The development of benzimidazoles as selective rho kinase inhibitors

Article abstract of DOI:10.1016/j.bmcl.2010.01.124

Rho Kinase (ROCK) is a serine/threonine kinase whose inhibition could prove beneficial in numerous therapeutic areas. We have developed a promising class of ATP-competitive inhibitors based upon a benzimidazole scaffold, which show excellent potency toward ROCK (IC₅₀ <10 nM). This report details the optimization of selectivity for ROCK over other related kinases such as Protein kinase A (PKA).

Full text of DOI:10.1016/j.bmcl.2010.01.124

Bioorganic & Medicinal Chemistry Letters 20 (2010) 1939–1943
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
The development of benzimidazoles as selective rho kinase inhibitors
E. Hampton Sessions, Michael Smolinski, Bo Wang, Bozena Frackowiak, Sarwat Chowdhury, Yan Yin,
Yen Ting Chen, Claudia Ruiz, Li Lin, Jennifer Pocas, Thomas Schröter, Michael D. Cameron, Philip LoGrasso,
*
Yangbo Feng, Thomas D. Bannister
Department of Molecular Therapeutics and Translational Research Institute, The Scripps Research Institute, Scripps Florida, 130 Scripps Way, Jupiter, FL 33458, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Rho Kinase (ROCK) is a serine/threonine kinase whose inhibition could prove beneficial in numerous
therapeutic areas. We have developed a promising class of ATP-competitive inhibitors based upon a
benzimidazole scaffold, which show excellent potency toward ROCK (IC50 <10 nM). This report details
the optimization of selectivity for ROCK over other related kinases such as Protein kinase A (PKA).
Ó 2010 Elsevier Ltd. All rights reserved.
Received 22 September 2009
Revised 21 January 2010
Accepted 27 January 2010
Available online 1 February 2010
Keywords:
Benzimidazole
ROCK
Rho Kinase
Glaucoma
Chroman
Rho Kinase (ROCK) is one of the over 500 known constituents in
the human kinome. This serine/threonine kinase is ubiquitously
expressed throughout vascular tissue and plays an important role
in essential signal transduction pathways.1 Upon activation by
RhoA, ROCK mediates several fundamental cellular processes such
as contraction, migration, adhesion, and cytoskeleton regulation.
As such, the inhibition of ROCK has been hypothesized to be of
potential therapeutic utility for a range of diseases including
stroke,2 hypertension,3 multiple sclerosis,4 asthma,5 erectile
dysfunction,6 glaucoma,7 and central nervous system disorders.8
Currently, the only clinically approved ROCK inhibitor is fasudil,
which is used in Japan for the treatment of cerebral vasospasm.9
Many groups, including ours, have pursued small molecule
inhibitors of ROCK based upon several different scaffolds including
indazoles,10 isoquinolines,11 aminofurazans,12 chromanamides,13
and benzimidazoles.14 Though structurally distinct, these series
all act as ATP-competitive ROCK inhibitors. Close homology of
the ATP binding pockets within the human kinome makes kinase
selectivity a major issue in the development of small molecule
inhibitors. We previously reported that benzimidazole-based
ROCK inhibitors (Fig. 1) have excellent potency in both biochemical
(IC50 <10 nM) and cell-based (IC50 <100 nM) assays.14 Molecular
modeling studies and preliminary SAR indicated that the high po-
tency was largely due to three key interactions within the ROCK15
binding pocket: (i) H-bonding between the pyridyl nitrogen and
M172 of the hinge region, (ii) water-mediated H-bonding of an
imidazole nitrogen and L121, and (iii) hydrophobic interactions be-
tween the aryl moiety and the P-loop. To evaluate their selectivity
for ROCK over other members of the kinome, the potency against
the closely related protein kinase A (PKA) was also determined.16,17
Unfortunately, the initial compounds synthesized proved to be rel-
atively potent (IC50 <500 nM) inhibitors of PKA also. We hoped to
improve upon the selectivity for ROCK over PKA, which could also
provide greater selectivity over less closely related kinases.
In a related aniline-based scaffold (Fig. 2), the addition of a polar
side chain (e.g., N,N-dimethylaminoethoxy in 2) dramatically im-
proved selectivity for ROCK over PKA (472-fold vs 63-fold for the
unsubstituted analog).13 This improved selectivity was attributed
to the interaction of the polar side chain with an aspartic acid res-
idue (D176) in the ROCK binding pocket that is not present in PKA.
* Corresponding author. Tel.: +1 561 228 2206; fax: +1 561 228 3083.
E-mail address: tbannist@scripps.edu (T.D. Bannister).
Figure 1. Previously disclosed ROCK inhibitor 1 (X = CH2, O).
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.01.124

1940
E. H. Sessions et al. / Bioorg. Med. Chem. Lett. 20 (2010) 1939–1943
Scheme 2. General synthesis of chromans 10.
Figure 2. Key modeling interactions of inhibitor 2 with the ATP binding pocket of
ROCK (X = CH2, O).
Modeling suggests that the other key interactions in the aniline-
amide series are similar to those in the benzimidazole series.13
Incorporating an interaction analogous to the amine-D176 H-bond
into the benzimidazole scaffold might similarly improve the selec-
tivity against PKA. This Letter details our ultimately successful
efforts to identify low nanomolar inhibitors of ROCK with affinity
for PKA in the micromolar range.
Scheme 3. General synthesis of amides 12.
Table 1
Benzimidazole substituent effects and PKA selectivitya
The general synthetic route for the benzimidazoles is shown in
Scheme 1. Commercially available 4-bromo-2-fluoronitrobenz-
enes18 3 were converted into phenylenediamines 4 via nucleophilic
aromatic substitution followed by chemoselective reduction of the
nitro group. Peptide coupling with carboxylic acids 5 followed by
an acid catalyzed cyclodehydration formed the benzimidazole het-
erocycles 6. Palladium catalyzed cross-coupling reactions were then
utilizedtoinstalltheheteroaromatic hingebindingmoiety, resulting
in the ROCK inhibitors 7.19
Chroman acids 10 which were not commercially available were
synthesized as shown in Scheme 2. Salicylaldehydes 9 were formed
via an orthoformylation reaction from the corresponding phenol 8.
They were then heated with benzyl acrylate in the presence of
DABCO to give the corresponding chromene derivatives. Reduction
of both the olefin and the benzyl ester occurred upon exposure to
standard hydrogenation conditions, thus giving the chromans 10.20
A series of 6-carboxamide-substituted chromans were synthe-
sized as shown in Scheme 3.21 Access to the methyl ester 11 was
achieved through the standard route (Schemes 1 and 2). Saponifi-
cation of the ester, followed by HATU-mediated amide formation
gave the desired analogs 12.
#
Substitution
X
ROCK
(nM)
PKA
(nM)
PKA/
ROCK
13 None
14 7-Me
15 6-CF3
16 6-CF3
17 6-Me, 7-Me
18 None
O
O
O
O
O
7.5
240
2970
>20,000
>20,000
355
1380
3080
>20,000
>20,000
252
47
6
1
1
1
CH2 1.7
148
19
CH2 440
>20,000
45
20
CH2 183
CH2 4.1
O
O
1590
9
21 4-OMe
22 3-Me
23 1-Me
187
1960
3130
46
131
2
15
1780
24
25
26
O
O
O
24
850
35
5
190
680
1000
8810
13
27
O
130
6120
47
Drawing from our experience in the aniline-amide scaffold (i.e.,
2), it was felt that the addition of a side chain on the benzimidazole
core might lead to an improvement in selectivity for ROCK over
PKA. To test this hypothesis, alkyl or alkoxy groups were systemat-
ically incorporated onto the benzimidazole core of either benzodi-
oxane 13 or chroman 18 (Table 1). As previously noted,14 the
28
29
O
O
11
27
310
509
28
19
a
IC50 values are means of two or more experiments with errors within 143% of
the mean.
chroman variants were consistently more potent than benzodiox-
anes in the ROCK assays, but these analogs showed similar potency
in the PKA assay.22 Incorporation of a substituent at the 6- or 7-po-
sition of the benzimidazole reduced affinity for ROCK (cf. 14–17).
The lack of tolerance for even small groups on these positions
likely indicates that this portion of the inhibitor makes close con-
tact with the enzyme; this would be unexpected if the 6- or 7-posi-
tions projected toward the solvent exposed D176 residue. To allow
for a more direct comparison, two of the polar side chains from the
aniline series that were most effective in boosting selectivity over
PKA were also tested (e.g., 19, 20). Incorporation of these groups
led to significantly less affinity (>100-fold) for both ROCK and
PKA. It should be noted that these compounds also have a 60-meth-
oxy group on the chroman, a modification that generally boosts
ROCK potency and selectivity over PKA (discussed later in this Let-
ter, see Table 2), suggesting that without the 60-methoxy the PKA
Scheme 1. General synthesis of benzimidazoles 7.

E. H. Sessions et al. / Bioorg. Med. Chem. Lett. 20 (2010) 1939–1943
1941
Table 2
Having been unable to improve the selectivity for ROCK over
PKA through modifications of the benzimidazole core, attention
was turned to the chroman to achieve the same objective (Table
2). These explorations were primarily carried out with the amino-
pyrimidine group replacing the pyridine moiety due to the lower
levels of inhibition of cytochrome P450 enzymes observed in inhib-
itors bearing this group.25 An initial, limited screen of substituted
chromans led to the identification of 6-methoxychroman 30 as a
promising lead, with near 300-fold selectivity for PKA. The alter-
nate methoxy regioisomers (cf. 31–33) were found to be both less
potent against ROCK and less selective over PKA. These findings
support our modeling-based hypothesis that the chroman ring is
in close contact with the enzyme at the 7- and 8-positions.14 The
5-position, however, appears to be solvent exposed and therefore
shows greater tolerance toward substitution.
Exploration of chroman substituent effects on PKA selectivitya
#
Substitution
ROCK (nM)
PKA (nM)
PKA/ ROCK
30
31
32
33
34
35
36
37
38
6-OMe
5-OMe
7-OMe
8-OMe
none
6-O/Pr
6-F
6-Br
1.7
25
95
256
8.0
316
4.7
10
485
310
435
232
195
4320
147
82
285
12
4.6
0.9
24
14
31
8.2
17
6-CN
43
713
Having determined that the 6-position was optimal for substit-
uents, several derivatives were synthesized in the search for an im-
proved selectivity for ROCK over PKA. Increasing the size of the
alkoxy substituent led only to a large decrease in the ROCK potency
(e.g. 35). This lack of tolerance for larger alkoxy groups is interest-
ing given the high potency maintained with several other larger
substituents (vide infra, Table 3). Relatively small groups such as
F and Br were tolerated, but unsurprisingly did not improve upon
the selectivity of 30. Nitrile or sulfonamide incorporation resulted
39
40
923
23
40
41
2.7
4.0
691
803
256
200
42
43
1.2
0.8
497
78
414
98
in PKA affinities near 1 lM, however the accompanying increase in
the ROCK IC50 values negated any benefit. Greater success was ulti-
a
IC50 values are means of two or more experiments with errors within 139% of
Table 3
the mean.
Relative PKA selectivity of chroman-6-carboxamidesa
selectivity of analogs 19 and 20 would likely be even lower. The 4-
position of the benzimidazole core was next examined. Derivative
21 showed that a methoxy group was well tolerated, however the
selectivity against PKA was diminished relative to 18 (ꢀ3-fold de-
crease). Larger alkoxy groups might improve the selectivity,
though similar interactions might be achieved through more syn-
thetically-accessible N-3 substituted analogs. Overall no improve-
ment in the desired selectivity was found by adding substituents
on any of the three benzimidazole carbon atoms.
#
R
ROCK (nM)
129
PKA (nM)
1730
PKA/ROCK
13
44
45
46
47
1.1
<1
20
78
82
18
78
Substitutions on the two nitrogen atoms of the benzimidazole
core were next explored. A methyl group at N-1 was poorly toler-
ated,23 while the N-3 methyl derivative was well tolerated, exhib-
iting only a small decrease in affinity for ROCK and a larger
decrease in affinity for PKA.24 Many N-3 substituted analogs were
subsequently synthesized in search of further improvement in the
selectivity over PKA; only a representative sample of those tested
is shown in Table 1. As can be seen, while several substituents
were tolerated by ROCK, in no case was superior selectivity over
PKA achieved. The tolerance of fairly large groups (29, for example)
indicates that groups on N-3 were likely projected into the vacant,
solvent exposed portion of the binding pocket near D176 in ROCK,
however the lack of improved selectivity implies that no important
interaction with D176 was occurring. Thus addition of a polar side
chain to any position on the benzimidazole core, to date, has failed
to yield the same dramatic increase in PKA selectivity that was
seen in the previous aniline series. For example, the morpholino-
ethyl group and related amines confers added PKA selectivity with
no loss of ROCK affinity in the aniline series, while some potency is
lost with benzimidiazoles (see compound 27). This disparity sug-
gests that the precise orientations and conformational preferences
of the benzimidazole and aniline-based ROCK inhibitors are signif-
icantly different in the ATP binding pocket, despite their superficial
similarities.13b
0.78
105
48
49
50
1.7
700
305
442
412
709
0.43
0.25
1768
51
52
53
2.8
1.5
1.2
370
328
272
132
219
227
54
1.2
1530
1275
a
IC50 values are means of two or more experiments with errors within 92% of the
mean.

1942
E. H. Sessions et al. / Bioorg. Med. Chem. Lett. 20 (2010) 1939–1943
Table 4
Stability to human liver microsomes (HLM), in vivo pharmacokinetic data (Rat) and cell-based potencies (ppMLC) for chosen ROCK inhibitorsa
#
t1/2 HLM (min)
CI (mU min/kg)
Vss (L/kg)
t1/2 (h)
AUC po (
lM/h)
Cmax Po (lM)
Oral F%
ppMLC (nM)
30
42
50
53
54
28
79
1.5
100
15
17
29
58
27
19
0.9
1.4
5.9
4.6
0.5
2.3
2.0
3.6
3.0
0.7
1.9
0.90
35
<1
6
<1
0
85
210
250
2700
1200
0.01
0.08
0.01
0.0
0.009
0.005
0.003
0.0
a
Data was generated from three determinations. Dosed at 1.0 mg/kg (iv) Hid or 2.0 mg/kg (po).
mately achieved with 6-keto or 6-carboxamide derivatives. For in-
stance the 6-acetylchroman 40 showed low nanomolar affinity for
ROCK and a PKA selectivity ratio of 256. Primary amide 41 was
similar in potency and selectivity to acetylchroman 40. Secondary
amide 42 had increased ROCK and PKA potency with a selectivity
ratio of over 400. The importance of the carbonyl location was next
evaluated through the testing of homologue 43. This derivative
exhibited sub-nanomolar ROCK potency, but also has very high
affinity for PKA. These results led us to focus our efforts on an in-
depth exploration of the carboxamide-substituted chromans simi-
lar to 42.
achieved through the substitution of the chroman ring with vari-
ous carboxamide groups. These derivatives maintained low- or
sub-nanomolar potency against ROCK with approximately micro-
molar activity against PKA. Poor oral bioavailability precludes the
use of these inhibitors by oral delivery, however they might be
well-suited for topical applications such as for the treatment of
glaucoma. The in vivo evaluation of benzimidazole ROCK inhibitors
as anti-glaucoma agents is the subject of ongoing investigations to
be reported in future publications.
Acknowledgments
A diverse collection of amides was synthesized and assayed
against both ROCK and PKA; a representative sample is shown in
Table 3. Tertiary amides such as 44 were uniformly found to be
much less potent in both assays. Larger cycloalkyl amides such
as 45 possessed excellent ROCK potency, but the PKA potency
was also greatly increased compared to the smaller cyclopropyl
group of 42. Similarly, benzyl amide 46 and its pyridyl analog 47
both were potent ROCK inhibitors but had eroded selectivity ver-
sus PKA. Significant selectivity was found by further extending
the aryl moiety, for example, 48, raising the PKA IC50 to near micro-
molar levels while maintaining low nanomolar ROCK potency. The
corresponding pyridines showed a further 5–7-fold increase in
ROCK potency, with a smaller 2–3-fold increase in PKA potency
leading to excellent selectivity ratios. It is unclear whether or not
the pyridyl nitrogen is making a specific binding site interaction
which leads to the improved selectivity. To further explore the po-
tential for hydrogen bonding interactions within the enzyme’s
binding pocket, polar groups were incorporated and assayed. Smal-
ler polar chains (e.g., 51–53) were found to result in similar ROCK
and PKA affinities to the cyclopropylamide 42. The larger morpho-
lino group in 54, however, showed a remarkable 1300-fold prefer-
ence for ROCK over PKA. As a general trend, the larger 2-
ethylamine derivatives possessed more favorable selectivity pro-
files, with large distal functionality being optimal (e.g. 54).
The drug metabolism and pharmacokinetic (DMPK) properties
of several of the more selective benzimidazole-based inhibitors
were determined as shown in Table 4. The majority of synthesized
inhibitors showed good stability toward human liver microsomes
(HLM); the notable exceptions were the arylethylamide derivatives
such as 50. While the modestly PKA-selective methoxychroman 30
showed good systemic pharmacokinetic properties, the much more
selective amides had very poor oral bioavailability which limits
their usefulness as orally-administered agents. These inhibitors
could prove useful in topical formulations for local administration;
in such instances the poor oral bioavailability might be beneficial
in reducing potential side effects resulting from inadvertent sys-
temic exposure. The cell-based activity26 of these inhibitors was
also determined and found to be significantly lower than the
in vitro binding assay. An increase in the cell-based potency might
be necessary to further evaluate these compounds for topically
treated indications such as glaucoma.27
We would like to thank Professor Patrick Griffin and Professor
William Roush for their support and Dr. Derek Duckett and Ms.
Weimin Chen for the counter screening against p38 and JNK
kinases.
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C.; Lewis, R. L.; Mills, T. M.; Hellstrom, W. J. G.; Kadowitz, P. J. Proc. Natl. Acad.
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7. (a) Tanihara, H.; Inatani, M.; Honjo, M.; Tokushige, H.; Azuma, J.; Araie, M. Arch.
Ophthalmol. 2008, 3, 309; (b) Waki, M.; Yoshida, Y.; Oka, T.; Azuma, M. Curr. Eye
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8. Kubo, T.; Yamaguchi, A.; Iwata, N.; Yamashita, T. Ther. Clin. Risk Manag. 2008, 4,
605.
9. Shibuya, M.; Suzuki, Y.; Sugita, K.; Saito, I.; Sasaki, T.; Takakura, K.; Nagata, I.;
Kikuchi, H.; Takemae, T.; Hidaka, H., et al J. Neurosurg. 1992, 76, 571.
10. (a) Feng, Y.; Cameron, M.; Frackowiak, B.; Griffin, E.; Lin, L.; Ruiz, C.; Schröter,
T.; LoGrasso, P. Biorg. Med. Chem. Lett. 2007, 17, 2355; (b) Goodman, K. B.; Cui,
H.; Dowdell, S. E.; Gaitanopoulos, D. E.; Ivy, R. L.; Sehon, C. A.; Stavenger, R. A.;
Wang, G. Z.; Viet, A. Q.; Xu, W.; Ye, G.; Semus, S. F.; Evans, C.; Fries, H. E.;
Jolivette, L. J.; Kirkpatrick, R. B.; Dul, E.; Khandekar, S. S.; Yi, T.; Jung, D. K.;
Wright, L. L.; Smith, G. K.; Behm, D. J.; Bentley, R.; Doe, C. P.; Hu, E.; Lee, D. J.
Med. Chem. 2007, 50, 6.
11. Iwakubo, M.; Takami, A.; Okada, Y.; Kawata, T.; Tagami, Y.; Sato, M.; Sugiyama,
T.; Fukushima, K.; Taya, S.; Amano, M.; Kaibuchi, K.; Iijima, H. Bioorg. Med.
Chem. 2007, 15, 1022.
12. Stavenger, R. A.; Cui, H.; Dowdell, S. E.; Franz, R. G.; Gaitanopoulos, D. E.;
Goodman, K. B.; Hilfiker, M. A.; Ivy, R. L.; Leber, J. D.; Marino, J. P., Jr.; Oh, H.-Y.;
Viet, A. Q.; Xu, W.; Ye, G.; Zhang, D.; Zhao, Y.; Jolivette, L. J.; Head, M. S.; Semus,
S. F.; Elkins, P. A.; Kirkpatrick, R. B.; Dul, E.; Khandekar, S. S.; Yi, T.; Jung, D. K.;
Wright, L. L.; Smith, G. K.; Behm, D. J.; Doe, C. P.; Bentley, R.; Chen, Z. X.; Hu, E.;
Lee, D. J. Med. Chem. 2007, 50, 2.
13. (a) Chen, Y.; Bannister, T.; Weiser, A.; Griffin, E.; Lin, L.; Ruiz, C.; Cameron, M.;
Schürer, S.; Duckett, D.; Schröter, T.; LoGrasso, P.; Feng, Y. Biorg. Med. Chem.
Lett. 2008, 18, 6406; (b) More detailed modeling studies for several classes of
ROCK inhibitors, including the benzimidazoles, will be reported in the future.
14. Sessions, E.; Yin, Y.; Bannister, T.; Weiser, A.; Griffin, E.; Pocas, J.; Cameron, M.;
Ruiz, C.; Lin, L.; Schürer, S.; Schröter, T.; LoGrasso, P.; Feng, Y. Biorg. Med. Chem.
Lett. 2008, 18, 6390.
15. In this Letter, only the affinity for the ROCK-II isoform is reported. ROCK-I
affinity was monitored as well, but isoform selectivity in these series did not
exceed 10-fold.
In summary, benzimidazole-based ROCK inhibitors with high
potency have been optimized to improve selectivity over the
closely related kinase PKA.17 The enhanced selectivity was

E. H. Sessions et al. / Bioorg. Med. Chem. Lett. 20 (2010) 1939–1943
The 6-cyanochroman was synthesized as follows:
1943
16. ROCK and PKA assays were performed as described in: Schröter, T.; Minond, D.;
Weiser, A.; Dao, C.; Habel, J.; Spicer, T.; Chase, P.; Baillargeon, P.; Scampavia, L.;
Schürer, S. C.; Chung, C.; Mader, C.; Southern, M.; Tsinoremas, N.; LoGrasso, P.;
Hodder, P. J. Biomol. Screen. 2008, 13, 17.
17. In many instances the compounds reported herein were also evaluated versus
a selected panel of other kinases (e.g., MRCKa, AKT, p38, JNK1, JNK3, etc.). PKA
inhibition was the most frequently seen unwanted activity in early leads and
so was used as a surrogate marker for potential broad kinase selectivity. In the
amide series (see Fig. 2 and Ref. 13) enhanced selectivity for PKA was found to
generally correlate with selectivity for many other kinases of concern.
18. 2-Amino-4-bromo-3-methoxy-1-nitrobenzene is not commercially available
and was synthesized according to: Mallory, F.; Wood, C.; Hurwitz, B. J. Org.
Chem. 1964, 29, 2605.
21. The 6-acetylchroman 40 was also synthesized from this route. The Weinreb
amide was made as shown, followed by methyl ketone formation via treatment
with MeMgBr in THF.
22. The corresponding 2-substituted chromans and tetrahydronaphthalenes were
also briefly examined and shown to be less potent and selective compared to
the 3-substituted chromans.
19. (a) Compounds 19 and 20 were made similarly to Scheme 1, however the
starting material 4-bromo 2-6-difluoronitrobenzene was used as shown below.
23. Larger substituents at N-1 were similarly inactive, data not shown. For clarity
in this discussion, the pyridyl-containing site is consistently termed C-5 rather
than renumbering all sites after defining the unsubstituted N as N-1.
24. This promising compound was unfortunately also found to be
inhibitor of the cytochrome P450 enzymes.
a strong
25. 4-Pyridyl-containing inhibitors in this series commonly inhibit one or more of
CYP450 isoforms 2C9, 2D6, 3A4, and 1A2. Aminopyrimidine analogs of these
same compounds are known to lack CYP450 affinity (cf. Table 1, Ref. 14).
26. Schröter, T.; Griffin, E.; Weiser, A.; Feng, Y.; LoGrasso, P. Biochem. Biophys. Res.
Commun. 2008, 374, 356.
(b) Compound 23 was made similarly to Scheme 1, however 4-bromo-1-fluoro-2-
nitrobenzene was used as the starting material.
20. The 6-sulfonamidechroman was synthesized as below:
27. Our earlier work (Ref. 14) showed that fluorine-containing analogs in this
series show significant (even 10-fold) improvement in potency in cell-based
assays. There is also a general trend for enhanced cell-based potency among
analogs with low total polar surface area (tPSA).