Effect of linker substitution on the binding of butorphan univalent and bivalent ligands to opioid receptors

Article abstract of DOI:10.1016/j.bmcl.2010.01.101

A series of bivalent hydroxy ether butorphan ligands were prepared and their binding affinities at the opioid receptors determined. Addition of a hydroxy group to a hydrocarbon chain can potentiate binding affinity up to 27- and 86-fold at the mu and kappa opioid receptors, respectively. Two bivalent ligands with sub-nanomolar binding affinity at the mu and kappa opioid receptors were discovered.

Full text of DOI:10.1016/j.bmcl.2010.01.101

Bioorganic & Medicinal Chemistry Letters 20 (2010) 1507–1509
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Effect of linker substitution on the binding of butorphan univalent
and bivalent ligands to opioid receptors
b
b
a
Brian S. Fulton a, , Brian L. Knapp , Jean M. Bidlack , John L. Neumeyer
*
a Alcohol and Drug Abuse Research Center, McLean Hospital, Harvard Medical School, 115 Mill Street, Belmont, MA 02478, USA
b Department of Pharmacology and Physiology, School of Medicine and Dentistry, University of Rochester, Rochester, NY 14642, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of bivalent hydroxy ether butorphan ligands were prepared and their binding affinities at the opi-
oid receptors determined. Addition of a hydroxy group to a hydrocarbon chain can potentiate binding
affinity up to 27- and 86-fold at the mu and kappa opioid receptors, respectively. Two bivalent ligands
with sub-nanomolar binding affinity at the mu and kappa opioid receptors were discovered.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 21 December 2009
Revised 15 January 2010
Accepted 19 January 2010
Available online 25 January 2010
Keywords:
Opioid
Bivalent
Univalent
Ligand
Addiction
Butorphan
The analgesic, euphoric, and addictive properties of analgesic
opioids such as morphine are thought to be due primarily to the
H
H
interaction of the opioid with the mu (
not proven possible to separate the analgesic and addictive proper-
ties of opioid analgesics. Activation though of the kappa ( ) opioid
l
) opioid receptor.1 It has
HO
HO
j
2
1
receptor or delta (d) opioid receptor can modulate the agonist
properties of opioid analgesics. Behavioral pharmacological studies
in nonhuman primates have shown that dual acting compounds
Figure 1. Structures of butorphan (1) and cyclorphan (2).
that are agonists or partial agonists at both the
l
opioid receptor
and
j
opioid receptor could be useful as medications in the treat-
good binding affinity, selectivity, and potency whereas those con-
nected by an ether linkage (Fig. 2) lost binding affinity.7–9
ment of drug abuse and as analgesics.2 Opioid receptor dimeriza-
tion has been suggested to explain the apparent opioid subtype
selectivity observed with different pharmacological agents.3,4 The
possibility of the G-protein coupled receptor opioid heterodimers
as drug targets has been reviewed.5 Previous reports from our lab-
We have recently reported where the addition of methyl groups
adjacent to the hydrolytically labile ester linkage increased stabil-
ity while partially affecting binding affinity.10 To further investi-
gate the importance of linker type on binding affinity a series of
butorphan bivalent ligands have been prepared where hydroxy
ether linkages replaced ether linkages. The incorporation of a hy-
droxy group was dictated in part to determine if a hydrogen-bond
oratory indicated that the opioid
phan (1) (Fig. 1) has a more promising profile of activity than the
opioid antagonist/
agonist cyclorphan (2).2,6
This finding led to the synthesis of a series of homobivalent li-
gands incorporating butorphan as the pharmacophore connected
by linking spacers of various lengths. From these studies it was ob-
served that bivalent ligands connected by an ester linkage retained
l partial agonist/j agonist butor-
l
j
N
H
N
H
O
O
n
3 (MCL-154) n = 8
4 MCL-153) n = 2
* Corresponding author at present address: Center for Drug Discovery, North-
eastern University, 360 Huntington Avenue, 116 Mugar Hall, Boston, MA 02115,
United States. Tel.: +1 617 475 5232.
E-mail address: b.fulton@neu.edu (B.S. Fulton).
Figure 2. Structures of butorphan bivalent ether ligands.
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.01.101

1508
B. S. Fulton et al. / Bioorg. Med. Chem. Lett. 20 (2010) 1507–1509
N
H
N
H
i
+
TsO
OTs
1
1
O
O
5
N
N
H
N
H
H
i
O
+
O
O
O
OH
14
6
N
7
n = 1
n = 2
n = 3
N
i
8
9
O
O
H
+
H
1
10 n = 4
11 n = 5
12 n = 6
O
O
n
n
OH
OH
n = 1-6
N
N
H
O
O
i
H
+
1
O
O
O
O
O
O
OH
OH
13
Scheme 1. Synthesis of the bivalent ligands. Reagents and conditions: (i) NaH, DMF, 80 °C, 24–48 h.
donor group (e.g., OH) could help restore the binding affinity that
was lost upon conversion of the phenol group to ether.
The affinity and selectivity of the compounds synthesized were
evaluated for binding affinities at all three opioid receptor types (
l,
The bivalent ligands were synthesized by coupling butorphan
with electrophiles under basic conditions (Scheme 1). Compound
5 was prepared in 66% yield by reacting 2 equiv of the sodium salt
of butorphan (sodium hydride in dimethylformamide) with the bis-
tosylate of 1,3-propanediol. Its hydroxy derivative, 6, was prepared
by reacting butorphan with the glycidyl ether of butorphan (14). The
bivalent ligands 7–12 were prepared in variable yields of 15% (10) to
70% (12) by reacting the sodium salt of butorphan with a homolo-
gous series of terminal bisepoxides. The difference in yields was
due mainly to the need for multiple chromatographic purifications
for some compounds. The bisepoxides in turn were prepared by
d, and
j
) using a previously described procedure (Table 1).9 As
shown previously, linking butorphan via an ether hydrocarbon
chain greatly reduced the binding affinities relative to butorphan.9
Replacing the ether linkage by an ester linkage restored the bind-
ing affinity. The current results show that the binding affinity can
be restored upon the introduction of a hydroxy group on the car-
bon atom beta to the ether linkage. This type of linkage is attractive
as it will not be hydrolyzed by esterases thus producing a more
chemically and metabolically stable bivalent ligand.
Figure 4 clearly shows that potent binding affinity can be re-
stored upon introduction of the hydroxy group.
epoxidation of
a
,x
-diolefins with m-chloroperbenzoic acid under
The most striking results are in the 10-carbon series. The ether
standard conditions in dichloromethane. Bivalent ligand 13was pre-
pared in 72% yield by reacting 2 equiv of butorphan with commer-
cially available bis[4-(glycidyloxy)phenyl]methane. The bivalent
ligands were obtained and tested as inseparable diastereomeric
mixtures.
The univalent butorphan compounds (Fig. 3) were prepared to
investigate the importance of a second butorphan unit on binding
affinity to the opioid receptors. Compound 14 is readily prepared in
77% yield by reacting butorphan with epichlorohydrin. Butorphan
was reacted with 1,2-epoxydecane, decanoyl chloride, and 1-bro-
modecane to form 15–17, respectively. Compound 18 was pre-
pared in 50% yield by reacting butorphan with 1,2-epoxy-3-
phenoxypropane.
based compound 3 had a Ki value of 66 and 120 nM at the l and j
opioid receptors, respectively; introduction of the beta-hydroxy
groups (12) increased binding affinity by 27- and 86-fold at the
Table 1
Ki values for the inhibition of
l, d and j opioid binding to CHO membranes by uni-
and bivalent ligands
Compound
Ki (nM) SE
Selectivity
l/d/k
l
d
j
1a
0.23 0.01
66 4.3
29 1.2
7.1 0.19
0.95 0.16
1.2 0.07
17 1.9
5.9 0.55
2500 91
730 29
180 3.3
37 3.3
33 5.0
420 14
160 7.5
74 1.9
99 5.4
27 1.9
29 1.4
200 21
52 2.8
20 1.2
920 149
86 11
0.079 0.003
120 8.2
18 1.3
3/75/1
3b (MCL-154)
1/38/1.8
1.6/40/1
1.1/29/1
1/39/1
7/194/1
1.4/35/1
1.2/16/1
1/23/1
1.2/25/1
1.7/19/1
1/121/1.4
1/27/5
4b (MCL-153)
5
6
7
8
9
10
11
12
13
14
15
16
17
18
6.2 0.53
0.99 0.022
0.17 0.089
12 0.87
9.8 0.56
3.3 0.23
3.9 0.12
1.4 0.15
0.34 0.064
36 0.52
8.3 0.68
0.2 0.023
52 5.1
N
12 1.0
H
R
O
3.2 0.41
4.8 0.16
2.4 0.43
0.24 0.036
7.3 1.0
7.4 0.26
0.21 0.017
70 7.0
14 R =
O
1/7/1.1
H2C
1/100/1
1.3/180/1
1/13/3
15 R = CH2CHOHC8H17
16 R = COC9H19
17 R = C10H21
6.5 0.28
22 1.2
18 R CH2CHOHCH2OC6H5
a
Ref. 6.
Ref. 9.
b
Figure 3. Structures of univalent butorphan ligands.

B. S. Fulton et al. / Bioorg. Med. Chem. Lett. 20 (2010) 1507–1509
1509
ported.13 The introduction of a hydroxy group in long chain univa-
lent and bivalent butorphan ligands can increase binding affinity to
the opioid receptors. The results as a whole suggest that univalent
butorphan ligands can bind as strongly as bivalent butorphan li-
gands to the opioid receptors. Their exact mode of binding,
whether in the opioid binding pocket or to allosteric sites, is un-
known and is under investigation.
Acknowledgment
This work was supported in part by NIH grants R01-DA14251
(J.L.N.), K05-DA 00360 (J.M.B.), and T32 DA007252 (B.S.F.). Levor-
phanol tartrate was generously donated by Mallinckrodt Inc.
Figure 4. Effect of binding affinity on chain length and presence of a hydroxy group.
References and notes
l
and j opioid receptors, respectively. Additional evidence of the
importance of the hydroxyl group can be seen from the C10 univa-
lent series. Compound 17, the des-hydroxy derivative of 15, has
1. Koob, G. F.; Le Moal, M. Neurobiology of Addiction; Academic Press: New York,
2006, pp 121–172.
2. Bowen, C. A.; Negus, S. S.; Zong, R.; Neumeyer, J. L.; Bidlack, J. M.; Mello, N. K.
Neuropsychopharmacology 2003, 28, 1125.
3. Portoghese, P. S.; Lunzer, M. M. Eur. J. Pharmacol. 2003, 467, 233.
4. Waldhoer, M.; Fong, J.; Jones, R. M.; Lunzer, M. M.; Sharma, S. K.; Kostenis, E.
Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 9050.
5. Gomes, I.; Gupta, A.; Filipovska, J.; Szeto, H. H.; Pintar, J. E.; Devi, L. A. A. Proc.
Natl. Acad. Sci. U.S.A. 2004, 101, 5135.
6. Neumeyer, J. L.; Bidlack, J. M.; Zong, R.; Bakthavachalam, V.; Gao, P.; Cohen, D.
J.; Negus, S. S.; Mello, N. K. J. Med. Chem. 2000, 43, 114.
7. Mathews, J. L.; Fulton, B. S.; Negus, S. S.; Neumeyer, J. L.; Bidlack, J. M.
Neurochemical Res. 2008, 33, 2142.
8. Mathews, J. L.; Peng, X.; Xiong, W.; Zhang, A.; Negus, S. S.; Neumeyer, J. L.;
Bidlack, J. M. J. Pharmacol. Exp. Ther. 2005, 315, 821.
9. Neumeyer, J. L.; Zhang, A.; Xiong, W.; Gu, X. H.; Hilbert, J. E.; Knapp, B. I.; Negus,
S. S.; Mello, N. K.; Bidlack, J. M. J. Med. Chem. 2003, 46, 5162.
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Med. Chem. 2009, 52, 7389.
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10-fold weaker binding affinity at the
l opioid receptor. Interest-
ingly, the second butorphan unit in the bivalent ligand does not ap-
pear to be necessary for binding affinity. The bivalent monohydroxy
ligand 6 has 7.5-fold greater binding affinity at the
l receptor than
its des-hydroxy analog 5. However, the univalent hydroxy ether
18 (equal-potent to C3 des-hydroxy 5) has only 7.5-fold weaker
binding affinity at the
ether 6. The univalent hydroxy ether analog 15 had approximately
3–6-fold weaker binding affinity at the and receptors than the
bivalent ligand 12. Interestingly, the decyl ester 16 was one of the
most potent compounds with sub-nanomolar affinity at the and
opioid receptors suggesting that the presence of a hydrogen-bond
l opioid receptor than the bivalent hydroxy
l
j
l
j
donating group is not strictly required for strong binding affinity.
The hydrophobicity of substituents attached to the phenolic oxygen
has been shownto be importantfor the bindingof morphinans to the
opioid receptors.11,12 We have previously shown that strong binding
affinity can be obtained with univalent ligands compared to their
bivalent analogs.9,10
As shown in Figure 1 there does not appear to be any relation-
ship between linker length and binding affinity. The bivalent li-
gands that bound strongest to the opioid receptors were at
opposite extremes of linker length. Compound 13, with 17 atoms
between the phenol oxygens of butorphan, has a Ki value of
12. Wentland, M. P.; VanAlstine, M.; Kucejko, R.; Lou, R.; Cohen, D. J.; Parkhill, A. L.;
Bidlack, J. M. J. Med. Chem. 2006, 49(5), 635.
13. Bis-((À)-N-cyclobutylmethylmorphinan-3-oxy)
decane-2,9-diol
(12):
To
butorphan (83 mg, 0.27 mmol) in DMF (3 mL) was added hexane washed
sodium hydride (10 mg, 0.42 mmol) and the slurry was stirred at room
temperature for thirty minutes. To the resultant solution was added 1,9-
diepoxydecane (23 mg, 0.13 mmol) in DMF (0.5 mL) and the solution was
heated at 80 °C for 48 h. The reaction was cooled to room temperature,
quenched with water, and extracted twice with ethyl acetate. The organic
layers were combined and washed three times with brine, dried over sodium
sulfate, filtered, and concentrated in vacuo to give 98 mg of a light yellow foam.
The compound was purified by flash chromatography (EtOAc/Et3N = 100/1) to
give 73 mg (71%) as an oil. 1H NMR (CDCl3, 300 MHz): d 7.02 (d, J = 9 Hz, 2H),
6.82 (s, 2H), 6.73 (d, J = 8.4 Hz, 2H), 4.1–3.9 (m, 4H), 3.8–3.7 (m, 2H), 2.92 (d,
J = 18 Hz, 2H), 2.77 (br d s, 2H), 2.56–2.06 (m, 14H), 2.05–1.0 (m, 46H) ppm. 13C
NMR (CDCl3, 75 MHz) d 157.1, 142.3, 130.8, 128.7, 111.9, 111.5, 72.5, 70.4, 61.8,
56.1, 46.1, 45.3, 42.2, 38.0, 36.9, 35.2, 33.4, 29.8, 28.1, 27.1, 26.8, 25.7, 24.3,
22.5, 19.1 ppm. Anal. Calcd for C52H76N2O4ÁHCl: C, 75.28; H, 9.35; N, 3.38.
Found: C, 75.29; H, 9.28; N, 3.48.
0.24 nM at the
tween the phenol oxygens, has a Ki value of 0.95 nM at the
l
opioid receptor while 6, with only 3 atoms be-
opioid
l
receptor, a 4.5-fold difference. A similar trend was observed for
bivalent ester butorphan ligands.9
In summary, the synthesis and pharmacological evaluation of
series of bivalent ligands for the opioid receptors has been re-