The resistance of human glioblastoma multiforme (GBM) to traditional chemotherapy and radiotherapy makes the establishment of novel therapeutics an important goal of research. A recent publication by Samuel Rabkin's and Robert Martuza's group at Harvard Medical School in Boston, Massachusetts (Cancer Res 69:3472– 3481, 2009) found that oncolytic herpes simplex virus (oHSV), a genetically modified virus capable of killing neoplastic cells while leaving normal cells intact, was effective at killing GBM-derived tumor stem cells (TSCs) and also inhibited the self-renewal capacity of surviving cells. TSCs were generated from GBM patients and oHSVs were used to infect them. Efficient infection was proven by infecting TSCs with an enhanced green fluorescent protein (EGFP) on a viral vector. Replication competent oHSV vector carrying EGFP showed an increase in cells with EGFP over time, showing that the virus was able to replicate after infection. By coculturing cells infected with EGFP on a viral vector and uninfected cells with an orange dye, the authors were able to prove cell to cell spread of the virus by identifying cells with both orange dye and vector present in 48 hours. Finally, costaining for EGFP and 7-AAD, which stains for nonviable cells, showed an increase in non-viable cells at the 3-day mark.
Intratumoral injection of G47 prolongs survival of mice bearing glioblastoma-derived cancer stem-like cells (GBM-SC) xenografts. A and B, Kaplan-Meier survival curves of the mice bearing GBM-SCs xenografts treated with oncolytic herpes simplex virus (oHSV) vectors. Arrows, time of virus injection. C and D, G47 infects GBM-SC tumors in vivo. C, X-gal staining of the sections revealed an extensive infection of tumor tissue that displays a progression along white matter tracts (left). LV, lateral ventricle. Asterisk, injection track. Scale bar, 200 μm. Efficient in vivo infection by G47 is shown at higher power magnification (middle), whereas no lacZ positivity is seen in a phosphate-buffered saline-treated section (right). Scale bars, 50 μm. D, immunofluorescent staining showing colocalized detection of β- galactosidase (Cy3, red) and human nuclei (fluorescein isothiocyanate, green) on a G47-infected brain section. Scale bar, 50 μm. Reproduced with permission from Cancer Res 69:3472–3481, 2009.
Intratumoral injection of G47 prolongs survival of mice bearing glioblastoma-derived cancer stem-like cells (GBM-SC) xenografts. A and B, Kaplan-Meier survival curves of the mice bearing GBM-SCs xenografts treated with oncolytic herpes simplex virus (oHSV) vectors. Arrows, time of virus injection. C and D, G47 infects GBM-SC tumors in vivo. C, X-gal staining of the sections revealed an extensive infection of tumor tissue that displays a progression along white matter tracts (left). LV, lateral ventricle. Asterisk, injection track. Scale bar, 200 μm. Efficient in vivo infection by G47 is shown at higher power magnification (middle), whereas no lacZ positivity is seen in a phosphate-buffered saline-treated section (right). Scale bars, 50 μm. D, immunofluorescent staining showing colocalized detection of β- galactosidase (Cy3, red) and human nuclei (fluorescein isothiocyanate, green) on a G47-infected brain section. Scale bar, 50 μm. Reproduced with permission from Cancer Res 69:3472–3481, 2009.
The study also characterized the cytotoxic properties of oHSVs and postulated that because they kill cancer cells through oncolysis, as opposed to other cytocidal mechanisms for which resistance has already been proven in GBMs, oHSVs may prove efficacious where other therapies have failed. Viral mutation studies suggested that there was a close correlation between viral replication and cell killing efficiency. Intracerebral xenografts of TSCs in mice formed invasive tumors, but injection of oHSV resulted in prolongation of survival, compared to treatment with phosphate-buffered saline (PBS) injection.
The results of this study are promising. Although the treatment was not curative, the evidence that oHSV injection into solid GBM tumor mass may have in vivo efficacy in prolonging survival is highly encouraging. Possible methods for increasing efficacy, including convection-enhanced delivery and pharmacological immune suppression to allow for viral spread, were discussed. Possible complications to this therapy include a tendency for oHSV mutants to illicit and angiogenic response in vivo, which is counterproductive in treating GBMs, which show a preference for the perivascular niche. Another problem is the actual and the perceived dangers of injecting live virus cells into a patient. These complications not withstanding, this still represents an encouraging novel therapeutic area of research in an otherwise uniformly lethal disease.
