Radiosynthesis and in vivo evaluation of a novel σ1 selective PET ligand

Evaluation of [18F]F-TZ3108 for PET Imaging of Metabolic-Associated Fatty Liver Disease

PURPOSE

Sigma-1 receptor (Sig-1R), a chaperone that resides at the mitochondrion-associated endoplasmic reticulum (ER) membrane, is an ER stress biomarker. It is thought that ER stress plays a critical role in the progression of metabolic-associated fatty liver disease (MAFLD). The aim of this study was to evaluate a positron emission tomography (PET) tracer [¹⁸F]F-TZ3108 targeting Sig-1R for MAFLD.

PROCEDURES

The mouse model of MAFLD was established by feeding high-fat diet (HFD) for 12 weeks. Dynamic (0-60 min) PET/CT scans were performed after intravenous injection of 2-deoxy-2[¹⁸F]fluoro-D-glucose ([¹⁸F]-FDG) and [¹⁸F]F-TZ3108. Tracer kinetic modeling was performed for quantification of the PET/CT imaging of the liver. Post-PET biodistribution, the liver tissue western blotting (WB), and immunofluorescence (IF) were performed to compare the expression of Sig-1R levels in the organs harvested from both MAFLD and age-matched control mice.

RESULTS

The micro PET/CT imaging revealed a significantly decreased uptake of [¹⁸F]F-TZ3108 in the livers of the MAFLD group compared to the healthy controls, while the uptake of [¹⁸F]-FDG in the livers was not significantly different between the two groups. Based on the tracer kinetic modeling, the binding disassociate rate (k₄) for [¹⁸F]F-TZ3108 was significantly increased in MAFLD group compared to healthy controls. The volume distribution (V_T), and the non-displacement binding potential (BP_ND) revealed significantly decrease in MAFLD compared to healthy controls respectively. The post-PET biodistribution (%ID/g) of [¹⁸F]F-TZ3108 in the livers of MAFLD mice was significantly reduced nearly twofold than that in the livers of control mice. WB and IF experiments further confirmed the reduction of Sig-1R expression in the MAFLD group.

CONCLUSIONS

The expression of Sig-1R in the liver, measured by the PET tracer, [¹⁸F]F-TZ3108, was significantly decreased in mouse model of MAFLD. The [¹⁸F]F-TZ3108 PET/CT imaging may provide a novel means of visualization for ER stress in MAFLD or other diseases in vivo.

Chiral resolution of serial potent and selective σ1 ligands and biological evaluation of (-)-[18F]TZ3108 in rodent and the nonhuman primate brain

Twelve optically pure enantiomers were obtained using either crystallization or chiral high performance liquid chromatography (HPLC) separation methodologies to resolve six racemic sigma-1 (σ₁) receptor ligands. The in vitro binding affinities of each enantiomer for σ₁, σ₂ receptors and vesicular acetylcholine transporter (VAChT) were determined. Out of the 12 optically pure enantiomers, five displayed very high affinities for σ₁ (K_i<2nM) and high selectivity for σ₁ versus σ₂ and VAChT (>100-fold). The minus enantiomer, (-)-14a ((-)-TZ3108) (K_i-σ1=1.8±0.4nM, K_i-σ2=6960±810nM, K_i-VAChT=980±87nM), was chosen for radiolabeling and further in vivo evaluation in rodents and nonhuman primates (NHPs). A biodistribution study in Sprague Dawley rats showed brain uptake (%ID/gram) of (-)-[¹⁸F]TZ3108 reached 1.285±0.062 at 5min and 0.802±0.129 at 120min. NHP microPET imaging studies revealed higher brain uptake of (-)-[¹⁸F]TZ3108 and more favorable pharmacokinetics compared to its racemic counterpart. Pretreatment of the animal using two structurally different σ₁ ligands significantly decreased accumulation of (-)-[¹⁸F]TZ3108 in the brain. Together, our in vivo evaluation results suggest that (-)-[¹⁸F]TZ3108 is a promising positron emission tomography (PET) tracer for quantifying σ₁ receptor in the brain.