Lu-labeled low-molecular-weight agents for PSMA-targeted radiopharmaceutical therapy.

Purpose: To develop a prostate-specific membrane antigen (PSMA)-targeted radiotherapeutic for metastatic castration-resistant prostate cancer (mCRPC) with optimized efficacy and minimized toxicity employing the β-particle radiation of Lu.

Methods: We synthesized 14 new PSMA-targeted, Lu-labeled radioligands (Lu-L1-Lu-L14) using different chelating agents and linkers. We evaluated them in vitro using human prostate cancer PSMA(+) PC3 PIP and PSMA(-) PC3 flu cells and in corresponding flank tumor models. Efficacy and toxicity after 8 weeks were evaluated at a single administration of 111 MBq for Lu-L1, Lu-L3, Lu-L5 and Lu-PSMA-617. Efficacy of Lu-L1 was further investigated using different doses, and long-term toxicity was determined in healthy immunocompetent mice.

Results: Radioligands were produced in high radiochemical yield and purity. Cell uptake and internalization indicated specific uptake only in PSMA(+) PC3 cells. Lu-L1, Lu-L3 and Lu-L5 demonstrated comparable uptake to Lu-PSMA-617 and Lu-PSMA-I&T in PSMA-expressing tumors up to 72 h post-injection. Lu-L1, Lu-L3 and Lu-L5 also demonstrated efficient tumor regression at 8 weeks. Lu-L1 enabled the highest survival rate. Necropsy studies of the treated group at 8 weeks revealed subacute damage to lacrimal glands and testes. No radiation nephropathy was observed 1 year post-treatment in healthy mice receiving 111 MBq of Lu-L1, most likely related to the fast renal clearance of this agent.

Conclusions: Lu-L1 is a viable clinical candidate for radionuclide therapy of PSMA-expressing malignancies because of its high tumor-targeting ability and low off-target radiotoxic effects.