Therapeutic Efficiency and Safety of a Second-Generation Replication-Conditional HSV1 Vector for Brain Tumor Gene Therapy

Abstract

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.

A herpes simplex virus type 1 mutant deleted for γ34.5 and LAT kills glioma cells in vitro and is inhibited for in vivo reactivation

“…into BALB /c mice. Kramm et al 8 independently reported a second virus also defective for 34.5 and RR. Although both of these viruses appeared to have increased safety compared to hrR3, the additional mutation decreased therapeutic efficacy, presumably because of its very poor growth characteristics.…”

“…6,7 Mutants containing deletions in two viral genes have been developed for potential use against malignant brain tumors by two groups 8,9 to further increase safety by altering multiple mechanisms of virulence. Both viruses have a deletion in the neurovirulence gene 34.5 and in the ICP6 gene, which encodes for the large subunit of the HSV-1 ribonucleotide reductase ( RR ).…”

“…Deleting both 34.5 and RR resulted in mutants that grew poorly and demonstrated reduced therapeutic potential. 8 We previously constructed the HSV-1 strain McKraebased mutant d34.5. This mutant is deleted for both copies of 34.5.…”

Evaluation of ganciclovir-mediated enhancement of the antitumoral effect in oncolytic, multimutated herpes simplex virus type 1 (G207) therapy of brain tumors

“…25 A difficulty in evaluating the efficacy of replication-competent HSV-1 in vivo has been the lack of suitable immunocompetent animal tumor models due to the relative resistance of rodent cells to HSV-1 infection. The 9L rat gliosarcoma model has been used for replication-competent HSV studies 36,45,46 as well as HSV-tk/GCV therapy, [5][6][7]10,11,14 but the cell line is highly resistant to HSV-1, 36 especially G207 (our unpublished data), and is immunogenic; 58 therefore, it is not suitable for G207 studies. A/J is one of the most sensitive mouse strains to HSV-1 infection, 49 and syngeneic N18 tumors in A/J mice are very responsive to G207 treatment.…”

“…MGH-1, a multimutated HSV-1 vector with the same mutation as G207, exhibited no therapeutic effect in the 9L tumor model with or without GCV administration. 36 The resistance of 9L cells to MGH-1 seemed to largely account for the lack of efficacy. It has also been shown that enhancement of the effect of GCV on hrR3-mediated killing was observed only in 9L cells, and not in various human colorectal cancer cells in culture or in nude mice.…”