Synthesis and evaluation of new¹⁸F-labelled acetamidobenzoxazolone-based radioligands for imaging of the translocator protein (18 kDa, TSPO) in the brain

Abstract

The visualization of the activated microglia/TSPO is one of the main aspects of neuroimaging. Here we describe two new (18)F-labelled molecules, 2-[5-(4-[(18)F]fluoroethoxyphenyl)- ([(18)F]2) and 2-[5-(4-[(18)F]fluoropropyloxyphenyl)- ([(18)F]3) -2-oxo-1,3-benzoxazol-3(2H)-yl]-N-methyl-N-phenylacetamide as novel PET ligands for imaging the translocator protein (18 kDa, TSPO) in the brain. The three-D pharmacophore evaluation and docking studies suggested their high affinity for the TSPO and in vitro binding assays of the TSPO showed binding affinities 6.6 ± 0.7 nM and 16.7 ± 2.5 nM for 2 and 3, respectively. The radiochemical yields for [(18)F]2 and [(18)F]3 were found to be 22 ± 4% (n = 8) and 5 ± 2% (n = 5), respectively at EOB. The radiochemical purity for both was found ≥98% and the specific activity was in the range of 98-364 GBq μmol(-1) at EOS. In vitro autoradiography with an ischemic rat brain showed significantly increased binding on the ipsilateral side compared to the contralateral side. The specificity of [(18)F]2 and [(18)F]3 for binding TSPO was confirmed using the TSPO ligands PK11195 and MBMP. The biodistribution patterns of both PET ligands were evaluated in normal mice by 1 h dynamic PET imaging. In the brain, regional radioactivity reached the maximum very rapidly within 0-4 min for both ligands, similar to (R)[(11)C]PK11195. The metabolite study of [(18)F]2 also favoured a more favourable profile for quantification in comparison to (R)[(11)C]PK11195. In summary, these data indicated that [(18)F]2 and [(18)F]3 have good potential to work as PET ligands, therefore there are merits to use these radioligands for the in vivo evaluation in animal models to see their efficacy in the living brain.

Radiosynthesis and Preclinical Evaluation of [18F]F-DPA, A Novel Pyrazolo[1,5a]pyrimidine Acetamide TSPO Radioligand, in Healthy Sprague Dawley Rats

“…These include the atypical benzodiazepine [ 11 C]Ro5-4864 [8], the vinca alkaloid [ 11 C]Vinpocetine [9], dihydro-9H-purinacetamides (for example, N -benzyl- N -ethyl-2-(7-[ 11 C]methyl-8-oxo-2-phenylpurin-9-yl)acetamide, known as [ 11 C]Emapunil [10]), aryloxyanilides (such as [ 11 C]PBR28 [11] and [ 18 F]FEPPA [12]), indoleacetamides (such as [ 18 F]GE-180 [13] and [ 11 C]SSR180575 [14]), acetamidobenzoxazolones [15] without forgetting pyrazolopyrimidines, and bioisoteric structures (imidazopyridines, for example). The latter classes indeed include an impressive list of PET-radioligands, among them, the 5,7-dimethylpyrazolo[1,5-a]pyrimidin-3-yl)acetamides [ 11 C]DPA-713 [16], [ 18 F]DPA-714 [17], and [ 18 F]PBR146 [18], and the 6-chloro-imidazo[1,2-a]pyridin-3-yl)acetamides [ 11 C]CLINME [19], [ 18 F]PBR102 [18] and [ 18 F]PBR111 [20].…”

Modified benzoxazolone (ABO‐AA) based single photon emission computed tomography (SPECT) probes for 18 kDa translocator protein

“…Encouraged by the results of our recently developed TSPO ligands having TSPO affinity in nanomolar range (Tiwari, Fujinaga, et al, ; Tiwari, Yui, et al, ; Tiwari, Yui, et al, ), we directed our efforts towards development of analogs of ABO for 99m Tc labeling to target 18 kDa TSPO. The basis of our research was the successful outcome of radioligands [ 11 C]MBMP and [ 18 F]FE/FPBMP showing promise for TSPO visualization and quantification.…”

“…Further, it was also compared with PET and SPECT ligands of ABO category. The release kinetic rates between 15 and 60 min for 18 F‐FEBMP (Tiwari, Ji, et al, ), 11 C‐MBMP (Tiwari, Fujinaga, et al, ), 18 F‐2‐(5‐(6‐Fluoropyridin‐3‐yl)‐2‐oxobenzo[d]oxazol‐3(2H)‐yl)‐N‐methyl‐N‐phenylacetamide (Fujinaga et al, ), and 99m Tc‐MBIP‐Cl are 0.175%ID/g/min, 0.150%ID/g/min, 0.070%ID/g/min, and 0.085%ID/g/min in heart, this ratio became more comparable as 0.382%ID/g/min, 0.375%ID/g/min, 0.155%ID/g/min and 0.231%ID/g/min in lungs. In liver and kidney, release kinetic rates were approximately 3–30 folds faster in 99m Tc‐MBIP‐Cl which makes it interesting for further evaluation.…”

“…A new skeleton acetamidobenzoxazolone (ABO) which has been derived from RoS‐4,864 by opening the diazepine ring and reorganizing it (Fukaya et al, ; Fukaya et al, ) to overcome the problems of intersubject variability and nonspecific binding (Tiwari, Yui, et al, ). In recent past, our group has explored this skeleton for diagnostic application using positron emission tomography (PET) with 11 C and 18 F radionuclides such as 2‐[5‐(4‐[ 11 C]Methoxyphenyl)‐2‐oxo‐1,3‐benzoxazol‐3(2 H )‐yl]‐ N ‐methyl‐ N ‐phenylacetamide ( 11 C‐MBMP), 2‐[5‐(4‐[ 18 F] fluoroethoxy/propoxyphenyl)‐2‐oxo‐1,3‐benzoxazol‐3(2 H )‐yl]‐ N ‐methyl‐ N ‐phenylacetamide ( 18 F‐FEBMP and 18 F‐FPBMP) and using single photon emission computed tomography (SPECT) with 99m Tc such as 99m Tc‐MBIP (Kumari et al, ; Srivastava, Kaul, Ojha, Kumar, & Tiwari, ; Tiwari, Fujinaga, et al, ; Tiwari, Yui, et al, ; Tiwari, Ji, et al, ).…”

Modified benzoxazolone derivative as 18‐kDa TSPO ligand

“…Therefore, a new candidate as TSPO ligand with different structural skeleton is desirable. Recently, we have synthesized a new series of PET ligands 2‐[5‐(4‐methoxyphenyl)‐ 2‐oxo‐1,3‐benzoxazol‐3( 2H )‐yl]‐ N ‐methyl‐ N ‐phenyl acetamide labelled with carbon‐11 ([ 11 C]MBMP) and two new 18F‐labelled molecules, 2‐[5‐(4‐[ 18 F]fluoroethoxy‐ ([ 18 F]FEBMP) and 2‐[5‐(4‐[ 18 F]fluoropropyloxy‐ ([ 18 F]FPBMP) ‐2‐oxo‐1,3‐benzoxazol‐3( 2H )‐yl)‐ N ‐methyl‐ N ‐phenyl acetamide (Figure ) and evaluated in vitro and in vivo for imaging of TSPO expression in brain . The in vitro autoradiography and in vivo PET study demonstrated appropriate specific binding and higher binding potential of [ 11 C]MBMP toward TSPO in the ischemic rat brain but moderate kinetics in primates (unpublished data).…”