Prostate-specific membrane antigen (PSMA) is a well-recognized target for identification and therapy of a variety of cancers. Here we report five ⁶⁴Cu-labeled inhibitors of PSMA, [⁶⁴Cu]3–7, which are based on the lysine–glutamate urea scaffold and utilize a variety of macrocyclic chelators, namely NOTA(3), PCTA(4), Oxo-DO3A(5), CB-TE2A(6), and DOTA(7), in an effort to determine which provides the most suitable pharmacokinetics for in vivo PET imaging. [⁶⁴Cu]3–7 were prepared in high radiochemical yield (60–90%) and purity (>95%). Positron emission tomography (PET) imaging studies of [⁶⁴Cu]3–7 revealed specific accumulation in PSMA-expressing xenografts (PSMA+ PC3 PIP) relative to isogenic control tumor (PSMA– PC3 flu) and background tissue. The favorable kinetics and high image contrast provided by CB-TE2A chelated [⁶⁴Cu]6 suggest it as the most promising among the candidates tested. That could be due to the higher stability of [⁶⁴Cu]CB-TE2A as compared with [⁶⁴Cu]NOTA, [⁶⁴Cu]PCTA, [⁶⁴Cu]Oxo-DO3A, and [⁶⁴Cu]DOTA chelates in vivo.

Targeted radiopharmaceutical therapy (TRT) using α-particle radiation is a promising approach for treating both large and micrometastatic lesions. We developed prostate-specific membrane antigen (PSMA)–targeted low-molecular-weight agents for ²¹²Pb-based TRT of patients with prostate cancer (PC) by evaluating the matching γ-emitting surrogate, ²⁰³Pb. Methods: Five rationally designed low-molecular-weight ligands (L1-L5) were synthesized using the lysine-urea-glutamate scaffold, and PSMA inhibition constants were determined. Tissue biodistribution and SPECT/CT imaging of ²⁰³Pb-L1–²⁰³Pb-L5 were performed on mice bearing PSMA(+) PC3 PIP and PSMA(−) PC3 flu flank xenografts. The absorbed radiation dose of the corresponding ²¹²Pb-labeled analogs was determined using the biodistribution data. Antitumor efficacy of ²¹²Pb-L2 was evaluated in PSMA(+) PC3 PIP and PSMA(−) PC3 flu tumor models and in the PSMA(+) luciferase-expressing micrometastatic model. ²¹²Pb-L2 was also evaluated for dose-escalated, long-term toxicity. Results: All new ligands were obtained in high yield and purity. PSMA inhibitory activities ranged from 0.10 to 17 nM. ²⁰³Pb-L1–²⁰³Pb-L5 were synthesized in high radiochemical yield and specific activity. Whole-body clearance of ²⁰³Pb-L1–²⁰³Pb-L5 was fast. The absorbed dose coefficients (mGy/kBq) of the tumor and kidneys were highest for ²⁰³Pb-L5 (31.0, 15.2) and lowest for ²⁰³Pb-L2 (8.0, 4.2). The tumor-to-kidney absorbed dose ratio was higher for ²⁰³Pb-L3 (3.2) and ²⁰³Pb-L4 (3.6) than for the other agents, but with lower tumor-to-blood ratios. PSMA(+) tumor lesions were visualized through SPECT/CT as early as 0.5 h after injection. A proof-of-concept therapy study with a single administration of ²¹²Pb-L2 demonstrated dose-dependent inhibition of tumor growth in the PSMA(+) flank tumor model. ²¹²Pb-L2 also demonstrated an increased survival benefit in the micrometastatic model compared with ¹⁷⁷Lu-PSMA-617. Long-term toxicity studies in healthy, immunocompetent CD-1 mice revealed kidney as the dose-limiting organ. Conclusion: ²⁰³Pb-L1–²⁰³Pb-L5 demonstrated favorable pharmacokinetics for ²¹²Pb-based TRT. The antitumor efficacy of ²¹²Pb-L2 supports the corresponding ²⁰³Pb/²¹²Pb theranostic pair for PSMA-based α-particle TRT in advanced PC.