Optimized human CYP4B1 in combination with the alkylator prodrug 4-ipomeanol serves as a novel suicide gene system for adoptive T-cell therapies.

Engineering autologous or allogeneic T cells to express a suicide gene can control potential toxicity in adoptive T-cell therapies. We recently reported the development of a novel human suicide gene system that is based on an orphan human cytochrome P450 enzyme, CYP4B1, and the naturally occurring alkylator prodrug 4-ipomeanol. The goal of this study was to systematically develop a clinically applicable self-inactivating lentiviral vector for efficient co-expression of CYP4B1 as an ER-located protein with two distinct types of cell surface proteins, either MACS selection genes for donor lymphocyte infusions after allogeneic stem cell transplantation or chimeric antigen receptors for retargeting primary T cells. The U3 region of the myeloproliferative sarcoma virus in combination with the T2A site was found to drive high-level expression of our CYP4B1 mutant with truncated CD34 or CD271 as MACS suitable selection markers. This lentiviral vector backbone was also well suited for co-expression of CYP4B1 with a codon-optimized CD19 chimeric antigen receptor (CAR) construct. Finally, 4-ipomeanol efficiently induced apoptosis in primary T cells that co-express mutant CYP4B1 and the divergently located MACS selection and CAR genes. In conclusion, we here developed a clinically suited lentiviral vector that supports high-level co-expression of cell surface proteins with a potent novel human suicide gene.

No Significant Cytotoxic Effect of the EZH2 Inhibitor Tazemetostat (EPZ-6438) on Pediatric Glioma Cells with Wildtype Histone 3 or Mutated Histone 3.3.

BACKGROUND: Glioblastoma multiforme (GBM) and diffuse intrinsic pontine glioma (DIPG) belong to the most aggressive cancers in children with poor prognosis and limited therapeutic options. Therapeutic targeting of epigenetic proteins may offer new treatment options. Preclinical studies identified Enhancer of Zeste Homolog 2 (EZH2) within polycomb repressor complex 2 (PRC2) as a potential epigenetic anti-tumor target in adult GBM cells but similar inhibition studies in pediatric GBM/DIPG were still missing. Moreover, approximately 30% of pediatric high grade gliomas (pedHGG) including GBM and DIPG harbor a lysine 27 mutation (K27M) in histone 3.3 (H3.3) which is correlated with poor outcome and was shown to influence EZH2 function. PATIENTS, MATERIALS AND METHODS: The present study investigated the correlation of expression of EZH2 and other PRC2 genes (EZH1, SUZ12, EED) with overall survival of pediatric GBM patients and the cytotoxic impact of EZH2 inhibition by the novel agent Tazemetostat in pediatric GBM/DIPG cells harboring either a H3.3 mutation or a H3 wildtype. RESULTS: EZH2 gene expression does not correlate with survival of pedHGG patients, and EZH2 inhibition does not induce significant cytotoxicity in pedHGG cells independently of H3.3 mutations. DISCUSSION AND CONCLUSION: We suggest that EZH2 inhibition might not offer an effective single agent treatment option for paedHGG patients. However, the therapeutic efficacy in combination with cytotoxic and/or other epigenetically active agents still has to be elucidated.

Cerebellar location may predict an unfavourable prognosis in paediatric high-grade glioma.

BACKGROUND: High-grade glioma (HGG) of the cerebellum accounts for only 5% of paediatric HGG. Since little is known about these tumours, the present study aimed at their further characterisation. METHODS: Twenty-nine paediatric patients with centrally reviewed cerebellar HGG were identified from the HIT-GBM/HIT-HGG database. Clinical and epidemiological data were compared with those of 180 paediatric patients with cortical HGG. RESULTS: Patients with cerebellar tumours were younger (median age of 7.6 vs 11.7 years, P=0.028), but both groups did not differ significantly with regard to gender, tumour predisposing syndromes, secondary HGG, primary metastasis, tumour grading, extent of tumour resection, chemotherapy regimen, or radiotherapy. Except for an increased incidence of anaplastic pilocytic astrocytoma (APA) in the cerebellar subset (20.7% vs 3.3%; P<0.001), histological entities were similarly distributed in both groups. As expected, tumour grading had a prognostic relevance on survival. Compared with cortical HGG, overall survival in the cerebellar location was significantly worse (median overall survival: 0.92 ± 0.02 vs 2.03 ± 0.32 years; P=0.0064), and tumour location in the cerebellum had an independent poor prognostic significance as shown by Cox-regression analysis (P=0.019). CONCLUSION: High-grade glioma represents a group of tumours with an obviously site-specific heterogeneity associated with a worse survival in cerebellar location.

Craniospinal irradiation with concurrent temozolomide and nimotuzumab in a child with primary metastatic diffuse intrinsic pontine glioma. A compassionate use treatment.

Primary metastatic diffuse intrinsic pontine glioma (DIPG) is relatively rare and associated with a dismal prognosis. Combining craniospinal irradiation (CSI) with concurrent temozolomide and nimotuzumab therapy may slightly improve tumor control and overall survival. However, little is known about the feasibility and toxicity of this treatment approach. Here, we describe the case of an 8-year-old girl with primary metastatic DIPG who received craniospinal radiotherapy, a local boost, and concurrent temozolomide and nimotuzumab treatment based on an individual therapy recommendation. Radiotherapy could be completed without any interruption. However, concurrent temozolomide had to be disrupted several times due to considerable acute myelotoxicity (grade III-IV).Maintenance immunochemotherapy could be started with a delay of 5 days and was performed according to treatment schedule. The disease could be stabilized for a few months. A routine MRI scan finally depicted disease progression 5.7 months after the start of irradiation. The patient died 1.9 months later.

Always expect the unexpected: lung abscess due to pseudomonas aeruginosa mimicking pulmonary aspergilloma in acute B-cell leukemia.

We report on a case of Pseudomonas aeruginosa sepsis and consecutive lung abscess in a 13-year-old patient with acute B-cell leukemia. At first, radiographic findings strongly suggested presence of pulmonary aspergilloma and only microbiological testing of the surgically enucleated mass revealed the correct underlying pathogen and confirmed final diagnosis.

High dose methotrexate treatment in childhood ALL: pilot study on the impact of the MTHFR 677C>T and 1298A>C polymorphisms on MTX-related toxicity.

Methotrexate (MTX) is commonly administered in high doses for treatment of childhood acute lymphoblastic leukemia (ALL). The aim of this analysis was to study the influence of 2 common MTHFR polymorphisms (MTHFR 677C>T and 1298 A>C) on MTX toxicity in children with ALL.Retrospective analysis of 129 MTX courses in 34 pediatric patients with ALL.677C>T variants (CT or TT) were found in 19 (14 heterozygous, 5 homozygous) and 1298A>C variants (AC or CC) in 20 (16 heterozygous, 4 homozygous) patients. The MTHFR 677C>T wild type was associated with an increased frequency of grade III and IV leukopenia (60% vs. 31%, p<0.05) compared to the variants. The rate of severe infections (21% vs. 0%, p<0.05) and grade III-IV anemia (26% vs. 5%, p<0.05) was increased in carriers of the MTHFR 677C>T wild type compared to patients with the TT variant. Grade III-IV anemia was more frequent in patients with the MTHFR 1298A>C CC variant compared to the wild type (56% vs. 21%, p<0.05). The differences were not significant in a patient-based analysis.MTX related toxicity might be influenced by the MTHFR 677C>T or the MTHFR 1298A>C polymorphisms. Differences in MTX toxicity are only partially explainable by these 2 polymorphisms.

Leukapheresis and exchange transfusion in children with acute leukemia and hyperleukocytosis. A single center experience.

BACKGROUND: The risk of severe complications or death during the initial period of acute leukemia was markedly decreased due to the progress in pediatric oncology and use of simple measures like hyperhydration, forced diuresis, treatment of hyperuricemia, correction of electrolyte and coagulation disturbances and the careful use of antileukemic drugs. The incidence of leukostasis and tumor lysis syndrome depends on absolute initial white blood cell counts and the underlying type of leukemia. Leukapheresis or exchange transfusion may improve the prognosis of high risk patients. METHODS: Records of all pediatric patients who were newly diagnosed with acute leukemia between 1 / 1998 und 12 / 2008 were retrospectively reviewed for presence of hyperleukocytosis(white blood cell count > 100 GPT / l) at diagnosis and subsequent leukapheresis or exchange transfusion in regards to the clinical outcome. RESULTS: At diagnosis 11 (14 % ) of 77 children with acute leukemia (7 acute lymphoblastic leukemia / ALL; 4 acute myeloblastic leukemia /AML) had hyperleukocytosis. 4 patients (2 ALL, 2 AML) received exchange transfusion and 2 others (1 ALL, 1 AML) underwent leukapheresis. Marked cytoreduction was achieved in all patients within 24 h after therapy initiation. There were no procedure-related adverse events. Symptoms due to hyperleukocytosis markedly improved after cytoreduction. CONCLUSION: Leukapheresis or exchange transfusion together with conservative management and specific oncological therapy may contribute to rapid leukocyte reduction with acceptable risk. The exact impact of leukapheresis or exchange transfusion on short and long term outcome in pediatric patients with acute leukemia and initial hyperleukocytosis has to be evaluated in future multicentre studies or by the formation of clinical registries.

Folylpolyglutamyl synthetase gene transfer and glioma antifolate sensitivity in culture and in vivo.

BACKGROUND: Although antifolates are popular agents for use in chemotherapy, they display minimal toxicity against slow-growing tumors and are toxic to actively replicating cells in normal tissues. These drugs are converted intracellularly into polyglutamate derivatives by the enzyme folylpolyglutamyl synthetase (FPGS). Because tumors with high expression of FPGS often respond to nontoxic antifolate doses, we investigated whether augmenting tumoral FPGS activity by gene delivery would enhance tumoral antifolate sensitivity. METHODS: 9L rat gliosarcoma cells were stably transfected with a human FPGS complementary DNA (cDNA), producing 9L/FPGS cells. The sensitivity of these cells to the antifolates methotrexate and edatrexate was measured in culture and in subcutaneous tumors, as was their ability to increase the chemosensitivity of nearby nontransfected cells, i.e., a bystander effect. The antifolate sensitivity of nonselected cells transduced with a hybrid amplicon vector that expressed FPGS was also ascertained. RESULTS: In comparison with 9L cells, 9L/FPGS cells displayed enhanced sensitivity to 4-hour pulses of antifolate. Subcutaneous 9L/FPGS tumors responded as well to methotrexate given every third day as 9L tumors did to daily treatment. A modest bystander effect was observed with edatrexate treatment in culture and in vivo. The observed bystander effect appeared to result from the release of antifolates by transfected cells after the removal of extracellular drug. In culture, enhanced antifolate sensitivity was also seen in other stably transfected rodent and human glioma cell lines, including one with high pre-existing FPGS activity, and in canine and human glioblastoma cell lines transduced with a vector bearing FPGS cDNA. CONCLUSIONS: FPGS gene delivery enhances the antifolate sensitivity of several glioma cell lines and merits further evaluation as a therapeutic strategy.

Synergistic anticancer effects of ganciclovir/thymidine kinase and 5-fluorocytosine/cytosine deaminase gene therapies.

BACKGROUND: A bacterial enzyme, Escherichia coli cytosine deaminase, which converts the prodrug 5-fluorocytosine into the toxic drug 5-fluorouracil, and a viral enzyme, herpes simplex virus thymidine kinase, which converts ganciclovir from an inactive prodrug to a cytotoxic agent by phosphorylation, are being actively investigated for use in gene therapy for cancer. The purpose of this study was to determine whether combining these prodrug-activating gene therapies might result in enhanced anticancer effects. METHODS: Rat 9L gliosarcoma cells were transfected with plasmids containing the E. coli cytosine deaminase gene (9L/CD cells), with plasmids containing the herpes simplex virus thymidine kinase gene (9L/TK cells), or with both expression plasmids (9L/CD-TK cells). The drug sensitivities of the cell lines were evaluated; in addition, the sensitivities of 9L and 9L/CD-TK cells mixed in varied proportions were measured. The effects of prodrug treatment on 9L/CD-TK tumor growth (i.e., size and volume) in nude mice were monitored. The isobologram method of Loewe and the multiple drug-effect analysis method of Chou-Talalay were used to measure the interaction between the two prodrug-activating gene therapies. To elucidate the mechanism of interaction, the phosphorylation of ganciclovir in 9L/CD-TK cells after varying prodrug treatments was studied. RESULTS AND CONCLUSIONS: The presence of transfected cytosine deaminase and thymidine kinase genes in 9L gliosarcoma cells reduced cell survival, both in vitro and in vivo, following treatment with the relevant prodrugs; the effects of the two components appeared to be synergistic and related mechanistically to the enhancement of ganciclovir phosphorylation by thymidine kinase following 5-fluorouracil treatment.

Long-term survival of rats harboring brain neoplasms treated with ganciclovir and a herpes simplex virus vector that retains an intact thymidine kinase gene.

Survival of rats harboring cerebral 9L gliosarcomas can be significantly extended by an intratumoral inoculation with a herpes simplex virus vector, designated as hrR3. This vector, which bears the lacZ reporter gene, is defective in the gene encoding ribonucleotide reductase, allowing for replication in dividing tumor cells but not in postmitotic neural cells. It also possesses an intact viral thymidine kinase (TK) gene, which confers chemosensitivity to ganciclovir. In this study, the ability of ganciclovir to potentiate the antitumor effect of hrR3 was evaluated. In culture, there was a 23% decrease in the growth of 9L cells treated with hrR3 plus ganciclovir compared to hrR3 alone (P < 0.01). The combination of hrR3 plus ganciclovir led to the long-term survival of 48% of rats harboring intracerebral 9L gliosarcomas compared to 20% survival in the hrR3 group (P < 0.05). Ganciclovir treatment had no effect on the growth of tumor cells in vitro or in vivo when a herpes simplex virus vector with a defective TK gene was used. Immunocytochemistry confirmed selective expression of the TK gene in cells within the tumor. These findings indicate that the TK gene can potentiate the antitumor effect of the hrR3 herpes simplex virus vector and provide the basis for placing additional therapeutic genes in the genome of hrR3.

Selective uptake of viral and monocrystalline particles delivered intra-arterially to experimental brain neoplasms.

In this study we investigated the intra-arterial delivery of viral and nonviral particles to experimental brain tumors. A herpes simplex virus (HSV) vector and monocrystalline iron oxide nanoparticles (MION) were injected into the internal carotid artery of Fisher 344 rats harboring intracerebral 9L gliosarcomas, using bradykinin to disrupt the blood-tumor barrier. Brain and internal organs were stained both for virus-mediated gene expression and for iron. Quantitative comparisons of gene expression and MION uptake with and without blood-tumor barrier disruption were performed in the center and at the periphery of the tumor mass, as well as in normal brain. In addition, MION distribution was traced in vivo by MR imaging. Delivery of HSV into 9L gliosarcoma cells was greatly enhanced by intra-carotid bradykinin infusion. Virus-mediated expression of the HSV-thymidine kinase (TK) and beta-galactosidase gene products was highest at the tumor periphery as compared to the tumor center. Selective HSV infection of multiple tumor foci was achieved in both hemispheres without affecting normal brain. MION uptake was high at the tumor periphery even without blood-tumor barrier disruption. Bradykinin increased MION uptake predominantly in the center of the tumor with virtually no effect at the periphery. These findings show that selective blood-tumor barrier disruption by bradykinin can be used to enhance HSV-mediated gene delivery to tumor cells in the periphery of brain tumors. A crucial aspect in the treatment of malignant brain tumors is the eradication of tumor cells infiltrating the brain; bradykinin may facilitate access of vectors to these areas by selective disruption of their neovasculature.

Herpes vector-mediated delivery of marker genes to disseminated central nervous system tumors.

The present study investigated the ability of a recombinant herpes simplex virus type 1 (HSV) vector to deliver genes into disseminated brain tumor foci through intrathecal injection of the vector. The animal model was designed to simulate brain tumors with cerebrospinal fluid (CSF) metastases, which are found especially in the pediatric population. 9L gliosarcoma cells were injected both into the right frontal lobe and in through the cisterna magna of adult rats. The HSV vector, hrR3, was inoculated intrathecally 5 days later. This vector is defective in the gene for ribonucleotide reductase, and, therefore, replicates preferentially in dividing cells; it retains an intact HSV-thymidine kinase gene (HSV-tk). Two days after injection of the vector, immunohistochemical staining for HSV thymidine kinase (HSV-TK) revealed expression in frontal tumors, as well as in leptomeningeal tumor foci along the entire neuroaxis. HSV-TK-immunopositive cells were most frequent in small tumors contacting the CSF pathways. Frontal lobe tumors showed the highest density of HSV-TK-immunopositive cells around their periphery with little expression in central parts. Some paraventricular neurons temporarily showed HSV-TK-immunolabeling at this early time point. The number of HSV-TK-immunopositive tumor cells markedly decreased 5 days after injection of the HSV vector. In all animals, some toxicity was observed in the first 2-4 days after virus injection with extensive leptomeningeal inflammation. In conclusion, intrathecal application of HSV vectors can mediate widespread transfer of the therapeutic HSV-tk gene into disseminated tumors throughout the brain and CSF pathways. Although there was marked toxicity associated with intrathecal injection of this vector, this mode of gene delivery offers a promising approach for treatment of CSF-metastases in conjunction with development of less toxic vectors.

Long-term survival in a rodent model of disseminated brain tumors by combined intrathecal delivery of herpes vectors and ganciclovir treatment.

Brain tumors that have disseminated into cerebrospinal fluid (CSF) pathways are an unresolved therapeutic problem, especially in pediatric neurooncology. Here a gene therapy approach using the herpes simplex virus type 1 thymidine kinase (HSV-TK)/ganciclovir (GCV) paradigm was tested using an HSV vector in a rodent model of disseminated central nervous system tumors. 9L-gliosarcoma cells were implanted simultaneously into the brain and the CSF of syngeneic rats. Five days later, resulting intracerebral and leptomeningeal tumors were treated by intrathecal injection of a replication-conditional HSV vector. This vector was defective for the ribonucleotide reductase gene, but contained an intact HSV-tk gene. Systemic GCV treatment was started 2 days after vector application and continued for 14 days. Tumor-free, long-term survival (LTS) was achieved in 90% of the animals treated with this combined therapeutic approach, whereas only 30% LTS was found in animals that had received the vector alone and 10% LTS in untreated animals. This therapeutic response probably involves oncolytic, on-site replication of the vector, activation of GCV by a HSV-TK, and a strong immune response both to the vector and to 9L cells. Apparent vector-related mortality was observed in 20% of animals without subsequent GCV therapy, but no vector-related mortality was found when the animals were treated with GCV after vector application. Given the successful outcome of this experimental treatment and the apparent potential of GCV to control HSV-related toxicity, intrathecal application of HSV vectors combined with GCV treatment may be a promising approach for treatment of disseminated brain tumors.

New prodrug activation gene therapy for cancer using cytochrome P450 4B1 and 2-aminoanthracene/4-ipomeanol.

Vector-mediated transfer of prodrug-activating genes provides a promising means of cancer gene therapy. In a search for more selective and more potent bioactivating enzymes for gene therapy of malignant brain tumors, the toxicity-generating capacity of the rabbit cytochrome P450 isozyme CYP4B1 was investigated. Rabbit CYP4B1, but not rat or human isozymes, efficiently converts the inert prodrugs, 2-aminoanthracene (2-AA) and 4-ipomeanol (4-IM), into highly toxic alkylating metabolites. Toxicity of these two prodrugs was evaluated in culture in parental and genetically modified rodent (9L) and human (U87) glioma cell lines stably expressing CYP4B1, and in vivo in a subcutaneous 9L tumor model in nude mice. The most sensitive CYP4B1-expressing glioma clone, 9L4B1-60, displayed an LD50 of 2.5 microM for 2-AA and 4-IM after 48 h of prodrug incubation, whereas 20 times higher prodrug concentrations did not cause any significant toxicity to control cells. Substantial killing of control tumor cells by 2-AA was achieved by co-culturing these cells with CYP4B1-expressing cells at a ratio of 100:1, and toxic metabolites could be transferred through medium. In both CYP4B1-expressing cells and co-cultured control cells, prodrug bioactivation was associated with DNA fragmentation, as assayed by fluorescent TUNEL assays and by annexin V staining. Alkaline elution of cellular DNA after exposure to 4-IM revealed extensive protein-DNA crosslinking with single-strand breakage. Growth of 9L-4B1 tumors in nude mice was inhibited by intraperitoneal injection of 4-IM with minimal side effects. Potential advantages of the CYP4B1 gene therapy paradigm include: the low concentrations of prodrug needed to kill sensitized tumor cells; low prodrug conversion by human isozymes, thus reducing toxicity to normal cells; a tumor-killing bystander effect that can occur even without cell-to-cell contact; and the utilization of lipophilic prodrugs that can penetrate the blood-brain barrier.

Therapeutic efficiency and safety of a second-generation replication-conditional HSV1 vector for brain tumor gene therapy.

A second-generation replication-conditional herpes simplex virus type 1 (HSV) vector defective for both ribonucleotide reductase (RR) and the neurovirulence factor gamma34.5 was generated and tested for therapeutic safety and efficiency in two different experimental brain tumor models. In culture, cytotoxic activity of this double mutant HSV vector, MGH-1, for 9L gliosarcoma cells was similar to that of the HSV mutant, R3616, which is defective only for gamma34.5, but was significantly weaker than that of the HSV mutant hrR3, which is defective only for RR. The diminished tumoricidal effect of the gamma34.5 mutants could be accounted for by their reduced ability to replicate in 9L cells. The MGH-1 vector did not achieve significant prolongation of survival in vivo in the syngeneic 9L rat gliosarcoma model for either single brain tumor focus or multiple intracerebral and leptomeningeal tumors, when the vector was applied intratumorally or intrathecally, respectively, and with or without subsequent ganciclovir (GCV) treatment. In identical 9L brain tumor models with single and multiple foci, application of hrR3 with or without GCV was previously shown to result in marked long-term survival. Contrary to the findings with intrathecal injection of hrR3, no vector-related mortality was observed in any animals treated with MGH-1. Thus, in these rat brain tumor models, the double mutant, replication-conditional HSV vector MGH-1 showed a higher therapeutic safety than the RR-minus vector, hrR3, but had clearly decreased therapeutic efficiency compared to hrR3. The development of new HSV vectors for brain tumor gene therapy will require a balance between maximizing therapeutic efficacy and minimizing toxicity to the brain. Standardized application in brain tumor models as presented here will help to screen new HSV vectors for these requirements.

Gene therapy for brain tumors.

Gene therapy has opened new doors for treatment of neoplastic diseases. This new approach seems very attractive, especially for glioblastomas, since treatment of these brain tumors has failed using conventional therapy regimens. Many different modes of gene therapy for brain tumors have been tested in culture and in vivo. Many of these approaches are based on previously established anti-neoplastic principles, like prodrug activating enzymes, inhibition of tumor neovascularization, and enhancement of the normally weak anti-tumor immune response. Delivery of genes to tumor cells has been mediated by a number of viral and synthetic vectors. The most widely used paradigm is based on the activation of ganciclovir to a cytotoxic compound by a viral enzyme, thymidine kinase, which is expressed by tumor cells, after the gene has been introduced by a retroviral vector. This paradigm has proven to be a potent therapy with minimal side effects in several rodent brain tumor models, and has proceeded to phase 1 clinical trials. In this review, current gene therapy strategies and vector systems for treatment of brain tumors will be described and discussed in light of further developments needed to make this new treatment modality clinically efficacious.

Temozolomide enhances herpes simplex virus thymidine kinase/ganciclovir therapy of malignant glioma.

Gene therapy for malignant glioma with the herpes simplex virus thymidine kinase/ganciclovir (HSV-tk/GCV) system is already in the stage of clinical trials, but still needs major improvement to achieve greater clinical efficacy. The aim of this study was to determine whether combining HSV-tk/GCV gene therapy with temozolomide (TMZ), an alkylating drug clinically proven to be efficient in recurrent high-grade gliomas, would result in enhanced antitumor effect in malignant glioma in culture and in vivo. Human U87MG glioblastoma (GBM) cells with or without expression of HSV-tk were treated with different concentrations of GCV, TMZ, or both drugs. Cell viability was accessed by an automated microplate assay (MTT). The isobologram method and the combination index (CI) method of Chou-Talalay were used to measure the interactions between the two drugs when applied simultaneously. U87-tk and control U87 cells (5x10(6) each) were implanted in the flanks of nude mice, and animals were treated with GCV or TMZ or with both drugs. All tumors were measured and weighed at specified time points. IC(50) for GCV was 511 microM in control U87 cells and 14.3 microM in U87-tk cells, resulting in 35.7-fold increase of toxicity in the HSV-tk-expressing cells. TMZ had an IC(50) of 20.2 mM in control cells and 2.35 mM in U87-tk cells, resulting in 8.6-fold increase in sensitivity of the HSV-tk-expressing cells. TMZ and HSV-tk/GCV actions were synergistic (CI<1) in both control and U87-tk cells with higher synergism in U87-tk cells at high effect levels. Tumors expressing HSV-tk and treated with TMZ and GCV were significantly smaller than those treated by TMZ, but not by GCV. There was also a significant difference between the weight of HSV-tk expressing versus control tumors treated with TMZ, with GCV, or with both drugs. These data demonstrate synergism between HSV-tk/GCV and TMZ and higher sensitivity against TMZ in HSV-tk-expressing GBM cells. The potential importance for clinical studies combining both local tumor gene therapy and systemic chemotherapy should be explored further.

Retrovirus-mediated gene therapy of experimental brain neoplasms using the herpes simplex virus-thymidine kinase/ganciclovir paradigm.

Recent results in experimental brain tumors indicate that transfer of sensitizing genes to tumor cells in vivo with subsequent drug treatment can reduce tumor masses and prolong the survival of rodents. In the present study, the 9L rat gliosarcoma model was used to evaluate the therapeutic effectiveness of the herpes simplex virus-thymidine kinase (HSV-tk) gene, delivered by a retrovirus vector, against tumor cells in the rat brain after systemic application of the nucleoside analogue ganciclovir (GCV). The HSV-tk gene was inserted into a retroviral vector (pMFG), which was produced using the amphotropic packaging cell line CRIP-MFG-S-HSV-TK. Packaging cells were implanted into established 9L tumors in the brains of syngeneic rats to effect gene delivery to tumor cells, followed by intraperitoneal GCV injections. Treated animals survived significantly longer (more than twice as long) than did the control groups. Brains from GCV-treated and nontreated animals were examined immunohistochemically at different time intervals after grafting of CRIP-MFG-S-HSV-TK cells and GCV treatment. Tumors in GCV-treated animals were significantly smaller as compared with nontreated animals at all time points. Sections stained immunohistochemically for HSV-TK confirmed gene transfer to tumor cells, which could be distinguished from packaging cells by different morphology and immunohistochemical staining for the retroviral envelope protein gp70. Approximately 45% of the cells in tumors implanted with CRIP-MFG-S-HSV-TK cells, but not treated with GCV, showed immunocytochemical staining for HSV-TK, demonstrating a high-efficiency of retrovirus-mediated gene transfer. Tumors in rats treated with packaging cells and GCV showed only 9% HSV-TK-positive cells after treatment, indicating that most cells expressing the HSV-tk gene were killed. The success of this therapeutic modality in experimental animals depends in large parts on the high efficiency of gene delivery and on the immune response against tumor cells.

Vaccination for experimental gliomas using GM-CSF-transduced glioma cells.

Brain tumors have an immunoprivileged status which contributes to their refractoriness to treatment. In this study, immune rejection of GL261 glioma tumors in the mouse brain was achieved by subcutaneous vaccination with GM-CSF-transduced glioma cells. Cultured GL261 cells were transduced to secrete murine GM-CSF using a retrovirus vector, then irradiated, and injected subcutaneously into H-2 matched C57BL/6 mice. In prevaccination studies, the median survival time (MST) of animals vaccinated with 5 x 10(4) or 5 x 10(5) GM-CSF-transduced cells 7 days prior to intracranial injection of 10(6) nontransduced, nonirradiated GL261 cells was significantly prolonged by 45-50% compared with animals vaccinated in parallel with nontransduced, irradiated glioma cells. In treatment of established gliomas, the MST of animals, which were treated subcutaneously with 5 X 10(6) irradiated GM-CSF-transduced cells 3 days after intracranial injection of 2 x 10(4) nontransduced cells, was prolonged significantly by 36% compared with animals treated with the same number of nontransduced, irradiated cells or to sham-treated animals. In prevaccination studies, histology of brain tumors 4 days after intracranial tumor cell injection revealed infiltrates of CD8+ lymphocytes and eosinophils, the latter exclusively in animals vaccinated with GM-CSF-transduced cells, Thus, subcutaneous injection of irradiated GM-CSF-transduced glioma cells can induce a potent immune response to intracranial gliomas both as a vaccination against subsequent intracranial glioma cell implantation and for treatment of established intracranial glioma.