Novel rhenium complexes derived from α-tropanol as potential ligands for the dopamine transporter

Article abstract of DOI:10.1016/S0968-0896(98)00144-8

A series of rhenium complexes was synthesized as model compounds for the corresponding radioactive technetium-99m complexes for preliminary biological investigations. In a '3+1' mixed-ligand approach the tropanol molecule was linked with the metal core through a ω-mercaptoester group as monodentate ligand. Bis(thioethyl)sulphide was used as the tridentate dithiol ligand to block the remaining free coordination sites. The ω-mercaptoesters were synthesized via the trityl-protected precursors. Binding tests on the cloned human dopamine transporter revealed moderate binding affinities for some of the prepared compounds. The complexes were characterised by X-ray and the lipophilicity as well as pK(a) values were determined. Copyright (C) 1998 Elsevier Science Ltd.

Full text of DOI:10.1016/S0968-0896(98)00144-8

BIOORGANIC &
MEDICINAL
CHEMISTRY
Bioorganic & Medicinal Chemistry 6 (1998) 1663±1672
Novel Rhenium Complexes Derived from ꢀ-Tropanol as
Potential Ligands for the Dopamine Transporter
Alexander Hoepping,a,* Peter Brust,a Ralf Berger,a Peter Leibnitz,b
Hartmut Spies,a Susanne Machill,c Dieter Schellerc and Bernd Johannsena
aForschungszentrum Rossendorf, Institut fur Bioanorganische und Radiopharmazeutische Chemie, POB 51 01 19, D-01314 Dresden
È
bBundesanstalt fur Materialforschung, D-12489 Berlin
È
cTechnische Universitat Dresden, Institut fur Analytische Chemie, D-01062 Dresden
È
È
Received 13 January 1998; accepted 23 March 1998
AbstractÐA series of rhenium complexes was synthesized as model compounds for the corresponding radioactive
technetium-99m complexes for preliminary biological investigations. In a `3+1' mixed-ligand approach the tropanol
molecule was linked with the metal core through a o-mercaptoester group as monodentate ligand. Bis(thioethyl)sul-
phide was used as the tridentate dithiol ligand to block the remaining free coordination sites. The o-mercaptoesters
were synthesized via the trityl-protected precursors. Binding tests on the cloned human dopamine transporter revealed
moderate binding anities for some of the prepared compounds. The complexes were characterised by X-ray and the
lipophilicity as well as pKa values were determined. # 1998 Elsevier Science Ltd. All rights reserved.
Introduction
parameters like size and lipophilicity. Therefore, rhe-
nium compounds serve as ideal model compounds for
the analogous technetium-99m complexes.
Since the readily available radionuclide technetium-99m
is the preferred radionuclide for nuclear medical diag-
nosis, there is great interest in the development of novel
imaging agents based on this nuclide. Currently over
85% of clinical nuclear medicine procedures use tech-
netium-99m because of its ideal gamma emission of
140 keV and its good availability by the Mo-99/Tc-99m
generator. Since the both most common nuclides of
technetium, technetium-99m and technetium-99 are
radioactive the handling a€ords great care and is sub-
jected to some restrictions. The chemistry of the third-
row congener rhenium is well investigated and o€ers the
advantage of work with `cold' isotopes. The chemical
behaviour of rhenium and technetium is nearly iden-
tical. In particular both elements form complexes with
the same coordination geometry. This results in similar
Our investigations aimed at the development of new
technetium compounds that speci®cally bind to the
dopamine transporter and we spend our e€ort on rhe-
nium complexes as predecessors in investigations that
will ®nally be extended to technetium.
Dopaminergic
neurotransmission,
among
other
mechanisms, depends on the ecient removal of the
transmitter from the synaptic cleft by a re-uptake
mechanism into the dopaminergic neuron. There is
considerable interest in this uptake system because of
profound e€ects of drugs such as cocaine that inhibit
this process. Moreover, protection against the action of
the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydro-
pyridine (MPTP), which elicits syndromes similar to
Parkinson's disease (PD), can be achieved with selective
dopamine uptake inhibitors.1 This fact contributed to
the development of current hypotheses regarding the
aetiology of PD, which are focused on the potential
contributions of environmental toxins.2
Key words: Dopamine transporter; rhenium; technetium;
SPECT ; a-tropanol.
*Correspondence sauthor. Tel.: +49 351 2603694; Fax: +49
351 2603232; E-mail: hoepping@fz-rossendorf.de
0968-0896/98/$Ðsee front matter # 1998 Elsevier Science Ltd. All rights reserved
PII: S0968-0896(98)00144-8

1664
A. Hoepping et al./Bioorg. Med. Chem. 6 (1998) 1663±1672
In the past 10 years, emission tomography has con-
siderably contributed to the understanding of parkin-
sonism.3 Several cocaine analogues have been recently
synthesized as potent dopamine transporter (DAT)
ligands for positron emission tomography (PET)4±10
and single photon emission computed tomography
(SPECT).11±14 123I]b-CIT [N-methyl-2b-carbomethoxy-
[
3b-(4-iodophenyl) tropane] has been successfully used
for SPECT imaging of DAT in the living human
brain.15 However the ligand also displays considerable
anity towards the serotonin transporter (5-HTT).16 b-
CFT [N-methyl-2b-carbomethoxy-3b-(4-¯uorophenyl)-
tropane] (or WIN 35,428) has a higher selectivity and
was recently labeled with 99mTc.14 Speci®c accumulation
in the striatum as the target tissue was shown. Even
though substituents in 2-position seem to be necessary
for high anity, there are also ligands without sub-
stituents in 2-position such as benztropine analogues.17
Scheme 1. Synthesis of o-mercaptoesters of tropane. Z: a=0,
b=CH2, c=C2H4, d=C3H6, e=C4H8, f=1,3-phenyl, g=1,4-
phenyl.
plexes were obtained by reaction of the tridentate dithiol
ligand with tetrachlorooxorhenate, providing a pre-
formed metal unit22 with a remaining chloro atom,
which is substituted by the monothiol bearing the tro-
pane ring. An intriguing advantage is the lack of stereo-
isomers in relation to the chelate unit, which obviates
the need for lengthy purifcation procedures. This would
make potential high anity compounds particularly
useful for clinical use.
As we linked the tropane molecule with the metal core
by an ester chain the required thiol group was intro-
duced into the o-position of the acid moiety (Scheme 1).
The most striking advantage of the benztropines is their
selectivity towards the DAT against 5-HTT and the
norepinephrine transporter. On the other hand, they
show a very high anity to the muscarinic m1 receptor.
It has also been suggested that the benztropines may
interact on the DAT at a di€erent binding site than
cocaine.17 Further examples show the possibility of
accommodating bulky p-electron-rich substituents in 3-
position without a decrease in anity.18 Therefore, we
decided to mimic one or both of the phenyl rings in the
benztropines with the p-electron-rich chelate moiety.19
The synthesis of tropane esters is the key step in the
whole reaction sequence. The best results were attained
by mixing equimolar amounts of tropanol hydro-
chloride and the appropriate o-halogen-substituted acid
chloride at elevated temperatures.23 The yields are very
high and no epimerisation could be observed, as estab-
lished by 1H NMR spectra. The pseudo-triplett found
around 5 ppm is typical for the b-orientation of the
proton in 3-position in contrast to a multiplett usually
found for its a-orientation.24 Also in the following steps
no additional peak for this proton suggesting an epi-
merisation could be found. Finally, the X-ray structure
prove this results to have been correct. However, we
observed an unexpected e€ect: the partial exchange of
the o-bromo atom of the haloester by chlorine. This was
found by the chemical shifts in the 1H NMR spectra.
Results and Discussion
Chemistry
The `3+1' mixed-ligand concept20,21 is a very con-
venient way to prepare a large number of compounds
for a primary screening. `3+1' mixed-ligand complexes
are easily accessible by combining appropriate tri-
dentate and monodentate ligands. `3+1' complexes,
especially, formed by use of bis(thioethyl)sulphide as the
tridentate ligand are very stable. Accordingly, the com-
1
Thus, we observed four triplets in the H NMR spectra
of 3-halopropionic acid trop-3-yl ester. Two of the tri-
plets have an intensity of about 66% each and the other
two 33% each. By separate preparation of the 3-chloro

A. Hoepping et al./Bioorg. Med. Chem. 6 (1998) 1663±1672
1665
Scheme 2. Synthesis of the complexes.
propionic acid trop-3-yl ester according to the same
procedure the two triplets with the higher intensity were
assigned to this compound. That means that the amount
of the bromo compound was approximately 33%. This
fact was observed with all aliphatic bromoesters. Ele-
mental analyses indicate a 1/1 ratio of the two halogens,
so the salts are mainly hydrobromides (66%). The
halogen exchange is probably caused by elimination of
HBr and subsequent addition of HCl in an anti-Mar-
kovnikov manner or by a simple SN2 reaction. Separa-
tion of the two compounds by simple recrystallization
proved impossible but was actually unnecessary because
further reactions are not a€ected by this fact. Therefore,
no attempts were made to study the mechanism of the
halogen exchange. In the further reaction steps a mix-
ture of the bromo and chloro esters was used.
column chromatography and the purity determined by
Elmann's test.
Complexation was performed according to a typical
`3+1' procedure by reacting the mercaptane with 3-
thiapentane-1,5-dithiolato-oxorhenium (V) [ReOSSSCl]
in acetonitrile until a red colour appeared22 (Scheme 2).
Flash chromatography yields the pure complexes which
were crystallized after being allowed to stand by slow
evaporation of the solvent. Some of the complexes were
crystallized as oxalates or hydrochlorides. Complex 6f
crystallized in such a manner yielding suitable crystals
for X-ray analysis (Fig. 1). X-ray crystallography dis-
plays the expected square pyramidal structure of the
complex unit and the 3a-orientation of the ester group
at the tropane ring.
The introduction of the mercapto group was achieved
by reacting the o-halogen substituted tropane esters
with triphenyl methane thiol and potassium tert-but-
oxide in DMF at temperatures between 50 and 60 ꢀC.
Flash chromatography gave the S-protected tropane
mercapto esters in yields up to 80% as waxy or glassy
solids which were dicult to purify. The deprotection of
the mercapto group worked best with mercury acetate in
tri¯uoroacetic acid and subsequent reaction with
hydrogen sulphide. The mercaptans were puri®ed by
An important aspect for all CNS-imaging agents is the
brain uptake. In the case of passive transport brain
uptake is in¯uenced by such parameters as lipophilicity
or pKa values. In order to ®nd correlations between
structure and brain uptake for all new complexes devel-
oped by our working group the DHPLC, PHPLC and
pKa(c) values were routinously estimated by HPLC
methods. These values were derived from the retention
times according to the equation log P=a log k0+b,
where k0=(tR t0)/t0 represents the capacity factor, tR is
the retention time of the sample and t0 the retention
time of methanol as an unretained solute. With a given
ionizable compound
results. From the turning point of this curve the appar-
ent pKa value (pKHPLC was derived. detailed
a sigmoidal DHPLC/pH curve
)
A
Table 1. Selected bond length and angles of complex 6f
Bond length (A)
Bond angles (ꢀ)
Re-O (1)
Re-S (3)
Re-S (1)
Re-S (4)
Re-S (2)
1.684
2.2825
2.2905
2.310
2.383
O (1)-Re-S (3)
115.69
115.42
128.82
104.85
87.87
O (1)-Re-S (1)
S (3)-Re-S (1)
O (1)-Re-S (4)
S (3)-Re-S (4)
S (1)-Re-S (4)
O (1)-Re-S (2)
S (3)-Re-S (2)
S (1)-Re-S (2)
S (4)-Re-S (2)
81.74
101.06
84.35
83.8
153.81
Figure 1. Powdercell plot of complex 6f.

1666
A. Hoepping et al./Bioorg. Med. Chem. 6 (1998) 1663±1672
Table 2. PHPLC, DHPLC and pKa(c) values of the rhenium
complexes
®nd any evidence for a second binding component
which may be due to the use of 1 mM GBR12909 for
de®nition of non-speci®c binding.
Compl
Z
PHPLC
DHPLC at pH 7.4
pKa(c)
In Table 3 the IC50 values of compounds 6a±6g are
compared with GBR12909, a drug which is known to
bind to dopamine transporters with a high anity.27±29
Also the IC50 value of cocaine is given for comparison.
As also found by others it has a much lower anity in
the binding assay.26±29 Re complexes 6d±6f exhibited an
anity to the DA transporter which is about 1000 times
lower of that observed for GBR12909. For complexes
6a±c and 6e no anities were estimated below to 10 mM.
Inclusion of a phenyl moiety in the chain did not sig-
ni®cantly improve the anity of the complexes to the
DAT.
6a
6b
6c
6d
6e
6f
0
34
36
2
2
9.34
9.51
9.44
9.48
9.85
9.04
9.45
CH2
C2H4
C3H6
94
6
163
330
398
478
7
C4H7
1.3-phenyl
1.4-phenyl
9
25
18
6g
description of the procedure is given in the experimental
part. Some complexes were used in the HPLC-investi-
gations in the form of their salts. Since the complexes
are dissolved in an excess of bu€er, they show identical
behaviour on the HPLC column as free bases.
Conclusions
Whereas the pKa(c) values are more or less in the same
order at about 9 (Cocaine: 8.525), the PHPLC and con-
comitantly the DHPLC values at pH 7.4 increased with
the number of carbon atoms in the ester chain. Hence
the compounds 6f and 6g should have a lipophilicity
high enough to penetrate the blood±brain barrier in
reasonable amounts. For all other compounds the lipo-
philicity is too low and the basicity is too high as
The preferred radioligands for the DA transporter are
based on the 4-¯uorophenyl derivative (WIN 35,428)29
and the 4-iodophenyl derivative (RTI-55) of cocaine.30
Based on these compounds, potential SPECT imaging
agents like [99mTc]Technepine14 and [99mTc]TRODAT31
have been developed showing the possibility to incor-
porate a metal as a radioactive label preserving the bio-
logical activity. However, with cocaine as the usual
starting material, synthetic ¯exibility is naturally
restricted and alternative synthesis strategies have
already been considered.32,33 The rationale of the pre-
sent study was the synthesis of DA transporter ane
rhenium complexes starting from the tropanol molecule
in order to avoid stereochemical problems and to check
whether substitution in 3a-position is a suitable strategy
for the design of Tc complexes with anity to the DA
transporter. Newmann17 and Meltzer34 already showed
that with di€erent substituents in the 3a position in the
benztropine series the anity di€ers by 3 orders of
magnitude. Therefore structural variations should be
promising at this site. We recently showed that aromatic
moieties in the ketanserine molecule may be replaced by
Re or Tc chelates with preservation of the receptor a-
nity.35 Similar features of the complexes are expected in
our study. QSAR models for the phenyl-tropane series
suggest that distribution properties such as hydro-
phobicity are important contributors to binding at the
DAT.36 On the other hand, Davies et al.32 concluded
from their studies that the DAT has a relatively strict
requirement as regards size around the 3-position. It is
also known that excessive bulk leads to reduced
potency.36 A certain spacer length may, therefore, be
required for hydrophobic interactions. Complexes (6d,e)
with a longer spacer length displayed better anities
than complexes with a short spacer length (6a±c).
Inclusion of the Re chelate in 4-position of the phenyl
expressed by the distribution coecient DHPLC
.
Receptor binding anity
The anity constants of the binding of compounds 6a±
6g to cloned human dopamine transporters are shown in
Table 3 in terms of the IC50 values. For the binding of
[3H]WIN35,428 to cloned human dopamine transpor-
ters, a one-site model was assumed as suggested by
Reith et al.26 Under other circumstances the same
authors have also reported a second low-anity com-
ponent of the [3H]WIN35,428 binding27 with a Kd
between 3 and 8 mM. In our binding assay we did not
Table 3. IC50 values for inhibition of [3H]WIN35,428 binding
on cloned human dopamine transporters
Ligand
IC50
GBR12909
Cocaine
2.35‹0.35 nM
306‹48 nM
6a
6b
6c
6d
6e
6f
>10 mM
>10 mM
>10 mM
17‹11 mM
5.3‹4.5 mM
2.4‹1.6 mM
>10 mM
6g

A. Hoepping et al./Bioorg. Med. Chem. 6 (1998) 1663±1672
1667
ring 6g instead of in 3-position as in compound 6f
reduces the anity to the DAT. To explain this fact
further investigations have to be carried out.
3-Halopropionic acid trop-3ꢀ-yl ester 3b (mixture of
compounds)1y: 1/3 3-bromopropionic acid trop-3ꢀ-yl
ester x HCl. 2/3 3-Chloropropionic acid trop-3ꢀ-yl ester
x HBr. Yield: 8.3 g (88%); 1H NMR d 2.23 (d,
J=16.33, 2H), 2.4±2.52 (m, 4H), 2.58 (dt, J1=16.24,
J2=3.96, 2H), 2.9 (s, 3H), 2.95 (t, J=6.24, 2H, int.
66%), 3.09 (d, J=6.34, 2H, int. 33%), 3.74 (d, J=6.34,
2H, int. 33%), 3,9 (d, J=6.24, 2H, int. 66%), 4.02 (bs,
2H), 5.18 (t, J=5, 1H). Anal. calcd for C11H19NO2BrCl:
C, 42.26; H, 6.13; N, 4.48. Found C, 42.07; H, 6.31; N
4.45.
In conclusion, our data obtained show that it is possible
to obtain Re complexes with DA transporter anity by
substitution at the 3-position of the tropanol molecule
even if the anity is to low to permit radio-
pharmaceutical drug development. On the other hand
our data show that a lack of a substituent in 2-position
of the tropan ring seems not to be tolerated by the
DAT. However, the results are considered a good start-
ing point for further optimization of the substituents at
the 3-position of the tropane ring at derivatives
obtained from naturally occuring cocaine. In this case,
additional substitutions between the 2- and 4-positions
of the phenyl ring could provide further information
about what it takes to obtain high-anity compounds.
4-Halobutyric acid trop-3ꢀ-yl ester 3c (mixture of
compounds)1y: 1/3 4-bromobutyric acid trop-3ꢀ-yl ester
x HCl. 2/3 4-Chlorobutyric acid trop-3ꢀ-yl ester x HBr.
1
Yield: 7.9 g (80%); H NMR d 2.1±2.52 (m, 10H), 2.64
(t, J=7.31, 2H), 2.9 (s, 3H), 3.61 (t, J=6.51, 2H int.
33%), 3.72 (t, J=6.4, 2H, int. 66%), 4.0 (bs, 2H), 5.18
(t, J=4.88, 1H). Anal. calcd for C12H21NO2BrCl: C,
44.12; H, 6.48; N, 4.29; Found C, 43.64; H, 6.56; N,
4.25.
Experimental
General
5-Halopentanoic acid trop-3ꢀ-yl ester 3d (mixture of
compounds)1y: 1/3 5-bromopentanoic acid trop-3ꢀ-yl
ester x HCl. 2/3 5-Chloropentanoic acid trop-3ꢀ-yl ester
All reagents were of commercial grade unless otherwise
stated. DMF was dried over molecular sieves. Flash
chromatography was performed on silica gel (E. Merck)
70±230 mesh. IR spectra were recorded on an M 80
(Carl Zeiss Jena). NMR spectra were recorded on a
Bruker MSL 300 and a Bruker DRX 500 in chloroform
or methanol (hydrochlorides and oxalates) as solvent.
1H spectra were obtained at 500.13 MHz unless other-
wise noted and 13C spectra were obtained at
125.77 MHz. Coupling constants are given in Hz. Mass
spectra were recorded on a MAT 95 spectrometer from
Finnigan by the FAB method with positive ions in gly-
cerol as matrix. Elemental analyses were carried out on
a LECO Elemental Analyzer CHNS-932 and are within
a range of 0.4%. For thiols and tritylthioethers, no ele-
mental analyses were carried out.
1
x HBr. Yield: 9.36 g (92%); H NMR d 1.88 (m, 4H),
1.98 (m, 2H, int. 33%), 2,2 (d, J=16,17, 2H), 2,37±2.56
(m, 8H), 2.88 (s, 3H), 3.55 (t, J=6.35, 2H int. 33%),
3.67 (t, J=5.78, 2H), 3,99 (bs, 2H), 5.13 (t, J=4,81,
1H). Anal. calcd for C13H23NO2BrCl: C, 46.01; H, 6.23;
N, 4.13. Found C, 45.2; H, 6.84; N, 4.13.
5-Chlorohexanoic acid trop-3ꢀ-yl ester 3e. Yield: 6.6 g
(71%), mp: 158±161 ꢀC, IR: n-CO: 1736; 1H NMR d
1.58 (m, 2H), 1.76 (qui, 2H), 2,2 (d, J=16,17, 2H), 2.37±
2.56 (m, 8H), 2.88 (s, 3H), 3.55 (t, J=6.35, 2H), 3.67 (t,
J=5.78, 2H), 3,99 (bs, 2H), 5.13 (t, J=4,81, 1H); Anal.
calcd for C14H25NO2Cl2: C, 54.20; H, 8.12; N, 4.51.
Found C, 53.98; H, 8.17; N, 4.62.
3-(Chloromethyl)benzoic acid trop-3ꢀ-yl ester 3f. Yield:
9.6 g (97%); mp: 195±197 ꢀC; IR: n-CO: 1712 cm 1; H
1
Esters of tropanol
NMR d 2.38 (d, J=16.29, 2H), 2.46±2.58 (m, 4H), 2.64
(dt, J1=16.32, J2=4.09, 2H), 2.92 (s, 3H), 4.05 (bs, 2H),
4.81 (s, 2H), 5.38 (t, J=4.84, 1H), 7.61 (t, J=7.7, 1H),
7.77 (d, J=7.73, 1H), 8.06, (d, J=7.9, 1H). Anal. calcd
for C16H21NO2Cl2: C, 58.19; H, 6.41; N, 4.24. Found C,
58.01; H, 6.17; N, 4.04.
Preparation of the o-halide esters;23 general procedure:
30 mmol tropanole hydrochloride and 30 mmol of the
appropriate acid chloride were mixed in an autoclave
bottle and heated in an oven for 3 h at 140 ꢀC. The
ester halogenides were then recrystallised from ethanol/
ether.
4-(Chloromethyl)benzoic acid trop-3ꢀ-yl ester 3g. Yield:
8.8 g (88%); mp: 232±235 ꢀC; IR: n-CO: 1716 cm 1; H
NMR d 2.36 (d, J=16.34, 2H), 2.46±2.65 (m, 6H), 2.92
(s, 3H), 4.04 (bs, 2H), 4.79 (s, 2H), 5.38 (t, J=4.89, 1H),
1
2-Chloroacetic acid trop-3ꢀ-yl ester hydrochloride 3a.
1
Yield: 5.8 g (76%); mp: 215±220 ꢀC; IR: n-CO: 1732 cm
;
1H NMR d 2.2 (d, J=16.68, 2H), 2.3±2.63 (m, 6H), 2.9
(s, 3H), 3.97 (bs, 2H), 4.33 (s, 2H), 5.21 (t, J=4.94).
Anal. calcd for C10H17NO2Cl: C, 47.26; H, 6.74; N,
5.51. Found C, 46.92; H, 6.84; N 5.46.
ydue to halogen exchange

1668
A. Hoepping et al./Bioorg. Med. Chem. 6 (1998) 1663±1672
7.65 (d, J=8,3, 2H), 8.1, (d, J=8.3, 1H). Anal. calcd for
C16H21NO2Cl2: C, 58.19; H, 6.41; N, 4.24. Found C,
58.21; H, 6.33; N, 4.16.
4); 1H NMR d 1.25 (m, 2H), 1.38 (m, 2H), 1.48 (qui,
J=7.62, 2H), 1.83 (d, J=15.52, 2H), 2.12 (m, 6H), 2.18
(t, J=7.54, 2H), 2.56 (s, 3H), 2.68 (d, J=14.82, 2H),
3.47 (bs, 2H), 5.04 (t, J=4.77, 1H), 7.1±7.45 (m, 15H);
13C NMR d 24.23, 24.35, 24.88, 28.18, 28.37, 31.70,
34.43, 34.88, 39.14, 61.11, 65.22, 66.42, 126.53, 127.78,
129.52, 144.90, 172.15.
Preparation of the !-thiotritylated esters. Sixteen milli-
moles triphenyl methane thiol and 30 mmol potassium
tert-butoxide were dissolved in 50 mL DMF and stirred
for 15 min. Then 15 mmol of the tropane ester hydrohalide
were added portionwise over 15 min and the resulting
mixture was stirred for 2 h at room temperature and an
additional hour at 50±60 ꢀC. After cooling to room
temperature, 50 mL of dichloromethane were added and
the whole mixture was poured into a cold saturated
solution of ammonium chloride. The aqueous layer was
extracted three times with 20 mL of dichloromethane.
The organic layer was washed twice with brine and
dried over sodium sulphate. After removing the solvent
the residue was chromatographed on silica gel by gra-
dient elution, employing ®rst ethyl acetate and then
methanol/dichloromethane/triethyl amine 80/20/0.1 to
give thick yellow oils, which became glassy upon standing.
3-(Triphenylmethylmercaptomethyl)benzoic acid trop-3ꢀ-yl
1
ester 4f. Yield: 7.3 g (86%); Rf=0.33; H NMR d 2.12
(d, J=16.81, 2H), 2.28 (m, 2H), 2.45 (d, J=8.52, 2H),
2.79 (d, J=3.34, 3H), 3.18 (d, J=14.23, 2H), 3.36 (s,
2H), 3.82 (bs, 2H), 5.37 (bs, 1H), 7.2±7.8 (m, 19H); 13C
NMR 24.61, 34.66, 36.58, 39.37 62.27, 65.29, 67.61,
126.82, 127.73, 127.91, 128.75, 129.52, 130.01, 130.15,
134.11, 138.09, 144.44, 164.97.
4-(Triphenylmethylmercaptomethyl)benzoic acid trop-3ꢀ-yl
1
ester 4g. Yield: 6.83 g (80%); Rf=0.19; H NMR d 1.82
(m, 2H), 2.05 (m, 4H), 2.23 (dt, J1=14.83, J2=4.23,
2H), 2.31 (s, 3H), 3.15 (bs, 2H), 3.37 (s, 2H), 5.24 (t,
J=5.13), 7.15±7.9 (m 19H); 25.59, 36.43, 36.58, 40.25,
59.69, 67.49, 67.75, 126.65, 127.76, 127.83, 129.02,
129.34, 129.99, 142.36, 144.31, 165.31.
2-Triphenylmethylmercaptoacetic acid trop-3ꢀ-yl ester
4a. Yield: 1.3 g (19%); Rf=0.16; 1H NMR d 1.8 (d,
J=15.77, 2H), 1.95±2.1 (m, 4H), 2.57 (s, 3H), 2.7 (m,
2H), 2.9 (s, 2H), 3.49 (bs, 2H), 4.91 (t, J=4.77, 1H),
7.15±7.35 (m, 15H); 13C NMR d 24.38, 33.97, 34.7, 38.7,
61.42, 65.82, 67.27, 126.97, 128.02, 129.33, 143.7,
168.03.
Preparation of the mercaptans. Five millimoles of the
compounds 4a±4g were dissolved in 15 mL tri¯uoro-
acetic acid and cooled to 0 ꢀC. Then 0.2 mL anisole and
5.5 mmol mercury acetate were added while stirring. The
whole mixture was stirred for 2 h at room temperature.
The solvent was removed in vacuo and ether was added
to the residue. The white precipitate formed was stirred
for 1 h at room temperature and then isolated by suction
®ltration. The mercury salt was dissolved in 30 mL
ethanol and hydrogen sulphide was bubbled through the
solution for 20 min. The black precipitate was ®ltered
through a thick pad of Celite and the ®ltrate was evap-
orated in vacuo. The residue was dissolved in 20 mL
dichloromethane and washed with saturated Na2CO3
solution and brine. The organic layer was separated,
dried and the solvent was removed. The colourless oil
obtained was puri®ed by ¯ash-chromatography on
silica-gel employing methanol/chloroform=1:4 as eluent.
The purity of the compounds obtained by this proce-
dure was in the most cases not sucient to perform
elemental analysis, but the quality was sucient for the
next reaction step.
3-Triphenylmethylmercaptopropionic acid trop-3ꢀ-yl
ester 4b. Yield: 6.1 g (81%); Rf=0.2; 1H NMR d 1.62
(d, J=14.85, 2H), 1.76 (m, 2H), 1.94 (m, 2H), 2.07 (dt,
J1=14.85, J2=4.38, 2H), 2.19 (t, J=7.55 2H), 2.24 (s
3H), 2.45 (t, J=7.55 2H), 3.05 (bs, 2H), 4.92 (t, J=5.28,
1H), 7.18±7.43 (m, 15H); 13C NMR d 25.48, 26.93, 34.1,
36.28, 40.23, 59.73, 66.75, 67.57, 126.64, 127.68, 129.48,
144.54, 170.62.
4-Triphenylmethylmercaptobutyric acid trop-3ꢀ-yl ester
4c. Yield: 4.0 g (52%); Rf=0.19; 1H NMR
(200.13 MHz) d 1.45±2.1 (m, 14H), 2.2 (s, 3H), 2.95 (bs,
2H), 4.85 (t, J=5.28, 1H), 7.0±7.4 (m, 15H); 13C NMR
d 23.76, 25.54, 31.22, 33.92, 36.52, 40.39, 59.65, 66.48,
67.32, 126.53, 127.76, 129.44, 144.70, 171.65.
5-Triphenylmethylmercaptopentanoic acid trop-3ꢀ-yl ester
1
4d. Yield: 5.7 g (72%); Rf=0.3; H NMR (300.13 MHz)
d 1.35 (m, 2H), 1.5 (m, 2H), 1.9 (s, 2H), 2.05 (m, 8H),
2.45 (s, 3H), 3.38 (bs, 2H), 4.93 (t, J=5.03, 1H), 7.1±7.5
(m, 15H); 13C NMR d 23.12, 23.95, 24.93, 27.90, 31.29,
34.07, 34.69, 38.72, 60.38, 65.42, 66.39, 126.47, 127.71,
129.41, 144.73, 171.90.
2-Mercaptoethanoic acid trop-3ꢀ-yl ester 5a. Yield: ca.
500 mg impure product, only MS-spectra was recorded,
MS: M=215,31, M++1 (21), 216 (100), 217 (14), 218
(4).
3-Mercaptopropanoic acid trop-3ꢀ-yl ester 5b. Yield:
0.99 g (88%); Rf=0.19; 1H NMR d 1.64 (t, J=8.15,
1H), 2.0 (d, J=15.74, 2H), 2.2±2.4 (m, 4H), 2.62 (t,
6-Triphenylmethylmercaptohexanoic acid trop-3ꢀ-yl ester
4e. Yield: 6.1 g (79%); Rf=0.41(methanol 1/chloroform

A. Hoepping et al./Bioorg. Med. Chem. 6 (1998) 1663±1672
1669
J=6.63, 2H), 2.73±2.77 (m, 5H), 2.95 (d, J=13.53, 2H),
3.78 (bs, 2H), 5.13, (bs, 1H); 13C d 19.55, 24.28, 34.49,
38.49, 38.55, 39.27, 62.11, 64.85, 170.16.
(3-Thiapentane-1.5-dithiolato)(3-thiolato-ethanoic acid trop-
3ꢀ-yl-ester) oxorhenium(V) 6a. Yield: 42 mg (73.9%)
dark red crystals, mp: 186±188 ꢀC (decomp), IR: n-CO:
1704 cm 1, ReO: 968 cm 1; Rf=0.07; 1H NMR d 1.72
(d, J=14.8, 2H), 1.92±2.12 (m, 8H), 2.24 (s, 3H), 3.05
(bs, 2H), 3.1 (m,2H), 3.93 (m, 2H), 4.28 (dd, J1=13.14,
J2=4.61, 2H), 4.03 (s, 2H), 5.02 (t, J=4.32, 1H); 13C
NMR d 25.6, 36.48, 40.17, 40.36, 43.66, 46.76, 59.78,
68.32, 170.97; MS: M=568.81, M++1: 568 (48.3); 569
(7.6); 570 (100); 571 (15.5); 572 (14.0); 573 (1.7). Anal.
calcd for C14H24NO3S4Re: C, 29.56; H, 4.25; N, 2.46.
Found C, 29.80; H, 4.33; N, 2.42.
4-Mercaptobutyric acid trop-3ꢀ-yl ester 5c. Yield: 0.83 g
1
(68%); Rf=0.2; H NMR d 1.32 (t, J=8.07, 1H), 1.82
(d, J=15.48, 2H), 2.1 (qui, J=7.05, 2H) 2.15 (bs, 4H),
2.41 (t, J=7.13, 2H), 2.55±2.65 (m, 2H), 2.58 (s, 3H),
2.7 (t, J=6.98, 2H), 3.55 (bs, 2H), 5.05, (bs, 1H); 13C
NMR d 23.9, 24.9, 32.84, 34.91, 37.57, 39.07, 60.77,
65.87, 171.68.
5-Mercaptopentanoic acid trop-3ꢀ-yl ester 5d. Yield:
1.09 g (85%); Rf=0.28; 1H NMR d 1.34 (t, J=8.03,
1H), 1.55±1.75 (m, 4H), 1.96 (d, J=16.02, 2H) 2.17±2.38
(m, 6H) 2.5 (q, J=7.1, 2H), 2.65 (m, 2H), 2.7 (s, 3H),
3.76 (bs, 2H), 5.05, (t, J=4.58, 1H); 13C NMR d 23.28,
24.02, 24.22, 33.07, 33.88, 34.69, 38.91, 61.99, 64.26,
171,83.
(3-Thiapentane-1.5-dithiolato)(3-thiolato-propanoic acid
trop-3ꢀ-yl-ester) oxorhenium(V) 6b. Yield: 28.6 mg
(49%, free base), mp: 149±154 ꢀC (oxalate), IR (KBr): n-
CO: 1724 cm 1, ReO: 960 cm 1; Rf=0.23; 1H NMR
(CD3OD) d 2.13±2.15 (m, 4H), 2.32±2.36 (m, 2H), 2.43±
2.47 (m, 2H), 2.52±2.57 (m, 2H), 2.85 (s, 3H, CH3), 2.98
(t, J=6.85, 2H, CH2), 3.13 (td, J1=13.65, J2=2.78,
2H), 3.93 (bs, 2H), 4.06 (t, J=6.85, 2H, CH2), 4.12
(dd, J1=10.74, J2=3.37, 2H), 4.37 (dd, J1=13,
J2=4.77), 5.13 (t, J=4.6, 1H); 13C NMR d (CD3OD):
24.89, 32.60, 36.03, 39.07, 39.46, 47.41, 48.49, 63.88,
65.63, 165.46, 172.97; MS: M=583.03, M++1: 582
(54.3), 583 (10.5), 584 (100), 585 (19), 586 (17), 587 (3),
588 (1.4). Anal. calcd for C17H28NO7S4Re (oxalate): C,
30.35; H, 4.19; N, 2.08. Found C, 30.41; H, 4.33; N,
2.20.
6-Mercaptohexanoic acid trop-3ꢀ-yl ester 5e. Yield:
0.7 g (52%); Rf=0.19; 1H d 1.3 (m, 2H), 1.4 (m, 2H),
1.57±1.7 (m, 6H), 1.85±2.05 (m, 4H), 2.13 (m, 2H), 2.27
(t, J=7.7, 2H), 2.28 (s, 3H), 3.1 (bs, 2H), 4.98, (t,
J=5.3, 1H). 13C NMR d 24.34, 24.49, 27.82, 27.97,
33.55, 34.74, 36.49, 40.29, 59.83, 67.21, 172.63.
3-(Mercaptomethyl)benzoic acid trop-3ꢀ-yl ester 5f. Yield:
0.98 g (67%); Rf=0.28; 1H NMR d 1.78 (t, J=7.73, 1H),
2.14 (d, J=15.88, 2H), 2.33 (m, 2H), 2.44 (m, 2H), 2.77
(d, J=4.76, 3H), 3.73 (d, J=7.72, 2H), 3.85 (bs, 2H),
5.33 (t, J=3.48, 1H), 7.42 (t, J=7.7, 1H), 7,55 (d, J=6.7,
1H), 7,85 (d, J=7.79, 1H), 7.94 (s, 1H); 13C NMR d
24.44, 28.51, 34.73, 38.96, 61.92, 65.23, 127.84, 129.029,
120.07, 130.17, 133.09, 141.86, 165.02.
(3-Thiapentane-1.5-dithiolato)(4-thiolato-butyric acid trop-
3(ꢀ-yl-ester) oxorhenium(V) 6c. Yield: 43.3 mg (72.6%);
mp: 131±135 ꢀC; IR: n-CO: 1728 cm 1; ReO: 960 cm
;
1
1
Rf=0.2; H d 1.71 (d, J=14.9, 2H), 1.96 (m, 6H), 2.13
(dt, J1=14.84, J2=3.91, 2H), 2.22 (qui, J=7.35, 2H),
2.29 (s, 3H), 2.52 (t, J=7.45, 2H), 3.08 (m, 2H), 3.12
(bs, 2H), 3.84 (t, J=7.1, 2H), 3.91 (m, 2H), 4.27 (dd,
J1=13.17, J2=4.8, 2H), 4.98 (t, J=5.13, 1H); 13C NMR
d 25.51, 28.04, 33.86, 36.42, 36.7, 40.26, 43.70, 46.68,
59.93, 66.98, 172.52; MS: M=597.058, M++1: 596
(42.6); 597 (4.1); 598 (100); 599 (7.5); 600 (12.2); 601 (1).
Anal. calcd for C16H28NO3S4Re: C, 32.2.; H, 4.73; N,
2.35. Found C, 32.17; H, 4.97; N, 2.27.
4-(Mercaptomethyl)benzoic acid trop-3ꢀ-yl ester 5g. Yield:
1
0.38 g (26%); Rf=0.12; H NMR d 1.75±1.85 (m, 3H),
2.0±2.12 (m, 4H), 2.22 (dt, J1=14.92, J2=4.28, 2H), 2.3
(s, 1H), 3.15 (s, 1H), 3.76 (s, 1H), 5.24 (t, J=5.22, 1H),
7.39 (d, J=8.2, 2H), 7.96 (d, J=8.2, 2H), 13C NMR d
25.79, 28.69, 36.67, 40.42, 59.81, 68.06, 128.13, 129.86,
142.43, 146.21, 165,49.
Preparation of the complexes22. o-Mercapto acid trop-
3a-yl-ester (150 mmol) was added while stirring to a boil-
ing solution of 38.9 mg (100 mmol) chloro(3-tiapentane-
1,5-dithiolato)oxorhenium (V) in 5 mL acetonitrile. The
mixture was heated to re¯ux for 5 min. The solvent was
removed in vacuo and the residue was puri®ed by ¯ash-
chromatography using methanol/chloroform=1:4 as
eluent.
(3-Thiapentane-1.5-dithiolato)(5-thiolato-pentanoic acid
trop-3ꢀ-yl-ester) oxorhenium(V) 6d. Yield: 21 mg
(34.4%) mp (oxalate): 172±74 ꢀC, IR: n-CO: 1724 cm
,
1
ReO: 960 cm 1; Rf=0.25; 1H NMR d 1.78±1.82 (m,
4H), 1.9 (d, J=15.77, 2H), 2.15±2.25 (m, 6H), 2.37 (m,
4H), 2.66 (s, 3H), 3.02 (dd, J1=14.13, J2=4.13, 2H),
3.66 (t, J=6.87, 2H), 3.78 (bs, 2H), 4.06 (dd, J1=10.6,
J2=3.46, 2H), 4.28 (dd, J1=12.85, J2=4.49, 2H), 4.92
(bs, 1H); 13C NMR d (CDCl3): 24.1, 24.45, 32.25, 34.54,
34.73, 36.76, 39.24, 43.72, 46.72, 62.40, 64.26, 172.06;
MS: M=610.89, M++1: 610 (50.1); 611 (5.7); 612
(100); 613 (10); 614 (12.6); 614 (0.9). Anal. calcd for
The dark red oils obtained were crystallised upon slow
evaporation of the solvent or as oxalic acid salts or
hydrochlorides.

1670
A. Hoepping et al./Bioorg. Med. Chem. 6 (1998) 1663±1672
C19H32NO7S4Re (oxalate): C, 32.56; H, 4.60; N, 2.00.
Found C, 32.31; H, 4.55; N, 1.97.
investigation may be obtained from the Fachinforma-
tionszentrum Karlsruhe, D-76344 Eggenstein-Leopold-
shafen (Germany), on quoting the depository number
CSD-40 70 92.
(3-Thiapentane-1.5-dithiolato)(6-thiolato-hexanoic acid
trop-3ꢀ-yl-ester) oxorhenium(V) 6e. Yield: 53.4 mg
(81%) mp (hydrochloride): 193±195 ꢀC; IR: n-CO:
1726 cm 1; ReO: 960 cm 1; Rf=0.29; 1H d 1.46 (d,
J=6.92, 2H), 1.6 (d, J=7.63, 2H), 1.76 (d, J=7.3, 2H),
1.92 (bd, 2H), 2.1±3.3 (m, 6H), 2.34 (d, J=7.37, 2H),
2.65 (bs, 3H), 3.02 (td, J1=13.32, J2=4.2, 2H), 3.32 (bs,
2H), 3.64 (t, J=7.32), 3.81 (bs, 2H), 4.06 (dd, J1=10.52,
J2=3.27, 2H), 4.28 (dd, J1=12.91, J2=4.58, 2H), 4.92
(t, J=4.6, 1H); 13C NMR d 23.1, 23.95, 27.71, 32.18,
33.83, 34.02, 35.96, 37.8, 43.02, 45.52, 61.17, 63.91,
171.81. Anal. calcd for C18H33ClNO3S4Re (hydro-
chloride): C, 32.67; H, 5.03; N, 2.12. Found C, 32.71; H,
5.02; N, 2.10.
Crystallographic and re®nement details:
Formula:
C20H28NO3ReS4
644.87
Formula weight
Space group
Lattice parameter
P 1 2_1/c 1
A=11.374,
b=17.945,
c=12.068,
b=106.920
monoclinic
2356.53 (2)A3
4
Crystal system
Volume of cell
Z=
X-ray density (g/cm3)
Mass absorpt. coef. (cm2/g)
Absorption coecient
F (000)
Crystal size
Theta range for data collection 1.87 to 26.99 deg.
1.8177
72.19
5.532 mm
1272
(3-Thiapentane-1.5-dithiolato)(3-thiolatomethylbenzoic
acid trop-3ꢀ-yl-ester) oxorhenium(V) 6f. Yield: 28.6 mg
(44.3%) , mp: 206±208 ꢀC; IR: n-CO: 1712 cm 1; ReO:
1
1
960 cm 1; Rf=0.15; H NMR d 1.83 (d, J=14.63, 2H),
0.09Â0.23Â1.08 mm
1.97 (m, 2H), 2.07 (m, 4H), 2.21 (dt, J1=14.61,
J2=4.68, 2H), 2.29 (s, 3H), 3.1 (m, 2H), 3.14 (bs, 2H),
3.89 (m, 2H), 4.27 (dd, J1=13.15, J2=4.66, 2H), 5.05 (s,
2H), 5.23 (t, J=5.3, 1H), 7.39 (t, J=7.66, 1H), 7.66 (d,
J=7.67, 1H), 7.86 (d, J=7.71, 1H), 8.09 (s, 1H); 13C
NMR d 25.84, 36.56, 40.35, 42.32, 43.69, 46.72, 59.8,
67.94, 127.68, 128.55, 130.26 130.87, 133.77, 142.31,
165.87; MS: M=645.05 M++1: 644 (44.5); 645 (8.4);
646 (100); 647 (6.77); 648 (6.5). Anal. calcd for
C20H28NO3S4Re: C, 37.25; H, 4.38; N, 2.17. Found C,
36.97; H, 4.43; N, 1.99.
Index range
h: 0±14, k: 0±22,
l:±15±14
5382
Re¯ections collected
Independent re¯ections
5128
[R(int)=0.0205]
Full-matrix least-
squares on F2
5128/0/266
1.045
Re®nement method
Data/restraints/parameters
Goodness-of-®t on F2
Final R indices [I>2sigma (I)] R1=o.0263,
wR2=0.0585
(3-Thiapentane-1.5-dithiolato)(4 -thiolatomethylbenzoic
acid trop-3ꢀ-yl-ester) oxorhenium(V) 6g. Yield: mp:
Largest di€. peak and hole
1.017 and
0.666 e.A
3
215±217 ꢀC, IR: n-CO: 1708 cm 1, ReO: 960 cm
;
1
1
Rf=0.18; H: 1.86 (d, J=15.02; 2H), 1.98 (m, 2H), 2.11
(bs, 4H), 2.32 (bd, J=15.06, 2H), 2.36 (s, 3H), 3.1 (m,
2H), 3.24 (bs, 2H), 3.9 (m, 2H), 4.28 (dd, J1=13.24,
J2=4.73, 2H), 5.04 (s, 2H); 5.24 (t, J=5.16, 1H), 7.53
(d, J=8.2, 2H), 7.95 (d, J=8.2, 2H); 13C NRM d 25.61,
36.31, 40.16, 42.26, 43.68, 46.76, 60.18, 67.35, 128.86,
129.33, 129.61, 147.26, 165.68; MS: M=645.05,
M++1: 644 (47.5); 645 (12.1); 646 (100); 647 (19.1); 648
(13.2); 649 (1.3). Anal. calcd for C20H28NO3S4Re: C,
37.25; H, 4.38; N, 2.17. Found C, 36.87; H, 4.22; N,
2.15.
Radioligand binding assays. Radioligand binding assays
were performed on cloned human dopamine transpor-
ters (Receptor Biology, Inc., Baltimore, USA). Binding
of [3H]WIN35,428 (83.5 Ci/mmol, DuPont, Dreieich,
Germany) to the transporters was performed according
to published methods.26,37 Brie¯y, about 2 mg of mem-
brane, bu€er (50 mM Tris±HCl, 100 mM NaCl, pH 7.4)
and unlabeled ligands were incubated in a total volume
of 1.0 mL for 2 h at 4 ꢀC. Nonspeci®c binding was
determined in the presence of 1 mM GBR-12909
(Research Biochemicals International, Natick, USA).
The binding assays were terminated by rapid ®ltration
over GF/B glass ®bre ®lters (Whatman, Maidstone,
UK) on the Brandel cell harvester. The ®lters were pre-
soaked in 0.3% polyethylene imine. Following four
washes with 4 mL portions of ice-cold bu€er (see above)
the ®lter paper containing the membrane-bound
X-ray crystal structure analysis of compound 6f. X-ray
crystallographic data were obtained on a CAD4 dif-
fractometer at 293 K using graphite-monochromatic
Cu-Ka radiation of a wavelength of 0.71069 A. Single
crystals suitable for X-ray structure analysis were
obtained by slow evaporation of a solution of 6f in
chloroform. Further details of the crystal structure

A. Hoepping et al./Bioorg. Med. Chem. 6 (1998) 1663±1672
1671
[3H]WIN35,428 was transferred into 10 mL of liquid
References and Notes
scintillation cocktail (Ultima-Gold, Packard), and the
radioactivity on each ®lter was counted in a Canberra-
Packard liquid scintillation counter (TRI-CARB
2100TR). Aliquots of the incubation ¯uid were mea-
sured as well. Corrections were made for the binding of
[3H]WIN35,428 to the ®lters. Increasing concentrations
(between 0.01 nM and 10 mM) of the unlabelled ligands,
GBR 12909 (purchased from RBI) and cocaine (pur-
chased from Th. Geyer, Rennningen, Germany) were
competed against a ®xed concentration of the radi-
olabeled ligand (total bindingꢁ1500 dpm). From the
competition curves IC50 values were calculated using the
program Fig. P (Biosoft).
1. Horn, A. S. Prog. Neurobiol. 1990, 34, 387.
2. Jenner, P.; Shapira, A. H. V.; Marsden, C. D. Neurology
1992, 42, 2241.
3. Tedro€, J.; Aquilonius, S. M.; Hartvig, P. G.; Langstrom, B.
In Parkinson's Disease; Battistin, L.; Scarlato, G.; Caraceni, T.;
Ruggieri, S.; Eds; Lippincott-Raven Publ, 1996; Vol. 69,
p 443.
4. Yu, D. W.; Gatley, S. J.; Wolf, A. P.; MacGregor, R. R.;
Dewey, S. L.; Fowler, J. S.; Schlyer, D. J. J. Med. Chem. 1992,
35, 2178.
5. Muller, L.; Halldin, C.; Farde, L.; Karlsson, P.; Hall, H.;
Swahn, C. G.; Neumeyer, J.; Gao, Y.; Milius, R. Nucl. Med.
Biol. 1993, 20, 249.
6. Lundkvist, C.; Halldin, C.; Swahn C-G.; Hall, H.; Karlson,
P.; Nakashima, Y.; Wang, S.; Milius, R. A.; Neumeyer, J. L.;
Farde, L. Nucl. Med. Biol. 1995, 22, 905.
Determination of lipophilicity and pKa values. Reversed-
phase HPLC was used to determine the lipophilicity of
the rhenium complexes synthesized. Lipophilicity is
expressed by the partition coecient P (log P), which
refers to the neutral state of the solute or the distribu-
tion coecient D, which is obtained at a given pH and
may thus result from the contributions of the cationic
and neutral forms. Using HPLC various unknown fac-
tors caused and in¯uenced by this system have to be
taken into consideration.38±40 Hence, only apparent
partition and distribution coecients designated in this
work as PHPLC and DHPLC were obtained. These values
were derived from the retention times according to the
equation log P=a logk0+b, where k0=(tR t0)/t0 repre-
sents the capacity factor, tR is the retention time of the
sample and t0 the retention time of methanol as an
unretained solute. Aniline (0.9), benzene (2.13), and
bromobenzene (2.99) served as log P references. With a
given ionizable compound a sigmoidal DHPLC/pH curve
results. From the turning point of this curve the appar-
ent pKa value (pKHPLC) was derived. Since measure-
ments are performed in organic/aqueous solutions,
values were corrected by comparing the experimental
values with those obtained for amines. A plot of the pKa
values of these reference substances over the measured
pKHPLC gives a good correlation following the relation-
ship pKa=0.988 pKHPLC+1.501 (R=0.992). Thus, a
calibration curve is available for estimating the pKa(c)
values from the measured pKHPLC values of the
complexes.
7. Halldin, C.; Farde, L.; Lundkvist, C.; Ginovart, N.;
Nakashima, Y.; Karlson, P.; Swahn, C.-G. Synapse 1996,
22, 386.
8. Hume, S. P.; Luthra, S. K.; Brown, D. J.; Opacka-Ju€ry, J.;
Osman, S.; Ashworth, S.; Myers, R.; Brady, F.; Kuhar, M. J.;
Brooks, D. J. Nucl. Med. Biol. 1996, 23, 377.
9. Wilson, A. A.; DaSilva, J. N.; Houle, S. Nucl. Med. Biol.
1996, 23, 141.
10. Ametamey, S. M.; Halldin, C.; Swahn, C.-G.; Hall, H.;
Schubiger, P. A.; Farde, L. Nucl. Med. Biol. 1995, 22, 959.
11. Metwally, S. A. M.; Gatley, S. J.; Wolf, A. P.; Yu, D. W.
J. Labelled Comp. Radiopharm. 1992, 31, 219.
12. Shaya, E. K.; Sche€el, U.; Dannals, R. F.; Ricaurte, G. A.;
Carrol, F. Y.; Wagner, H. N. Jr.,. Kuhar, M. J.; Wong, D. F.
Synapse 1992, 10, 169.
13. Meegalla, S.; Ploessl, K.; Kung, M.-P.; Stevenson, D. A.;
Liable-Sands, L. M.; Rheingold, A. L.; Kung, H. F. J. Am.
Chem. Soc. 1995, 117, 11037.
14. Madras, B. K.; Jones, A. G.; Mahmood, A.; Zimmerman,
R. E.; Garada, B.; Holman, B. L.; Davison, A.; Blundell, P.;
Meltzer, P. C. Synapse (N.Y.) 1996, 22, 239.
15. Kuikka, J. T.; Tiihonen, J.; Bergstrom, K. A.; Karhu, J.;
Hartikainen; P.; Vinamaki, H.; Lansimies, E.; Lehtonen, J.;
Hakola, P. J. Nucl. Med. 1995, 22, 346.
16. Neumeyer, J. L.; Tamagnan, G.; Wang, S.; Gao, Y.;
Milius, R. A.; Kula, N. S.; Baldessarini, R. J. J. Med. Chem.
1996, 39(2), 543.
17. Newman, A. H.; Kline, R. H.; Allen, A. C.; Izenwasser, S.;
George, C.; Katz, J. L. J. Med. Chem. 1995, 38, 3933.
18. Aronson, B.; Enmon, J. L.; Izenwasser, S.; Katz, J. L.;
Kelkar, S. V.; Luo, L.; Nolan, S. P.; Trudell, M. L. J. Med.
Chem. 1996, 39, 1560.
The above lipophilicity and ionization constants were
determined by using the Perkin-Elmer HPLC Controller
System Model 1022 equipped with a UV/VIS spectro-
meter detector at 254 nm and a Hamilton PRP-1 column
(250Â4.1 mm; 10 mm). As mobile phase an isocratic elu-
ent (acetonitrile and 0.01 M phosphate or citrate bu€er,
3:1, v/v) was applied with a ¯ow rate of 1.5 mL/min.
Before each application the certain pH values of the
eluents were measured by using a glass electrode, which
was calibrated with standard bu€ers daily.
19. Hoepping, A.; Spies, H.; Johannsen, B. Bioorg. Med.
Chem. Lett. 1996, 6, 2871.
20. Pietzsch, H. J.; Spies, H.; Ho€mann, S. Inorg. Chim. Acta
1989, 161, 15.
21. Spies, H.; Fietz, T.; Glaser, M.; Pietzsch, H. J.; Johannsen,
B. In Technetium and Rhenium in Chemistry and Nuclear Medi-
cine; Nicolini, N.; Bandoli, G.; Mazzi, U.; Eds; SG Editoriali:
Padova, 1995; Vol. 4, p 243.

1672
A. Hoepping et al./Bioorg. Med. Chem. 6 (1998) 1663±1672
22. Fietz, T.; Spies, H.; Pietzsch, H.-J.; Leibnitz, P. Inorg.
Chim. Acta 1995, 231, 233.
32. Davies, H. M. L.; Saikali, E.; Huby, N. J. S.; Gilliatt, V. J.;
Matasi, J. J.; Sexton, T.; Childers, S. R. J. Med. Chem. 1994,
37, 1262.
23. Wolfenstein, R.; Rolle, J. Chem. Ber. 1908, 41, 737.
33. Kozikowski, A. P.; Araldi, G. L.; Ball, R. G. J. Org. Chem.
1997, 62, 503.
24. Munoz, O.; Piovano, M.; Garbarino, J.; Hellwing, V.;
Breitmaier, E. Phytochem. 1996, 43, 709.
34. Meltzer, P. C.; Liang, A. Y.; Madras, B. K. J. Med. Chem.
1996, 39, 371.
25. Katzung, B. G. In Basic and Clinical Pharmacology; Kat-
zung, B. G., Ed.; Appleton & Lange: Norwalk, 1995; pp 1±8.
35. Johannsen, B.; Scheunemann, M.; Spies, H.; Brust, P.;
Wober, J.; Syhre, R.; Pietzsch, H. J. Nucl. Med. Biol. 1996, 23,
429.
26. Reith, M. E. A.; Xu, C. and Co€ey, L. L. Biochem. Phar-
macol. 1996, 52, 1435.
27. Reith, M. E. A.; Xu, C.; Zhang, L. A.; Co€ey, L. L. Nau-
nyn-Schmiedebergs Archives of Pharmacology 1996, 354, 295.
36. Carroll, F. I.; Gao, Y.; Rahman, M. A.; Abraham, P.;
Parham, K.; Lewin, A. H.; Boja, J. W.; Kuhar, M. J. J. Med.
Chem. 1991, 34, 2719.
28. Madras, B. K.; Spealman, R. D.; Fahey, M. A.; Neumeyer,
J. L.; Saha, J. K.; Milius, R. A. Mol. Pharmacol. 1989, 36,
518.
37. Madras, B. K.; Spealman, R. D.; Fahey, M. A.; Neumeyer,
J. L.; Saha, J. K. and Milius, R. A. Mol. Pharmacol. 1989, 36,
518.
29. Gracz, L. M.; Madras, B. K. J. Pharmacol. Exp. Ther.
1995, 273, 1224.
38. Stylli, C.; Theobald, A. E. Appl. Radiat. Isot., Int. J.
Radiat. Appl. Instrum. Part A 1987, 38, 701.
30. Boja, J. W.; Patel, A.; Carroll, F. I.; Rahman, M. A.; Phi-
lip, A.; Lewin, A. H.; Kopajtic, T. A.; Kuhar, M. J. Eur. J.
Pharmacol. 1991, 194, 133.
39. Thevenon-Emeric, G.; Tchapla, A.; Martin, M. J. Chro-
matog. 1991, 550, 267.
31. Kung, M. P.; Stevenson, D. A.; Plossl, K.; Meegalla, S. K.;
Beckwith, A.; Essman, W. D.; Mu, M.; Lucki, I.; Kung, H. F.
Eur. J. Nucl. Med. 1997, 24, 372.
40. Pagliara, A.; Khamis, E.; Trinh, A.; Carrupt, P.-A.; Tsai,
R.-S.; Testa, B. J. Liquid Chromatog. 1995, 181, 721.