Rapid ¹³C Hyperpolarization of the TCA Cycle Intermediate α-Ketoglutarate via SABRE-SHEATH

Abstract

α-Ketoglutarate is a key biomolecule involved in a number of metabolic pathways─most notably the TCA cycle. Abnormal α-ketoglutarate metabolism has also been linked with cancer. Here, isotopic labeling was employed to synthesize [1-¹³C,5-¹²C,D₄]α-ketoglutarate with the future goal of utilizing its [1-¹³C]-hyperpolarized state for real-time metabolic imaging of α-ketoglutarate analytes and its downstream metabolites <i>in vivo</i>. The signal amplification by reversible exchange in shield enables alignment transfer to heteronuclei (SABRE-SHEATH) hyperpolarization technique was used to create 9.7% [1-¹³C] polarization in 1 minute in this isotopologue. The efficient ¹³C hyperpolarization, which utilizes parahydrogen as the source of nuclear spin order, is also supported by favorable relaxation dynamics at 0.4 μT field (the optimal polarization transfer field): the exponential ¹³C polarization buildup constant <i>T</i>_b is 11.0 ± 0.4 s whereas the ¹³C polarization decay constant <i>T</i>₁ is 18.5 ± 0.7 s. An even higher ¹³C polarization value of 17.3% was achieved using natural-abundance α-ketoglutarate disodium salt, with overall similar relaxation dynamics at 0.4 μT field, indicating that substrate deuteration leads only to a slight increase (∼1.2-fold) in the relaxation rates for ¹³C nuclei separated by three chemical bonds. Instead, the gain in polarization (natural abundance versus [1-¹³C]-labeled) is rationalized through the smaller heat capacity of the "spin bath" comprising available ¹³C spins that must be hyperpolarized by the same number of parahydrogen present in each sample, in line with previous ¹⁵N SABRE-SHEATH studies. Remarkably, the C-2 carbon was not hyperpolarized in both α-ketoglutarate isotopologues studied; this observation is in sharp contrast with previously reported SABRE-SHEATH pyruvate studies, indicating that the catalyst-binding dynamics of C-2 in α-ketoglutarate differ from that in pyruvate. We also demonstrate that ¹³C spectroscopic characterization of α-ketoglutarate and pyruvate analytes can be performed at natural ¹³C abundance with an estimated detection limit of 80 micromolar concentration × *%<i>P</i>_13C. All in all, the fundamental studies reported here enable a wide range of research communities with a new hyperpolarized contrast agent potentially useful for metabolic imaging of brain function, cancer, and other metabolically challenging diseases.