75. Molecular Imaging of an EGFP-DRD2-Expressing Vaccinia Virus

Abstract

Background: As oncolytic vectors for cancer therapy come to clinical trials, noninvasive monitoring of vector biodistribution will be necessary. Oncolytic vaccinia viruses (rVV) preferentially target, infect and replicate in tumours, leading to an amplification of expressed genes, and ultimately protein products, which can be targets for imaging. The creation of a vaccinia virus vector which will both treat tumours and allow non-invasive imaging is a novel and exciting concept. Purpose: To develop non-invasive strategies of monitoring rVV in vivo and to investigate the sensitivity of imaging, earliest time point and persistence of gene expression. Methods: Athymic mice were inoculated subcutaneously with 105 murine colon cancer cells (MC-38). Upon tumor formation (5–7 mm), increasing doses of vvDD-EGFP-DRD2 were injected intraperitoneally. Six days later, mice were anesthetized and imaged. To determine the earliest time-point of EGFP detection and persistence, mice implanted with tumors and injected with virus were imaged 48 hours post-injection of the virus and then every 24 hours thereafter until fluorescence of protein products (EGFP) completely dissipated (optical imaging). In vitro binding studies were done to confirm that the radioligand 123I[IBF] specifically binds tumour cells infected with the DRD2-expressing virus. In vivo studies are currently being performed to examine biodistribution and specificity of the 123I[IBF] in nude mice after delivery of vvDD-EGFP-DRD2. Results: EGFP expression intensified with increasing viral dose to a mean maximum intensity of 63 52 units/cm2 of tumour area (at 109 pfu). Interestingly, fluorescence intensity varied inversely with tumour size. Mice injected with a control virus showed no fluorescence. Gene expression was detected as early as 48 hrs post viral injection, peaked at 9d and slowly dissipated by day 30. Physical tumour size was measured and correlated with the intensity of gene expression. In vitro binding studies confirmed that the virally-expressed DRD2 receptor has a high affinity for 123I[IBF], and in vivo studies are on-going. Conclusions: Optical and nuclear imaging of rVV expressing EGFP and DRD2 provides a non-invasive assessment of vector biodistribution and persistence, and potentially will be able to image tumour responses. Thus, we feel oncolytic vaccinia virus is a promising vector for cancer therapy.