Engineering Cancer Selective Virotherapies: Are the Pieces of the Puzzle Falling into Place?

Abstract

Advances in gene therapy, synthetic biology, cancer genomics, and patient-derived cancer models have expanded the repertoire of strategies for targeting human cancers using viral vectors. Novel capsids, synthetic promoters, and therapeutic payloads are being developed and assessed through approaches such as rational design, pooled library screening, and directed evolution. Ultimately, the goal is to generate precision-engineered viruses that target different facets of tumor cell biology, without compromising normal tissue and organ function. In this study, we briefly review the opportunities for engineering cancer selectivity into viral vectors at both the cell extrinsic and intrinsic level. Such stringently tumor-targeted vectors can subsequently act as platforms for the delivery of potent therapeutic transgenes, including the exciting prospect of immunotherapeutic payloads. These have the potential to eradicate nontransduced cells through stimulation of systemic anticancer immune responses, thereby side-stepping the inherent challenge of achieving gene delivery to the entire cancer cell population. We discuss the importance of using advanced primary human cellular models, such as patient-derived cultures and organoids, to enable rapid screening and triage of novel candidates using disease-relevant models. We believe this combination of improved delivery and selectivity, through novel capsids and promoters, coupled with more potent choices for the combinations of immunotherapy-based payloads seems capable of finally delivering innovative new gene therapies for oncology. Many pieces of the puzzle of how to build a virus capable of targeting human cancers appear to be falling into place.

Keywords: AAV; adenovirus; cancer; capsid engineering; directed evolution; enhancer; gene therapy; immunotherapy; oncolytics; promoter; rational design; therapeutic payloads.

Figure 1.

Summary of extrinsic and intrinsic strategies for engineering cancer-selective virotherapies. Extrinsic methods may encompass combinations of route of delivery, pseudotyping approaches, as well as knowledge-guided rational or semirational vector engineering approaches. Intrinsic selectivity may be achieved through combinations of tissue- or tumor-specific enhancer or promoter sequences, gene circuits, or alterations in early viral genes that enable selective viral replication within tumor cells.

Figure 2.

Strategies for the development of genetically re-targeted cancer virotherapy vectors. Knowledge of capsid protein structure can be harnessed to aid in the rational design of virotherapy vectors with novel binding interactions (1). Alternatively, a structure-free directed evolution approach may be employed, in which initial diverse pools of potential viral vectors are subjected to selection on cell populations of interest, either in vivo or in vitro. Virions recovered from this can be sequenced and further diversified (e.g., by error-prone PCR) followed by use as the input for subsequent rounds of selection. After multiple rounds, viruses with improved replication and infection kinetics in the cell line of interest can be isolated (2). These two approaches can also be combined in a powerful semirational design approach. Here, structural knowledge can guide the insertion of random targeting molecule libraries into permissive regions of the viral capsid. Insertions that improve cancer-selective targeting can be selected for by subjecting the resulting viral pool to high-throughput directed evolution screening (3). PCR, polymerase chain reaction.

Figure 3.

Arming cancer virotherapies with therapeutic payloads. Cancer virotherapy vectors can mediate efficient delivery of therapeutic transgenes that target multiple different aspects of tumor cell biology. This includes factors that inhibit angiogenesis, re-invigorate the anticancer immune response, promote tumor cell lysis through localized activation of cytotoxic drugs, and target the dense immunosuppressive stroma.