Bioorganic & Medicinal Chemistry Letters
Generation of novel radiolabeled opiates through site-selective iodination
Susruta Majumdar a, Maxim Burgman a, Nathan Haselton a, Steven Grinnell a,b, Julia Ocampo a,
Anna Rose Pasternak a, Gavril W Pasternak a,b,
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a Molecular Pharmacology and Chemistry Program and Department of Neurology, Memorial Sloan-Kettering Cancer Center, 1275 York Ave., New York, NY 10065, USA
b Department of Neurology and Neuroscience, Weill Cornell Medical College, New York, NY 10065, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Tritiated opioid radioligands have proven valuable in exploring opioid binding sites. However, tritium has
many limitations. Its low specific activity and limited counting efficiency makes it difficult to examine
low abundant, high affinity sites and its disposal is problematic due to the need to use organic scintillants
and its relatively long half-life. To overcome these issues, we have synthesized both unlabeled and car-
rier-free radioiodinated iodobenzoyl derivatives of 6b-naltrexamine (125I-BNtxA, 18), 6b-naloxamine
Received 8 February 2011
Revised 28 April 2011
Accepted 2 May 2011
Available online 8 May 2011
(
125I-BNalA, 19) and 6b-oxymorphamine (125I-BOxyA, 20) with specific activities of 2100 Ci/mmol. To
Keywords:
optimize the utility of the radioligand, we designed a synthesis in which the radiolabel is incorporated
in the last synthetic step, which required the selective iodination of the benzoyl moiety without incorpo-
ration into the phenolic A ring. Competition studies demonstrated high affinity of the unlabelled com-
pounds for opioid receptors in transfected cell lines, as did the direct binding of the 125I-ligands to the
opioid receptors. The radioligand displayed very high sensitivity, enabling a marked reduction in tissue,
as well as excellent signal/noise characteristics. These new 125I-radioligands should prove valuable in
future studies of opioid binding sites.
Opioid receptor
Radioligand
Iodination
Ó 2011 Elsevier Ltd. All rights reserved.
The opioid receptors are G-protein coupled receptors which
mediate pain relief through both the central and peripheral nervous
systems.1,2 The opiate antagonists naltrexone (Ntx, 3) and naloxone
(Nal, 4) and the agonist oxymorphone (Oxy, 5) are potent and widely
used opiates. Three families of opiate receptors have been identified
and cloned: mu (MOR-1), kappa1 (KOR-1) and delta (DOR-1). Radio-
ligands were critical in the initial demonstration of the opioid recep-
tors. Early attempts to detect binding sites using 14C-labeled
opioids3 were not successful due to the low specific activity of the
radioligands combined with the high affinity and low abundance
of the sites. A number of selective tritiated ligands are currently used
mu,4–6 delta,7,8 and kappa receptors9–11 proposed from both bio-
chemical and cloning studies. The use of tritium also poses practical
issues. These range from detection issues, such as the need to utilize
scintillation fluor for counting which destroys the labeled sample
and special films and long exposures for autoradiography, to its dis-
posal, which is problematic due to the long half-life and the need to
use organic scintillation fluors. To avoid these issues, we developed
high affinity 125I-labeled opiate ligands that obviate many of these
problems. They have high specific activity (2100 Ci/mmol), which
permits the examination of binding at very low concentrations
and can detect sites of low abundance. They can be counted without
the need for scintillation fluor and their disposal is facilitated by
their short half-life.
to label mu ([
D
-Ala2,MePhe4,Gly(ol)5]enkephalin; DAMGO), kappa1
(U50,488H) and delta ([
D
-Pen2, -Pen5]enkephalin; DPDPE) recep-
D
tors. However, tritium presents a number of limitations. While their
specific activity, typically around 50 Ci/mmol, is sufficient to iden-
tify the receptors in brain tissue or cell lines expressing them, exam-
ination of binding sites with very low abundance and high affinity is
limited and may impact the ability to identify proposed subtypes of
Traditionally incorporating a radioactive 125I into a molecule re-
quires the direct iodination of a phenolic or an aromatic amine.
Problems can arise if a molecule has more than one potential site
of incorporation. This is particularly difficult if iodination of one
of the sites adversely impacts binding affinity or functional activity,
as is the case with the A ring of opiates. Our goal to develop 125I-la-
beled opioids suitable for general use required an approach to
incorporate the iodine as the final synthetic step into a specific loca-
tion of the molecule. Prior studies suggested benzoic acid substitu-
tions at the 6-position of the opiate scaffold can maintain
activity.12,13 We now report the design and synthesis of three novel,
high affinity 125I-labeled opiates based upon 6-substituted amines
of naltrexone (6), naloxone (7), and oxymorphone (8).
Abbreviations: NHS, N-hydroxy succinimide; THF, Tetrahydrofuran; DCC,
Dicyclohexyl carbodiimide; NH4OAc, Ammonium acetate; NaBH3CN, Sodium cya-
noborohydride; MeOH, Methanol; rt, room temperature; DCM, Methylene chloride;
DIEA, N,N-Diisopropyl ethyl amine; n-Bu3SnCl, Tributyl tin chloride; BuLi, Butyl
lithium; Na2S2O5, Sodium metabisulfite; AcOH, Acetic acid.
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Corresponding author. Tel.: +1 646 888 2165.
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.