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

Abstract

The prostate-specific membrane antigen (PSMA)-targeted radiohybrid (rh) ligand [¹⁷⁷Lu]Lu-rhPSMA-7.3 has recently been assessed in a pretherapeutic dosimetry study on prostate cancer patients. In comparison to [¹⁷⁷Lu]Lu-PSMA I&T, application of [¹⁷⁷Lu]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 ¹⁷⁷Lu-labeled rhPSMA ligands is still missing. Thus, this study aimed to identify the rhPSMA ligand with the most favorable pharmacokinetics for ¹⁷⁷Lu-radioligand therapy. Methods: The 4 isomers of [¹⁷⁷Lu]Lu-rhPSMA-7 (namely [¹⁷⁷Lu]Lu-rhPSMA-7.1, -7.2, -7.3, and -7.4), along with the novel radiohybrid ligands [¹⁷⁷Lu]Lu-rhPSMA-10.1 and -10.2, were compared with the state-of-the-art compounds [¹⁷⁷Lu]Lu-PSMA I&T and [¹⁷⁷Lu]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. Results: ¹⁷⁷Lu 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 [¹⁷⁷Lu]Lu-rhPSMA-7 isomers demonstrated the highest AMW and thus strongest HSA interactions, [¹⁷⁷Lu]Lu-rhPSMA-10.1 showed an AMW lower than for [¹⁷⁷Lu]Lu-rhPSMA-7.3 but higher than for the ¹⁷⁷Lu-labeled references PSMA I&T and PSMA-617. In biodistribution studies, [¹⁷⁷Lu]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. Conclusion: Clinical investigation of [¹⁷⁷Lu]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.

Keywords: PSMA; prostate cancer; radiohybrid; radioligand therapy; rhPSMA.