A structure-activity relationship study of novel phenylacetamides which are sodium channel blockers

Article abstract of DOI:10.1021/jm950467y

A structure-activity relationship study of a series of novel Na⁺ channel blockers, structurally related to N-[3-(2,6-dimethyl-1-piperidinyl)propyl]- α-phenylbenzeneacetamide (1, PD85639) is described. The diphenylacetic acid portion of the molecule was left unchanged throughout the study, while structural features in the amine portion and the amide alkyl linkage of the molecule were modified. The compounds were tested for inhibition of veratridine-stimulated Na⁺ influx in CHO cells expressing type IIA Na⁺ channels. Several derivatives show a trend toward more potent Na⁺ channel blockade activity with increasing lipophilicity of the amine portion of the molecule. The presence of a phenyl ring near the amine increases inhibitory potency. A three-carbon spacer between the amide and amine is optimal, and a secondary amide linkage is preferred.

Full text of DOI:10.1021/jm950467y

1514
J . Med. Chem. 1996, 39, 1514-1520
A Str u ctu r e-Activity Rela tion sh ip Stu d y of Novel P h en yla ceta m id es Wh ich Ar e
Sod iu m Ch a n n el Block er s
Ioannis Roufos,† Sheryl Hays,*,† and Roy D. Schwarz‡
Departments of Chemistry and Pharmacology, Parke-Davis Pharmaceutical Research, Division of Warner-Lambert Company,
2800 Plymouth Road, Ann Arbor, Michigan 48105
Received J une 26, 1995X
A structure-activity relationship study of a series of novel Na+ channel blockers, structurally
related to N-[3-(2,6-dimethyl-1-piperidinyl)propyl]-R-phenylbenzeneacetamide (1, PD85639) is
described. The diphenylacetic acid portion of the molecule was left unchanged throughout the
study, while structural features in the amine portion and the amide alkyl linkage of the molecule
were modified. The compounds were tested for inhibition of veratridine-stimulated Na+ influx
in CHO cells expressing type IIA Na+ channels. Several derivatives show a trend toward more
potent Na+ channel blockade activity with increasing lipophilicity of the amine portion of the
molecule. The presence of a phenyl ring near the amine increases inhibitory potency. A three-
carbon spacer between the amide and amine is optimal, and a secondary amide linkage is
preferred.
In tr od u ction
follows: neurotoxin receptor site 1, where water-soluble
tetrodotoxin and saxitoxin act by blocking sodium
permeation; neurotoxin receptor site 2, where lipid-
soluble toxins like veratridine, aconitine, and batra-
chotoxin act causing prolonged activation; neurotoxin
receptor site 3, where R-scorpion toxin and sea anemone
neumatocytes bind inhibiting inactivation; neurotoxin
receptor site 4, where â-scorpion toxins act by shifting
activation; and finally, neurotoxin site 5, where cigua-
toxins act to prolong activation.
Voltage-dependent sodium channels have been im-
plicated as the site of action for many types of drugs
including antiarrhythmics, anticonvulsants, and local
anesthetics.1 Additionally, sodium channel blockers
have demonstrated neuroprotection in experimental
models of ischemic stroke.2,3 Sodium channel blocking
agents are believed to act by interfering with the rapid
influx of Na+ ions which is responsible for the genera-
tion of action potentials in excitable cells.4 Sodium
channels in neuronal cells are large trimeric glycopro-
teins (containing a 260 kDa R subunit, a 36 kDa â1
subunit, and a 33 kDa â2 subunit) having different
electrophysiological and pharmacological properties from
sodium channels in cardiac and skeletal muscle (which
contain two subunits). Different subtypes of sodium
channels have been cloned (I, II, IIA, and III from rat
brain) with the type I found primarily in neuronal cell
bodies and type II in the axons of the neuronal cells.5
In some species (e.g. eel electroplax) the sodium channel
consists of a single R subunit. When the R subunit alone
(type IIA) was cloned and expressed in Chinese hamster
ovary (CHO) cells, fully functional sodium channels
were obtained.6,7
The sodium channel can shift between three distinct
conformational states: active, resting, and inactivated.
After the neuronal cell membrane is depolarized, the
permeability of the membrane to sodium ions increases,
with sodium ions rushing through into the cell, and
shortly after that in a rate-and voltage-dependent
manner, the permeability to sodium returns to its
original resting level.1 The availability of specific neu-
rotoxins has contributed to a better understanding of
voltage-sensitive sodium channels. These toxins can
distinguish not only different types of channels but also
different states of the channels by either inducing or
preventing them from adopting a specific conformational
state. The sites of their action8-12 are defined as
The site of action of the class III antiarrhythmic,
anticonvulsant, and local anesthetic drugs is believed
to be the intracellular side of the sodium channel. They
allosterically inhibit interaction with neurotoxin recep-
tor site 2, in a frequency and voltage-dependent man-
ner.10 Selective high-affinity binding of these inhibitors
to the inactivated state of the sodium channel is believed
to be the property that allows them to selectively block
abnormally firing sodium channels without inhibiting
normal cardiac and neuronal sodium channel functions.1
N-[3-(2,6-Dimethyl-1-piperidinyl)propyl]-R-phenylben-
zeneacetamide (PD85639, 1) has been shown to act as
a local anesthetic with high selectivity for the type II
sodium channels.13,14 As part of a previous structure-
activity relationship (SAR) study on derivatives of 1, we
reported that these compounds inhibited [3H]batra-
chotoxin binding in rat neocortical membranes and Na+
influx in CHO cells expressing the R subunit of type IIA
sodium channels.15 Some derivatives also inhibited
veratridine- and hypoxia-induced lactate dehydrogenase
(LDH) release from cell cultures, which has been used
as a measure of neuroprotection.16-18 These data sug-
gest that they may function as neuroprotecting agents
and potentially be useful for the treatment of ischemic
stroke. The previous SAR study of derivatives of 1
involved changes in the diphenylacetyl portion of the
molecule.15 In the present study, structural modifica-
tions were explored in the linkage (part B) and amine
portion (part C) of compound 1, while the diphenylacetic
acid moiety (part A) was held unchanged throughout
the series. The synthesis and sodium channel blockade
activity of these derivatives are described.
† Department of Chemistry.
‡ Department of Pharmacology.
X Abstract published in Advance ACS Abstracts, February 15, 1996.
0022-2623/96/1839-1514$12.00/0 © 1996 American Chemical Society

Novel Phenylacetamides as Sodium Channel Blockers
J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 7 1515
Sch em e 1
diamine in dichloromethane (Scheme 3, procedure A)
or by reacting diphenylacetic acid, 1,1′-carbonyldiimid-
azole (CDI) and the diamine in dimethylformamide
(Scheme 3, procedure B).
Ch em istr y
Biologica l Testin g
Inhibition of [3H]batrachotoxin ([3H]BTX) binding in
rat neocortical membranes has been used as a rapid
screening method for identifying compounds that block
voltage-sensitive sodium channels.20,21 The existence of
a good correlation (r ) 0.841) between inhibition of [3H]-
BTX binding and inhibition of veratridine-stimulated
Na+ influx (NaFl) in CHO cells expressing the R subunit
of rat brain type IIA sodium channels has been previ-
ously demonstrated.15 In this report the ability of the
synthesized compounds to block Na+ influx (NaFl) was
used exclusively to determine their interaction with the
sodium channel. After application of test compounds
to the CHO cells for 5 min, the cells were treated with
a veratridine solution containing R-scorpion toxin, which
results in opening of the sodium channels. A [14C]-
guanidinium solution was then added to the cells, the
inhibition of the [14C]guanidinium influx was used to
measure sodium channel blockade,22 and the IC50 values
were determined by linear regression analysis (see
Table 2).
To study the effect of the spacer and amine portions
of 1, a variety of targets were designed and synthesized.
The diamines that were not commercially available were
prepared as shown in Scheme 1. The starting amines
(2A-I) were treated with acrylonitrile in methanol to
afford the nitriles in good yield, and the nitriles were
subsequently converted to the diamines (3A-I) by
Raney nickel catalyzed hydrogenation. The products
(3A-I) from this procedure are shown in Table 1.
The targeted compounds (7-28) were prepared as
shown in Schemes 2 and 3. The diphenylacetic acid 4
was treated with endo-N-hydroxy-5-norborene-2,3-di-
carbodiimide 519 in the presence of 1,3-dicyclohexylcar-
bodiamide (DCC) to form the activated ester 6 as shown
in Scheme 2. The ester 6 was mixed with 3-aminopro-
pionitrile to afford the amide which was then reduced
via catalytic hydrogenation to produce the primary
amine 7. The remainder of the amide targets were
synthesized either by coupling of the activated ester
derivative of diphenylacetic acid 6 with the appropriate
Ta ble 1. Structure and Physical Constants of Compounds Prepared by Scheme 1

1516 J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 7
Roufos et al.
Sch em e 2
Sch em e 3
Resu lts a n d Discu ssion
An enormous difference in activity was found between
the various piperazine derivatives. The 4-methylpip-
erazine analog 15 was essentially inactive (IC50 > 200
µM) while the 4-(2-methoxy)phenyl analog 16 has activ-
ity (IC50 ) 1.2 µM) on the same order as the parent 1
and the benzylpiperazine analog 17 (IC50 ) 0.7 µM) has
activity approaching that of flunarizine or pimozide.
This large discrepancy in activity of these compounds
may be a function of the basicity of the piperazine
nitrogen atoms. The most basic center in 15 is the
N-methyl nitrogen atom whereas the electron-with-
drawing effects of the aryl and benzyl groups make the
corresponding nitrogen atoms of 16 and 17 less basic.23
As a result, the protonation of compound 15 would occur
on the methylated nitrogen atom while the positive
charge would reside on the alternative nitrogen of the
other derivatives. This change in the position of pro-
tonation would certainly affect alignment of the com-
pound within the channel. The phenyl rings may also
play a critical role in anchoring inhibitors 16 and 17
into the Na+ channel. This is further supported by the
good activity observed for piperidine analogs such as
compounds 18-22 which contain a phenyl moiety in this
region of the molecule. These derivatives have IC50s <
2.2 µM and 20 (IC50 ) 0.6 µM) was the most potent of
any of the new analogs synthesized. Compound 22
differs only from 18 in the methyl group that it has in
the spacer. The two compounds are essentially equi-
potent (IC50’s 1.5 and 1.1 µM, respectively). Derivative
23 with the benzimidazolone substitution in the 4-posi-
tion of the piperidine ring mimics the heterocyclic
The local anesthetic 1 inhibits Na+ influx in CHO cells
expressing the R subunit of type IIA sodium channels
with an IC50 of 2.7 µM. The vasodilator flunarizine,
which is characterized principally as a Ca2+ channel
blocker, is also a very potent Na+ blocker (IC50 ) 0.42
µM) as is the antipsychotic drug, pimozide (IC50 ) 0.34
µM). Sodium channel blockade was determined experi-
mentally for these reference compounds and can be
viewed in Table 2. All of these compounds contain a
diphenylmethyl moiety, an amine, and a lipophilic tail.
The primary amine 7 (IC50 ) 40 µM), the dimethyl-
amino analog 8 (IC50 ) 20 µM), the diethylamino
derivative 9 (IC50 ) 4.4 µM), and the methylbenzyl-
amine derivative 10 (IC50 ) 1.2 µM) show a trend
toward more potent Na+ channel blockade activity with
the increasing lipophilicity of the amine portion of the
molecule. While the piperidine, 2,5-dimethylpyrroli-
dine, and the morpholine derivatives 11, 12, and 13
demonstrated good inhibition of Na+ influx (IC50 ) 5.3,
4.6, and 6.5 µM, respectively), only the slightly more
lipophilic 2,6-dimethylmorpholine derivative 14 was
equipotent with the parent compound 1 (IC50 ) 2.7 µM).

Novel Phenylacetamides as Sodium Channel Blockers
J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 7 1517
stirred at room temperature for 24 h. The volatiles were
removed under reduced pressure, and the residue was purified
either by distillation or chromatography. The purified product
was dissolved in MeOH (saturated with NH3 to reduce the
formation of secondary and tertiary amines) and was hydro-
genated over Raney Ni. The catalyst was filtered off, and the
residue was purified by either chromatography or distillation.
1,7-Dim eth yl-3,5-d ioxo-4-a za tr icyclo[5.2.1.02,6]d ec-8-en -
4-yld ip h en yla cetic Acid Ester (6) (Sch em e 2).19 Diphen-
ylacetic acid 3 (84.8 g, 0.4 mol) and endo-N-hydroxy-5-
norborene-2,3-dicarboximide (2; 80.55 g, 0.45 mol) were
dissolved in THF/1,4-dioxane (1:1) (1.2 L) cooled to 0 °C in an
ice water bath and treated with 1,3-dicyclohexylcarbodiimide
(92.7 g, 0.45 mol). The mixture was stirred for 16 h at room
temperature, the white solid precipitate was filtered off, and
the filtrates were washed with a 5% NaHCO3, water, and
saturated NaCl and dried (Na2SO4). The volatiles were
evaporated under reduced pressure, and the residue was
recrystallized from CH2Cl2 to afford 125.8 g of analytically pure
product (84.3% yield). mp 188-190 °C. Anal. (C23H19N1O4)
C, H, N.
N-(3-Am in opr opyl)-2,2-diph en ylacetam ide (7) (Sch em e
2). The activated ester 6 (7.46 g, 20 mmol), 3-aminopropioni-
trile fumarate (3.68 g, 20 mmol), and triethylamine (10 mL,
72 mmol) were dissolved in CH2Cl2 (100 mL) and stirred
together for 16 h, and the reaction was worked up as in
procedure A to give 4.65 g of an analytically pure white solid:
mp 123 °C. Anal. (C17H16N2O) C, H, N. This product was
dissolved in CH3OH saturated with NH3 (100 mL) and
hydrogenated over 1.5 g of Raney Nickel catalyst. After the
catalyst was filtered off, the volatiles were removed under
reduced pressure to afford the product, which was taken up
in acetone (10 mL), and the oxalate salt was formed by
treatment with oxalic acid (720 mg) in acetone (10 mL). The
solid which precipitated was recrystallized from CH3OH/Et2O
to afford pure product 7 (2.55 g, 35.5% overall yield): mp 217-
220 °C; IR (KBr) 1644 cm-1; NMR (DMSO-d6) δ 1.62-1.65 (m,
2Η), 2.66-2.70 (m, 2H), 3.07-3.11 (m, 2H), 4.89 (s, 1H), 7.15
(m, 12H). Anal. C17H20N2O‚C2H2O4 (C, H, N).
Exa m p le of P r oced u r e A (Sch em e 3). N-[3-(4-Ben -
zylp ip er id in -1-yl)p r op yl]-2,2-d ip h en yla ceta m id e (20). Ac-
tivated ester 6 (3.1 g, 10 mmol) and amine 3G (2.3 g, 10 mmol)
were dissolved in CH2Cl2 (25 mL) and stirred at 23 °C for 18
h. The reaction mixture was washed with 5% aqueous
NaHCO3 (20 mL), H2O (20 mL), and saturated NaCl (20 mL),
and the organic phase was dried over Na2SO4. The volatiles
were removed under reduced pressure, and the crude residue
(5.2 g) was recrystallized from hot 95% EtOH. The off-white
solid was collected by filtration (2.18 g, 51% yield): mp 95-
97 °C; IR (KBr) 1639 cm-1; NMR (CDCl3) δ 1.00-1.10 (m, 2Η),
1.40-1.63 (m, 5H), 1.71-1.77 (t, 2H), 2.27-2.31 (t, 2H), 2.47
(d, 2H), 2.76 (d, 2H), 3.1-3.5 (m, 2H), 4.77 (s, 1H), 7.06-7.31
(m, 16H). Anal. C29H34N2O (C, H, N).
Exa m p le of P r oced u r e B (Sch em e 3). 2,2-Dip h en yl-
N-(3-p ip er id in -1-ylp r op yl)a ceta m id e (11). The diphenyl-
acetic acid (2.17 g, 10 mmol) and 1,1′-carbonyldiimidazole (2.43
g, 15 mmol) were stirred together in dry DMF (20 mL) at 23
°C until gas evolution had ceased (about 20 min). A solution
of the 1-piperidinepropanamine (30 mmol) in dry DMF (20 mL)
was added and the mixture was stirred at 23 °C under a
nitrogen atmosphere. After 30 min the reaction mixture was
poured into an excess of 0.5 N Na2CO3 solution, and the white
solid that precipitated was collected by filtration, washed with
water, and dried under reduced pressure to afford the product.
The product recrystallized from EtOH/H2O to afford the amine
11 as a fine cotton-like white solid (2.2 g, 65% yield): TLC
(SiO2; MeOH/CH2Cl2, 1:12); mp 101-103 °C; IR (KBr) 1640
cm-1; NMR (CDCl3) δ 1.35-1.46 (m, 6Η), 1.6 (m, 2H), 2.25-
2.34 (m, 6H), 3.35 (m, 2H), 4.8 (s, 1H), 7.2-7.3 (m, 11H). Anal.
C22H28N2O (C, H, N).
portion of pimozide. This compound (IC50 ) 0.72 µM)
like pimozide demonstrates potent sodium channel
inhibition.
A number of derivatives of 1 were targeted that
explored the linkage between the diphenylacetyl and
amine functionalities. If the propyl spacer of 11 (IC50
) 5.3 µM) is shortened to two carbons (analog 24), or
lengthened to four carbons (analog 25), the compounds
decrease their ability to block Na+ channel activity
(IC50s ) 7.2 and 10.0 µM). This suggests that the three
carbon spacer is optimal. When the spacer is part of a
conformationally restricted ring such as 26 or 27 (IC50s
) 32 and 95 µM), which may function to reorient the
tertiary amine at the Na+ channel binding site, inhibi-
tory activity is decreased. A secondary amide linkage
would seem optimal since methyl alkylation of the
amide 8 (IC50 ) 20 µM) to afford the tertiary amide 28
resulted in a complete loss of activity (compound 28; IC50
> 200 µM).
Most clinically useful local anesthetics consist of both
hydrophilic (usually a tertiary amine) and hydrophobic
(usually an aromatic ring) domains that are linked by
an amide or ester bond and separated by an alkyl
chain.24,25 This simplified description of local anesthetic
structure-activity relations (SAR) is appropriate for
derivatives of 1. While the amine moiety is hydrophilic
in its protonated state, additional lipophilicity in the
amine portion of the molecule greatly enhances sodium
channel inhibition for this group of compounds. Rags-
dale and co-workers26 have identified phenylalanine1764
and tyrosine1774 as determinants of the local anesthetic
binding site of the voltage-dependent Na+ channel.
Substitution of either of these residues with alanine
destabilizes drug binding by reducing the hydrophobic-
ity and aromaticity at these positions in the channel.
Hydrophobic moieties at either end of the local anes-
thetic molecule that could interact with these residues
is clearly beneficial for Na+ blockade activity. This
suggests that an appropriately positioned phenyl ring
in the amine portion of these analogs may be beneficial
as a result of interactions with other aromatic moieties
in the channel. In conclusion, several compounds more
active than 1 in blocking the veratridine-stimulated Na+
influx have been identified. On the basis of previous
functional data,15 these derivatives should also be
neuroprotectant in cell-based and animal models of
neuronal injury.
Exp er im en ta l Section
Ch em istr y. All melting points were determined on a
MELT-TEMP II capillary melting point apparatus and are
uncorrected. Infrared (IR) spectra were determined in KBr
on a Mattson Cygnus 100 or a Nicolet MX1 instrument.
Proton magnetic resonance (NMR) were recorded either on a
Varian XL-300 or a Bruker AM250 spectrometer; shifts are
reported in δ units relative to internal tetramethylsilane. All
mass spectra were obtained on a Finnigan 4500 GCMS or a
VG analytical 7070 E/F spectrometer. Elemental analyses
were performed on a CEC Model 240 elemental analyzer, and
they were within 0.4% of the theoretical values. Medium-
pressure liquid chromatography utilized E. Merck silica gel,
230-400 mesh. All reactions were run under N2, unless
indicated otherwise. All analytical (C, H, N) and spectroscopic
(1H NMR, IR, MS) data were in agreement with the proposed
structures.
Biologica l Meth od s. In h ibition of Ver a tr id in e-In -
d u ced Na + In flu x. These experiments were performed using
the cell line CNaIIA-1, derived from a Chinese hamster ovary
(CHO) cell line (CHO-K1; American Type Cultures) which were
transfected with the vector ZEM2580 containing a cDNA
encoding the rat brain IIA Na+ channel.5 The rat IIA sequence
Syn th esis of Am in es (3A-I, Sch em e 1). The appropriate
starting amine (2A-I) and an equimolar amount of acryloni-
trile were added to dry MeOH at 0 °C, and the mixture was

1518 J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 7
Roufos et al.
Ta ble 2. Physical Constants and Biological Activity of Compounds 7-28

Novel Phenylacetamides as Sodium Channel Blockers
J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 7 1519
Ta ble 2 (Continued)
used contains the natural leucine at position 860, conferring
normal voltage-dependent properties. CNaIIA-1 cells were
cultured in 12-well plates at 37 °C and 5% CO2 in RPMI
medium1640 (GIBCO) containing 10% fetal calf serum, 2 mL
of geneticin solution (5 mg/1 mL), and penicillin/streptomycin
(final concentration 20 units/mL of penicillin G sodium and
20 µg/mL streptomycin sulfate). Incubation experiments were
run in a final volume of 500 µL. Freshly oxygenated KRH
buffer (425 µL; pH ) 7.4), drug solution in DMSO/H2O:1/9 (25
µL), and veratridine/R-scorpion venom solution (25 µL of a 0.6
mM; final concentration 30 µM) were incubated for 11 min at
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