Comparative Analysis of Human Nucleoside Kinase-Based Reporter Systems for PET Imaging

Purpose: Radionuclide-based reporter gene imaging has the sensitivity to monitor gene- and cell-based therapies in human subjects. Potential immunogenicity of current viral transgenes warrants development of human-based reporter systems. We compared human nucleoside kinase reporters to a panel of nucleoside analogs of FEAU, FMAU, and FIAU, including the first in vivo assessment of L-[¹⁸F]FEAU.

Procedures: Human isogenic U87 cell lines were transduced to express different human reporter genes including dCK-R104M/D133A (dCKDM), dCK-R104Q/D133N (dCKep16A), dCK-A100V/R104M/D133A (dCK3M), and TK2-N93D/L109F (TK2DM), and wild-type dCK (dCK) and herpes simplex virus type-1 (HSVTK) reporter gene as references. In vitro cell uptake assays were performed with [¹⁸F]FEAU, L-[¹⁸F]FEAU, [¹⁴C]FMAU, L-[¹⁸F]FMAU, and [¹²⁴I]FIAU. Micro-positron emission tomography/X-ray computed tomography imaging of xenograft-bearing nu/nu mice was conducted with [¹⁸F]FEAU, L-[¹⁸F]FEAU, L-[¹⁸F]FMAU, and [¹²⁴I]FIAU on consecutive days. A cell viability assay was also performed to assess sensitivities to gemcitabine and bromovinyldeoxyuridine (BVdU).

Results: In vitro, dCKep16A and dCKDM with [¹⁸F]FEAU exhibited the highest sensitivity and selectivity of the human reporters, second only to HSVTK/[¹⁸F]FEAU. L-[¹⁸F]FEAU biodistribution in mice was on par with [¹⁸F]FEAU and L-[¹⁸F]FMAU. L-[¹⁸F]FMAU uptake in isogenic xenografts was highest for all human reporter genes. However, [¹⁸F]FEAU was the most selective of the short half-life reporter probes due to its minimal recognition by human dCK and relative sensitivity, whereas [¹²⁴I]FIAU permitted imaging at a later time point, improving signal-to-background ratio. Of the human reporter genes, dCKep16A consistently outperformed the other tested reporters. Reporter genes of interest increased potency to the nucleoside analog prodrugs gemcitabine and BVdU.

Conclusions: We demonstrate that human nucleoside kinase reporter systems vary significantly in their sensitivity and selectivity for in vivo imaging. The sufficiently high signal-to-background ratios and enhanced suicide gene potential support clinical translation.