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.