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New hybrid virus provides targeted molecular imaging of cancer

April 24, 2006

HOUSTON-Researchers at The University of Texas M. D. Anderson Cancer Center have created a new class of hybrid virus and demonstrated its ability to find, highlight, and deliver genes to tumors in mice.

Researchers say the advance, reported in the journal Cell, is potentially an important step in making human cancer both more visible and accessible to treatment; it may also allow prediction and monitoring of how specific anti-cancer agents are actually working.

"In tumor-bearing mice, we show that this hybrid virus can target tumors systemically to deliver an imaging or therapeutic gene," says the co-leader of the study, Renata Pasqualini, Ph.D., professor of Medicine and Cancer Biology. "The signal is specific only to tumors, so one can monitor drug effectiveness at the molecular level."

The team created and characterized the hybrid viruses by combining genetic elements and biological attributes of an animal virus (adeno-associated virus, or AAV) with those of a bacterial virus (phage). Unlike animal viruses that infect mammalian cells, bacterial viruses have evolved to infect only bacterial hosts. The paper shows how particles of the hybrid virus, called AAV phage or AAVP, can serve as a vehicle for targeted delivery of genes to experimental tumors in mice and to the tumors' blood vessel supply, providing a strategy for finding tumors and genetically marking them for imaging on a clinic-ready body scanner.

The AAVP hybrid combines the ability of the bacterial virus to target specific tissues with the capability of the mammalian virus to actually deliver genes to cells. The crucial vehicles, or vectors, in the AAVP hybrid retained the properties of their respective parental viruses, which the researchers called a surprising outcome.

"This is only a proof-of-concept, and although we have yet to translate these hybrid viruses for use in humans, we hope that this new system will have future clinical applications," says Wadih Arap, M.D., the co-leader of the study and professor of Medicine and Cancer Biology. "In addition to the obvious biological interest, when the vector is refined for patient use, it could perhaps help us diagnose, monitor and treat human tumors more accurately."

The finding is the latest in a series of studies by Pasqualini and Arap that are built around their discovery that the human vasculature system contains unique molecular addresses. Organs and specialized tissues also have specific "zip codes" on their blood vessels, as do tumor blood vessels. Knowing this, Pasqualini and Arap designed, constructed, evaluated, and validated the targeted AAVP system over the past several years. Amin Hajitou, Ph.D., a post-doctoral fellow in the Arap/Pasqualini laboratory and first author of the Cell study says, "we were pleased by the strong effects of gene transfer in mouse models of common diseases such as breast and prostate cancer."

Their next step was to work closely with the team of M. D. Anderson researcher Juri Gelovani, M.D., Ph.D., chair of the Department of Experimental Diagnostic Imaging, a pioneer in development of molecular-genetic imaging tools.

"We could see by using positron emission tomography that the reporter and therapeutic genes were being expressed throughout the tumors in the animals," Gelovani says. "This is an example of the so-called "theragnostic" approach, a combination of the words therapeutic and diagnostic."

Next, the international collaborative research team plans to evaluate the safety and efficacy of other hybrid vectors in animal models. The ultimate goal is to adapt and optimize the AAVP-based targeting prototype for use in patients.

University of Texas M. D. Anderson Cancer Center