Reprogrammed viruses as cancer therapeutics: targeted, armed and shielded

Abstract

Virotherapy is currently undergoing a renaissance, based on our improved understanding of virus biology and genetics and our better knowledge of many different types of cancer. Viruses can be reprogrammed into oncolytic vectors by combining three types of modification: targeting, arming and shielding. Targeting introduces multiple layers of cancer specificity and improves safety and efficacy; arming occurs through the expression of prodrug convertases and cytokines; and coating with polymers and the sequential usage of different envelopes or capsids provides shielding from the host immune response. Virus-based therapeutics are beginning to find their place in cancer clinical practice, in combination with chemotherapy and radiation.

Figure 1. Oncolytic viruses that are currently used in cancer clinical trials

The major characteristics of seven families of oncolytic viruses are summarized. Recombinant strains of all the DNA viruses shown are currently in clinical trials, whereas among the RNA viruses shown, only recombinant MV is in clinical trials; recombinant poliovirus and VSV are in pre-clinical trials and non-engineered strains of reovirus and NDV are in Phase I–II clinical trials. HSV1, herpes simplex virus 1; MV, measles virus; NDV, Newcastle disease virus; VSV, vesicular stomatitis virus.

Figure 2. Four layers of specificity for retargeting viral tropism

a | Virus particle activation can be reprogrammed to depend on proteases that are secreted by cancer cells. Activation occurs through matrix metalloproteases (MMPs) that are located in the tumour matrix. b | Recombinant viruses can be engineered to enter cells through a designated receptor rather than through the natural attachment protein. c | Viral transcription and replication can be made dependent on tissue- or cancer-specific promoters. d | Viruses with modifications or deletions of their immune-evasion proteins replicate preferentially in certain transformed cells. Not every targeting strategy can be applied to every virus, but more and more viruses with combined layers of specificity are being engineered for specific clinical trials.

Figure 3. MV particle structure, genome organization and targeting approaches

The MV genome has six genes that code for eight proteins. The first gene codes for the nucleocapsid protein (N) that encapsidates the genomic RNA. The last gene (L) codes for the polymerase protein that replicates and transcribes the genome together with the phosphoprotein (P), a polymerase cofactor. The matrix protein (M) organizes virus particle assembly. The two glycoproteins haemagglutinin (H) and the fusion (F) protein contact the receptor and execute membrane fusion, respectively. Two non-structural proteins that are coded by the P gene (C and V) control the innate immune response. a | Three-dimensional structure of MV H. Residues that are necessary for signalling lymphocytic activation molecule (SLAM)-dependent or CD46-dependent fusion are in red. The site of addition of the single-chain fragment variable (scFv) is in black. b | A schematic of the P and V proteins that are encoded by the P gene. These proteins share their amino-terminal domain, but differ at the carboxyl terminus. The residues of the V and P common domain that are important for the interaction with STAT1 (signal transducer and activator of transcription 1) have been characterized. Three amino acids in a conserved hexapeptide are shown in purple. Data from the Horvath group indicate that STAT2 and MDA5 interact with different sequences in the unique cysteine-rich domain of V (A. Ramachandran, J-P. Parisien and C.M. Horvath, unpublished observations). c | Schematic of the MV F protein and amino acid sequences of its cleavage site. The standard F protein is cleaved into F₁ and F₂ fragments by furin, a ubiquitous protease. Furin cleavage occurs even after a hexameric peptide that codes for a matrix metalloproteinase 2 (MMP2) cleavage site is introduced (F-MMP), but the resulting F₁ protein, which is extended by six residues, is inactive. Trimming of three amino-terminal residues by MMP2 cleavage confers function to F-MMP.

Figure 4. Adenovirus particle structure, genome organization and targeting approaches

An icosahedral, non-enveloped adenovirus (Ad) particle is shown. The key genes in the viral genome that are relevant to the four targeting approaches discussed in the main text are indicated. a | Cancer-specific transcription and replication targeting are applied to the E1 and E4 genes. b | Cancer-specific proteolytic activation has not yet been attempted. c | Cancer-specific receptor attachment is mediated by genetic or chemical modification of the IX, penton, hexon or fibre proteins or genes. d | Preferential spread targeting can be based on over-expression of the ADP gene and the insertion of exogenous genes into the viral genome. ITR, inverted terminal repeat; MMP, matrix metalloproteinase; Pro, protease.

Figure 5. Strategies to improve oncolytic virus efficacy

a | Shielding the virus against antibodies. Pre-existing neutralizing antibodies in humans can interfere with efficacy. Changing virus serotypes and coating particles with shielding polymers can address the neutralizing-antibody problem. b | Transient immunosuppression of the host. Infected cells can be attacked by macrophages, T cells and natural killer cells. Transient immunosuppression interferes with the activation and ability of these cells to recognize and/or kill infected cells and restrict oncolytic efficacy.