Review article on treatments for overactive bladder.

1. Review
An overview of potassium
channel activators for the
treatment of overactive bladder:
1. Introduction a survey of new structures
2. The ATP-dependent potassium
channel openers 2000 – 2005
3. A-type potassium channel
Thomas M Argentieri† & John A Butera
activators †GlobalBusiness Development, Wyeth Pharmaceuticals, CC-3208, 500 Arcola Road, Collegeville,
f
4. KCNQ potassium channel PA 19426, USA
activators
For over 25 years, the muscarinic receptor antagonists have been the first-line
5. The calcium-dependent
o
potassium channel openers
treatment for overactive bladder (OAB). However, severe dry mouth and
exacerbation of disease symptoms with prolonged use can result in compli-
6. Expert opinion
o
ance rates as low as 30%. Approximately 10 years ago, investigators began to
look for alternative approaches to the treatment of overactive bladder. In the
P r
recent past, it was demonstrated that ATP-dependent potassium channel
openers designed as antihypertensive agents inhibited the hyperactivity asso-
ciated with the symptoms of overactive bladder. Newer agents targeting
other bladder potassium channels and/or bladder-specific channel subtypes
represent an exciting new frontier for the effective treatment of OAB, with
o r
the potential of having an improved side effect profile compared to the
antimuscarinic agents. In this review the authors examine the patent litera-
ture surrounding the discovery and development of several novel classes of
compounds targeting the various potassium channels associated with urinary
h
bladder activity and excitability. When appropriate, the primary literature is
t
also touched upon to highlight pertinent structure–activity relationships
(SAR) associated with these newly disclosed chemotypes.
u
Keywords: bladder, IK, KATP, KCNQ, Maxi K, potassium channel, SK
Expert Opin. Ther. Patents (2006) 16(5):xxx-xxx
A 1. Introduction
In recent years, the unmet medical need of urinary incontinence has been widely
recognised. There are three major variants of the disease: stress incontinence, over-
active bladder (OAB; also known as urge incontinence, neurogenic bladder, hyper-
active bladder etc.) and mixed incontinence. Stress incontinence is most commonly
associated with involuntary urine loss associated with increases in intra-abdominal
pressure associated with sneezing, laughing, bending to pick-up objects etc. In this
condition the urethral closing pressure is insufficient to adequately contain the
increase in bladder pressure that results from the increased intra-abdominal pressure.
Surgery for stress incontinence remains the treatment of choice, although the first
pharmacological treatment for stress incontinence (the serotonin noradrenaline
re-uptake inhibitor, duloxetine) [1] was recently approved in Europe. OAB is mainly
associated with an abnormal sense of urinary urgency unrelated to bladder urine vol-
ume. It is believed that this sense of urgency is the result of abnormally contracting
bladder smooth muscle. Because of this, patients with OAB often lose the ability to
recognise when their bladder is full and often have associated involuntary urine loss.
Patients with mixed incontinence have a combination of insufficient urethral closing
pressure and OAB.
10.1517/13543776.16.5.xxx © 2006 Informa UK Ltd ISSN 1354-3776 1

2. An overview of potassium channel activators for the treatment of overactive bladder
For over 25 years, the muscarinic receptor antagonists have channel (KATP) openers [9-11]. These are molecules that appear
been the first-line treatment for OAB [2-6]. As the muscarinic to preferentially hyperpolarise bladder smooth muscle over
receptor is the primary efferent receptor on bladder smooth vascular smooth muscle and cardiac muscle, thus minimising
muscle, antimuscarinic agents are very effective at inhibiting the risk associated with cardiovascular side effects. Since the
bladder contractility and preventing the abnormal sense of discovery of KATP openers aimed at treating hypertension,
urgency. However, there are two shortcomings with this treat- much information has been generated around the molecular
ment approach. First, although abnormal contractility is effec- determinants of this channel in the bladder. It is now believed
tively inhibited, so is normal contractility. In extreme cases, this that the KATP channel consists of the inward rectifying pore
can lead to increased residual void volumes, and an exacerba- (Kir) and one or more sulfonyl urea receptors (SUR) [12].
tion of the disease. Second, because the muscarinic M3 receptor Channels derived from Kir 6.2/SUR2A are found mostly in
is the major receptor subtype in the bladder and in the salivary cardiac muscle, whereas Kir 6.2/SUR1 underlies the channel
gland, severe dry mouth is often an intolerable side effect associ- found in the pancreatic β-islet cell. The Kir 6.2/SUR2B com-
ated with the use of antimuscarinic agents. In fact, because of plex is the most common channel subtype associated with uri-
this and other antimuscarinic side effects, the compliance with
these agents can be as low as 30%. Although the compliance
with newer antimuscarinics has improved with their improved
pharmacokinetic profiles, investigators have continued to look
o f
nary and vascular smooth muscle [13]. Recent work has
revealed the presence of a SUR2B splice variant in bladder
with an exon 17 deletion. This splice variant (SUR2B∆17)
has been shown to be the predominant form in bladder
o
for alternative approaches to the treatment of OAB. smooth muscle over other tissues such as vascular smooth
Potassium channels have been a target of interest for the muscle [14]. With this knowledge, it is conceivable to identify
r
treatment of OAB for a number of years [7,8]. As in most con- KATP openers that are more selective for the bladder variant
tractile tissues, bladder smooth muscle potassium channels and have an improved side effect profile.
P
play an important role in determining and regulating the
excitability of the cell. These channels are largely responsible 2.1 Dihydropyridines and closely-related analogues
for determining the cell resting potential and modulating con- During the period covered by this review, researchers at
r
tractility by acting as an excitability ‘brake’. Initially, the phar- Abbott have capitalised widely on the reported bladder-selec-
macology of the ATP-dependent potassium channel was the tive acridinedione KATP channel opener, ZM-244085 (1) [15].
o
most well understood. Once recognised that these channels A plethora of symmetrical and non-symmetrical structural
are prominent in the bladder, numerous investigators variants of the tricyclic dihydropyridine-based class of KATP
attempted to design activators that would hyperpolarise blad- channel openers have been reported. Simply by contracting
der smooth muscle. Finding active molecules with this
t h
approach was relatively easy; however, finding compounds
that made useful drugs was substantially more difficult
the flanking cyclohexenone rings to cyclopentenone, com-
pounds 2 – 4 were reported to potently hyperpolarise
guinea-pig bladder cells with EC50 values of 0.071, 0.053 and
u
because of the ubiquitous nature of this channel (in particular, 0.013 µM, respectively, in a fluorescence-imaging plate reader
their location in cardiac and vascular tissues). It quickly (FLIPR) assay utilising DiBAC4(3) as an anionic potentiomet-
became apparent that targeting this channel would result in ric probe [101]. Compounds 2 and 3 were reported to inhibit
A
unacceptable cardiovascular side effects. KCl-induced contractions in isolated Landrace pig bladder
Recent advances in molecular biology have allowed for the strips with EC50 values of 0.16 and 0.25 µM, respectively.
rapid identification of other potassium channel targets and, in Due to the enhanced solubility associated with these mole-
particular, allowed for structure-based drug design. Since the cules versus the acridinedione analogue 1, scientists at Abbott
ATP-dependent potassium channel was first identified as a were able to demonstrate in vivo activity with compound 3.
target, a number of other potassium channels have been iden- Upon intravenous administration, compound 3 produced a
tified in bladder smooth muscle that may be more amenable reduction in the area under the curve (AUC ED30 = 0.1
to useful drug development. Bladder potassium channels can µmol/kg) in an in vivo rat isovolumetric contraction model
now be divided into four broad categories, these include: i) after ligation of the urethra.
the ATP-dependent channels; ii) the voltage-gated rectifier Modification of one flanking cyclohexenone ring into a thio-
family of potassium channels (KV); iii) the KCNQ family of pyrano-1,1-dioxide ring system afforded a series of potent KATP
potassium channels; and iv) the calcium-dependent channels channel openers exemplified by compound 5 (EC50 = 0.38 µM
(KCa). An excellent review on potassium channel targets for in guinea-pig bladder cell membrane hyperpolarisation FLIPR
urological disorders has recently been presented [8]. assay) [102]. Structure–activity relationship (SAR) studies [16]
around this core have shown that: i) replacement of either or
2. The
ATP-dependent potassium channel both flanking rings with a five-membered ring is tolerated; ii)
openers expansion of the sulfone-containing ring into a seven-mem-
bered ring reduces activity; iii) the optimal substitution pattern
A substantial body of literature exists around the discovery of for the phenyl ring is 3-bromo-4-fluoro; iv) differences in the
so-called second generation ATP-dependent potassium in vitro potencies for each pair of enantiomers ranged 3- to
2 Expert Opin. Ther. Patents (2006) 16(5)

3. Argentieri & Butera
N R2 F F
R1 Br Br
O O O O
O O O O O
O
S S
N N
H N N
H H H
1 ZM-244085 2 R1 = Br, R2 = F 5 6 A-278637
3 R1 = NO2, R2 = Cl
4 R1 = I, R2 = F
f
F F F F
Br Br Br Br
O O O O
O
O
S
o
O
o O
N
N
r
O
O O O
N N N N
H H H H
Br
7
F
8
F
Br
r P 9
F
I
10
F
Br
O O
NH
O
h o O
NH
O O
NH
R
N
O
N
t
O
N N N N N N
N
H H H H
A
11
u 12
10-fold; and v) activities were reversed by addition of glyburide,
a selective blocker of the KATP channel. Compound 6
(A-278637; FLIPR EC50 = 0.123 µM) was identified as a com-
13 14 R = H
15 R = CH3
Elaboration of the dihydropyridine core into a fused pyra-
zolo-pyrimidine scaffold afforded compound 10 (FLIPR EC50
= 0.03 µM), which retains potent KATP channel opening
pound that selectively inhibited spontaneous bladder contrac- activity that is reversed by addition of glyburide [107]. The
tions over those elicited by electrical stimulation. The compound was reported to be equipotent with cromakalim at
compound was also found to be selective in vivo by inhibiting inhibiting the contraction of isolated bladder strips in vitro
the AUC in the obstructed pig assay (by 75%) at plasma con- (EC50 = 0.34 µM) [19].
centrations that did not cause significant effects (< 10%) on A key liability seen with most dihydropyridines from the
mean arterial pressure in a separate group of conscious pigs. Ca2+ channel blocker and the K+ channel opener classes of
In an attempt to further expand the SAR of the dihydro- molecules is their susceptibility to CYP3A4-mediated meta-
pyridine core [17,18], additional heteroatomic motifs have bolic oxidation to pyridine [20]. To address this, researchers at
been incorporated into the flanking rings, including lac- Abbott fused a substituted pyrazolone ring onto the dihydro-
tones, pyranones, lactams and piperidinones. This has led pyridine core as a handle to sterically block and electronically
to the characterisation of numerous potent and selective inhibit this oxidative metabolic pathway. Compounds such as
bladder relaxants as exemplified by compounds 7 (FLIPR 11 – 13 were reported to show significantly enhanced meta-
EC50 = 0.006 µM; pig detrusor strip EC50 = 0.135 µM), 8 bolic stability after incubation in human liver microsomes
(FLIPR EC50 = 0.027 µM; pig detrusor strip EC50 = 0.126 when compared to nifedipine, felodipine, and the above-men-
µM) and 9 (FLIPR EC50 = 0.005 µM; pig detrusor strip tioned lactone analogue 7 [21]. The methylated pyrazolones 11
EC50 = 0.068 µM) [103-106]. – 13 retained their ability to potently hyperpolarise guinea-pig
Expert Opin. Ther. Patents (2006) 16(5) 3

4. An overview of potassium channel activators for the treatment of overactive bladder
O O
O O O O
Cl
N N N H N H
H H N N N N N N
H H H H
Cl
O O
16 17 18
f
N
N N N
O N N
o
N N N
N N N H H N
N
H H H H H
R
O O
19 N
O
20
O
r o 21
P
N N N N N
S N
r
S
22 R = CH3 24
o
23 R = H
h
bladder smooth muscle cells, with potencies (EC50) of 0.55, analogue. However, the bladder strip efficacy of analogue 18
0.05 and 0.07 µM, respectively, in the FLIPR assay [108,109]. (ED50 value of 0.68 µM and an 88% maximal effect),
Bicyclic analogues, such as compounds 14 and 15, have
as demonstrated by the reported EC50 value for compound 14
in the FLIPR assay (0.017 µM), and the reported EC50 value t
been reported to retain their KATP channel opening properties
u
for methyl analogue 15 in the isolated pig bladder strip assay
correlated better with the FLIPR assay.
Within the putative KATP channel opener pharmacophore,
the bioisosteric relationship of the 1,2-diamino-cyclobutenedi-
one and the N-cyanoguanidine moieties has been demon-
strated [22]. Thus, N-cyanoguanidine analogues related to this
(0.01 µM) [110,111].
2.21 1,2-Diaminocyclobutenediones and
closely-related analogues A
Numerous reports by Wyeth on the SAR of the cyclobutene-
dione-class of KATP channel openers have demonstrated the
class of aminal derivatives have been reported to hyperpolarise
bladder smooth muscle cells. Compound 19 was reported to
have an EC50 value of 0.57 µM in the FLIPR assay. Although
this activity did not correlate well with the EC50 value in the
isolated bladder strip assay (17 µM), the compound was found
to be effective at lowering the AUC (EC30 = 0.70 µmol/kg)
potential utility of this scaffold for the generation of selective in vivo in the isovolumetric contraction model in male rats
bladder smooth muscle relaxants [22,23]. KCO-616 (16) has after intravenous administration [114,115].
been shown to possess in vivo efficacy in preclinical models of Researchers at Wyeth have developed alternative scaffolds
urge incontinence at doses that cause no significant effects on to the squaric acid diamide derivatives as demonstrated by
mean arterial blood pressure [24]. A related series of 3,4-diamino-pyrazoles 20 and 21, reported to relax rat detru-
cyclobutenediones possessing an aminal moiety has been sor strips that were precontracted by treatment with KCl with
reported by researchers at Abbott [112,113]. Pyridyl squarate 17 IC50 values of 4.9 and 4.3 µM, respectively [116]. Compound
was reported to cause a hyperpolarisation in guinea-pig blad- 20 was shown to be effective in vivo in a rat hypertrophied
der cells with an EC50 value of 0.15 µM in the FLIPR assay, bladder model described by Malmgren [25]. Upon oral admin-
whilst derivative 18 had an EC50 value of 0.093 µM. Com- istration at 10 mg/kg, pyrazole 20 caused a 36% reduction in
pound 17, however, was found to only weakly inhibit electri- the frequency of spontaneous bladder contractions in rats
cal field-stimulated contractions in isolated pig bladder strips with hypertrophied bladders due to urethral ligation.
(ED50 = 19 µM) with only ∼ 54% of maximal efficacy when A putative mimetic of the diaminocyclobutenediones
benchmarked against the effects of P-1075, a pinacidil related to compound 16 and its analogues may be the
4 Expert Opin. Ther. Patents (2006) 16(5)

5. Argentieri & Butera
1,3-disubstituted-2-thioxo-imidazolidine-4,5-diones as dem- O O
onstrated by the reported bladder relaxant activity observed
with compounds 22 – 24. The compounds were active in vitro NH OH R1 NH OH
with rat bladder inhibitory IC50 values of 3.3, 3.4 and CF3 CF3
3.6 µM, respectively [117]. Compound 22 was reported to be CF3 R2 CF3
active in vivo at reducing both the frequency (- 84%) and
the amplitude (- 54%) of spontaneous bladder contractions
in the rat hypertrophied bladder model after intravenous 25 26 R1 = C2H5, R2 = H
administration at 3 mg/kg. 27 R1 = Ph, R2 = H
28 R1 = 2-Furyl, R2 = H
2.3 Naphthyl amides 29 R1 = 5-Oxazolyl, R2 = H
A series of naphthyl amides has been disclosed and character- 30 R1 = CH3, R2 = 4-Br
ised by Abbott as a new class of KATP channel openers [118].
Although no biological data are disclosed in the patent, the
compounds are claimed to be active in FLIPR and pig bladder
strip relaxation assays. The lead compound in this series has
been identified as A-151892 (25). Extensive in vitro and
o f
have been the target of at least one group at Kyowa Hakko
Kogyo. The role of this channel in bladder sensory afferents
has been described in adult rats [28]. A-type channels are a
member of the Kv4 family of potassium channels present in
o
in vivo data for this particular lead has recently been pub- a number of excitable tissues, including smooth muscle,
lished [26]. The compound was found to potently enhance a neuronal and epithelial tissues. A-type currents are charac-
r
glyburide-sensitive whole-cell current and hyperpolarise terised by their rapid inactivation. This inactivation is mod-
guinea-pig bladder smooth muscle cells (FLIPR EC50 = ulated by several subunits and accessory proteins that could
P
0.023 µM). Binding to the dihydropyridine KATP binding site be targets for pharmacological manipulation of channel
was demonstrated by displacing [125I]A-312110 (a standard, conductance [29].
non-subtype specific KATP channel agonist) with a Ki value of Scientists from Kyowa Hakko Kogyo and the University of
r
0.035 µM. In functional assays, A-151892 relaxed both carba- Pittsburgh have disclosed tertiary carbinol 31 (KW-7158) as a
chol and electric field-stimulated contractions in isolated pig potent enhancer of the slowly-inactivating A-type K+ channels
o
bladder strips in a glyburide-reversible manner with IC50 val- in the afferent pathways, and have demonstrated utility for
ues of 0.047 and 0.095 µM, respectively. The compound was OAB [119] and bladder irritation accompanied by prostatic
also shown to be effective in vivo at suppressing spontaneous hyperplasia [120]. The compound was shown to suppress affer-
t h
bladder contractions in both obstructed pigs and rats with
ED35 values of 8.9 nmol/kg (i.v.) and 36 nmol/kg (i.v.),
respectively. The compound was found to have only marginal
ent nerve activity as measured by the increase in transient out-
ward current by 20 – 50% at concentrations of 0.05 – 1 µM
in rat dorsal root ganglia. The compound also shortened
u
selectivity in vivo as demonstrated by a mean arterial pressure action potential duration by 25 – 40% in the same system [30].
(MAP) ED10 value of 30 nmol/kg in the pig. The compound was shown to be active in models of bladder
The in vitro SAR surrounding this novel chemotype for hyperactivity [31] and is currently in Phase II clinical trials for
A
KATP channel opening activity has been reported in a separate urinary incontinence [32].
publication [27]. In this work, it was shown that the com-
pounds hyperpolarised Ltk cells that were stably transfected 4. KCNQ potassium channel activators
with Kir6.2/SUR2B. A-151892 had a FLIPR EC50 value of
0.018 µM in this assay and, whilst the corresponding propi- Recently data have emerged that support a role for certain
onic amide (26) was equipotent (0.015 µM), the activity KCNQ channel subtypes in bladder smooth muscle [33]. Pre-
quickly dropped off with larger or branched R1 groups. Potent viously, KCNQ channels had been shown to be widely dis-
hyperpolarising activity was recovered with R1 as phenyl (27, tributed in neuronal tissues and have been the target of
0.044 µM), or a variety of heterocycles such as 2-furyl (28, therapies for epilepsy [34]. Currently five KCNQ channel sub-
0.009 µM) or 5-oxazolyl (29, 0.024 µM). Substitution on the types have been identified: KCNQ1, KCNQ2, KCNQ3,
naphthyl ring was poorly tolerated as demonstrated by the KCNQ4 and KCNQ5. All KCNQ channels can couple to
sharp loss of activity seen with compound 30 (5.31 µM) or muscarinic receptors and form M-currents that are gated
any of several other examples where R2 does not equal H. (inhibited) by acetylcholine. Work in human bladder smooth
Selected potent compounds in this series were shown to relax muscle cells has provided molecular evidence for the existence
isolated pig bladder strips. of KCNQ3 and KCNQ5 subtypes and electrophysiological
evidence for an ion current sensitive to the selective KCNQ
3. A-type potassium channel activators channel blocker XE-991 [35] and to acetylcholine. The
KCNQ channel activator retigabine was also shown to inhibit
Sensory C-fibres that contribute to the afferent limb of the spontaneous non-voiding bladder contractions in rats [33,36].
micturition reflex contain A-type potassium channels that These data suggest that KCNQ channel currents underlie a
Expert Opin. Ther. Patents (2006) 16(5) 5

6. An overview of potassium channel activators for the treatment of overactive bladder
O O
and a β subunit KCMB1 [41-43]. This channel is blocked by
S the scorpion venoms and activated by several natural and syn-
thetic compounds, and both subunits have been shown to
play a significant role in bladder function [44]. NS-1619 is a
OH
S
benzimidazolone analogue that has been shown to relax
O HN guinea-pig detrusor strips, activate BKCa channels and block
CF3
calcium channels in isolated guinea-pig detrusor cells [45].
O
The bee venom apamin has a high affinity for the SKCa
31
channel, and has been shown to regulate bladder smooth
H
N O muscle [46,47]. Three SKCa mammalian genes, KCNN1,
KCNN2 and KCNN3, have been cloned, and the urinary
N NH2
O bladder appears to have all three isoforms. Studies modulating
H the expression of KCNN3 have demonstrated a clear role in
F
32
significant portion of human bladder smooth muscle resting
o f
the regulation of bladder function [48].
Substantially less information is available on the IKCa chan-
nel; however, their presence has been demonstrated in bladder
smooth muscle cells from human and mice [49,50]. To date,
o
membrane potential, and activation of these channels can one isoform (KCNN4) has been cloned.
reduce excitability of the cell. Thus, the KCNQ bladder
r
channel is an emerging target of interest for OAB. 5.1 BKCa compounds
A methods utility patent for the use of KCNQ channel A large number of novel structural classes affecting the BKCa
P
openers for maintaining bladder control was granted to channel with potential utility for treating OAB have been dis-
Wyeth [121]. Using qualitative rtPCR, KCNQ 1, KCNQ3 and closed during the period covered by this review. A unique class
KCNQ5 potassium channels were identified in rat urinary of 10H-benzo[4,5]furo[3,2-b]indole and 5,10-dihy-
r
bladder tissue with KCNQ5 exhibiting the highest expression dro-indeno[1,2-b]indole-1-carboxylic acids has been reported
levels. Whilst the patent specifically claimed KCNQ openers by Wyeth [122,123]. Benzofuranoindoles 33 – 35 were reported
o
of all subtypes, data were shown for retigabine (32), a to relax KCl-contracted rat bladder smooth muscle strips with
KCNQ2-5 channel opener that is being developed as an IC50 values of 15.1, 6.1 and 5.8 µM, respectively. In contrast
anti-epileptic agent [37,38]. Exposure of isolated rat bladder to the typical KCO, these agents were reported to be intrinsi-
smooth muscle cells to 10 µM retigabine resulted in a
t
17.8 mV membrane hyperpolarisation as measured by
patch-clamp experiments. This effect was reversed upon addi-
h cally selective for bladder smooth muscle, as demonstrated by
their poor relaxation activity against isolated rat aortic rings
(aortic IC50 values = 118, 128 and 268 µM, respectively) in
u
tion of 50 µM linopirdine, a known M-current antagonist. In the same assay. The SAR for this class of novel KCOs has been
functional studies on rat bladder strips, retigabine relaxed car- reported [51]. The imbedded 5,5-ring system is fairly tolerant
bachol-contracted tissue with an IC50 value of 3.5 µM. Addi- of structural modifications as evidenced by the similar activi-
A
tion of linopirdine or XE-991 also reversed this observed ties and, in most cases, selectivity of analogues 36 (bladder
relaxation. IC50 = 8.6 µM, bladder selectivity = 24.4-fold), 37 (bladder
IC50 = 5.2 µM, bladder selectivity = 3.3-fold) and 38 (bladder
5. The
calcium-dependent potassium channel IC50 = 4.4 µM, bladder selectivity = 24.8-fold). In voltage
openers clamp studies on isolated rat detrusor myocytes, compound
33 (10 µM) caused a 3.5-fold increase in outward current at
The role of KCa channels in the regulation of bladder resting +80 mV. Addition of iberiotoxin, a selective antagonist of the
potential and excitability has been well documented [39]. In BKCa channel, reversed this increase in outward current back
addition to their role in bladder smooth muscle, they are also to control values.
present in vascular and gastrointestinal smooth muscle, brain Wyeth also reported a series of anthranilic acid amides typ-
tissue, spinal cord and in neurons innervating the bladder [40]. ified by compound 39 [124]. The compound potently relaxed
Their presence in both bladder smooth muscle and neuronal KCl-induced contractions in rat bladder strips (IC50 =
tissues makes them an attractive target, as KCa activators could 0.52 µM) and exhibited some in vitro selectivity (aortic ring
impact OAB whether the underlying aetiology is either IC50 = 2.76 µM). Anthranilic amide 39 was shown to be effec-
neurogenic or myogenic in nature. tive in vivo in Malgrem’s rat hypertrophied bladder model.
KCa channels fall into three main subtypes based upon their Administration of 30 mg/kg (i.v.) of compound 39 caused a
conductance, Ca2+ sensitivity and voltage-dependence. These 70% reduction in the frequency of spontaneous bladder con-
are large conductance (BKCa), intermediate conductance tractions. Certain members of this structural class of KCOs
(IKCa) and small conductance (SKCa) channels. BKCa channels were also claimed to have an effect on swelling-induced chlo-
are composed of the pore forming KCa1.1 α subunit KCMA1 ride channel currents in isolated guinea-pig bladder cells.
6 Expert Opin. Ther. Patents (2006) 16(5)

7. Argentieri & Butera
O OH O OH
H
N Y F3C O OH
R R
H
N
O X
O
Cl
33 R = Br 36 R = Br, X = CH2, Y = NH 39
34 R = I 37 R = NO2, X = C(CH3)2, Y = NH
35 R = Cl 38 R = Br, X = NH, Y = CH2
Y O OH
Z N N
H O O
f
X N
HO S N
O
o
N N
H O
O S
o
CF3 41 42
40 Cl
Related analogues in which the double bond was constrained
in a five-membered ring (e.g., 40) were also reported to pos-
sess in vitro bladder relaxant activity [125].
A relatively minor structural modification of the KATP
P r
rabbit urinary bladder strips with an EC50 value of < 0.5
µM and inhibits substance P-induced spontaneous rhyth-
mic bladder contractions in vivo in anaesthetised rats after
intravenous administration (0.25 mg/kg) for > 20 minutes
3,4-diaminocyclobutenedione scaffold seen in compound 16
resulted in a series of 3-amino-4-phenyl-substituted
o
cyclobutenediones possessing a glyburide-insensitive bladder
relaxing effect. Compound 41 was reported to relax bladder r as measured by bladder cystometry.
Variants containing one heteroatom in the central hetero-
aroamatic ring were also disclosed [128]. Tri-substituted furan
analogue 43 represents an example from several series of fura-
h
smooth muscle strips with an IC50 value of 2.5 µM [126]. nyl, thienyl and imidazolyl templates featuring an acetic acid
This observed relaxation was reversed by iberiotoxin, a selec- moiety at the C-2-position. The compound was reported to
t
tive BKCa channel blocker. The compound demonstrated
in vitro selectivity for bladder tissue as evidenced by a higher
u
IC50 value in relaxing KCl-contracted aortic rings
(37.7 µM). Interestingly, the KATP agents based on com-
pound 16 were reported to be aortic selective in vitro. Recent
have the same level of activity as compound 42 in both the
in vitro (EC50 < 0.5 µM) and in vivo (inhibition of spontane-
ous contractions for at least 20 min after i.v. administration at
a dose of 0.25 mg/kg) assays. The in vivo effects of compound
43 were blunted in experiments where iberiotoxin was
A
SAR studies indicate that a pyridine ring is tolerated in place
of the phenyl ring, and that electron-donating groups are
preferred for activity [52]. Compound 41 (1 µM) was shown
to increase outward current (> +40 mV) in voltage-clamp
studies on isolated human bronchial smooth muscle cells.
This increase in outward current was reversed back to con-
pre-administered (s.c.) to the test animal; thus suggesting the
involvement of BKCa channels.
A related scaffold was recently disclosed by researchers at
Tanabe Seiyaku. The central heterocycle may contain two
heteroatoms adjacent to each other as seen in isoxazole deriva-
tive 44 [129]. The compound represents a series of analogues in
trol levels after addition of iberiotoxin; thus suggesting which the flanking phenyl groups are installed on adjacent
involvement of BKCa channels. atoms of the central ring and the hydrophilic moiety is trans-
Tanabe Seiyaku Co. has disclosed a large body of work posed from the central ring onto one of the phenyl groups.
covered in numerous patent applications describing new The in vitro and in vivo performances of compound 44 were
BKCa channel openers with potential to treat bladder insta- comparable to those of compounds 42 and 43.
bility. The general theme for these chemotypes focuses on Structural chemotypes represented by compounds 42 – 44
five-membered ring heterocycles containing two pendant are reminiscent of the widely familiar COX-2 inhibitor phar-
substituted aryl or heteroaryl groups in addition to a third macophore. In fact, Tanabe Seiyaku has recently filed a patent
substituent comprised of a hydrogen bond-donating moi- application for the utility of COX-2 inhibitors, such as
ety. Compound 42 represents a prototype from a series of celecoxib (45), as BKCa channel openers for the treatment of
thiazolyl, oxazolyl and imidazolyl acetic acid analogues urge urinary incontinence and bladder instability [130,131].
reported to possess bladder relaxant properties [127]. Data disclosed in the patent application suggest that these
Although specific data for compound 42 are not shown, compounds possess BKCa channel opening activity. Celecoxib
they report that the compound relaxes contracted isolated was shown to relax KCl-contracted isolated rabbit detrusor
Expert Opin. Ther. Patents (2006) 16(5) 7

8. An overview of potassium channel activators for the treatment of overactive bladder
CF3
O
N
N
HO O
N
O
H
S N
O
HO O S
Cl O
NH2
43 44 45
CF3
O
f
N N N
N
N OH
N N
O
O
S
O
O
S
N
H
N
o o N
H
N
r
NH2 NH2
46 47 48 49
N
N
N
Cl
Cl
H
N
O
r
O
P OH
N
F
N
F
N
o
N
H
H2N N O HN N O
h
50 51 52 O
t
O
53
A u
strips with an IC50 value of 9.24 µM. Transposing the methyl
group on the pyrazole C-5 phenyl ring from the para-position
to the ortho-position as in compound 46 (IC50 = 0.11 µM),
greatly enhances the BKCa opening activity. Isoxazole 47, a
sulfonamide analogue of compound 44, was reported to have
86Rb+ efflux in HEK293 cells stably transfected with the BKCa
α-subunit. The SAR of this class of compounds has been pub-
lished [55]. Compounds 48 – 50 emerge from the paper as
examples of the most potent analogues. Their EC50 values in
the 86Rb+ efflux assay using HEK293 cells transfected with the
a bladder strip inhibitory IC50 value of 1.53 µM. Compounds BKCa α-subunit are reported to be 5.13, 5.07 and 3.34 µM,
45 – 47 were also active in vivo by inhibiting sub- respectively. All three analogues increase outward current
stance-P-induced spontaneous bladder contractions in female (10 µM) by 75 – 108% over control values in the same
rats for a duration of 11, 8, and 16.7 minutes, respectively, HEK293 cells. The three analogues were also effective in a
after intravenous administration (0.25 mg/kg). The COX-2 functional assay that measures the ability of the compounds to
inhibitory activity of compounds 45 and 46 are comparable relax KCl-contracted isolated rat bladder strips with IC50
(0.04 and 0.069 µM, respectively) [53,54], thus suggesting a values in the micromolar range.
possible divergence in the SAR for the two pharmacological Researchers at GSK have disclosed a novel class of puta-
properties associated with this chemotype. tive BKCa channel openers [135]. The compounds can be
Abbott has disclosed a novel series of substituted described as substituted indoles with an aromatic group
2-amino-4-azaindoles as BKCa channel openers [132-134]. attached to the indole C-2-position, onto which is installed
Although there are no biological data presented in the patent an acidic headpiece such as a carboxyl group. Compound 51
applications, the most preferred compounds are claimed to is an example from the patent application. Although no data
have inhibitory IC50 values of < 5 µM in the isolated bladder are presented, the compounds are claimed to be active in
strip assay and EC50 values of < 5 µM in the electrophysiolog- relaxing KCl-contracted isolated rabbit urinary bladder
ical assays measuring the ability of the compounds to enhance strips and are claimed to activate the BKCa channel as
8 Expert Opin. Ther. Patents (2006) 16(5)

9. Argentieri & Butera
Table 1. BKCa channel openers without reported bladder data.
Company Structural class Assays Ref.
4SC AG Substituted Whole cell patch-clamp on CHO cells transfected with [138]
tetrahydro-isoquinolines α-hSlo and β-bSlo
Bristol-Myers Squibb Co. 3-Thio-4-aryl-quinolinones Patch-clamp; relaxation of rat corpus cavernosum and rat [139,140]
small intestine
Bristol-Myers Squibb Co. 1,3,4-Oxadiazolones Voltage-clamp on oocytes transfected with mSlo or hSlo [141]
4SC AG Indole derivatives Whole cell patch-clamp on CHO cells transfected with [142]
α-hSlo and β-bSlo
Bristol-Myers Squibb 4-Aryl-quinolinones Whole cell voltage clamp of oocytes expressing hSlo or [143]
mSlo
f
CHO: Chinese hamster ovary.
measured by patch-clamp studies on isolated rat bladder
smooth muscle cells.
Several Japanese patent applications by Yamanouchi
Pharmaceutical have been published covering substituted
o o
ATP-dependent potassium channel has been largely aban-
doned because even minor haemodynamic side effects have
been deemed unacceptable. At the time of writing, there were
no known ATP-dependent potassium channel openers in
pyridines as new BKCa openers with utility for treating urinary
incontinence and frequent urination [136,137]. Representative
compounds 52 and 53 are reported to inhibit KCl-induced
spontaneous contractions in isolated rat bladder strips with
IC50 values of 0.2 and 1.4 µM, respectively.
P r
development for OAB.
Recent advances in ion channel molecular biology have aided
the identification of other potassium channels, channel sub-
types and subunits in bladder smooth muscle and in bladder
sensory neurons. Unlike the ATP-dependent potassium chan-
Several patent applications have been published that dis-
close novel structures as BKCa channel openers, yet they do
o
not provide data from assays that would suggest utility for
these compounds as bladder relaxants. These applications are r nel, they appear to play a less dominant role in the cardiovascu-
lar system, particularly in the heart. A number of molecules
have been identified that activate calcium-dependent potassium
channels (KCa) and hyperpolarise bladder smooth muscle
h
included in Table 1. through an iberiotoxin-sensitive current. Preclinical data sug-
gest that KCa channel activators inhibit bladder contractility
5.2 IKCa and SKCa compounds
u t
There have been numerous reports during this review period
of novel structures selectively affecting the IKCa and SKCa
channels. Although these patent applications did not pro-
vide specifically relevant data to support the claims that the
in vitro and in vivo. Whilst KCa channels are prominent in vas-
cular smooth muscle, it remains to be seen whether these activa-
tors will have significant haemodynamic consequences.
Targeting neuronal potassium channels is an interesting
approach to treating urological diseases. Certainly the afferent
A
agents may have utility in the treatment of OAB, they did limb of the bladder micturition reflex can play an important
provide some data to suggest that the compounds possess role in the aetiology of OAB. Activating potassium channels
some cell membrane hyperpolarising properties and, as such, in rapidly firing, depolarised neurons in the Aδ and C-fibre
information abstracted from them is included in Table 2. bladder sensory afferents may be an effective means of treating
neurogenic bladder overactivity. It might be expected that
6. Expert opinion targeting neuronal channels would minimise cardiovascular
side effects; however, the possibility exists for the emergence
For a number of years now the research community has recog- of unwanted neuronal side effects. Successful development of
nised that urological disorders, such as OAB, represent a still the Kyowa Hakko Kogyo compound 31 (KW-7158) will be of
largely unmet medical need; the pharmaceutical industry is significant interest.
therefore still actively pursuing safer, more effective treat- The most recently identified potassium channel in bladder
ments with fewer side effects. Since 1987 [56] when the poten- smooth muscle is the KCNQ channel. Long recognised as a
tial for treating OAB with potassium channel activators was neuronal channel and target for diseases like epilepsy (see [57]),
first described, there has been tremendous interest in utilising this channel also appears to play a prominent role in main-
these targets as a means of stabilising bladder smooth muscle taining bladder resting potential. The KCNQ1 channel sub-
and inhibiting the non-voiding contractions associated with type mutation has been identified as an underlying cause of
urinary urgency. Studies with ATP-dependent potassium long QT syndrome (KvLQT); however, the KCNQ1 subtype
channel agonists have clearly demonstrated the effectiveness does not appear to have a significant presence in bladder
of such an approach. Whilst significant progress has been smooth muscle. Instead, data indicate that KCNQ3 and
made at minimising cardiovascular side effects, the KCNQ5 are the prominent bladder subtypes in human
Expert Opin. Ther. Patents (2006) 16(5) 9

10. An overview of potassium channel activators for the treatment of overactive bladder
Table 2. IKCa and/or SKCa channel openers without reported bladder data.
Company Structural class Channel Assays Ref.
Glaxo Wellcome Aryl carbamates and SKCa VIPR: activation of hSK1 K+ channels expressed in [144]
amides HEK293T cells
Glaxo Wellcome Bicyclic esters or amides IKCa/SKCa FLIPR: SK/IK stable cell lines in CHO-K1; Patch-clamp cell [145]
EP on hSK1-CHO cells; neuropathic/inflammatory pain
models (CCI and CFA)
NeuroSearch AS Benzamidazolones IKCa Whole cell activation of IKCa current expressed in HEK293 [146]
cells
NeuroSearch AS Oximes IKCa Whole cell activation of IKCa current expressed in HEK293 [147]
cells
f
NeuroSearch AS Amino benzothiozoles IKCa Whole cell activation of IKCa current expressed in HEK293 [148]
cells
o
CCI: Chronic constriction injury; CFA: Complete Freund's adjuvant; CHO: Chinese hamster ovary; EP: Electrophysiology; FLIPR: Fluorescence-imaging plate reader;
VIPR: Voltage ion probe reader.
bladder smooth muscle cells. In addition to the KCNQ chan-
nels located in bladder smooth muscle, neuronal KCNQ
channels are potentially targets for the treatment of OAB.
Conceivably, the right KCNQ channel activator would be o
unacceptable side effects is very low. In order to be successful,
r
new agents will need to be at least as efficacious as the
antimuscarinics, but have a significantly better side effect pro-
file. Substituting, for example, haemodynamic side effects for
efficacious for treating OAB, resulting from either myogenic
or neurogenic aetiologies.
In the past 25 or more years, there have been no real mech-
anistic advancements for the treatment of OAB. The potas-
sium channel openers have the potential to be more effective
P
dry mouth, is simply not acceptable. To date, the potassium
channel openers have not lived-up to their expectation to be
r
better, safer agents for OAB. However, the authors believe
potassium channel openers still represent enormous potential
as a treatment for OAB, especially as the understanding of the
and useful alternatives to the antimuscarinic agents that are
currently used today. A significant issue, as is always the case
with non-life-threatening diseases, is that the tolerance for
h o structure/function of existing potassium channels improves,
and newer channels or associated channel proteins are
characterised as potential drug targets.
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