Establishing the Lysine-rich Protein CEST Reporter Gene as a CEST MR Imaging Detector for Oncolytic Virotherapy

Abstract

Purpose:To (a) evaluate whether the lysine-rich protein (LRP) magnetic resonance (MR) imaging reporter gene can be engineered into G47D, a herpes simplex-derived oncolytic virus that is currently being tested in clinical trials, without disrupting its therapeutic effectiveness and (b) establish the ability of chemical exchange saturation transfer (CEST) MR imaging to demonstrate G47D-LRP. Materials and Methods:The institutional subcommittee for research animal care approved all in vivo procedures. Oncolytic herpes simplex virus G47D, which carried the LRP gene, was constructed and tested for its capacity to replicate in cancer cells and express LRP in vitro. The LRP gene was detected through CEST imaging of lysates derived from cells infected with G47D-LRP or the control G47D-empty virus. G47D-LRP was then tested for its therapeutic effectiveness and detection with CEST MR imaging in vivo. Images of rat gliomas were acquired before and 8-10 hours after injection of G47D-LRP (n = 7) or G47D-empty virus (n = 6). Group comparisons were analyzed with a paired t test. Results:No significant differences were observed in viral replication or therapeutic effectiveness between G47D-LRP and G47D-empty virus. An increase in CEST image contrast was observed in cell lysates (mean 6 standard deviation, 0.52% 6 0.06; P = .01) and in tumors (1.1% 6 0.3, P = .02) after infection with G47D-LRP but not G47D-empty viruses. No histopathologic differences were observed between tumors infected with G47D-LRP and G47D-empty virus. Conclusion:This study has demonstrated the ability of CEST MR imaging to show G47D-LRP at acute stages of viral infection. The introduction of the LRP transgene had no effect on the viral replication or therapeutic effectiveness. This can aid in development of the LRP gene as a reporter for the real-time detection of viral spread.q RSNA, 2015

Cellular and Molecular Imaging Using Chemical Exchange Saturation Transfer

“…This has been demonstrated for the LRP CEST reporter gene; following the initial study where it was shown that the contrast generated from LRP can be differentiated from controls, its practicality has been repeatedly demonstrated in live rodents and enabled collection of high temporal and spatial resolution images (76,77). Farrar and colleagues have confirmed that LRP can be visualized in live rats after delivery of an oncolytic virus, thus serving as a marker for treatment efficacy (78) (Fig. 4).…”

Going Deeper: Biomolecular Tools for Acoustic and Magnetic Imaging and Control of Cellular Function

“…This “catalytic” contrast scheme is called chemical exchange saturation transfer, or CEST. Proteins detectable with this method include a synthetic lysine-rich protein 29 and human protamine 30 .…”

Biomolecular MRI reporters: Evolution of new mechanisms

“…However, in in vivo applications, biomolecular CEST agents must also overcome background CEST contrast from endogenous proteins, making their detection at such low concentrations more difficult. Despite this limitation, protein-based CEST reporters have been used in several in vivo applications, and innovative pulse sequences and processing schemes have been developed to help maximize reporter-specific contrast [38–40]. …”