. Such gene therapy includes transferring genetic material into a host cell through viral (or bacterial) and non-viral vectors, immunomodulation of tumor cells or the host immune system, and manipulation of the tumor microenvironment, to reduce tumor vasculature or to increase tumor antigenicity for better recognition by the host immune system. Overall, modest success has been achieved with relatively minimal side effects. Previous approaches to cancer treatment, such as retrovirus integration into the host genome with the risk of mutagenesis and second malignancies, immunogenicity against the virus and/or tumor, and resistance to treatment with disease relapse, have markedly decreased with the new generation of viral and non-viral vectors. Several tumor-specific antibodies and genetically modified immune cells and vaccines have been developed, yet few are presently commercially available, while many others are still ongoing in clinical trials. It is anticipated that gene therapy will play an important role in future cancer therapy as part of a multimodality treatment, in combination with, or following other forms of cancer therapy, such as surgery, radiation and chemotherapy. The type and mode of gene therapy will be determined based on an individual's genomic constituents, as well as his or her tumor specifics, genetics, and host immune status, to design a multimodality treatment that is unique to each individual's specific needs.

Glioma virus therapies between bench and bedside

Despite extensive research, current glioma therapies are still unsatisfactory, and novel approaches are pressingly needed. In recent years, both nonreplicative viral vectors and replicating oncolytic viruses have been developed for brain cancer treatment, and the mechanistic background of their cytotoxicity has been unveiled. A growing number of clinical trials have convincingly established viral therapies to be safe in glioma patients, and maximum tolerated doses have generally not been reached. However, evidence for therapeutic benefit has been limited: new generations of therapeutic vectors need to be developed in order to target not only tumor cells but also the complex surrounding microenvironment. Such therapies could also direct long-lasting immune responses toward the tumor while reducing early antiviral reactions. Furthermore, viral delivery methods are to be improved and viral spread within the tumor will have to be enhanced. Here, we will review the outcome of completed glioma virus therapy trials as well as highlight the ongoing clinical activities. On this basis, we will give an overview of the numerous strategies to enhance therapeutic efficacy of new-generation viruses and novel treatment regimens. Finally, we will conclude with approaches that may be crucial to the development of successful glioma therapies in the future.

Intraprostatic distribution and long-term follow-up after AdV-tk immunotherapy as neoadjuvant to surgery in patients with prostate cancer

A phase I-II study to evaluate gene mediated cytotoxic immunotherapy in newly diagnosed prostate cancer before radical prostatectomy was conducted in Monterrey, Mexico.

Local bystander effect induces dormancy in human medullary thyroid carcinoma model in vivo

The extent of local bystander effect induced by fusion yeast cytosine deaminase::uracil phosphoribosyltransferase (yCD) in combination with 5-fluorocytosine (5FC) was evaluated in xenogeneic model of human medullary thyroid carcinoma (MTC). This approach to gene-directed enzyme/prodrug therapy (GDEPT) induces strong bystander cytotoxicity. Effector yCD-TT mixed with target EGFP-TT cells in a ratio 2:9 could achieve significant tumor regression and 14-fold decrease in serum marker calcitonin upon 5FC administration. Histopathological analysis unraveled that antitumor effect resulted in tumor dormancy and proliferation arrest of remaining tumor cell clusters in vivo. yCD/5FC combination represents another GDEPT approach to achieve tumor growth control in MTC.

Cytotoxic immunotherapy strategies for cancer: mechanisms and clinical development

Traditional therapies for cancer include surgery, chemotherapy, and radiation. Chemotherapy has widespread systemic cytotoxic effects against tumor cells but also affects normal cells. Radiation has more targeted local cytotoxicity but is limited to killing cells in the radiation field. Immunotherapy has the potential for systemic, specific killing of tumor cells. However, if the immune response is specific to a single antigen, tumor evasion can occur by down-regulation of that antigen. An immunotherapy approach that induces polyvalent immunity to autologous tumor antigens can provide a personalized vaccine with less potential for immunologic escape. A cytotoxic immunotherapy strategy creates such a tumor vaccine in situ. Immunogenic tumor cell death provides tumor antigen targets for the adaptive immune response and stimulates innate immunity. Attraction and activation of antigen presenting cells such as dendritic cells is important to process and present tumor antigens to T cells. These include cytotoxic T cells that kill tumor cells and T cells which positively and negatively regulate immunity. Tipping the balance in favor of anti-tumor immunity is an important aspect of an effective strategy. Clinically, immunotherapies may be most effective when combined with standard therapies in a complimentary way. An example is gene-mediated cytotoxic immunotherapy (GMCI) which uses an adenoviral vector, AdV-tk, to deliver a cytotoxic and immunostimulatory gene to tumor cells in vivo in combination with standard therapies creating an immunostimulatory milieu. This approach, studied extensively in animal models and early stage clinical trials, is now entering a definitive Phase 3 trial for prostate cancer.

Phase IB study of gene-mediated cytotoxic immunotherapy adjuvant to up-front surgery and intensive timing radiation for malignant glioma

PURPOSE

Despite aggressive therapies, median survival for malignant gliomas is less than 15 months. Patients with unmethylated O(6)-methylguanine-DNA methyltransferase (MGMT) fare worse, presumably because of temozolomide resistance. AdV-tk, an adenoviral vector containing the herpes simplex virus thymidine kinase gene, plus prodrug synergizes with surgery and chemoradiotherapy, kills tumor cells, has not shown MGMT dependency, and elicits an antitumor vaccine effect.

PATIENTS AND METHODS

Patients with newly diagnosed malignant glioma received AdV-tk at 3 × 10(10), 1 × 10(11), or 3 × 10(11) vector particles (vp) via tumor bed injection at time of surgery followed by 14 days of valacyclovir. Radiation was initiated within 9 days after AdV-tk injection to overlap with AdV-tk activity. Temozolomide was administered after completing valacyclovir treatment.

RESULTS

Accrual began December 2005 and was completed in 13 months. Thirteen patients were enrolled and 12 completed therapy, three at dose levels 1 and 2 and six at dose level 3. There were no dose-limiting or significant added toxicities. One patient withdrew before completing prodrug because of an unrelated surgical complication. Survival at 2 years was 33% and at 3 years was 25%. Patient-reported quality of life assessed with the Functional Assessment of Cancer Therapy-Brain (FACT-Br) was stable or improved after treatment. A significant CD3(+) T-cell infiltrate was found in four of four tumors analyzed after treatment. Three patients with MGMT unmethylated glioblastoma multiforme survived 6.5, 8.7, and 46.4 months.

CONCLUSION

AdV-tk plus valacyclovir can be safely delivered with surgery and accelerated radiation in newly diagnosed malignant gliomas. Temozolomide did not prevent immune responses. Although not powered for efficacy, the survival and MGMT independence trends are encouraging. A phase II trial is ongoing.

Antitumor activity of mutant bacterial cytosine deaminase gene for colon cancer

AIM: To evaluate bacterial cytosine deaminase (bCD) mutant D314A and 5-fluorocytosine (5-FC) for treatment of colon cancer in a mouse model.

METHODS: Recombinant lentivirus vectors that contained wild-type bCD gene (bCDwt), and bCD mutant D314A gene (bCD-D314A) with green fluorescence protein gene were constructed and used to infect human colon carcinoma LoVo cells, to generate stable transfected cells, LoVo/null, LoVo/bCDwt or LoVo/bCD-D314A. These were injected subcutaneously into Balb/c nude mice to establish xenograft models. Two weeks post-LoVo cell inoculation, PBS or 5-FC (500 mg/kg) was administered by intraperitoneal (i.p.) injection once daily for 14 d. On the day after LoVo cell injection, mice were monitored daily for tumor volume and survival.

RESULTS: Sequence analyses confirmed the construction of recombinant lentiviral plasmids that contained bCDwt or bCD-D314A. The lentiviral vector had high efficacy for gene delivery, and RT-PCR showed that bCDwt or bCD-D314A gene was transferred to LoVo cells. Among these treatment groups, gene delivery or 5-FC administration alone had no effect on tumor growth. However, bCDwt/5-FC or bCD-D314A/5-FC treatment inhibited tumor growth and prolonged survival of mice significantly (P < 0.05). Importantly, the tumor volume in the bCD-D314A/5-FC-treated group was lower than that in the bCDwt/5-FC group (P < 0.05), and bCD-D314A plus 5-FC significantly prolonged survival of mice in comparison with bCDwt plus 5-FC (P < 0.05).

CONCLUSION: The bCD mutant D314A enhanced significantly antitumor activity in human colon cancer xenograft models, which provides a promising approach for human colon carcinoma therapy.

Review article: gene therapy, recent developments and future prospects in gastrointestinal oncology

BACKGROUND

Gene therapy consists of the introduction of genetic material into cells for a therapeutic purpose. A wide range of gene therapy vectors have been developed and used for applications in gastrointestinal oncology.

AIM

To review recent developments and published clinical trials concerning the application of gene therapy in the treatment of liver, colon and pancreatic cancers.

METHODS

Search of the literature published in English using the PubMed database.

RESULTS

A large variety of therapeutic genes are under investigation, such as tumour suppressor, suicide, antiangiogenesis, inflammatory cytokine and micro-RNA genes. Recent progress concerns new vectors, such as oncolytic viruses, and the synergy between viral gene therapy, chemotherapy and radiation therapy. As evidence of these basic developments, recently published phase I and II clinical trials, using both single agents and combination strategies, in adjuvant or advanced disease settings, have shown encouraging results and good safety records.

CONCLUSIONS

Cancer gene therapy is not yet indicated in clinical practice. However, basic and clinical advances have been reported and gene therapy is a promising, new therapeutic approach for the treatment of gastrointestinal tumours.

ATRA enhances bystander effect of suicide gene therapy in the treatment of prostate cancer

OBJECTIVES

All-trans retinoic acid (ATRA) has been shown to inhibit the growth of many malignancies by altering gap junctional intercellular communication (GJIC) and the expression of connexin (Cx) 43. Here, we report that the alteration of GJIC by ATRA may directly enhance the bystander effect (BE) of suicide gene therapy against prostate cancer in vitro and in vivo.

METHODS

PC-3 cells were exposed to different concentrations of ATRA for varying lengths of time in culture. Flow cytometry was performed to measure Cx43-positive cells and the GJIC function of the cells was examined with the scrape-loading dye transfer assay. Cells were treated with ATRA in combination with an adenovirus/ganciclovir (Ad-TK/GCV) system encoding herpes simplex virus-thymidine kinase, and the BE was assessed in the treatment of androgen-independent prostate cancer both in vitro and in vivo. Semiquantitative reverse transcription polymerase chain reaction (RT-PCR) and immunohistochemistry were performed to assess the expression of Cx43 mRNA and protein in tumor tissues.

RESULTS

ATRA significantly increased the amount of Cx43-positive cells in a time- and dose-dependent manner (P < 0.05). GJIC functions were enhanced 3- to 5-fold in the presence of ATRA, although ATRA did not augment GCV toxicity of PC-3 cells. In the mixing assay, ATRA significantly increased cell killing when the ratio of TK-positive cells in the coculture ranged from 30% to 60% compared with ATRA-untreated cell (P < 0.05), and attained 50% cell killing cells when the ratio of TK-positive cell was 30%, but the same result did not appear until the ratio of TK-positive cell was up to 60% in the ATRA-untreated cell. Mice treated with a combination of ATRA and GCV had significantly smaller Ad-TK infected tumors than those treated with GCV or ATRA alone after 3-weeks of therapy (P < 0.05). However, from the fourth-week of therapy, there was no difference in tumor growth inhibition between GCV treatment and GCV + ATRA treatment (P > 0.05), as two tumors in the latter group started to grow more quickly than tumors in the control group. This phenomenon was not found in other groups.

CONCLUSIONS

ATRA could enhance the efficiency of cell killing in suicide gene therapy against prostate cancer by strengthening the BE in vitro and in vivo. Induction of Cxs and GJIC by ATRA might provide an element of selectivity to suicide gene therapy. Future studies should focus on safety and tailoring this cooperative therapy to the patient.

Gene therapy of liver cancer

The application of gene transfer technologies to the treatment of cancer has led to the development of new experimental approaches like gene directed enzyme/pro-drug therapy (GDEPT), inhibition of oncogenes and restoration of tumor-suppressor genes. In addition, gene therapy has a big impact on other fields like cancer immunotherapy, anti-angiogenic therapy and virotherapy. These strategies are being evaluated for the treatment of primary and metastatic liver cancer and some of them have reached clinical phases. We present a review on the basis and the actual status of gene therapy approaches applied to liver cancer.

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.

Increased anti-tumor effect by a combination of HSV thymidine kinase suicide gene therapy and interferon-gamma/GM-CSF cytokine gene therapy in CT26 tumor model

The potential therapeutic benefit of introducing IFN-gamma and GM-CSF genes in combination with the HSVtk suicide gene into subcutaneously implanted CT26 tumor cells was compared with that from each treatment alone. Cells, unmodified or retrovirally transduced with HSVtk or IFN-gamma/GM-CSF genes, were inoculated subcutaneously into syngeneic BALB/c mice in various combinations. HSVtk gene, with intraperitoneal ganciclovir treatment, reduced tumor volume by 81% at locally inoculated tumor sites (p < 0.01) and by 25% at distantly inoculated tumor sites (p = 0.052). IFN-gamma/GM-CSF genes showed a 56% tumor volume reduction at local tumor sites (p < 0.01) and 15% volume reduction at remote tumor sites, although this was not statistically significant. The combination of HSVtk (with GCV) and IFN-gamma/GM-CSF genes showed an 81% volume reduction at local tumor sites (p < 0.01) and a 43% volume reduction at remote tumor sites (p < 0.01). Thus, the combination of HSVtk and IFN-gamma/GM-CSF gene therapy produced greater therapeutic efficacy than either treatment alone.

Bystander effect contributes to the antitumor efficacy of CaSm antisense gene therapy in a preclinical model of advanced pancreatic cancer

Pancreatic adenocarcinoma (PC) is an aggressive malignancy resistant to standard treatment modalities. Previously, we have reported that cancer-associated Sm-like protein (CaSm) contributes to the neoplastic transformation of PC. In this study, we utilized a recently established preclinical model of PC to determine if molecular targeting of CaSm can serve as the basis for a novel PC therapy. In a subcutaneous tumor model, intratumoral administration of an adenoviral vector encoding CaSm antisense RNA (Ad-alphaCaSm) significantly inhibited Panc02 tumor growth. Furthermore, in a metastatic tumor model, systemic administration of Ad-alphaCaSm resulted in a significant decrease in the number of hepatic metastases and increased survival time. We assessed the efficiency of in vivo delivery and observed significant levels of vector transduction in tissues containing PC, as well as a bystander effect that was amplifying the efficacy of CaSm gene therapy. This bystander effect was also active in vitro and was shown to be at least partially independent of host-related mechanisms. We conclude that CaSm antisense gene therapy is an effective novel therapy for PC and that the antitumor efficacy is dependent on both direct and bystander mechanisms.

The controversial abscopal effect

The abscopal effect is potentially important for tumor control and is mediated through cytokines and/or the immune system, mainly cell-mediated immunity. It results from loss of growth stimulatory and/or immunosuppressive factors from the tumor. Until recently, the abscopal effect referred to the distant effects seen after local radiation therapy. However, the term should now be used interchangeably with distant bystander effect. Through analysis of distant bystander effects of other local therapies, we discuss the poorly understood and researched radiation-induced abscopal effect. Although the abscopal effect has been described in various malignancies, it is a rarely recognized clinical event. The abscopal effect is still extremely controversial with known data that both support and refute the concept.

Use of suicide genes for cancer gene therapy: study of the different approaches

Cancer is a disease of high incidence for which conventional treatments are not necessarily effective. There is a need for the development of new alternative strategies. Among them, suicide gene therapy has been developed. In this approach, a gene encoding for a protein toxic under particular conditions is delivered to the target cells, resulting in their death. Although this approach has been in development for a long time, new combinations with other gene therapy areas, such as selective replicative viruses, tumour targeting, or conventional treatments such as chemo- or radiotherapy, are currently being tested. This review will summarise some of these approaches.

Bystander-mediated regression of murine neuroblastoma via retroviral transfer of the HSV-TK gene

Selective introduction of genes conferring chemosensitivity into proliferating tumor cells may be used to treat cancer. We investigated the bystander effect of retrovirus-mediated gene transfer of herpes simplex virus thymidine kinase (HSV-TK) gene to murine neuroblastoma cell line (neuro-2a) in vitro and in vivo, and we examined whether the mechanism of bystander effect in neuroblastoma would also depend on connexin-dependent gap junction and/or immune response. A strong bystander effect was observed in vitro, whereby nontransduced tumor cells in proximity to transduced cells acquired susceptibility to ganciclovir (GCV) killing. Implanted mixtures of wildtype cells and HSV-TK transduced cells showed a potent bystander effect upon administration of GCV in A/J mice. HSV-TK/GCV system in murine neuroblastoma induced systemic immunity. Immunohistochemical staining showed many CD4+ and CD8+ cell infiltration but did not show anti-connexin 43+ cells. In conclusion, a strong bystander effect was observed in vitro and in vivo. The bystander effect in murine neuroblastoma might be dependent on immune response and/or on other mechanism such as protein phosphorylation or transfer of apoptotic vesicle, rather than connexin-dependent gap junction.

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Antitumor effect of secreted Flt3-ligand can act at distant tumor sites in a murine model of head and neck cancer

The Flt3 ligand (Flt3-L) manifests antitumor activity, presumably due to its capacity to recruit dendritic cells and cause their proliferation. To assess whether local production of Flt3-L can mediate a "distant bystander" effect, murine B4B8 squamous cell carcinoma cells were transfected with a plasmid encoding a secretory form of Flt3-L to produce B4B8FL cells. Similarly, B4B8FL and B4B8 cells were transfected with herpes simplex virus thymidine kinase (HSVTK) to produce B4B8TK and B4B8FL/TK cells, which should be sensitive to ganciclovir (GCV), to know whether the effects of Flt3-L and HSVTK/GCV would be synergistic. To test for a distant bystander effect in vivo, B4B8FL, B4B8TK, and B4B8FL/TK cells were injected subcutaneously into the left flank of syngeneic Balb/c mice, and naïve B4B8 cells were injected into the right flank. The formation of tumors derived from B4B8FL and B4B8FL/TK cells was significantly delayed in both flanks compared with naïve B4B8 and B4B8TK cells. Growth of B4B8TK tumors in the ipsilateral flank was retarded following GCV treatment, but in contrast to B4B8FL and B4B8FL/TK cells, no distant bystander effect in the contralateral flank was observed. Immunohistochemistry showed lymphocytic infiltrates in both flanks of the B4B8FL and B4B8FL/TK groups. The data indicate that in these cells, local secretion of Flt3-L is sufficient to evoke a distant bystander effect but that expression of HSVTK, even after GCV administration, is not. Furthermore, the combination of local Flt3-L and HSVTK production, together with GCV administration, does not enhance the distant bystander effect produced by Flt3-L alone.

Gene therapy for malignant liver disease

For most patients with advanced or multifocal hepatocellular carcinoma (HCC) or with metastatic malignant liver disease treatment options are limited, resulting in a poor prognosis. Novel therapeutic strategies such as gene therapy are therefore urgently required. Gene therapeutic approaches use gene delivery systems (vectors) to introduce DNA constructs as therapeutic agents into living cells. Antitumour strategies include the reintroduction of tumour suppressor genes into tumour cells, the expression of foreign enzymes to render tumours susceptible to treatment with chemotherapeutic agents and the enhancement of tumour immunogenicity by expressing immunomodulatory genes or by genetic vaccination with tumour antigens. Furthermore, gene therapy may be also used for anti-angiogenesis to reduce tumour growth and metastatic potential. Other novel approaches aim at the development of genetically altered replication competent viruses, which selectively replicate in tumour cells inducing cell lysis. Although most clinical trials of antitumour gene therapy so far have failed to induce strong therapeutic effects, further improvement of antitumour gene therapy may finally result in potent clinical treatment options for patients with malignant liver tumours.