A single injection of Evans blue modified 177Lu-PSMA-617 provides a radiotherapeutic cure for prostate-specific membrane antigen (PSMA) tumor xenografts in mice

313 Objectives: Background: Prostate cancer is the most frequent malignant tumor in men worldwide. Prostate-specific membrane antigen (PSMA), a cell-surface molecule specifically expressed by prostate tumors, was shown to be a valid target for radiotherapy in preclinical studies. Radiotherapy relies on delivery of radionuclide to receptor expressing tumors with limited off-target accumulation. In patients, anti-PSMA antibody labeled with 177Lu displayed beneficial anti-cancer activity but also produced bone marrow and hematology toxicity, possibly due to its long half-life in the blood (days). Several small molecules targeting PSMA were also evaluated in prostate cancer patients labeled with beta and alpha emitters. The most common radiotherapeutic agent is 177Lu-PSMA-617, which is under clinical evaluation in many countries. Most clinical trials with 177Lu-PSMA-617 were designed with several cycles of 177Lu-PSMA-617 per patient. Small molecules, in general, are cleared very quickly from the circulation. Therefore, radiotherapy using small molecules requires high doses and frequent administration, leading to systemic toxicity. Our goal was to prepare an analog of PSMA-617 with improved pharmacokinetics (PK) that would require less frequent administration and improve efficacy. Experimental: PSMA-617 was synthesized with a thiol group, instead of the DOTA, on the primary amine and, thereafter, reacted with a maleimide functional group of an Evans blue derivative, containing the necessary chelating DOTA, to give EB-PSMA-617. Tumor xenograft models (PC3-PIP) were established in nude mice. When the tumor reached 100-150 mm3, animals were divided into groups and treated with a single dose of saline, 177Lu-EB-PSMA 617 (7.4 MBq or 18.5 MBq), or 177Lu-PSMA-617 (18.5 MBq). The tumor size was monitored for 45 days or until size reached experimental end point. Results and discussion: EB-PSMA-617 retained EB binding to serum albumin as well as high internalization rate by tumor cells in vitro. Detailed PK studies showed that upon intravenous injection of 177Lu-EB-PSMA-617, it underwent equilibrium binding with serum albumin which extended its blood half-life thereby prolonging the time window for binding to PSMA. In the vicinity of the higher affinity PSMA, EB-PSMA-617 binds to the receptor expressing cells and subsequently internalized. The improved PK of EB-PSMA-617, demonstrated by both longer half-life and higher retention in the blood pool, resulted in significantly higher accumulation in PSMA+ tumors as evident from the area under the curve calculation and enhanced target to background ratios. As a result, the tumor radiotherapy efficacy was highly efficient and a single dose of 7.4 MBq 177Lu-EB-PSMA-617 was sufficient to eradicate established PMSA+ PC3-PIP tumors (Fig. 1) and significantly improve survival rates. In comparison, a similar dose of 177Lu-PSMA-617 radiotherapy only provided a temporary tumor-growth delay. No significant body weight changes were observed, suggesting that there was no gross toxicity. The radiopharmaceutical described here, 177Lu-EB-PSMA-617, may revolutionize radiotherapy of patients with PSMA-positive tumors by reducing both the amount of activity needed for therapy as well as the number of cycles. The general design may be applicable to other therapeutic PSMA ligands and can transform drugs into theranostic entities.