Importance of N- and C-terminal residues of substance P 1-7 for alleviating allodynia in mice after peripheral administration.

The heptapeptide SP (1, Arg-Pro-Lys-Pro-Gln-Gln-Phe) is the major bioactive metabolite formed after proteolytic processing of the neuropeptide substance P (SP, Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-Gly-Leu-Met-NH). The heptapeptide 1 frequently exhibits opposite effects to those induced by SP, such as exerting antinociception, or attenuating thermal hyperalgesia and mechanical allodynia. The heptapeptide SP amide (2, Arg-Pro-Lys-Pro-Gln-Gln-Phe-NH) is often more efficacious than 1 in experimental pain models. We have now assessed the anti-allodynic outcome after systemic administration of 2 and a series of Ala-substituted and truncated analogues of 2, in the spared nerve injury (SNI) mice model and the results obtained were correlated with in vitro plasma stability and permeability measurements. It is herein demonstrated that an intact Arg in SP amide analogues is fundamental for retaining a potent in vivo effect, while Lys of 2 is less important. A displacement with Ala or truncation rendered the peptide analogues either inactive or with a significantly attenuated in vivo activity. Thus, the pentapeptide SP amide (7, t=11.1 min) proven to be the major metabolite of 2, demonstrated an in vivo effect itself although considerably less significant than 2 in the SNI model. Intraperitoneal administration of 2 in a low dose furnished the most powerful anti-allodynic effect in the SNI model of all the analogous evaluated, despite a fast proteolysis of 2 in plasma (t=6.4 min). It is concluded that not only the C-terminal residue, that we previously demonstrated, but also the N-terminal with its basic side chain, are important for achieving effective pain relief. This information is of value for the further design process aimed at identifying more drug-like SP amide related peptidomimetics with pronounced anti-allodynic effects.