Synthesis and characterization of an ¹⁸F-labeled AmBF₃-conjugated albumin-binding peptide for imaging blood pool by PET/CT

Objectives: Imaging of the blood pool compartment is a useful nuclear medicine technique that can be used, for instance, to assess ventricular ejection fraction and wall motion. While this is usually accomplished with ^99mTc-labeled red blood cells, the technique requires multiple injections or manipulation of blood products. The development of a tracer that harnesses the resolution of PET/CT with a single injection would be of clinical interest. A class of pharmacodynamic modifiers, albumin binders (AB), can be used to bind a molecule of interest to circulating albumin, thus prolonging its residence time in blood. This can be used to lengthen the therapeutic effect of a drug but could also be used for blood pool imaging. The aim of the study was to evaluate an ¹⁸F-labeled albumin binder, HTK02172, for the task of blood pool imaging with PET/CT.

Methods: The protected azidoacetyl-conjugated albumin-binding peptide ((4-p-iodophenyl)butyl)-γGlu(OtBu)-Lys((4-p-iodophenyl)butyl)-γGlu(OtBu))-Lys(azidoacetyl)-OH containing two albumin-binding motifs 4-p-iodophenyl)butyl-γGlu was constructed on solid phase. The resulting peptide was simultaneously deprotected and cleaved from resin with trifluoroacetic acid. HTK02172, the radiolabeling precursor and standard, was obtained following a click reaction with N-propargyl-N,N-dimethylammoniomethyltrifluoroborate. Radiolabeling was achieved by ¹⁸F-¹⁹F isotope exchange reaction in aqueous solution (pH 2) at 80°C, and ¹⁸F-HTK02172 was purified by HPLC. Stability of ¹⁸F-HTK02172 was assessed in mouse plasma and monitored by HPLC. Two female NSG mice were imaged over 60-minutes in dynamic mode by PET/CT after injection of ¹⁸F-HTK02172. Afterwards, the mice were sacrificed by CO₂ inhalation, and major organs/tissues were harvested and counted on a gamma counter. On the PET images, activity in the blood compartment was measured by drawing a Region of Interest (ROI) of the heart. Analysis was performed with the R software (R Foundation for Statistical Computing, version 3.4.0).

Results: The synthesis yield for the non-radioactive standard was 37%. ¹⁸F-HTK02172 was obtained in 5.1 ± 2.3% decay-corrected radiochemical yields with > 96% radiochemical purity and 155 ± 16 GBq/μmol specific activity. The tracer was stable in mouse plasma, and no metabolites were observed after 60-minute incubation at 37°C. The organ uptake (%ID/g; mean ± s.d.) from the biodistribution was: blood 18.7 ± 1.73, intestines 11.57 ± 0.21, liver 4.89 ± 0.65, kidneys 5.44 ± 0.67, and muscle 0.40 ± 0.04. From the dynamic PET images, the heart was clearly visualized with good contrast. The activity remaining in the blood compartment (%ID/cc) based on drawn ROI was 19.30 ± 1.56, 15.65 ± 1.63, 12.90 ± 1.41, and 12.25 ± 1.48 at 15, 30, 45, and 60 minutes, respectively, and there was significant excretion of the tracer through the hepatobiliary system with subsequent accumulation in the bowel. Renal excretion was minimal on images. Conclusion: In this small pilot study, we demonstrated the feasibility of imaging blood pool with an ¹⁸F- labeled AmBF₃-conjugated albumin-binding peptide (HTK02172) by PET/CT. The radiotracer benefits from the resolution of PET imaging and requires only a single injection without manipulation of blood products, which is advantageous compared with technetium-labeled red blood cells.

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