Synthesis and evaluation of 18F- and 11C-labeled phenyl-galactopyranosides as potential probes for in vivo visualization of LacZ gene expression using positron emission tomography.

3-Hydroxy-2-nitrophenyl 2,3,4,6-tetra-O-acetyl-beta-D-galactopyranoside, a derivative of the chromogenic beta-galactosidase (beta-gal) substrate o-nitrophenyl beta-D-galactopyranoside (ONPG) was synthesized using a Koenigs-Knorr glycosylation reaction. It was alkylated with 2-[(18)F]fluoroethyl triflate or [(11)C]methyl triflate, followed by deacetylation of the sugar hydroxyl groups to obtain radiolabeled 3-(2'-[(18)F]fluoroethoxy)-2-nitrophenyl beta-D-galactopyranoside ([(18)F]-2c) and 3-[(11)C]methoxy-2-nitrophenyl beta- d-galactopyranoside ([(11)C]-3c), which were evaluated as potential reporter probes for in vivo visualization of LacZ gene expression with positron emission tomography (PET). In vitro, [(18)F]- 2c and [(11)C]-3c were good substrates of beta-gal and showed, respectively, a 7.5- and 2.5-fold higher uptake into beta-gal expressing cells (LacZ cells) compared to control cells. However, reversed-phase HPLC analysis of the LacZ cell lysate and supernatant showed that labeled 3-(2'-[(18)F]fluoroethoxy)-2-nitrophenol, the hydrolysis product formed by beta-gal-mediated cleavage of [(18)F]-2c, substantially leaked out of the cells, which would lead to loss of PET signal. In a microPET study of [(18)F]-2c in a mouse with a beta-gal expressing tumor, high retention was observed in liver and kidneys, but only negligible accumulation was seen in the tumor. As a general conclusion, it can be stated that the synthesized PET tracers [ (18)F]-2c and [(11)C]-3c are not suitable for use as LacZ reporter probes. Further structural modifications to improve the diffusion over the tumor cell membrane and to increase retention in beta-gal expressing cells may lead to more favorable in vivo imaging probes.