Hyperpolarized (1-¹³C)Alaninamide Is a Multifunctional In Vivo Sensor of Aminopeptidase N Activity, pH, and CO₂

Spin hyperpolarization enables real-time metabolic imaging of carbon-13-labeled substrates. While hyperpolarized l-(1-¹³C)alaninamide is a probe of the cell-surface tumor marker aminopeptidase-N (APN, CD13), its activity in vivo has not been described. Scanning the kidneys of rats infused with hyperpolarized alaninamide shows both conversion to [1-¹³C]alanine and several additional spectral peaks with distinct temporal dynamics. The (1-¹³C)alaninamide chemical shift is pH-sensitive, with a pK_a of 7.9 at 37 °C, and the peaks correspond to at least three different compartments of pH 7.46 ± 0.02 (1), 7.21 ± 0.02 (2), and 6.58 ± 0.05 (3). An additional peak was assigned to the carboxyamino adduct formed by reaction with dissolved CO₂. Spectroscopic imaging showed nonuniform distribution, with the low-pH signal more concentrated in the inner medulla. Treatment with the diuretic acetazolamide resulted in significant pH shifts in compartment 1 to 7.38 ± 0.03 (p = 0.0057) and compartment 3 to 6.80 ± 0.05 (p = 0.0019). While the pH of compartment 1 correlates with blood pH, the pH of compartment 3 did not correspond to the pH of urine. In vitro experiments show that alaninamide readily enters blood cells and can detect intracellular pH. While carbamate formation depends on pH and pCO₂, the carbamate-to-alaninamide ratio did not correlate with either arterial blood pH or pCO₂, suggesting that it may reflect variations in tissue pH and pCO₂. This study demonstrates the feasibility of using hyperpolarized sensors to simultaneously image enzyme activity, pCO₂, and pH in vivo.