esearchers screened a wide variety of drugs to see if any of them could reawaken the virus. They were fortunate in that one of the genes that is expressed upon viral lytic induction is EBV's thymidine kinase (EBV-TK), an enzyme that helps the virus begin to reproduce. This kinase is of interest because researchers know its "sister" kinase, the one produced by the herpes simplex virus, can be imaged by an injected radiolabeled chemical (FIAU), which can then be imaged using a gamma camera.
"To perform molecular-genetic imaging, we have always had to infect cells with active herpes simplex virus so that they can replicate, express TK, and only then could we use the FIAU tracer to make the cells light up," Pomper says. "So we were hoping to find a way to turn latent Epstein-Barr virus on in these cancers, and use the thymidine kinase it then produces to enable us to see the virus-associated tumors with radiolabeled FIAU."
The researchers screened 2,700 agents until they hit upon Velcade, a targeted chemotherapy drug already approved for use in multiple myeloma. "We were both surprised and lucky," he says. "Velcade is a proteasome inhibitor, but it also induces the lytic cycle thereby activating the TK in the Epstein-Barr virus. Once the TK is activated, we can image the tumors."
To test their findings, the researchers used mice carrying human Burkitt's lymphoma, a cancer often associated with Epstein-Barr viral infection. Tumors glowed in mice given Velcade followed by an injection of FIAU, but not in mice that weren't given Velcade. Mice whose Burkitt's lymphoma did not contain Epstein-Barr virus also did not respond to either Velcade or FIAU, according to researchers.
"Velcade woke up the virus in the tumors, which increased viral load by 12-fold, all the while cranking out TK," Pomper says. "An injection of FIAU made it easy to image the tumors with virus in them."
The method is highly sensitive, he says: as few
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