Aim/Introduction:

Radiolabeled fluorescent dyes are crucial for bimodal imaging and currently in demand bimodal imaging (scintigraphic and optical imaging). Organic fluorescent dyes show unique optical properties such as high quantum yields, suitable stokes shifts and high extinction coefficients (1). Organic fluorophores as ICG or 5-ALA are well-known for fluorescent light-guided intraoperative surgery. The goal of this work is the development of new near-infrared (NIR) dyes for scintigraphic and optical imaging. The dyes are belonging to the silicon-rhodamine familiy. We have synthesized these first-in-class dyes for their subsequent conjugation to the SPECT-compatible radiometal technetium-99m with the aim to elucidate their potential as sentinel lymph node detecting agents. Moreover, PET-compatible Si-rhodamine conjugation to exemplary prominent tumor targeting binding vectors such as the PSMA-617-binding motif has also been performed. The introduced small molecule based dyes are intended to be used for noninvasive SPECT imaging (prestaging) followed by light-guided R0-tumor-resection.

Materials and Methods:

The combination of scintigraphic and optical imaging leads to new approaches in tumor imaging and its resection. This powerful strategy enables the differentiation of healthy and affected tumor tissues. We have developed dyes with absorption and emission properties in the near-infrared region of ca. 660 nm. We utilized the dyes for copper(I)-catalyzed alkyne-azide [3+2]-cycloaddition to receive respective 1,2,3-triazoles for complexing the well-known SPECT-radiometal technetium-99m using the click-to-chelate concept (2). Furthermore we have designed another DOTA-functionalized 1,2,3-triazole for complexing the PET-radiometal gallium-68. Finally, the dyes were characterized using NMR-, UV/VIS/NIR-spectroscopy and mass spectrometry.

Results:

The conventional synthesis of novel Si-rhodamines through xanthone building blocks provided novel amino- and azide-functionalized Si-rhodamines with overall yields of 60%. The azide-functionalized Si-rhodamines were converted with PSMA-inhibitor functionalized alkynes adapted from the PSMA-617 binding motif and L-propargylglycine to the corresponding 1,2,3-triazoles (3). Already determined extinction coefficients up to 120.000 M^-1cm^-1 and quantum yields of 0.45 show promising results, making them potentially useful for optical imaging. Furthermore the dyes were prepared as precursors for technetium-99m and gallium-68-labeling. For that purpose the prominent PET-active gallium-68 chelator DOTA was used. Corresponding rhenium-Si-rhodamines [used as “cold” technetium-surrogate] were characterized as well.

Conclusion:

A variety of novel NIR fluorescent dyes based on the Si-rhodamine lead structure were synthesized and chemically characterized. We successfully developed non-radioactive rhenium analogues of putative NIR rhodamine dyes for bimodal scintigraphic and optical imaging. Furthermore, our first-in-class radiolabeled silicon dyes are subject of current and future biological evaluation.

References:

(1) T. Nagano et al., J. Am. Chem. Soc. 2012, 134, 5029–5031.

(2) T. M. Mindt et al., J. Am. Chem. Soc. 2006, 47, 15096–15097.

(3) K. Kopka et al., J. Nucl. Med. 2015, 56, 914–920.