Side-Chain Modified [^99mTc]Tc-DT1 Mimics: A Comparative Study in NTS₁R-Positive Models

Radiolabeled neurotensin analogs have been developed as candidates for theranostic use against neurotensin subtype 1 receptor (NTS₁R)-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 Lys⁷ in [^99mTc]Tc-[Lys⁷]DT1 (DT1, N₄-Gly-Arg-Arg-Pro-Tyr-Ile-Leu-OH, N₄ = 6-(carboxy)-1,4,8,11-tetraazaundecane) led to the fully stabilized [^99mTc]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 [^99mTc]Tc-DT1 without compromising pharmacokinetics, we now introduce three new [^99mTc]Tc-DT1 mimics, carrying different pendant groups at the ε-amine of Lys⁷: MPBA (4-(4-methylphenyl)butyric acid)-[^99mTc]Tc-DT10; MPBA via a PEG4-linker-[^99mTc]Tc-DT11; or a hydrophilic PEG6 chain-[^99mTc]Tc-DT12. The impact of these modifications on receptor affinity and internalization was studied in NTS₁R-positive cells. The effects on stability and AsPC-1 tumor uptake were assessed in mice without or during NEP/ACE inhibition. Unlike [^99mTc]Tc-DT10, the longer-chain modified [^99mTc]Tc-DT11 and [^99mTc]Tc-DT12 were significantly stabilized in vivo, resulting in markedly improved tumor uptake compared to [^99mTc]Tc-DT1. [^99mTc]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 NTS₁R-positive tumors using SPECT/CT.