Limited efficacy of the HSV-TK/GCV system for gene therapy of malignant gliomas and perspectives for the combined transduction of the interleukin-4 gene

Allergic Signs in Glioma Pathology: Current Knowledge and Future Perspectives

Historically restrained to immune defense against parasite infections, allergic inflammation has been recently rediscovered to protect from a wide array of environmental triggers, such as xenobiotics and carcinogens, which can induce DNA damage and ultimately lead to cancer development. Moreover, cells and mediators typical of allergic responses can importantly modulate the tissue inflammatory milieu, which represents a crucial gatekeeper towards the acquisition of malignancy by cancer cells through immune escape. Numerous studies have described an inverse association between allergies and glioma development. Mast cells, key players of allergic reactions, have been recently found at increased numbers in glioblastoma multiforme (GBM), the most common and lethal primary brain tumor, and they have been implicated in GBM pathogenesis. In this review, we summarize epidemiological studies and discuss the main evidence highlighting a potential interplay between allergic responses, and glioma formation and progression. Last, we draw future lines of research for better clarification whether and through which mechanisms allergic inflammation might impact on gliomagenesis. The comprehension of the immune mechanisms favoring or counteracting tumor growth might open the path to novel immunotherapy approaches.

Gene therapy and targeted toxins for glioma

The most common primary brain tumor in adults is glioblastoma. These tumors are highly invasive and aggressive with a mean survival time of 15-18 months from diagnosis to death. Current treatment modalities are unable to significantly prolong survival in patients diagnosed with glioblastoma. As such, glioma is an attractive target for developing novel therapeutic approaches utilizing gene therapy. This review will examine the available preclinical models for glioma including xenographs, syngeneic and genetic models. Several promising therapeutic targets are currently being pursued in pre-clinical investigations. These targets will be reviewed by mechanism of action, i.e., conditional cytotoxic, targeted toxins, oncolytic viruses, tumor suppressors/oncogenes, and immune stimulatory approaches. Preclinical gene therapy paradigms aim to determine which strategies will provide rapid tumor regression and long-term protection from recurrence. While a wide range of potential targets are being investigated preclinically, only the most efficacious are further transitioned into clinical trial paradigms. Clinical trials reported to date are summarized including results from conditionally cytotoxic, targeted toxins, oncolytic viruses and oncogene targeting approaches. Clinical trial results have not been as robust as preclinical models predicted; this could be due to the limitations of the GBM models employed. Once this is addressed, and we develop effective gene therapies in models that better replicate the clinical scenario, gene therapy will provide a powerful approach to treat and manage brain tumors.

Sitimagene ceradenovec: a gene-based drug for the treatment of operable high-grade glioma

The field of gene therapy for malignant glioma has made important advances since the first gene transfer studies were performed 20 years ago. Multiple Phase I/II trials and two Phase III trials have been performed and have demonstrated the feasibility and safety of intratumoral vector delivery in the brain. Sitimagene ceradenovec is an adenoviral vector encoding the herpes simplex thymidine kinase gene, developed by Ark Therapeutics Group plc (UK and Finland) for the treatment of patients with operable high-grade glioma. In preclinical and Phase I/II clinical studies, sitimagene ceradenovec exhibited a significant increase in survival. Although the preliminary results of a Phase III clinical study demonstrated a significant positive effect of sitimagene ceradenovec treatment on time to reintervention or death when compared with standard care treatment (hazard ratio: 1.43; 95% CI: 1.06-1.93; p < 0.05), the European Committee for Medicinal Products for Human Use did not consider the data to provide sufficient evidence of clinical benefit. Further clinical evaluation, powered to demonstrate a benefit on a robust end point, is required. This article focuses on sitimagene ceradenovec and provides an overview of the developments in the field of gene therapy for malignant glioma.

Exogenous wt-p53 enhances the antitumor effect of HSV-TK/GCV on C6 glioma cells

OBJECTIVE

To study on the antitumor effect of combining wt-p53 gene with suicide gene therapy (HSV-tk+GCV) for malignant gliomas.

METHODS

AdCMV-p53 was transfected into C6 glioma cells at MOI of (Multiplicity of infection) 0(G100), 10(TPG1), 100(TPG2), then AdCMV-tk was transducted to C6 glioma cells of G100, TPG1 and TPG2, respectively, at MOI of 100. The C6 glioma cells tranfected with both AdCMV-p53 and AdCMV-tk were exposed to various concentration of GCV. The cell survival rate was measured by MTT assay in vitro. Rat glioma model was established by injecting 5 x 10(5) C6 glioma cells into right caudate nucleus of SD rats. AdCMV-p53 and AdCMV-tk were injected into glioma on day 5 and 6, respectively. On day 7, ganciclovir (GCV) was administrated intraperitoneally at 15 mg/kg/day for 14 days. The survival time of all rats was observed. The growth of intracerebral tumors was monitored dynamically by enhanced MRI. Cell apoptosis was evaluated by TUNEL method. Expression of HSV-tk gene was identified by in situ hybridization and expression of exogenous p53 gene was detected with Western blotting.

RESULTS

In vitro, wt-p53 significantly enhanced antitumor effect of HSV-tk/GCV. The concentration of GCV for ID50 of TPG2 cells (0.001 microg/ml GCV) was 10 times lower than that for the cells of tk-GCV group (MOI = 100), while the concentration of GCV for ID100 of TPG2 (0.01 microg/ml GCV) and TPG1(0.1 microg/ml GCV) was 100 and 10 times lower than that for the cells of tk-GCV group (MOI = 100), respectively. Apoptosis of C6 glioma cells also could be induced by transfection with wt-p53 gene slightly. For in vivo study, the survival time of tumor-bearing rats treated with HSV-TK/GCV or wt-p53 combined with HSV-TK/GCV was significantly prolonged and the intracerebral tumors were regressed and disappeared earlier in the combined gene therapy group than those in the HSV-TK/GCV therapy group as shown in enhanced MRI. However, only half dose of GCV for the rats treated with both wt-p53 and HSV-TK/GCV was needed to obtain the same efficacy as those rats treated with HSV-TK/GCV alone. These results indicate that the transfection of wt-p53 potentiates the effect of HSV-TK/GCV therapy.

CONCLUSIONS

The combination of HSV-tk/GCV system with wt-p53 gene transduction is optimal for clinical therapeutic trials of suicide gene therapy for malignant gliomas.

Adeno-associated virus (AAV) serotypes 2, 4 and 5 display similar transduction profiles and penetrate solid tumor tissue in models of human glioma

BACKGROUND

Adeno-associated viral (AAV) vectors are potent delivery vehicles for gene transfer strategies directed at the central nervous system (CNS), muscle and liver. However, comparatively few studies have described AAV-mediated gene transfer to tumor tissues. We have previously demonstrated that while AAV2 and Adenoviral (Ad) 5 vectors have similar broad host ranges in tumor-derived cell lines, AAV2 was able to penetrate human glioblastoma biopsy spheroids and xenografts more efficiently than Ad 5 vectors. These results suggested that AAV vectors could be suitable for therapeutic gene delivery to solid tumor tissue. In the present work, the transduction efficacy of AAV serotypes 4 and 5 were compared to AAV2, both in vitro and in intracranial GBM xenografts derived from patient biopsies implanted into nude rats.

METHODS

AAV vector serotypes 2, 4, and 5 containing either the green fluorescent protein (GFP) or the bacterial beta-galactosidase (lacZ) reporter gene were added to five different human glioma cell lines, to multicellular spheroids generated from glioblastoma patient biopsies, and to spheroids xenografted intracranially in nude rats. Transduction efficiency was assessed by fluorescence imaging, histochemistry, immunohistochemistry and flow cytometry.

RESULTS

While all three AAV serotypes were able to transduce the glioma cell lines when added individually or when they were administered in concert, AAV2 transduced the glioma cells most effectively compared to AAV4 or AAV5. Upon infecting glioblastoma spheroids in vitro, all three AAV serotypes efficiently transduced cells located at the surface as well as within deeper layers of the spheroids. In addition, similarly to what was observed for AAV2 16, both AAV4 and AAV5 were able to transduce human glioblastoma xenografts implanted intracranially.

CONCLUSIONS

In addition to the widely used AAV2 serotype, AAV4 and AAV5 serotypes may also be used to transduce biologically diverse glioma cell lines. They also penetrate and transduce solid human tumor tissue derived from patient biopsies. Therefore, the data presented here provide a proof of principle for developing AAV4 and AAV5 as treatment vehicles for human malignant gliomas.

Gene therapy and targeted toxins for glioma

The most common primary brain tumor in adults is glioblastoma. These tumors are highly invasive and aggressive with a mean survival time of nine to twelve months from diagnosis to death. Current treatment modalities are unable to significantly prolong survival in patients diagnosed with glioblastoma. As such, glioma is an attractive target for developing novel therapeutic approaches utilizing gene therapy. This review will examine the available preclinical models for glioma including xenographs, syngeneic and genetic models. Several promising therapeutic targets are currently being pursued in pre-clinical investigations. These targets will be reviewed by mechanism of action, i.e., conditional cytotoxic, targeted toxins, oncolytic viruses, tumor suppressors/oncogenes, and immune stimulatory approaches. Preclinical gene therapy paradigms aim to determine which strategies will provide rapid tumor regression and long-term protection from recurrence. While a wide range of potential targets are being investigated preclinically, only the most efficacious are further transitioned into clinical trial paradigms. Clinical trials reported to date are summarized including results from conditionally cytotoxic, targeted toxins, oncolytic viruses and oncogene targeting approaches. Clinical trial results have not been as robust as preclinical models predicted, this could be due to the limitations of the GBM models employed. Once this is addressed, and we develop effective gene therapies in models that better replicate the clinical scenario, gene therapy will provide a powerful approach to treat and manage brain tumors.

Monocyte chemoattractant protein-1 gene delivery enhances antitumor effects of herpes simplex virus thymidine kinase/ganciclovir system in a model of colon cancer

Suicide gene therapy using the herpes simplex virus thymidine kinase/ganciclovir (HSV-tk/GCV) system is a well-characterized tool for cancer gene therapy; however, it does not yet exhibit sufficient efficacy to cure patients of malignancies. We have reported that adenovirally delivered monocyte chemoattractant protein (MCP)-1 augmented the antitumor effects of the HSV-tk/GCV system in an athymic nude mouse model. The current study, which uses an immunocompetent mouse model of colon cancer, was designed to evaluate the antitumor effects of MCP-1 gene delivery in conjunction with this suicide gene therapy system. Subcutaneous tumor foci were directly transduced with both recombinant adenoviruses (rAds) expressing an HSV-tk gene and either of the MCP-1, CD80 and LacZ genes, followed by GCV administration. The growth of tumors was markedly suppressed by codelivery of HSV-tk and MCP-1 genes, which was exclusively associated with the recruitment of monocytes/macrophages, T helper 1 (Th1) cytokine gene expression and cytotoxic activity of the splenocytes. Furthermore, the antitumor effects were more efficient than that obtained by the combination of HSV-tk and CD80 genes. These results suggest an immunomodulatory effect of MCP-1 in the context of suicide gene therapy of colon cancer via orchestration of innate and acquired immune responses.

Interleukin-4 gene transduced tumor cells promote a potent tumor-specific Th1-type response in cooperation with interferon-alpha transduction

To investigate antitumor mechanisms in interleukin (IL)-4 therapy, we established an IL-4-overexpressing MC38 murine colorectal cancer cell line (MC38-IL4). As a therapy against established tumors, MC38-IL4 cells were inoculated contralaterally 7 days after wild-type (MC38-WT) cells had been injected, significantly reducing growth of wild-type tumors (P=0.030). Immunohistochemical analysis showed numerous granulocytes infiltrating wild-type tumors of MC38-IL4-inoculated mice. Injection of MC38-IL4 cells in leukocyte-depleted mice confirmed that granulocytes were involved in IL-4-related primary antitumor effects. Inoculation of MC38-WT in leukocyte-depleted mice initially injected with MC38-IL4 suggested that T cells contributed to the antitumor effects. To investigate tumor-specific responses, we stimulated splenocytes of MC38-immune mice with MC38-IL4 cells in vitro, resulting in MC38-specific lysis (57.5+/-7.2%, effector to target ratio=20). Treatment of established wild-type tumors with MC38-IL4 in combination with interferon (IFN)-alpha-overexpressing MC38 cells (MC38-IFNalpha) significantly reduced the growth of wild-type tumors (P=0.009). In vitro IFN-gamma production by splenocytes from mice injected with both MC38-IL4 and -IFNalpha was greatly enhanced in comparison with MC38-IL4 alone, while IL-10 production was not increased. Thus, granulocytes concern early antitumor effects of IL-4 therapy. Subsequently, IL-4 induces long-lasting, tumor-specific immune responses. IL-4 appears to promote a T-helper 1-type antitumor immune response, which is enhanced in cooperation with IFN-alpha.

Widespread dispersion of adeno-associated virus serotype 1 and adeno-associated virus serotype 6 vectors in the rat central nervous system and in human glioblastoma multiforme xenografts

The transduction patterns of recombinant adeno-associated virus serotype 1 (AAV1) and serotype 6 (AAV6) vectors were assessed in human glioblastoma multiforme (GBM) cell lines, in human GBM biopsy spheroids, and in tumor xenografts growing in nude rat brains. All the cell lines tested (A172, D37, GaMg, HF66, and U373Mg) were found to be permissive to both AAV1 and AAV6 vectors, and thus displayed a transduction pattern similar to AAV2 vectors. For every cell line tested, the transduction efficiency displayed by AAV2 vectors was better than by isogenic and isopromoter AAV1 vectors. Transduction efficiency was dependent on the viral particle number used, suggesting that the receptors for these vectors are widely distributed in GBM tissues. Interestingly, AAV1, AAV2, and AAV6 vectors were able to infect and transduce the same cells when added simultaneously to monolayer cultures. Infection of human GBM biopsy spheroids with AAV1 and AAV6 vectors resulted in transgene expression both at the surface layers and in the core of the spheroids. Following injection of AAV1 and AAV6 vectors into human GBM biopsy xenografts growing in nude rat brains, reporter gene expression was seen both in the periphery as well as in the central regions of the tumors. When injected into the normal rat brain, both AAV1 and AAV6 vectors were found to transduce several central nervous system (CNS) regions. The presented results suggest a potential therapeutic role for AAV1 and AAV6 vectors in gene therapy for GBM and also for other CNS malignancies.

Recombinant adeno-associated virus (rAAV) expressing TFPI-2 inhibits invasion, angiogenesis and tumor growth in a human glioblastoma cell line

Recombinant adeno-associated viruses (rAAV) have become the vector of choice for many gene therapy protocols. rAAVs have a number of attractive features including long-term transgene expression and the ability to transduce both dividing and non-dividing cells. We have shown previously the anti-cancer role of tissue factor pathway inhibitor-2 (TFPI-2), a matrix-associated serine protease inhibitor, in human glioblastomas. As a result of our present study, in which 0.8-kb fragment of human TFPI-2 was cloned into the adeno-associated viral vectors (rAAA-TFPI-2), rAAV-TFPI-2 infection of SNB19 cells significantly increased TFPI-2 as determined by Western blotting. As assessed by spheroid and Matrigel assays, infection of SNB19 cells with rAAV-TFPI-2 significantly reduced migration and invasion in a dose-dependent manner. Tumor spheroids infected with rAAV-TFPI-2 and co-cultured with fetal rat brain aggregates did not invade rat brain aggregates, whereas 90-95% of the mock and AAV-CMV infected cells invaded rat brain aggregates. In vitro angiogenesis studies (tumor cells co-cultured with endothelial cells or endothelial cells seeded on matrigel) showed reduction of capillary-like structure formation in rAAV-TFPI-2-treated cells as compared to parental and mock-transfected cells. In in vivo angiogenesis results demonstrated the formation of microvessels in SNB19 parental cells and this formation was inhibited when the SNB19 cells were infected with rAAV-TFPI-2. Further, we observed a large reduction of tumor growth in SNB19 cells treated with rAAV-TFPI-2 virus injected intracerebrally when compared to controls. Our study demonstrates that rAAV-TFPI-2-mediated gene therapy offers a novel tool for the treatment of brain tumors.

Enhanced inhibition of syngeneic murine tumors by combinatorial therapy with genetically engineered HSV-1 expressing CCL2 and IL-12

Herpes simplex viruses type 1 (HSV-1) that lack the gamma(1)34.5 gene are unable to replicate in the central nervous system (CNS), but maintain replication competence in actively dividing tumors. To determine if antitumor therapy by M002, a gamma(1)34.5(-) HSV that expresses interleukin-12 (IL-12), could be augmented by combinatorial therapy with another gamma(1)34.5-deleted HSV-1 engineered to express the chemokine CCL2, Neuro-2a tumors were established subcutaneously in the syngeneic A/J mouse strain. Tumors received multiple injections intratumorally either of saline, the parent, non-cytokine-expressing virus R3659, M002, M010 (gamma(1)34.5(-) HSV expressing CCL2), or a combination of M002 and M010. Efficacies were evaluated by monitoring inhibition of tumor growth over time. Results demonstrated the following: (1) inhibition of tumor growth was most pronounced in tumors treated with a combination of M002 and M010; (2) enhanced tumor growth inhibition for the combinatorial treatment group was statistically significant compared to either M002 or M010 alone; and (3) the variability between slopes of the tumor growth rates within an individual treatment group appeared to be virus-dependent, and was reproducible between experiments. Our results demonstrate that combinatorial cytokine/chemokine gamma(1)34.5(-) HSV therapies can provide superior antitumor effects in experimental tumors as a model for malignancies arising in the brain.

Neuroradiographic changes following convection-enhanced delivery of the recombinant cytotoxin interleukin 13-PE38QQR for recurrent malignant glioma

OBJECT

Convection-enhanced delivery (CED) is a novel method for delivering therapeutic agents to infiltrative brain tumor cells. For agents administered by CED, changes on magnetic resonance (MR) imaging directly resulting from catheter placement, infusion, and the therapeutic compound may confound any interpretation of tumor progression. As part of an ongoing multiinstitutional Phase I study, 14 patients with recurrent malignant glioma underwent CED of interleukin (IL) 13-PE38QQR, a recombinant cytotoxin consisting of human IL-13 conjugated with a truncated Pseudomonas exotoxin. Serial neuroradiographic changes were assessed in this cohort of patients.

METHODS

Patients were treated in two groups: Group 1 patients received IL13-PE38QQR before and after tumor resection; Group 2 patients received infusion only after tumor resection. Preoperative and postinfusion MR images were obtained prospectively at specified regular intervals. Changes were noted along catheter tracks on postresection MR images obtained in all patients. A simple grading system was developed to describe these changes. When MR imaging changes appeared to be related to IL1 3-PE38QQR, patients were followed up without instituting new antitumor therapy.

CONCLUSIONS

As CED of therapeutic agents becomes more common, clinicians and investigators must become aware of associated neuroimaging changes that should be incorporated into toxicity assessment. We have developed a simple grading system to facilitate communication about these changes among investigators. Biological imaging modalities that could possibly distinguish these changes from recurrent tumor should be evaluated. In this study the authors demonstrate the challenges in determining efficacy when surrogate end points such as time to tumor progression as defined by new or progressive contrast enhancement on MR imaging are used with this treatment modality.

Antitumor immune responses mediated by adenoviral GDEPT using nitroreductase/CB1954 is enhanced by high-level coexpression of heat shock protein 70

Gene-directed enzyme prodrug therapy (GDEPT) is a promising approach to local management of cancer through targeted chemotherapy. Killing localized tumors by GDEPT in a manner that induces strong antitumor cellular immune responses might improve local management and allow benefit in disseminated cancer. Here we evaluated the combination of nitroreductase (NTR)/CB1954 GDEPT with high-level expression of heat shock protein 70 (HSP70, a stress protein that can shuttle cytosolic peptides into antigen-presenting cells) for induction of antitumor immunity using adenovirus gene delivery in an aggressive and nonimmunogenic BALB/c syngeneic 4T1 breast cancer model. The mechanism of cell death and spectrum of stress proteins induced are likely to be important determinants of the resulting immune responses. We showed that NTR/CB1954 treatment of 4T1 cells gave both apoptotic and nonapoptotic killing. In vivo killing of 4T1 cells expressing NTR gave weak antitumor immunity and very limited induction of stress proteins including HSP70. High-level coexpression of HSP70 during NTR/CB1954-mediated killing of 4T1 cells in vivo gave much greater protection from tumor challenge (67% long-term survivors compared to 17%) and induced 4T1-specific cytotoxic T-cell responses. The enhancement of antitumor responses resulting from HSP70 coexpression was similar to that conferred by coexpression of GM-CSF.

Current and future strategies for the treatment of malignant brain tumors

Glioblastoma (GB) is the most common subtype of primary brain tumor in adults. These tumors are highly invasive, very aggressive, and often infiltrate critical neurological areas within the brain. The mean survival time after diagnosis of GB has remained unchanged during the last few decades, in spite of advances in surgical techniques, radiotherapy, and also chemotherapy; patients' survival ranges from 9 to 12 months after initial diagnosis. In the same time frame, with our increasing understanding and knowledge of the physiopathology of several cancers, meaningful advances have been made in the treatment and control of several cancers, such as breast, prostate, and hematopoietic malignancies. Although a number of the genetic lesions present in GB have been elucidated and our understanding of the progressions of this cancer has increased dramatically over the last few years, it has not yet been possible to harness this information towards developing effective cures. In this review, we will focus on the classical ways in which GB is currently being treated, and will introduce a novel therapeutic modality, i.e., gene therapy, which we believe will be used in combination with classical treatment strategies to prolong the life-span of patients and to ultimately be able to control and/or cure these brain tumors. We will discuss the use of several vector systems that are needed to introduce the therapeutic genes within either the tumor mass, if these are not resectable, or the tumor bed, after successful tumor resection. We also discuss different therapeutic modalities that could be exploited using gene therapy, i.e., conditional cytotoxic approach, direct cytotoxicity, immunotherapy, inhibition of angiogenesis, and the use of pro-apoptotic genes. The advantages and disadvantages of each of the current vector systems available to transfer genes into the CNS are also discussed. With the advances in molecular techniques, both towards the elucidation of the physiopathology of GB and the development of novel, more efficient and less toxic vectors to deliver putative therapeutic genes into the CNS, it should be possible to develop new rationale and effective therapeutic approaches to treat this devastating cancer.

Dexamethasone inhibits the anti-tumor effect of interleukin 4 on rat experimental gliomas

Retroviral-mediated gene transfer of the IL-4 gene into experimental gliomas can cause tumor rejection, supporting the clinical use of this form of gene therapy for glioblastomas (GBM). In a clinical setting, the administration of dexamethasone (dex) is a standard procedure for GBM patients. This led us to examine the effects of dex on IL-4 gene therapy. We injected intracranially Fischer 344 rats with phosphate-buffered saline, 9L gliosarcoma cells mixed with E86.L4SN(200) cells (retroviral producer cells, RPC, transducing IL-4 cDNA) and 9L cells mixed with PA317.STK.SBA cells (control RPC expressing the HSV-tk gene). The rats from each group were treated with 0, 50, 100 or 250 microg dex/kg/day released by osmotic pumps implanted subcutaneously. While 80-100% of rats receiving 9L cells mixed with IL-4 RPC and not treated by dex survived for at least 2 months following tumor injection, only 50% and 17% of rats receiving 50 or 100 microg/kg/day of dex, respectively, reached this time point. These results indicate that dex significantly diminished the anti-tumor effect of IL-4. Thus, in a clinical setting, IL-4 gene transfer should be performed when low levels of dex are administered or in the absence of dex.

Transcellular transfer of active HSV-1 thymidine kinase mediated by an 11-amino-acid peptide from HIV-1 Tat

Suicide gene therapy using herpes simplex virus type-1 (HSV-1) thymidine kinase (TK) is a widely exploited approach for gene therapy of cancer and other hyperproliferative disorders. Despite its popularity, clinical success has been so far hampered mostly by the relative inefficiency of TK gene transfer and its limited bystander effect. Here we report that fusion of TK to an 11-amino-acid peptide from the basic domain of the HIV-1 Tat protein (Tat11) imparts cell membrane translocating ability to the enzyme and significantly increases its cytotoxic efficacy. In cells expressing Tat11-TK, this protein is found extracellularly, associated with cell surface heparan sulfate proteoglycans, and is released into the cell culture medium. Based on its interaction with HSPGs, the protein is then internalized by neighboring, nonexpressing cells, which become susceptible to cell death when treated with the nucleoside analogue acyclovir. As a consequence, co-cultures of wild-type cells with cells expressing Tat11-TK show increased sensitivity to ACV through a mechanism involving apoptosis. Modification of TK by fusion with Tat11 might constitute an important step for the optimization of TK suicide gene strategy for gene therapy of cellular proliferation.

Augmentation of local antitumor immunity in liver by interleukin-2 gene transfer via portal vein

Metastasis to the liver remains an important problem in the treatment of patients with gastrointestinal cancer. We examined the mechanism and effect on liver metastasis of in vivo interleukin-2 (IL-2) gene transfer to the liver. RCN-9 cells derived from F344 rat colon adenocarcinoma were injected into syngeneic rats via the ileocecal vein to induce liver tumors. A total of 2.5x10(9) pfu of adenovirus vector harboring the human IL-2 gene (AdCMVhIL-2), or 2.5x10(9) pfu of control vector encoding beta-galactosidase was administered before RCN-9 cell challenge. On day 14, mean tumor weight was 4.0+/-2.4 g in the control group, whereas IL-2-transduced livers had no tumors. Survival of AdCMVhIL-2-treated rats was significantly longer than that of control rats (P<.01). Flow cytometry demonstrated that the proportion of natural killer (NK) cells had increased among sinusoidal cells collected from IL-2-transduced livers. These cells were highly cytotoxic to RCN-9 cells in vitro in the presence of a physiological high concentration of recombinant IL-2. Preventative effects of IL-2 transduction of the liver against liver metastasis were lost after depletion of NK cells by treatment with anti-asialo GM1 antibodies. Our results indicate that IL-2 gene transfer to the liver prevents liver metastasis by continuously providing physiological high concentrations of IL-2 in the liver, thereby activating sinusoidal NK cells.

Adeno-associated viral vectors penetrate human solid tumor tissue in vivo more effectively than adenoviral vectors

The transduction efficiencies of adeno-associated viral vectors (AAV, serotype 2) and adenovirus vectors (ADV, serotype 5) were examined in three different models of cancer. First, we used flow cytometry to quantitate AAV-GFP or ADV-GFP transduction in 13 cell lines derived from malignant tissue (6 gliomas, 6 mammary cancers, and 1 leukemia). These experiments showed variable transduction efficiency (0%-81%) between the cell lines, with ADV being more effective compared to AAV in 9 of 13 cell lines. Second, spheroids prepared from human glioblastomas were infected with ADV or AAV expressing GFP or lacZ cassettes, and after 2 weeks, uniform reporter gene expression was observed on the spheroid. Whereas AAV produced consistent transduction throughout the spheroids, ADV infection was mainly limited to the outer cell layers of the spheroids, suggesting that AAV were more efficient at penetrating solid tumor tissue. Third, human biopsies from glioblastoma multiforme patients were xenografted into nude rats and grown for 4 weeks followed by viral vector injection. Combined use of high-resolution magnetic resonance imaging (MRI) and histologic analysis allowed the identification of transduced cells and their spatial distribution within the tumors. AAV-mediated transgene expression was observed in cell clusters through the entire tumor, while ADV-mediated transduction was restricted to cells at the tumor periphery. Thus, while AAV and ADV vectors may infect tumor-derived cell lines to a similar degree, AAV penetrated glioblastoma spheroids and xenografts more efficiently compared to ADV vectors. These results suggest that AAV may be suitable for therapeutic gene delivery to malignant tumors.

TRAIL enhances thymidine kinase/ganciclovir gene therapy of neuroblastoma cells

The clinical benefit of suicide gene therapy of tumors has been marginal, mostly due to the low gene transfer efficiency in vivo. The death-inducing ligand, TRAIL, effectively kills many tumor cell types, while sparing most normal tissues. We hypothesized that TRAIL may enhance HSV thymidine kinase/ganciclovir (TK/GCV) gene therapy of tumor cells by augmenting both target and bystander cell kill. Human SH-EP neuroblastoma cells expressing TK as well as bystander cells were effectively killed by apoptosis, and their clonogenicity was ablated following GCV. Human TRAIL enhanced TK/GCV-induced cell death and decreased clonogenicity of TK-expressing cells and also of bystander cells. Cooperation between TRAIL and TK/GCV depended both on caspase activation and on mitochondrial apoptogenic function because both the broad-spectrum caspase inhibitor zVAD.fmk and overexpression of Bcl-2 decreased enhancement of cell kill by TRAIL. Facilitation of TRAIL signalling by up-regulation of TRAIL receptors did not contribute to enhancement because cell surface expression of the agonistic TRAIL receptors 1 and 2 was not increased by TK/GCV. In conclusion, the concerted activation of caspases and the mitochondrial amplification of caspase activation by TK/GCV may explain the cooperative effect of TK/GCV and TRAIL on the kill of neuroblastoma cells. Because combined treatment also augmented the bystander cell kill, the addition of TRAIL may increase the efficacy of TK/GCV gene therapy of neuroblastoma.

Cytokine gene therapy of gliomas: effective induction of therapeutic immunity to intracranial tumors by peripheral immunization with interleukin-4 transduced glioma cells

To provide a means for comparing strategies for cytokine gene therapy against intracranial (i.c.) tumors, we generated rat gliosarcoma 9L cells transfected with interleukin-4 (9L-IL4), interleukin-12 (9L-IL12), granulocyte-macrophage colony-stimulating factor (9L-GMCSF) or interferon-alpha (9L-IFNalpha). To simulate direct and highly efficient cytokine gene delivery, cytokine transfected 9L tumors were implanted i.c. into syngeneic rats. i.c. injection led to tumor-outgrowth in the brain and killed most animals, whereas these cell lines were rejected following intradermal (i.d.) injection. Cytokine-expressing i.c. 9L tumors, however, had a greater degree of infiltration by immune cells compared with control, mock-transfected 9L-neo, but to a lesser degree than i.d. cytokine-expressing tumors. Tumor angiogenesis was suppressed in cytokine-transfected tumors. In a prophylaxis model, i.d. vaccination with 9L-IL4 resulted in long-term survival of 90% of rats challenged i.c. with parental 9L; whereas 40% of 9L-GM-CSF, 40% of 9L-IFNalpha and 0% of 9L-IL12-immunized rats were protected. In a therapy model (day 3 i.c. 9L tumors), only i.d. immunization with 9L-IL4 had long-term therapeutic benefits as 43% of rats survived >100 days. These data indicate that peripheral immunization with 9L-IL4 had the most potent therapeutic benefit among various cytokines and approaches tested against established, i.c. 9L tumors.

Herpes simplex virus thymidine kinase/ganciclovir-induced cell death is enhanced by co-expression of caspase-3 in ovarian carcinoma cells

There is a need to enhance the efficacy of genetic prodrug activation therapy using herpes simplex virus thymidine kinase (tk) and ganciclovir (GCV) following disappointing results in early clinical trials. tk/GCV has been shown to lead to the activation of caspase-3, a potent executor of apoptosis. We demonstrate that co-expression of pro-caspase-3 with tk/GCV leads to enhanced cell death in ovarian carcinoma cells in vitro. Following transfection with recombinant adenoviral vectors encoding tk, GCV treatment leads to greater cell death in pro-caspase-3-expressing clones of SKOV3 and IGROV1 than control cells, as well as more rapid activation of caspase-3 and more rapid cleavage of PARP. Flow cytometry suggests that there is a greater degree of S-phase block in the pro-caspase-3-expressing clones than in control cells following treatment with tk/GCV. None of these effects is seen following transfection with a control adenovirus that does not encode tk. The increased cell death, early caspase-3 activation and PARP cleavage, and flow cytometric changes seen in pro-caspase-3-expressing cells can be partially inhibited by treatment with benzyloxycarbonyl-val-ala-asp fluoromethylketone, a synthetic caspase inhibitor. Our data suggest that co-expression of pro-caspase-3 may lead to a significant enhancement of the efficacy of tk/GCV 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.

VP22 enhanced intercellular trafficking of HSV thymidine kinase reduced the level of ganciclovir needed to cause suicide cell death

BACKGROUND

The inefficiency of herpes simplex virus thymidine kinase (TK) gene transfer and toxicity of ganciclovir (GCV) at high concentrations in vivo limits the use of this suicide gene therapy approach for the treatment of cancers in clinical settings. To overcome the problem, we have sought evidence of amplification of cytotoxicity by co-transfer of the TK gene fused with the gene encoding HSV-1 structural protein VP22 which has a remarkable ability for intercellular trafficking.

METHODS

The expression of the fusion proteins from the chimeric VP22-TK or VP22-EGFP genes was shown by Western blot and VP22 promoted TK or EGFP intercellular trafficking by an indirect immunofluorescent assay. The cytotoxicity was demonstrated by a colorimetric cell proliferation assay followed by an assessment of the bystander effect on admixtures of transfected with non-transfected naive cells.

RESULTS

Our results show the expression of the VP22 fusion proteins and their spread to varying numbers of bystander cells (up to 30, observed in viable cells with VP22-EGFP as well as after methanol fixation), confirming that VP22 assisted intercellular trafficking of the fusion proteins. This VP22 promoted TK spreading resulted in killing by 2.5 microg/ml GCV of virtually all cells in cultures that had been transfected at an efficiency of only 27.5%. In contrast, fewer than 80% of cells were killed when transfected with 'tk alone' at the same efficiency. The cell killing effect was exponentially dependent on GCV concentration in cells transfected with 'tk alone' at GCV concentrations between 0.25 and 0.5 microg/ml, but not those transfected with VP22-TK, probably due to the continuously variable, high sensitivity of about 50% of cells. Even at low concentration of GCV (0.2 microg/ml), the enhancement of cell killing by VP22 was four-fold higher in cells transfected with VP22-TK than in cells transfected with 'tk alone'.

CONCLUSIONS

VP22 enhanced intercellular trafficking of TK and amplified the TK/GCV killing effect, especially in the lower range of GCV concentrations. This offers a new strategy to enhance the effectiveness of suicide gene therapy for the treatment of cancers.

Limitations of the C6/Wistar rat intracerebral glioma model: implications for evaluating immunotherapy

OBJECTIVE

Intracranial rat glioma models are a useful method for evaluating the efficacy and toxicity of novel therapies for malignant glioma. The C6/Wistar model has been used extensively as a reproducible in vivo model for studying primary brain tumors including anti-glioma immune responses. The objective of the present study is to provide in vivo evidence that the C6 rat glioma model is allogeneic within Wistar rats and is therefore inappropriate for evaluating immune responses.

METHODS

Growth patterns and immune responses of C6 cells implanted into the brain and flank of Wistar rats were analyzed and compared to an immunogenic syngeneic model (9L/Fischer).

RESULTS

Wistar rats with C6 tumors developed a potent humoral and cellular immune response to the tumor. Wistar rats given simultaneous flank and intracerebral tumors had a survival rate of 100% compared to an 11% survival rate in control animals receiving only intracranial C6 cells.

CONCLUSION

The C6 rat glioma induces a vigorous immune reaction that may mimic a specific anti-tumor response in Wistar rats. Efficacy of immunotherapy within this model must be cautiously interpreted.

Interleukin-18 gene transfer increases antitumor effects of suicide gene therapy through efficient induction of antitumor immunity

To increase the antitumor effects of cytosine deaminase (AdCD) gene therapy and induce more potent antitumor immunity, Th1 cytokine interleukin-18 encoded adenovirus (AdIL18) was combined with adenovirus encoding CD (AdCD) for the therapy of established murine B16 melanoma. Combination therapy of the tumor-bearing mice with AdIL 18 and AdCD/5FC inhibited the growth of the subcutaneous B16 tumors more significantly, compared with AdIL 18 or AdCD/5FC alone. In vivo depletion analysis with anti-CD4, anti-CD8 or anti-NK 1.1 McAb illustrated that both CD8+ T cells and CD4+ T cells played key roles in the augmented antitumor response of the combined therapy. Peptide/MHC tetramer represents a powerful and general tool for rapid, highly sensitive, and direct analysis of antigen-specific T cells. In this study, we prepared H-2Kb/TRP-2180-188 tetramer, which was demonstrated to bind H-2Kb-restricted, B16 melanoma-specific CD8+ T cells. B16 specific H-2Kb/TRP2180-188 tetramer was used to stain the tumor-specific CD8+ T cells and the results showed that CD8+ tetramer+ T cells were about 3-5% of the splenic CD8+ T cells derived from tumor-bearing mice after combined therapy. The CTL cytotoxicity was markedly induced in mice after combined therapy, suggesting efficient induction of tumor-specific CD8+ T cells after combined gene therapy with AdCD/5FC/AdIL18. IL-18 gene transfer could significantly augment the cytotoxicity of NK cells and macrophages, and increase the production of interleukin-2 and interferon-gamma, as compared with treatments with AdCD/5FC, AdlacZ/5FC or PBS. These data suggested that in vivo IL-18 gene transfer could augment the antitumor effects of CD suicide gene therapy through efficient induction of antitumor immunity.

Exploitation of immune mechanisms in the treatment of central nervous system cancer

Malignant gliomas are among the most common intrinsic brain tumors of both children and adults, and, because of unique aspects of their biology and anatomic site, they are the most refractory to conventional therapeutic strategies involving surgery, radiotherapy, or chemotherapy. Given the failure of standard therapies to improve the outlook of affected patients, significant attention has been focused on development of alternative treatments, particularly immunotherapy. Attempts have been made to treat gliomas using a variety of immunologically based strategies, including passive immunization, adoptive cellular immunotherapy, local and systemic delivery of biological response modifiers, and vaccination with tumor cells. Although preclinical modeling of these therapies provided an impetus for translation of their results into clinical protocols, these therapies have failed to yield consistently promising results in initial trials. However, significant insights into the immunobiology of the central nervous system (CNS) and gliomas have been gained from these studies, and have established that a number of immunobiological features of the brain and of gliomas themselves may be critical determinants in regulating efficacious treatment of these tumors. These include the following: (1) the presence of a blood-brain barrier that, although partially disrupted by the tumor, functions to exclude elements of the immune system from the tumor or brain parenchyma; (2) a lack of organized secondary lymphatic tissues supporting efficient immune responses locally in the CNS; (3) low levels of expression of major histocompatibility complex proteins in the CNS; (4) an apparent paucity of the most efficient antigen-presenting cells; and (5) glioma-derived immunosuppressive factors, such as transforming growth factor-beta, that interfere with the induction of local as well as systemic immune responses to the tumor. Recognition of these factors, and an appreciation of the underlying need for and validity of developing immunologically based therapies for gliomas, supports continued development of novel immunotherapeutic approaches, particularly those attempting to enhance the immunogenicity of glioma cells. This review addresses the current state of knowledge regarding the immunobiology of gliomas, recent developments in immunotherapy of gliomas, and promising future directions for development and implementation of cellular immunotherapy of gliomas.

Engineered herpes simplex virus expressing IL-12 in the treatment of experimental murine brain tumors

Genetically engineered, neuroattenuated herpes simplex viruses (HSVs) expressing various cytokines can improve survival when used in the treatment of experimental brain tumors. These attenuated viruses have both copies of gamma(1)34.5 deleted. Recently, we demonstrated increased survival of C57BL/6 mice bearing syngeneic GL-261 gliomas when treated with an engineered HSV expressing IL-4, as compared with treatment with the parent construct (gamma(1)34. 5(-)) alone or with a virus expressing IL-10. Herein, we report construction of a conditionally replication-competent mutant expressing both subunits of mIL-12 (M002) and its evaluation in a syngeneic neuroblastoma murine model. IL-12 induces a helper T cell subset type 1 response, which may induce more durable antitumor effects. In vitro studies showed that, when infected with M002, both Vero cells and murine Neuro-2a neuroblastoma cells produced physiologically relevant levels of IL-12 heterodimers, as determined by ELISA. M002 was cytotoxic for Neuro-2a cells and human glioma cell lines U251MG and D54MG. Neurotoxicity studies, as defined by plaque-forming units/LD(50), performed in HSV-1-sensitive A/J strain mice found that M002 was not toxic even at high doses. When evaluated in an intracranial syngeneic neuroblastoma murine model, median survival of M002-treated animals was significantly longer than the median survival of animals treated with R3659, the parent gamma(1)34.5(-) mutant lacking any cytokine gene insert. Immunohistochemical analysis of M002-treated tumors identified a pronounced influx of CD4(+) T cells and macrophages as well as CD8(+) cells when compared with an analysis of R3659-treated tumors. We conclude that M002 produced a survival benefit via oncolytic effects combined with immunologic effects meditated by helper T cells of subset type 1.

Adenovirus-mediated lymphotactin gene transfer improves therapeutic efficacy of cytosine deaminase suicide gene therapy in established murine colon carcinoma

Lymphotactin (Ltn) is the sole member of C chemokines which attracts T cells and NK cells specially. Ltn gene was transferred in vivo to improve the antitumor efficacy of cytosine deaminase (CD) gene therapy. Upregulation of CD80 and CD54 on murine CT26 colon carcinoma cells was observed after combined transfection with adenovirus encoding CD (AdCD) and adenovirus encoding murine Ltn (AdLtn) followed by administration of 5-fluorocytosine (5FC) in vitro. AdCD/5FC treatment also increased the expression of CD95 and induced obvious apoptosis of CT26 cells. After combined treatment with AdLtn and AdCD/5FC, the pre-established murine model with subcutaneous CT26 colon carcinoma exhibited most significant tumor growth inhibition, and four of eight tumor-bearing mice were tumor free, while tumors in other mice grew more progressively. Examination of lymphocyte infiltration and cytokine gene expression in tumor tissue revealed that tumors from AdLtn/AdCD/5FC-or AdLtn-treated mice were heavily infiltrated with CD4+, CD8+ T cells and NK cells, and IL-2 and IFN-gamma mRNA expression were present in parallel with T cell and NK cell infiltration. Splenic NK and CTL activities increased significantly after the combination therapy. In vivo depletion analysis showed that NK cells, CD4+ T cells and CD8+T cells participated in the antitumor effect of the host with CD8+T cells being the main T cell subset responsible for the enhanced antitumor immune response. These findings suggested that increased immunogenicity and induction of apoptosis of the tumor cells, and efficient induction of local and systemic antitumor immunity of the host might contribute to the enhanced antitumor effects of the combined Ltn and CD suicide therapy. Gene Therapy (2000) 7, 329-338.

Connexin 43-enhanced suicide gene therapy using herpesviral vectors

Tumor cell transduction with the herpes simplex virus (HSV) thymidine kinase (tk) gene and treatment with ganciclovir (GCV) is a widely studied cancer gene therapy. Connexin (Cx)-dependent gap junctions between cells facilitate the intercellular spread of TK-activated GCV, thereby creating a bystander effect that improves tumor cell killing. However, tumor cells often have reduced connexin expression, thus thwarting bystander killing and the effectiveness of TK/GCV gene therapy. To improve the effectiveness of this therapy, we compared an HSV vector (TOCX) expressing Cx43 in addition to TK with an isogenic tk vector (TOZ.1) for their abilities to induce bystander killing of Cx-positive U-87 MG human glioblastoma cells and Cx-negative L929 fibrosarcoma cells in vitro and in vivo. The results showed that low-multiplicity infection of U-87 MG cells with TOCX only minimally increased GCV-mediated cell death compared with infection by TOZ.1, consistent with the endogenous level of Cx in these cells. In contrast, bystander killing of L929 cells was markedly enhanced by vector-mediated expression of Cx. In vivo experiments in which U-87 MG cells were preinfected at low multiplicity and injected into the flanks of nude mice showed complete cures of all animals in the TOCX group following GCV treatment, whereas untreated animals uniformly formed fatal tumors. TOCX injection into U-87 MG intradermal and intracranial tumors resulted in prolonged survival of the host animals in a GCV-dependent manner. Together, these results suggest that the combination of TK and Cx may be beneficial for the treatment of human glioblastoma.

Ganciclovir-mediated in vivo elimination of myeloid leukemic cells expressing the HSVtk gene induces HSVtk loss variants

The in vivo elimination of suicide gene-expressing tumor cells with prodrug treatment can induce protective immunity against wild-type tumors. In this study, we determined the efficacy and safety of the in vivo elimination of HSVtk expressing cells with ganciclovir treatment of a bystander cell killing-insensitive leukemic cell line. The retroviral construct pLTk+NeoDeltaMo, containing the HSVtk gene and the NeoR gene in a bicistronic unit, was introduced into rat leukemic LT12 cells. LT12/Tk+N cells showed a 1000- to 10 000-fold increased sensitivity to ganciclovir in vitro. In vitro mixing experiments demonstrated that LT12 cells were not susceptible to bystander cell lysis by LT12/Tk+N-2 cells exposed to ganciclovir. Rats injected s.c. with cloned LT12/Tk+N-2 cells developed tumors reaching a diameter of 3-4 cm after 40 days. Rats treated with gan- ciclovir twice daily for 5 consecutive days starting at day 7 did not develop s.c. tumors. Large established s.c. LT12/Tk+N-2 tumors completely regressed after ganciclovir treatment. However, recurrences of s.c. tumors were observed that were no longer sensitive to ganciclovir treatment. In vitro analysis of aspirates from the recurrent tumors demonstrated loss of HSVtk expression. In vitro culture of LT12/Tk+N-2 cells in soft agar in the presence of ganciclovir indicated that the frequency with which HSVtk-loss variants occurred is approximately one per 104 cells. The in vivo occurrence of HSVtk-loss variants escaping ganciclovir-induced elimination may have important implications for vaccination protocols using HSVtk gene expressing tumor cells that are not susceptible to bystander cell killing.

Therapeutic efficacy of the thymidine kinase/ganciclovir system on large experimental gliomas: a nuclear magnetic resonance imaging study

Contradictory experimental results and human trials have questioned the clinical relevance of the HSVtk/ganciclovir system. To bypass the problem of transfection efficiency, we used a glioma cell line stably expressing the HSVtk gene, which was also fully characterized from gene to protein. We also designed a more clinically relevant experimental protocol, consisting of late GCV delivery on large tumor formations. In short-term studies, histological examination revealed a significant decrease in tumor volume in GCV-treated animals from day 1 or from day 10 after cell inoculation. We observed that late GCV delivery is as efficient as early delivery, probably because GCV can reach tumor cells more easily when neoangiogenesis occurs. In long-term experiments, the survival of treated rats bearing 15-day tumors was improved by 60% compared with C6 control animals. Surprisingly, a 30% survival rate was observed in C6TK control animals. Nuclear magnetic resonance imaging demonstrated, in all surviving animals, a complete regression of tumors without mass effect. These results clearly demonstrate that the HSVtk/GCV system remains a potent therapeutic strategy, even when tested in large tumors, in contrast with the microscopic tumor formations previously reported.

Effective cytokine gene therapy against an intracranial glioma using a retrovirally transduced IL-4 plus HSVtk tumor vaccine

To explore the potential for molecular immunotherapies in the treatment of malignant gliomas, we evaluated the efficacy of subcutaneous tumor cell vaccines in the treatment of intracranial 9L tumors, using 9L gliosarcoma cell lines stably transduced with the murine interleukin-4 cDNA (9L-IL4), the herpes simplex virus-thymidine kinase cDNA (9L-Tk) or both (9L-IL4-Tk). The expression of multiple genes from a single transcript was achieved by incorporating internal ribosomal entry site (IRES) cassettes in the retroviral constructs. Subcutaneous immunization of rats with nonirradiated 9L-IL4 cells or 9L-IL4-Tk cells followed by treatment with ganciclovir (GCV) completely protected the animals from a subsequent intracranial challenge with wild-type 9L cells. In contrast, only 50% of animals immunized with 9L-Tk cells and 0% of 9L-neo immunized animals rejected the same challenge with wild-type 9L. More importantly, treatment of established (day 3) intracranial 9L tumors with genetically engineered tumor cells resulted in long-term survival (> 100 days) for 25-43% of 9L-IL4-Tk immunized animals and for 27% of nonirradiated 9L-IL4 immunized animals. In striking contrast, no 9L-Tk, 9L-neo or irradiated 9L-IL4 immunized animals survived for more than 33 days. As a marker of a cellular immune response, splenocytes from nonirradiated 9L-IL4, 9L-Tk or 9L-IL4-Tk immunized animals produced interferon-gamma (IFN-gamma) in greater amounts than those from 9L-neo immunized or Hank's balanced salts solution (HBSS) treated animals when stimulated with wild-type 9L in vitro. Our findings support the use of tumor cell vaccines expressing the IL-4 and HSVtk genes for the treatment of malignant gliomas.

Combination gene delivery of the cell cycle inhibitor p27 with thymidine kinase enhances prodrug cytotoxicity

Cytoxicity induced by the herpesvirus thymidine kinase (TK) gene in combination with prodrugs is dependent on cell growth and leads to the elimination of genetically modified cells, thus limiting the duration of expression and efficacy of this treatment in vivo. Here, an effort was made to enhance TK/prodrug efficacy by coexpression of a cyclin-dependent kinase inhibitor (CKI), p27, to render cells resistant to TK/prodrug by inhibiting DNA synthesis. Expression of p27 by transfection substantially reduced cell cycle progression, and its activity was enhanced by mutations designed to stabilize the protein. Coexpression of p27 and TK or a p27/TK fusion protein led to greater prodrug cytotoxicity than that produced by TK alone in the Renca cell line, which is sensitive to bystander killing. Combination gene transfer of this CKI with TK therefore sustained the synthesis of TK by genetically modified cells to enhance the susceptibility of bystander cells to prodrug cytotoxicity and increased the efficacy of this gene transfer approach.

Gene therapy of malignant glioma: recent advances in experimental and clinical studies

Recent advances in molecular tumor biology and gene technology have provided the possibility to treat patients with malignant brain tumors by altering gene expression in tumor cells. Tumor development and progression involves alterations in a wide spectrum of genes, therefore a variety of gene therapy approaches for malignant gliomas have been proposed. In this review article, we discuss some principles of current gene therapeutic strategies that are under investigation in laboratories and in clinics. In addition, some general issues that remain to be resolved for clinical application of gene therapy in patients with malignant gliomas will be addressed.

Malignant glioma: should chemotherapy be overthrown by experimental treatments?

Despite more than two decades of clinical research with chemotherapy, the outcome of malignant gliomas remains poor. Recent years have seen major advances in elucidation of the biology of these tumors, which in turn have led to the current development of innovative therapeutic strategies. The question confronting us at the end of the 1990s is whether we should continue to use and investigate chemotherapy or whether the time has come for experimental treatments. As a contribution to this debate, we reviewed the abundant literature on chemotherapy of malignant glioma, paying special attention to methodological features. The new treatment approaches based on current knowledge about glioma biology are then briefly summarized. Assessment of more than 20 years of chemotherapy trials is discouraging despite a few areas of modest success. Only patients with specific histology (oligodendroglioma, anaplastic astrocytoma) and good prognostic factors (young age, good performance status) may benefit from chemotherapy, with a possible reversal of neurological dysfunction. However, the real impact on survival is small (anaplastic astrocytoma) or undefined (oligodendroglioma). Furthermore, it is unfortunately obvious that the outcome of glioblastoma patients is not significantly modified by chemotherapy. We believe the time has come to explore the potential of novel biological therapies in glioblastoma patients. This could also be proposed for anaplastic astrocytoma and oligodendroglioma patients after failure of chemotherapy.