Tolerability of [161Tb]Tb-SibuDAB in comparison to other PSMA-targeting radioligands in the preclinical setting

Purpose The successful preclinical efficacy studies prompted a Phase I clinical trial to investigate [ 161 Tb]Tb-SibuDAB. Deeper insight into potential undesired effects on normal tissues and organs remained to be investigated. The aim of this preclinical study was, therefore, to assess and compare the tolerability of [ 161 Tb]Tb-SibuDAB with that of [ 177 Lu]Lu-SibuDAB, [ 161 Tb]Tb-PSMA-I&T and [ 177 Lu]Lu-PSMA-I&T. Methods The time-dependent tissue distribution profiles of [ 161 Tb]Tb-SibuDAB and [ 161 Tb]Tb-PSMA-I&T were assessed in immunocompetent mice for dosimetry estimations. Substructural activity distribution in kidneys was further investigated ex vivo. In Study I, [ 161 Tb]Tb-SibuDAB, [ 177 Lu]Lu-SibuDAB, [ 161 Tb]Tb-PSMA-I&T and [ 177 Lu]Lu-PSMA-I&T were administered at 30 MBq/mouse while in Study II, additional activities of 15 MBq and 60 MBq [ 161 Tb]Tb-SibuDAB were applied. Blood cell counts were determined on Days 10, 28 and 56 after radioligand injection while blood and bone marrow smears, blood plasma biomarkers and selected organs were investigated at study end on Day 56. Results Organ uptake and absorbed doses were severalfold higher for SibuDAB than for PSMA-I&T and terbium-161 delivered about 40% higher organ doses than lutetium-177. Kidney accumulation of radioligands was mainly localized in the renal cortex and blockable with excess ligand in the case of PSMA-I&T but not for SibuDAB. In Study I, leukocyte and erythrocyte levels remained comparable between treated and control mice. Thrombocyte counts remained stable with PSMA-I&T radioligands, while SibuDAB radioligands caused 23–28% decrease by Day 10, with full recovery seen for [ 161 Tb]Tb-SibuDAB. In Study II, 60 MBq [ 161 Tb]Tb-SibuDAB did not significantly reduce leukocyte levels, but erythrocyte counts were marginally lower on Days 10 and 28. Thrombocytes decreased to 18%, 23% and 44% of control values on Day 10 after application of 15 MBq, 30 MBq and 60 MBq, respectively. Thrombocyte counts recovered over time, although average values in some groups remained modestly decreased on Day 56. In both studies, blood plasma biomarkers were similar between treated and control groups. Histological assessment of bone marrow, spleen, liver and kidney showed only minimal alterations (score ≤ 1/3). Conclusion Hematological changes were transient, but more pronounced after the application of SibuDAB than of PSMA-I&T, most likely due to the prolonged blood circulation. Other organs and tissues remained unaffected by any of the applied treatments. Substituting lutetium-177 with terbium-161 did not significantly alter the measured parameters. It remains to be demonstrated clinically whether 161 Tb-based radioligands can be administered at activity levels comparable to their 177 Lu-based analogues.