Table 2 T 1-relaxivities (r1) of GdDOTA-(BOM)4 in blood and plasma
NMR spectra were obtained on a Bruker Spectrospin
(20 MHz).
Avance DPX300 (Bruker GmbH, Germany). Electrospray (ES)
FT-ICR-MS mass spectra were recorded on a Bruker BioApex
4.7 T (Bruker GmbH, Germany).
r1(25 ЊC)/sϪ1 mMϪ1
r1(37 ЊC)/sϪ1 mMϪ1
Blood
Plasma
54.3
57.4
62.5
67.4
tert-Butyl DOTA-(BOM)4 (3)
A suspension of cyclen (1) (121 mg, 0.7 mmol), tert-butyl
3-benzyloxy-2-bromopropionate (2) (1.55 g, 4.9 mmol) and
potassium carbonate (677 mg, 4.9 mmol) in DMF (10 ml) was
heated to 100 ЊC under argon. After 24 h the mixture was co-
oled and filtered. The filtrate was evaporated in vacuo, and the
residue was submitted to flash chromatography (SiO2, CH2Cl2/
MeOH 9 : 1). Yield: 230 mg (30%). Mass spectrum (ES): m/z
1
555 [[M ϩ 2H]2ϩ, C64H94N4O12], 527, 499, 471, 443. H NMR
(300 MHz, CDCl3): δ 1.25–1.50 (m, 36H), 2.20–2.75 (m, 9H),
2.90–3.20 (m, 7H), 3.40–4.00 (m, 12H), 4.25–4.75 (m, 8H),
7.10–7.35 (m, 20H). 13C NMR (75 MHz, CDCl3): δ 27.70,
27.85, 27.98, 28.01, 45.75, 46.67, 47.00, 47.84, 60.67, 61.05,
61.25, 61.51, 61.99, 64.85, 65.29, 65.66, 73.27, 73.48, 73.60,
81.93, 82.11, 82.22, 83.34, 127.20, 127.30, 127.45, 127.52,
127.56, 127.67, 127.97, 128.24, 128.27, 128.32, 128.51, 136.60,
137.44, 137.60, 168.80, 172.63, 172.62.
DOTA-(BOM)4 (4)
tert-Butyl DOTA-(BOM)4 (3) (183 mg, 0.16 mmol) was dissol-
ved in trifluoroacetic acid (5 ml). After stirring at room temper-
ature for 16 h the solution was evaporated in vacuo. The residue
was dissolved in methanol (5 ml) and diethyl ether (50 ml) was
slowly added. The resulting precipitate was washed with diethyl
ether and dried. Yield: 73 mg (50%). Mass spectrum (ES): m/z
Fig. 3 T 1-relaxivity (r1) of GdDOTA-(BOM)4 in the presence of
3.5 mM BSA as a function of temperature (20 MHz, pH 7.4).
exhibit relatively long τM values, due to the dissociative mechan-
ism of water exchange.10 This means that the T 1-relaxivity
enhancement expected as a result of the increase in τR by the
formation of the GdDOTA-(BOM)4-BSA adduct is to some
extent quenched by the suboptimal τM value. When increasing
the temperature this value decreases, resulting in higher T 1-
relaxivities. At one point optimal water exchange conditions
appear, and the T 1-relaxivity becomes dependent on τR. This
effect has not been observed for low molecular weight Gd()
complexes, except for some neutral GdDTPA-bisamide deriv-
atives (DTPA = diethylenetriamine pentaacetic acid).
These results indicated that GdDOTA-(BOM)4 might have a
potential as a “blood-pool” imaging agent. To further study
this potential, measurements of the T 1-relaxivity in human
whole blood and plasma were carried out. The results are
summarised in Table 2.
The T 1-relaxivities in both blood and plasma were consider-
ably higher than the values obtained in 0.6 mM BSA, and even
somewhat higher than those obtained in 3.5 mM BSA. The
explanation might be that blood and plasma contain additional
proteins and lipoproteins other than albumin, increasing the
amount of bound Gd() complex. Another explanation can be
that GdDOTA-(BOM)4 binds more strongly to HSA than to
BSA.
In conclusion, a novel Gd() complex, GdDOTA-(BOM)4,
has been prepared. The complex exhibits strong interactions
with albumin and other blood components resulting in high
T 1-relaxivity values. The T 1-relaxivity is dependent on the
temperature, with increasing values from 5 ЊC to 37 ЊC. The
T 1-relaxivity obtained in blood, 62.5 sϪ1 mMϪ1 (20 MHz,
37 ЊC), makes it an interesting candidate as an MRI contrast
agent for angiographic applications.
885 [[M ϩ H]ϩ, C48H61N4O12], 795, 705, 617, 443 [M ϩ 2H]2ϩ
.
1H NMR (300 MHz, D2O): δ 2.25–3.50 (m, 16H), 3.51–4.50
(m, 12H), 4.70–5.25 (m, 8H), 7.25–8.25 (m, 20H). 13C NMR
(75 MHz, D2O): δ 48.02, 48.26, 64.40, 69.54, 73.78, 128.98,
129.13, 129.22, 129.54, 139.19, 178.95.
GdDOTA-(BOM)4 (5)
DOTA-(BOM)4 (4) (70 mg, 0.08 mmol) and GdCl3ؒ6H2O
(30 mg, 0.08 mmol) was dissolved in water (20 ml) and the pH
was adjusted to 7.8. After stirring at room temperature for 4 h
the temperature was raised to 80 ЊC for 1 h. The temperature
was then allowed to fall to room temperature overnight. The
pH of the solution was adjusted to 9.2, and the resulting
precipitate removed by filtration. The pH of the filtrate was
adjusted to 7.4. The filtrate was evaporated in vacuo yielding
60 mg of a slightly yellow solid.
Mass spectrum (ES): m/z 1040 [[M ϩ 2H]ϩ, C48H58N4O12Gd].
Relaxometric characterisation of GdDOTA-(BOM)4
The relaxation measurements were performed at 20 MHz on a
Bruker Minispec mq 20 NMR Analyzer (Bruker Analytik
GmbH, Rheinstetten, Germany). The temperature was con-
trolled with a HAAKE DC10 circulator (Gebrüder HAAKE
GmbH, Karlsruhe, Germany). The T 1-relaxation times were
obtained by the inversion recovery method, and the T 1-relaxiv-
ity (r1) was calculated using eqns. (1) and (2),
(1)
(2)
Experimental
Synthesis
Reagents were obtained from Aldrich Chemical Co. Inc., USA
or Fluka Chemie AG, Switzerland, and used as received. Cyclen
(1,4,7,10-tetraazacyclododecane) was a gift from Bracco S.p.A
(Milan, Italy).
where R1obs and R1m are the relaxation rates in sϪ1 of the sample
and the matrix, respectively, and C is the Gd() concentration
in mM.
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 1 7 0 7 – 1 7 1 0
1709