N-terminal tetrapeptide of dermorphin and D-Arg-substituted tetrapeptides: inactivation process of the antinociceptive activity by peptidase

Exploring μ-Opioid Receptor Splice Variants as a Specific Molecular Target for New Analgesics

Since a μ-opioid receptor gene containing multiple exons has been identified, the variety of splice variants for μ-opioid receptors have been reported in various species. Amidino-TAPA and IBNtxA have been discovered as new analgesics with different pharmacological profiles from morphine. These new analgesics show a very potent analgesic effect but do not have dependence liability. Interestingly, these analgesics show the selectivity to the morphine-insensitive μ-opioid receptor splice variants. The splice variants, sensitive to these new analgesics but insensitive to morphine, may be a better molecular target to develop the analgesics without side effects.

Dermorphin tetrapeptide analogs as potent and long-lasting analgesics with pharmacological profiles distinct from morphine

Dermorphin (Tyr-d-Ala-Phe-Gly-Tyr-Pro-Ser-NH(2)) is a heptapeptide isolated from amphibian skin. With a very high affinity and selectivity for μ-opioid receptors, dermorphin shows an extremely potent antinociceptive effect. The structure-activity relationship studies of dermorphin analogs clearly suggest that the N-terminal tetrapeptide is the minimal sequence for agonistic activity at μ-opioid receptors, and that the replacement of the d-Ala(2) residue with d-Arg(2) makes the tetrapeptides resistant to enzymatic metabolism. At present, only a handful of dermorphin N-terminal tetrapeptide analogs containing d-Arg(2) have been developed. The analogs show potent antinociceptive activity that is greater than that of morphine with various injection routes, and retain high affinity and selectivity for μ-opioid receptors. Interestingly, some analogs show pharmacological profiles that are distinct from the traditional μ-opioid receptor agonists morphine and [d-Ala(2),NMePhe(4),Gly-ol(5)]enkephalin (DAMGO). These analogs stimulate the release of dynorphins through the activation of μ-opioid receptors. The activation of κ-opioid receptors by dynorphins is suggested to reduce the side effects of μ-opioid receptor agonists, e.g., dependence or antinociceptive tolerance. The dermorphin N-terminal tetrapeptide analogs containing d-Arg(2) may provide a new target molecule for developing novel analgesics that have fewer side effects.

Differential inhibitory effects of mu-opioids on substance P- and capsaicin-induced nociceptive behavior in mice

The antinociceptive mechanisms of the selective mu-opioid receptor agonists [D-Ala2,NMePhe4,Gly(ol)5]enkephalin (DAMGO), H-Tyr-D-Arg-Phe-beta-Ala-OH (TAPA) or H-Tyr-D-Arg-Phe-beta-Ala-NH2 (TAPA-NH2) against substance P (SP)- or capsaicin-elicited nociceptive behaviors was investigated in mice. DAMGO, TAPA or TAPA-NH2 given intrathecally inhibited the nociceptive behaviors elicited by intrathecally administered SP or capsaicin, and these antinociceptive effects were completely eliminated by intrathecal co-administration with D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP), a selective mu-opioid receptor antagonist. Pretreatment subcutaneously with naloxonazine, a selective mu1-opioid receptor antagonist, partially attenuated the antinociceptive effect of TAPA-NH2, but not DAMGO and TAPA, against SP. However, the antinociception induced by TAPA, but not DAMGO and TAPA-NH2, against capsaicin was significantly inhibited by naloxonazine. On the other hand, co-administration intrathecally with Tyr-D-Pro-Trp-Gly-NH2 (D-Pro2-Tyr-W-MIF-1), a selective mu2-opioid receptor antagonist, significantly attenuated the antinociceptive effects of DAMGO, but not TAPA and TAPA-NH2, against capsaicin, while the antinociceptions induced by three opioid peptides against SP were significantly inhibited by D-Pro2-Tyr-W-MIF-1. These results suggest that differential inhibitory mechanisms on pre- and postsynaptic sites in the spinal cord contribute to the antinociceptive effects of the three mu-opioid peptides.

Blood-Brain Barrier Permeability of Novel [d-Arg2]Dermorphin (1-4) Analogs: Transport Property Is Related to the Slow Onset of Antinociceptive Activity in the Central Nervous System

To clarify the pharmacological characteristics of Nalpha-amidino-Tyr-D-Arg-Phe-betaAla-OH (ADAB) and Nalpha-amidino-Tyr-D-Arg-Phe-MebetaAla-OH (ADAMB), mu1-opioid receptor-selective [D-Arg2]dermorphin tetrapeptide analogs, the plasma pharmacokinetics, and the in vivo blood-brain barrier (BBB) transport of these peptides were quantitatively evaluated. The mechanism responsible for the BBB transport of these peptides was also examined. The in vivo BBB permeation influx rates of 125I-ADAB and 125I-ADAMB after an i.v. bolus injection into mice were determined to be 0.0515 +/- 0.0284 microl/(min.g of brain) and 0.0290 +/- 0.0059 microl/(min.g of brain), respectively, both rates being slower than that of 125I-Tyr-D-Arg-Phe-betaAla-OH (125I-TAPA), a [D-Arg2]dermorphin tetrapeptide analog. To elucidate the BBB transport mechanism of ADAB and ADAMB, a conditionally immortalized mouse brain capillary endothelial cell line (TM-BBB4) was used as an in vitro model of the BBB. The internalization of both 125I-ADAB and 125I-ADAMB into cells was concentration-dependent with half-saturation constant (Kd) values of 3.76 +/- 0.83 and 5.68 +/- 1.75 microM, respectively. The acid-resistant binding of both ADAB and ADAMB was significantly inhibited by dansylcadaverine (an endocytosis inhibitor) and poly-l-lysine and protamine (polycations), but it was not inhibited by 2,4-dinitrophenol, or at 4 degrees C. These results suggest that ADAB and ADAMB are transported through the BBB with slower permeation rates than that of TAPA, and this is likely to be a factor in the slow onset of their antinociceptive activity in the central nervous system. The mechanism of the BBB transport of these drugs is considered to be adsorptive-mediated endocytosis.

Differential involvement of μ1-opioid receptors in dermorphin tetrapeptide analogues-induced antinociception

The involvement of putative mu(1)-opioid receptors in the antinociception induced by the dermorphin tetrapeptide analogues Try-D-Arg-Phe-beta-Ala (TAPA) and Tyr-D-Arg-Phe-beta-Ala-NH(2) (TAPA-NH(2)) was determined in mice, using a tail-pressure test and a formalin test. TAPA and TAPA-NH(2) injected i.c.v. and i.t. produced dose-dependent antinociception in both assays. In the tail-pressure test, the antinociception induced by i.c.v. or i.t. injected TAPA, but not TAPA-NH(2), was significantly attenuated by pretreatment with naloxonazine, a selective antagonist for putative mu(1)-opioid receptors. Moreover, naloxonazine also significantly attenuated the antinociception induced by i.c.v. injected TAPA, but not TAPA-NH(2), in the formalin test. In contrast, the antinociception induced by both TAPA and TAPA-NH(2) given i.t. was significantly attenuated by pretreatment with naloxonazine in the formalin test. The present results suggest that TAPA and TAPA-NH(2) should be considered selective agonists for putative mu(1)- and mu(2)-opioid receptors, respectively. The C-terminal amidation of TAPA-NH(2) may be critical for distinguishing between putative mu(1)- and mu(2)-opioid receptors.

Synthesis and antinociceptive activity of orally active opioid peptides: improvement of oral bioavailability by esterification

To improve the oral bioavailability of a dermorphin tetrapeptide analog, N(alpha)-1-iminoethyl-Tyr-D-MetO-Phe-MebetaAla-OH (III), which has a potent analgesic activity after oral administration, various derivatives were synthesized to increase lipophilicity by esterification of the C-terminal carboxyl group and/or acylation of the phenolic hydroxyl group on Tyr1. Antinociceptive activity was evaluated after subcutaneous or oral administration using the mouse tail pressure test. As a result, increased antinociceptive activity after oral administration as well as an improved ED50(p.o.)/ED50(s.c.) ratio, which is an indicator of oral bioavailability, were found for some compounds. With regard to the improvement of bioavailability, derivatives with acylation of the phenolic hydroxyl group on Tyr1 showed better results than derivatives with esterification of the C-terminal carboxyl group. In particular, an ED50(p.o.)/ED50(s.c.) ratio equivalent to that of morphine was found for an acetylated derivative, N(alpha)-1-iminoethyl-Tyr(COMe)-D-MetO-Phe-MebetaAla-OH (7a), as well as for a methoxycarbonylated derivative, N(alpha)-1-iminoethyl-Tyr(CO2Me)-D-MetO-Phe-MebetaAla-OH (7l).

Blood-brain barrier transport of a novel micro 1-specific opioid peptide, H-Tyr-D-Arg-Phe-beta-Ala-OH (TAPA)

The purpose of this study was to clarify the mechanism of the blood-brain barrier (BBB) transport of H-Tyr-D-Arg-Phe-beta-Ala-OH (TAPA), which is a novel dermorphin analog with high affinity for the micro 1-opioid receptor. The in vivo BBB permeation influx rate of [125I]TAPA after an i.v. bolus injection (7.3 pmol/g body weight) into mice was estimated to be 0.265 +/- 0.025 microL/(min.g of brain). The influx rate of [125I]TAPA was reduced 70% by the coadministration of unlabeled TAPA (33 nmol/g of brain), suggesting the existence of a specific transport system for TAPA at the BBB. In order to elucidate the BBB transport mechanism of TAPA, a conditionally immortalized mouse brain capillary endothelial cell line (TM-BBB4) was used as an in vitro model of the BBB. The acid-resistant binding of [125I]TAPA, which represents the internalization of the peptide into cells, was temperature- and concentration-dependent with a half-saturation constant of 10.0 +/- 1.7 microm. The acid-resistant binding of TAPA was significantly inhibited by 2,4-dinitrophenol, dansylcadaverine (an endocytosis inhibitor) and poly-l-lysine and protamine (polycations). These results suggest that TAPA is transported through the BBB by adsorptive-mediated endocytosis, which is triggered by binding of the peptide to negatively charged sites on the surface of brain capillary endothelial cells. Blood-brain barrier transport via adsorptive-mediated endocytosis plays a key role in the expression of the potent opioid activity of TAPA in the CNS.

Structure-activity relationships (SAR) of [D-Arg(2)]dermorphin(1-4) analogues, N(alpha)-amidino-Tyr-D-Arg-Phe-X

In investigating the development of compounds with potent analgesic effects after oral administration, 74 C-terminal analogues (N(alpha)-amidino-Tyr-D-Arg-Phe-X), based on the structure of N(alpha)-amidino-Tyr-D-Arg-Phe-Me beta Ala-OH (ADAMB), were synthesized. Their analgesic activity was evaluated using the mouse-tail pressure test after both subcutaneous and oral administration, and the structure-activity relationships (SAR) were examined in detail. The results clearly indicated that compounds containing beta-amino acid without a side chain at the X position are preferable for expression of potent analgesic activity, and that the free carboxyl group is superior in its analgesic activity to that of the esterified or amidated carboxy group at the C-terminal. In addition, N-methylation of the amide bond at the 4th position contributed to improved analgesic activity. These results indicated that the strong and long-lasting analgesic effect of ADAMB is expressed by the synergistic effects of N(alpha)-amidination, the N-methylation of the amide bond at the 4th position and the carbon chain length (beta-Ala) of the residue at the 4th position, and that this is the most suitable structure.

Selective antagonism by naloxonazine of antinociception by Tyr-D-Arg-Phe-beta-Ala, a novel dermorphin analogue with high affinity at mu-opioid receptors

To examine the role of mu-opioid receptor subtypes, we assessed the antinociceptive effect of H-Tyr-D-Arg-Phe-beta-Ala-OH (TAPA), an analogue of dermorphin N-terminal peptide in mice, using the tail-flick test. Intracerebroventricularly (i.c.v.) or intrathecally (i.t.) injected TAPA produced potent antinociception with tail-flick as a thermal noxious stimulus. The selective mu(1)-opioid receptor antagonist, naloxonazine (35 mg/kg, s.c.), or the selective mu-opioid receptor antagonist, beta-funaltrexamine, 24 h before testing antagonized the antinociceptive effect of i.t. or i.c.v. TAPA on the response to noxious stimuli. Pretreatment with beta-funaltrexamine completely antagonized the antinociception by both i.c.v. and i.t. administered TAPA and [D-Ala(2), Me-Phe(4), Gly(ol)(5)]enkephalin (DAMGO). Especially in the tail-flick test, pretreatment with naloxonazine produced a marked rightward displacement of the i.t. TAPA dose-response curve for antinociception. Though DAMGO is a highly selective mu-opioid receptor agonist, pretreatment with naloxonazine partially blocked the antinociceptive response to DAMGO after i.c.v., but not after i. t. injection. These results indicate that TAPA can act as a highly selective mu(1)-opioid receptor agonist (notable naloxonazine-sensitive receptor agonist) at not only the supraspinal level, but also the spinal level. These data also reveal different antinociceptive mechanisms for DAMGO and for TAPA.