Hyperpolarization without persistent radicals for in vivo real-time metabolic imaging

Abstract

Hyperpolarized substrates prepared via dissolution dynamic nuclear polarization have been proposed as magnetic resonance imaging (MRI) agents for cancer or cardiac failure diagnosis and therapy monitoring through the detection of metabolic impairments in vivo. The use of potentially toxic persistent radicals to hyperpolarize substrates was hitherto required. We demonstrate that by shining UV light for an hour on a frozen pure endogenous substance, namely the glucose metabolic product pyruvic acid, it is possible to generate a concentration of photo-induced radicals that is large enough to highly enhance the (13)C polarization of the substance via dynamic nuclear polarization. These radicals recombine upon dissolution and a solution composed of purely endogenous products is obtained for performing in vivo metabolic hyperpolarized (13)C MRI with high spatial resolution. Our method opens the way to safe and straightforward preclinical and clinical applications of hyperpolarized MRI because the filtering procedure mandatory for clinical applications and the associated pharmacological tests necessary to prevent contamination are eliminated, concurrently allowing a decrease in the delay between preparation and injection of the imaging agents for improved in vivo sensitivity.

Keywords: acetate; lactate; metabolism; nuclear magnetic resonance; pyruvate.

Fig. 1.

Radical generation and ESR analysis at 77 K (A) Radical concentration in pure PA as a function of UV irradiation time as determined by X-band ESR. (Inset) Structural formula of [1-¹³C]PA. (B) X-band ESR spectrum of natural abundance PA after 60 min of UV irradiation. (C) X-band ESR spectrum of [1-¹³C]PA after 60 min of UV irradiation.

Fig. 2.

¹³C MR spectrum of dissolved UV-irradiated [U-¹³C]PA. Room-temperature high-resolution ¹³C MR spectrum of UV-irradiated (60-min irradiation at 77 K) uniformly ¹³C-labeled PA melted in D₂O (0.9 M) measured in a 400-MHz MR spectrometer.

Fig. 3.

Microwave spectra at 5 T and 4.2 K. Comparison between the microwave spectra measured by ¹³C MR in UV-irradiated [1-¹³C]PA (full circles), [1-¹³C]PA doped with 16-mM OX063 trityl radicals (stars), and a frozen 3-M sodium [1-¹³C]pyruvate aqueous solution doped with 50 mM TEMPO nitroxyl radicals (open circles). The frequency separation Δ between the central and the left or right vertical dotted lines corresponds to the ¹H MR frequency at 5 T.

Fig. 4.

In vivo real-time pyruvate metabolism. In vivo ¹³C MR data recorded following the injection of 300 μL of 80 ± 5 mM [1-¹³C]pyruvate solution hyperpolarized by DNP using polarizing agents created by UV irradiation. (A, Upper) Series of ¹³C spectra acquired in the mouse head every 3 s. (A, Lower) Sum of all 25 displayed spectra. (B and C) The 5-mm³ resolution bSSFP ¹³C images recorded using interleaved selective excitations of [1-¹³C]pyruvate (B) and [1-¹³C]lactate (C) superimposed to ¹H anatomical images of the mouse head.