Stealth Virus for Cancer Therapy

The promise of gene therapy to treat cancer has always been limited by the vector used to deliver the rectified genetic payload. While off-targeting has always been a major concern, no less of an issue is the host immune system destroying the therapeutic virus before it has had a chance to complete its mission.

Now, researchers from the University of Zurich have engineered a protein coat shielding for the most commonly used vector in clinical gene therapy, human adenovirus type 5, for improved cancer drug delivery. The findings from the new study were released in Nature Communications, in an article entitled “Adenoviral Vector with Shield and Adapter Increases Tumor Specificity and Escapes Liver and Immune Control.”

There are innumerable different viruses that can be utilized for gene therapy efforts, but the human adenovirus 5, which normally causes the symptoms of a typical cold, has one substantial advantage: Its genome can be replaced completely by an artificial one that contains only “useful” genes. Without any of the viral genes left, the virus can no longer replicate and trigger diseases. In addition, the genome of the adenovirus is very large and does not integrate into human chromosomes.

Until recently, the use of adenoviruses in tumor therapy has been very limited. They lack the ability to infect cancer cells and therefore cannot inject the genetic blueprints for the therapeutic molecules to fight the disease. Moreover, adenoviruses are efficiently neutralized by the immune system and very rapidly eliminated by the liver. This new study shows how researchers succeeded in rebuilding these viruses so that they effectively recognize and infect tumor cells. “For this purpose, we have created molecules that act as an adapter between the virus and the tumor cell,” explained lead study investigator Markus Schmid, Ph.D., professor in the department of biochemistry at the University of Zurich.

The adapters, which cling very tightly to the coat of the virus, can—depending on their version—bind to different surface molecules on the tumor cell. The scientists tested adapters for several receptors such as human epidermal growth factor receptor 2 (HER2) and epidermal growth factor receptor (EGFR), which are present on several types of cancer cells. Only viruses equipped with these adapters could infect the tumor cells.

“[We engineered] a high-affinity protein coat, shielding the most commonly used vector in clinical gene therapy, human adenovirus type 5,” the authors wrote. “Using electron microscopy and crystallography we demonstrate a massive coverage of the virion surface through the hexon-shielding scFv fragment, trimerized to exploit the hexon symmetry and gain avidity. The shield reduces virion clearance in the liver. When the shielded particles are equipped with adaptor proteins, the virions deliver their payload genes into human cancer cells expressing HER2 or EGFR.”

The researchers subsequently hid the virus under a novel protein coat, which serves as camouflage for the virus and which protects it from the immune system. As a basis for this shield, the researchers used an existing antibody that they redesigned.

“When the shielded particles are equipped with adaptor proteins, the virions deliver their payload genes into human cancer cells expressing HER2 or EGFR,” the authors explained. “The combination of shield and adapter also increases viral gene delivery to xenografted tumors in vivo, reduces liver off-targeting and immune neutralization. Our study highlights the power of protein engineering for viral vectors overcoming the challenges of local and systemic viral gene therapies.”

Interestingly, the shield not only protects the redesigned virus from the immune cells but also prevents the virus from being eliminated by the liver, which normally quickly removes unmodified adenoviruses from the bloodstream, often making therapeutic applications impossible. With its shield and its adapter, these viral gene shuttles efficiently infected tumor cells in laboratory animals.

Using these stealth gene shuttles, the UZH scientists want to develop novel therapies for several types of cancer. The numerous advantages of adenoviruses will likely help to tackle one of the greatest problems of cancer medicine—the development of resistance against drugs.

“With this gene shuttle, we have opened up many avenues to treat aggressive cancers in the future, since we can make the body itself produce a whole cocktail of therapeutics directly in the tumor,” concluded senior study investigator Andreas Plückthun, Ph.D., director in the department of biochemistry at the University of Zurich.