A series of 13- and 14-membered cyclic enkephalin analogs based on the moderately $\mu$ selective prototype compound Tyr-C[D-$A_2bu$-Gly-Phe-Leu] 8a were synthesized to investigate the structure-activity relationship. The modifications of sequence were mainly focused on two positions 3 and 5, critical for the selective recognition for $\mu$ and $\delta$ opioid receptors. The substitution of hydrophobic $Leu^5$ with hydrophilic $Asp^5$ derivatives led to Tyr-C[D-$A_2bu$-Gly-Phe-Asp(N-Me)] 7 and Tyr-C[D-Glu-Phe-gPhe-rAsp(O-Me)] 5, the peptides with a large affinity losses at both $\mu$ and $\delta$ receptors. The substitution of $Phe^3$ with $Gly^3$ led to Tyr-C[D-Glu-Gly-gPhe-rLeu] 3 and Tyr-C[D-Glu-Gly-gPhe-D-rLeu] 4, the peptides with large affinity losses at $\mu$ receptors, indicating the critical role of phenyl ring of $Phe^3$ for $\mu$ receptor affinities. One atom reduction of the ring size from 14-membered analogs Tyr-C[D-Glu-Phe-gPhe-(L and D)-rLeu] 6a, 6b to 13-membered analogs Tyr-C[D-Asp-Phe-gPhe-(L and D)-rLeu] 1, 2 reduced the affinity at both $\mu$ and $\delta$ receptors, but increased the potency in the nociceptive assay, indicating the ring constrain is attributed to high nociceptive potency of the analogs. For the influence of D- or L-chirality of $Leu^5$ on the receptor selectivity, regardless of chirality and ring size, all cyclic diastereomers displayed marked $\mu$ selectivity with low potencies at the $\delta$ receptor. The retro-inverso analogs display similar or more active at $\mu$ receptor, but less active at $\delta$ receptor than the parent analogs.
一系列基于中等选择性的μ受体原型化合物Tyr-C[D-
-Gly-Phe-Leu] 8a的13和14成员环状类脑啡肽类似物被合成,以研究结构-活性关系。序列的修改主要集中在3和5位,这两位对于选择性识别μ和δ阿片受体至关重要。将疏水性Leu^5替换为亲水性Asp^5衍生物,导致Tyr-C[D--Gly-Phe-Asp(N-Me)] 7和Tyr-C[D-Glu-Phe-gPhe-rAsp(O-Me)] 5的合成,这些肽在μ和δ受体上的亲和力均大幅下降。将Phe^3替换为Gly^3,得到Tyr-C[D-Glu-Gly-gPhe-rLeu] 3和Tyr-C[D-Glu-Gly-gPhe-D-rLeu] 4,这些肽在μ受体上的亲和力显著下降,表明Phe^3的苯环在μ受体亲和力中起着关键作用。将环的大小从14成员类比物Tyr-C[D-Glu-Phe-gPhe-(L和D)-rLeu] 6a、6b减少一个原子到13成员类比物Tyr-C[D-Asp-Phe-gPhe-(L和D)-rLeu] 1、2,在μ和δ受体上的亲和力均下降,但在痛觉测定中效力提高,表明环的约束是类比物高疼痛效能的原因。至于Leu^5的D或L手性对受体选择性的影响,无论手性和环的大小,所有环状二叠体均表现出明显的μ选择性,并在δ受体上显示低效能。逆转对映异构体在μ受体上的活性与父类类似物相似或更强,但在δ受体上的活性低于父类类似物。