Normal brain cells contribute to the bystander effect in suicide gene therapy of malignant glioma.

PURPOSE: Lentiviral vectors pseudotyped with glycoproteins of the lymphocytic choriomeningitis virus (LCMV-GP) are promising candidates for gene therapy of malignant glioma, as they specifically and efficiently transduce glioma cells in vitro and in vivo. Here, we evaluated the therapeutic efficacy of LCMV-GP and vesicular stomatitis virus glycoprotein (VSV-G) pseudotyped vectors. EXPERIMENTAL DESIGN: Therapeutic efficacy was tested for unmodified (9L) and DsRed-modified (9LDsRed) gliomas using the suicide gene thymidine kinase of the herpes simplex virus type 1 (HSV-1-tk). Positron emission tomography (PET) and magnetic resonance imaging were done to analyze transduction of tumors and monitor therapeutic outcome. RESULTS: LCMV-GP pseudotypes mediated a successful eradication of 9LDsRed tumors with 100% of long-term survivors. Before initiation of ganciclovir treatment, a strong HSV-1-tk expression within the tumor was detected by noninvasive PET using the tracer 9-[4-[(18)F]fluoro-3-(hydroxymethyl)butyl]guanine. Therapeutic outcome was successfully monitored by magnetic resonance imaging and PET imaging and correlated with the histopathologic data. In the 9L model, LCMV-GP and VSV-G pseudotyped lentiviral vectors displayed similar therapeutic efficacy. Further studies revealed that normal brain cells transduced with VSV-G pseudotypes were not eliminated by ganciclovir treatment and contributed significantly to the bystander killing of tumor cells. CONCLUSIONS: Suicide gene transfer using pseudotyped lentiviral vectors was very effective in the treatment of rat glioma and therefore is an attractive therapeutic strategy also in human glioblastoma especially in conjunction with an imaging-guided approach. In addition, high selectivity of gene transfer to tumor cells may not always be desirable for therapeutic genes that exert a clear bystander effect.

Bystander killing of malignant glioma by bone marrow-derived tumor-infiltrating progenitor cells expressing a suicide gene.

Adult stem cells are promising cellular vehicles for therapy of malignant gliomas as they have the ability to migrate into these tumors and even track infiltrating tumor cells. However, their clinical use is limited by a low passaging capacity that impedes large-scale production. In the present study, a bone marrow-derived, highly proliferative subpopulation of mesenchymal stem cells (MSCs)-here termed bone marrow-derived tumor-infiltrating cells (BM-TICs)-was genetically modified for the treatment of malignant glioma. Upon injection into the tumor or the vicinity of the tumor, BM-TICs infiltrated solid parts as well as the border of rat 9L glioma. After intra-tumoral injection, BM-TICs expressing the thymidine kinase of herpes simplex virus (HSV-tk) and enhanced green fluorescent protein (BM-TIC-tk-GFP) were detected by non-invasive positron emission tomography (PET) using the tracer 9-[4-[(18)F]fluoro-3-hydroxymethyl)butyl]guanine ([(18)F]FHBG). A therapeutic effect was demonstrated in vitro and in vivo by BM-TICs expressing HSV-tk through bystander-mediated glioma cell killing. Therapeutic efficacy was monitored by PET as well as by magnetic resonance imaging (MRI) and strongly correlated with histological analysis. In conclusion, BM-TICs expressing a suicide gene were highly effective in the treatment of malignant glioma in a rat model and therefore hold great potential for the therapy of malignant brain tumors in humans.

18F-fluoro-L-thymidine and 11C-methylmethionine as markers of increased transport and proliferation in brain tumors.

UNLABELLED: Because of the high glucose metabolism in normal brain tissue 18F-FDG is not the ideal tracer for the detection of gliomas. Methyl-11C-l-methionine (11C-MET) is better suited for imaging the extent of gliomas, because it is transported specifically into tumors but only insignificantly into normal brain. 3'-Deoxy-3'-18F-fluorothymidine (18F-FLT) has been introduced as a proliferation marker in a variety of neoplasias and has promising potential for the detection of brain tumors, because its uptake in normal brain is low. Additionally, the longer half-life might permit differentiation between transport and intracellular phosphorylation. METHODS: PET of 18F-FLT and 11C-MET was performed on 23 patients (age range, 20-70 y) with histologically verified gliomas of different grades. On all patients, conventional MRI was performed, and 16 patients additionally underwent contrast-enhanced imaging. Images were coregistered, and the volumes of abnormality were defined for PET and MRI. Uptake ratios and standardized uptake values (SUVs) of various tumors and regions were assessed by region-of-interest analysis. Kinetic modeling was performed on 14 patients for regional time-activity curves of 18F-FLT from tumorous and normal brain tissue. RESULTS: Sensitivity for the detection of tumors was lower for 18F-FLT than for 11C-MET (78.3% vs. 91.3%), especially for low-grade astrocytomas. Tumor volumes detected by 18F-FLT and 11C-MET were larger than tumor regions displaying gadolinium enhancement (P<0.01). Uptake ratios of 18F-FLT were higher than uptake ratios of 11C-MET (P<0.01). Uptake ratios of 18F-FLT were higher in glioblastomas than in astrocytomas (P<0.01). Absolute radiotracer uptake of 18F-FLT was low and significantly lower than that of 11C-MET (SUV, 1.3+/-0.7 vs. 3.1+/-1.0; P<0.01). Some tumor regions were detected only by either 18F-FLT (7 patients) or 11C-MET (13 patients). Kinetic modeling revealed that 18F-FLT uptake in tumor tissue seems to be predominantly due to elevated transport and net influx. However, a moderate correlation was found between uptake ratio and phosphorylation rate k3 (r=0.65 and P=0.01 for grade II-IV gliomas; r=0.76 and P<0.01 for grade III-IV tumors). CONCLUSION: 18F-FLT is a promising tracer for the detection and characterization of primary central nervous system tumors and might help to differentiate between low- and high-grade gliomas. 18F-FLT uptake is mainly due to increased transport, but irreversible incorporation by phosphorylation might also contribute. In some tumors and tumor areas, 18F-FLT uptake is not related to 11C-MET uptake. In view of the high sensitivity and specificity of 11C-MET PET for imaging of gliomas, it cannot be excluded that 18F-FLT PET was false positive in these areas. However, the discrepancies observed for the various imaging modalities (18F-FLT and 11C-MET PET as well as gadolinium-enhanced MRI) yield complementary information on the activity and the extent of gliomas and might improve early evaluation of treatment effects, especially in patients with high-grade gliomas. Further studies are needed, including coregistered histology and kinetic analysis in patients undergoing chemotherapy.