446
Antitumor Steroids
3. The compounds should not produce a
rapid loss of binding ability of their
steroidal vector.
4. The concentration of receptors should
be sufFciently high to produce an ef-
fective cellular uptake of the cytotoxic
moiety. This concentration should be
maintained until this cytotoxic moiety
has reached its target, potential degra-
dation of the receptor (downregulation)
induced by the steroidal vector should
be limited.
The following paragraphs analyze how
such conditions could be satisFed for
(anti)estrogenic conjugates.
3.1
Main Structural Requirements for Estradiol
Binding
The main and unique characteristic of E
2
is its lack of binding afFnity for steroid
hormone receptors other than its own
receptor. The phenolic group in C
3
,no
t
recorded in the other steroid hormone,
generates this big difference. Binding data
of E
2
and derivatives led to the following
conclusions.
OH
HO
AB
CD
2
1
4
3
10
5
6
7
8
14
15
16
17
18
13
12
11
9
E
2
3.1.1
A-ring
C
1
is
intolerant
of
both
hydrophobic
and polar substituents, suggesting close
contacts
with
the
HBD.
The
hydroxy
group in C
3
donates a proton to Glu353
and Arg394 (see above), and this bond
is
donated
syn
to
the
C
2
–C
3
bond.
C
4
may
be
in
close
proximity
to
a
cysteine residue, based on inactivation
assays
of
the
receptor
by
4-mercuri-
E
2
. The A-ring electron cloud may be
engaged
in
a
weak
polar
interaction
with a slightly positively charged recep-
tor residue, situated on the
β
-face of
the steroid.
3.1.2
B-ring
C
6
seems
to
be
intolerant
for
both
polar and nonpolar substituents, once
again suggesting close interaction of this
area
to
the
receptor
essential
spatial
volume. Large aliphatic chains at C
7
α
are well tolerated (i.e. pure antiestrogens
ICI
164,384
and
ICI
182,780;
±ig. 5),
they
localize
within
a
subsite
of
the
HBD accepting 11
β
nonpolar substituents
(rotation of the E
2
core around a virtual
symmetry axis that runs roughly through
C
3
and
C
17
)
.
Positions
in
C
7
β
and
C
8
α
are unexplored and C
9
α
has low
steric tolerance.
3.1.3
C-ring
A
preformed
hydrophobic
subsite
of
the HBD has a high steric tolerance
for 11
β
residues. It can accommodate
aromatic substituents bearing aliphatic
side chains (pure antiestrogen RU 58,668;
±ig. 5) while still retaining high afFn-
ity; it is intolerant of polar groups. The
covalent binding of tamoxifen aziridine
has suggested an accessible receptor nu-
cleophile in C
11
β
vicinity; such a hy-
pothesis was rejected since 11
β
-methyl
aziridine
estradiol
does
not
covalently
label
ER.
Moreover,
11
β
-chloromethyl
and
bromoacetyl
analogs
also
fail
to
covalently label the receptor. The 11
β
-
chloromethyl substituent of E
2
produces,
however, a
quasi
-irreversible binding of
the hormone with the receptor. Interac-
tion of the chloride atom with a putative
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