Preclinical dosimetric studies of 177Lu-scFvD2B and comparison with 177Lu-PSMA-617 and 177Lu-iPSMA endoradiotherapeutic agents

Purpose: Internal dosimetry has become a very important tool to evaluate the risks and benefits of new endoradiotherapeutic agents. Nowadays, some of the most successful targeted radionuclide therapy (TRT) agents are 177Lu-DOTA conjugates based on low molecular weight (LMW) Glu-ureido PSMA inhibitors. It has, however, been demonstrated that the DOTA chelating moiety reduces the internalization of the LMW-PSMA agent and its radiation dose to the tumor. Previously, we reported that 177Lu-scFvD2B, an antibody-based construct, demonstrated statistically significant higher cell uptake and internalization in LNCaP prostate cancer (PCa) cells (PSMA-positive) when compared to the LMW-PSMA agents, 177Lu-PSMA-617 and 177Lu-iPSMA, two of the endoradiotherapeutic agents which currently are the most used in PCa therapy. The aim of this study is to estimate the preclinical 177Lu-scFvD2B organ and tumor-absorbed doses, and to compare the values with those of 177Lu-PSMA-617 and 177Lu-iPSMA. Methods: 177Lu-scFvD2B, 177Lu-PSMA-617, and 177Lu-iPSMA were prepared and their radiochemical purity determined. Biodistribution studies of each radiopharmaceutical were then carried out in healthy mice to define the main source organs (SO) and to calculate the number of disintegrations in each source organs per unit of administered activity (NSO). Absorbed dose in the main organs were then calculated for each 177Lu-conjugate by means of OLINDA/EXM 2.1.1 software, using the calculated NSO for both the adult male and the mouse phantoms as program inputs. Images of mice bearing micropulmonary tumors injected with 177Lu-conjugates were also obtained. Tumor standardized uptake values (SUV) for the different conjugates, obtained from the 3D SPECT image reconstruction of these mice, were used as the number of disintegrations in a tumor site per unit of administered activity (NT). The tumor-absorbed dose was calculated using the published electron dose S-values for sphere models with diameters ranging from 10 µm to 10 mm and considering a uniform activity distribution and tumor density equivalent to water density. Results: All 177Lu-labeled agents were obtained in high yield (98%). Dosimetric studies carried out using mouse phantoms demonstrated that organ absorbed doses of 177Lu-scFvD2B were from 1.4 to 2.3 times higher than those for 177Lu-iPSMA and from 1.5 to 2.6 times higher than those for 177Lu-PSMA-617. However, the 177Lu-scFvD2B values of tumor-absorbed doses for all investigated tumor sizes were from 2.8 to 3.0 times greater than those calculated for 177Lu-iPSMA and 177Lu-PSMA-617, respectively. Moreover, 177Lu-scFvD2B showed the highest tumor/kidney ratio when compared to those reported for 177Lu-albumin conjugates. Conclusions: In this preclinical study, we demonstrated the potential of 177Lu-scFvD2B as a therapeutic agent for PSMA-expressing tumors, due to its higher tumor-absorbed dose when compared with 177Lu-LMW agents.