Discovery of novel pyridyl carboxamides as potent CCR5 antagonists and optimization of their pharmacokinetic profile in rats

Article abstract of DOI:10.1016/j.bmcl.2011.08.080

A novel series of pyridyl carboxamide-based CCR5 inhibitors was designed, synthesized, and demonstrated to be highly potent against HIV-1 infection in both HOS and PBL assays. Attempts to evaluate this series of compounds in a rat PK model revealed its instability in rat plasma. A hypothesis for this liability was proposed, and strategies to overcome this issue were pursued, leading to discovery of highly potent 40 and 41, which featured dramatically improved rat PK profiles. 2011 Elsevier Ltd. All rights reserved.

Full text of DOI:10.1016/j.bmcl.2011.08.080

Bioorganic & Medicinal Chemistry Letters 21 (2011) 6470–6475
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Discovery of novel pyridyl carboxamides as potent CCR5 antagonists
and optimization of their pharmacokinetic profile in rats
Maosheng Duan a, , Wieslaw M. Kazmierski a, Pek Y. Chong a, Felix DeAnda a, Mark Edelstein a, Rob Ferris a,
⇑
Jennifer Peckham a, Pat Wheelan a, Zhiping Xiong a, Huichang Zhang a, Rena Nishizawa b,
Yoshikazu Takaoka b
a Infectious Disease Medicine Discovery & Development, GlaxoSmithKline, Five Moore Drive, Research Triangle Park, NC 27706, USA
b Medicinal Chemistry Research Laboratory, Ono Pharmaceutical Co., Ltd, Shimamoto, Mishima, Osaka 618-8585, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel series of pyridyl carboxamide-based CCR5 inhibitors was designed, synthesized, and demon-
strated to be highly potent against HIV-1 infection in both HOS and PBL assays. Attempts to evaluate this
series of compounds in a rat PK model revealed its instability in rat plasma. A hypothesis for this liability
was proposed, and strategies to overcome this issue were pursued, leading to discovery of highly potent
40 and 41, which featured dramatically improved rat PK profiles.
Received 29 May 2011
Revised 15 August 2011
Accepted 17 August 2011
Available online 23 August 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Pyridyl carboxamide
CCR5
Antagonist
Pharmacokinetics
HIV-1
Chemokine
World Health Organization (WHO) estimated that by the end of
2009, 33.4 million people worldwide lived with HIV. 2.7 Million
became newly infected patients in 2008, and 2 million people died
of AIDS related illness in the same year.1 HIV/AIDS continues to be
a threat to global public health. Current anti-HIV medicines, target-
ing the HIV virus itself, serve as the first-line treatment and have
dramatically increased the life expectancy of patients and signifi-
cantly reduced morbidity, however, mutant resistance and patient
compliance due to drug side effects are serious issues. The chemo-
kine receptor R5 (CCR5) has been identified as a major co-receptor
for M-tropic human immunodeficiency virus (HIV) entry into host
cells and plays an essential role in the early stage of HIV infec-
tions.2 Therefore, CCR5 antagonists have been intensively pursued
as anti-HIV agents.3 Although several small molecules have been
advanced into clinical developments and Maraviroc became the
first marketed CCR5 inhibitor,4 novel CCR5 antagonists with im-
proved PK and safety profiles are still needed to expand the usage
of this class of antiretrovirals.
been previously described.7 In brief, the HOS assay is a reporter-
based assay in which human osteosarcoma cells expressing human
CCR5 and human CD4 and containing an HIV LTR-luciferase repor-
ter construct are infected with R5-tropic strains of HIV (e.g., Ba-L).
Infectious events are indicated by activation of the luciferase re-
porter after 4 days in culture. The PBL assay is an HIV infection as-
say in which peripheral blood lymphocytes are inoculated with
HIV-1 for 7 days followed by the collection and measurement of
progeny virus particles in the cell-free supernatants. The tropane
benzimidazole analogue 1 exhibited potent antiviral activity in
both HOS and PBL cell assays against the Ba-L strain of HIV-1,
but its development was terminated due to toxicity concerns. Sub-
stantial modifications of the tropane benzoimidazole scaffold led
to the discovery of novel p-cyanobenzyl urea derivative 2, which
also displayed good antiviral and PK profiles. During the course
of our work, scientists at ONO Pharmaceuticals disclosed a related
series of CCR5 antagonists in their patent publication.8 Interest-
ingly, the ONO CCR5 molecules contained a urea moiety similar
to that in 2 and in order to further expand the urea SAR, we em-
barked on a mix-and-match strategy. m-Fluorophenyl/phenyl (ring
D) was retained for DMPK purpose while introduction of the het-
ero-aryl pyridine ring (ring E) with cyano (CN) or carboxamide
substituents in ring E could reduce the overall hydrophobicity.
We initially synthesized the prototype cyano compound 5 and
amide 6 using chemistry similar to that in Scheme 1 (Fig. 2). As
expected, 5 was potent in HOS assay (IC50 = 44 nM), but quite
We previously reported our discovery of several series of CCR5
antagonists, exemplified by tropane benzimidazole analogue (1)5
and p-cyanobenzyl urea derivative (2)6 (Fig. 1). Our SAR was funda-
mentally guided by two cell based antiviral assays which have
⇑
Corresponding author.
E-mail address: duanmaosheng@hdbiosciences.com (M. Duan).
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.08.080

M. Duan et al. / Bioorg. Med. Chem. Lett. 21 (2011) 6470–6475
6471
R'
R
N
O
N
O
F
O
Y
O
O
S
HO
N
N
N
O
N
F
X
N
H
R"
H
N
Cl
N
O
ONO series
H
N
N
H
F
N
N
H
CN
F
HOS IC50 4.3nM; PBL IC50 3.5nM
HOS IC50 8 nM; PBL IC50 8.1nM
2
1
O
R'
R'
X
X
CN
R
R
N
NH2
N
O
N
O
N
A
B
C
E
A
B
C
E
N
N
W
N
N
O
W
O
H
H
Y
Y
D
D
F(H)
F(H)
4
3
Figure 1. Design of the pyridyl carboxamide series.
unexpectedly, amide 6 was 4 times more potent than 5 (HOS
IC50 = 10 nM). We thus focused our efforts on the carboxamide
moiety.
Our optimization began with the modulation of ring A by differ-
ent substituents as depicted in Scheme 1. Thus, commercially
available 4-Boc-piperidone was coupled with 3-fluoroaniline by
an efficient reductive amination to yield aniline 7. Meanwhile,
5-amino-2-pyridinecarbonitrile was reacted with excess phosgene
in presence of pyridine to generate isocyante 8. The subsequent
reaction of 7 and 8 at room temperature overnight provided urea
9. After the oxidative hydrolysis of CN in 9 with urea hydrogen per-
oxide (UHP), the resulting carboxamide was de-protected to afford
key intermediate 10. Preparation of aldehyde 14 started from phe-
nol 119 and 6-chloropyridine 12.10 The 6-chloro substituent in 12
Boc
N
NH2
Boc
N
N
+
O
a
N
CN
2HCl
Boc
N
O
N
O
HN
O
NH2
F
c
N
N
d
F
H
N
N
7
H
N
CN
N
CN
b
F
F
H2N
N
9
10
O
8
O
H
H
N
H
N
CN
e
N
S
f
CN
S
N
H
N
+
S
N
O
O
O
O
O
O
Cl
OH
O
O
X
X
X
13
11
12
14
O
H
N
g
N
S
N
O
NH2
N
O
O
X=
H
15a
Me 15b
N
N
O
H
X
Cl
F
15c
15d
F
15a-d
Scheme 1. Reagents and conditions: (a) NaBH(OAc)3, HOAc, CH2Cl2, rt, 91%; (b) phosgene, pyridine, 0 °C to rt, 24 h; (c) 7 and 8, CH2Cl2, 24 h, 70% over (b) and (c); (d) (i) UHP,
K2CO3, acetone/water rt, 24 h, 99%, (ii) 4 M HCl in doxane, quant.; (e) Cs2CO3, DMF, 80 °C, overnight, 80–90%; (f) Dibal-H, 0 °C, CH2Cl2, 80%; (g) NaBH(OAc)3, Et3N, CH2Cl2, rt,
overnight, 60–75%.

6472
M. Duan et al. / Bioorg. Med. Chem. Lett. 21 (2011) 6470–6475
H
O
H
N
N
CN
N
S
N
N
O
N
S
N
O
NH2
N
O
O
O
O
N
N
O
N
N
H
O
H
5
6
Figure 2. Prototype cyano compound 5 and amide 6.
O
H
H
O
H
N
N
N
S
S
OEt
N
b
OH
N
+
a
S
OEt
N
O
O
O
O
O
O
N
Cl
OH
N
O
N
O
19
17
16
18
O
H
N
N
H
c
S
H
N
O
NH2
N
d
N
O
O
S
O
N
N
N
N
O
O
O
H
N
O
F
20
21
Scheme 2. Reagents and conditions: (a) Cs2CO3, DMF, 80 °C, overnight, 66%; (b) Dibal-H, À78–0 °C, CH2Cl2, 42%; (c) MnO2, CH2Cl2, rt, 42%; (d) 10, NaBH(OAc)3, Et3N, CH2Cl2,
rt, overnight, 30%.
was displaced at elevated temperature with the phenoxide gener-
ated in situ from 11, yielding the biaryl ether 13. The cyano group
in 13 was then subjected to Dibal-H reduction to give aldehyde 14.
With both precursors 10 and 14 in hand, final compounds 15a–d
were assembled by a standard reductive alkylation protocol with
NaBH(OAc)3.
We next focused our efforts on ring B with the aim of attenuat-
ing the basicity of the aryl nitrogen atom and optimizing the alkyl
substituent. To this end, 2-chloropyrimidine 17, prepared accord-
ing to the literature procedure,11 was condensed with phenol 16
in a similar way to one described in the synthesis 13, thus, afford-
ing biaryl ether 18. Subsequent Dibal-H reduction of ethyl ester
group of 18, followed by a MnO2 oxidation of the resultant primary
alcohol, yielded pyrimidine aldehyde 20 whose next reductive
animation with 10 furnished 21 (Scheme 2).
next dehydration-chlorination with POCl3 set the stage for Suzuki
coupling. After the resulting c-Pr derivative 24 was converted into
aldehyde 25, des-F urea 10, synthesized in identical fashion to 10,
was reductively alkylated with 25 to provide 26.
The synthesized compounds were tested for their antiviral
activity in both HOS (n = 2) and PBL cells (n P 4) against the Ba-L
strain of HIV-1.7 Compound induced cellular toxicity was also
investigated using CellTiter-Glo reagent (Promega). None of the
compounds induced cell death up to 1 lM (data not shown) indi-
cating that the observed potency is the result of an antiviral effect
and not toxicity (Table 1). To our satisfaction, these compounds
demonstrated high antiviral potencies. Additional substitution on
ring A had little effect on potency (15a vs 15b–d). Replacement
of ring B pyridine with a pyrimidine gave similar result in com-
pound 21. Installation of cPr instead of Me in ring B (26) resulted
in a slight loss of potency in PBL assay presumably due to missing
m-F in ring D.
With their excellent antiviral potencies, this class of the com-
pounds was subsequently advanced to pharmacokinetic evalua-
tions in Sprague Dawley rats at 1 mg/kg using 5% mannitol with
0.05% acetic acid as dosing vehicle. Surprisingly, there was little
HOS and PBL data for 6 and 15a revealed that the methyl group
at position 2 of ring B enhanced antiviral activity substantially
though m-F in 15a may also have its effect on potency. Hence,
we attempted to introduce larger groups, such as c-Pr (Scheme
3). Oxidation of 6-chloro-3-pyridinecarbonitrile under UHP condi-
tion gave pyridine N-oxide 22. Mild biaryl ether formation and
Cl
H
CN
H
CN
O
e
N
N
f, g
CN
N
CN
h
N
S
S
N
N
O
O
O
O
Cl
Cl
O
O
22
23
24
O
H
H
i
N
N
H
N
j
S
N
O
NH2
N
O
O
S
O
N
O
O
N
N
O
H
O
25
26
Scheme 3. Reagents and conditions: (e) UHP, TFAA, CH2Cl2, 0 °C to rt, 82%; (f) 16, Cs2CO3, DMF, 45 °C, 4 h, 90%; (g) POCl3, reflux, 1 h, 86%; (h) c-PrB(OH)2, PEPPSI™-IPr, K3PO4,
toluene, 100 °C, 20 h, 71%; (i) Dibal-H, 0 °C, CH2Cl2, 94%; (j) des-F 10, NaBH(OAc)3, Et3N, CH2Cl2, rt.

M. Duan et al. / Bioorg. Med. Chem. Lett. 21 (2011) 6470–6475
6473
Table 1
Preparation of 2-amido-3-methyl-pyridiyl urea 34 began with a
nitro reduction of commercially available 2-bromo-3-methyl-5-
nitropyridine. Urea 32 was next obtained from the resulting amine
31 and 5, following a similar protocol described in Scheme 1. After
the installation of the cyano moiety using Zn(CN)2/Pd, the resultant
2-cyano-3 methyl pyridyl 33 was then converted into 34 by a
two-step sequence similar to that employed towards 30 (Scheme
5).
Antiviral Potency of 15a–d, 21, and 26 in HOS and PBL cell assays
Compounds
HOSa IC50 (nM)
PBLb IC50 (nM)
15a
15b
15c
15d
21
1.0
1.0
1.2
1.2
2.4
1.0
0.8
0.6
1.1
0.5
0.9
2.2
26
Synthesis of 2-amido-3-fluoro-pyridyl urea 39 proved to be a
challenge (Scheme 6). The nitration of the pyridine N-oxide 35 ob-
tained from an oxidation of 2-cyano-3 fluoropyridine under UHP
condition was highly troublesome and gave the desired nitro com-
pound 36 in only 9% yield under our best conditions. Reduction of
the newly introduced nitro and deoxygenation of pyridine-N-oxide
were achieved in one pot with Fe/HOAc at elevated temperature.
The resulting 5-amino-3-fluoro-2-pyridinecarbonitrile 37 was then
progressed to 39 using the same chemistry as described in Scheme
5. With all ring E modified urea moiety in hand, final compounds
40, 41, and 42 were synthesized by reductive amination with
aldehyde 14 (Fig. 4).
The additional methylation on ring E in compounds 40 and 41
led to a modest potency loss (ꢀ4-fold), while the fluorinated 42
was equipotent to parent 15a (Table 3). These compounds were
thus advanced to rat plasma stability screening. We were very
pleased to find that 40 and 41 demonstrated a substantial
improvement in the rat plasma stability despite that the observed
percentage increase for 41 over the time may suggest a slow disso-
lution of this compound. On the contrary, 42 exhibited a profile
similar to 15a (Table 2). We concluded that while the 6-methyl
moiety was likely to provide the direct steric hindence to enzymes;
the 3-methyl moiety was likely to exert its action by disrupting the
stability of planarity of complex a, Figure 4. On the other hand,
introduction of F in position 3 of ring E restored the five-mem-
bered-ring bident chelating environment due to the weak intramo-
lecular hydrogen bonding or electrostatic interaction between F
and the proton of the amide, hence, resulting in a quick hydrolysis
of the amide.14 Compounds 40, 41, and 42 were further evaluated
in rat PK models. Compound 40 had an excellent PK profile in rat
with low clearance and high bioavailability (49%). Compound 41
was also bioavailable though its Cl was slightly higher (Table 4).
Compound 42 exposure from either iv or p.o. dosing could not be
measured due to its instability in rat plasma.
a
IC50 values for the HOS assay represent the concentration of inhibitor that
prevents 50% of virus entry and subsequent activation of an HIV-LTR-Luciferase
reporter.12
b
IC50 values for the PBL assay represent the concentration of inhibitor that
prevents 50% of virus replication.12
Table 2
Stability of 15a, 40, 41, and 42 in Sprague Dawley rat plasma
Time (min)
Percent remaining (%)
15a
40
41
42
0
5
100
85
67
44
25
11
3
100
94
91
95
84
74
71
100
90
88
100
86
71
42
26
12
4
10
20
30
45
60
94
120
123
134
or no exposure for all these compounds in either iv or p.o. doses.
Detailed in vitro study indicated that the amide moiety on ring E
was rapidly converted into the corresponding carboxylic acid in
rat plasma (Table 2, 15a), a liability for the entire class of com-
pounds. One possible explanation of this unexpected amide liabil-
ity is that both the carbonyl oxygen of the amide group and the
pyridine nitrogen12 may chelate metal ions present in biological
milieu and form a quasi-planar five-membered ring (Fig. 3a). Such
chelation would lead to activation of the amide moiety and
accelerated amide hydrolysis in presence of plasma enzymes.13
To further test this hypothesis and to enable the future rat toxicity
study, we designed analogues that either introduce steric hin-
drance adjacent to the pyridine N (Fig. 3b), attempted to disrupt
the presumed co-planarity between the carbonyl oxygen and pyr-
idine nitrogen through steric interference of neighboring group
(Fig. 3c), or installed a putative intra-molecular H-bond (or electro-
static interaction) to restore the chelating environment (Fig. 3d).
Syntheses of these proposed molecules started with the prepara-
tions of the individual urea portions as shown in Scheme 4. Follow-
ing bis-Boc protection of 2-methyl-3-aminopyridine, an oxidation
with UHP was carried out, yielding pyridine N-oxide 27. Treatment
of 27 with TMSCN in presence of triethylamine installed the cyano
group at position 6. Upon removal of bis-Boc, the resulting 2-
methyl-3-amino-6-cyanopyridine dihydrochloride 28 was sub-
jected to urea formation. Subsequent oxidative hydrolysis of the
resulting 29 followed by a routine Boc deprotection to furnish 30.
In summary, a novel class of pyridyl carboxamides as potent
CCR5 antagonists (15a–d) was discovered. This class of the com-
pounds demonstrated high antiviral activity in both HOS and PBL
assays, but its exposure in rat PK could not be evaluated due to a
rapid metabolic hydrolysis of its amide. To resolve this issue, we
pursued several chemical modifications. While it is unclear
whether these analogues are stable in plasma because of hindrance
to deaminases or due to destabilization of putative five-member
chelate, which would render the amide highly prone to hydrolysis,
resulting in potent inhibitors 40 and 41 and successfully resolving
the amide instability issue in rat plasma. This allowed us to further
progress compounds 40 and 41 into pre-clinical development.
M 2+
O
O
O
O
N
N
N
H
H2N
O
N
N
H2N
O
N
N
H
O
N
H2N
H3C
O
N
N
H
N
N
H
N
F
N
H
N
N
H
N
F
F
F
F
c
b
d
a
Figure 3. Metal ion chelating hypothesis (a) and the proposals to disrupt/restore such chelating environments (b, c, and d).

6474
M. Duan et al. / Bioorg. Med. Chem. Lett. 21 (2011) 6470–6475
O
Boc
f, g
NC
N
2HCl
NH
O
N
O
N
N
NC
N
N
H2N
O
e
a,b
c, d
N(Boc)2
N
F
N
N
F
N
NH2
2HCl
NH2
29
28
27
30
Scheme 4. Reagents and conditions: (a) Boc2O, CH2Cl2, 40 °C, 3 h, 38%; (b) m-CPBA, CH2Cl2, overnight; (c) TMSCN, Et3N, acetonitrile, reflux, 60%; (d) 4 M HCl in dioxane,
quant.; (e) phosgene, pyridine, CH2Cl2; then 7, 80%; (f) UHP, K2CO3 acetone/water, rt, overnight, 90%; (g) 4 M HCl in dioxane, quant.
O
NC
N
Boc
k, l
N
Br
N
Boc
Br
N
O
N
H2N
O
NH
2HCl
Br
N
N
O
h
i
j
N
F
N
N
N
N
F
N
NH2
NO2
F
31
32
33
34
Scheme 5. Reagents and conditions: (h) Fe, HOAc/H2O, 40 °C, 1 h, quant.; (i) phosgene, pyridine, CH2Cl2; then, 5, 40 °C, overnight, 87%; (j) Zn(CN)2, Pd2(dba)3, DPPF, DMF,
95 °C, 48 h, 70%; (k) UHP, K2CO3 acetone/water, rt, overnight, 70%; (l) 4 M HCl in dioxane, quant.
O
N
O
O
NH
H2N
O
NC
F
N
N+
NC
F
N
NC
F
N
NC
F
N+
NBoc
q, r
O
p
NC
F
o
n
m
N
F
N
2HCl
H
N
N
NH2
NO2
H
39
F
38
35
36
37
F
Scheme 6. Reagents and conditions: (m) UHP, TFAA, CH2Cl2, 0 °C to rt, 74%; (n) Bu4NNO3, TFAA, CH2Cl2, 0 °C, 9%; (o) Fe, HOAc, 40 °C, 32%; (p) phosgene, pyridine, CH2Cl2; then
5, 99%; (q) UHP, K2CO3 acetone/water, rt, overnight; (r) 4 M HCl in dioxane, 79% over two steps.
O
H
N
N
O
H2N
O
N
N
S
O
N
H
N
O
F
40
O
O
F
H
N
O
N
H
N
O
H2N
N
O
N
S
N
H2N
O
N
N
S
O
O
N
N
O
H
N
H
N
O
F
F
41
42
Figure 4. Final compounds of modification on ring E.
Table 4
Pharmacokinetics of 40 and 41 in rat at 1 mg/kga
Table 3
Antiviral potency of 40, 41, and 42 in HOS and PBL cell assays
Compounds
T1/2 (h)
Clearance (mL/min/kg)
F%
Compounds
HOS IC50 (nM)
PBL IC50 (nM)
40
41
2.8
2.0
11.7
37
49
30
40
41
42
3.6
1.9
1.2
3.9
5.3
1.1
a
Sprague Dawley rats at 1 mg/kg using 5% mannitol with 0.05% acetic acid as
dose vehicle.

M. Duan et al. / Bioorg. Med. Chem. Lett. 21 (2011) 6470–6475
6475
8. Kusuda, S.; Nishiyama, T.; Hashimura, K.; Ueda, J.; Shibayama, S. PCT Int. Appl.
(2007), WO 2007105637.
9. Preparation of 11: see J. Med. Chem. 1999, 42, 1041.
Further work and data for these compounds will be published in
due time.
10. Preparation of 12: see Synthesis, 1991, 894.
11. Preparation of 17: see J. Med. Chem. 2000, 43, 3995.
12. To probe how the pyridyl nitrogen assists the hydrolysis, compound 43 was
synthesized. This compound demonstrated 53% of bioavailability in rat with
decent clearance (Cl = 15 mL/min/kg) and half life (t1/2 = 3.9 h):
Reference and notes
1. http://www.who.int/hiv/data/2009_global_summary.png.
2. Dragic, T.; Litwin, V.; Allaway, P. G.; Martin, R. S.; Huang, Y.; Nagashima, A. K.;
Cayanan, C.; Maddaon, J. P.; Koup, A. R.; Moore, P. J.; et al. Nature 1996, 381, 667.
3. See a review: Kazmierski, W. M.; Gudmundsson, K. S.; Piscitelli, S. C. Annu. Rep.
Med. Chem. 2007, 42, 301.
4. Price, D. A.; Armour, D.; De Groot, M.; Leishman, D.; Napier, C.; Perros, M.;
Stamman, B. L.; Wood, A. Bioorg, Med. Chem. Lett. 2006, 16, 4633.
5. Kazmierski, W. M.; Anderson, D. L.; Aquino, C.; Chauder, B. A.; Duan, M.; Ferris,
R.; Kenakin, T.; Kobe, C. S.; Lang, D. G.; Micintyre, M. S.; Peckham, J.; Watson, C.;
Wheelan, P.; Spalstenstein, A.; Wire, M. B.; Svolto, A.; Youngman, M. J. Med.
Chem. 2011 (accepted).
O
H
F
N
O
H2N
O
N
N
S
O
F
N
H
N
O
F
6. (a) Duan, M.; Peckham, J.; Edelstein, M.; Ferris, R.; Kazmierski, W. M.;
Spaltenstein, A.; Wheelan, P.; Xiong, Z. Bioorg. Med. Chem. Lett. 2010, 20,
7397; (b) Duan, M.; Peckham, J.; Edelstein, M.; Ferris, R.; Kazmierski, W. M.;
Spaltenstein, A.; Wheelan, P.; Xiong, Z. Bioorg. Med. Chem. Lett. 2010, 20, 7401.
7. Muniz-Medina, V.; Jones, S.; Maglich, J.; Galardi, C.; Hollinggswirth, R.;
Kazmierski, W.; Ferris, R.; Edelstein, M.; Chiswell, K.; Kenakin, T. Mol.
Pharmacol. 2009, 75, 490.
43
13. The enzymes could be metallo-dependent hydrolases, such as deaminase,
protease, etc.
14. We have to concede that F of ring E in 42 may be not large enough to disrupt
the putative five-membered complex and is just like a H in 15a.