Previous studies have described in vitro serial passage of a Deltagamma(1)34.5 herpes simplex virus type 1 (HSV-1) strain in SK-N-SH
neuroblastoma cells and selection of mutants that have acquired the ability to infect and replicate in this previously nonpermissive cell line. Here we describe the selection of a mutant HSV-1 strain by in vivo serial passage, which prolongs survival in two separate experimental murine
brain tumor models. Two conditionally replication-competent Deltagamma(1)34.5 viruses, M002, which expresses murine
interleukin-12, and its parent virus, R3659, were serially passaged within human
malignant glioma D54-MG cell lines in vitro or flank
tumor xenografts in vivo. The major findings are (i) viruses passaged in vivo demonstrate decreased neurovirulence, whereas those passaged in vitro demonstrate a partial recovery of the neurovirulence associated with HSV-1; and (ii) vvD54-M002, the virus selected after in vivo serial passage of M002 in D54-MG
tumors, improves survival in two independent murine
brain tumor models compared to the parent (unpassaged) M002. Additionally, in vitro-passaged, but not in vivo-passaged, M002 displayed changes in the
protein synthesis profile in previously nonpermissive cell lines, as well as early U(S)11 transcription. Thus, a mutant HSV-1 strain expressing a foreign gene can be selected for enhanced antitumor efficacy via in vivo serial passage within flank D54-MG
tumor xenografts. The enhanced antitumor efficacy of vvD54-M002 is not due to restoration of
protein synthesis or early U(S)11 expression. This finding emphasizes the contribution of the in vivo
tumor environment for selecting novel oncolytic HSV specifically adapted for
tumor cell destruction in vivo.