PT  - JOURNAL ARTICLE
AU  - Loriann Mahurter
AU  - Carrie Garceau
AU  - Jacqueline Marino
AU  - Helmut Schmidhammer
AU  - Géza Tóth
AU  - Gavril W. Pasternak
TI  - Separation of Binding Affinity and Intrinsic Activity of the Potent μ-Opioid 14-Methoxymetopon
AID  - 10.1124/jpet.106.105395
DP  - 2006 Oct 01
TA  - Journal of Pharmacology and Experimental Therapeutics
PG  - 247--253
VI  - 319
IP  - 1
4099  - http://jpet.aspetjournals.org/content/319/1/247.short
4100  - http://jpet.aspetjournals.org/content/319/1/247.full
SO  - J Pharmacol Exp Ther2006 Oct 01; 319
AB  - Receptor binding studies of 5,14-O-dimethyloxymorphone (14-methoxymetopon) in brain membranes have established its high affinity for μ-binding sites, but its analgesic potency far exceeds the modest increase in binding affinity relative to other opioids. The current study has established the selectivity of [3H]14-methoxymetopon for μ sites in calf striatal membranes and for a number of full-length splice variants of the cloned murine μ-opioid receptor 1 (mMOR-1) in transfected cell lines. The binding affinity of [3H]14-methoxymetopon for the variants expressed in Chinese hamster ovary cells was quite high, with KD values around 0.2 nM for all of the variants with the exception of mMOR-1F (KD of 1.2 nM). The affinity for most of the expressed variants was greater than that seen in the brain membranes (KD of 0.99 nM). Functionally, in guanosine 5′-O-(3-[35S]thio)triphosphate ([35S]GTPγS) binding assays with the MOR-1 variants, 14-methoxymetopon and the μ-opioid peptide [d-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO) showed similar efficacies, as determined by maximal stimulation, but 14-methoxymetopon was up to 65-fold more potent than DAMGO. The greatest difference was seen with mMOR-1E and the least with mMOR-1C, which displayed only a 10-fold difference. These potency differences in the stimulation of [35S]GTPγS binding far exceeded the differences in binding affinity. The differences between 14-methoxymetopon and DAMGO remained after normalizing the potency shifts based upon receptor binding affinities and varied from 1.2-fold with mMOR-1C to 21-fold for mMOR-1 and 42-fold with mMOR-1F. Thus, 14-methoxymetopon is a potent agonist against all of the mMOR-1 splice variants, but its potency ranged widely despite similar binding affinities for most of the variants and may give insight into its unusual pharmacological profile. The American Society for Pharmacology and Experimental Therapeutics