Table 2. Binding affinity at human opioid receptors.

	Ki (nM)
Drug	μ	δ	κ	ORL1	Reference
Buprenorphine	0.0027	33	0.0021	25,000	[60]
Steady state free Cmax = 0.87 nM	0.52				[61]
	0.08	0.42	0.11	285	[37]
	1.5	6.1	2.5	77.4	[62]
	1.5	4.5	0.8		[63]
	0.21				[64]
	0.21	2.9	0.62		[65]
	1.8				[66]
	1.5	4.5	0.8		[67]
Norbuprenorphine	0.0018	1300	0.0013		[60]
Steady state total Cmax = 22 nM	0.07	3.17	0.91	7330	[37]
Methadone	14				[61]
Free Cmax = 438 nM	3.7				[68]
	5.6				[69]
	0.6	132.2	323.5		[63]
	3.37				[37]
	7.7				[66]
	3.38				[64]
Naltrexone	1.1				[61]
Free Cmax = 28 nM	0.26	117	5.15		[70]
	0.2	10.8	4		[63]
	0.5		1		[71]
Naloxone	3.3				[61]
Free Cmax = 13 nM	0.8				[68]
	0.79	76	1.1		[72]
	2.4				[73]
	1.4	67.5	2.5		[63]
	3				[74]
	1.51				[64]
	7		4		[71]
	0.66	120	1.2		[65]
	0.6				[66]