Probes for narcotic receptor mediated phenomena. 18. Epimeric 6 alpha- and 6 beta-iodo-3,14-dihydroxy-17-(cyclopropylmethyl)-4,5 alpha-epoxymorphinans as potential ligands for opioid receptor single photon emission computed tomography: synthesis, evaluation, and radiochemistry of [125I]-6 beta-iodo-3,14-dihydroxy-17-(cyclopropylmethyl)-4,5 alpha-epoxymorphinan.
The epimeric 6 beta- and 6 alpha-iodo-3,14-dihydroxy-17-(cyclopropylmethyl)-4,5 alpha-epoxymorphinans (1, ioxy) and (2, epioxy), respectively, were each synthesized in five steps starting with naltrexone. The configuration of the 6-iodo group of 1 was unequivocally determined to be beta-based on single crystal X-ray analysis of its precursor 3-acetoxy-6 beta-iodo-14-hydroxy-17-(cyclopropylmethyl)-4,5 alpha-epoxymorphinan (10). Both 1 and 2 as well as their corresponding 3-O-acetates 10 and 11 were found to readily cross the blood-brain barrier and completely reverse the analgesic effects of a 10 mg/kg intraperitoneal dose of morphine sulfate as determined by the paw withdrawal latency test. Compounds 1 and 2 were found to bind with high affinity to mu, delta, and kappa receptors in vitro. In general, 1 and 2 exhibited higher affinity for mu and kappa receptors than naltrexone while the 6 beta-iodo epimer 1 (ioxy) was more potent than its epimer 2. In a comparison of the 6 beta-halogen substituent on binding affinity across opioid receptor subtypes, it was generally found that I greater than Br greater than F. On the basis of the results of in vitro and in vivo testing, 1 was selected as a target for radioiodination and evaluation as a potential single photon emission computed tomography imaging agent for opioid receptors. Carrier-free [125I]-1 was synthesized in near quantitative yield by the sequence of reaction of excess 3-acetoxy-6 alpha-[[(trifluoromethyl)sulfonyl]oxy]-14-hydroxy-17- (cyclopropylmethyl)-4,5 alpha-epoxymorphinan (8) with anhydrous Na125I in dry acetonitrile for 90 min at 76 degrees C followed by deacetylation of the product with 1:1 aqueous ammonia/acetonitrile at 25 degrees C. The potential of [125I]-1 as an in vivo imaging agent for opioid receptors is evaluated and discussed.
de Costa
B R
BR
Laboratory of Medicinal Chemistry, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892.
Iadarola
M J
MJ
Rothman
R B
RB
Berman
K F
KF
George
C
C
Newman
A H
AH
Mahboubi
A
A
Jacobson
A E
AE
Rice
K C
KC
D000818:Animals; D001665:Binding Sites; D001812:Blood-Brain Barrier; D001921:Brain / Q000000981:diagnostic imaging / Q000378:metabolism; D002851:Chromatography, High Pressure Liquid; D006168:Guinea Pigs; D007202:Indicators and Reagents; D007457:Iodine Radioisotopes; D008297:Male; D015394:Molecular Structure; D009019:Morphinans / Q000138:chemical synthesis / Q000378:metabolism* / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D011957:Receptors, Opioid / Q000378:metabolism*; D015899:Tomography, Emission-Computed, Single-Photon; D014961:X-Ray Diffraction
D007202:Indicators and Reagents; D007457:Iodine Radioisotopes; D009019:Morphinans; D011957:Receptors, Opioid; C075927:6-iodo-3,14-dihydroxy-17-(cyclopropylmethyl)-4,5-epoxymorphinan