Examining Absorbed Doses of Indigenously Developed Lu-PSMA-617 in Metastatic Castration-Resistant Prostate Cancer Patients at Baseline and During Course of Peptide Receptor Radioligand Therapy.

The objective of this study was to estimate the absorbed doses to the normal organs and tumor lesions in metastatic castration-resistant prostate cancer (mCRPC) patients treated with indigenously developed Lu-PSMA-617 that could establish optimal treatment protocol with minimum risk to the dose-limiting organs. Furthermore, attempt was also made to compare radiation absorbed doses for normal organs and tumor lesions in subsequent cycles of Lu-PSMA-617 peptide receptor radioligand therapy (PRLT) in the same group of patients during the course of treatment. A total of 30 patients of proven mCRPC were enrolled for this prospective study. These patients received up to 5 cycles of treatment with Lu-PSMA-617 PRLT (1 cycle for 13 patients, 2 cycles for 9 patients, 3 cycles for 3 patients, and 5 cycles for 5 patients), at 11-12-week intervals between the two successive therapies. The patients underwent postadministration whole-body scintigraphy at five time points: 0.5 (prevoid), 2, 12, 24, and 72/96 h (postvoid). From time-activity curves generated by drawing regions of interests on the images, number of disintegrations was determined. Tumor masses were estimated from pretherapeutic Ga-PSMA-11 positron emission tomography-computed tomography images. Absorbed doses for organs and tumors were calculated using OLINDA 2.0 software. The average activity of Lu-PSMA-617 (mean ± SD) administered per patient per cycle was 4.94 ± 0.45 GBq. The mean absorbed organ doses (mean ± SD) from first therapy cycle in Gy/GBq were as follows: kidneys 0.52 ± 0.16, spleen 0.17 ± 0.07, liver 0.08 ± 0.05, salivary glands 0.53 ± 0.30, lacrimal glands 1.45 ± 0.85, nasal mucosa membrane 0.46 ± 0.19, urinary bladder 0.23 ± 0.02, and bone marrow 0.04 ± 0.03. The mean effective dose for whole body from first therapy cycle was 0.05 ± 0.03 Sv/GBq. Among all the normal organs, lacrimal glands received the highest absorbed dose. The median dose for all lesions, bone lesions, lymph nodes, primary site, liver lesion, lung lesion, and soft tissue deposit from first therapy cycle was determined to be 4.17, 4.23, 3.96, 4.36, 10.27, 0.78, and 4.68 Gy/GBq respectively. Absorbed doses received by the normal organs in five consecutive cycles follow three different trends, (a) for kidneys, salivary glands, and nasal mucous membrane, absorbed doses increased from first therapy cycle to second therapy cycle and then slowly decreased in subsequently therapy cycles; (b) for spleen, liver, and lacrimal glands, absorbed doses decreased with the successive therapy cycles; and (c) in case of bone marrow, bladder, and whole body, mean absorbed dose almost remained constant in each therapy cycle. Absorbed doses to the lesions gradually decreased with increase of the number of therapy cycles. The organ and tumor absorbed doses of Lu-PSMA-617 in mCRPC patients were found to be comparable to the data reported in the literature. The highest absorbed organ dose was observed in lacrimal glands and being a dose limiting organ, a cumulative activity up to 32.5 GBq (878 mCi) of Lu-PSMA-617 in 4-5 therapy cycles appears safe and feasible to achieve full therapeutic window.