The interaction of N alpha-alkylenkephalins with opiate receptors. Tissue-dependent shifts in the opiate activity of methionine-enkephalin following N alpha-alkylation

An Effective and Safe Enkephalin Analog for Antinociception

Opioids account for 69,000 overdose deaths per annum worldwide and cause serious side effects. Safer analgesics are urgently needed. The endogenous opioid peptide Leu-Enkephalin (Leu-ENK) is ineffective when introduced peripherally due to poor stability and limited membrane permeability. We developed a focused library of Leu-ENK analogs containing small hydrophobic modifications. N-pivaloyl analog KK-103 showed the highest binding affinity to the delta opioid receptor (68% relative to Leu-ENK) and an extended plasma half-life of 37 h. In the murine hot-plate model, subcutaneous KK-103 showed 10-fold improved anticonception (142%MPE·h) compared to Leu-ENK (14%MPE·h). In the formalin model, KK-103 reduced the licking and biting time to ~50% relative to the vehicle group. KK-103 was shown to act through the opioid receptors in the central nervous system. In contrast to morphine, KK-103 was longer-lasting and did not induce breathing depression, physical dependence, and tolerance, showing potential as a safe and effective analgesic.

Further studies of tyrosine surrogates in opioid receptor peptide ligands

A series of opioid peptide ligands containing modified N-terminal tyrosine (Tyr) residues was prepared and evaluated against cloned human mu, delta, and kappa opioid receptors. This work extends the recent discovery that (S)-4-carboxamidophenylalanine (Cpa) is an effective tyrosine bioisostere. Amino acids containing negatively charged functional groups in place of tyrosine's phenolic hydroxyl lacked receptor affinity, while exchange of Tyr for (S)-4-aminophenylalanine was modestly successful. Peptides containing the new amino acids, (S)-4-carboxamido-2,6-dimethylphenylalanine (Cdp) and (S)-beta-(2-aminobenzo[d]thiazol-6-yl)alanine (Aba), displayed binding (K(i)) and functional (EC(50)) profiles comparable to the parent ligands at the three receptors. Cdp represents the best performing Tyr surrogate in terms of overall activity, while Cpa and Aba show a subtle proclivity toward the delta receptor.

Studies on the structure–activity relationship of 2′,6′-dimethyl-l-tyrosine (Dmt) derivatives: bioactivity profile of H–Dmt–NH–CH3

The 2',6'-dimethyl-l-tyrosine (Dmt) enhances receptor affinity, functional bioactivity and in vivo analgesia of opioid peptides. To further investigate its direct influence on these opioid parameters, we developed a series of compounds (H-Dmt-NH-X). Among them, H-Dmt-NH-CH(3) showed the highest affinity (K(i)mu=7.45 nM) equal to that of morphine, partial mu-opioid agonism (E(max)=66.6%) in vitro and a moderate antinociception in mice.

(4-Carboxamido)phenylalanine is a surrogate for tyrosine in opioid receptor peptide ligands

(S)-4-(Carboxamido)phenylalanine (Cpa) is examined as a bioisosteric replacement for the terminal tyrosine (Tyr) residue in a variety of known peptide ligands for the mu, delta and kappa opioid receptors. The Cpa-containing peptides, assayed against cloned human opioid receptors, display comparable binding affinity (Ki), and agonist potency (EC50) to the parent ligands at the three receptors. Cpa analogs of delta selective peptides show an increase in delta selectivity relative to the mu receptor. Cpa is the first example of an amino acid that acts as a surrogate for Tyr in opioid peptide ligands, challenging the long-standing belief that a phenolic residue is required for high affinity binding.

Ethanol ingestive behavior as a function of central neurotransmission

Uncontrollable alcohol ingestive behavior has been linked to deficits of central neurotransmission. The pineal gland plays an important role in modulating ethanol intake in numerous animal species. The opioidergic (i.e. beta-endorphin, enkephalin, and dynorphin) system is involved in both the actions of alcohol and opiates, as well as craving and/or genetic predisposition towards abuse of these two agents. Furthermore, there is significant evidence to link ingestive behaviors with the ventral tegmental accumbens-hypothalamic axis, whereby the biogenic amines dopamine and serotonin are reciprocally involved. Evidence is presented which implicates the striatum and the hypothalamus as possible specific loci for regional differences between alcohol-preferring and alcohol-nonpreferring mice. We believe that photoperiod-induced alcohol ingestive behavior may involve alterations in both pineal and hypothalamic opioid peptides.

Tyr1-substituted and fluorescent Pya1-enkephalins bind strongly and selectively to mu and delta opiate receptors

The fluorescent enkephalins in which an essential Tyr1 residue is replaced by L-1-pyrenylalanine (Pya) were synthesized and examined in the receptor binding assays. [Pya1, Leu5]Enkephalin and its methyl ester showed binding characteristics specific for the opiate receptors, exhibiting a potent inhibition of Tyr1-containing enkephalins. Surprisingly, the methyl ester displayed almost the same potencies to those of DAGO-enkephalin. This analog bound 24-fold more strongly to mu than to delta-receptors. C-terminal free analog Pya1-Enk-OH was delta-preferential with a fairly good affinity. These results indicate that Tyr1 in enkephalin is not necessary to recognition of the opiate receptors.

The interactions of 1-alkyl-4,4-diphenylpiperidines with opiate receptors

1-Alkyl-4,4-diphenylpiperidines inhibit the binding of mu- and delta-opiate receptor agonists to crude synaptosomal membranes of rat brain. Opiate receptor affinity is dependent on the nature of the 1-alkyl substitution. In displacing [3H]etorphine, [3H]dihydromorphine and [3H][D-Ala2,D-Leu5] enkephalin, the 1-methyl-derivative was most effective (IC50 [3H]etorphine = 9 microM, IC50 [3H]dihydromorphine = 0.71 microM, IC50 [3H][D-Ala2,D-Leu5]enkephalin = 2.1 microM). 1-t-Butyl-4,4-diphenylpiperidine and its 1-isopropyl analogue, used as antiparkinsonism drugs, exhibit distinctly lower inhibiting concentration values (50% inhibition, IC50 values).

Endogenous opiates: 1981

This paper is the fourth of an annual series reviewing the research concerning the endogenous opiate peptides. This installment covers only work published during 1981 and attempts to provide a comprehensive, but not exhaustive, survey of the area. Previous papers in the series have dealt with research done before 1981. Topics concerning endogenous opiates reviewed here include a delineation of their receptors, their distribution, their precursors and degradation, behavioral effects resulting from their administration, their possible involvement in physiological responses, and their interactions with other peptides and hormones. Due to the burgeoning literature in this field, the comprehensive nature of this review in the future will be limited to considerations of behavioral phenomena related to the endogenous opiates.