Esterase-Catalyzed Production of Hyperpolarized ¹³C-Enriched Carbon Dioxide in Tissues for Measuring pH

¹³C Magnetic resonance imaging of hyperpolarized (HP) ¹³C-enriched bicarbonate (H¹³CO₃^-) and carbon dioxide (¹³CO₂) is a novel and sensitive technique for tissue pH mapping in vivo. Administration of the HP physiological buffer pair is attractive, but poor polarization and the short T₁ of ¹³C-enriched inorganic bicarbonate salts are major drawbacks for this approach. Here, we report a new class of mixed anhydrides for esterase-catalyzed production of highly polarized ¹³CO₂ and H¹³CO₃^- in tissue. A series of precursors with different alkoxy and acyl groups were synthesized and tested for chemical stability and T₁. ¹³C-enriched ethyl acetyl carbonate (¹³C-EAC) was found to be the most suitable candidate due to the relatively long T₁ and good chemical stability. Our results showed that ¹³C-EAC can be efficiently and rapidly polarized using BDPA. HP ¹³C-EAC was rapidly hydrolyzed by esterase to ¹³C-enriched monoacetyl carbonate (¹³C-MAC), which then decomposed to HP ¹³CO₂. Equilibrium between the newly produced ¹³CO₂ and H¹³CO₃^- was quickly established by carbonic anhydrase, producing a physiological buffer pair with ¹³C NMR signals that can be quantified for pH measurements. Finally, in vivo tissue pH measurements using HP ¹³C-EAC was successfully demonstrated in the liver of healthy rats. These results suggest that HP ¹³C-EAC is a novel imaging probe for in vivo pH measurements.