Pharmacokinetic-Modifying Linker Design Modulates Performance of Multivalent PSMA/GRPR-Cell-Targeting 177Lu Radiopharmaceuticals

Background/Objectives: A series of PSMA/GRPR-targeting compounds were selected based on modifications to their respective pharmacokinetic-modifying linking groups and the structure of the PSMA-binding moiety, to examine how these alterations impact binding affinity and specificity for preclinical prostate cancer models. Methods: The multi-receptor targeting heterodimers PSMA/2PSMA-X-DOTA-Y-BBN (where X is 6-aminohexanoic acid (6AHX) or polyethylene glycol-4 (PEG4) and Y is either 8-aminooctanoic acid (8AOC) or carboxy piperidine (CP)) were radiolabeled with the theranostic isotope [177Lu]Lu. Each heterodimer construct was evaluated through characterization, stability, whole-cell in vitro studies, biodistribution in tumor-bearing mice, and in vivo small animal SPECT/CT molecular imaging. Results: In vitro evaluation of 2PSMA-6AHX-DOTA-8AOC-BBN demonstrated moderate IC50 values of 200 ± 20 nM in LNCaP cells and 21 ± 1 nM in PC3 cells. Subsequent biodistribution studies demonstrated selective tumor uptake at 4 h post-intravenous injection with uptake of 9.52 ± 2.59%ID/g in GRPR[+] PC3 tumors and 6.37 ± 0.64%ID/g in PSMA[+] PC3-PIP tumors. At 24 h post-injection, high tumor-to-blood ratios (874.0 ± 276 in PC3 and 344.3 ± 85 in PC3-PIP) and tumor-to-muscle ratios (739.8 ± 277 in PC3 and 403.5 ± 66 in PC3-PIP) were observed, further supporting the theranostic feasibility of the radiolabeled heterodimer. Small animal SPECT/CT imaging confirmed tumor targeting specificity of the heterobivalent peptide. Conclusions: Evaluation in PSMA/GRPR-expressing tumor models identified 2PSMA-6AHX-DOTA-8AOC-BBN as a leading candidate, having enhanced tumor uptake and favorable pharmacokinetics in tumor-bearing mice. These investigations advance structure–activity relationship understanding and support heterobivalent radiopharmaceuticals for precision imaging and targeted prostate cancer therapy.