Bis-quinolinium cyclophanes: 6, 10-diaza-3(1, 3), 8(1, 4)-dibenzena-1, 5(1, 4)diquinolinacyclodecaphane (UCL 1684), the first nanomolar, non-peptidic blocker of the apamin-sensitive Ca2+-activated K+ channel

Full text of DOI:10.1021/jm970571a

2
J . Med. Chem. 1998, 41, 2-5
Communications to the Editor
Ch a r t 1
Bis-Qu in olin iu m Cyclop h a n es:
6,10-Dia za -3(1,3),8(1,4)-d iben zen a -1,5(1,4)-
d iqu in olin a cyclod eca p h a n e (UCL 1684),
th e F ir st Na n om ola r , Non -P ep tid ic
Block er of th e Ap a m in -Sen sitive
Ca 2+-Activa ted K+ Ch a n n el
J oaquin Campos Rosa,†,‡ Dimitrios Galanakis,†,§
C. Robin Ganellin,*,† Philip M. Dunn,| and
Donald H. J enkinson|
Departments of Chemistry and Pharmacology,
University College London, Gower Street,
London WC1E 6BT, U.K., Departamento de Quimica
Organica, Facultad de Farmacia, Universidad de Granada,
Campus de Cartuja, s/ n, 18071 Granada, Spain, and
Department of Pharmaceutical Chemistry, School of
Pharmacy, Aristotelian University of Thessaloniki,
Thessaloniki, 540 06, Greece
a
L ) aromatic rings, as indicated in Table 1.
in Xenopus oocytes of SKCa channel-forming peptides
from rat and human brain.15
Received August 25, 1997
Small conductance Ca2+-activated K+ (SKCa) channels
comprise a widely distributed but relatively little stud-
ied class of K+ channels.1 Selective blockade of SKCa
channels may find application in the therapy of myo-
tonic muscular dystrophy,2 gastrointestinal dysmotili-
ties (Dunn, P. M.; J enkinson, D. H., unpublished
results), disorders of memory,3 narcolepsy,4 and alcohol
abuse.5 Furthermore, SKCa channel blockers will be
invaluable tools for further investigations into the role
of this K+ channel subtype in physiological and patho-
physiological processes.
Molecular modeling studies suggest that the cyclo-
phanes of type 2 are more rigid than their noncyclic
analogues 1 (Chart 1). Thus, the number of distinct
conformations within 3 kcal/mol of the calculated global
minimum (i.e. the conformations that have a >99%
probability of existing at 37 °C) is 2-10 times smaller
for cyclophanes 2 compared with the respective noncyc-
lic analogues 1 (Galanakis, D., unpublished results).
However, cyclophanes 2 retain substantial conforma-
tional mobility, mainly due to the presence of the long
aliphatic chain A.
Blockers of SKCa channels include natural peptidic
toxins such as apamin,6 leiurotoxin I,7 PO5,8 and Ts K,9
as well as nonpeptidic compounds such as the neuro-
muscular blockers tubocurarine, pancuronium, and
atracurium.10 Furthermore, the antiseptic compound,
dequalinium (Chart 1), has been shown to be a relatively
potent and selective SKCa channel blocker11 and has
constituted a lead for the development of several series
of novel SKCa channel blockers.12 We have recently
reported the design of novel bis-quinolinium cyclophanes
of the general structure 2 (L ) group having two
aromatic rings, Chart 1) which are submicromolar
blockers of the SKCa channel.12i In addition, we have
shown that one of them, 2 [UCL 1530, L ) methyl-
enebis(benzene-1,4-diyl)], shows selectivity for neuronal
over hepatocyte SKCa channels,13 which adds to the
functional evidence for the existence of SKCa channel
isoforms in different tissues.14 Such selective com-
pounds should prove to be useful pharmacological tools,
especially in view of the recent cloning and expression
In the present study we have sought to reduce further
the conformational freedom of the quinolinium groups
of series 2 by replacing part of the aliphatic chain with
aromatic rings (A). Initially, we synthesized bis-quino-
linium cyclophanes of the general structure 3 (Chart 1
and Table 1) in which each of the linkers A and L
contain two aromatic rings. Subsequently, we reduced
the size of the linkers by synthesizing cyclophanes in
which A and L are benzene rings.
Ch em istr y. The cyclophanes 3a -g (Table 1) were
synthesized according to Scheme 1. The conversion of
the novel diquinolines 4a -d to the desired cyclophanes
was carried out under high dilution conditions (1-2
mM). The preparations of the necessary dibromides
4,4′-bis(bromomethyl)biphenyl16 and bis[p-(bromometh-
yl)diphenyl]methane17 have been described previously.
The requisite diamines 5a and 5b have been reported18
but were prepared via the two alternative routes shown
in Scheme 2. Higher overall yields were obtained
through the diazide route. Amines 5c and 5d were
commercially available. Final products were purified
by high performance liquid chromatography and iso-
lated and analyzed as solid trifluoroacetate hydrates.
Biologica l Testin g. The SKCa blocking action of the
compounds was assessed from their ability to inhibit the
after-hyperpolarization (AHP) in cultured rat sympa-
* Address for correspondence: University College London, Depart-
ment of Chemistry, Christopher Ingold Laboratories, 20 Gordon St.,
London WC1H 0AJ , U.K.
† Department of Chemistry, University College London.
| Department of Pharmacology, University College London.
‡ Universidad de Granada.
§ Aristotelian University of Thessaloniki.
S0022-2623(97)00571-2 CCC: $15.00 © 1998 American Chemical Society
Published on Web 01/01/1998

Communications to the Editor
J ournal of Medicinal Chemistry, 1998, Vol. 41, No. 1
3
Ta ble 1. Compound Structures and Biological Results for Inhibition of Afterhyperpolarization in Cultured Rat Sympathetic
Neurones
Sch em e 1
thetic neurones as described previously.11b Briefly, each
compound was tested at two to four concentrations on
at least three cells. Two to four compounds were
examined on a batch of cultured neurones, and in each
such series of experiments, dequalinium was also in-
cluded as a reference compound. The Hill equation was
fitted to the data to obtain estimates of the IC50.
However, because there was some variation in the
potency of dequalinium during the course of the study,
equieffective molar ratios (EMR: relative to dequal-
inium) were also obtained by simultaneous nonlinear
least-squares fitting of the data with the Hill equation.
These are also listed in Table 1, and it is these values
which have been used for the comparison between
compounds.
cyclophanes and the results of biological testing are
presented in Table 1. In those cases where the linkers
A and L are large groups containing two or more
aromatic rings (3e-g), the activity of the compound does
not seem to be critically dependent upon the nature of
the linkers. Hence, cyclophanes 3e-g have similar
potencies. However, this is not the case when the
linkers A and L are benzene rings (3a -d ). The blocking
potency of these smaller cyclophanes is remarkably
sensitive to the substitution pattern of the benzene
rings. This is exemplified with 3a and 3b in which a
change in the substitution pattern of A from meta to
para causes a 22-fold increase in potency. Compound
3b (UCL 1684) is the most potent nonpeptidic SKCa
channel blocker reported to date, being 100 times more
potent than dequalinium. Furthermore, having an IC50
Resu lts a n d Discu ssion . The structures of the

4
J ournal of Medicinal Chemistry, 1998, Vol. 41, No. 1
Communications to the Editor
Sch em e 2a
a
(i) NaN3, DMF, 90 °C, 17 h; 6a , 97%; 6b, 95%; (ii) LiAlH4, THF, Reflux, 1 h; 5a , 93%; 5b, 87%; (iii) Potassium Phthalimide, DMF, 90
°C, 40 min; 7a , 87%; 7b, 88%; (iv) H2NNH2‚H2O, DMSO, 120 °C, 1.5 h; 5a , 98%; 5b, 35%.
of 3 nM, it constitutes the first synthetic compound to
have a potency similar to that of apamin (IC50 ) 1 nM).6
The results for the small cyclophanes 3a -d constitute
a significant advancement in the structure-activity
relationships of the SKCa channel blockers, when com-
pared with the existing knowledge in the field. So far,
large structural changes in the linker(s) joining the two
quinolinium groups in several series of SKCa channel
blockers have been shown to have only a small effect
on the potency of the molecule. Thus, in dequalinium
homologues of structure 1 (Chart 1) in which L was
(CH2)n, the activity of the compound has been reported
to be independent of the length of the methylene chain
for 5 < n < 12, while a small decrease in potency has
been observed in the 4- and 3-methylene homologues.12f
Furthermore, in a series of blockers of the general
structure 1 which incorporated a large number of
semirigid linkers L containing one, two, or three aro-
matic rings, the change in the biological activity of the
molecules was remarkably small, the maximum varia-
tion only being approximately 10-fold.12g These results
are attributable to the residual conformational mobility
of the two quinolinium groups and to the extensive
delocalization of the positive charge within these
groups.12g The cyclophanes of series 2 were synthesized
in an attempt to reduce further the conformational
mobility of the charged heterocycles.12i However, the
structure-activity trends in series 2 were similar to
those observed in the noncyclic analogues 1, although
one compound demonstrated an interesting selectivity
profile for blocking the SKCa channel in different tissues.
Further rigidification in the linker A of series 2 via
replacement of most of the methylene chain by groups
having two aromatic rings (compounds 3e-g) has not
resulted in significant changes in activity. In this
context, the small cyclophanes 3a -d represent the first
example of closely related analogues in terms of struc-
ture, which show substantially different potencies for
blocking the SKCa channel. Preliminary molecular
modeling studies indicate that the two quinolinium
rings in 3a -d may be “cis” (synperiplanar) or “trans”
(antiperiplanar) to each other, or the planes of the two
quinolinium rings may form a dihedral angle of ap-
proximately 90° (“orthogonal” conformation). Whereas
3a and 3b can adopt stable cis and trans conformations
of approximately equal energies, 3c and 3d adopt
orthogonal and trans conformations, also of similar
energies. Clearly, these findings do not account for the
differences between the potencies of these compounds
as channel blockers.
Con clu sion . The synthesis and pharmacological
testing of novel bis-quinolinium cyclophanes as blockers
of the SKCa channel has been described. Compound 3b
is the most potent nonpeptidic blocker reported to date,
showing activity in the low nanomolar range and similar
to that of apamin. Furthermore, the results with the
present series add significantly to the structure-activity
knowledge in the field, since they incorporate the first
example of molecules in which the activity depends
critically on the nature of the linkers joining the two
quinolinium groups.
Ack n ow led gm en t. This work was partially sup-
ported by the Wellcome Trust including fellowships to
D.G. and P.M.D. The award of a grant from the Spanish
DGICYT to J .C.R. is acknowledged.
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