Authors: Ling Qiu | Wei Wang | Ke Li | Ying Peng | Gao‐Chao Lv | Qingzhu Liu | Feng Gao | Yann Seimbille | Minhao Xie | Jianguo Lin
Source: Theranostics (article, 2019-01-01)
Field: Chemistry — Spectroscopy

Rational design of caspase-responsive smart molecular probe for positron emission tomography imaging of drug-induced apoptosis <b>Purpose:</b> Positron emission tomography (PET) imaging of apoptosis is very important for early evaluation of tumor therapeutic efficacy. A stimuli-responsive probe based on the peptide sequence Asp-Glu-Val-Asp (DEVD), [<sup>18</sup>F]DEVD-Cys(StBu)-PPG(CBT)-AmBF<sub>3</sub> ([<sup>18</sup>F]<b>1</b>), for PET imaging of tumor apoptosis was designed and prepared. This study aimed to develop a novel smart probe using a convenient radiosynthesis method and to fully examine the sensitivity and specificity of the probe response to the tumor treatment. <b>Methods:</b> The radiolabelling precursor DEVD-Cys(StBu)-PPG(CBT)-AmBF<sub>3</sub> (<b>1</b>) was synthesized through multistep reactions. The reduction together with caspase-controlled macrocyclization and self-assembly of <b>1</b> was characterized and validated <i>in vitro</i>. After [<sup>18</sup>F]fluorination in the buffer (pH= 2.5), the radiolabelling yield (RLY), radiochemical purity (RCP) and stability of the probe [<sup>18</sup>F]<b>1</b> in PBS and mouse serum were investigated by radio-HPLC. The sensitivity and specificity of [<sup>18</sup>F]<b>1</b> for detecting the drug-induced apoptosis was fully evaluated <i>in vitro</i> and <i>in vivo</i>. The effect of cold precursor <b>1</b> on the cell uptake and tumor imaging of [<sup>18</sup>F]<b>1</b> was also assessed. The level of activated caspase-3 in Hela cells and tumors with or without apoptosis induction was analyzed and compared by western blotting and histological staining. <b>Results:</b> The whole radiosynthesis process of [<sup>18</sup>F]<b>1</b> was around 25 min with RLY of 50%, RCP of over 99% and specific activity of 1.45 ± 0.4 Ci/µmol. The probe was very stable in both PBS and mouse serum within 4 h. It can be activated by caspase-3 and then undergo an intermolecular cyclization to form nanosized particles. The retained [<sup>18</sup>F]<b>1</b> in DOX-treated HeLa cells was 2.2 folds of that in untreated cells. Within 1 h microPET imaging of the untreated Hela-bearing mice, the injection of [<sup>18</sup>F]<b>1</b> resulted in the increase of the uptake ratio of tumor to muscle (T/M) only from 1.74 to 2.18, while in the DOX-treated Hela-bearing mice T/M increased from 1.88 to 10.52 and the co-injection of [<sup>18</sup>F]<b>1</b> and <b>1</b> even led to the increase of T/M from 3.08 to 14.81. <b>Conclusions:</b> A caspase-responsive smart PET probe [<sup>18</sup>F]<b>1</b> was designed and prepared in a kit-like manner. Co-injection of [<sup>18</sup>F]<b>1</b> and 1 generated remarkably enhanced tumor uptake and signal-to-noise ratio in the tumor-bearing mice with drug-induced apoptosis, which correlated well with the expression level of activated caspase-3. This early readout of treatment response ensured that the probe [<sup>18</sup>F]<b>1</b> could serve as a promising PET imaging probe for timely and noninvasive evaluation of tumor therapy. Apoptosis ; Positron emission tomography ; Radiosynthesis ; In vivo ; HeLa ; Pet imaging ; In vitro ; Chemistry ; Caspase 3 ; Preclinical imaging ; Molecular biology ; Caspase ; Cancer research ; Programmed cell death ; Nuclear medicine ; Biochemistry ; Medicine ; Biology Molecular Sensors and Ion Detection ; Advanced biosensing and bioanalysis techniques ; Luminescence and Fluorescent Materials