oncolytic adenovirus oncolytic adenovirus adenovirus varieties have been explored extensively as a viral vector for gene therapy and also as an oncolytic virus. further information: 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 (sfda) 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 (onyx-015), could replicate selectively in p53 deficient cells. a conditionally replicative adenovirus (crad) with a 24 base pair deletion in the retinoblastoma-binding domain of the e1a protein (ad5- δ24e3), 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 (ad5), whose binding to host cells is initiated by interactions between the cellular coxsackie virus and adenovirus receptor (car), 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 bi-specific adapter molecules can be administered along with the virus to redirect viral coat protein tropism. these molecules are fusion proteins that are made up of an antibody raised against the knob domain of the adenovirus coat protein, fused to a natural ligand for a cell-surface receptor. the use of adapter molecules has been shown to increase viral transduction. however, adapters add complexity to the system, and the effect of adapter molecule binding on the stability of the virus is uncertain. coat-protein modification this method involves genetically modifying the fiber knob domain of the viral coat protein to alter its specificity. short peptides added to the c-terminal end of the coat protein successfully altered viral tropism. the addition of larger peptides to the c-terminus is not viable because it reduces adenovirus integrity, possibly due to an effect on fiber trimerisation. the fiber protein also contains an hi-loop structure, which can tolerate peptide insertions of up to 100 residues without any negative effects on adenovirus integrity. an rgd motif inserted into the hi loop of the fiber knob protein, shifts specificity toward integrins, which are frequently over-expressed in oesophageal adenocarcinoma. when combined with a form of non-transductional targeting, these viruses proved to be effective and selective therapeutic agents for oesophageal adenocarcinoma. transcriptional targeting this approach takes advantage of deregulated promoter to drive and control the expression of adenoviral genes. for instance, cyclooxygenase-2 enzyme (cox-2) expression is elevated in a range of cancers, and has low liver expression, making it a suitable tumour-specific promoter. adcox2lluc is a crad targeted against oesophageal adenocarcinoma by placing the early genes under the control of a cox-2 promoter (adenoviruses have two early genes, e1a and e1b, that are essential for replication). when combined with transductional targeting, adcox2lluc showed potential for treatment of oesophageal adenocarcinoma. cox-2 is also a possible tumour-specific promoter candidate for other cancer types, including ovarian cancer. a suitable tumour-specific promoter for prostate cancer is prostate-specific antigen (psa), whose expression is greatly elevated in prostate cancer. cn706 is a crad with a psa tumour-specific promoter driving expression of the adenoviral e1a gene, required for viral replication. the cn706 titre is significantly greater in psa-positive cells. post-transcriptional detargeting oncolytic adenovirus controlled by microrna response element another layer of regulation that has emerged to control adenoviral replication is the use of micrornas (mirna) artificial target sites or mirna response elements (mres). differential expression of mirnas between healthy tissues and tumors permit to engineer oncolytic viruses in order to have their ability to replicate impaired in those tissues of interest while allowing its replication in the tumor cells. tissue/cell-typeenriched mirnause of the mre