ABSTRACT
Title
Endomorphin-1 analogues as novel and highly selective MOPr agonists: design and characterization of innovative analgesic drugs
Authors
A. Bedini1, L. Gentilucci2, R. De Marco2 and S. Spampinato1
1Dept. of Pharmacology, University of Bologna, via Irnerio 48, 40126 Bologna, Italy
2Dept. of Chemistry “G. Ciamician”, University of Bologna, via Selmi 2, 40126 Bologna, Italy
1Dept. of Pharmacology, University of Bologna, via Irnerio 48, 40126 Bologna, Italy
2Dept. of Chemistry “G. Ciamician”, University of Bologna, via Selmi 2, 40126 Bologna, Italy
Abstract
Morphine, a prototypical mu-opioid receptor (MOPr) agonist, is widely used as highly effective pain reliever, albeit its chronic administration leads to the development of tolerance and addiction. Because of morphine’s several side effects on respiratory, cardiovascular and gastrointestinal systems, its rewarding, addictive and immunomodulatory properties, and its poor effect on neuropathic pain, the discovery of novel opioid analgesics is a very important challenge for pharmaceutical and clinical research.
Endomorphin-1 (EM-1) and -2 (EM-2) are endogenous peptides discovered in the brain, with a high selectivity towards MOPr (Zadina et al., 1997); unlike morphine, endomorphins appear to be effective in reducing neuropathic pain and their analgesic effect seems to be dissociated by immunomodulatory, cardiovascular and respiratory effects (Przewlocki and Przewlocka, 2001). For these reasons, they have been conjectured as secondary effect-free pharmacological tools to be used in place of morphine. However, natural endomorphins show poor bioavailability and rapid degradation in vivo; therefore, they require chemical modifications to be employed as novel potential analgesics.
A few years ago we found a novel EM-1 analogue, c[Tyr-D-Pro-D-Trp-Phe-Gly] (c[YpwFG]), which displayed good affinity and high selectivity to MOPr (Ki 34 nM) and showed an agonist behaviour (Cardillo et al., 2004), and we provided insights into its interaction to MOPr (Gentilucci et al., 2008), thus building up a general model to design new MOPr-active ligands; more recently, we showed that c[YpwFG] could significantly reduce nociception in a mouse model of visceral pain in which EM-1 resulted ineffective (Bedini et al., 2010), indicating c[YpwFG] as an interesting lead compound to develop innovative analgesics.
On the basis of these findings we designed a series of novel EM-1 analogues derived from the parental compound c[YpwFG], aiming to find novel molecules with higher MOPr affinity and better analgesic effects in vivo. This search led to generate c[Tyr-Gly-D-Trp-Phe-Gly] (c[YGwFG]), which showed a 10-fold higher affinity for MOPr (Ki 3,6 nM), as compared to the parental c[YpwFG], as well as a partial agonist behaviour (IC50 31.4 nM; Emax 65%) and discrete analgesic properties in vivo, as measured in the tail flick test (EC50 9,6 mg/kg).
As these novel EM-1 analogues bind to and activate MOPr mainly through the side chains of the amino acids 2 (w) and 3 (F), we synthesized and tested short di- and tri-peptides containing the sequence D-Trp-Phe (wF), aiming to determine the minimal features necessary for their pharmacological effects. This study led to new linear peptides: wF and wFG. Both compounds displayed good affinity to MOPr and partial agonist behaviour; furthermore they showed mild analgesic effects in an animal model of central pain (tail-flick test), with wFG determining 55% of the maximal possible analgesic effect, whereas wF only 16%. Interestingly, both wFG and wF could almost completely abolish nociception in an animal model of visceral pain. These findings demonstrate that the novel EM-1 analogues are highly effective towards visceral pain and suggest that their peripheral analgesic effects could be due to a scarce crossing of blood brain barrier. For this reason, we designed a novel pool of EM-1 analogues which are predicted to better distribute to the central nervous system; MOPr selectivity and in vivo effects of this latter group of compounds will be also discussed.
In conclusion, we developed novel EM-1 analogues which selectively bind to MOPr with high affinity and determine significant analgesic effects in different pain models, thereby providing the basis to the rational design of innovative opioid analgesics.
Zadina et al. (1997), Nature 386: 499-502
Przewlocki et al. (2001), Eur J Pharmacol 429 :79-91
Cardillo et al. (2004), J Med Chem 47: 5198-203
Gentilucci et al. (2008), FEBS J. 275: 2315-2337
Bedini et al. (2010), Peptides 31: 2135-40
Endomorphin-1 (EM-1) and -2 (EM-2) are endogenous peptides discovered in the brain, with a high selectivity towards MOPr (Zadina et al., 1997); unlike morphine, endomorphins appear to be effective in reducing neuropathic pain and their analgesic effect seems to be dissociated by immunomodulatory, cardiovascular and respiratory effects (Przewlocki and Przewlocka, 2001). For these reasons, they have been conjectured as secondary effect-free pharmacological tools to be used in place of morphine. However, natural endomorphins show poor bioavailability and rapid degradation in vivo; therefore, they require chemical modifications to be employed as novel potential analgesics.
A few years ago we found a novel EM-1 analogue, c[Tyr-D-Pro-D-Trp-Phe-Gly] (c[YpwFG]), which displayed good affinity and high selectivity to MOPr (Ki 34 nM) and showed an agonist behaviour (Cardillo et al., 2004), and we provided insights into its interaction to MOPr (Gentilucci et al., 2008), thus building up a general model to design new MOPr-active ligands; more recently, we showed that c[YpwFG] could significantly reduce nociception in a mouse model of visceral pain in which EM-1 resulted ineffective (Bedini et al., 2010), indicating c[YpwFG] as an interesting lead compound to develop innovative analgesics.
On the basis of these findings we designed a series of novel EM-1 analogues derived from the parental compound c[YpwFG], aiming to find novel molecules with higher MOPr affinity and better analgesic effects in vivo. This search led to generate c[Tyr-Gly-D-Trp-Phe-Gly] (c[YGwFG]), which showed a 10-fold higher affinity for MOPr (Ki 3,6 nM), as compared to the parental c[YpwFG], as well as a partial agonist behaviour (IC50 31.4 nM; Emax 65%) and discrete analgesic properties in vivo, as measured in the tail flick test (EC50 9,6 mg/kg).
As these novel EM-1 analogues bind to and activate MOPr mainly through the side chains of the amino acids 2 (w) and 3 (F), we synthesized and tested short di- and tri-peptides containing the sequence D-Trp-Phe (wF), aiming to determine the minimal features necessary for their pharmacological effects. This study led to new linear peptides: wF and wFG. Both compounds displayed good affinity to MOPr and partial agonist behaviour; furthermore they showed mild analgesic effects in an animal model of central pain (tail-flick test), with wFG determining 55% of the maximal possible analgesic effect, whereas wF only 16%. Interestingly, both wFG and wF could almost completely abolish nociception in an animal model of visceral pain. These findings demonstrate that the novel EM-1 analogues are highly effective towards visceral pain and suggest that their peripheral analgesic effects could be due to a scarce crossing of blood brain barrier. For this reason, we designed a novel pool of EM-1 analogues which are predicted to better distribute to the central nervous system; MOPr selectivity and in vivo effects of this latter group of compounds will be also discussed.
In conclusion, we developed novel EM-1 analogues which selectively bind to MOPr with high affinity and determine significant analgesic effects in different pain models, thereby providing the basis to the rational design of innovative opioid analgesics.
Zadina et al. (1997), Nature 386: 499-502
Przewlocki et al. (2001), Eur J Pharmacol 429 :79-91
Cardillo et al. (2004), J Med Chem 47: 5198-203
Gentilucci et al. (2008), FEBS J. 275: 2315-2337
Bedini et al. (2010), Peptides 31: 2135-40