Abstract
Radiotracers incorporating the urea-based Glu-NH-C(O)-NH-Lys group have gained prominence due to their role in targeting prostate-specific membrane antigen (PSMA)-a clinical biomarker of prostate cancer. Here, the synthesis, radiolabeling, and in vitro and in vivo characterization of two (68)Ga-radiolabeled Glu-NH-C(O)-NH-Lys radiotracers conjugated to the desferrioxamine B (DFO) chelate were evaluated. Two linker groups based on amide bond and thiourea coupling chemistries were employed to develop (68)Ga-DFO-Nsucc-PSMA ((68)Ga-4) and (68)Ga-DFO- pNCS-Bn-PSMA ((68)Ga-7), respectively. Radiosynthesis proceeded quantitatively at room temperature with high radiochemical yields, chemical/radiochemical purities, and specific activities. Pharmacokinetic profiles of (68)Ga-4 and (68)Ga-7 were assessed using positron-emission tomography (PET) in mice bearing subcutaneous LNCaP tumors. Data were compared to the current clinical benchmark radiotracer (68)Ga-HBED-CC-PSMA ((68)Ga-1) (HBED = N,N'-Bis(2-hydroxy-5-(ethylene-beta-carboxy)benzyl)ethylenediamine N,N'-diacetic acid). Results indicated that the target binding affinity, protein association, blood pool and background organ clearance properties, and uptake in PSMA-positive lesions are strongly dependent on the nature of the chelate, the linker, and the spacer groups. Protein dissociation constants ( K(d) values) were found to be predictive of pharmacokinetics in vivo. Compared to (68)Ga-1, (68)Ga-4 and (68)Ga-7 resulted in decreased tumor uptake but enhanced blood pool clearance and reduced residence time in the kidney. The study highlights the importance of maximizing protein binding affinity during radiotracer optimization.