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Comparison of a Short Versus Long Stokes Shift Near-Infrared Dye During Intraoperative Molecular Imaging

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Intraoperative molecular imaging (IMI) utilizes optical dyes that accumulate within tumors to assist with detection during a cancer operation. IMI can detect disease not visualized preoperatively, as well as positive margins. However, these dyes are limited by autofluorescence, signal reflection, and photon-scatter. We hypothesize that a novel dye with a wide separation between excitation and emission spectra, SS180, would help overcome these obstacles.


Two targeted molecular contrast agents, OTL38 and SS180, were selected for this study. Both dyes had the same targeting ligand to folate receptor alpha (FRα). OTL38, a well-annotated IMI agent in human trials, has a Stokes shift of 22 nm, whereas SS180, the new dye, has a Stokes shift of 129 nm. Cell lines were tested for FRα expression and incubated with dyes to demonstrate receptor-dependent binding. Cells were incubated in various concentrations of the dyes to compare dose- and time-dependent binding. Finally, cells tagged with the dyes were injected subcutaneously in a murine model to estimate tumor burden necessary to generate fluorescent signal.


Cellular studies demonstrated that SS180 binds cells in a dose-, receptor-, and time-dependent manner and exhibits higher mean fluorescence intensities by flow cytometry when compared with OTL38 for each time point and concentration. In an in vivo flank tumor model, SS180 had a higher tumor-to-background ratio (TBR) than OTL38, though not statistically significant (p = 0.08). Ex vivo, OTL38 had a higher TBR than SS180 (p = 0.02). The subcutaneous model revealed that SS180 had a higher TBR at 5 × 106 cells than OTL38 (p = 0.05). No toxicity was observed in the animals.


SS180 exhibits greater TBRs in vivo, but not ex vivo. These findings suggest that SS180 may have weaker fluorescence, but superior contrast. Studies in large animal models and clinical trials may better elucidate the clinical value of a long Stokes shift.

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CC was supported by the National Center for Advancing Translational Sciences of the National Institutes of Health under award number TL1TR001880. SS and PL were supported by the National Institutes of Health Biomedical Research Partnership, R01 CA193556. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

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Correspondence to Christopher J. Corbett.

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Philip Low is on the Board of Directors at On Target Laboratories, the manufacturers of all the study drugs utilized in this report. The remaining authors declare that they have no conflict of interest.

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Corbett, C.J., Frenzel Sulyok, L.G., Predina, J.D. et al. Comparison of a Short Versus Long Stokes Shift Near-Infrared Dye During Intraoperative Molecular Imaging. Mol Imaging Biol 22, 144–155 (2020). https://doi.org/10.1007/s11307-019-01434-2

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Key words

  • Oncology
  • Surgery
  • Imaging
  • Fluorescence
  • Small molecule
  • Dye