Structure-Activity studies for a novel series of tricyclic dihydropyrimidines as KATP channel openers (KCOs)

Article abstract of DOI:10.1016/S0960-894X(02)00207-X

A novel series of tricyclic dihydropyrimidines was synthesized and evaluated for activity as K_ATP channel openers. The functional activity of several compounds, for example 6A (EC₅₀=30 nM) and its enantiomers exceeded cromakalim.

Full text of DOI:10.1016/S0960-894X(02)00207-X

Bioorganic & Medicinal Chemistry Letters 12 (2002) 1481–1484
Structure–Activity Studies for a Novel Series of Tricyclic
Dihydropyrimidines as KATP Channel Openers (KCOs)
Irene Drizin,* Mark W. Holladay, Lin Yi, Henry Q. Zhang, Sujatha Gopalakrishnan,
Murali Gopalakrishnan, Kristi L. Whiteaker, Steven A. Buckner,
JamesP. Sullivan and William A. Carroll
Neuroscience Research, GPRD, Abbott Laboratories, 100 Abbott Park Road, Abbott Park, IL 60064-6101, USA
Received 22 January 2002; accepted 12 March 2002
Abstract—A novel series of tricyclic dihydropyrimidines was synthesized and evaluated for activity as KATP channel openers. The
functional activity of several compounds, for example 6A (EC50=30 nM) and itsenantiomersexceeded cromakalim. # 2002
Elsevier Science Ltd. All rights reserved.
Potassium channels play an important role in regulating
cell membrane excitability, action potential regulation
and epithelial electrolyte transport. KATP channel open-
ers are amongst the most widely explored channels with
regard to their potential for the treatment of various
diseases such as, for example, bladder overactivity.1
hybrid of ZM 244085 and 1 may display enhanced KATP
properties. The proposed structure 2 would have an
electron deficient aromatic ring positioned orthogonally
to a second ring with a hydrogen bond acceptor group.
Thisperpendicular arrangement ishypotheiszed to be
the pharmacophore responsible for the KATP activity of
cromakalim-like compounds.6 In thisarticle we wish to
report the synthesis and SAR studies of the novel series
of tricyclic dihydropyrimidines 2 that were prepared
based on this concept.
Bladder overactivity is a condition associated with the
spontaneous contractions of its smooth muscle. KATP
channels exist in the bladder and the ability of potas-
sium channel openers to hyperpolarize cells and relax
smooth muscle may provide a method to ameliorate or
prevent bladder overactivity.
The synthesis of the proposed hybrid structure 2 was
accomplished by a three component Hantzsch-type
reaction asdepicted in Scheme 2. A mixture of aldehyde
3, 1,3-cyclohexanedione 4 and 3-aminopyrazole 5 was
heated at reflux in ethanol. The resulting products:
dihydropyrimidine (structure A) and dihydropyridine
(structure B) were separated either by crystallization or
chromatography on silica gel.
Since a small clinical study with cromakalim indicated
potential uesfulnessof KCO’sfor the treatment of
detrusor instability, several novel series of KATP openers
were developed.2 Amongst them ZD 6169,3 ZM 244085,4
WAY-133536, and WAY-151616,5 have been reported to
activate the bladder KATP channel in vitro and selectively
inhibit bladder function in vivo.
SAR studies were conducted by varying the three com-
ponentsof the reaction hsown in Scheme 2. In the
course of SAR studies on the left hand portion it was
found that the ratio of the productsformed depended
on the ring size and the substitution of the starting
diketone. In the case of five-membered rings, like 1,3-
cyclopentanedione and tetrahydrothiophene-3-oxo-1,1-
dioxide,7 dihydropyridines( B) were the only products
formed. Introduction of sulfur in the cyclohexanedione
ring asin 23A slightly diminished the ratio of dihydro-
pyrimidines( A). Addition of gem-dimethyl groupsto
Our effort wasaimed at finding novel structuresexhibit-
ing KATP activity. The screening of our in house library
of compoundsidentified pyrazolopyrimidine 1, a com-
pound with moderate potency. We reasoned that tricyclic
dihydropyrimidines 2 (Scheme 1), being a structural
*Corresponding author. Fax: +1-847-935-5466; e-mail: irene.drizin@
abbott.com
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00207-X

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I. Drizin et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1481–1484
the cyclohexanedione ring asin 21A and 22A also low-
ered the ratio of A/B.
potassium channels was measured by evaluating chan-
gesin membrane potential uisng DIBAC dye in a 96-
well cell-based kinetic assay system, Fluorescence Ima-
ging Plate Reader (FLIPR). Changesin fluorecsence
were measured by comparison to the effect elicited by
P1075 (a potent KCO). The maximal response of each
compound, expressed as % relative to P1075, was equal
or above 80%. The observed effects were reversed by
glyburide, confirming a KATP mechanism.
Optimization of the aromatic substitution was realized
by using a variety of commercially available aldehydes.
Replacement of 3-aminopyrazole with 3-amino-1,2,4-tri-
azole or 5-aminotetrazole afforded 25 and 26, res pec-
tively. Bromination of 6A with bromine in
dichloromethane yielded the bromo derivative 27. The
3,4-dichlorosubstituted analogue 8 served as a starting
material for the synthesis of various aryl and hetero-
substituted derivatives by the Suzuki method (Scheme 3).
The more potent compoundswere alos evaluated in
vitro using tissue strips obtained from Landrace pig
bladders.9 Tissues were stimulated by a low-frequency
current that produced a stable twitch response. P1075
completely eliminated the stimulated twitch response in
a dose-dependant fashion. The maximal efficacy of each
compounds was expressed in comparison to P1075.
The separation of enantiomers of the most potent com-
pound 6A wasaccomplished by chiral chromatography
on a Chiracel OD column. The absolute stereo-
chemistry of the enantiomers was determined by X-ray
crystallography.
Table 1 summarizes the effect of various substituents on
the aromatic ring. Electron withdrawing groupsin the
3,4-position as in 6A, 8, 10, 12, and 17 enhanced the
potency. Monosubstituted compounds 13, 14, 15, and
Compoundswere evaluated for potiaussm channel
opening activity using primary cultured guinea pig
urinary bladder (GPB) cells.8 Functional activity at
Scheme 3. Reagentsand conditions: (i) Br , CH2Cl2; (ii) Suzuki reaction.
2
Table 1. SAR of the aromatic substitution
Scheme 1.
Compd
R1
—
EC50 (nM)a
3800
ZM244085
(À)Cromakalim
—
3-Br, 4-F
3-Br, 4-F
3-Br, 4-F
3-Br, 4-F
3-Cl, 4-F
3,4-diCl
3-Cl
3-CF3, 4-Cl
3-Br
3-CF3, 4-F
3-CN
3-Me
425
30
6A10
6B11
(S)-6A
(R)-6A
7
>10,000
262
155
256b
8
9
10
11
12
13
14
15
16
17
130
1100b
187b
550b
290b
2860
>10,000
1700
7800
310
3-OCF3
4-F
3-NO2, 4-Cl
aValuesare meansof three experiments.
bValuesare meansof two experiments.
Scheme 2. Reagentsand conditions: (a) EtOH, reflux.

I. Drizin et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1481–1484
1483
16 were significantly less active. This finding is in
agreement with the previously described trends for the
aromatic substitution around ZM 244085.6
Insertion of sulfur in the cyclohexanone ring (23A) was
well tolerated.
Cyclohexanone proved to be the optimal ring fusion for
the core structure, affording the most potent compound in
the series (6A). Resolution of the compound 6A provided a
set of enantiomers, both of which were active as KCOs.
Exploration of the core ring system was conducted with
the optimized aromatic substitution (3-Br, 4-F). Table 2
summarizes the results of this effort. It should be noted
that all the dihydropyridinesof tsructure B were inac-
tive irrelevant of the changesin the left-hand portion of
the molecule. The following trendswere obesrved in
dihydropyrimidines A. Introduction of dimethyl groups
a to the carbonyl group, asin 21A, had no effect on
potency, but when these groups were in the b position
(22A) the activity dropped 300-fold. Replacement of
keto-group with sulfone (19A) also lowered the potency.
With the aromatic substitution and left hand portion
optimized we explored the limitationsof the right-hand
portion of the molecule. Judging by the striking differ-
ence in potency between 6A and 6B we surmised that
the lone electron pair residing on the pyrazole nitrogen
in 6A mimicsthe effect of carbonyl group that ispresent
in ZM 244085.
Generally, the right hand portion of the molecule
proved to be more sensitive to changes. Steric bulk
represented by a t-butyl group (asin 24) diminished the
KATP activity and introduction of additional nitrogen
atomsin the ring ( 25, and 26) led to a complete loss of
activity. Asisevident from Table 3, the only compound
retaining activity, wasbromo derivative 27. A library of
variously substituted phenyl and heterosubstituted
compoundswasprepared, (Scheme 3), however, none of
Table 2. SAR of the left-hand portiona
the new compoundsexhibited K
activity.
ATP
Table 3. SAR of the right hand portion
Compd
Type A EC50 (nM)
Ratio A/B
6A
30
—
3:1
B
Compd
R
EC50 (nM)a
18A
19A
20A
21A
22A
23A
24
25
26
3-Br, 4-F
3-Br, 4-F
3-Br, 4-F
12,000
>10,000
>10,000
910
—
4:1
B
35
2:1
1:2
1:1
27
28
3-Br, 4-F
3-Cl, 4-Cl
394
9700
3200
40b
29
3-Cl, 4-Cl
12,000
aValuesare meansof three experiments.
bValuesare meansof two experiments.
aValuesare meansof two experiments.

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I. Drizin et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1481–1484
Table 4. Functional KCO activity in isolated bladder strips
2. Coghlan, M. J.; Carroll, W. A.; Gopalakrishnan, M. J.
Med. Chem. 2001, 44, 1628.
Compd
Efficacy (%P1075)
PEC50
3. (a) Chess-Williams, R.; Martin, S. W.; Korstanje, C.;
Chapple, C. R. Br. J. Urol. 1999, 83, 1050. (b) Zografos, P.;
Li, J. H.; Kau, S. T. Pharmacology 1992, 45, 216. (c) Martin,
S. W.; Radley, S. C.; Chess-Williams, R.; Korstanje, C.;
Chapple, C. R. Br. J. Urol. 1997, 80, 405.
(À)Cromakalim
6A
95
96
6.34
6.47
6.67
6.34
6.0
(R)-6A
(S)-6A
21A
100
95
75
4. Li, J. H. Cardiovasc. Drug Rev. 1997, 15, 220.
5. (a) Butera, J. A.; Antane, M. M.; Antane, S. A.; Argentieri,
T. M.; Freeden, C.; Graceffa, R. F.; Hirth, B. H.; Jenkins, D.;
Lennox, J. R.; Matelan, E.; Norton, N. W.; Quagliato, D.;
Sheldon, J. H.; Spinelli, W.; Warga, D.; Wojdan, A.; Woods,
M. J. Med. Chem. 2000, 43, 1187. (b) Gilbert, A. M.; Antane,
M. M.; Argentieri, T. M.; Butera, J. A.; Francisco, G. D.;
Freeden, C.; Gundersen, E. G.; Graceffa, R. F.; Herbst, D.;
Hirth, B. H.; Lennox, J. R.; McFarlane, G.; Norton, N. W.;
Quagliato, D.; Sheldon, J. H.; Warga, D.; Wojdan, A.;
Woods, M. J. Med. Chem. 2000, 43, 1203.
6. Frank, C. A.; Forst, J. M.; Grant, T.; Harris, R. J.; Kau,
S. T.; Li, J. H.; Ohnmacht, C. J.; Smith, R. W.; Trainor, D. A.;
Trivedi, S. Bioorg. Med. Chem. Lett. 1993, 12, 2725.
7. Carroll, W. A.; Agrios, K. A.; Altenbach, R. J.; Drizin,
I.; Kort, M. F. US 6,191,140, 2001; Chem. Abstr. 2001,
131202.
The most potent compounds of the series were eval-
uated for their KATP activity in pig bladder strips. The
results are presented in Table 4. The lead compound 6A
and itsenantiomersdemonstrate activity comparable to
cromakalim.
In summary, we identified tricyclic dihydropyrimidines as
a new class of potassium channel openers. The tricyclic
core of 6A wasfound to be an iostsere of the acridine-
dione of ZM 244085 with the pyrazole nitrogen replacing
the carbonyl group. The synthetic route utilized for the
series allowed us a quick analysis of the optimal aromatic
substitution and the steric and electronic limitations of
the pharmacophore.
8. Gopalakrishnan, M.; Whiteaker, K.; Molinari, E.; Davis-
Taber, R.; Scott, V. E.; Shieh, C.-C.; Buckner, S. A.; Milicic,
I.; Cain, J.; Postl, S.; Sullivan, J. P.; Brioni, J. D. J. Pharm.
Exp. Ther. 1999, 289, 551.
Acknowledgement
9. Buckner, S. A.; Milicic, I.; Daza, A.; Davis-Taber, R.;
Scott, V. E.; Sullivan, J. P.; Brioni, J. D. Eur. J. Pharmacol.
2000, 400, 287.
The authorswould like to thank Rodger Henry for the
X-ray structure determination.
10. 6A 1H NMR (DMSO-d6) d 1.94 (m, 2H), 2.25 (m, 2H),
2.63 (m, 2H), 5.72 (d, 1H), 6.19 (s, 1H), 7.1 (m, 1H), 7.23 (t,
1H), 7.31 (d, 1H), 7.4 (dd, 1H), 10.55 (s, 1H).
11. 6B 1H NMR (DMSO-d6) d 1.89 (m, 2H), 2.19 (m, 2H),
2.58 (m, 2H), 5.08 (s, 1H), 7.12 (m, 1H), 7.19 (t, 1H), 7.33 (s,
1H), 7.39 (dd, 1H), 9.9 (s, 1H).
References and Notes
1. Andersson, K.-E. Urology 1997, 50, 74.