Synthesis and Preclinical Evaluation of ¹⁷⁷Lu-labeled Radiohybrid PSMA Ligands (rhPSMAs) for Endoradiotherapy of Prostate Cancer

Abstract

Introduction: The prostate-specific membrane antigen (PSMA)-targeted radiohybrid (rh) ligand [¹⁷⁷Lu]Lu-rhPSMA-7.3 has recently been assessed in a pretherapeutic dosimetry study in 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. Aim: 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 most favorable pharmacokinetics for ¹⁷⁷Lu-radioligand therapy. Methods: The four 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 to the state-of-the-art compounds [¹⁷⁷Lu]Lu-PSMA I&T and [¹⁷⁷Lu]Lu-PSMA-617. The comparative evaluation comprised affinity studies (IC50) 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 µSPECT imaging was performed in LNCaP-tumor bearing mice at 24 h post injection. Results: ¹⁷⁷Lu-labeling of the radiohybrids was carried out 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, most pronounced differences were identified regarding the HSA-related AMW. While [¹⁷⁷Lu]Lu-rhPSMA-7 isomers demonstrated the highest AMW and thus strongest HSA-interactions, [¹⁷⁷Lu]Lu-rhPSMA-10.1 showed an AMW lower than [¹⁷⁷Lu]Lu-rhPSMA-7.3 but higher than 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 in order to determine if the favorable pharmacokinetics observed in mice will also result in high tumor uptake and decreased absorbed dose to kidneys and other non-target tissues in patients.