Lu-177-PSMA-617 Prostate-Specific Membrane Antigen Inhibitor Therapy in Patients with Castration-Resistant Prostate Cancer: Stability, Bio-distribution and Dosimetry

Objective:The aim of the study was to estimate the radiation-absorbed doses and to study the in vivo and in vitro stability as well as pharmacokinetic characteristics of lutetium-177 (Lu-177) prostate-specific membrane antigen (PSMA)-617.Methods: For this purpose, 7 patients who underwent Lu-177-PSMA therapy were included into the study.The injected Lu-177-PSMA-617 activity ranged from 3.6 to 7.4 GBq with a mean of 5.2±1.8GBq.The stability of radiotracer in saline was calculated up to 48 h.The stability was also calculated in blood and urine samples.Post-therapeutic dosimetry was performed based on whole body and single photon emission computed tomography/computed tomography (SPECT/CT) scans on dualheaded SPECT/CT system.Results: The radiochemical yield of Lu-177-PSMA-617 was >99%.It remained stable in saline up to 48 h.Analyses of the blood and urine samples showed a single radioactivity peak even at 24 hours after injection.Half-life of the distribution and elimination phases were calculated to be 0.16±0.09and 10.8±2.5 hours, respectively.The mean excretion rate was 56.5±8.8%ranging from 41.5% to 65.4% at 24 h.Highest radiation estimated doses were calculated for parotid glands and kidneys (1.90±1.19 and 0.82±0.25 Gy/GBq respectively).Radiation dose given to the bone marrow was significantly lower than those of kidney and parotid glands (p<0.05)(0.030±0.008Gy/GBq).Conclusion: Lu-177-PSMA-617 is a highly stable compound both in vitro and in vivo.Lu-177-PSMA-617 therapy seems to be a safe method for the treatment of castration-resistant prostate cancer patients.The fractionation regime that enables the longest duration of tumor control and/or survival will have to be developed in further studies.