Abstract
There has been increasing interest in oncolytic virotherapy for the treatment of cancer over the past decade. Reovirus is a ubiquitous double-stranded RNA virus that is oncolytic. It has been shown to selectively kill cancer cells in murine, and human models. The mechanism by which reovirus selectively targets and kills cancer cells is slowly being elucidated. Activation of the Ras pathway in transformed cells plays a part in the permissivity of cancer cells to reovirus, in part through the inability of Ras-activated cells to phosphorylate cellular PKR. Our microarray analysis found increased expression of epidermal growth factor receptor (EGFR) in B16 mouse melanoma cells after exposure to reovirus. As EGFR is a component of the Ras pathway, we explored this increased expression and found that in certain cell lines there was an increase in EGFR expression after reovirus exposure both by PCR and western blot analysis. Data is presented looking at the effect of silencing EGFR on cell survival. Early human trials indicate that while reovirus is safe and capable of inducing cancer cell death, single agent activity is likely to be limited. Attention has now focused on combination strategies. We evaluated the combination of reovirus with rapamycin in the B16.F10 murine model of malignant melanoma based on potential mechanisms by which mTOR inhibitors might enhance viral oncolysis. These include cell cycle arrest, targeting of alternative signalling pathways, and suppression of the antiviral immune response. Rapamycin attenuated viral replication if given prior to or concomitantly with reovirus and similarly reduced reovirus-induced apoptotic cell death. However, we found clear evidence of synergistic antitumour effects of the combination both in vitro and in vivo, which was sequence dependent only in the in vitro setting. Rapamycin showed no systemic immunomodulation and cell cycle effects of reovirus (increased G0/G1 fraction) were unaffected by concomitant or sequential exposure of rapamycin. We also conducted a multicentre, phase 1 dose escalation study designed to assess the safety of combining reovirus with docetaxel chemotherapy in patients with advanced cancer. Patients received 75mg/m2 of docetaxel, day 1, and escalating doses of reovirus up to 3 x 1010 TCID50, day 1-5, every 3 weeks. 25 patients were enrolled, with 23 completing at least one cycle and 16 suitable for response assessment. Dose-limiting toxicity of grade 4 neutropaenia was seen in one patient but the maximum tolerated dose was not reached. Antitumour activity was seen with one complete response and 3 partial responses. A disease control rate of 88% was observed. The combination of reovirus and docetaxel was concluded to be safe, with evidence of objective disease responses, and warrants further evaluation in a phase II study at a recommended schedule of 75mg/m2 of docetaxel, 3 weekly and reovirus 3 x 1010 TCID50 day 1-5, every 3 weeks.