Convenient one-pot synthesis of 1-(4-(4-(2-[18F]fluoroethoxy)phenyl)piperazin-1-yl)ethanone ([18F]FEt-PPZ) for imaging tumors expressing sigma-1 receptors

Abstract

The overexpression of the sigma-1 receptor (σ1R) in a variety of cancers makes it a potential target for developing anticancer drugs or diagnostic/therapeutic radiopharmaceuticals. Molecules with piperidine or piperazine substructures have shown high binding affinity towards σ1R. Docking and MD simulation studies on commercially available piperidine/piperazine-containing compounds identified 1-(4-(4-hydroxyphenyl)piperazin-1-yl)ethanone (PPZ) as a potential ligand for targeting σ1R. Docking studies on its fluorinated analogue, 1-(4-(4-(2-fluoroethoxy)phenyl)piperazin-1-yl)ethanone (FEt-PPZ), showed good affinity towards σ1R, comparable to that of a clinically evaluated radiopharmaceuticals such as [¹⁸F]fluspidine. The root mean square deviation (RMSD) value of 2 A° in the MD simulations over a period of 50 ns indicated the excellent stability of the FEt-PPZ-σ1R receptor–ligand complex. Based on these results, the radiosynthesis of [¹⁸F]FEt-PPZ was carried out to evaluate its potential to target σ1R. The radiotracer was prepared through a one-pot, two-step method using a GE PETtrace cyclotron and PETtracer synthetic module with an activity yield of 32% ± 5.8% (n = 6). The radiochemical purity (RCP) of [¹⁸F]FEt-PPZ was >95%, as assessed through radioTLC and radio-HPLC. In vitro cell binding studies on [¹⁸F]FEt-PPZ in U-87 MG and B16F10 cells showed maximum uptakes of 13.28% ± 1.04% and 10.74% ± 0.82%, respectively, at 60 min post-incubation. Inhibition studies carried out in the presence of an excess non-radioactive compound (FEt-PPZ) showed a reduction of ∼55% in radiotracer uptake in glioma cells and ∼48% in melanoma cells, indicating its specificity towards σ1R. Bio-distribution and small animal PET/CT imaging studies on melanoma-tumor-bearing C57BL6 mice showed a significant tumor uptake (10.55% ± 1.3% ID per g organ) at 60 min post-injection and an hepatic and renal mode of excretion. Preclinical studies on [¹⁸F]FEt-PPZ developed using an in silico approach demonstrated its potential to target σ1R, thus warranting further investigation.