Targeted, Molecular Europium (III) Probes Enable Luminescence-Guided Surgery and 1 Photon Post-Surgical Luminescence Microscopy of Solid Tumors
Résumé
Discrete luminescent lanthanide complexes represent
a potential alternative to organic chromophores due to their
tunability of optical properties, insensitivity to photobleaching, and
large pseudo-Stokes shifts. Previously, we demonstrated that the
lack of depth penetration of UV excitation required to sensitize
discrete terbium and europium complexes can be overcome using
Cherenkov radiation emitted by clinically employed radioisotopes
in situ. Here, we show that the second-generation europium
complexes [Eu(III)(pcta-PEPA2)] and [Eu(III)(tacn-pic-PEPA2)] (Φ
= 57% and 76%, respectively) lower the limit of detection (LoD) to
1 nmol in the presence of 10 μCi of Cherenkov emitting isotopes,
18F and 68Ga. Bifunctionalization provides access to cysteine-linked peptide conjugates with comparable brightness and LoD. The
conjugate, [Eu(tacn-(pic-PSMA)-PEPA2)], displays high binding affinity to prostate-specific membrane antigen (PSMA)-expressing
PC-3 prostate cancer cells in vitro and can be visualized in the membrane-bound state using confocal microscopy. Biodistribution
studies with the [86Y][Y(III)(tacn-(pic-PSMA)-PEPA2)] analogue in a mouse xenograft model were employed to study
pharmacokinetics. Systemic administration of the targeted Cherenkov emitter, [68Ga][Ga(III)(PSMA-617)], followed by intratumoral
injection or topical application of 20 or 10 nmol [Eu(III)(tacn-(pic-PSMA)-PEPA2)], respectively, in live mice resulted in statistically
significant signal enhancement using conventional small animal imaging (620 nm bandpass filter). Optical imaging informed
successful tumor resection. Ex vivo imaging of the fixed tumor tissue with 1 and 2 photon excitation further reveals the accumulation
of the administered Eu(III) complex in target tissues. This work represents a significant step toward the application of luminescent
lanthanide complexes for optical imaging in a clinical setting.