Design and application of oncolytic HSV vectors for glioblastoma therapy

Abstract

Glioblastoma multiforme is one of the most common human brain tumors. The tumor is generally highly infiltrative, making it extremely difficult to treat by surgical resection or radiotherapy. This feature contributes to recurrence and a very poor prognosis. Few anticancer drugs have been shown to alter rapid tumor growth and none are ultimately effective. Oncolytic vectors have been employed as a treatment alternative based on the ability to tailor virus replication to tumor cells. The human neurotropic herpes simplex virus (HSV) is especially attractive for development of oncolytic vectors (oHSV) because this virus is highly infectious, replicates rapidly and can be readily modified to achieve vector attenuation in normal brain tissue. Tumor specificity can be achieved by deleting viral genes that are only required for virus replication in normal cells and permit mutant virus replication selectively in tumor cells. The anti-tumor activity of oHSV can be enhanced by arming the vector with genes that either activate chemotherapeutic drugs within the tumor tissue or promote anti-tumor immunity. In this review, we describe current designs of oHSV and the experience thus far with their potential utility for glioblastoma therapy. In addition, we discuss the impediments to vector effectiveness and describe our view of future developments in vector improvement.

Figure 1. Herpes simplex virus (HSV) genome and the location of the essential and accessory genes that are manipulated in the generation of replication defective or oncolytic vectors

The HSV-1 genome is comprised of 152 kb of linear, dsDNA arranged as long and short unique segments (U_L and U_S) flanked by inverted repeated sequences (TR_L, TR_S, IR_L and IR_S). The locations of two essential immediate-early genes (ICP4 and ICP27) are indicated below the viral genome (these genes have been deleted to create replication defective vectors), and the locations of several accessory, or nonessential, genes that have been manipulated to create oncolytic vectors are indicated above the viral genome. TR_L: Terminal repeat long; TR_S: Terminal repeat short; IR_L: Internal repeat long; IR_S: Internal repeat short; U_L: Long unique segment; U_S: Short unique segment.

Figure 2. Nonessential viral proteins that are needed to counteract the dsRNA-dependent PKR pathway that is activated upon viral infection

The schematic depicts the PKR pathway that is initiated following virus infection. Expression of PKR is activated by the interferon signaling pathway that is initiated upon viral infection. PKR activation (phosphorylation) is induced in response to dsRNA that is produced in virally infected cells. Active PKR phosphorylates eIF2α and thereby inhibits protein synthesis and blocks the production of viral proteins. PKR also activates the transcription factor NF-κB and induces an antiviral immune response. In order to counteract this pathway, γ34.5 retargets protein phosphatase-1α to induce dephosphorylation of eIF2α. US11 (if present with immediate-early gene kinetics) is able to inhibit PKR by a different mechanism, and the multifunctional ICP0 protein is also thought to play a role in inactivation of the interferon and PKR pathways. Activation of the Ras signaling pathway inhibits PKR activity and complements for the lack of γ34.5 (this example illustrates Ras activation after binding of EGF to its receptor). HSV: Herpes simplex virus; NF: Nuclear factor; PKR: RNA-dependent protein kinase.