Table 1.

Receptor Binding and Functional Bioactivity.

		Receptor affinitya (nM)		Selectivity		Functional bioactivity (nM)
no.	Structure	$K_{\text{i}}^{\delta }$	$K_{\text{i}}^{\mu }$	δ/μ	μ/δ	MVD (IC50)b	MVD (Ke)c	GPI (IC50)b
	H-Dmt-Tic-Gly-NH-CH2-Phd	0.031	0.16	–	5.3		0.56	2.69
	H-Dmt-Tic-Gly-NH-Ph d	0.042	0.16	–	3.6	3.02	-	2.57
	H-Dmt-Tic-Gly*-Bidd	0.035	0.50	–	14	0.13	-	26.9
	H-Tyr-Pro-Phe-Phe-NH2e (EM-2)	9,200	0.69	13,300	-	344	-	5.79
	H-Tyr-Pro-Trp-Phe-NH2e (EM-1)	1,500	0.36	4,200	-	36.3	-	10.1
1	H-Dmt-Aba-Gly-NH-CH2-Ph	11.0 ± 2.3 (3)	0.46 ± 0.07 (3)	23.9	-	830 ± 70		51 ± 5
2	H-Dmt-Aba-Gly-NH-Ph	27.2 ± 5.7 (3)	1.48 ± 0.11 (3)	18.4	-	Partial Agonist (max 40%) IC50 = 650 nM		95 ± 7.5
3	H-Dmt-Aba-Gly*-Bid	427 ± 38 (3)	19.9 ± 0.57 (3)	21.5		Partial Agonist (max 40%) IC50 = 2,700 nM		231 ± 15

^aThe K_i values (nM) were determined according to Chang and Prusoff ⁴⁸ as detailed in the Experimental Section. The mean ± SE with n repetitions in parenthesis is based on independent duplicate binding assays with five to eight peptide doses using several different synaptosomal preparations.

^bAgonist activity was expressed as IC₅₀ obtained from dose-response curves. These values represent the mean ± SE for at least five fresh tissue semples. Deltorphin C and dermorphin were the internal standards for MVD (δ-opioid receptor bioactivity) and GPI (μ-opioid receptor bioactivity) tissue preparation, respectively.

^cThe K_e (equilibrium dissociation constant) values of opioid antagonists against the agonists (deltorphin C and dermorphin) were determined by the method of Kosterlitz and Watt.⁵¹

^dData taken from Balboni, G. et al.¹¹