PSMA-targeted¹⁸F-labeled Radiohybrid Inhibitors: Concept, preclinical evaluation and first proof of concept study in men

Objectives: With the aim to develop a platform technology that allows for fast and efficient labeling of peptide and peptide-like radiopharmaceuticals with either ¹⁸F or radiometals, we have synthesized and evaluated novel radiohybrid PSMA (rhPSMA) ligands. A unique feature of such rhPSMA-ligands is that both, fluorine and the metal, are always present, but only one of them as radioactive isotope (e.g. [¹⁸F,^natGa]rhPSMA or [¹⁹F,⁶⁸Ga]rhPSMA). In this study we present the results of the development and evaluation of a series of F/Ga-rhPSMA ligands. Methods: Six different radiohybrid-PSMA ligands that contain a PSMA-addressing urea motif, a silicon fluoride acceptor moiety for ¹⁸F-labeling by isotopic exchange (IE) and a chelator for complexation with trivalent metal ions were synthesized by SPPS. ¹⁸F-labeling of [¹⁹F,^natGa]rhPSMA-7 by IE was carried out at room temperature within 5 min. The overall production was completed in less than 15 min (80±5% RCY). PSMA-affinities (IC₅₀) and internalization studies (37°C, 60 min) were carried out on LNCaP cells. In addition, binding to human serum albumin and log P values were quantified. μPET imaging and biodistribution was carried out on LNCaP tumor-bearing SCID mice. Proof of concept (PoC) in men was performed by [¹⁸F,^natGa]rhPSMA-7 in a patient suffering from mCRPC. Results: All compounds showed suitable hydrophilicity (log P: -2.6 to -3.5) and >94% binding to human serum albumin. The novel rhPSMA inhibitors showed PSMA affinities between 3.0 and 11 nM and rapid cell internalization (33-177%). Biodistribution and μPET-scans of rhPSMA-7 and 10 in mice showed high tumor accumulation, fast renal excretion, low activity accumulation in non-target tissues and no elevated ¹⁸F-uptake in bone. Based on their high affinity, fast internalization and excellent biodistribution, the DOTA- and DOTAGA-derived inhibitors rhPSMA-7 and 10 were found to be the most promising candidates for first PoC studies in men. In the proof of concept study in a patient [¹⁸F,^natGa]rhPSMA-7 demonstrated fast blood clearance via the kidneys, showed almost no activity transfer into the bladder (up to 120 min p.i.) and allowed high-contrast imaging of LN and bone metastasis. Conclusion: Radiohybrid PSMA ligands represent a unique class of either paired diagnostic (e.g. F/Ga-rhPSMA) or true theranostic (e.g. F/Lu-rhPSMA) ligands. [¹⁸F,^natGa]rhPSMA-7 and [¹⁸F,^natGa]rhPSMA-10 were found to have favorable pharmacokinetic profiles in men, show high stability towards in-vivo defluorination (no bone uptake) and almost no activity in the ureters and bladder. Based on these characteristics and the simple high yield production, [¹⁸F,^natGa]rhPSMA-7 has been transferred into detailed clinical studies. Noteworthy, due to the identity of ¹⁸F- and ⁶⁸Ga-labeled rhPSAM-7, experiences obtained with one of these compounds can be transferred to the other twin.