Reovirus and other oncolytic viruses for the targeted treatment of cancer

Oncolytic virus therapy: A new era of cancer treatment at dawn

Oncolytic virus therapy is perhaps the next major breakthrough in cancer treatment following the success in immunotherapy using immune checkpoint inhibitors. Oncolytic viruses are defined as genetically engineered or naturally occurring viruses that selectively replicate in and kill cancer cells without harming the normal tissues. T‐Vec (talimogene laherparepvec), a second‐generation oncolytic herpes simplex virus type 1 (HSV‐1) armed with GM‐CSF, was recently approved as the first oncolytic virus drug in the USA and Europe. The phase III trial proved that local intralesional injections with T‐Vec in advanced malignant melanoma patients can not only suppress the growth of injected tumors but also act systemically and prolong overall survival. Other oncolytic viruses that are closing in on drug approval in North America and Europe include vaccinia virus JX‐594 (pexastimogene devacirepvec) for hepatocellular carcinoma, GM‐CSF‐expressing adenovirus CG0070 for bladder cancer, and Reolysin (pelareorep), a wild‐type variant of reovirus, for head and neck cancer. In Japan, a phase II clinical trial of G47∆, a third‐generation oncolytic HSV‐1, is ongoing in glioblastoma patients. G47∆ was recently designated as a “Sakigake” breakthrough therapy drug in Japan. This new system by the Japanese government should provide G47∆ with priority reviews and a fast‐track drug approval by the regulatory authorities. Whereas numerous oncolytic viruses have been subjected to clinical trials, the common feature that is expected to play a major role in prolonging the survival of cancer patients is an induction of specific antitumor immunity in the course of tumor‐specific viral replication. It appears that it will not be long before oncolytic virus therapy becomes a standard therapeutic option for all cancer patients.

Clinical development of reovirus for cancer therapy: An oncolytic virus with immune-mediated antitumor activity

Reovirus is a double-stranded RNA virus with demonstrated oncolysis or preferential replication in cancer cells. The oncolytic properties of reovirus appear to be dependent, in part, on activated Ras signaling. In addition, Ras-transformation promotes reovirus oncolysis by affecting several steps of the viral life cycle. Reovirus-mediated immune responses can present barriers to tumor targeting, serve protective functions against reovirus systemic toxicity, and contribute to therapeutic efficacy through antitumor immune-mediated effects via innate and adaptive responses. Preclinical studies have demonstrated the broad anticancer activity of wild-type, unmodified type 3 Dearing strain reovirus (Reolysin^®) across a spectrum of malignancies. The development of reovirus as an anticancer agent and available clinical data reported from 22 clinical trials will be reviewed.

The oncolytic virus, pelareorep, as a novel anticancer agent: a review

Pelareorep (REOLYSIN®) is an investigational new drug, a proprietary formulation consisting of a live, replication-competent, naturally occurring Reovirus Type 3 Dearing strain. Through several preclinical studies it was determined that reovirus can exhibit profound cytotoxic effects on cancer cells predominantly with an activated RAS-signalling pathway. Moreover, it was discovered that reoviruses can "hitchhike" on peripheral blood mononuclear cells and dendritic cells, thereby evading neutralizing antibodies of the host immune system. Cell carriage, targeted delivery, triggering host immune response and other inherent characteristics of the reovirus led to its further advancement into cancer therapy. When injected into Sprague-Dawley rats, the viral routes of clearance, predominantly through the spleen and liver, remained consistent with earlier studies. Toxicology findings were considered incidental and not associated with pelareorep when tested in animal models. Pelareorep demonstrated a high level of homogeneity at the amino acid level and genetic stability when compared to the master and working virus banks. The drug is manufactured in a 100 L bioreactor after which it is purified and formulated for use in pre-clinical, clinical and research studies. Over the past few decades, we have witnessed a paradigm shift from conventional therapy to the conceivable use of oncolytic viruses for the treatment of cancer. This review will detail pre-clinical evidence of anticancer activity of pelareorep that has led to extensive clinical development. Several Phase I-II clinical trials have been completed or are ongoing in cancer patients on a broad spectrum of solid tumors and hematologic malignancies.

Activated ras signaling pathways and reovirus oncolysis: an update on the mechanism of preferential reovirus replication in cancer cells

The development of wild-type, unmodified Type 3 Dearing strain reovirus as an anticancer agent has currently expanded to 32 clinical trials (both completed and ongoing) involving reovirus in the treatment of cancer. It has been more than 30 years since the potential of reovirus as an anticancer agent was first identified in studies that demonstrated the preferential replication of reovirus in transformed cell lines but not in normal cells. Later investigations have revealed the involvement of activated Ras signaling pathways (both upstream and downstream) and key steps of the reovirus infectious cycle in promoting preferential replication in cancer cells with reovirus-induced cancer cell death occurring through necrotic, apoptotic, and autophagic pathways. There is increasing evidence that reovirus-induced antitumor immunity involving both innate and adaptive responses also contributes to therapeutic efficacy though this discussion is beyond the scope of this article. Here, we review our current understanding of the mechanism of oncolysis contributing to the broad anticancer activity of reovirus. Further understanding of reovirus oncolysis is critical in enhancing the clinical development and efficacy of reovirus.

Two-stage phase I dose-escalation study of intratumoral reovirus type 3 dearing and palliative radiotherapy in patients with advanced cancers

PURPOSE

To determine the safety and feasibility of combining intratumoral reovirus and radiotherapy in patients with advanced cancer and to assess viral biodistribution, reoviral replication in tumors, and antiviral immune responses.

EXPERIMENTAL DESIGN

Patients with measurable disease amenable to palliative radiotherapy were enrolled. In the first stage, patients received radiotherapy (20 Gy in five fractions) plus two intratumoral injections of RT3D at doses between 1 x 10(8) and 1 x 10(10) TCID(50). In the second stage, the radiotherapy dose was increased (36 Gy in 12 fractions) and patients received two, four, or six doses of RT3D at 1 x 10(10) TCID(50). End points were safety, viral replication, immunogenicity, and antitumoral activity.

RESULTS

Twenty-three patients with various solid tumors were treated. Dose-limiting toxicity was not seen. The most common toxicities were grade 2 (or lower) pyrexia, influenza-like symptoms, vomiting, asymptomatic lymphopenia, and neutropenia. There was no exacerbation of the acute radiation reaction. Reverse transcription-PCR (RT-PCR) studies of blood, urine, stool, and sputum were negative for viral shedding. In the low-dose (20 Gy in five fractions) radiation group, two of seven evaluable patients had a partial response and five had stable disease. In the high-dose (36 Gy in 12 fractions) radiation group, five of seven evaluable patients had partial response and two stable disease.

CONCLUSIONS

The combination of intratumoral RT3D and radiotherapy was well tolerated. The favorable toxicity profile and lack of vector shedding means that this combination should be evaluated in newly diagnosed patients receiving radiotherapy with curative intent.

Selective in vitro cytotoxic effect of human cancer cells by bluetongue virus-10

Bluetongue viruses (BTVs) infect primarily domestic cattle and wild ruminants but have never been shown to infect normal human cells. Thus, humans are sero-negative towards BTVs. The selective and differential effects of BTV serotype 10 (BTV-10) infection were investigated with five cell lines including primary human embryo lung fibroblast (HEL) and primary murine embryos fibroblast(MEF), human hepatic carcinoma 3B cell line (Hep-3B), human lung carcinoma cell line (A549) and mouse fibroblast cell line (NIH 3T3). In this study, comparative analyses of differential cytopathic effects (CPEs), survival rates using different Multiplicities of Infection (MOI), ultra-structural changes by transmission electron microscopy, and the preferential cell cycle changes of infected cells by flow cytometry were made among these cells. Detection of the presence of BTV genome and kinetic analysis of virus titers in TCID50 were also made. We provided the first analytical demonstration and evidence that BTV-10 could selectively infect and degrade human cancer cells but not cultured primary normal cells. No CPE or viral mRNAs could be detected within these normal cells, while various degrees of CPE could be found in Hep-3B and A549, as well as in NIH 3T3 under similar conditions. Before death, BTV-infected human cancer cells were directly arrested in the sub-G1 phase and the diversity of BTV infection as shown by the MTT method had significant difference (F = 95.635, p < 0.01). Above results suggested that this viral dose-dependent cytotoxic effect is caused by both effective virion amplification and induced apoptosis. Cellular distinctive transformation status may contribute to the selectivity. Thus, selective degradation of human cancer cells but not normal diploid cells by the newly discovered oncolytic potential of BTV would provide a very attractive approach for cancer therapy in the future.

Reovirus therapy in cancer: has the orphan virus found a home?

There has been great interest in the development of oncolytic viruses - viruses that selectively destroy tumour cells - as cancer therapeutics. Reovirus holds great promise as an anticancer therapy, not just because it is a wild type virus that inherently displays selective tumour cytotoxicity in cancers with active Ras signalling pathways but also because it results only in relatively benign infections with few minor symptoms. As many tumours have an activated Ras pathway, the potential for utilizing reovirus as an effective anticancer agent is substantial. The several challenges that need to be overcome in the development of oncolytic viruses as anticancer agents, including issues of systemic toxicity, tumour selectivity and immune response, are addressed in this review. Clinical studies with the objective of developing Reolysin (human reovirus serotype 3 Dearing) as a human cancer therapeutic are currently underway. The first human Phase I study with intravenous Reolysin has now been completed and further studies, including Phase I and II clinical trials using Reolysin alone and in combination with radiation or chemotherapy, delivered via local or systemic intravenous administration, have commenced.

A phase I study of intravenous oncolytic reovirus type 3 Dearing in patients with advanced cancer

PURPOSE

To determine the safety and feasibility of daily i.v. administration of wild-type oncolytic reovirus (type 3 Dearing) to patients with advanced cancer, assess viral excretion kinetics and antiviral immune responses, identify tumor localization and replication, and describe antitumor activity.

EXPERIMENTAL DESIGN

Patients received escalating doses of reovirus up to 3 x 10(10) TCID(50) for 5 consecutive days every 4 weeks. Viral excretion was assessed by reverse transcription-PCR and antibody response by cytotoxicity neutralization assay. Pretreatment and post-treatment tumor biopsies were obtained to measure viral uptake and replication.

RESULTS

Thirty-three patients received 76 courses of reovirus from 1 x 10(8) for 1 day up to 3 x 10(10) TCID(50) for 5 days, repeated every four weeks. Dose-limiting toxicity was not seen. Common grade 1 to 2 toxicities included fever, fatigue, and headache, which were dose and cycle independent. Viral excretion at day 15 was not detected by reverse transcription-PCR at 25 cycles and only in 5 patients at 35 cycles. Neutralizing antibodies were detected in all patients and peaked at 4 weeks. Viral localization and replication in tumor biopsies were confirmed in 3 patients. Antitumor activity was seen by radiologic and tumor marker (carcinoembryonic antigen, CA19.9, and prostate-specific antigen) evaluation.

CONCLUSIONS

Oncolytic reovirus can be safely and repeatedly administered by i.v. injection at doses up to 3 x 10(10) TCID(50) for 5 days every 4 weeks without evidence of severe toxicities. Productive reoviral infection of metastatic tumor deposits was confirmed. Reovirus is a safe agent that warrants further evaluation in phase II studies.