Douglas E. Brough Ph.D. CSO GenVec
Making a Comeback in the 21st Century
Adenovectors are hot again. Two decades after the initial flurry of excitement about gene-based medicine, researchers and the media are once again enthusiastic about the promise of the field to deliver an array of medical breakthroughs. Driving this renaissance are a host of advances—such as the development of CRISPR/Cas9 and other gene-editing technologies such as TALEN, Zinc Finger, and ARCUS—that offer an unprecedented level of accuracy for researchers who seek to modify the genome. And along with these techniques has come a new generation of adenovectors. Also known as viral vectors, these are viruses molecularly engineered to carry specific genes or antigens into the body. The new generation of vectors appears to eliminate certain drawbacks that slowed down progress in the field for years.
Promise and Pitfalls of Early Adenovectors
Adenovectors have been developed as gene delivery vehicles since the early 1980s, with their potential as in vivo tools supported by the work of many researchers. Most early vectors were based on human adenovirus serotypes 5 (Ad5) and 2 (Ad2) of species C1. It was recognized that deleting the E1 and/or E3 regions of their genomes rendered these vectors replication-deficient and able to propagate only in certain cell lines. Among other advantages, these vectors were found to have the ability to infect both dividing and non-dividing cells and to deliver their gene payload effectively to cell nuclei2.
Unfortunately, a series of drawbacks soon manifested themselves. One of the advantages of early adenovectors was that the key cell surface receptors are present on a large number of cells; however, this precluded targeting the vectors to specific cell types3. Conversely, some cell types representing important targets for gene transfer expressed only low levels of cellular receptors, leading to inefficient infection. In addition, a disturbing trend of inflammatory-type toxicities was observed, based on the activation of immune responses to the vectors4. Overcoming the immunogenicity associated with viral vectors represented a major challenge.
In addition to these hurdles were a handful of high-profile case studies that tarnished the initial promise of the field. In 1999, for example, 18-year-old Jesse Gelsinger, who suffered from a liver enzyme deficiency, died four days after treatment with gene therapy. It was determined that the administration of an adenovector had triggered a massive inflammatory response leading to multi-organ failure5. And in 2007, the Merck Ad5/HIV Step Study was halted early when patients who were previously Ad5-seropositive were found to have an increased risk of HIV-1 acquisition6. The case is still being debated in the scientific literature.
What’s New and on the Horizon
Today, advances in adenovector design have substantially improved their therapeutic potential and are being applied in many areas. Let us consider one specific potential application: treatment of hearing loss. To restore hearing, it is necessary to generate new functional hair cells in the inner ear; one potential route to cell regeneration is to induce a phenotypic transdifferentiation of non-sensory cells that remain in the deaf cochlea. Research has determined that the Atoh1 (atonal) gene induces regeneration of hair cells and substantially improves hearing thresholds in the mature deaf inner ear after delivery to non-sensory cells. The data suggest a new therapeutic approach through the use of adenovectors7.
Additional research has resulted in the development of a mouse model of vestibular aminoglycoside ototoxicity and demonstrated that delivery of an advanced-generation adenovector expressing Atoh1 results in the regeneration of vestibular hair cells; furthermore, mice treated with Atoh1 have been found to recover balance function8. Balance disorders resulting from vestibular hair cell loss are debilitating, so regeneration of these cells is being studied as a route to therapy and early results are promising9. This work is especially important given the fact that there are currently no dedicated pharmaceuticals that target the inner ear, so the development of a series of adenovectors that transduce the inner ear could be used as the basis of a new type of gene therapy10.
This type of therapy is made possible by a new generation of adenovectors that exhibit excellent safety profiles, facilitated by additional viral genome deletions compared to first-generation vectors. These new vectors also exhibit efficient transduction in dividing and non-dividing cells, a low probability of disturbance of vital cellular genes, and the ability to be grown in high titers. In addition, some more recently discovered vectors have no or very low seroprevalence in the human population. Techniques have been perfected for adjusting the tropism of adenovectors, thus allowing for more cell-specific gene delivery, while the additional deletions to the viral genome facilitate large packaging capacity. The new generation of adenovectors could potentially form the basis of a platform with applications not only in therapeutic gene delivery but also in a host of other fields such as vaccines, regenerative medicine, cell therapy, oncolytics, gene editing, immunotherapy and nucleic acid therapeutics.
For example, recent research has focused on the use of gorilla adenovectors with distinct advantages for molecular vaccines, including high-level durable antibody responses and high-level T-cell responses from a single administration11. The research suggests these new adenovirus serotypes, which were isolated from the wild gorilla and are similar to species C human adenovirus, have very low seroprevalence in human populations. Work to date with these vectors, involving challenges using respiratory syncytial virus (RSV) and other agents in animal models, has proven promising as a method of boosting immunogenicity.
It seems as if the gene-based medicine revolution may finally be here for good—and advances in the effective use of adenovectors for new therapeutics and vaccines are a major component of it all.
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6 Michael NL. Rare serotype adenoviral vectors for HIV vaccine development. J Clin Invest. 2012 Jan:122(1):25-27.
7 Izumikawa M, Minoda R, Kawamoto K, et al. Auditory hair cell replacement and hearing improvement by Atoh1 gene therapy in deaf mammals. Nat Med. 2005 Mar;11(3):271-276.
8 Baker K, Brough DE, Staecker H. Repair of the vestibular system via adenovector delivery of atoh1: a potential treatment for balance disorders. In Ryan AF (ed): Gene Therapy of Cochlear Deafness. Adv Otorhinolaryngol. Basel, Karger 2009;66:52-63.
9 Schlecker C, Praetorius M, Brough DE. Selective atonal gene delivery improves balance function in a mouse model of vestibular disease. Gene Ther. 2011 Sep;18(9):884-890.
10 Staecker H, Praetorius M, Brough DE. Development of gene therapy for inner ear disease: using bilateral vestibular hypofunction as a vehicle for translational research. Hear Res. 2011 Jun;276(1-2):44-51.
11 Brough DE. Gorilla adenovirus vectors for molecular therapeutics and vaccines. Vaccines R&D Conference. November 2015.