THERAPY OF EXPERIMENTAL HUMAN BRAIN-TUMORS USING A NEUROATTENUATED HERPES-SIMPLEX VIRUS MUTANT

BACKGROUND: Engineered herpes simplex virus (HSV) strains previously h ave been shown to offer a potential therapeutic alternative to convent ional treatment modalities for brain tumors. Because HSV Type 1 strain 1716 has a deletion in the gamma 34.5 neurovirulence gene that render s it avirulent in the mouse central nervous system, we have assessed i ts potential to induce selective lysis of tumor cells versus neurons i n vitro and in vivo. EXPERIMENTAL DESIGN: To do this, we studied paren tal 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 aci d. Intracerebral grafts of NT2 cells into the brains of nude mice resu lted in lethal brain tumors, and grafts of NT2N cells resulted in the integration of NT2N cells. RESULTS: In vitro studies showed that strai n 1716 replicates in and spreads on monolayers of NT2 cells but not in NT2N cells. In vivo, strain 1716 replicated preferentially in NT2 tum ors as evidenced by immunohistochemical staining for viral antigens, b y in situ hybridization for HSV-specific transcripts, and by titration of virus from brains with tumor after intracranial injection of the v irus into these mice. The temporal regression of NT2 tumors in mice tr eated with strain 1716 was demonstrated in vivo by magnetic resonance imaging. Electron microscopy and studies of DNA fragmentation suggeste d that regression of NT2 brain tumors in strain 1716-treated mice was mainly caused by a nonapoptotic, lytic mode of cell death. Finally, st rain 1716-treated NT2 tumor-bearing mice survived more than twice as l ong as mock-treated tumor-bearing mice, and these differences in survi val (25 vs. 9 weeks) were statistically significant (p < 0.03). CONCLU SIONS: We conclude from these studies that strain 1716 induces regress ion of human neural tumors established in the brains of nude mice, res ulting in their prolonged survival.