Optical imaging probes are indispensable tools for the visualization of biomarkers of disease without perturbing the biological system. Luminescent lanthanides exhibit many properties of ideal optical imaging probes such as low limits of detection, minimal photobleaching and emit in the first or second biological window (550-800 nm and 1000-1350 nm). However, as f-f transitions are Laporte-forbidden, organic chromophores absorbing in the 200-400 nm range, known as antennae, are necessary for efficient excitation of these complexes. These excitation wavelengths are not compatible with biological applications due to the strong absorbance of proteins and hemoglobin at wavelengths below 550 nm; as a consequence, discrete luminescent lanthanide complexes have remained unexplored for in vivo optical imaging applications.<br/>To circumvent this, we have successfully demonstrated that lanthanide-based complexes can be excited in situ using Cherenkov radiation. Cherenkov radiation is generated by charged particles emitted during the decay of radioisotopes in dielectric media in the form of the emission of short wavelength light with a maximum below 400 nm. In situ excitation of lanthanide complexes with Cherenkov radiation energy transfer (CRET) eliminates the need for external high energy, short wavelength excitation. Following our initial proof of concept, we have demonstrated that CRET is suitable to excite discrete, polyazamacrocycle based Tb and Eu complexes with detection limits of 5 nmol and 10 nmol, respectively.<br/>Here, we demonstrate that multiplexed imaging with Tb and Eu complexes is feasible in vitro. Following phantom imaging experiments, we assessed tissue permeability of <sup>18</sup>F-mediated, CRET-mediated excitation of peptide-functionalized Tb and Eu complexes and for the first time, demonstrate a persistent in vivo detectable optical signal in murine, prostate-specific membrane antigen-expressing xenografts using conventional, excitationless optical, small animal imaging. This work paves the way for the optical, intrasurgical molecular imaging with discrete luminescent lanthanide complexes.

In situ excitation of luminescent lanthanide complexes for in vivo bioimaging

ACS Fall 2021

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