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Z. Gao et al. / Bioorg. Med. Chem. Lett. xxx (2013) xxx–xxx
Previous work12,13 from our group described a series of H3R
antagonists/inverse agonists, represented by 5-fluoro-2-methyl-
N-[2-methyl-4-(2-methyl-[1, phenyl]
30]bipyrrolidinyl-10-yl)
H3C
O
CH3
N
N
H
benzamide (1), that displayed oral efficacy in a mouse food intake
inhibition model. In our on-going program aimed at the discovery
of a ‘best in class’ H3R antagonist/inverse agonist, that is potent
with lower species discrepancy; high selectivity towards a panel
of GPCRs, ion channels, enzymes and kinases, particularly biogenic
amine receptors; desirable PK profile suitable for qd dosing in
human; and acceptable neuropsychological, behavioral, and car-
diovascular safety in experimental animal models; and superior
hERG channel selectivity. Besides these specific requirements, we
set up an additional criterion of a lower risk of potential phospho-
lipidosis induction, one of the common issues of H3R antagonists/
inverse agonists reported in the literature.14,15 Herein, we describe
the optimization sequence leading to 5a(SS) (SAR110068).
We took the multipronged strategy in which the H3R affinities
and calculated physico-chemical properties, such as molecular
weight, number of hydrogen bond donors and acceptors, number
of rotatable bonds, clogP, and polar surface area (calculated using
ACD/Labs methods) were considered in a ballanced manner. The
present work was the continuation of optimization effort from
the previous series to only solve the hERG channel selectivity issue.
The plan was not to modify the Me-pyrrolidine or its vicinity but
only the amide moiety. Therefore, the pKa, polar surface area, and
molecule weight were slightly different or the same among the li-
gands we discussed herein. As said, the optimization was mainly
driven by H3R affinity and clogP.
From previous SAR, it was evident that the left side aromatic
ring with small lipophilic substituents was required for good H3R
affinity. Unfortunately, lipophilic substituents in the aromatic ring
also adversely increased hERG channel affinity and phospholipido-
sis liability. Levoin et al.16 reported a QSAR approach in which the
authors concluded that lipophilic character of the molecules (the
sum of atomic polarizabilities, clogP, clogD), as well as aromatic
tendency had the greatest influence over hERG affinity.
Phospholipidosis is a storage disorder resulting in excessive
accumulation of phospholipids in lysosomes of the tissues. The
cause is not well defined. However, the amphiphilic type of mole-
cules display high risk for induction of phospholipidosis. In order to
increase our odds to identify an ideal molecule with the lowest risk
of phospholipidosis induction potential, we chose to use the in sil-
ico phospholipidosis model.17,18 Given the fact that H3R is a bio-
genic amine receptor and our current lead compound possessed
a basic amine, it was hypothesized that increasing the polarity
(lowering clogP), and reduce the aromaticity, should be the most
direct approach to bring the calculated values into a more desirable
range. To this end, it was envisioned that replacing substituted aryl
moiety with cyclic-non-aromatic residues in the terminal end of
the amide (Fig. 1) would achieve the objective mentioned above.
Consequently, a new series of cyclic-non-aromatic amide 2 was
designed.
N
H3C
F
1
h-H3R binding Ki = 8.6 nM;
rh-H3R binding Ki = 1.2 nM;
r-H3R binding Ki = 16.5 nM;
rh-H3R GTPγS: EC50 = 1.1 nM
hERG IC50 = 0.48 uM
H3C
O
N
N
H
Cyclic-
Non-Ar
N
H3C
2
Figure 1. Structure of the lead 1 and the optimization strategy.
moiety. The compound was active with a Ki of 9.0 nM. Encouraged
by this data, we decided to explore further on cyclic-non-aromatic
carboxamides. A decrease of structural flexibility by introducing a
bridge in the cyclohexyl ring (2c) did not enhance the affinity. In-
stead, the affinity decreased by twofold compared to 2a, indicating
that steric bulkness was not well tolerated in this region of the li-
gand. This observation was echoed by the fact that compound 2f
was equipotent to 2a, while 2j displayed a fourfold decrease in
affinity. However, extension of the cycloalkyl by a methylene lin-
ker is tolerated as exemplified by 2m. Another trend of the SAR
was that the analogs of R1 = 20-Me were more potent than analogs
of R1 = 30-Me (2c vs 2d; 2f vs 2h; 2j vs 2k; 2m vs 2n), while the
analogs of R1 = H were comparable in H3R affinity with that of
R1 = 20-Me. This trend was consistent with the previous SAR stud-
ied with the aryl amide series in which the 20-methyl was impor-
tant in enhancing activity at H3R and also improving the
metabolic stability. The enhancing effect of the 20-Me might attri-
bute to its ability of maintaining the preferred confirmation of the
ligand to better fit the contour of the H3R receptor as hypothesized
previously.12
The clogP (data see Table 1) was not in the desired range of 1.5–
2.5 for CNS penetration for most of the compounds in Table 1. Fur-
ther optimization was necessary.
In order to decrease clogP while maintain H3R affinity for this
series of the compounds, the strategy would be to make the small-
est structural modification possible. To achieve this objective,
introduction of heteroatoms into the left side cycloalkyl moiety
was proposed. Thus, compound 4a (Table 2) was synthesized and
tested. The affinity was, fortunately, comparable with the cyclic
carboxamide 2a. The analog 4d was comparable with 4a in H3R
affinity, while 4g was an eightfold less potent than 4a (Table 2).
Similarly, the analogs of R1 = 20-Me were more potent than analogs
of R1 = 30-Me (4a vs 4b; 4d vs 4e). However, 4g was comparable
with 4h in this case; while the analogs of R1 = H were comparable
with that of R1 = 20-Me (4c vs 4d).
The syntheses of the analogs 2a–2n are described in Scheme 1.
Aniline 3 (R1 = H, 20-CH3, 20-CF3, and 30-CH3)12,13 was coupled with
proper acids or acid chlorides to obtain the desired compounds in
high yield (75–82%).
Reagents and conditions: (a) RCOCl, CH2Cl2, pyridine, rt, 16 h,
62–88% yield; or RCO2H, CH2Cl2, DMF, EDCꢀHCl, HOBt, N-methyl-
morpholine, rt, overnight, 75–82% yield.
Compounds (2a–2n) were then evaluated in an H3R binding
assay13 by displacement of [3H]N-
a-methylhistamine in mem-
branes isolated from a CHO cell line stably transfected with the
rhesus monkey H3 receptors (rh-H3R) (Table 1).
The first compound synthesized and tested was 2a, a cyclohex-
anecarboxylic amide instead of substituted phenyl carboxylic
amide, to ensure similar relative size of the ligand’s terminal
We then turned our attention towards the stereochemistry ef-
fect on H3R affinity in this series. In this endeavor, we narrowed
down to the analogs where R1 = 20-Me because of its superior
H3R affinity and metabolic benefits as mentioned above. Thus
two stereoisomers, 5a(SS) and 5a(RS), corresponding to the more
Gao, Zhongli
