Glioblastoma, also known as glioblastoma multiforme (GBM) or grade IV astrocytoma, is the most common and deadliest of primary brain tumors in adults with 14.6‐month median survival time and a 2% 5‐year survival rate (Kwiatkowska
et al,
2013). This depressing prognosis occurs with current, aggressive gold standard treatment regimens, such as chemotherapy. Even more recent trials with anti‐angiogenic and molecularly targeted therapies have shown only slight improvements in GBM outcome (Gilbert
et al,
2014). Therefore, novel remedies that effectively treat or improve/complement existing treatments are pressingly needed. Temozolomide (TMZ), an alkylating agent able to cross the blood–brain barrier (BBB), is well‐tolerated and widely used as part of standard chemotherapy for newly diagnosed and recurrent GBM, but has limited efficacy (Thon
et al,
2013; Messaoudi
et al,
2015; Taal
et al,
2015). We hypothesized that combining TMZ chemotherapy with gene therapy should yield a synergistic effect against GBM. Gene therapy for GBM has been attempted for the past 24 years, but progress has been hindered mostly by lack of tumor selectivity and inefficiency of eukaryotic viral vectors (Natsume & Yoshida,
2008; Kwiatkowska
et al,
2013). Gene therapy of brain tumors is further hampered by the BBB (Maguire
et al,
2014). Although this could be circumvented by delivery of vectors locally, the invasiveness of repeated intracerebral injections and the diffuse/not discrete nature of GBM place great limitations. We have introduced a unique prokaryotic viral‐based approach of systemic intravenous gene delivery to target tumors specifically by using the harmless and non‐pathogenic filamentous M13 bacteriophage, a virus that infects only bacterial cells and lacks native tropism to normal tissues in human and eukaryotes in general (Asavarut & Hajitou,
2014). A key strategy of our systemic delivery platform involves the construction of a hybrid bacteriophage vector termed AAV‐phage, or AAVP (Fig
EV1A). In this vector, the single‐stranded genome of human adeno‐associated virus (AAV) was inserted within the M13 phage single‐stranded genome, resulting in a phage capsid which incorporates AAV genomes(s) (Hajitou
et al,
2006,
2007). Importantly, the phage capsid was engineered to display the CDCRGDCFC (RGD4C) ligand (Fig
EV1A) that binds the heterodimer α
vβ
3 integrin cell surface receptor, which is overexpressed on tumor cells and supporting angiogenic vasculature in most tumor types including human GBM (Hajitou
et al,
2006; Schnell
et al,
2008). Upon binding to α
vβ
3 integrin receptor and subsequent entry of the RGD4C/AAVP viral particles into cells, the AAV genome is released to express genes in tumors from a cytomegalovirus,
CMV, promoter (Hajitou
et al,
2006). To date, we and collaborators have established that these vectors target various preclinical models of human cancer including soft tissue sarcoma, melanoma, breast, prostate, and pancreatic cancers (Hajitou
et al,
2006,
2008; Tandle
et al,
2009; Yuan
et al,
2013; Dobroff
et al,
2016; Smith
et al,
2016). Additionally, a study by the National Cancer Institute, USA, using our vector in pet dogs with natural cancers demonstrated that RGD4C/AAVP delivered the cytokine, tumor necrosis factor‐alpha (
TNFα), selectively, to spontaneous cancers (Paoloni
et al,
2009). Remarkably, repeated vector dosing proved safe and resulted in complete tumor eradication in several dogs with aggressive cancers (Paoloni
et al,
2009). To improve the vector platform for use in targeted gene therapy against GBM, we have refined the technology by replacing the
CMV promoter with the tumor‐specific
Grp78 promoter and designed the dual tumor targeting RGD4C/AAVP‐
Grp78 vector (Kia
et al,
2012,
2013). This novel vector effectively ensured further tumor selectivity, through transcriptional targeting as previously reported (Kia
et al,
2012). We also demonstrated that the RGD4C/AAVP‐
Grp78 vector provides much longer lasting transgene expression than the RGD4C/AAVP‐
CMV vector carrying a
CMV promoter
, in vitro and
in vivo in subcutaneous GBM following intravenous administration (Kia
et al,
2012). The
Grp78 promoter is marginally active in healthy tissues; however, potent activation has been observed in aggressive tumors, including GBM (Dong
et al,
2004; Pyrko
et al,
2007; Virrey
et al,
2008), and is induced by stress and conditions of tumor microenvironment such as glucose deprivation, chronic anoxia, and acidic pH. It is further worth considering that the induction of the
Grp78 gene expression and activation confers drug resistance in a variety of human tumors, including gliomas (Li & Lee,
2006; Lee,
2007; Pyrko
et al,
2007).
Grp78 can also be induced by TMZ in GBM (Pyrko
et al,
2007), and its activation has been associated with GBM resistance to TMZ (Pyrko
et al,
2007; Virrey
et al,
2008). Thus, further increase in gene expression from the RGD4C/AAVP‐
Grp78 can be ensured through TMZ activation of the
Grp78 promoter. Consequently, we postulated that RGD4C/AAVP‐
Grp78 is a suitable candidate for use in combination with TMZ against GBM. Herein, we investigated the effects of combining TMZ chemotherapy and targeted gene therapy with RGD4C/AAVP‐
Grp78, termed chemovirotherapy, against intracranial orthotopic models of human glioblastoma. Firstly, we demonstrated that a list of various human glioblastoma cell lines as well as primary GBM and primary GBM stem cells express α
vβ
3 integrin, a receptor of RGD4C, resulting in ligand‐directed gene delivery by RGD4C/AAVP
in vitro. Then, we showed that TMZ treatment of human glioblastoma cell lines increased expression of the endogenous
Grp78, through the UPR (unfolded protein response) pathway, subsequently stimulating RGD4C/AAVP‐
Grp78‐mediated gene expression. We also found that TMZ amplified the destruction of GBM cells in combination with RGD4C/AAVP‐
Grp78‐HSVtk encoding the
Herpes simplex virus type I thymidine kinase in the presence of ganciclovir (GCV); we used the
HSVtk mutant SR39 (Black
et al,
2001). Next, we confirmed that RGD4C/AAVP‐
Grp78 targets orthotopic glioblastoma in mice after intravenous administration selectively binding to tumor cells and tumor vasculature without accumulation in the healthy brains. Additionally, the combination of TMZ and RGD4C/AAVP‐
Grp78‐HSVtk/GCV administered systemically elicited strong anti‐tumor activity against intracranial glioblastoma established
in vivo from GBM cell lines and primary GBM, and in both immunodeficient and immunocompetent mice. Unless technically, the
in vivo effect was measured synergistic, compared to TMZ or RGD4C/AAVP‐
Grp78‐HSVtk/GCV alone. The HSVtk enzyme phosphorylates prodrug nucleoside analogues such as GCV and converts them into nucleoside analogue triphosphates. These triphosphate GCV compounds are then incorporated into the cellular genome, inhibit DNA polymerase, and subsequently induce cell death by apoptosis (Hamel
et al,
1996; Natsume & Yoshida,
2008). It is important to note that the HSVtk/GCV approach also elicits a bystander effect, which means that cells containing the HSVtk kill neighboring non‐transduced cells through the gap junctional intercellular communications (GJIC) that allow the transfer of the converted cytotoxic drug and/or toxic metabolites between these cells as we have previously reported (Trepel
et al,
2009; Duarte
et al,
2012). After a few days, “this bystander effect” results in increased cell killing by the RGD4C/AAVP‐
HSVtk vector and may potentially overcome the requirement for all malignant cells to be transduced in order to achieve meaningful tumor regression. Altogether, these findings indicate that this combination therapy strategy offers significant translational potential in the treatment regime for GBM patients.