Dermorphin Tetrapeptide Research Articles

Synthesis and evaluation of peptide–fentanyl analogue conjugates as dual µ/δ-opioid receptor agonists for the treatment of pain

Novel peptide-fentanyl analogue conjugates were synthesized by the covalent coupling of carfentanyl derivatives to the C-terminus or N-terminus of the conformationally constrained dermorphin tetrapeptide BVD03 via a chemical linker. The carfentanyl-related analogues displayed distinct binding and functional activities at µ/δ opioid receptors (MOR/DOR) and antinociceptive effects when conjugated to the peptide. The most potent compound, SW-LJ-11, displayed mixed MOR/DOR agonist properties in the low nanomolar range and significant analgesic efficacy in vivo in four classic mouse models of pain. Interestingly, SW-LJ-11 did not exhibit any physical dependence or respiratory depression, in contrast to an equipotent analgesic dose of morphine or BVD03, indicating that the use of opioid peptide–fentanyl analogue conjugates as dual MOR/DOR agonists may be a promising strategy for obtaining safer opioids.

Superpotent [Dmt1]Dermorphin Tetrapeptides Containing the 4-Aminotetrahydro-2-benzazepin-3-one Scaffold with Mixed μ/δ Opioid Receptor Agonistic Properties

Novel dermorphin tetrapeptides are described in which Tyr(1) is replaced by Dmt(1), where d-Ala(2) and Gly(4) are N-methylated, and where Phe(3)-Gly(4) residue is substituted by the constrained Aba(3)-Gly(4) peptidomimetic. Most of these peptidic ligands displayed binding affinities in the nanomolar range for both μ- and δ-opioid receptors but no detectable affinity for the κ-opioid receptor. Measurements of cAMP accumulation, phosphorylation of extracellular signal-regulated kinase (ERK1/2) in HEK293 cells stably expressing each of these receptors individually, and functional screening in primary neuronal cultures confirmed the potent agonistic properties of these peptides. The most potent ligand H-Dmt-NMe-d-Ala-Aba-Gly-NH(2) (BVD03) displayed mixed μ/δ opioid agonist properties with picomolar functional potencies. Functional electrophysiological in vitro assays using primary cortical and spinal cord networks showed that this analogue possessed electrophysiological similarity toward gabapentin and sufentanil, which makes it an interesting candidate for further study as an analgesic for neuropathic pain.

Involvement of mouse μ-opioid receptor splice variants in the spinal antinociception induced by the dermorphin tetrapeptide analog amidino-TAPA

The involvement of the mouse μ-opioid receptor (mMOR-1) splice variants in the antinociceptive effect of intrathecally (i.t.) administered Nα-amidino-Tyr-D-Arg-Phe-β-Ala (amidino-TAPA) and [D-Ala2,N-MePhe4,Gly-ol5]enkephalin (DAMGO) was investigated in mice by monitoring the recovery from acute antinociceptive tolerance to amidino-TAPA and DAMGO. A single i.t. pretreatment with DAMGO produced an acute antinociceptive tolerance, which peaked at 2h and disappeared within 5h after the pretreatment. In contrast, a single i.t. pretreatment with amidino-TAPA produced an acute antinociceptive tolerance, which disappeared within 3h after the pretreatment. The concomitant i.t. pretreatment with an antisense oligodeoxynucleotide (ODN) for exon-1, exon-12, exon-13 or exon-14 of mMOR-1 maintained the acute antinociceptive tolerance to amidino-TAPA for 24h after the pretreatment. On the other hand, the concomitant i.t. pretreatment with an antisense ODN for exon-1 of mMOR-1, but not an antisense ODN for exon-12, exon-13 or exon-14 of mMOR-1, maintained the acute antinociceptive tolerance to DAMGO for 24h after the pretreatment. The present results suggest that the spinal antinociception of amidino-TAPA is partially mediated through the activation of the amidino-TAPA-sensitive and DAMGO-insensitive mMOR-1 splice variants MOR-1J, MOR-1K and MOR-1L, which contain the sequence encoded by exon-12, exon-13 and exon-14, respectively.

Blood−Brain Barrier Penetration by Two Dermorphin Tetrapeptide Analogues: Role of Lipophilicity vs Structural Flexibility

Two dermorphin analogues having an almost identical structure but different structural flexibility were compared for opioid activity. In 1 the aromatic side chains were incorporated into a lactam structure, while in 2 N-amide alkylation was retained but the side chains were flexible. Both compounds produced comparable antinociceptive effects in the mouse tail flick test after peripheral administration. This indicates that lipophilicity, rather than side chain flexibility, is the key determinant for blood-CNS barrier penetration.

Involvement of endogenous opioid peptides in the antinociception induced by the novel dermorphin tetrapeptide analog amidino-TAPA

The antinociceptive effect of i.t. administered N α-amidino-Tyr- d-Arg-Phe-β-Ala (amidino-TAPA), an N-terminal tetrapeptide analog of dermorphin, was characterized in ddY mice. In the opioid receptor ligand-binding assays using mouse brain membranes, amidino-TAPA showed a very high affinity for μ-opioid receptors, a low affinity to δ-opioid receptors and no affinity for κ-opioid receptors. In the mouse tail-flick test, i.t. treatment with amidino-TAPA produced a potent antinociception. The antinociception induced by amidino-TAPA was significantly attenuated by i.t. pretreatment with the μ-opioid receptor antagonist β-funaltrexamine, the κ-opioid receptor antagonist nor-binaltorphimine and the δ-opioid receptor antagonist naltrindole. Moreover, the antinociception induced by amidino-TAPA was significantly attenuated by i.t. pretreatment with antisera against the endogenous κ-opioid peptides dynorphin A, dynorphin B and α-neo-endorphin; and the endogenous δ-opioid peptide [Leu 5]enkephalin. In mice lacking prodynorphin, the precursor of the endogenous κ-opioid peptides, the antinociceptive effect of amidino-TAPA was significantly attenuated compared to that in wild-type C57BL/6J mice. However, there was no difference in G-protein activation by amidino-TAPA in the spinal cord membranes from prodynorphin knockout mice and C57BL/6J mice. The present results suggest that the spinal antinociception induced by the μ-opioid receptor selective peptide amidino-TAPA is mediated in part by the release of endogenous opioid peptides in the spinal cord, which is caused by the direct stimulation of μ-opioid receptors.

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 μ 1-opioid receptors in dermorphin tetrapeptide analogues-induced antinociception

The involvement of putative μ 1-opioid receptors in the antinociception induced by the dermorphin tetrapeptide analogues Try- d-Arg-Phe-β-Ala (TAPA) and Tyr- d-Arg-Phe-β-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 μ 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 μ 1- and μ 2-opioid receptors, respectively. The C-terminal amidation of TAPA-NH 2 may be critical for distinguishing between putative μ 1- and μ 2-opioid receptors.

Dermorphin tetrapeptide analogues with 2′,6′-dimethylphenylalanine (Dmp) substituted for aromatic amino acids have high μ opioid receptor binding and biological activities 1

To investigate the value of the 2′,6′-dimethylphenylalanine (Dmp) residue as an aromatic amino acid substitution, we prepared analogues of the μ opioid receptor-selective dermorphin tetrapeptide Tyr- d-Arg-Phe-βAla-NH 2 (YRFB) in which Dmp or its d-isomer replaced Tyr 1 or Phe 3. Replacing Phe 3 with Dmp essentially tripled μ receptor affinity and the receptor's in vitro biological activities as determined with the guinea pig ileum (GPI) assay but did not change δ receptor affinity. Despite an inversion of the d configuration at this position, μ receptor affinity and selectivity remained comparable with those of the l-isomer. Replacing the N-terminal Tyr residue with Dmp produced a slightly improved μ receptor affinity and a potent GPI activity, even though the substituted compound lacks the side chain phenolic hydroxyl group at the N-terminal residue. Dual substitution of Dmp for Tyr 1 and Phe 3 produced significantly improved μ receptor affinity and selectivity compared with the singly substituted analogues. Subcutaneous injection of the two analogues, [Dmp 3]YRFB and [Dmp 1]YRFB, in mice produced potent analgesic activities that were greater than morphine in the formalin test. These lines of evidence suggest that the Dmp residue would be an effective aromatic amino acid surrogate for both Tyr and Phe in the design and development of novel opioid mimetics.

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).

Synthesis and structure-activity relationships of an orally available and long-acting analgesic peptide, N(alpha)-amidino-Tyr-D-Arg-Phe-MebetaAla-OH (ADAMB).

A novel dermorphin tetrapeptide N(alpha)-amidino-Tyr-D-Arg-Phe-MebetaAla-OH (ADAMB) was designed based on the structures of several dermorphin tetrapeptide analogues, including N(alpha)-amidino-Tyr-D-Arg-Phe-Gly-OH (ADA-DER), H-Tyr-D-Arg-Phe-betaAla-OH (TAPA), and H-Tyr-D-Arg-Phe-Sar-OH (DAS-DER). These parent compounds were known to show a weak oral analgesic activity in animals and/or to possess a different mechanism of analgesia from other mu-opioid peptides. Six analogues of ADAMB were also synthesized to investigate the effect on potency of N-terminal amidination and N-methyl-beta-alanine (MebetaAla) substitution at position 4. Compounds were assessed using the tail pressure test in mice after subcutaneous and oral administration. Among the peptides tested, ADAMB showed the strongest oral antinociceptive activity, with an ED(50) of 5.8 vs 22.2 mg/kg for morphine, as well as a 38-fold stronger activity after subcutaneous administration. ADAMB also showed long-lasting antinociceptive activity, with 50% of the maximum effect persisting in the tail pressure test at 10 h after oral administration (10 mg/kg). In contrast, orally administered morphine (80 mg/kg) showed a rapid decrease of activity in the same test and its antinociceptive effect disappeared within 4 h. When the antinociceptive effect of ADAMB was compared with that of analogues possessing betaAla(4) (1) or Sar(4) (2), as well as analogues with N-substitution (3-6), it was found that both the N(alpha)-amidino substitution and the MebetaAla(4) were synergistically involved in creating ADAMB's exceptionally high antinociceptive activity.

Single diastereomeric desaminotyrosylalanyl tetra‐ and heptapeptides with opioid antagonistic activity

The N-terminal dipeptide Tyr-d-Ala of a mu-selective agonist, dermorphin tetrapeptide (DT, H-Tyr-D-Ala-Phe-Gly-NH2) and delta-selective agonist deltorphin C (DEL-C, H-Tyr-D-Ala-Phe-Asp-Val-Val- Gly-NH2) was changed into an aminodiacyl moiety. The relevant synthetic step is a nucleophilic substitution of bromine from a chiral 2-bromopropanamide by the amino group of tyrosine, with overall retention of configuration. The resulting pseudo tetra- and heptapeptides I-VI were characterized for mu- and delta-opioid receptor binding properties using [3H]DAGO and [3H]DPDPE, respectively, and in a bioassay using guinea pig ileum (GPI) and mouse vas deferens (MVD). As a result of chemical alteration of N-terminal depeptide moiety, all synthesized analogs showed considerable reduction in opioid receptor affinity compared to mu- and delta-prototypes (500-fold on the mu-site, analog I, and 125-fold on the delta-site, analog IV). Interestingly, analogs I and IV showed moderate antagonist activity, respectively, on GPI and MVD, with pA2 values of 6.05 and 6.82. Analog IV did not exhibit the delta-antagonist potency and delta-selectivity of TIPP peptides.

Opioid ligands with extraordinarily high μ-selectivity: Dermorphin tetrapeptides containing thymine-modified alanine residues

Four new [ d-MetO 2]dermorphin tetrapeptides with substituted N- and C-terminal groups and a thymine-modified alanine residue at position 4 were prepared and tested for their activity. All analogues were found to be μ-opioid receptor ligands. Two of them, H-Tyr- d-MetO-Phe-TalNHR (R=H, Ad) displayed an extremely highu μ-opioid receptor selectivity comparable with that of the most μ-selective agonists among opioid peptides.

Tolerance and cross-tolerance to the antinociceptive effects of [ d-Arg 2]-dermorphin tetrapeptide analogue and morphine

Rats were given repeated subcutaneous injections of [ d-Arg 2, Sar 4]-dermorphin (1–4) [DAS-DER (1–4)] and/or morphine over a period of 4 or 7 days. Antinociception was determined at 90 min for DAS-DER (1–4) and 30 min for morphine after each morning injection (9:00 a.m.), using the tail-flick and digit pinching tests in rats. Subcutaneous administration of DAS-DER (1–4) and morphine produced the development of antinociceptive tolerance, respectively. A marked tolerance to DAS-DER (1–4) and morphine was seen in rats made tolerant to morphine. However, administration of morphine produced no significant decrement in the antinociceptive activity in rats made tolerant to DAS-DER (1–4). These results suggest that the site of action of DAS-DER (1–4) may be more limited than that of morphine in the nociceptive pathways, for lack of its antinociceptive efficacy in morphine-tolerant rats.

Spinal antinociception: Comparison of a dermorphin tetrapeptide analogue, [ d-arginine 2, sarcosine 4]-dermorphin (1–4) and morphine in rats

Intracerebroventricular injections of [ d-arginine 2, sarcosine 4]-dermorphin (1–4)(DAS-DER 1–4) and morphine produced a dose-dependent inhibition of the tail-flick response to thermal stimulation. The ED 50 value for each drug was 3.23 (1.35–7.73) nmol/rat and 32.0 (13.3–76.6) nmol/rat, respectively. When injected into the spinal subarachnoid space, the ED 50 value was 0.035 (0.015–0.086) nmol/rat for the tetrapeptide and 11.9 (5.7–25.2) nmol/rat for morphine, respectively. Antinociception induced by DAS-DER 1–4 and morphine, through the intracerebroventricular and intrathecal routes, was clearly reduced by pretreatment with a small dose of naloxone. After spinal transection, the antinociceptive potency of systemically-administered morphine was significantly reduced while that of DAS-DER 1–4 was unaltered. The activity of DAS-DER 1–4 and morphine was also reversed by naloxone in spinal animals. It is concluded that DAS-DER 1–4, a dermorphin analogue, has a minor supraspinal action but acts mainly at the level of the spinal cord, in contrast to the action of morphine.

O-094 - [D-Arg2]-dermorphin tetrapeptide analogs: the action mechanism of a potent and prolonged antinociceptive activity

O-094 - [D-Arg2]-dermorphin tetrapeptide analogs: the action mechanism of a potent and prolonged antinociceptive activity

Antinociceptive cross-tolerance between [D-Arg 2]-dermorphin tetrapeptide analogs and morphine

Cross-tolerance between [D-Arg 2]-dermorphin tetrapeptide analogs and morphine with respect to antinociception was examined in the present set of experiments. Systemic administration of H-Tyr-D-Arg-Phe-Gly-NH 2 (TDAPG-NH 2), H-Tyr-D-Arg-Phe-β-Ala-OH (TDAPA) or morphine over a period of 5 days produced the development of tolerance. In the cross-tolerance study, antinociception after subcutaneous (SC), intracerebroventricular (ICV) and intrathecal (IT) administrations of TDAPG-NH 2 and TDAPA in morphine-tolerant mice was not significantly different from their respective effects in saline-pretreated control mice. A marked tolerance to SC- and ICV-administered morphine was seen in mice made tolerant to TDAPG-NH 2 and TDAPA. However, IT administration of morphine produced no significant decrement in the antinociceptive activity in mice made tolerant to TDAPG-NH 2 and TDAPA. These data indicate that [D-Arg 2]-dermorphin tetrapeptide analogs can produce significant antinociception in morphine-tolerant mice.

A new class opioid peptide, [D-Arg 2, β-Ala 4]-dermorphin tetrapeptide; Physical dependence liability in mice

Development of morphine-like physical dependence of the [D-Arg 2, β-Ala 4]-dermorphin tetrapeptide (H-Tyr-D-Arg-Phe-β-Ala-OH) has been evaluated and compared with the physical dependence liability of morphine or pentazocine. Degree of the physical dependence was assessed by the naloxone-precipitated jumping behaviour in mice after treatment of a single dose of each compound. The number of jumps and the time of latency to first jump were recorded in this experiment. Number of jumps in a group pretreated with the peptide showed less than that in morphine-treated group. In addition, latency to the appearance of the first jump in the peptide-treated mice was later than that in the morphine-treated group. The present results indicate that the physical dependence induced by [D-Arg 2, β-Ala 4]-dermorphin tetrapeptide may be less marked than that produced by morphine. It is also interesting to note that the antinociceptive effect of this opioid peptide is more powerful and of longer duration than that induced by morphine or pentazocine.

Comparison of the antinociceptive effects of new [D-Arg 2]-dermorphin tetrapeptide analogs and morphine in mice

The antinociceptive effects of synthetic dermorphin tetrapeptide analogs containing D-Arg in position 2, H-Tyr-D-Arg-Phe-Gly-NH 2 and H-Tyr-D-Arg-Phe-β-Ala-OH, were measured in mice by the tail-pressure test. The antinociceptive effect produced by intracerebroventricular (ICV), intrathecal (IT) and subcutaneous (SC) administration of either peptide was greater than that produced by morphine. Oral (PO) administration of the peptides showed approximately the same antinociceptive potency as morphine. In addition, the antinociceptive effect produced by SC and PO administration of either peptide was of longer duration than morphine. Pretreatment with naloxone resulted in nearly complete antagonism of the antinociceptive effects produced by ICV and IT administration of either peptide or morphine. Dose ratios (ICV/IT) of H-Tyr-D-Arg-Phe-Gly-NH 2 and H-Tyr-D-Arg-Phe-β-Ala-OH, which were calculated from the AD 50 (Antinociceptive Dose=50% MPE) values, were 5.8 and 6.2, respectively, whereas that of morphine was only 1.46. These results suggest that the mechanisms of the antinociceptive effects of [D-Arg 2]-dermorphin tetrapeptide analogs differ from morphine, and that these peptides may possess higher affinities than does morphine for opioid receptors in the spinal cord.

Antinociception and physical dependence produced by [D-Arg2] dermorphin tetrapeptide analogues and morphine in rats.

1. The antinociceptive effects of [D-Arg2] dermorphin tetrapeptide analogues, H-Tyr-D-Arg-Phe-Gly-NH2 and H-Tyr-D-Arg-Phe-beta-Ala-OH when administered subcutaneously (s.c.) in rats were measured by the tail-flick test. In addition, the appearance of typical withdrawal signs upon cessation of administration or on subsequent treatment with naloxone were measured after chronic administration of either peptide or morphine. 2. The dose of peptides and of morphine in the physical dependence test was determined from the AD50 to inhibit the tail-flick test in rats. Doses from 4 to 64 times the AD50 doses were employed in the s.c. administration schedules. 3. The intensity of the antinociception induced by either peptide was greater than that produced by morphine. Moreover, the antinociception induced by the peptides was of much longer duration than that produced by morphine. 4. Abrupt withdrawal after chronic administration of either peptide produced only slight loss of body weight. In contrast, morphine withdrawal produced sharp loss of body weight. 5. Naloxone precipitated withdrawal signs after chronic administration of either peptide were less intense than those after chronic morphine. 6. These results suggest that the antinociception produced by these peptides is more intense and of longer duration than that produced by morphine. It is also interesting to note that the physical dependence produced by these peptides is less marked than that produced by morphine.

Synthesis and activity profiles of new dermorphin-(1-4) peptide analogues.

A new series of 12 dermorphin tetrapeptides, W-Tyr-D-MetO-Phe-Xaa-Y (W = H, H2NC = (NH); Xaa = Gly, Sar, D-Ala; Y = OH, OCH3, NH2) were prepared by traditional methods in solution and tested for opioid activity. In binding studies based on displacement of mu, delta, and kappa opioid receptor selective radiolabels from guinea pig brain membranes, the new analogues showed a negligible affinity for the kappa binding site and a preference for mu- over delta-receptors with an evident dependence on N- and/or C-terminal modifications; H-Tyr-D-MetO-Phe-Gly-OCH3 was shown to be one of the most selective mu-receptor ligands reported to date. All these tetrapeptides display dose-related naloxone-reversible antinociceptive effects following intracerebroventricular (icv) or subcutaneous (sc) administrations in mice. In comparison to morphine, H-Tyr-D-MetO-Phe-Sar-NH2 and the guanidino derivative H2NC = (NH)-Tyr-D-MetO-Phe-Gly-NH2 showed lower affinity for mu, delta, and kappa sites but exceptionally stronger analgesia: respectively they are 560 and 1550 times as potent an analgesic as morphine. Among analogues tested after sc administration, H-Tyr-D-MetO-Phe-Sar-NH2 and H-Tyr-D-MetO-Phe-D-Ala-OH displayed the highest activities; they were respectively 22 and 30 times more potent than morphine on a molar basis. These results indicate that N- or C-terminal modifications and substitution at position 2 or 4 of dermorphin-(1-4) peptide do not only influence the affinity of the resulting analogues to opioid receptors but also may favorably alter their pharmacokinetic properties.