An ultimate goal of the developers of cancer genetherapy/virotherapy is to develop
a device enabling systemic treatment of the patients with advanced or spread diseases.
The targeting at the level of infection, which can define the initial distribution following intravenous
administration, is considered to be the most important for realization of systemic
therapy. Adenovirus which has very high in vivo transduction efficiency is an attractive vector
for designing novel cancer therapeutics for systemic therapy, and we and others have
been working on this issue in the field of oncolytic adenovirus (OAd). However, the success rate of incorporation
of targeting ligand to OAd without losing potency has been extremely low. We thus decided to run a
screening of the ligand sequence in the adenoviral capsid format, and chose to replace 7 amino acids sequence
mediating the initial binding of the virus to the receptor of adenovirus. By modifications and optimization of
the production procedure, we achieved >1010 diversity, which permits the full coverage of 7 random amino
acid library. Our high-throughput screening system is powerful enough to enable the screen of the adenovirusformat
high-diversity library, and identified the specific targeting ligand in a highly efficient manner.
In in vivo models, Infectivity Selective OAd (ISOAd) showed strong and selective anti-tumor effect in the
xenotrafts of MSLN-positive pancreatic cancer cell line. The intravenously injected ISOAd targeting mesothelin
showed a surprisingly strong antitumor effect, which was equivalent or stronger than that of intratumoral
injection. These data indicate the possibility of systemic clinical application with ISOAd. While OAd is
a very promising direction, the application of this novel series of vectors is not limited to OAd. Our screening
strategy also produces many targeting ligands for therapeutic vector development. Our novel ligand identification
system has broad implications to various purposes, and further development is anticipated.