Discovery of 3-aminopiperidines as potent, selective, and orally bioavailable dipeptidyl peptidase IV inhibitors

Article abstract of DOI:10.1016/j.bmcl.2007.05.087

Substituted 3-aminopiperidines 3 were evaluated as DPP-4 inhibitors. The inhibitors showed good DPP-4 potency with superb selectivity over other peptidases (QPP, DPP8, and DPP9). Selected DPP-4 inhibitors were further evaluated for their hERG potassium ch

Full text of DOI:10.1016/j.bmcl.2007.05.087

Bioorganic & Medicinal Chemistry Letters 17 (2007) 4579–4583
Discovery of 3-aminopiperidines as potent, selective, and
orally bioavailable dipeptidyl peptidase IV inhibitors
Jason M. Cox,a,* Bart Harper,a Anthony Mastracchio,a, Barbara Leiting,b
Ranabir Sinha Roy,b Reshma A. Patel,b Joseph K. Wu,b Kathryn A. Lyons,a
Huaibing He,a Shiyao Xu,c Bing Zhu,c Nancy A. Thornberry,b
Ann E. Webera and Scott D. Edmondsona
aMerck Research Laboratories, Department of Medicinal Chemistry, PO Box 2000, Rahway, NJ 07065, USA
bMerck Research Laboratories, Department of Metabolic Disorders, PO Box 2000, Rahway, NJ 07065, USA
cDepartment of Drug Metabolism Merck & Co. Inc., PO Box 2000, Rahway, NJ 07065, USA
Received 2 April 2007; accepted 29 May 2007
Available online 2 June 2007
Abstract—Substituted 3-aminopiperidines 3 were evaluated as DPP-4 inhibitors. The inhibitors showed good DPP-4 potency with
superb selectivity over other peptidases (QPP, DPP8, and DPP9). Selected DPP-4 inhibitors were further evaluated for their hERG
potassium channel, calcium channel, Cyp2D6, and pharmacokinetic profiles.
Ó 2007 Elsevier Ltd. All rights reserved.
Type 2 diabetes mellitus is a growing health problem,
with an estimated 170 million people worldwide who
suffer from the disease.1 Upon ingestion of food, the
incretin hormones glucagon-like peptide 1 (GLP-1)
and glucose-dependent insulinotropic polypeptide
(GIP) are released and regulate insulin secretion in a glu-
cose-dependent fashion.2 Infusion of GLP-1 was shown
to stimulate insulin biosynthesis, inhibit glucagon
secretion, slow gastric emptying, reduce appetite, and
stimulate regeneration and differentiation of islet b-cells.
Unfortunately, due to rapid (<2 min) inactivation of
GLP-1 by the serine protease dipeptidyl peptidase IV
(DPP-4), the reduction of blood glucose and hemoglo-
bin A1c levels can only be achieved by constant infusion
of GLP-1.2b Inhibition of DPP-4 leads to an increased
level of endogenous GLP-1 and GIP, allowing their
therapeutic benefits to be realized.2,4 Furthermore,
DPP-4 inhibitors are a clinically proven, novel therapeu-
tic approach to the treatment of type 2 diabetes.3
Initial work from our laboratories focused on the devel-
opment of DPP-4 inhibitors lacking an electrophilic
nitrile functionality. These efforts culminated in the dis-
covery of sitagliptin 1 (DPP-4 IC50 18 nM),5 the first
DPP-4 inhibitor approved by the FDA for the treatment
of type 2 diabetes (Fig. 1). In a continued search for
structurally diverse inhibitors,6 DPP-4 inhibitor 2
(DPP-4 IC50 21 nM) was designed using the sitagliptin
X-ray crystallographic structure along with molecular
modeling.7 Replacement of the central cyclohexylamine
in 2 with a 3-aminopiperidine led to the discovery of
3-aminopiperidines 3 as potent, selective, and orally bio-
available DPP-4 inhibitors.8
The synthesis of 3-aminopiperidine-based DPP-4 inhibi-
tors commenced with a Horner–Wadsworth–Emmons
reaction of aldehyde 4 (Scheme 1). 9 Subsequent Michael
F
F
F
F
F
F
F
F
F
NH2
O
NH2
NH2
N
Keywords: Dipeptidyl peptidase IV; DPP-4; DPP8; DPP9; QPP;
Aminopiperidine.
N
N
N
N
R
N
N
*
N
N
Corresponding author. Tel.: +1 732 594 4425; fax: +1 732 594
5350; e-mail: jason_cox@merck.com
CF3
CF3
sitagliptin (1)
2
3
Present address: Department of Chemistry, Princeton University,
USA.
Figure 1. Select DPP-4 inhibitors.
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.05.087

4580
J. M. Cox et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4579–4583
F
F
F
O
F
F
F
O
CN
CO2Me
NHBoc
N
MeO
Cl
N
X
a, b
H
c-e
CO2Me
a, b
f, g
F
F
10
+
F
NH
F
F
N
X
NO2
F
F
F
O
11a X=NHMe
11b X=CONH2
11c X=CN
12a-c
NH2
4
5
6
12a
36
12b
38
F
F
c-e
F
f, e
F
F
CO2H
NHBoc
NBn
NHBoc
NBn
j
h, i
F
+ TFA-
NH3
F
NBn
O
F
F
g, e
O
12c
F
N
N
7
8
9
N
F
F
F
F
N
H
F
F
+ TFA-
NHBoc
NH
NH3
N
37
m, n
k, l
Scheme 2. Synthesis of DPP-4 inhibitors 36–38. Reagents and condi-
tions: (a) DIEA, tol, D; (b) Ra–Ni, THF/MeOH (10:1) 40 °C or NiCl2,
NaBH4, MeOH/THF (3:1), 0 °C; (c) c-PrCO2H, EDC, HOBt, DIEA,
DCM; (d) AcOH, D; (e) TFA, DCM; (f) (MeO)3CMe, tol, D; (g) c-
PrMgX, THF, rt; MeOCHO, 50 °C.
R
13-35, 39-46
10
Scheme 1. Synthesis of aminopiperidine DPP-4 inhibitors 13–35, 39–
46. Reagents and conditions: (a) (EtO)2POCH2CO2Et, DBU, THF; (b)
NCCH2CO2Me, NaOMe, MeOH, D; (c) H2 (50 psi), PtO2, HCl,
MeOH; (d) K2CO3, tol/MeOH (1:1), D; (e) Me3SiCHN2, Et2O/MeOH
(1:1), 0 °C ! rt; (f) BnBr, NaHMDS, THF/DMF (5:1), ꢀ78 °C ! rt;
(g) LiOH (1 N), THF/MeOH (3:1), 60 °C; (h) DPPA, TEA, tol; BnOH,
D; (i) H2 (45 psi), Pd(OH)2, Boc2O, MeOH; (j) BH3, THF, D; HCl D;
Boc2O, EtOAc; (k) Chiral HPLC (ChiralCel AD column, 5% IPA/
heptane); (l) H2, Pd(OH)2, MeOH; (m) RX, TEA or DIEA, tol/DMF
(5:1), D; (n) TFA, DCM.
against DASH family members,14 including DPP8,15
DPP9,16 FAP,17 PEP, and other proline specific enzymes
with DPP-4-like activity,18 including QPP (DPP-II).14,19
Selectivity over DPP8 and DPP9 is of particular concern
because inhibition of these enzymes is associated with
toxicity in preclinical species and the relevance of this
toxicity to humans is not yet known.20 Inhibition data
for DPP-4, QPP, DPP8, and DPP9 are presented in sub-
sequent tables (FAP and PEP activity was generally
weak >35 lM).
addition, using the sodium anion of methyl cyanoacetate,
provided diester 5. Reduction of the nitrile, cyclization,
and ester regeneration yielded d-lactam 6, with >5:1
trans/cis selectivity.10 Protection of the d-lactam nitrogen
as its benzyl amide, followed by saponification, afforded
acid 7. With 7 in hand, we were poised to carry out a Cur-
tius rearrangement.11 In the event, treatment of 7 with di-
phenyl phosphoryl azide (DPPA) and triethylamine
(TEA), followed by trapping of the resulting isocyanate
with benzyl alcohol (BnOH), and protecting group ex-
change, provided lactam 8. Borane reduction of the lac-
tam carbonyl and reprotection of the primary amine
afforded piperidine 9. The enantiomers of racemic piperi-
dine 9 were separated using chiral HPLC to afford
(2R,3R)-9. Hydrogenolysis of the secondary benzyl amine
provided key intermediate 10. Analogs 13–35 and 39–46
were synthesized by refluxing piperidine 10 with the
appropriate halo-heterocycle,12 followed by tert-butyl-
carbonyl (Boc) removal using trifluoroacetic acid (TFA).
Our initial SAR breakthrough was the discovery that 2-
pyridyl substitution played a significant role in DPP-4
potency and selectivity (Table 1).21 2-Pyridyl substituted
analog 14 had an almost fourfold increase in potency
over phenyl ring analog 13, and greater than twofold in-
crease in potency over 3- and 4-pyridyl analogs 15 and
16, respectively. The pyridyl substitution also improved
Table 1. Monocyclic substitution effects on selected DPP-4 inhibitors
F
F
+ TFA-
NH3
F
N
R
Compound
R
IC50 (lM)
DPP-4 QPP DPP8 DPP9
Additional analogs were synthesized using intermediates
12a–c (Scheme 2). Intermediates 12a–c were prepared by
thermal reaction of chlorides 11a–c12 with piperidine 10
followed by nitro reduction. Analog 36 was prepared by
HOBt/EDC coupling of 12a and cyclopropyl acid, fol-
lowed by cyclization with acetic acid and Boc removal.
Treatment of 12b with trimethyl orthoformate and sub-
sequent deprotection yielded analog 38. Nitrile 12c was
converted to analog 37 via cyclopropyl Grignard addi-
tion to the nitrile, in situ trapping of the resulting imine,
and deprotection.
13
14
15
16
17
18
19
20
21
22
23
24
25
Ph
2-pyr
0.210
0.058
0.144
0.125
0.111
0.067
0.463
0.650
0.014
5.5
33
34
54
43
39
38
50
82
46
3-pyr
4-pyr
36
35
3-CF3-2-pyr
4-CF3-2-pyr
5-CF3-2-pyr
6-CF3-2-pyr
4-CN-2-pyr
5.3 >100
28 7.6
21
13
77
57
>100
>100
>100
>100
40
5.8
9.6
2,4-Pyrimidine 0.037
2,6-Pyrimidine 0.090
>100
54
33
60
33
76
40
67
58
40
44
2,5-Pyrazine 0.082
2,3-Pyridazine 0.063
Compounds 13–46 were evaluated for in vitro inhibition
of human DPP-4.13 Each inhibitor was also tested
>100

J. M. Cox et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4579–4583
4581
selectivity over QPP while maintaining good selectivity
over DPP8 and DPP9, regardless of position. Substitu-
ents at position 4 maintained potency but at the cost
of selectivity over the other enzymes (18 and 21), and
a trifluoromethyl substituent at other positions de-
creased DPP-4 potency (17, 19, and 20). Analogs 22–
25 contained a second nitrogen in the ring and in general
maintained selectivity and potency, with pyrimidine 22
(DPP-4 IC50 37 nM; QPP, DPP8, DPP9 IC50 > 60 lM)
showing the best overall profile in the monocyclic series.
in the monocyclic series (DPP-4 IC50 54 nM), but suf-
fered degradation in selectivity (QPP, DPP8, DPP9
IC50 10–21 lM), while inhibitor 27 significantly lost
activity against DPP-4 (IC50 3.7 lM).
The 6,6-biaryl series (28–32 and 37) showed good po-
tency (DPP-4 IC50 7–130 nM) and variable selectivity,
with analog 32 showing the best profile in the series.
6,5-Bicyclic compounds 33–36 possessed similar potency
and selectivity as the 6,6 series with inhibitor 34 showing
a superb profile (DPP-4 IC50 5 nM; QPP, DPP8, DPP9
IC50 > 100 lM).
We next turned our attention to bicyclic inhibitors con-
taining a 2-pyridyl derived moiety (Table 2). The first
two inhibitors synthesized in this series were analogs
26 and 27. Inhibitor 26 maintained the potency observed
Concurrent with our investigation of DPP-4 inhibitors
outlined in Table 2, we examined the profiles of bicyclic
lactams 38–41 (Table 3). Gratifyingly, the bicyclic lac-
tams were potent (DPP-4 IC50 1.2–8 nM) and in general
had good to excellent selectivity (QPP, DPP8, DPP9
IC50 P 19 lM).
Table 2. Bicyclic substitution effects on selected DPP-4 inhibitors
F
F
+ TFA-
NH3
Similar to the bicyclic lactam series, the triazolo-pyrida-
zine series showed good DPP-4 potency and selectivity
(Table 4). Potency increased 3- to 5-fold by the addition
of alkyl substituents at the 3 position (43–45), with re-
spect to compound 42. With the exception of 46, selec-
tivity over the other peptidase screened was superb
(QPP, DPP8, DPP9 IC50 P 74 lM) in the series.
F
N
R
Compound
R
IC50 (lM)
DPP-4 QPP DPP8 DPP9
N
26
27
28
29
30
31
32
33
34
35
36
0.054
21
10
13
Representative analogs that possessed superior potency
and selectivity profiles were evaluated for hERG potas-
sium channel,22 rabbit L-type calcium channel,23 and
Cyp2D6 activity24 (Table 5), and pharmacokinetic prop-
erties (Table 6). Ion channel and Cyp2D6 potencies were
variable for the selected DPP-4 inhibitors (hERG IC50
8.1–100 lM, Ca IC50 1.0–100 lM, Cyp2D6 IC50 0.1–
55.4 lM) with 35 and 39 showing the cleanest overall
profile.
3.7
32
>100 >100
N
N
N
N
N
0.007
0.012
0.13
35
25
10
50
37
27
N
N
Table 6 depicts the rat pharmacokinetic properties.
Compounds 32, 35, and 39 suffered from rapid clearance
N
N
58
66 >100
>100 >100
54 >100
N
N
Table 3. Bicyclic lactams as potent and selective DPP-4 inhibitors
0.043
0.013
0.022
0.005
0.026
0.002
87
F
F
N
N
N
+ TFA-
NH3
N
N
93
F
N
N
N
R
Compound
R
IC50 (lM)
DPP-4 QPP DPP8 DPP9
N
N
64
49
75
CF3
N
O
N
N
N
N
N
N
>100
>100
41
>100 >100
CF3
38
39
40
0.007
46
80
>100
19
NH
N
N
N
N
N
30
43
57
N
0.0012 >100 >100
N
N
O
O
N
N
N
N
>100
0.008
0.005
>100 >100
57 >100
>100
N
O
N
37
0.008
9.9
28
32
N
41
>100
N
N
N
N

4582
J. M. Cox et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4579–4583
Table 4. Substituted triazolo-pyridazines as selective DPP-4 inhibitors
(2.0 lM). Current efforts are focused on improving
potassium channel, calcium channel, and Cyp2D6 selec-
tivity, while maintaining DPP-4 potency, selectivity over
other DASH proteins, and excellent pharmacokinetics.
These and other discoveries will be reported in due
course.
F
F
+ TFA-
NH3
R
3
F
N
N
N
N
N
Compound
R
IC50 (lM)
Acknowledgments
DPP-4
QPP
DPP8
DPP9
The authors thank Phil Eskola, Gina Black, Joe Leone,
Daniel Kim, and Derek von Langen of Synthetic Ser-
vices Group for large scale synthetic support and Ber-
nard Choi and Eric Streckfuss of Analytical Support
Group for open access LC/MS service. The authors also
thank Tesfaye Biftu for consultation during manuscript
preparation.
42
43
44
45
46
H
Me
0.031
0.006
0.006
0.009
0.016
>100
74
90
>100
>100
>100
>100
40
>100
>100
91
Et
CF3
4-F–Ph
96
15
90
>100
Table 5. Ion channel and Cyp2D6 activity of selected DPP-4 inhibitors
Compound
hERG IC50
(lM)
Ca IC50
(lM)
Cyp2D6 IC50
(lM)
References and notes
22
32
34
35
39
40
43
26.1
9.9
26.9
4.1
0.95
11.6
0.1
1. World Health Organization, 2006 <http://www.who.int/
diabetes/en/>.
8.1
1.0
2. For lead GLP-1 references, see: (a) Chia, C. W.; Egan, J.
M. Drug Discovery Today: Disease Mech. 2005, 2, 295; (b)
Holst, J. J. Curr. Opin. Endocrin. Diabetes 2005, 12, 56; (c)
Knudsen, L. B. J. Med. Chem. 2004, 47, 4128; (d) Vahl, T.
P.; D’Alessio, D. A. Expert Opin. Investig. Drugs 2004, 13,
177.
100
100
33.2
20.3
100
84.6
18.5
2.0
20.6
55.4
16.2
8.8
3. For lead DPP-4 references, see: (a) Deacon, C. F.; Holst,
J. J. Int. J. Biochem. Cell Biol. 2006, 38, 831; (b) Mentlein,
R. Expert Opin. Investig. Drugs 2005, 14, 57; (c) Weber, A.
E. J. Med. Chem. 2004, 47, 4135.
Table 6. Pharmacokinetic properties of selected DPP-4 inhibitors in
the rat (1/2 mpk iv/po)
Compound
Clp
(mL/min/kg)
t1/2
F
po AUCnorm
(lM h kg/mg)
4. Ahren, B.; Landin-Olsson, M.; Jansson, P.-A.; Svensson,
M.; Holmes, D.; Schweizer, A. J. Clin. Endocrin. Metab.
2004, 89, 2078.
5. Kim, D.; Wang, L.; Beconi, M.; Eiermann, G. J.; Fisher,
M. H.; He, H.; Hickey, G. J.; Kowalchick, J. E.; Leiting,
B.; Lyons, K.; Marsilio, F.; McCann, M.; Patel, R. A.;
Petrov, A.; Scapin, G.; Patel, S. B.; Sinha Roy, R.; Wu, J.
K.; Wyvratt, M. J.; Shang, B. B.; Zhu, L.; Thornberry, N.
A.; Weber, A. E. J. Med. Chem. 2005, 48, 141, and
references therein.
(h)
(%)
32
35
39
40
43
62
54
35
18
12
1.4
1.5
1.6
1.4
4.7
45
19
0.34
0.17
0.15
1.73
4.3
12
74
100
6. For previous structurally diverse DPP-4 inhibitors from
our laboratories see: (a) Edmondson, S. D.; Mastracchio,
A.; Mathvink, R. J.; He, J.; Harper, B.; Park, Y.-J.;
Beconi, M.; Di Salvo, J.; Eiermann, G. J.; He, H.; Leiting,
B.; Leone, J. F.; Levorse, D. A.; Lyons, K.; Patel, R. A.;
Patel, S. B.; Petrov, A.; Scapin, G.; Shang, J.; Sinha Roy,
R.; Smith, A.; Wu, J. K.; Xu, S.; Zhu, B.; Thornberry, N.
A.; Weber, A. E. J. Med. Chem. 2006, 49, 3614, and
references therein; (b) Biftu, T.; Feng, D.; Qian, X.; Liang,
G.-B.; Kieczykowski, G.; Eiermann, G.; He, H.; Leiting,
B.; Lyons, K.; Petrov, A.; Sinha Roy, R.; Zhang, B.;
Scapin, G.; Patel, S.; Gao, Y.-D.; Singh, S.; Wu, J.;
Zhang, X.; Thornberry, N. A.; Weber, A. E. Bioorg. Med.
Chem. Lett. 2006, 17, 49, and references therein.
7. Scapin, G.; Singh, S.; Feng, D.; Becker, J.; Doss, G.;
Eiermann, G.; He, H.; Lyons, K.; Patel, S.; Petrov, A.;
Sinha Roy, R.; Shang, B.; Wu, J.; Zhang, X.; Thornberry,
N. A.; Weber, A. E.; Biftu, T. Bioorg. Med. Chem. Lett.
2007, 17, 3384.
(35–62 mL/min/kg), short half-life (1.4–1.6 h), low oral
bioavailability (12–45%), and low normalized oral
AUC (0.15–0.34 lM h kg/mg). Compound 40 showed
an improved pharmacokinetic profile, but had a short
half-life (1.4 h). DPP-4 inhibitor 43 demonstrated the
best overall rat pharmacokinetic profile with low clear-
ance (12 mL/min/kg), good half-life (4.7 h), high oral
bioavailability (100%), and high normalized oral AUC
(4.3 lM h kg/mg).
In summary, we have discovered a novel series of 3-
aminopiperidines that are potent and selective DPP-4
inhibitors. The 2-pyridyl substitution proved to be a
key discovery in increasing the potency and selectivity
of this structural class. Further investigation afforded
bicyclic inhibitors that have increased DPP-4 potency
(IC50 < 10 nM) and selectivity (IC50 > 50 lM) over
other DASH proteins. While compounds 35 and 39
demonstrated low activity at ion channels and Cyp2D6,
they suffered from poor pharmacokinetic properties. On
the other hand, inhibitor 43 demonstrated excellent
pharmacokinetics but poor Ca channel selectivity
8. Structurally related aminopiperidines were recently dis-
closed, see: Pei, Z.; Li, X.;von Geldern, T. W.; Longenecker,
K.; Pireh, D.; Stewart, K. D.; Backes, B. J.; Lai, C.; Lubben,
T. H.; Ballaron, S. J.; Beno, D. W. A.; Kempf-Grote, A. J.;
Sham, H. L.; Trevillyan, J. M. J. Med. Chem. 2007, 50, 1983.

J. M. Cox et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4579–4583
4583
9. Cox, J. M.; Edmondson, S. D.; Harper, B.; Weber, A. E.
PCT Int. Appl. WO 039325, 2006.
19. McDonald, J. K.; Leibach, F. H.; Grindeland, R. E.; Ellis,
S. J. Biol. Chem. 1968, 243, 4143.
10. Moos, W. H.; Gless, R. D.; Rapoport, H. J. Org. Chem.
1981, 46, 5064.
11. Evans, D. A.; Wu, L. D.; Wiener, J. J. M.; Johnson, J. S.;
Ripin, D. H. B.; Tedrow, J. S. J. Org. Chem. 1999, 64, 6411.
12. Halo-heterocycles were commercially available or could be
made readily (see Ref. 8).
13. Leiting, B.; Pryor, K. D.; Wu, J. K.; Marsilio, F.; Patel, R.
A.; Craik, C. S.; Ellman, J. A.; Cummings, R. T.;
Thornberry, N. A. Biochem. J. 2003, 371, 525.
14. Rosenblum, J. S.; Kozarich, J. W. Curr. Opin. Chem. Biol.
2003, 7, 1.
15. Abbot, C. A.; Yu, D. M.; Woollatt, E.; Sutherland, G. R.;
McCaughan, G. W.; Gorrell, M. D. Eur. J. Biochem. 2000,
267, 6140.
16. Olsen, C.; Wagtmann, N. Gene 2002, 299, 185.
17. Scallan, M. J.; Raj, B. K. M.; Calvo, B.; Garin-Chesa, P.;
Sanz-Moncasi, M. P.; Healey, J. H.; Old, L. J.; Rettig, W.
J. Proc. Natl. Acad. Sci. U.S.A. 1994, 91, 5657.
18. (a) Sedo, A.; Malik, R. Biochim. Biophys. Acta 2001, 1550,
107; (b) Rosenblum, J. S.; Kozarich, J. W. Curr. Opin.
Chem. Biol. 2003, 7, 496.
20. Lankas, G.; Leiting, B.; Sinha Roy, R.; Eiermann, G.;
Biftu, T.; Chan, C.-C.; Edmondson, S. D.; Feeney, W.
P.; He, H.; Ippolito, D. E.; Kim, D.; Lyons, K. A.; Ok,
H. O.; Patel, R. A.; Petrov, A. N.; Pryor, K. A.; Qian,
X.; Reigle, L.; Woods, A.; Wu, J. K.; Zaller, D.; Shang,
X.; Zhu, L.; Weber, A. E.; Thornberry, N. A. Diabetes
2005, 54, 2988.
1
21. All final compounds were characterized by H NMR and
LC–MS.
22. IC50 determinations for hERG were carried out as
described in Friesen, R. W.; Ducharme, Y.; Ball, R. G.;
Blouin, M.; Boulet, L.; Cote, B.; Frenette, R.; Girard, M.;
Guay, D.; Huang, Z.; Jones, T. R.; Laliberte, F.; Lynch, J.
J.; Mancini, J.; Martins, E.; Masson, P.; Muise, E.; Pon,
D. J.; Siegel, P. K. S.; Styhler, A.; Tsou, N. N.; Turner, M.
J.; Young, R. N.; Girard, Y. J. Med. Chem. 2003, 46,
2017.
23. IC50 determinations for rabbit calcium channel were
carried out as described in: Schoemaker, H.; Hicks, P.;
Langer, S. J. Cardiovasc. Pharm. 1987, 9, 173.
24. Rendic, S. Drug Metab. Rev. 2002, 34, 83.