Biphalin is a dimericenkephalin endogenous peptide (Tyr-D-Ala-Gly-Phe-NH)2 composed of two tetrapeptides derived from enkephalins, connected 'tail-to-tail' by a hydrazide bridge.[1] The presence of two distinct pharmacophores confers on biphalin a high affinity for both μ and δ opioid receptors (with an EC50 of about 1–5 nM for both μ and δ receptors), therefore it has analgesic activity.[2]
Biphalin presents a considerable antinociceptive profile. In fact, when administered intracerebroventricularly in mice, biphalin displays a potency almost 7-fold greater than that of the ultra-potent alkaloid agonist, etorphine and 7000-fold greater than morphine; biphalin and morphine were found to be equipotent after intraperitoneal administration. The extraordinary in vivo potency shown by this compound is coupled with low side-effects, in particular, to produce no dependency in chronic use.[3] For these reasons, several efforts have been carried out in order to obtain more information about structure-activity relationship (SAR). Results clearly indicate that, at least for μ receptor binding, the presence of two pharmacophores is not necessary;[2]Tyr1 is indispensable for analgesic activity, while replacing Phe at the position 4 and 4' with non-aromatic, but lipophilic amino acids does not greatly change the binding properties[2] and in general 4,4' positions are found to be important to design biphalin analogues with increased potency and modified μ/δ selectivity.[4][5] The hydrazide linker is not fundamental for activity or binding, and it can be conveniently substituted by different conformationally constrained cycloaliphatic diamine linkers.[6]
References
^Flippen-Anderson JL, Deschamps JR, George C, Hruby VJ, Misicka A, Lipkowski AW (March 2002). "Crystal structure of biphalin sulfate: a multireceptor opioid peptide". The Journal of Peptide Research. 59 (3): 123–33. doi:10.1034/j.1399-3011.2002.01967.x. PMID11985706.
^ abcLipkowski AW, Misicka A, Davis P, Stropova D, Janders J, Lachwa M, et al. (September 1999). "Biological activity of fragments and analogues of the potent dimeric opioid peptide, biphalin". Bioorganic & Medicinal Chemistry Letters. 9 (18): 2763–6. doi:10.1016/S0960-894X(99)00464-3. PMID10509931.
^Horan PJ, Mattia A, Bilsky EJ, Weber S, Davis TP, Yamamura HI, et al. (June 1993). "Antinociceptive profile of biphalin, a dimeric enkephalin analog". The Journal of Pharmacology and Experimental Therapeutics. 265 (3): 1446–54. PMID8389867.
^Li G, Haq W, Xiang L, Lou BS, Hughes R, De Leon IA, et al. (March 1998). "Modifications of the 4,4'-residues and SAR studies of Biphalin, a highly potent opioid receptor active peptide". Bioorganic & Medicinal Chemistry Letters. 8 (5): 555–60. doi:10.1016/S0960-894X(98)00065-1. PMID9871617.
^Mollica A, Davis P, Ma SW, Lai J, Porreca F, Hruby VJ (May 2005). "Synthesis and biological evaluation of new biphalin analogues with non-hydrazine linkers". Bioorganic & Medicinal Chemistry Letters. 15 (10): 2471–5. doi:10.1016/j.bmcl.2005.03.067. PMID15863299.
