Gene Therapy for Meningeal Carcinomatosis 
1.2 Strategy of Gene Therapy 
Gene transfer in the subarachnoid space has several advantages of safety and potential 
efficacy. First, retroviral vectors only integrate, and therefore express, vector genes in 
proliferating cells. In the subarachnoid space, the tumor seeds comprise the most mitotically active 
cells. Among the normal tissues, only the choroid plexus, macrophage-derived cells and 
endothelial cells are at risk of being transduced. Therefore, the possibility of specific and 
preferential transduction of tumor cells is enhanced. Second, the CNS, including the subarachnoid 
space, is an immunologically privileged site due to the blood-CSF barrier. This should enhance 
survival of the xenogeneic murine cells and viral vectors and permit an increased transduction rate 
of the tumor. Subsequently, the producer cells and tumor cells that integrate and express HS-tk 
will be destroyed by GCV. In LMC, malignant cells, which grow as thin sheets on the 
leptomeninges, are exposed to the cerebrospinal fluid, (18) allowing circulating vector particles to 
reach the tumor cells and facilitating the distribution of the HStk gene into tumor cells. 
1.3 Description of Viral Vector 
GITKSVNa is a retroviral vector derived from the Moloney murine leukemia virus 
(MoMLV). This vector contains 1) herpes thymidine kinase (HS-tk) gene cDNA that is transcribed 
from the viral LTR and 2) the bacterial neomycin resistance (NeoR) gene transcribed from an 
internal SV40 (simian virus 40) early promoter (LTR-HS-tk-SV-NeoR-LTR) in the G1 vector 
backbone (Genetic Therapy Inc., Gaithersburg, MD). This Gl-based vector has been modified for 
increased safety by alteration of the gag start codon to a stop codon, and by elimination of viral 
sequences needed for formation of a replication-competent virus. This has been shown to decrease 
the potential for helper virus production from producer cells which contain the vector. No 
replication-competent virus has been detected in vectors administered to patients or following 
administration of the vector to animals or humans. 
1.4 Preclinical Studies 
(For detailed description of the pre-clinical efficacy and toxicity studies refer to appendix II) 
Summary of Preclinical Efficacy Studies 
In meningeal carcinomatosis retroviral vector-producer cells survive in the subarachnoid space 
long enough to distribute the vector in the CSF, to reach malignant tumor cells in the leptomeninges, 
and selectively infect and transfer a gene of interest to these cells. Gene transfer experiments with the 
lacZ gene and in vitro retroviral titer measurements showed that retroviral vectors survive in the CSF, 
retain their infectivity, and successfully transduce tumor cells. Significant antitumor effect was 
demonstrated by in situ, intrathecal, transduction with the herpes simplex-thymidine kinase gene 
followed with ganciclovir therapy in a rat model. Prolongation of survival was observed when 
ganciclovir was given systemically, but not intrathecally. Assessment of the antitumor efficacy of the 
GlTklSvN.7 HStk vector-producer cells demonstrated that intrathecal injection of the clone 7 cells 
resulted in significant prolongation of survival after i.p. GCV therapy. These results support the 
potential application of gene therapy using the HStk/GCV system for treatment of meningeal 
carcinomatosis. 
Summary of Preclinical Toxicity Studies 
Toxicity studies in rats indicated that no significant adverse effects were associated with 
intrathecal administration of the xenogeneic retroviral vector-producer cells, systemic GCV 
administration, or combination of the two. No transduction of normal leptomeningeal or 
subarachnoid structures throughout the brain and spinal cord occurred. Injection of HStk vector- 
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