Clinical translation of 18F-fluoropivalate - a PET tracer for imaging short-chain fatty acid metabolism: safety, biodistribution, and dosimetry in fed and fasted healthy volunteers

Abstract

Background: Fatty acids derived de novo or taken up from the extracellular space are an essential source of nutrient for cell growth and proliferation. Radiopharmaceuticals including ¹¹C-acetate, and ¹⁸F-FAC (2-¹⁸F-fluoroacetate), have previously been used to study short-chain fatty acid (SCFA) metabolism. We developed ¹⁸F-fluoropivalate (¹⁸F-FPIA; 3-¹⁸F-fluoro-2,2-dimethylpropionic acid) bearing a gem-dimethyl substituent to assert metabolic stability for studying SCFA metabolism. We report the safety, biodistribution, and internal radiation dosimetry profile of ¹⁸F-FPIA in 24 healthy volunteers and the effect of dietary conditions.

Materials and methods: Healthy volunteer male and female subjects were enrolled (n = 24), and grouped into 12 fed and 12 fasted. Non-esterified fatty acids (NEFA) and carnitine blood measurements were assessed. Subjects received 159.48 MBq (range, 47.31-164.66 MBq) of ¹⁸F-FPIA. Radiochemical purity was > 99%. Safety data were obtained during and 24 h after radiotracer administration. Subjects underwent detailed multiple whole-body PET/CT scanning with sampling of venous bloods for radioactivity and radioactive metabolite quantification. Regions of interest were defined to derive individual and mean organ residence times; effective dose was calculated using OLINDA 1.1.

Results: All subjects tolerated ¹⁸F-FPIA with no adverse events. Over 90% of radiotracer was present in plasma at 60 min post-injection. The organs receiving highest absorbed dose (in mGy/MBq) were the liver (0.070 ± 0.023), kidneys (0.043 ± 0.013), gallbladder wall (0.026 ± 0.003), and urinary bladder (0.021 ± 0.004); otherwise there was low tissue uptake. The calculated effective dose using mean organ residence times over all 24 subjects was 0.0154 mSv/MBq (SD ± 0.0010). No differences in biodistribution or dosimetry were seen in fed and fasted subjects, though systemic NEFA and carnitine levels reflected fasted and fed states.

Conclusion: The favourable safety, imaging, and dosimetric profile makes ¹⁸F-FPIA a promising candidate radiotracer for tracing SCFA metabolism.

Keywords: 18F-FPIA; 18F-Fluoropivalate; Carnitine; Dosimetry; Positron emission tomography; Short chain fatty acid metabolism.

Fig. 1

Chemical structure of ¹⁸F-fluoropivalate (¹⁸F-FPIA; 3-¹⁸F-fluoro-2,2-dimethylpropionic acid)

Fig. 2

Metabolite analysis of ¹⁸F-FPIA in a healthy volunteer (fed). (A–G) Typical high-performance liquid chromatogram of ¹⁸F-FPIA in plasma at 5, 10, 15, 30, 60, 90, and 120 min time points. Red arrows indicate parent/unmetabolized ¹⁸F-FPIA. Scaling of D–G adjusted to allow for visualization of metabolite peaks (blue arrow). (H) Percentage parent plasma fraction. Over all subjects, 90% parent radiotracer is present after 60 and over 80% at 120 min. In all fed subjects the mean percentage parent tracer after 120 min (94.4% ± 1.44) and in fasted subjects (95.9% ± 1.57). CPM = counts per minute

Fig. 3

Time course biodistribution of ¹⁸F-FPIA in female and male healthy subjects. Maximum-intensity-projection images of ¹⁸F-FPIA in (A) female fasted healthy subject and (B) male fed healthy subject. p.i. = post injection

Fig. 4

Mean decay-corrected time–activity curves (TAC) for main source organs (A–C). Except where indicated, the TACs shown are for fed subjects (n = 12 patients). Graph (A), also shows the main difference in ¹⁸F-FPIA uptake between fed and fasted subjects. Fasted subjects showed lower uptake over the course of scanning study

Fig. 5

Urinary bladder radioactivity in all subjects. Graph shows variable radioactivity in all subjects within the study. Bladder radioactivity represents the total activity in a standard sized bladder

Fig. 6

Non-esterified fatty acid, insulin and glucose measurements in blood of fed (A–C) and fasted (D–F) healthy volunteer subjects

Fig. 7

Acyl carnitine measurements in fed and fasted healthy volunteer subjects. Pre-scan (baseline), 1 h p.i. and end of scan blood samples were taken for analysis. Acetyl carnitine (C2, short chain fatty acid), (C3–C5, sum of short chain carboxylic acids C3, C4, C5; without C2), free carnitine (FC) and total carnitine (TC) are shown. Overall, levels of C2, FC and TC were higher in fasted subjects at baseline, suggesting a flux in the fatty acid oxidation pathway

Fig. 8

Heat map of Acylcarnitine measurement. Fed and fasted subjects 1–24 are shown (left hand column). For each subject, there are 3 measurements per subject i.e. 1 pre-scan, 1 h p.i. and end of scan. Acetyl carnitine (C2), short chain fatty acids (C3–C5), medium chain fatty acids (C6–C8), Long chain fatty acids (C10, C12, C14, C16, and C18), Free carnitine (FC), Total carnitine (TC)