Synthesis and Preclinical Evaluation of [18F]SiFA-PSMA Inhibitors in a Prostate Cancer Model

Positron emission tomography (PET) imaging of prostate-specific membrane antigen (PSMA) with gallium-68 (68Ga) and fluorine-18 (18F) radiotracers has aroused tremendous interest over the past few years. The use of organosilicon-[18F]­fluoride acceptors (SiFA) conjugated to urea-based peptidomimetic PSMA inhibitors provides a “kit-like” multidose synthesis technology. Nine novel 18F-labeled SiFA-bearing PSMA inhibitors with different linker moieties were synthesized and analyzed for their in vitro binding against [125I]­I-TAAG-PSMA in LNCaP cells. IC50 values ranged from 58–570 nM. Among all compounds, [18F]­SiFA-Asp2-PEG3-PSMA (IC50 = 125 nM) showed the highest tumor uptake in LNCaP tumors (SUV60min 0.73). A substantial increase in molar activity (Am) (from 7.5 ± 0.5 to 86 ± 3 GBq/μmol) led to a significant increase in LNCaP tumor uptake (SUV60min 1.18; Δ 0.45 corresponding to +62%). In vivo blocking with DCFPyL resulted in −32% uptake after 60 min. The SiFA-isotopic exchange chemistry offers a method that is readily adaptable for a “kit-type” labeling procedure and clinical translation.