Design, Synthesis, Molecular Dynamics Simulation, and Functional Evaluation of a Novel Series of 26RFa Peptide Analogues Containing a Mono- or Polyalkyl Guanidino Arginine Derivative

26RFa, the endogenous QRFPR ligand,
is implicated in several physiological
and pathological conditions such as the regulation of glucose homeostasis
and bone mineralization; hence, QRFPR ligands display therapeutic
potential. At the molecular level, functional interaction occurs between
residues Arg25 of 26RFa and Gln125 of QRFPR.
We have designed 26RFa(20–26) analogues incorporating
arginine derivatives modified by alkylated substituents. We found
that the Arg25 side chain length was necessary to retain
the activity of 26RFa(20–26) and that N-monoalkylation of arginine was accommodated by the QRFPR active
site. In particular, [(Me)ωArg25]­26RFa(20–26) (5b, LV-2186) appeared to be 25-fold
more potent than 26RFa(20–26) and displayed a position
in a QRFPR homology model slightly different to that of the unmodified
heptapeptide. Other peptides were less potent than 26RFa(20–26), exhibited partial agonistic activity, or were totally inactive
in accordance to different ligand-bound structures. In vivo, [(Me)ωArg25]­26RFa(20–26) exerted
a delayed 26RFa-like hypoglycemic effect. Finally, N-methyl substituted arginine-containing peptides represent lead compounds
for further development of QRFPR agonists.