Discovery of potent, selective, bioavailable phosphodiesterase 2 (PDE2) inhibitors active in an osteoarthritis pain model, Part I: Transformation of selective pyrazolodiazepinone phosphodiesterase 4 (PDE4) inhibitors into selective PDE2 inhibitors

Article abstract of DOI:10.1016/j.bmcl.2013.03.072

We identified potent, selective PDE2 inhibitors by optimizing residual PDE2 activity in a series of PDE4 inhibitors, while simultaneously minimizing PDE4 activity. These newly designed PDE2 inhibitors bind to the PDE2 enzyme in a cGMP-like mode in contrast to the cAMP-like binding mode found in PDE4. Structure activity relationship studies coupled with an inhibitor bound crystal structure in the active site of the catalytic domain of PDE2 identified structural features required to minimize PDE4 inhibition while simultaneously maximizing PDE2 inhibition.

Full text of DOI:10.1016/j.bmcl.2013.03.072

Bioorganic & Medicinal Chemistry Letters 23 (2013) 3438–3442
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Discovery of potent, selective, bioavailable phosphodiesterase 2
(PDE2) inhibitors active in an osteoarthritis pain model, Part I:
Transformation of selective pyrazolodiazepinone phosphodiesterase
4 (PDE4) inhibitors into selective PDE2 inhibitors
b
a
a
d
Mark S. Plummer a, , Joseph Cornicelli , Howard Roark , Donald J. Skalitzky , Charles J. Stankovic ,
Susan Bove b, Jayvardhan Pandit c, Annise Goodman a, James Hicks a, Aurash Shahripour a, David Beidler b,
Xiao Kang Lu b, Brian Sanchez b, Christopher Whitehead a, Ron Sarver e, Timothy Braden b,
Richard Gowan b, Xi Qiang Shen b, Katherine Welch b, Adam Ogden d, Nalini Sadagopan d,
Heidi Baum d, Howard Miller d, Craig Banotai e, Cindy Spessard e, Sandra Lightle e
⇑
a Department of Chemistry, Pfizer Global Research and Development, Ann Arbor, MI 48105, USA
b Inflammation Pharmacology, Pfizer Global Research and Development, Ann Arbor, MI 48105, USA
c Discovery Technology, Pfizer Global Research and Development, Groton CT 06340, USA
d Pharmacodynamics and Drug Metabolism, Pfizer Global Research and Development, Ann Arbor, MI 48105, USA
e Discovery Technology, Pfizer Global Research and Development, Ann Arbor, MI 48105, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 28 March 2013
We identified potent, selective PDE2 inhibitors by optimizing residual PDE2 activity in a series of PDE4
inhibitors, while simultaneously minimizing PDE4 activity. These newly designed PDE2 inhibitors bind
to the PDE2 enzyme in a cGMP-like mode in contrast to the cAMP-like binding mode found in PDE4.
Structure activity relationship studies coupled with an inhibitor bound crystal structure in the active site
of the catalytic domain of PDE2 identified structural features required to minimize PDE4 inhibition while
simultaneously maximizing PDE2 inhibition.
Keywords:
Phosphodiesterase 2 (PDE2)inhibition
Structure-based drug design
Osteoarthritis(OA)pain
Ó 2013 Elsevier Ltd. All rights reserved.
Despite the number of analgesic medicines and their various
mechanisms of action, inflammatory pain is a condition that has
significant unmet medical need and afflicts a large percentage of
the population. This percentage will increase as the population
ages. The efficacy of current medications is often limited and side
effects can hinder utility.1 Thus, it is important to identify new
medicines with novel mechanisms of action to effectively treat
patients with chronic pain. As part of an indications discovery ef-
fort to identify novel analgesic mechanisms, we observed antinoci-
ceptive activity in a selective phosphodiesterase 2 (PDE2) oxindole
inhibitor 12 (Scheme 1) in an acute model of inflammatory pain.
The role of PDE2 in the production and modulation of pain is
inferred from a number of observations: (1) PDE2 is highly
expressed in the brain, spinal cord, dorsal root ganglia (DRG), as
well as non-pain related tissues including adrenal cortex, heart,
and platelets,3 (2) selective inhibition of PDE2 elevates cGMP in
cells or subcellular compartments where it is located,4 (3) the
non-hydrolyzable cGMP analog, 8-bromo-cGMP, reduces
nociceptive behavior in rats at low doses in formalin-induced
inflammatory pain, but causes hyperalgesia when injected at high-
er doses.5 In contrast to results obtained with oxindole 1, EHNA
(erythro-9-(2-hydroxy-3-nonyl)adenine) 2 (Figure 1), which is a
weak inhibitor of PDE2, was reported to have no effect on nocicep-
tive behavior,5 (4) PDE2 mRNA expression, however, is increased
after formalin injection.6 To establish in vivo proof of concept in
our animal models of OA pain we sought a nanomolar potent orally
bioavailable PDE2 selective (>100Â versus other PDEs) inhibitor
that was non brain penetrant. Low brain penetration would avoid
potential CNS side effects while allowing a PDE2 inhibitor to act
at the DRG. Notably several organizations have actively pursued
selective PDE2 inhibitors and their patent activity is reported in
the Ref. and notes.
PDE2 is unique among the known 11 PDE isozymes as it has
specificity for both cAMP and cGMP while being cGMP stimulated8,
such that increased concentrations of cGMP produce an increased
rate of cAMP hydrolysis. The increase in enzyme activity is attrib-
uted to allosteric interactions initiated by the binding of cGMP to
the GAF B domain of the enzyme.7 Employing the apo crystal struc-
ture of the PDE2 catalytic domain enabled a structure based
⇑
Corresponding author.
E-mail address: marksplummer@gmail.com (M.S. Plummer).
0960-894X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2013.03.072

M. S. Plummer et al. / Bioorg. Med. Chem. Lett. 23 (2013) 3438–3442
3439
O
O
H
H
N
N
O
ONa
O
O
N
OEt
NO2
N
b
OEt
a
OEt
c
OEt
O
EtO
O
7
6
9
OTHP
OTHP
OTHP
HO
O
O
N
N
N
N
e
d
OH
OEt
H2N
.HCl
+
O
NO2
NO2
10
11
12
OTHP
HO
O
N
O
g
N
f
NH
N
N
HN
NO2
N
O
OTHP
N
OTHP
N
13
14
O
CF3
O
O
N
N
O
N
HN
NH2
HN
N
Sn
N
OEt
O
O
NO2
15
13a
8
Scheme 1. (a) NaOEt/EtOH, (72%); (b) N2H4 H2O, EtOH (98%); (c) Cu(NO3)2 , CHCl3/TFAA (90%): (d) resin-PPh3/DIAD/0-RT (83%); (e) NaOH/MeOH/H2O (100%); (f) EDAC HCl/
HOBt/NMM/CH2Cl2 (76%); (g) SnCl22 Â H2O, EtOH/reflux (50%).
O
N
HN
NH2
N
N
N
S
N
N
N
O
N
N
NH
O
OH
O
H3CO
N
OH
OCH3
Oxindole 1
PDE2 IC50 = 40nM
3
EHNA 2
PDE2 IC50 = 800nM
Bay 60-7550
PDE2 IC50 = 4.7nM
Figure 1. Known PDE2 inhibitors: oxindole2, EHNA7, and bay 60–7550.8
OH
O
discovery program. We sought to establish selective PDE2 inhibi-
tion as a mechanism by which to treat inflammatory pain.
Although the PDE2 selective oxindole 1 has many characteristics
of an acceptable pharmacological inhibitor, we required alternative
chemical matter with broader SAR and improved potency. A class
of potent imidazotriazinone PDE2 inhibitors represented by Bay
60-7550 3 show moderate to highly selective (50 to 800-fold)
PDE2 inhibition and were also considered as lead chemical matter.9
In vivo studies with this agent suggest it may have beneficial
effects in cognitive disorders, however, 3 was discounted as a lead
due to reported CNS effects and poor metabolic stability. Mining
Pfizer’s chemical database revealed a series of phosphodiesterase
4 (PDE4) diazepinone inhibitors10 which often had low levels of
PDE2 inhibition (Figure 2). Two diazepinones from the historic
PDE4 program had advanced to become clinical candidates, and
demonstrated excellent pharmacokinetic properties as represented
by 4.
N
O
HN
N
N
HN
N
N
N
HO
O
F
5
4
PDE4 IC50 = 31nM
PDE2 IC50 = 259nM
PDE4/PDE2 = 0.12X
PDE4 IC50 = 1.2nM
PDE2 IC50 = 840nM
PDE4/PDE2 = 0.0015X
>5uM against 9 other PDEs
Rat bioavaiablility 80%
PO T1/2 = 5.0h
IV T1/2 = 1.9h
Figure 2. PDE4 inhibitors possessing residual PDE2 activity as starting points for
the discovery of potent, selective PDE2 inhibitors.

3440
M. S. Plummer et al. / Bioorg. Med. Chem. Lett. 23 (2013) 3438–3442
Tyr329
HO
provided the intermediate trifluoroacetylated nitropyrazole 8,
which was hydrolyzed in situ by rapid stirring with water to give
the desired nitropyrazole 9. Selective N-alkylation of 9 under Mits-
unobu conditions provided exclusively regioisomer, 10, in which
alkylation occurred adjacent to the ester functionality. The use of
resin bound triphenylphosphine in the Mitsunobu reaction simpli-
fied the first chromatographic purification in the reaction sequence.
The purified ester 10 was hydrolyzed to acid 11 and was coupled
with the aminoketone hydrochloride 12, giving the amide 13. The
crude nitroamide 13 was then subjected to a one-pot tin(II) chloride
reduction and cyclization in refluxing ethanol providing the target
diazepinone 14 in about 20% overall yield from diethyl oxalate.
Noteworthy is that the tin(II) chloride reduction and cyclization is
absolutely critical to the successful synthesis of the diazepinone ring
as reduction by Pd/C or Raney nickel provided only nitro reduction.
Cyclization of the amine 13a from these catalytic reductions was
attempted without success under a variety of dehydrating condi-
tions. Interestingly, the amine intermediate when treated with tin
chloride did undergo cyclization suggesting that the aza-tin ylide
15 might be a key intermediate.
Preliminary SAR derived from the diazepinone analogs synthe-
sized in the historic PDE4 project indicated that hydrogen bond-
accepting 4-carboxyphenyl and pyridine groups were preferred
aryl groups for PDE4 inhibition, while electron rich rings such as
4-methoxyphenyl favored PDE2 (Figure 4). Branched alkyl groups
were preferred for PDE2 at the pyrazole 3-position, while smaller
groups lead to enhanced PDE4 inhibition. 1-Ethyl pyrazole substi-
tution was greatly preferred for PDE4 activity whereas larger or
more polar groups like hydroxyethyl increased PDE2 inhibition,
though the range of groups at the pyrazole N-1 position in the
historic data was limited.
To begin transforming diazepinones from PDE4 selective inhib-
itors into a PDE2-selective series, we expanded upon the trends
seen in the historical compounds. We desired to maintain drug-like
attributes of 4 but also wanted to increase hydrogen bond donor
count and polar surface area recognizing the need for peripherally
restricted inhibitors. Thus, we targeted for synthesis compounds
where the aryl group was varied and favored PDE2 inhibition in
combination with either an N1 hydroxyethyl or hydroxypropyl
pyrazole moiety providing compounds 16–21 (Table 1).
Clear SAR trends emerged from this first set of compounds,
giving rise to diazepinones that were selective for PDE2 over
PDE4. The hydroxyethyl compounds 16–18 exhibited increased
inhibition of PDE2 over the hydroxypropyl analogs 19–21 (Table 1).
H
O
H
Asn321
O
Gln859
H
H
Gln369
N
NH2
H
O
O
H
N
H
O
N
N
O
N
H
N
H
H
N
N
N
N
N
O
H2N
OH
OH
O
Tyr827
O
P
O
O-
O-
O
P
O
O
O
PDE4, cAMP Specific
PDE2 binding to cGMP
Figure 3. Modeled hydrogen bonding interactions of cAMP with PDE4, and cGMP
with PDE2 showing the switch in orientation of the conserved glutamine.
Structure activity relationships (SAR) in the PDE4 diazepinone
series indicated that good initial potency for PDE2 existed as
shown by 5 (Figure 2). Additionally, PDE4 diazepinone inhibitors
had been crystallized in the PDE4 catalytic domain and provided
a strong basis for a future structure-enabled project once PDE2
cocrystals were obtained. Early demonstration of good PDE2 selec-
tivity especially over the PDE4 isoforms was of importance to avoid
severe emesis seen with PDE4 inhibition.
Since PDE4 specifically hydrolyzes cAMP, while PDE2 hydro-
lyzes both cAMP and cGMP, we postulated that the diazepinone-
based inhibitors might bind in a unique mode to PDE2 relative to
their binding mode to all of the PDE4 subtypes. After extensive
modeling of diazepinone-based inhibitors in the active site of the
PDE2 apo catalytic domain, we predicted that diazepinone inhibi-
tors would bind to PDE2 in a cGMP-like binding mode, orthogonal
to that found in the PDE4 subtypes due to alternative orientations
of a key glutamine residue (Gln 859 in PDE2), referred to as the
‘glutamine switch’ (Figure 3).11 This offered a strong basis for
decreasing the undesired PDE4 activity while increasing PDE2
inhibition.
Compounds in this series were synthesized as shown in
Scheme 1. 3-Ethyl heptanone was condensed with diethyl oxalate
in ethanol to give the sodium salt 6 which was neutralized with
aqueous hydrochloric acid and treated with hydrazine hydrate at
0 °C to provide pyrazole 7. Pyrazole 7 was nitrated in chloroform
with copper(II) nitrate and trifluoroacetic anhydride. This procedure
HO
R1
=
>>
R1
O
N
NH
R3
N
R2
=
~
~
>>
R2
N
R3 =
>
>
>>
~
N
H3CO
OCH3
CO2H
OCH3
N
N
Fig. 4. PDE2 SAR from existing PDE4 inhibitors rank ordered by PDE2 inhibition.

M. S. Plummer et al. / Bioorg. Med. Chem. Lett. 23 (2013) 3438–3442
3441
Table 1
Diazepinone analogs targeting PDE2 activity
OMe
OMe
Ar=
N
N
HO
O
Compound
16
54
0.32
538
11
17
23
18
102
0.38
76
0.7
20
PDE2 IC50 (nM)
Ligand efficiency
PDE4 IC50 (nM)
Selectivity(PDE4/PDE2)
HLM t1/2 (min)
cLogP
0.36
48
2
23
2.4
NH
N
N
N
5
2.9
Ar
2.7
HO
Compound
PDE2 IC50 (nM)
19
328
0.28
10800
33
6
3.2
20
93
0.32
365
4
44
2.7
21
44
0.33
1390
3
36
3.0
O
Ligand efficiency
PDE4 IC50 (nM)
Selectivity(PDE4/PDE2)
HLM t1/2 (min)
cLogP
NH
N
N
N
Ar
Table 2
4-Phenylimidazole SAR
Notably the hydroxypropyl analogs were somewhat more selec-
tive. This selectivity was accomplished primarily through poorer
PDE4 inhibition rather than increased PDE2 inhibition. The most
selective compound 19 has modest 33-fold PDE2 selectivity and
submicromolar inhibition demonstrating a good reversal of selec-
tivity of 275-fold compared with starting PDE4 inhibitor 5. All of
the compounds, however, showed unfavorable metabolic stability,
with the phenylimidazole group of 16 and 19 correlating with the
poorest human liver microsome (HLM) stability. Nevertheless, the
ligand efficiency of this initial set of compounds was generally
greater than 0.30, the widely regarded lower limit for good lead
matter.12
Several analogs of imidazole 16 were synthesized to determine
whether sterically encumbering the imidazole group or blocking
the predicted metabolically labile carbon in the 3-pentyl append-
age of the pyrazole would improve metabolic stability or PDE2
selectivity (Table 2).
We attempted to improve metabolic stability by blocking the
metabolically labile benzylic position in 16, as in 22 to no avail.
Neither dimethyl nor isopropyl substitution on the imidazole, as
in 23 and 24, affected metabolic stability, potency or selectivity.
These observations led us to explore the more metabolically stable
3,4-dimethoxyphenyl ring in 17 and 20 even though these com-
pounds had poor selectivity. Additionally, changing the branching
of the pyrazole 3-alkyl appendage in the imidazole series offered
no advantage; we opted for the tertiary butyl group as in 24 since
this group has the lowest molecular weight. Although the phenyl-
imidazole diazepinones did not provide a path forward, they dem-
onstrated modest, reliable PDE2 selectivity and demonstrated that
the PDE4 selective starting compound could be made PDE2 selec-
tive with small substituent changes. Therefore, we crystallized 22
in the PDE2 catalytic domain and determined the X-ray structure
of the inhibitor-protein complex at 1.7 Å resolution, PBP ID 4JIB.
As predicted, the diazepinone PDE2 inhibitors bind to the cata-
lytic domain in a cGMP-like binding mode shown by the orienta-
tion of Gln859 (Figure 5). The hydroxyethyl side chain points to
the interior of the binding pocket, with a hydrogen bond interac-
tion between the hydroxyl group of the inhibitor and Gln812. In
PDE4, the residue corresponding to Gln812 is a proline which can-
not form such a hydrogen bond. This additional interaction and the
size of the surrounding pocket offer a point of differentiation, when
compared to PDE4 protein and the PDE4 selective diazepinones
that bind orthogonally, that contributes to the enhanced PDE2
HO
O
NH
N
N
N
R1
R2
23
Compound
R1
16
22
24
H
R2
N
N
N
N
N
N
N
N
PDE2 IC50 (nM)
PDE4B IC50 (nM)
Selectivity
HLM t1/2 (min)
cLogP
54
45
378
8
105
1950
19
9
3.1
97
330
3
12
3.2
583
11
5
7
2.9
2.8
selectivity when hydroxyl side chains are employed. The crystal
structure also reveals several hydrophobic residues adjacent to
the phenyl group of the phenylimidazole moiety at the entrance
to the binding pocket, suggesting an additional area to further opti-
mize PDE2 inhibition (see Part 2).
In summary, SAR studies of a series of pyrazolodiazepinone
inhibitors selective for PDE4, having residual PDE2 activity, re-
sulted in the discovery of a series of potent pyrazolodiazepinone
inhibitors. The inhibitors possess modest selectivity and potency
for PDE2 but represent a 275-fold reversal in PDE4 selectivity in fa-
vor of PDE2 selectivity. These SAR studies were based upon the
hypothesis that PDE2 selectivity could be achieved with pyrazolo-
diazepinone inhibitors by a cGMP-like binding mode which is
orthogonal to that of a cAMP binding mode seen for this family
of inhibitors in PDE4. A crystal structure of a PDE2 selective inhib-
itor bound to the active site of the PDE2 catalytic domain is consis-
tent with this hypothesis. Although the prototype inhibitor, 22,
possessed poor HLM stability it provided structural clues from
which to build inhibitors with increased potency, selectivity and
metabolic stability. Our efforts to improve the potency, selectivity,
and metabolic stability of these inhibitors and to examine their

3442
M. S. Plummer et al. / Bioorg. Med. Chem. Lett. 23 (2013) 3438–3442
Figure 5. Crystal structure, PBP ID 4JIB, of 22 (yellow) at 1.7 Å resolution in the PDE2 catalytic domain (green) with waters removed for clarity. The conserved glutamine
(Gln859), and the residues that define the ‘hydrophobic clamp’ (Phe862, Phe830, Ile 826) are represented as blue sticks. The two metal ions Zn (yellow) and Mg (green) are
represented as spheres. Hydrogen bonding to Gln859 clearly demonstrates cGMP like orientation. An additional hydrogen bond is observed between the hydroxyethyl side
chain on the inhibitor with Gln812 in the back of the binding pocket (not shown).
effects in a pain model of osteoarthritis are described in an accom-
panying publication.
PDE2 Patent activity
(a). Bourguignon, J. -J.; Lugnier, C.; Abarghaz, M.; Lagouge, Y.; Wagner, P.;
Mondadori, C.; Macher, J. -P.; Schultz, D.; Raboisson, P. WO 2004041258 A2
20040521; PCT Int. Appl. 2004.
References and notes
(b). Abarghaz, M.; Biondi, S.; Duranton, J.; Limanton, E.; Mondadori, C.; Wagner, P.
EP 1548011 A1 20050629, Eur. Pat. Appl. 2005.
(c). Schmidt, B.; Weinbrenner, S.; Flockerzi, D.; Kuelzer, R.; Tenor, H.; Kley, H. -P.
WO 2006024640 A2 20060309, PCT Int. Appl. 2006.
(d). Schmidt, B.; Weinbrenner, S.; Flockerzi, D.; Kuelzer, R.; Tenor, H.; Kley, H. -P.
WO 2006072615 A2 20060713, PCT Int. Appl. 2006.
(e). De Leon, Pablo; Egbertson, Melissa; Hills, Ivory D.; Johnson, Adam Wayne;
Machacek, Michelle. WO 2011011312 A1 20110127, PCT Int. Appl. 2011.
(f). Helal, Christopher John; Chappie, Thomas Allen; Humphrey, John Michael;
Verhoest, Patrick Robert; Yang, Eddie. US 20120214791 A1 20120823, U.S. Pat.
Appl. Publ. 2012.
1. Childers, W. E.; Gilbert, A. M.; Kennedy, J. D.; Whiteside, G. T. Expert Opin. Ther.
Pat. 2008, 18, 1027.
2. Chambers, R. J.; Abrams, K.; Garceau, N. Y.; Kamath, A. V.; Manley, C. M.; Lilley,
S. C.; Otte, D. A.; Scott, D. O.; Sheils, A. L.; Tess, D. A.; Vellekoop, A. S.; Zhang, Y.;
Lam, K. T. Bioorg. Med. Chem. Lett. 2006, 16, 307.
3. Gupta, R.; Kumar, G.; Kumar, R. S. Methods Find Exp. Clin. Pharmacol. 2005, 27,
101.
4. de Vente, J.; Markerink-van Ittersum, M.; Vles, J. S. H. J. Chem. Neuroanat. 2006,
31, 263.
5. Tegeder, I.; Schmidtko, A.; Niederberger, E.; Ruth, P.; Geisslinger, G. Neurosci.
Lett. 2002, 332, 146.
6. Shi, X.; Li, X.; Clark, J. D. Mol. Pain 2005, 1, 33.
(g). Helal, Christopher J.; Chappie, Thomas Allen; Humphrey, John M. WO
2012168817 A1 20121213, PCT Int. Appl. 2012.
7. Podzuweit, T.; Nennstiel, P.; Muller, A. Cell. Signalling 1995, 7, 733.
8. Martinez, S. E.; Wu, A. Y.; Glavas, N. A.; Tang, X.-B.; Turley, S.; Hol, W. G.; Beavo,
J. A. PNAS 2002, 99, 13260.
9. Boess, F. G.; Hendrix, M.; Van der Staay, F.-J.; Erb, C.; Schreiber, R.; van Stavern,
W.; de Vente, J.; Prickaerts, J.; Blokland, A.; Koenig, G. Neuropharmacology 2004,
47, 1081.
10. Burnouf, C.; Berecibar, A.; Navet, M. PCT Int. Appl. 2001, 34, 211.
WO2001049689.
11. Zhang, K. Y. J.; Card, G. L.; Suzuki, Y.; Artis, D. R.; Fong, D.; Gillette, S.; Hsieh, D.;
Neiman, J.; West, B. L.; Zhang, C.; Milburn, M. V.; Kim, S.-H.; Schlessinger, J.;
Bollag, G. Mol. Cell 2004, 15, 279.
(h). Niewohner, Ulrich; Schaub, Dagmar; Hendrix, Martin; Konig, Gerhard; Bos,
Frank-Gerhard; Van Der Staay, Franz-Josef; Schreiber, Rudy; Schlemmer,
Karl-Heinz; Grosser, Rolf. PCT WO 02/50078 A1, 2002.
(i). Tenor, Hermann; Baer, Thomas; Hatzelmann, Armin; WO 2004/089953 A1.
(j). Niewohner, Ulrich; Bischoff, Erwin; Hutter, Joachim; Perzborn, Elisabeth;
Schutz, Helmut. US 5,861,396, 1999.
(k). Haning, Helmut; Niewohner, Ulrich; Rosentreter, Ulrich; Schenke, Thomas;
Keldenich, Jorg; Bischoff, Erwin; Schlemmer, Karl-Heinz; Schutz, Helmuth;
Thomas, Gunter. U.S. 6.174,884 B1, 2001.
(l). Boss, Frank-Gerhard; Hendrix, Martin; Konig, Gerhard; Niewohner, Ulrich;
Schlemmer, Karl-Heinz; Schreiber, Rudy; Van Der Staay, Franz-Josef; Schauss,
Dagmar. Pub. No.: US 2002/0132754 A1, 2002.
12. Hopkins, A. L.; Groom, C. R.; Alex, A. Ligand efficiency: a useful metric for lead
selection Drug Disc. Today 2004, 9(10), 430.