Therapy of experimental human brain tumors using a neuroattenuated herpes simplex virus mutant

Engineered herpes simplex virus (HSV) strains previously have been shown to offer a potential therapeutic alternative to conventional treatment modalities for brain tumors. Because HSV Type 1 strain 1716 has a deletion in the gamma 34.5 neurovirulence gene that renders it avirulent in the mouse central nervous system, we have assessed its potential to induce selective lysis of tumor cells versus neurons in vitro and in vivo.

To do this, we studied parental HSV-1 strain 17+ and strain 1716 using human embryonal carcinoma cells (NT2 cells). These cells resemble neuronal progenitor cells and can be induced to differentiate into neurons (NT2N) with retinoic acid. Intracerebral grafts of NT2 cells into the brains of nude mice resulted in lethal brain tumors, and grafts of NT2N cells resulted in the integration of NT2N cells.

In vitro studies showed that strain 1716 replicates in and spreads on monolayers of NT2 cells but not in NT2N cells. In vivo, strain 1716 replicated preferentially in NT2 tumors as evidenced by immunohistochemical staining for viral antigens, by in situ hybridization for HSV-specific transcripts, and by titration of virus from brains with tumor after intracranial injection of the virus into these mice. The temporal regression of NT2 tumors in mice treated with strain 1716 was demonstrated in vivo by magnetic resonance imaging. Electron microscopy and studies of DNA fragmentation suggested that regression of NT2 brain tumors in strain 1716-treated mice was mainly caused by a nonapoptotic, lytic mode of cell death. Finally, strain 1716-treated NT2 tumor-bearing mice survived more than twice as long as mock-treated tumor-bearing mice, and these differences in survival (25 vs. 9 weeks) were statistically significant (p < 0.03).

We conclude from these studies that strain 1716 induces regression of human neural tumors established in the brains of nude mice, resulting in their prolonged survival.