Design and Investigation of a [¹⁸F]-Labeled Benzamide Derivative as a High Affinity Dual Sigma Receptor Subtype Radioligand for Prostate Tumor Imaging

Abstract

High overexpression of sigma (σ) receptors (σ1 and σ2 subtypes) in a variety of human solid tumors has prompted the development of σ receptor-targeting radioligands, as imaging agents for tumor detection. A majority of these radioligands to date target the σ2 receptor, a potential marker of tumor proliferative status. The identification of approximately equal proportions of both σ receptor subtypes in prostate tumors suggests that a high affinity, dual σ receptor-targeting radioligand could potentially provide enhanced tumor targeting efficacy in prostate cancer. To accomplish this goal, we designed a series of ligands which bind to both σ receptor subtypes with high affinity. Ligand 3a in this series, displaying optimal dual σ receptor subtype affinity (σ1, 6.3 nM; σ2, 10.2 nM) was radiolabeled with fluorine-18 (18F) to give [18F]3a and evaluated as a σ receptor-targeting radioligand in the mouse PC-3 prostate tumor model. Cellular assays with PC-3 cells demonstrated that a major proportion of [18F]3a was localized to cell surface σ receptors, while ∼10% of [18F]3a was internalized within cells after incubation for 3.5 h. Serial PET imaging in mice bearing PC-3 tumors revealed that uptake of [18F]3a was 1.6 ± 0.8, 4.4 ± 0.3, and 3.6 ± 0.6% ID/g (% injection dose per gram) in σ receptor-positive prostate tumors at 15 min, 1.5 h, and 3.5 h postinjection, respectively (n = 3) resulting in clear tumor visualization. Blocking studies conducted with haloperidol (a nonselective inhibitor for both σ receptor subtypes) confirmed that the uptake of [18F]3a was σ receptor-mediated. Histology analysis confirmed similar expression of σ1 and σ2 in PC-3 tumors which was significantly greater than its expression in normal organs/tissues such as liver, kidney, and muscle. Metabolite studies revealed that >50% of radioactivity in PC-3 tumors at 30 min postinjection represented intact [18F]3a. Prominent σ receptor-specific uptake of [18F]3a in prostate tumors and its subsequent clear visualization with PET imaging indicate potential utility for the diagnosis of prostate carcinoma.

Synthesis and evaluation of an 18F‐labeled trifluoroborate derivative of 2‐nitroimidazole for imaging tumor hypoxia with positron emission tomography

“…A cell‐based uptake study was performed following literature procedures . HT‐29 cells were harvested by cell dissociation buffer (Life Technologies, Carlsbad, CA, USA) and resuspended in PBS at a final concentration of 1.5 × 10 6 cell/mL.…”

“…With suboptimal logP value 18 F‐AmBF 3 ‐Bu‐2NI might not be able to freely diffuse across the cell membrane of hypoxic cancer cells and reach the intracellular target, nitroreductase. To further confirm this, a cell uptake study was carried out following literature procedures . HT‐29 cells were incubated with 18 F‐AmBF 3 ‐Bu‐2NI for up to 3 hours.…”

“…To further confirm this, a cell uptake study was carried out following FIGURE 6 Time activity curves of 18 F-AmBF 3 -Bu-2NI A, heart, liver and kidney, and B, tumor, muscle, bone, and brain literature procedures. 32 HT-29 cells were incubated with 18 F-AmBF 3 -Bu-2NI for up to 3 hours. At pre-determined time points, an aliquot of cell suspension was taken out, centrifuged, and the cell pellet was further washed with acidic solution to remove membrane-bound radioactivity.…”

“…A cell-based uptake study was performed following literature procedures. 32 HT-29 cells were harvested by cell dissociation buffer (Life Technologies, Carlsbad, CA, USA) and resuspended in PBS at a final concentration of 1.5 × 10 6 cell/mL. The cells (2.5 mL of the cell suspension) were then incubated with an aliquot (10 μL) of 18 F-AmBF 3 -Bu-2NI (final concentration: 2.0 × 10 5 cpm/mL) at 27°C.…”

Synthesis and Biological Evaluation of Sigma‐1 (σ1) Receptor Ligands Based on Phenyl‐1,2,4‐oxadiazole Derivatives

“…[8][9][10] Ligands acting on the σ 2 receptor have played a potential role in cancer imaging and treatment. [11,12] Neuropathic pain is an important health problem. [13] Until now, tricyclic antidepressants, opioids, and anticonvulsants have been used to treat chronic pain; however, these agents are not effective in all cases.…”