Synthesis and Preclinical Evaluation of 177 Lu-Labeled Radiohybrid PSMA Ligands for Endoradiotherapy of Prostate Cancer

The prostate-specific membrane antigen (PSMA)-targeted radiohybrid (rh) ligand [ Lu]Lu-rhPSMA-7.3 has recently been assessed in a pretherapeutic dosimetry study on prostate cancer patients. In comparison to [ Lu]Lu-PSMA I&T, application of [ Lu]Lu-rhPSMA-7.3 resulted in a significantly improved tumor dose but also higher kidney accumulation. Although rhPSMA-7.3 has been initially selected as the lead compound for diagnostic application based on the characterization of its gallium complex, a systematic comparison of the most promising Lu-labeled rhPSMA ligands is still missing. Thus, this study aimed to identify the rhPSMA ligand with the most favorable pharmacokinetics for Lu-radioligand therapy. The 4 isomers of [ Lu]Lu-rhPSMA-7 (namely [ Lu]Lu-rhPSMA-7.1, -7.2, -7.3, and -7.4), along with the novel radiohybrid ligands [ Lu]Lu-rhPSMA-10.1 and -10.2, were compared with the state-of-the-art compounds [ Lu]Lu-PSMA I&T and [ Lu]Lu-PSMA-617. The comparative evaluation comprised affinity studies (half-maximal inhibitory concentration) and internalization experiments on LNCaP cells, as well as lipophilicity measurements. In addition, we determined the apparent molecular weight (AMW) of each tracer as a parameter for human serum albumin (HSA) binding. Biodistribution studies and small-animal SPECT imaging were performed on LNCaP-tumor bearing mice at 24 h after injection. Lu labeling of the radiohybrids was performed according to the established procedures for the currently established PSMA-targeted ligands. All ligands showed potent binding to PSMA-expressing LNCaP cells, with affinities in the low nanomolar range and high internalization rates. Surprisingly, the most pronounced differences regarded the HSA-related AMW. Although [ Lu]Lu-rhPSMA-7 isomers demonstrated the highest AMW and thus strongest HSA interactions, [ Lu]Lu-rhPSMA-10.1 showed an AMW lower than for [ Lu]Lu-rhPSMA-7.3 but higher than for the Lu-labeled references PSMA I&T and PSMA-617. In biodistribution studies, [ Lu]Lu-rhPSMA-10.1 exhibited the lowest kidney uptake and fastest excretion from the blood pool of all rhPSMA ligands while preserving a high tumor accumulation. Clinical investigation of [ Lu]Lu-rhPSMA-10.1 is highly warranted to determine whether the favorable pharmacokinetics observed in mice will also result in high tumor uptake and decreased absorbed dose to kidneys and other nontarget tissues in patients.