INTRODUCTION

The goal of this study was to conduct studies in normal, healthy volunteers to quantitatively assess normal values, develop techniques and obtain optimal scanning parameters on a 3 Tesla MRI using hyperpolarized (HP) ¹³C-pyruvate. Three classes of experiments were performed: technique validation, RF coil performance assessments, and normal values measurements. Here we report our experience to date regarding safety, tolerability and reproducibility for imaging of the head, chest, abdomen and pelvis with HP ¹³C-MRI.

METHODS

Human Subjects: These volunteer studies were conducted under the aegis of a UCSF IRB-approved protocol. A total of 15 adults (5 females, 10 males) were interviewed to confirm the absence of any significant current or past health problems (i.e., no history of cancer, kidney disease, heart disease, diabetes or other major medical issues) and consented for the HP ¹³C-MRI Study. On day of study, subjects had vital signs and weight measured. A peripheral intravenous catheter was placed and subjects were positioned inside a 3T MRI scanner with appropriate ¹H and ¹³C coils for the body section of interest to be scanned.
Sample Preparation: The sample preparation was carried out in collaboration with UCSF Radiopharmacy. Each sample consisted of a mixture of GMP grade [1-¹³C]- or [2-¹³C]pyruvic acid (14.2 M; MilliporeSigma Isotec) and 15 mM Electron Paramagnetic Agent (EPA), (AH111501, GE Healthcare). Samples were prepared the same morning of each study and inserted under aseptic conditions into pharmacy kits from GE Healthcare.
Polarization: A SPINlab polarizer (GE Healthcare) was used to polarize [1-¹³C]pyruvate at 5T and 0.8 K. The sample was irradiated with microwave at fMW= 140.02 GHz for 2 hours, achieving a ¹³C polarization of ~40% in the liquid state.
Pre-injection Quality Check & Injection: Following dissolution, the EPA was filtered mechanically and quality control carried out prior to injection: pH, pyruvate and EPA concentrations, polarization, and temperature were measured and compared to ranges of acceptability. In parallel, integrity of the 0.2 μm sterile filter was tested following manufacturer specifications to a pressure-stress of 50 psi. After release by the pharmacist, ~250 mM pyruvate was injected at a dose of 0.43 mL/kg and a rate of 5 mL/s, followed by a 20 mL saline flush.
Acquisition: All experiments were performed on a 3T MR scanner (MR750, GE Healthcare) with multi-nuclear excitation and 32-channel reception capability. Brain studies were performed using a birdcage for RF excitation and either a 24-channel or 32-channel coil for data acquisition. All other studies utilized a clamshell volume coil for RF transmit and either an 8- or 16-channel coil for receive. Images of pyruvate, lactate, bicarbonate or alanine (dependent on organ of interest) were acquired using either spectroscopic imaging or rapid imaging approaches (EPI/Spiral^1,2, SSFP³, EPSI⁴) with a 2-3 s temporal resolution. Scan parameters for a standard acquisition and coil configurations are provided in Table 1.
Reconstruction and Analysis: Multi-channel ¹³C data was combined using coil sensitivity maps estimate from the pyruvate signal⁵. An inputless two-site exchange model⁶ was applied to quantitatively characterize pyruvate-to-lactate conversion rate, k_PL.

RESULTS AND DISCUSSION

We have conducted HP ¹³C-MRI studies in 15 separate normal, healthy individuals to date, performing a total of 71 HP ¹³C-agent injections. While both [1-¹³C]-Pyruvate and [2-¹³C]-pyruvate⁷ agents were employed, the majority of the studies have been with the C1 compound (61 injections/scans) and only these data are detailed here. The body regions/organs studied included head/brain, chest/heart, abdomen/liver and pelvis/kidney. Several of these individuals received 2 injections 15-60 min apart and also over multiple different sessions. Two subjects have received over a dozen injections: one has received 14 injections over 10 sessions; another has received 19 injections over 12 sessions. No adverse effects have been observed in either of these individuals, nor in any of the others studied to date. Only minor, transient alterations in sensations have been noted by subjects, such as disturbances in taste (usually “metallic” in character) lasting only a few seconds or a sensation of cold or warmth in the arms or upper body (again, lasting only a few seconds).

Conversion rates of pyruvate-to-lactate (k_PL) or pyruvate-to-bicarbonate (k_PB), and/or ratios of Lactate/Pyruvate (Lac/Pyr) and Bicarbonate/Pyruvate (Bic/Pyr) ratios were estimated (Figure 1). From available data in the brain (Figure 2, Table 3), test-retest reproducibility was observed in two brain subjects between studies at different times (same day) and different days using various C-13 transceiver setups.

CONCLUSIONS

This study employed HP ¹³C-MRI to interrogate typical metabolism levels of pyruvate-to-lactate and pyruvate-to-bicarbonate conversion rates in the major body regions and organs in normal, healthy adults. No adverse events were observed in any of the subjects. These results add to earlier phase 1 studies conducted in cancer patients⁸ and other smaller cohorts of healthy volunteers^9,10.11 which demonstrated the safety of this metabolic imaging modality on selected organs. In summary, HP ¹³C-MRI with ¹³C-pyruvate appears to be extremely safe and well tolerated over all major body regions and shows reproducibility in the most studied normal organ to date. While still preliminary, the results from this project are promising and should provide critical normative data for on-going and future studies of disease with HP ¹³C-MRI.

Acknowledgements

This work was funded by the NIH (U01EB026412, P41EB013598) and the UCSF NICO project.