Abstract
SIGNIFICANCE: In previous studies, we reported on a new untargeted contrast agent, TMR-PEG1k, that exhibits high and durable diagnostic performance in glioma models starting within minutes of administration. This agent uses a fluorophore in the visible regime, which, although it helps ensure high-resolution imaging of superficial tissue, precludes the detection of subsurface structures due to the limited optical penetration. Thus, development of a near-infrared (NIR) version of the agent that retains the properties of TMR-PEG1k would combine the favorable diagnostic performance of TMR-PEG1k with sensitivity to subsurface pathologies enabled by NIR imaging. AIM: We aim to determine whether exchanging the fluorophore on TMR-PEG1k from tetramethylrhodamine to cyanine 7 would retain the highly favorable imaging properties exhibited by TMR-PEG1k. APPROACH: Mice ( N = 5 ) with orthotopic gliomas expressing green fluorescent protein (GFP) were co-administered TMR-PEG1k and Cy7-PEG1k. After 30 min, animals were euthanized, and the whole-body specimens were imaged using fluorescence cryotomography to recover co-registered three-dimensional (3D) fluorescence distributions of all fluorescent reporters. We evaluated the two agents using tumor-to-background ratio (TBR), contrast-to-noise ratio (CNR), area under the receiver operating characteristic curve (ROC-AUC), and normalized cross-correlation with GFP fluorescence (CC to GFP). RESULTS: Although the imaging system exhibited higher sensitivity to Cy7-PEG1k in phantoms, the 3D cryotomography results showed dramatic differences in the properties of the two agents in vivo. TMR-PEG1k produced highly selective tumor enhancement and concordance with GFP, whereas Cy7-PEG1k showed much lower selectivity, lower signal intensity, and produced no enhancement in many tumor regions. These observations were confirmed by the evaluation metrics, which were found to be (1) TBR = 5.2 versus 1.7 ( p = 0.0037 ), (2) CNR = 17.7 versus 3.8 ( p = 0.046 ), (3) ROC-AUC = 0.999 versus 0.821, and (4) CC to GFP = 0.90 versus 0.52, for TMR-PEG1k versus Cy7-PEG1k, respectively. CONCLUSIONS: Cy7-PEG1k did not retain the favorable properties exhibited by TMR-PEG1k and thus is not a suitable NIR analog for this agent.