Date Published: November 14, 2012
Publisher: Hindawi Publishing Corporation
Author(s): M. H. Verheije, P. J. M. Rottier.
Oncolytic virus therapy is based on the ability of viruses to effectively infect and kill tumor cells without destroying the normal tissues. While some viruses seem to have a natural preference for tumor cells, most viruses require the modification of their tropism to specifically enter and replicate in such cells. This review aims to describe the transductional targeting strategies currently employed to specifically redirect viruses towards surface receptors on tumor cells. Three major strategies can be distinguished; they involve (i) the incorporation of new targeting specificity into a viral surface protein, (ii) the incorporation of a scaffold into a viral surface protein to allow the attachment of targeting moieties, and (iii) the use of bispecific adapters to mediate targeting of a virus to a specified moiety on a tumor cell. Of each strategy key features, advantages and limitations are discussed and examples are given. Because of their potential to cause sustained, multiround infection—a desirable characteristic for eradicating tumors—particular attention is given to viruses engineered to become self-targeted by the genomic expression of a bispecific adapter protein.
Cancer is one of the major health problems of our times. Though the prognosis for people diagnosed with, at least some forms of, cancer has increased considerably, it is more typical a disease of which treatment is initially effective, to be followed later by an irreversible and eventually fatal relapse. Already for decades, cancer treatment is based on three types of approaches: surgery, radio-, and chemotherapy. While the scientific and technological advancements have improved the efficacy of each of these classical approaches tremendously, and while also some new therapies have evolved including immunotherapy, the treatments apparently fail to eradicate all residual tumor cells or metastases completely. Therefore, additional means are urgently required to support or replace the conventional therapies. Hence, a variety of new approaches is currently being explored, one of which is based on the use of viruses.
The most popular approach to generate oncolytic viruses has been by adapting their surface-exposed components. Viral surface proteins can be modified to express ligands that bind to receptors preferentially or exclusively expressed on tumor cells. Viruses can be genetically adapted to express those modifications to redirect them towards tumor cells (Figures 1(a) and 1(b)). The main advantage of this strategy is that the targeting specificity is inherent to the viral genome and will, thus, be maintained upon replication. Progeny virus is then able to infect neighboring cells harboring the target receptor, thereby establishing a multiround infection that will be maintained until no further tumor target cells remain. For this strategy, the ability to genetically modify the viral genome is crucial. Furthermore, detailed structural information about the viral surface protein to be modified is indispensible to predict at which location targeting motifs might be tolerated and will be exposed. Such motifs should not only allow binding of the modified virion to the cells but should also not be detrimental to the entry mechanism of the particular virus, not interfering for instance with the fusion of viral and cellular membrane. In addition, the targeting ligand introduced into the viral protein will have to meet size limitations. Small peptides, thus, seem the first and most obvious choice. The development of targeting strategies of viruses is, however, severely limited by a shortage of naturally existing molecules available for use as targeting ligands. Therefore, other sources of binding ligands have been investigated and incorporated into viral proteins for this purpose. These include (parts of) antibodies, like scFvs (single-chain variable fragments, composed of a fusion of the variable regions of the heavy (VH) and light chains (VL) of an immunoglobulin) or Fabs (antigen-binding fragments, composed of one constant and one variable domain from each heavy and light chain of the antibody). The feasibility of modifying viral coat proteins has been demonstrated for a number of viruses as is summarized below.
Rather than incorporating specific tumor targeting information into a viral surface protein, an alternative approach involves the incorporation of a scaffold moiety into such a protein to which subsequently various types of targeting modules can be linked (schematically depicted in Figure 1(c)). The main strategic difference relative to the previous method is that the moiety incorporated is not a tumor ligand itself but represents an attachment site for exogenously provided targeting moieties that, besides to the scaffold, also bind to the receptor of interest (compare Figures 1(b) and 1(c)). An essential operational limitation is that this targeting strategy provides viruses that can only establish single-round infection, remaining dependent on the external supply of the targeting module. Yet, it has the advantage of flexibility as the targeting device, binding always to the same, previously modified viral protein, can be changed relatively easy. Some of these strategies are based on antibody targeting, giving the opportunity to redirect the oncolytic virus to virtually every tumor surface epitope. A particular application based on this principle relies on the biotin-(strept)avidin-coupling method (reviewed in ).
Appealing as the idea of oncolytic virotherapy may be, its realization is still disconcertingly remote. What this overview emphatically reveals is that the field of retargeting of viruses for therapeutic use is in its early infancy. In fact, for the majority of the viruses studied, scientists are still struggling with the most fundamental aspects of changing their cell tropism. Robust platforms for retargeting have actually not been established yet for any of the viruses. On the positive side, the feasibility of retargeting was, at least in vitro, demonstrated for an increasing number of viruses in the past years. Clearly the most attractive goal will be to generate oncolytic viruses that carry the retargeting information in their genome. Only then will the viruses be able to sustain their replication in the tumor tissue, irrespective of the retargeting principle used, that is, whether through the modification of the viral attachment protein or through expression of an adapter protein. However, (pre)clinical data on the efficacy—let alone safety—of such transductionally targeted viruses in vivo are very limited. With one exception (a transductionally and transcriptionally targeted adenovirus ), none of such genetically modified, tropism modified oncolytic viruses have entered phase I clinical trial. This makes it virtually impossible to compare the viruses reviewed here. Thus, many hurdles have yet to be overcome before new oncolytic viruses will reach the clinic. Below, some of the important future tasks and challenges for the field are summarized.