Single Low-Dose Injection of Evans Blue Modified PSMA-617 Radioligand Therapy Eliminates Prostate-Specific Membrane Antigen Positive Tumors

Prostate Cancer is the most frequently diagnosed malignant tumor in men worldwide. Prostate-specific membrane antigen (PSMA) is a surface molecule specifically expressed by prostate tumors that has been shown to be a valid target for internal radionuclide therapy in both preclinical and clinical settings. The most common radiotherapeutic agent is the small molecule ¹⁷⁷Lu-PSMA-617, which is under clinical evaluation in multiple countries. Nevertheless, its efficacy in causing tumor regression is still suboptimal, even when administered in several cycles per patient, perhaps due to poor pharmacokinetics (PK), which limits uptake by the tumor cells. We postulated that the addition of the Evans blue (EB) moiety to PSMA-617 would improve the PK by extending circulation half-life, which would increase tumor uptake and improve radiotherapeutic efficacy. PSMA-617 was modified by conjugation of a 2-thiol acetate group onto the primary amine and thereafter reacted with a maleimide functional group of an EB derivative, to give EB-PSMA-617. The PK and radiotherapeutic efficacy of ⁹⁰Y- or ¹⁷⁷Lu-EB-PSMA-617 was compared to the clinically used radiopharmaceutical ⁹⁰Y- or ¹⁷⁷Lu- PSMA-617 in PC3-PIP tumor-bearing mice. EB-PSMA-617 retained binding to serum albumin as well as a high internalization rate by tumor cells. Upon injection, metal-labeled EB-PSMA-617 demonstrated an extended blood half-life compared to PSMA-617 and, thereby, prolonged the time window for binding to PSMA. The improved PK of EB-PSMA-617 resulted in significantly higher accumulation in PSMA⁺ tumors and highly effective radiotherapeutic efficacy. Remarkably, a single dose of 1.85 MBq of ⁹⁰Y- or ¹⁷⁷Lu-EB-PSMA-617 was sufficient to eradicate established PMSA⁺ tumors in mice. No significant body weight loss was observed, suggesting little to no gross toxicity. The construct described here, EB-PSMA-617, may improve the radiotherapeutic efficacy for patients with PSMA-positive tumors by reducing both the amount of activity needed for therapy as well as the frequency of administration, as compared to PSMA-617.