  The endogenous tridecapeptide neurotensin (NT) has emerged as an important
inhibitory modulator of pain transmission, exerting its analgesic action
through the activation of the G protein-coupled receptors, NTS1 and NTS2.
Whereas both NT receptors mediate the analgesic effects of NT, NTS1 activation
also produces hypotension and hypothermia, which may represent obstacles for
the development of new pain medications. In the present study, we implemented
various chemical strategies to improve the metabolic stability of the
biologically active fragment NT(8-13) and assessed their NTS1/NTS2 relative
binding affinities. We then determined their ability to reduce the nociceptive
behaviors in acute, tonic, and chronic pain models and to modulate blood
pressure and body temperature. To this end, we synthesized a series of NT(8-13)
analogs carrying a reduced amide bond at Lys8-Lys9 and harboring site-selective
modifications with unnatural amino acids, such as silaproline (Sip) and
trimethylsilylalanine (TMSAla). Incorporation of Sip and TMSAla respectively in
positions 10 and 13 of NT(8-13) combined with the Lys8-Lys9 reduced amine bond
(JMV5296) greatly prolonged the plasma half-life time over 20 hours. These
modifications also led to a 25-fold peptide selectivity toward NTS2. More
importantly, central delivery of JMV5296 was able to induce a strong
antinociceptive effect in acute (tail-flick), tonic (formalin), and chronic
inflammatory (CFA) pain models without inducing hypothermia. Altogether, these
results demonstrate that the chemically-modified NT(8-13) analog JMV5296
exhibits a better therapeutic profile and may thus represent a promising avenue
to guide the development of new stable NT agonists and improve pain management.
