HSV-1 as a novel therapy for breast cancer meningeal metastases

Meningeal metastasis is a fatal complication of breast cancer that affects 5–8% of patients. When cancer cells seed in the meninges, their subsequent growth results in severe neurological complications involving the cranial nerves, cerebrum and spinal cord, limiting life expectancy to less than 4 months. The incidences of meningeal metastases increase with prolonged lifespan resulting from treatment advances for primary breast cancer and their metastases. Currently, there is no cure. Aggressive multimodal therapies such as radiation and chemotherapy (intra-cerebrospinal fluid (CSF) and systemic) are ineffective. Therapeutic agents are often quickly cleared from the CSF, while higher doses that can achieve a therapeutic response are highly toxic. The secure guarding of the subarachnoid space by the blood–brain barrier on one side and the blood–CSF barrier on the other prevents chemotherapy from reaching cancer cells in the meninges. These challenges with treating meningeal metastases highlight the urgent need for a new therapeutic modality. An ideal treatment would be an agent that avoids rapid clearance, remains within the CSF, reaches the meninges and selectively destroys tumor cells. Replication conditional oncolytic herpes simplex virus type 1 (HSV-1) may be effective in this regard. Viral oncolysis, the destruction of cancer cells by replicating virus, is under clinical investigation for cancers that are unresponsive to current therapies. It is based on the model of multiple cycles of lytic virus replication in cancer cells that amplify the injected dose. The therapeutic potential of oncolytic HSV-1 for breast cancer meningeal metastases is discussed here. HSV-1 could be a potential novel treatment for meningeal metastases that can be translated to the clinic.