The radiolabeling of silicon rhodamines for multimodal PET/ SPECT- and NIR optical imaging

Objectives: The combination of non-invasive molecular imaging (PET/SPECT)- and optical imaging (OI) techniques for tumor identification and resection are emerging. This powerful strategy promises to precisely differentiate between healthy and affected tumor tissues which is of most relevance for preoperative planning (prestaging) followed by R0-tumor resection via image-guided intraoperative surgery. The goal of this work is the development of radiolabeled near-infrared (NIR) fluorophores for PET/SPECT and optical imaging. The fluorophores were prepared for radiolabeling with the positron emitter fluorine-18 and with the gamma emitter iodine-123 for SPECT imaging with the aim to elucidate their potential as imaging agents for the detection of tumor tissues. Moreover, the radiolabeled dyes are intended to be bioconjugated to the PSMA-1007 binding motif, as a generic prominent tumor targeting vector for enrichment in prostate tumors (1).
Methods: We have developed fluorophores belonging to the silicon rhodamine (SiR) family with optical properties in the NIR spectral range (2). The photostable fluorophores have been characterized using NMR-, UV/VIS/NIR-spectroscopy and mass spectrometry. Furthermore the SiRs were radiolabeled by using the approach for copper-mediated radiolabeling of arylboronic acids, functioning as precursors both for fluorine-18 and iodine-123 labeling (3). The radiolabeling conditions were optimized based on radiochemical conversions (RCC) as analyzed by using radio-HPLC and radio-TLC. The model and lead compound [18F]F-SiR was isolated by radio-HPLC followed by SPE for further in vitro experiments, e.g. in vitro stability was determined in human serum.
Results: Novel boronic acid functionalized SiRs with chemical yields up to 68% were received via multistep organic syntheses. The blue dyes show high extinction coefficients up to 95.000 M-1cm-1, quantum yields of 0.33 and high photo stability making them useful for NIR optical imaging. After careful optimization of the radiolabeling conditions, [18F]F-SiR was obtained in isolated radiochemical yields of up to 31% and a molar activity of 70 GBq/µmol. Moreover, radioiodination experiments led to [123I]I-SiRs with radiochemical conversions higher than 90% (figure 1).
Demonstrated by human serum stability, the lead fluorophore [18F]F-SiR showed promising performance for in vitro and in vivo experiments.
The current work is focused on the synthesis of 18F/123I-radiolabeled SiRs and their reference analogs containing an active ester for bioconjugation with prominent biological vectors (e.g. PSMA-1007 motif) to perform first proof-of-concept studies.
Conclusions: The very first radiolabeled NIR fluorophores based on the SiR lead structure were synthesized and their labeling efficiencies for radiofluorination and radioiodination were evaluated. Ideal optical- and radiolabeling properties show promising features for further bioconjugation with prominent target vectors (e.g. PSMA-1007 binding motif) and biological evaluation of the novel SiR-PSMA-1007 conjugates in vitro and in vivo.
Acknowledgements:
This project is supported by the Wilhelm Sander-Stiftung for a grant on dual-labeled tumor tracers, grant number 2018.024.1.
References:
(1) K. Kopka et al., J. Nucl. Med. 2015, 56, 914–920.
(2) T. Nagano et al., J. Am. Chem. Soc. 2012, 134, 5029–5031.
(3) B. Neumaier et al., Chem. Eur. J. 2017, 23, 3251–3556.