Novel Fe³⁺-Based ¹H MRI β-Galactosidase Reporter Molecules**

There is increasing interest in the development of reporter agents to reveal enzyme activity in vivo using small animal imaging. We have previously demonstrated the feasibility of detecting lacZ gene activity using the commercially available 3,4-cyclohexenoesculetin-β-D-galactopyranoside (S-Gal™) as a ¹H MRI reporter. Specifically, β-galactosidase (β-gal) releases the aglycone, which forms an MR contrast-inducing paramagnetic precipitate in the presence of Fe³⁺. Contrast was primarily T₂-weighted signal loss, but T₁ effects were also observed. Since T₁-contrast generally provides signal enhancement as opposed to loss, it appeared attractive to explore whether analogues could be generated with enhanced characteristics. We now report the design and successful synthesis of novel analogues together with characterization of ¹H MRI contrast based on both T₁ and T₂ response to β-gal activity in vitro for the lead agent.