Table 2. In Vitro Agonist Activity at the Opioid Receptors of 6-Amino Acid (2–21)- and 6-Dipeptide-Substituted Derivatives (22a/b and 23a), and Reference Compound 1^a.

		MOR		DOR		KOR
compd	amino acid substitution at position 6	EC50 (nM)	% stim.	EC50 (nM)	% stim.	EC50 (nM)	% stim.
1		3.83 ± 0.95	97 ± 3	37.3 ± 8.5	106 ± 1	116 ± 31	77 ± 5
2a	α-Gly	1.16 ± 0.59	99 ± 4	9.61 ± 4.14	103 ± 1	399 ± 119	87 ± 4
2b	β-Gly	3.78 ± 0.73	98 ± 9	7.92 ± 1.63	103 ± 7	361 ± 154	82 ± 9
3a	α-l-Ala	1.34 ± 0.17	97 ± 5	9.55 ± 4.47	93 ± 9	214 ± 71	51 ± 3
3b	β-l-Ala	6.24 ± 0.79	87 ± 6	5.20 ± 1.70	104 ± 6	392 ± 160	64 ± 9
4a	α-l-Phe	0.38 ± 0.19	93 ± 8	0.39 ± 0.19	102 ± 2	219 ± 2	39 ± 12
4b	β-l-Phe	6.76 ± 3.16	99 ± 8	0.48 ± 0.16	94 ± 8	1172 ± 505	81 ± 9
6a	α-l-Ser	1.60 ± 0.59	87 ± 5	13.9 ± 1.5	101 ± 2	1213 ± 200	44 ± 5.2
6b	β-l-Ser	3.56 ± 0.76	101 ± 9	6.98 ± 4.24	98 ± 1	201 ± 116	88 ± 2
7a	α-l-Val	10.5 ± 4.0	95 ± 8	33.8 ± 8.9	91 ± 2	462 ± 126	51 ± 0.5
7b	β-l-Val	11.7 ± 5.7	84 ± 4	5.73 ± 2.48	96 ± 9	1117 ± 411	68 ± 10
8a	α-l-Lys	2.25 ± 0.07	90 ± 8	152 ± 2	106 ± 4	118 ± 26	79 ± 13
8b	β-l-Lys	6.85 ± 1.39	90 ± 7	45.1 ± 22.7	93 ± 8	525 ± 24	62 ± 7
9a	α-l-Tyr	1.87 ± 0.44	92 ± 2	1.76 ± 0.22	88 ± 6	100 ± 6	62 ± 8
9b	β-l-Tyr	24.7 ± 9.8	93 ± 7	6.23 ± 1.03	95 ± 0.8	774 ± 57	60 ± 3
10a	α-l-Trp	0.51 ± 0.34	93 ± 9	2.52 ± 1.41	102 ± 8	70.1 ± 20.5	61 ± 8
10b	β-l-Trp	1.64 ± 0.43	101 ± 11	2.18 ± 0.72	96 ± 5	181 ± 49	87 ± 8
11a	α-l-Asn	0.83 ± 0.17	99 ± 0.2	9.78 ± 3.32	106 ± 6	81.7 ± 12.3	67 ± 12
11b	β-l-Asn	2.04 ± 0.44	96 ± 14	3.18 ± 0.93	88 ± 5	923 ± 11	71 ± 9
12a	α-l-Gln	2.27 ± 0.11	90 ± 8	7.80 ± 3.61	104 ± 10	185 ± 30	70 ± 3
12b	β-l-Gln	9.54 ± 1.15	98 ± 4	3.96 ± 0.07	103 ± 9	1410 ± 418	63 ± 17
13a	α-l-Asp	4.10 ± 1.29	90 ± 7	10.1 ± 3.8	97 ± 3	2991 ± 659	83 ± 17
13b	β-l-Asp	1.45 ± 0.01	74 ± 3	11.8 ± 2.2	101 ± 1	753 ± 358	49 ± 7
14a	α-l-Glu	3.11 ± 1.32	105 ± 3	10.8 ± 2.1	98 ± 3	1167 ± 448	68 ± 5
14b	β-l-Glu	12.7 ± 4.7	98 ± 4	4.60 ± 1.79	101 ± 3	2233 ± 238	76 ± 0.7
15a	α-d-Ala	1.44 ± 0.49	100 ± 8	24.3 ± 5.3	106 ± 7	254 ± 118	67 ± 13
15b	β-d-Ala	15.4 ± 8.4	102 ± 9	5.46 ± 2.68	106 ± 6	1001 ± 192	86 ± 2
16a	α-d-Val	4.51 ± 2.35	105 ± 4	1.12 ± 0.69	93 ± 4	2218 ± 818	96 ± 16
16b	β-d-Val	2.38 ± 0.77	101 ± 5	1.30 ± 0.64	99 ± 0.2	1278 ± 283	98 ± 16
17a	α-d-Phe	0.77 ± 0.24	96 ± 4	8.36 ± 0.28	95 ± 10	215 ± 32	80 ± 11
17b	β-d-Phe	0.68 ± 0.29	78 ± 5	1.71 ± 0.78	96 ± 1	611 ± 350	92 ± 10
18a	α-l-Chg	2.88 ± 1.21	86 ± 8	20.5 ± 8.0	101 ± 1	250 ± 80	52 ± 6
18b	β-l-Chg	5.33 ± 0.89	86 ± 9	3.57 ± 1.63	96 ± 10	282 ± 58	59 ± 7
19a	α-l-Abu	5.12 ± 0.09	88 ± 2	98.2 ± 5.1	104 ± 9	942 ± 235	61 ± 2
19b	β-l-Abu	5.47 ± 1.18	83 ± 2	2.22 ± 1.64	100 ± 3	572 ± 18	72 ± 0.5
20a	α-β-Ala	3.52 ± 1.65	99 ± 7	96.4 ± 8.7	97 ± 2	186 ± 15	78 ± 9
20b	β-β-Ala	5.74 ± 0.47	78 ± 6	20.1 ± 10.7	99 ± 3	622 ± 311	67 ± 12
21a	α-GABA	2.88 ± 0.82	103 ± 4	34.4 ± 6.5	103 ± 1	2034 ± 253	104 ± 9
21b	β-GABA	12.3 ± 6.8	86 ± 7	6.06 ± 1.07	103 ± 6	3396 ± 1982	74 ± 5
22a	α-l-Val-l-Tyr	0.89 ± 0.04	84 ± 7	1.16 ± 0.60	88 ± 5	330 ± 179	50 ± 2
22b	β-l-Val-l-Tyr	0.16 ± 0.06	73 ± 5	1.56 ± 0.83	89 ± 7	1884 ± 594	63 ± 0.1
23a	β-Gly-Gly	4.39 ± 0.54	85 ± 9	2.84 ± 1.63	102 ± 7	885 ± 298	75 ± 0.7

^a[³⁵S]GTPγS binding assays were performed with membranes from CHO stably expressing the human opioid receptors. Percentage stimulation (% stim.) is presented relative to the reference full agonists DAMGO (MOR), DPDPE (DOR), and U69,593 (KOR). Values represent the mean ± SEM of three to four independent experiments each performed in duplicate.