S-Gal®, a novel¹H MRI reporter for β-galactosidase

Reporter genes and associated enzyme activity are becoming increasingly significant for research in vivo. The lacZ gene and β-galactosidase (β-gal) expression have long been exploited as reporters of biologic manipulation at the molecular level, and a noninvasive detection strategy based on proton MRI is particularly attractive. 3,4-Cyclohexenoesculetin β-D-galactopyranoside (S-Gal®) is a commercial histologic stain, which forms a black precipitate in the presence of β-gal and ferric ions, suggesting potential detectability by MRI. Generation of the precipitate is now shown to cause strong T₂* relaxation, revealing β-gal activity. A series of tests with the enzyme in vitro and with tumor cells shows that this approach can be used as an assay for β-gal activity. Proof of principle is shown in human breast tumor xenografts in mice. Upon direct injection of a mixture of 3,4-cyclohexenoesculetin β-D-galactopyranoside and ferric ammonium citrate, intense contrast was observed immediately in MCF7-lacZ tumors, but not in wild-type tumors. 3,4-Cyclohexenoesculetin β-D-galactopyranoside activation in combination with ferric ions introduces a novel approach for assaying enzyme activity by MRI in vivo.