Toward Parahydrogen-Induced Polarization of Acetoacetate

Hyperpolarized acetoacetate is a promising probe for detection of redox status inside mitochondria using 13C MRI. In this pilot study we used parahydrogen-induced polarization by means of side arm hydrogenation (PHIP-SAH) approach for hyperpolarization of [1,3-13C2]acetoacetate. The propargyl [1,3-13C2]acetoacetate precursor was synthesized and subjected to homogeneous hydrogenation with parahydrogen. After optimization, the observed 1H polarization was 25.5% while achieving 100% chemical conversion of propargyl moiety and 80% selectivity to allyl [1,3-13C2]acetoacetate product. After hydrogenation, linear magnetic field sweep was performed for 1H-to-13C polarization transfer in allyl [1,3-13C2]acetoacetate with maximum observed 13C polarizations: P(13C-1) = 4.1% and P(13C-3) = 2.7%. Next, we analyzed the products of allyl [1,3-13C2]acetoacetate hydrolysis under different conditions. Alkaline hydrolysis results in the formation of stereoisomeric enolates of allyl [1,3-13C2]acetoacetate, which can then be converted into hyperpolarized [1,3-13C2]acetoacetate under harsh alkaline conditions with subsequent decomposition to hyperpolarized [2-13C]acetone and [13C]carbonate. These results represent a step toward expanding the PHIP-SAH agent portfolio beyond pyruvate, here specifically towards establishing acetoacetate as a promising candidate for future redox and ROS-sensitive 13C MRI studies.