Design, synthesis, and preliminary biological evaluation of new isoform-selective f-current blockers

Article abstract of DOI:10.1021/jm1006758

New I_f blockers have been designed and tested on HEK293 cells stably expressing the HCN1, HCN2, and HCN4 channels to find compounds able to discriminate among the channel isoforms. Among the synthesized compounds, the cis-butene derivative (R)-5 shows some preference for HCN2 while the pseudodimeric product (R)-6 shows selectivity for HCN1. These compounds can be important pharmacological tools to study the channels in native tissues and may be useful to design safe drugs.

Full text of DOI:10.1021/jm1006758

J. Med. Chem. 2010, 53, 6773–6777 6773
DOI: 10.1021/jm1006758
Design, Synthesis, and Preliminary Biological Evaluation of New Isoform-Selective f-Current Blockers
Michele Melchiorre,†,§ Martina Del Lungo,‡,§ Luca Guandalini,† Elisabetta Martini,† Silvia Dei,† Dina Manetti,†
Serena Scapecchi,† Elisabetta Teodori,† Laura Sartiani,‡ Alessandro Mugelli,‡ Elisabetta Cerbai,‡ and
Maria Novella Romanelli*,†
†Laboratory of Design, Synthesis, and Study of Biologically Active Heterocycles (HeteroBioLab), Department of Pharmaceutical Sciences,
Via Ugo Schiff 6, 50019 Sesto Fiorentino (FI), Italy, and ‡Center of Molecular Medicine (C.I.M.M.B.A.), University of Florence,
Viale G. Pieraccini 6, 50139 Firenze, Italy. §These authors contributed equally.
Received April 9, 2010
New If blockers have been designed and tested on HEK293 cells stably expressing the HCN1, HCN2,
and HCN4 channels to find compounds able to discriminate among the channel isoforms. Among the
synthesized compounds, the cis-butene derivative (R)-5 shows some preference for HCN2 while the
pseudodimeric product (R)-6 shows selectivity for HCN1. These compounds can be important
pharmacological tools to study the channels in native tissues and may be useful to design safe drugs.
Introduction
The interest in HCN channels modulators has recently
grown because of the emerging evidence that these channels
are involved in the function of sensory neurons and in pain
perception,11 making them attractive targets for the discovery
of drugs not only for heart diseases12 but also for the manage-
ment of neuropathic pain.13,14 In addition, the analysis of
transgenic HCN knockout mice has shed more light into the
physiological role of HCN channels.3 Besides heart, HCN
channels regulate rhythmic activity in other excitable cells
such as neurons and photoreceptors. They have a role in den-
dritic integration, long-term potentiation, synaptic transmis-
sion, and the control of working memory, and they have been
found in pancreatic β-cells, although their role is still un-
known.4,15 The different isoforms vary in their distribution
within tissues;therefore, isoform-selectivesubstances couldbe
important pharmacological tools to study the channels in
native tissues and may be useful to design safe drugs.
The design of selective HCN blockers is hampered by the
fact that most of the phenylalkylamines synthesized so far
have been tested in vivo or in vitro mainly for their ability to
reduce heart rate, and only a few compounds have been tested
on different HCN isoforms. Since the negative chronotropic
activity may be due to several mechanisms,16 it is obviously
quite difficult to derive structure-activity relationships for
the interaction with the molecular target of interest (i.e., HCN)
from the data available in the literature. Zatebradine, cilo-
bradine, ivabradine, and 1, the most studied compounds, have
a different ability to block If, but they are not able to dis-
criminate among HCN channel isoforms.17,18 Regarding the
site of interaction of these substances on the channel, there is
evidence that it is located into the channel pore and that the
compounds must cross the cell membrane to interact with
it;1,19,20 therefore, an important property of HCN blockers is
lipophilicity. In addition, at the site of interaction some
differences exist within the four isoforms that can be exploited
to design selective ligands.19-21
The “funnycurrent” (If) isa mixed Naþ/Kþ inwardcurrent,
activated upon hyperpolarization, directly modulated by
cAMP and regulated by neurotransmitter receptors coupled
to cyclic nucleotide second messengers. (If)1 is carried by the
HCNa (hyperpolarization-activated cyclic nucleotide gated)
channels; they belong to the superfamily of cyclic nucleotide-
gated (CNG) channels and consist of four subunits, each
showing six transmembrane domains (S1-6), a voltage sensor
(S4), and a cyclic nucleotide-binding site.2 In mammals, the
HCN channel family is formed by four members (HCN1-4)
that are differently distributed within tissues such as central
and peripheralnervoussystem andinthe heart.3 Whenexpres-
sed in heterologous cells, the subunits form homomeric
channels displaying the main biophysical properties of native
If but differing from each other mainly with regard to their
speed of activation and the extent by which they are modu-
lated by cAMP.4 There is evidence that in vivo the four HCN
subunits can combine into heterotetrameric channels whose
stoichiometry is not known (see ref 4 and references therein).
If plays a major role in the generation of pacemaker activity
and in the regulation of heart rate, and it can be blocked by
specific bradycardic agents, a group of structurally diverse
drugs among which are 1 (ZD7288)5 and the phenylalkyla-
mines reported in Chart 1: zatebradine, cilobradine, and iva-
bradine. While cilobradine recently entered clinical trials,6 the
development of zatebradine stopped because of adverse side
effects on vision,7 probably caused by inhibition of HCN
channels in retinal rod.8 Ivabradine, whose pharmacological
profile is safer than that of zatebradine,9,10 has been approved
in 2005 by EMEA for the treatment of stable angina pectoris
and is the only specific bradycardic agent that is currently
available for clinical use.
*To whom correspondence should be addressed. Phone: þ39 055
4573691. Fax: þ39 055 4573671. E-mail: novella.romanelli@unifi.it.
a Abbreviations: HCN, hyperpolarization-activated cyclic nucleotide
gated; CNG, cyclic nucleotide-gated; HEK, human embryonic kidney.
To search for subtype-selective If blockers, we decided to
design new phenylalkylamines by applying classical approaches
r
2010 American Chemical Society
Published on Web 08/26/2010
pubs.acs.org/jmc

6774 Journal of Medicinal Chemistry, 2010, Vol. 53, No. 18
Melchiorre et al.
Chart 1
Scheme 1a
a Reagents and conditions: (a) t-BuOK, cis- or trans-1,4-dichlorobutene; (b) N-methylhomoveratrylamine; (c) (R)-11; (d) homoveratrylamine;
(e) (R)-13; (f) CH2O, HCOOH. The S enantiomers were prepared in the same way, using (S)-11 (step c) and (S)-13 (step e).
of medicinal chemistry. In a previous paper we disclosed a
structural analogue of zatebradine (2, Chart 1) that was able
to reduce heart rate in guinea pig spontaneously beating
right atria and to block If in ventricular cardiomyocyte of
old spontaneously hypertensive rats, with a potency compa-
rable to that of the lead.22 When this compound and its cis
isomer 3 were tested on HCN isoforms expressed in HEK293
cells, it was found that, contrary to what happens for ivabra-
dine, they show some preference in the blockade of channel
isoforms (see Results and Discussion). These findings pro-
mpted us to make some additional structural manipulations
tosee if selectivitycould be achieved. Therefore, a chiralcenter
was placed on the phenethyl moiety of 2 and 3, giving the
enantiomers of 4 and 5. The activity of the serendipitously
discovered 6 prompted us to synthesize also the nonchiral
analog 7 and the trans-derivative 8. The structures of the new
compounds are shown in Scheme 1.
alkene 10; the low yield of the reaction can be accounted for
the competition with the double addition of 9 to the cis-
dichlorobutene, yielding 10a. Changes in the ratio between
reactants or reverse addition of the anion to the halogen
derivative did not stop the formation of the byproduct. The
formation of the dimeric product was not observed when the
reaction was carried out on trans-1,4-dichlorobutene.22 Reac-
tion of 10 with commercially available N-methylhomovera-
trylamine gave 3 in good yields.
(R)- and (S)-2-(3,4-Dimethoxyphenyl)propan-1-amine (11,
Scheme 2) were prepared following the procedure reported by
Riggs,24 with a slight modification. The mixture of N-((S)-1-
phenylethyl)-2-(RS)-(3,4-dimethoxyphenyl)propan-1-amine
was separated by column chromatography, yielding the two
diastereoisomers (SR)-12 and (SS)-12 which were catalyti-
cally hydrogenated to yield the (R)- and (S)-11. The enantio-
meric excess was the same as for the commercially available
(S)-1-phenylethylamine (98%). In fact, after chromatographic
separation, inthe 1H NMR spectrumof (SR)-12 or(SS)-12no
trace of the other diastereoisomer was detectable. Treatments
of compounds (SR)-12 and (SS)-12 with formaldehyde and
formic acid followed by catalytic hydrogenation afforded
(R)- and (S)-13.25,26
Chemistry
The compounds were prepared starting from 923 according
to the synthetic pathway reported in Scheme 1. N-Alkylation
of 9 with commercially available cis-1,4-dichlorobutene gave

Brief Article
Journal of Medicinal Chemistry, 2010, Vol. 53, No. 18 6775
Scheme 2a
a Reagents and conditions: (a) LDA, THF, MeI; (b) SOCl2, CHCl3, (S)-1-phenylethylamine; (c) BH3 THF; (d) chromatographic separation;
(e) HCHO, HCOOH; (f) H2/Pd(OH)2/C.
3
Table 1. Effect of Compounds 1-7 and Ivabradine on mHCN1 and
Selected compounds were tested on guinea pig sponta-
neously beating atria to evaluate their negative chronotropic
activity on a tissue expressing native HCN channels. Com-
pounds were tested at increasing doses (10-9-10-4 M) to
measure the decrease in atrial rate. The potency of the drug is
definedasEC50 (Table2), anditwas evaluated byfittingof the
concentration-effect curve with the Hill equation.
hHCN4a
compd (5 μM)
mHCN1b
hHCN4c
2
35 ( 17
53 ( 1
85 ( 7
85 ( 0.3
63 ( 1
76 ( 3
27 ( 3
76 ( 0.2
82 ( 3
60 ( 6
93 ( 13
47 ( 3
53 ( 5
59 ( 0.5
72 ( 4
71 ( 9
83 ( 0.1
91 ( 2
93 ( 1
92 ( 1
75 ( 1
73 ( 18
83 ( 13
49 ( 2
3
(R)-4
(S)-4
(R)-5
(S)-5
(R)--6
(S)-6
7
Results and Discussion
The If blocking ability of the compounds at 5 μM is
reported in Table 1. The synthesized compounds were able
to block If at 5 μM, with the exception of (S)-5, (R)-6, and
(S)-6 on HCN4 and (S)-8 on HCN1. Some compounds were
more active on HCN1 than on HCN4 (2, (R)-5, (S)-5, (R)-6,
(S)-6, and (R)-8), while (R)-4, (S)-4, 7, and (S)-8 were more
active on HCN4. On HCN1, 2 and (R)-6 were able to inhibit
f-current to a larger extent with respect to ivabradine, which,
as expected, did not show selectivity, the extent of the inhibi-
tion being similar on both HCN isoforms. The stereogenic
center is important for the interaction with the channel, since
for 5, 6, and 8 the R forms were more active than their S
enantiomers. Also, the configuration of the double bond
affected the biological activity, since the cis-butene derivatives
were more active than their trans isomers (compare 5 and 6
with 4 and 8, respectively). In this respect, 2 and 3 were an
exception. In fact, the trans-derivative 2 was more active than
its cis isomer 3 on HCN1, while the two compounds showed
the same activity on HCN4.
In order to better evaluate the potency and the selectivity of
our compounds, for some of them the EC50 was determined
on the three available isoforms (Table 2) and used to calculate
the selectivity ratio (Figure 1). Compounds 2, 3, and (R)-5
were able to block If, with potency similar to ivabradine
at least on one isoform. Compound 2 preferentially bloc-
ked the HCN1 channel, its EC50 values being, on this subtype,
3 and 7 times lower than on HCN4 and HCN2, respectively.
The shift of the configuration of the double bond from trans
(2) to cis (3) changed the preference for HCN isoforms, since 3
was equallyactive on HCN1 andHCN4and 4 times less active
on HCN2. The introduction of a methyl group on the phe-
nylethylmoiety changed the selectivityprofile of 3: (R)-5 was 4
and 10 times more potent on HCN2 than on HCN1 and
HCN4, respectively. The duplication of the benzazepinylbu-
tenyl group gave interesting results. While 7showed low potency
on the three isoforms, (R)-6 was 7 times more potent than
ivabradine on HCN1. In addition, its potency on HCN1 was
170 times higher than on HCN4 and 30 times higher than on
HCN2, therefore displaying high selectivity for the HCN1
(R)-8
(S)-8
ivabradine
a Results are expressed as percentage of f-current amplitude at -120
mV after application of a 5 μM drugs. Values are normalized to control
and represent the mean ( SEM of three to five experiments. b Mouse
HCN channel expressed in HEK 293 cells. c Human HCN channel
expressed in HEK 293 cells.
Reaction of 10 with (R)-11 in 1:1 ratio (Scheme 1) gave a
mixture of compounds (R)-6 and (R)-14, which were sepa-
rated by column chromatography. (R)-14 was then methy-
lated using formaldehyde and formic acid, obtaining (R)-5.
With the same procedure, starting from 10 and (S)-11, (S)-6,
(S)-14, and after methylation, (S)-5 were obtained. Com-
pounds (R)- and (S)-4 were prepared by reaction of 1522 with
(R)- and (S)-13. Reaction of 10 with commercially available
homoveratrylamine in 2:1 ratio gave 7 in good yield. (R)-8 and
(S)-8 were obtained in the same way starting from 15 and
(R)- or (S)-11.
Pharmacology
To measure the inhibition of the hyperpolarization-acti-
vated current, patch-clamp experiments were performed on
mouse HCN1 and human HCN4 isoforms heterologously
expressed in HEK293 cells. As a preliminary screening, the
compounds were tested at 5 μM to check their ability to block
f-current (Table 1). For the most interesting compounds, the
EC50 has been determined on mHCN1 and hHCN4 and
also on mHCN2 heterologously expressed in HEK293 cells
(Table 2). Unfortunately HCN3 was not available. The selec-
tivity ratios have been calculated as the ratio between the EC50
values and are reported in Figure 1. Ivabradine was taken as
control. This compound has been previously tested on this cell
line, transfected with hHCN1-4 cDNA18 or with mHCN1
and hHCN4 cDNA.27 It was found to block If with EC50
values in the micromolar range, although it was not able to
discriminate among the isoforms.

6776 Journal of Medicinal Chemistry, 2010, Vol. 53, No. 18
Melchiorre et al.
Table 2. If Blocking Potency and Negative Chronotropic Activity of Selected Compounds in Comparison with Ivabradine and Zatebradine
compd
EC50 (μM) mHCN1a
ERb
EC50 (μM) mHCN2a
ERb
EC50 (μM) hHCN4c
ERb
EC50 (μM) guinea pig atriad
2
3
2.31 ( 0.37
5.60 ( 0.26
9.41 ( 0.25
31.27 ( 6.69
0.60 ( 0.07
17.94 ( 0.48
106.9 ( 49.9
4.50 ( 0.44
16.78 ( 0.9
17.22 ( 1.74
24.58 ( 4.89
2.3 ( 0.60
25.47 ( 3.7
18.3 ( 0.14
50.11 ( 11.9
147.4 ( 0.62
4.52 ( 2.82
5.45 ( 1.04
7.23 ( 2.60
7.14 ( 0.11
11.3 ( 3.3e
15.8 ( 1.7
7.1 ( 1.8
nd
(R)-5
(S)-5
3
11
3
24.94 ( 0.10
31.59 ( 3.49
103.78 ( 29.80
135.73 ( 74.70
70.67 ( 23.80
4.28 ( 0.40
1
1
(R)-6
(S)-6
30
113.2 ( 36.1
nd
nd
7
ivabradine
zatebradine
nd
8.57 ( 1.85
13.4 ( 8.7e
a Mouse HCN channel expressed in HEK 293 cells. b Eudismic ratio (ER): ratio between the activity of the two enantiomers. c Human HCN channel
expressed in HEK 293 cells. d Decrease of heart rate in spontaneously beating isolated guinea pig atria. Values represent the mean ( SEM of three to
four experiments. nd: not determined. e From ref 22.
tions. Moreover, they may be useful as drugs, providing they
have high specificity for HCN with respect to other ion chan-
nels. For instance, a HCN1 selective blocker may be useful for
the treatment of neuropathic pain,14 while a HCN4 selective
inhibitor can be useful to control sinus node rhythm and
ventricular arrhythmia.32 Our data show that it is possible to
achieve selectivity, at least on the homomeric channels, by
manipulating the chemical structure of phenylalkylamines
related to zatebradine. Compounds 2-8 represents analogues
with different steric and conformational characteristics, and
some of them (2, (R)-5, and (R)-6) show a different preference
for the homomeric HCN channel isoforms. Selectivity may be
related to the ability of these compounds to adopt different
conformations, to the presence of specific interactions due to
the introduction of additional moieties, or to the different
Figure 1. Ratio between the EC50 values on the three channel
isoforms of selected compounds.
shape or volume of the molecules. Work is underway to derive
sound structure-activity relationships in this class of com-
pounds in order to optimize their potency and selectivity and
to test them for their activity toward other ion currents.
isoform (Figure 1). The study of the enantiomers of 5 and 6
shows that the interaction was enantioselective on those iso-
forms where the compounds showed higher potency, such as
HCN2 for 5 (ER = 11) and HCN1 for 6 (ER = 30). In both
cases, the R isomer is the eutomer. This is interesting, since the
stereogenic center is located at the same position as in ivabra-
dine, which has the S configuration. This enantiomer has been
developed as drug because of its higher specificity for HCN
channels with respect to other channels, since its R-isomer
(R-60) also interacts with Kþ channels.9,28,29
Compounds 3, (R)-5, and (R)-6 were further evaluated on
spontaneously beating guinea pig right atria to test their
activity on a native tissue; the results are reported in Table 2.
Zatebradine and compound 2 were used as a reference in this
test. Although the distribution of HCN channel isoforms in
guinea pig heart is not known, in the sinoatrial node of all the
species so far examined the most abundant isoform is
HCN4,30 and the bradycardic effect of the compounds likely
relies on their activity on this subtype. Indeed, 2 and 3 are
almost equipotent with zatebradine in both their negative
chronotropic activity and HCN4 blocking potency, while the
poor activity of (R)-6 on heart reflects its low potency on
the HCN4 isoform, suggesting that the selectivity found in
cellular test is maintained also in native tissues. The negative
chronotropic potency of (R)-5 is unexpectedly high if com-
pared to its HCN4 blocking ability. In this regard, however, it
must be mentioned that a recent report showed evidence that
in mouse heart HCN2 protein coassembles with HCN4 to
form functional heteromeric HCN channels.31
Experimental Section
Chemistry: General Information. All melting points were
€
taken on a Buchi apparatus and are uncorrected. NMR spectra
were recorded on a Gemini 200 spectrometer (200 MHz for 1H
NMR, 50 MHz for 13C) or on a Bruker Avance 400 spectrometer
(400 MHz for 1H NMR, 100 MHz for 13C). Chromatographic
separations were performed on a silica gel column by gravity
chromatography (Kieselgel 40, 0.063-0.200 mm; Merck) or
flash chromatography (Kieselgel 40, 0.040-0.063 mm; Merck).
Yields are given after purification, unless differently stated.
Where analyses are indicated by symbols, the analytical results
are within 0.4% of the theoretical values. The synthesis of (R)-6
is described below as representative. The syntheses of all other
compounds and the NMR spectra of (R)-6 are reported in the
Supporting Information.
(R)-N,N-Bis[(Z)-4-(7,8-dimethoxy-2-oxo-1,3-dihydrobenzo-
[d]azepin-3-yl)but-2-enyl]-2-(3,4-dimethoxyphenyl)propanamine
[(R)-6]. To a solution of 10 (0.25 g, 0.82 mmol) in anhydrous
CH3CN (10 mL) were added anhydrous triethylamine (0.083 g,
0.82 mmol) and (R)-11 (0.82 mmol, 0.16 g). The mixture was
left stirring under nitrogen at room temperature overnight.
Then the solvent was removed by rotary evaporation, and the
residue was dissolved in dichloromethane and washed with 2 M
NaOH (3 ꢀ 15 mL). The organic layers were collected, dried on
Na2SO4, and the solvent was evaporated under vacuum. The
residue was then purified by flash chromatography (dichloro-
methane/methanol/ammonia, 95/5/0.5). The first eluting com-
pound is (R)-6 (Rf = 0.71, 12% yield, mp 97-98 °C) The second
eluting compound is (R)-14 (Rf = 0.42, yellow oil, 8% yield).
Selective compounds may be important tools to study the
physiology and the stoichiometry of the channels in native
tissues and to study the channel under pathological condi-
(R)-6 (hydrochloride): [R]20 -22.25° (c 0.5, CH3OH). Anal.
D
(C43H52ClN3O8) C, H, N.

Brief Article
Journal of Medicinal Chemistry, 2010, Vol. 53, No. 18 6777
Acknowledgment. We acknowledge the support of Nor-
macor (Contract LSH M/CT/2006/018676) and particularly
Prof. Martin Biel (University of Munich, Germany) for provi-
ding stably transfected HCN1, HCN2, and HCN4 HEK293
cells.
activated cyclic nucleotide-gated channels in rat pancreatic β cells.
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