Side-Chain Modified [(99m)Tc]Tc-DT1 Mimics: A Comparative Study in NTS(1)R-Positive Models

Radiolabeled neurotensin analogs have been developed as candidates for theranostic use against neurotensin subtype 1 receptor (NTS(1)R)-expressing cancer. However, their fast degradation by two major peptidases, neprilysin (NEP) and angiotensin-converting enzyme (ACE), has hitherto limited clinical success. We have recently shown that palmitoylation at the ε-amine of Lys(7) in [(99m)Tc]Tc-[Lys(7)]DT1 (DT1, N(4)-Gly-Arg-Arg-Pro-Tyr-Ile-Leu-OH, N(4) = 6-(carboxy)-1,4,8,11-tetraazaundecane) led to the fully stabilized [(99m)Tc]Tc-DT9 analog, displaying high uptake in human pancreatic cancer AsPC-1 xenografts but unfavorable pharmacokinetics in mice. Aiming to improve the in vivo stability of [(99m)Tc]Tc-DT1 without compromising pharmacokinetics, we now introduce three new [(99m)Tc]Tc-DT1 mimics, carrying different pendant groups at the ε-amine of Lys(7): MPBA (4-(4-methylphenyl)butyric acid)—[(99m)Tc]Tc-DT10; MPBA via a PEG4-linker—[(99m)Tc]Tc-DT11; or a hydrophilic PEG6 chain—[(99m)Tc]Tc-DT12. The impact of these modifications on receptor affinity and internalization was studied in NTS(1)R-positive cells. The effects on stability and AsPC-1 tumor uptake were assessed in mice without or during NEP/ACE inhibition. Unlike [(99m)Tc]Tc-DT10, the longer-chain modified [(99m)Tc]Tc-DT11 and [(99m)Tc]Tc-DT12 were significantly stabilized in vivo, resulting in markedly improved tumor uptake compared to [(99m)Tc]Tc-DT1. [(99m)Tc]Tc-DT11 was found to achieve the highest AsPC-1 tumor values and good pharmacokinetics, either without or during NEP inhibition, qualifying for further validation in patients with NTS(1)R-positive tumors using SPECT/CT.