Oncolytic adenovirus
varieties have been explored extensively as a viral vector for gene therapy and also as an oncolytic virus.
Of the many different viruses being explored for oncolytic potential, an adenovirus was the first to be approved by a regulatory agency, the genetically modified H101 strain. It gained regulatory approval in 2005 from China's State Food and Drug Administration for the treatment of head and neck cancer.
Engineering of oncolytic adenovirus
Adenoviruses have so far been through three generations of development. Some of the strategies for modification of adenoviruses are described below.Attenuation
For adenovirus replication to occur, the host cell must be induced into S phase by viral proteins interfering with cell cycle proteins. The adenoviral E1A gene is responsible for inactivation of several proteins, including retinoblastoma, allowing entry into S-phase. The adenovirus E1B55kDa gene cooperates with another adenoviral product, E4ORF6, to inactivate p53, thus preventing apoptosis. It was initially proposed that an adenovirus mutant lacking the E1B55kDa gene, dl1520, could replicate selectively in p53 deficient cells.A conditionally replicative adenovirus with a 24 base pair deletion in the retinoblastoma-binding domain of the E1A protein, is unable to silence retinoblastoma, and therefore unable to induce S-phase in host cells. This restricts Ad5-Δ24E3 to replication only in proliferating cells, such as tumour cells.
Targeting
The most commonly used group of adenoviruses is serotype 5, whose binding to host cells is initiated by interactions between the cellular coxsackie virus and adenovirus receptor, and the knob domain of the adenovirus coat protein trimer. CAR is necessary for adenovirus infection. Although expressed widely in epithelial cells, CAR expression in tumours is extremely variable, leading to resistance to Ad5 infection. Retargeting of Ad5 from CAR, to another receptor that is ubiquitously expressed on cancer cells, may overcome this resistance.- Adapter molecules
- Coat-protein modification
- Transcriptional targeting
- Post-Transcriptional detargeting
| Tissue/cell-type | Enriched miRNA | Use of the MRE | References |
| Liver | miR-122 | Prevent liver toxicity, hepatotoxicity | |
| Muscle | miR-133, miR-206 | Prevent muscle inflammation, myositis | |
| Pancreas | miR-148a | Promote pancreatic tumor targeting | |
| Prostate | miR-143, miR-145 | Promote prostate tumor targeting | |
| Neuron | miR-124 | Promote astrocyte targeting |
Examples
Oncorine (H101)
H101 and the very similar Onyx-015 have been engineered to remove a viral defense mechanism that interacts with a normal human gene p53, which is very frequently dysregulated in cancer cells. Despite the promises of early in vivo lab work, these viruses do not specifically infect cancer cells, but they still kill cancer cells preferentially. While overall survival rates are not known, short-term response rates are approximately doubled for H101 plus chemotherapy when compared to chemotherapy alone. It appears to work best when injected directly into a tumour, and when any resulting fever is not suppressed. Systemic therapy is desirable for treating metastatic disease. It is now marketed under the brand name Oncorine.Onyx-015 (''dl''1520)
Onyx-015 is an experimental oncolytic virus created by genetically engineering an adenovirus. It has been trialed as a possible treatment for cancer. The E1B-55kDa gene has been deleted allowing the virus to selectively replicate in and lyse p53-deficient cancer cells.Directed Evolution
Traditional research has focussed on species C Adenovirus serotype 5 for creating oncolytic vaccines for the potential use as cancer treatment. However, recent data suggests that it may not be the best virus serotype for deriving all oncolytic agents for treating human malignancies. For example, oncolytic vaccines based on the Ad5 serotype have relatively poor clinical efficacy as monotherapies. The need for increased potency has led to an expanded search involving a larger number of less well studied adenovirus serotypes.ColoAd1
One non-species C oncolytic adenovirus currently in development is ColoAd1. It was created using a process of “directed evolution”. This involves the creation of new viral variants or serotypes specifically directed against tumour cells via rounds of directed selection using large populations of randomly generated recombinant precursor viruses. The increased biodiversity produced by the initial homologous recombination step provides a large random pool of viral candidates which can then be passed through a series of selection steps designed to lead towards a pre-specified outcome without requiring any previous knowledge of the resultant viral mechanisms that are responsible for that outcome.One particular application of this approach produced ColoAd1, which is a novel Ad11p/Ad3 chimeric Group B oncolytic virus with specificity for human colon cancer and a broad spectrum of anti-cancer activity in common solid tumours. The therapeutic efficacy of ColoAd1 is currently being evaluated in three ongoing clinical trials.
ColoAd1 potency can be further enhanced via the use of therapeutic transgenes, which can be introduced into the ColoAd1 genome without compromising the selectivity or activity of the virus. Recent studies with ColoAd1 have shown a unique mechanism of cell death similar to Oncosis with expression of inflammatory cell death markers and cell membrane blistering and have highlighted mechanisms by which ColoAd1 alters host cell metabolism to facilitate replication.
Background
s form in cells when mutations in genes involved in cell cycle control and apoptosis accumulate over time. Most tumours studied, have defects in the p53 tumor suppressor pathway. p53 is a transcription factor that plays a role in apoptosis, cell cycle and DNA repair. It blocks cell progression in response to cellular stress or DNA damage. Many viruses replicate by altering the cell cycle and exploiting the same pathways that are altered in cancer cells. E1B proteins produced by adenoviruses protect the infected cell by binding to and degrading the p53 transcription factors, preventing it from targeting the cell for apoptosis. This allows the virus to replicate, package its genome, lyse the cell and spread to new cells.This gave rise to the idea that an altered adenovirus could be used to target and eliminate cancer cells. Onyx-015 is an adenovirus that was developed in 1987 with the function of the E1B gene knocked out, meaning cells infected with Onyx-015 are incapable of blocking p53's function. If Onyx-015 infects a normal cell, with a functioning p53 gene, it will be prevented from multiplying by the action of the p53 transcription factor. However, if Onyx-015 infects a p53 deficient cell it should be able to survive and replicate, resulting in selective destruction of cancer cells.