Yung‐Nien Chang

Glial cell line-derived neurotrophic factor (GDNF) supports growth and survival of dopaminergic (DA) neurons. A replication-defective adenoviral (Ad) vector encoding human GDNF injected near the rat substantia nigra was found to protect DA neurons from the progressive degeneration induced by the neurotoxin 6-hydroxydopamine (6-OHDA) injected into the striatum. Ad GDNF gene therapy reduced loss of DA neurons approximately threefold 6 weeks after 6-OHDA lesion, as compared with no treatment or injection of Ad lacZ or Ad mGDNF (encoding a biologically inactive deletion mutant GDNF). These results suggest that Ad vector-mediated GDNF gene therapy may slow the DA neuronal cell loss in humans with Parkinson's disease.

Previously, we observed that an adenoviral (Ad) vector encoding human glial cell line-derived neurotrophic factor (GDNF), injected near the rat substantia nigra (SN), protects SN dopaminergic (DA) neuronal soma from 6-hydroxydopamine (6-OHDA)-induced degeneration. In the present study, the effects of Ad GDNF injected into the striatum, the site of DA nerve terminals, were assessed in the same lesion model. So that effects on cell survival could be assessed without relying on DA phenotypic markers, fluorogold (FG) was infused bilaterally into striatae to retrogradely label DA neurons. Ad GDNF or control treatment (Ad mGDNF, encoding a deletion mutant GDNF, Ad lacZ, vehicle, or no injection) was injected unilaterally into the striatum near one FG site. Progressive degeneration of DA neurons was initiated 7 days later by unilateral injection of 6-OHDA at this FG site. At 42 days after 6-OHDA, Ad GDNF prevented the death of 40% of susceptible DA neurons that projected to the lesion site. Ad GDNF prevented the development of behavioral asymmetries which depend on striatal dopamine, including limb use asymmetries during spontaneous movements along vertical surfaces and amphetamine-induced rotation. Both behavioral asymmetries were exhibited by control-treated, lesioned rats. Interestingly, these behavioral protections occurred in the absence of an increase in the density of DA nerve fibers in the striatum of Ad GDNF-treated rats. ELISA measurements of transgene proteins showed that nanogram quantities of GDNF and lacZ transgene were present in the striatum for 7 weeks, and picogram quantities of GDNF in the SN due to retrograde transport of vector and/or transgene protein. These studies demonstrate that Ad GDNF can sustain increased levels of biosynthesized GDNF in the terminal region of DA neurons for at least 7 weeks and that this GDNF slows the degeneration of DA neurons and prevents the appearance of dopamine dependent motor asymmetries in a rat model of Parkinson's disease (PD). GDNF gene therapy targeted to the striatum, a more surgically accessible site than the SN, may be clinically applicable to humans with PD.

Transfer of the herpes simplex virus type-1 thymidine kinase (HSV-tk) gene into tumor cells followed by ganciclovir (GCV) administration, will provide selective tumor cell killing. We studied the effect of herpes simplex virus thymidine kinase (HSV-tk) expression level on the HSV-tk/GCV-mediated “bystander effect.” Clones of HSV-tk-transduced rat glioma cells (9L) were isolated that stably expressed with different levels of HSV-tk. All clones studied had similar sensitivity to ganciclovir with IC50 values ranging from 0.45 to 1.3 μM. Within certain enzyme level thresholds, in vitro evaluation of the bystander effect has shown that clones with higher level of HSV-tk expression exhibited a better bystander effect. Interestingly, the bystander effect was observed between different cell types. Both the transduction efficiency and bystander effect are essential factors for the success of the antitumor effect by the HSV-tk/prodrug GCV suicide gene system. Many studies have described the herpes simplex virus type-1 thymidine kinase and prodrug ganciclovir (GCV) system for antitumor effect Complete tumor regression does not require that all tumor cells express herpes simplex virus thymidine kinase (HSV-tk). Tumor cells in close proximity to HSV-tk-expressing tumor cells become GCV-sensitive through a phenomenon described as the “bystander effect.” The bystander effect can function between different cell types. Combination of the direct cytotoxicity of target cells via the HSV-tk/prodrug GCV system and the bystander effect are essential in achieving antitumor effects in vitro and in vivo. In this study, we demonstrate the effect of HSV-tk expression level on the HSV-tk/GCV-mediated bystander effect Within a certain level of TK enzyme level threshold, the higher levels of HSV-tk expression correlated with better bystander effect in mediating cell killing.

BACKGROUND: A stable packaging cell line facilitates large-scale lentivirus vector manufacture. However, it has been difficult to produce clinical-scale HIV-1-based lentiviral vectors using a packaging cell line, in part due to toxicity of packaging genes, and gene silencing that occurs during the long culture period necessary for sequential addition of packaging constructs. METHODS: To avoid these problems, we developed a three-level cascade gene regulation system designed to remove tetracycline transactivator (tTA) from cytomegalovirus immediate early promoter (CMV)-controlled expression to reduce cytotoxicity from constitutive expression of tTA and leaky expression of packaging genes. We also performed a one-step integration of the three packaging plasmids to shorten the culture time for clonal selection. RESULTS: Although leaky expression of p24 and vector production still occurred despite the three-level regulation system, little cytotoxicity was observed and producer cells could be expanded for large-scale production. Producer cells yielded remarkably stable vector production over a period greater than 11 days with the highest titer 3.5 x 10(7) transducing units (TU)/ml and p24 300 ng/ml, yielding 2.2 x 10(11) TU and 1.8 milligram (mg) p24 from one cell factory. No replication-competent lentivirus (RCL) was detected. Long-term analysis demonstrated that, although the cells are genetically stable, partial gene silencing occurs after 2-3 months in culture; however, the one-step construct integration allowed prolonged vector production before significant gene silencing. Concentrated vector resulted in 90% transduction in CD4+ lymphocytes at 20 TU per cell. CD34+ progenitor cells were transduced at 41-46% efficiency, and long-term initiating culture (LTC-IC) was transduced at 45-51%. CONCLUSIONS: These results demonstrate for the first time HIV-1-based lentiviral vector production on the large scale using a packaging cell line.

We report the design of a unique two-plasmid production system for the first lentiviral vector to be evaluated in humans, VRX496. VRX496 is an optimized VSV-G pseudotyped vector derived from HIV-1 that expresses antisense to the HIV envelope gene. We found that a two-plasmid approach to production resulted in higher vector production titers when compared with a three-plasmid approach, which is particularly important for vector production at the large scale. Therefore, we carefully designed a single packaging construct, VIRPAC, for safety by reducing its homology with VRX496 and by insertion of functionally validated genetic elements designed to reduce the risk of generation of a replication-competent lentivirus (RCL). A native cis-acting ribozyme is used to prevent read through into the envelope gene from the upstream gag-pol genes in the packaging vector, thus preventing RNAs containing gag-pol and env together for comparable safety to a three-plasmid system. We demonstrate that there is no significant in vivo vector mobilization using a primary SCID-hu mouse transplantation model, which correlates with the presence of an anti-HIV payload and suggests that inclusion of antisense may be a useful tool to restrict mobilization in other vector constructs. Gene transfer is achieved using a one-step transduction procedure that is simple and clinically translatable, which reaches stable transduction efficiencies of >99% in CD4+ T lymphocytes within 3 days of culture initiation.