S. Kim et al. / Bioorg. Med. Chem. Lett. 14 (2004) 2741–2745
2743
prudent to explore the effect of the addition of substit-
uents, R1. As can be seen in Table 1, the presence of a
methyl group (compound 2) was tolerated. However, the
simple extension to an ethyl group (compound 3) not
only diminished the affinity to both receptors, but also
lowered the selectivity over ERb. Conversely, a signifi-
cant increase in binding activity (ERa ¼ 0.5 nM) was
observed upon introduction of an electronegative sub-
stituent, such as fluorine and chlorine (compounds 5 and
6); albeit, with reduced selectivity (ca. 3–10-fold). A
plausible rationale for the latter maybe, that a reduction
in the electron density on sulfur may, in turn, reduce the
level of the electrostatic repulsion with the Met 366
residue in ERb and thereby allow for a greater affinity to
it. Interestingly, this remarkable binding affinity to ERa
only correlated with the in vivo antagonist/agonist
activity profile of compound 5, as evidenced by the
comparison of the uterine weight data (89% inhibition
and ꢀ2.0% agonism for compound 5 versus 68% inhi-
bition and 12% agonism for compound 6). A similar
trend was observed for the C-6 hydroxylated derivatives
12 and 13.
Sprague–Dawley rats, and the results are depicted in
Table 2. Noteworthy are the two sets of structurally
close derivatives, which primarily differ in the position
of the phenolicoxygen group but exhibit extremely
contrasting oral bioavailabilities: F ¼ 31% and 22% for
compounds 11 and 12, respectively, as compared to
F ¼ 0%, 6%, and 4.4% for compounds 1, 2, and 5,
respectively. Further, as previously reported by us,1b
only the [2S,3R] enantiomer, 11D, reproduced the
activities exhibited by the racemate (11D vs 11L).
Likewise compound 12D was found to possess good oral
bioavailability (F ¼ 37%). It is of interest to note that
similar observations were denoted in the pharmacoki-
netics of similarly substituted cis-tetrahydronaphtha-
lenes and in particular lasofoxifen,5 in which the
dramatically improved oral bioavailability was attrib-
uted to a reduction in intestinal wall, enantioselective
glucuronidation. Certainly, the structural model pro-
posed by Rosati et al.,5 for the resistance to gut wall
glucuronidation, which featured nonplanar topology
and axial/equatorial disposition of the pendant aryl
groups, can be superimposed on the dihydrobenzo-
xathiin class. However, it also would appear that the
topological issues effecting glucuronidation are even
more subtle since the remarkable difference between the
C-6 and C-7 phenolicdihydrobenzoxathiins transcends
the scope of this model.
On the other hand, substituents R4 led to comparable
binding activities to 1 and 11, but significantly impacted
the ability to inhibit the estradiol stimulated prolifera-
tion in the uterus (29% inhibition for 9 and 0% inhibi-
tion for 17). This suggested that subtle conformational
differences of the ligand–receptor complex resulting
from a minor change in the ligand may alter the nature
of the interactions with the transcriptional machinery in
the cellular assay and thus the in vivo antagonism in the
uterine weight gain assay.4 In the case of compound 4,
there appears to be an inconsistency in the correlation
between the binding activity and anti-uterotropic activ-
ity. Thus, compound 4, with weak binding activity
(ERa ¼ 25 nM) and modest alpha-selectivity, exhibited
an equal level of antagonism in the uterine weight gain
assay (64% inhibition) as compared to 1. Alternative
substituent arrangements, as with compounds 7, 8, 14,
and 15, were without improvement.
A recent report from this laboratory disclosed that the
40-hydroxyphenyl group at C-3 of the dihydro-
benzoxathiin 11 was superior to alkyl, cycloalkyl, and
heterocyclic replacements with respect to alpha-selec-
tivity and in vivo efficacy.1c The results of extended
studies at fine-tuning the C-3 aryl pendant group are
described in Table 3. The incorporation of the electro-
negative substituent fluorine (20) maintained the binding
activity, as compared to 11, while the presence of the p-
OMe functionality in 19 led to a significant decrease in
both potency and alpha-selectivity (ERa ¼ 41 nM).
However, compound 20 failed to inhibit the estradiol
mediated proliferation of the uterus, which paralleled
the poor activity observed in the functional assay (HEK-
293 ERa ¼ 94 nM). Noticeably, the ER binding activity/
alpha-selectivity was regained by the introduction of a
Having potent SERAMs in hand, the pharmacokinetic
profile of selected compounds was assessed in female
Table 2. Pharmacokinetic data for selected compounds
Compound
C2;3
Binding affinitya
ERa ERb
Pharmacokinetic parametersb
Uterine weight assay
(po)e % inhibition/
% control @ 1 mpk
F (%)
T1=2 (h)
Clp (mL/min/kg)
11
11D
11L
12
12D
1
Æcisc
3.0
0.8
23.0
0.8
1.0
1.6
2.0
0.5
143 (n ¼ 5) 31
45 (n ¼ 36) 62
3.8
3.4
1.6
2.4
3.1
2.2
2.7
5.9
5
10
77/5.0f
99/9.0
16/1.0f
76/7.0f
100/1.0
ND
[2S,3R]c
[2R,3S]c
Æcis
287
13.2
18
10
22
37
0
31
16
[2S,3R]d
Æcis
18
44.4
46.2
13
154
51
2
Æcis
6
4.4
ND
ND
5
Æcis
41
a IC50 (nM), see Table 1.
b In female, Sprague–Dawley rats following intravenous dosing at 1 mpk (n ¼ 2) and oral dosing at 2 mpk (n ¼ 3).
c See Ref. [1b]; Absolute stereochemistry of 11D was determined by X-ray crystallography.
d
½a þ285.8 (c 0.875) in MeOH; The absolute stereochemistry of 12D was assigned based on analogy with 11D and biological data.
D
e See Table 1.
f Dosed sc at 1 mpk, see Table 1.