Toward Stability Enhancement of NTS1R-Targeted Radioligands: Structural Interventions on [Tc-99m]Tc-DT1

The neurotensin subtype 1 receptor (NTS1R) is overexpressed in a number of human tumors, thereby representing a valid target for cancer theranostics with radiolabeled neurotensin (NT) analogs like [Tc-99m]Tc-DT1 (DT1, N-4-Gly(7)-NT(8-13)). Thus far, the fast degradation of intravenously injected NT-radioligands by neprilysin (NEP) and angiotensin-converting enzyme (ACE) has compromised their clinical applicability. Aiming at metabolic stability enhancements, we herein introduce (i) DT7 ([DAsn(14)]DT1) and (ii) DT8 ([beta-Homoleucine(13)]DT1), modified at the C-terminus, along with (iii) DT9 ([(palmitoyl)Lys(7)]DT1), carrying an albumin-binding domain (ABD) at Lys(7). The biological profiles of the new [Tc-99m]Tc-radioligands were compared with [Tc-99m]Tc-DT1, using NTS1R-expressing AsPC-1 cells and mice models without or during NEP/ACE inhibition. The radioligands showed enhanced in vivo stability vs. [Tc-99m]Tc-DT1, with [Tc-99m]Tc-DT9 displaying full resistance to both peptidases. Furthermore, [Tc-99m]Tc-DT9 achieved the highest cell internalization and tumor uptake even without NEP/ACE-inhibition but with unfavorably high background radioactivity levels. Hence, unlike C-terminal modification, the introduction of a pendant ABD group in the linker turned out to be the most promising strategy toward metabolic stability, cell uptake, and tumor accumulation of [Tc-99m]Tc-DT1 mimics. To improve the observed suboptimal pharmacokinetics of [Tc-99m]Tc-DT9, the replacement of palmitoyl on Lys(7) by other ABD groups is currently being pursued.