Margret Schottelius, Alexander Wurzer, Katharina Wissmiller, Roswitha Beck, Maximilian Koch, Dimitrios Gkorpas, Johannes Notni, Tessa Buckle, Matthias N. van Oosterom, Katja Steiger, Vasilis Ntziachristos, Markus Schwaiger, Fijs W.B. van Leeuwen, Hans-Jürgen Wester. (11-09-2018).

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Department: 

Research Area B

Abstract: 

The PSMA-targeted radiotracers 68Ga/177Lu-PSMA-I&T and 99mTc-PSMA-I&S are currently successfully employed for clinical PET imaging, radionuclide therapy and radioguided surgery of metastatic prostate cancer. To additionally exploit the high sensitivity and spatial resolution of fluorescence imaging for improved surgical guidance, a first PSMA-I&T-based hybrid tracer, PSMA-I&F (DOTAGA-k(Sulfo-Cy5)-y-nal-k-Sub-KuE), has been developed and evaluated.

The in vitro PSMA-targeting efficiency of PSMA-I&F, the reference PSMA-I&T and their corresponding natGa-/68Ga- and natLu/177Lu-counterparts was determined in LNCaP cells via competitive binding assays (IC50), dual-tracer radioligand- and fluorescence internalization studies. Biodistribution and small-animal PET imaging studies were performed in CB17-SCID and LNCaP xenograft bearing SHO mice, respectively, and complemented by intraoperative far-red fluorescence imaging using a clinical laparoscope. Additionally, fully automated serial cryosectioning and fluorescence imaging of one tumor-bearing animal as well as PSMA immunohistochemistry (IHC) and fluorescence microscopy of organ cryosections (tumor, kidney, spleen) were also performed.

Compared to the parent PSMA-I&T analogs, the PSMA-affinities of PSMA-I&F and its natGa-/natLu-complexes remained high and unaffected by dye conjugation (7.9<IC50<10.5 nM for all ligands). The same was observed for the internalization of 68Ga- and 177Lu-PSMA-I&F. In vivo, blood clearance of 68Ga- and 177Lu-PSMA-I&F was only slightly delayed by high plasma protein binding (94-95%), and very low accumulation in non-target organs was observed already at 1h p.i.. Dynamic PET imaging confirmed PSMA-specific (as demonstrated by coinjection of 2-PMPA) uptake into the LNCaP xenograft (4.5±1.8%iD/g) and the kidneys (106±23%iD/g). Tumor/background ratios of 2.1, 5.2, 9.6 and 9.6 for blood, liver, intestines and muscle, respectively, at 1h p.i. led to excellent imaging contrast in 68Ga-PSMA-I&F PET and in intraoperative fluorescence imaging. Furthermore, fluorescence imaging of tissue cryosections allowed high-resolution visualization of intraorgan PSMA-I&F distribution in vivo and its correlation with PSMA expression as determined by IHC.

Thus, with its high PSMA-targeting efficiency and favorable pharmacokinetic profile, 68Ga/177Lu-PSMA-I&F serves as an excellent proof-of-concept compound for the general feasibility of PSMA-I&T-based hybrid imaging. The PSMA-I&T scaffold represents a versatile PSMA-targeted lead structure, allowing relatively straightforward adaptation to the different structural requirements of dedicated nuclear or hybrid imaging agents.