The cyclic peptide [2,6-dimethyl-Tyr1,D-Pen2,D-Pen5]enkephalin (2) was synthesized by solid-phase techniques and contains the optically pure unnatural amino acid 2,6-dimethyltyrosine (DMT) as a replacement for the Tyr1 residue of [D-Pen2,D-Pen5]enkephalin (DPDPE, 1). This structural modification resulted in a 10-fold increase in the potency of 2 at the delta opioid receptor and a 35-fold increase in potency at the mu receptor while substantial delta receptor selectivity was maintained. In addition, 2 was 86-fold more effective than 1 at inhibiting electrically stimulated contractions of the mouse vas deferens. In the hot plate test, 2 was 7-fold more potent than 1 after intracerebroventricular administration in the mouse. While 1 was inactive following systemic administration of doses as high as 30 mg/kg, subcutaneous administration of 2 significantly inhibited writhing with an ED50 of 2.6 mg/kg. These results demonstrate that the potency and systemic activity of DPDPE are significantly increased by replacement of Tyr1 with DMT.

8-Chlorodibenz[b,f][1,4]oxazepine-10(11H)-carboxylic acid, 2-acetylhydrazide (1, SC-19220) has been previously reported by us and others to be a PGE2 antagonist selective for the EP1 receptor subtype with antinociceptive activities. Analogs of SC-19220, in which the acetyl moiety has been replaced with pyridylpropionyl groups and their homologs, have been synthesized as illustrated by compounds 13 and 29. These and other members of this series have been shown to be efficacious analgesics and PGE2 antagonists of the EP1 subtype. This report discusses the structure activity relationships within this series.

8-Chlorodibenz[b,f][1,4]oxazepine-10(11H)-carboxylic acid, 2-[1-oxo-3-(4-pyridinyl)propyl]hydrazide, monohydrochloride (1, SC-51089) is a functional PGE2 antagonist selective for the EP1 receptor subtype with antinociceptive activity. During metabolism in cultured rat hepatocytes, SC-51089, which contains a diacylhydrazine moiety, has been shown to release hydrazine. Analogs of SC-51089, in which the diacylhydrazine functionality has been replaced by isosteric and isoelectronic groups, have been synthesized and have been shown to be analgesics and PGE2 antagonists of the EP1 subtype. This report discusses the structure-activity relationships within these series.

No abstract

A number of analogues of the recently disclosed analgesic dipeptide 2,6-dimethyl-L-tyrosyl-D-alanine-phenylpropylamide (SC-39566, 2) were prepared. These analogues contained oxymethylene, aminomethylene, ketomethylene, bismethylene, and trans double bond (including vinyl fluoride) isosteric replacements for the amide bond between the D-alanine and phenylpropylamine units in 2. These compounds were tested in opioid binding assays and in the mouse writhing assay for analgesic activity. Though not as potent as 2, the oxymethylene, and trans double bond isosteres showed analgesic activity. The aminomethylene analogues also showed binding activity in subnanomolar concentrations at the mu receptor. The amide bond between 2,6-dimethyl-L-tyrosine and D-alanine units seems to be critical for opioid activity.

In order to develop systemically-active opioid peptides, the delta-selective, opioid pentapeptide [D-Pen2,D-Pen5]-enkephalin (DPDPE) was modified by esterification and by substitution of 2',6'-dimethyltyrosine for tyrosine to yield 4. Compound 4 was on the order of 8- and 800-fold more active than DPDPE in both delta and mu opioid radioligand binding assays, respectively, in rat neural membrane suspensions. Compound 4 was considerably more potent than DPDPE at inhibiting contractions of electrically-stimulated mouse vas deferens in vitro, and this effect was very sensitive to naltrindole, a delta-selective opioid antagonist. These observations can be taken as indication that 4 exerts its effects through delta opioid receptors. This interpretation is supported by the finding that the EC50 value of 4 derived in the smooth muscle assay is very similar to that derived in NG108-15 neuroblastoma cells, a preparation devoid of mu receptors. Unlike DPDPE, 4 exhibited significant, naloxone-sensitive, antinociceptive activity when administered systemically, as measured by inhibition of phenylbenzquinone-induced stretching in mice (ED50 = 2.1 mg/kg). Compound 4 also displayed significant antinociceptive activity following systemic administration as measured by its action in mice to increase latencies for tail withdrawal from radiant heat (ED50 = 50 mg/kg). Compound 4 did not produce morphine-like discriminative stimulus effects in rats trained to discriminate 3.0 mg/kg morphine from vehicle at doses ranging from 30 to 120 mg/kg. This observation can be interpreted as indication that within this dosage range there is an absence of morphine-like subjective effects. Physical dependence, however, could be induced in mice at higher doses of 4 under a progressively-graded, 4-day dose regimen. Congeners of 4 with amide bond surrogates for the Gly-Phe amide bond (oxymethylene, trans-double bond, and bismethylene isosteres) in the cyclic core of DPDPE were prepared in an attempt to increase the antinociceptive activity of 4. While some of the congeners were active in the in vitro assays, they did not display significant antinociceptive activity following systemic administration. The preparation of all the compounds was accomplished by solution-phase methods. The mechanisms which might underlie the biological and systemic activity of 4 are discussed.

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A number of O- and N-alkylated derivatives of the antinociceptive, orally active, mu-opioid-selective truncated enkephalin analog L-2,6-dimethyltyrosyl-N-(3-phenylpropyl)-D-alaninamide (2, SC-39566) were synthesized to explore the structure-activity relationships of the series. The parent molecule is quite forgiving of substitution on the tyrosyl phenolic moiety and on the alanyl nitrogen. The tyrosyl and (phenylpropyl)amide NH sites, however, appear to be critical to interactions with the receptor, for even modest changes at these sites cause great loss of binding potency.