Inorganic Chemistry
177Lu]Lu3+ also yielded high molar activity (28.2 GBq/μmol
Article
HCl solution or from Curium Pharma as 0.02 M HCl solution.
177LuCl3 was produced by reactor-based indirect strategy53,54
[
at [H6dappa] = 10−6 M with a RCY > 90%). The formation
constant and pM value of Y3+-dappa were the lowest of the
studied metal ions, and similarly the radiolabeling of [90Y]Y3+
resulted in the lowest RCYs and molar activities. Very mild and
rapid radiolabeling was observed for all radionuclides studied,
as only 1 min at room temperature was required to reach the
maximum RCY. The kinetic inertness of the [111In3+][In-
(dappa)]3− complex was explored via serum stability studies. It
was observed that rapid decomplexation occurred upon
introduction of human serum proteins to a solution containing
[
176Yb(n,γ)177Yb → 177Lu, no carrier added; specific activity 3800−
3000 GBq/mg] and was purchased from ITG (Isotope Technologies
Garching) GmbH as 0.04 M HCl solution. 90YCl3 [90Sr/90Y
generator; carrier free] was purchased from Eckert & Ziegler
Strahlen- and Medizintechnik AG as 0.04 M HCl solution.
Radionuclide solutions were used within one-half-life (upon arrival)
to minimize reduction in specific activity.
Synthesis and Characterization. Methyl-6-bromomethylpico-
linate (1). Compound 1 was prepared according to the literature
preparation with appropriate characteristic spectra.55
(2-Carboxyethyl)phosphinic Acid (2). Compound 2 was prepared
according to the literature preparation with appropriate characteristic
spectra.33
[
111In][In(dappa)]3−, with near 50% degradation in 1 h, an
outcome highlighting that thermodynamic stability is not a
valid predictor or kinetic inertness or in vivo stability. DFT
calculations revealed the rapid dissociation is likely a result of
poor encapsulation of [111In]In3+ in the binding pocket of
H6dappa. This is unlike what has previously been observed
with H4octapa,30 leading to the conclusion that the
replacement of carboxylic acids with phosphinates has an
undesirable kinetic effect for “pa” family targeted radionuclide
delivery, likely as a result of the steric and electronic effects of
these newly introduced functional groups. Moreover, coordi-
nation by the aliphatic carboxylic acids likely results in the
formation of fluxional isomers at physiological pH; future
studies with amide-functionalized H6dappa are of further
interest to probe the influence of these proximal acid groups.
Overall, H6dappa may be better suited for applications
requiring rapid metal coordination.
N,N′-Dibenzylethane-1,2-diamine·2HCl (3). Ethylene diamine
(3.00 g, 3.34 mL, 49.9 mmol) and benzaldehyde (11.7 g, 11.2 mL,
110 mmol) were dissolved in MeOH (150 mL) in a round-bottom
flask. The solution was stirred and heated to reflux for 5 h, during
which time the yellow solution darkened into a yellow-orange color.
The solution was then allowed to cool to ambient temperature and
then cooled to 0 °C in an ice bath. NaBH4 (6.61 g, 175 mmol) was
added in several portions, and the solution was allowed to warm to
ambient temperature and stirred for 24 h. After LR-MS revealed
complete reduction of the imine intermediate, the solution was
evaporated via rotary evaporator to yield a yellow oil. Aqueous HCl (3
M, 50 mL) was slowly added to the oil to yield a white solid that was
filtered out and washed with acetone to yield 3 as a dihydrochloric
acid salt (13.1 g, 95%). 1H NMR (300 MHz, 298 K, D2O + NaOD):
δ 7.51−7.46 (m, 10H), 4.23 (s, 4 H), 3.37 (s, 4H). 13C{1H} NMR
(100 MHz, 298 K, D2O + NaOD): 131.3, 129.6, 129.6, 129.3, 51.6,
43.0. LR-ESI-MS: calcd for [C16H20N2 + H]+: 241.2; found [M +
H]+: 241.3. Elemental analysis: calcd % for C16H20N2·2HCl: C 61.35,
H 7.08, N 8.94; found: C 61.67, H 6.93, N 8.94.
Dimethyl-6,6′([ethane-1,2-diylbis{benzylazanediyl}]bis-
[methylene])dipicolinate (4). Compounds 1 (3.40 g, 14.7 mmol) and
3 (1.87 g, 5.97 mmol) were dissolved in ACN (120 mL) in a round-
bottom flask. K2CO3 (5.61 g, 40.6 mmol) was then added, and the
solution was stirred and heated to reflux for 3 d. The reaction mixture
was quenched with H2O (100 mL), and then DCM (140 mL) was
added for the first extraction. The phases were separated and the
aqueous phase was further washed with DCM (3 × 100 mL); the
combined organic phases were dried over MgSO4, filtered, and loaded
onto Celite and dried. The product was purified via column
chromatography using a silica column (CombiFlash Rf automated
column system, 120 g gold silica column, 100% hexane to 100%
EtOAc). The product fractions were rotary evaporated to yield an oil,
which solidified to a yellow solid upon standing at ambient
EXPERIMENTAL SECTION
■
Materials and Methods. All solvents and reagents were
purchased from commercial suppliers (Sigma-Aldrich, Fisher
Scientific, TCI America, Alfa Aesar, AK Scientific, Fluka) and were
used as received. Human serum was purchased frozen from Sigma-
Aldrich. Synthetic reactions were monitored by TLC (MERCK
Silicagel 60 F254, aluminum sheet). Radiolabeling reactions were
monitored by TLC (Silicagel 60 RP-18 F254S, aluminum sheet) and
HPLC (Knauer Smartline System consisting of Smartline 1000 pump,
K2501 diode array detector, Raytest Ramona Star activity detector,
Chromgate 2.8 software and a Smartline 5000 manager with a Zobax
SB-C18 column; Agilent 4.6 × 250 mm, 5 μm). Radio-TLC
chromatograms were scanned using a radioisotope thin layer analyzer
(BIOSCAN system 200 imaging scanner, Rita Star or Fuji BAS-
1800II, raytest); evaluation program AIDA. Flash chromatography
was performed using Redisep Rf HP silica columns and a Teledyne
Isco (Lincoln, NE) Combiflash Rf automated system. Water used was
ultrapure (18.2 MΩ cm−1 at 298 K, Milli-Q, Millipore, Billerica, MA).
1H, 13C{1H}, and 31P{1H} NMR spectra were recorded at ambient
temperature on Bruker AV300 and AV400 instruments; unless
otherwise specified the NMR spectra are expressed on the δ scale and
referenced to residual solvent peaks. Low resolution (LR) mass
spectrometry was performed using a Waters ZG spectrometer with an
ESCI electrospray/chemical-ionization source, and high-resolution
electrospray ionization mass spectrometry (HR-ESI-MS) was
performed on a Micromass LCT time-of-flight instrument at the
Department of Chemistry, University of British Columbia. Micro-
analyses for C, H, and N were performed on a Carlo Erba elemental
analyzer EA 1108. The HPLC system used for analysis and
purification of nonradioactive compounds consisted of a Waters 600
controller, A Waters 2487 dual wavelength absorbance detector and a
Waters delta 600 pump with a Phenomenex Synergi 4 μm hydro-RP
80 Å column (250 mm × 21.2 mm semipreparative) or a Zorbax
(Agilent) 300SB-C18, 300 Å, 5 μm, 9.4 mm × 250 mm column were
used for purification of H6dappa (6). 111InCl3 was cyclotron-produced
1
temperature (1.82 g, 57%). H NMR (400 MHz, 298 K, CDCl3): δ
7.95 (t, J = 4.4 Hz, 1H), 7.69 (d, J = 4.5 Hz, 2H), 7.30−7.15 (m,
∼5H; overlap with CDCl3), 3.97 (s, 3H), 3.82 (s, 2H), 3.57 (s, 2H),
2.68 (s, 2H). 13C{1H} NMR (100 MHz, 298 K, CDCl3): 165.8, 161.1,
147.1, 139.0, 137.3, 128.7, 128.3, 127.0, 125.9, 123.5, 60.5, 59.1, 52.9,
52.0. LR-ESI-MS: calcd for [C32H34N4O4 + H]+: 539.3; found [M +
H]+: 539.3.
Dimethyl-6,6′-([ethane-1,2-diylbis{azanediyl}]bis[methylene])-
dipicolinate (5). Compound 4 (1.18 g, 2.19 mmol) was dissolved in
50 mL of glacial acetic acid in a two-neck round-bottom flask. The
flask was purged with N2 and 10% w/w Pd/C (375 mg, 0.35 mmol)
added under a stream of N2. The flask was thrice purged with N2 and
then filled with H2 from a balloon. The mixture was stirred at room
temperature overnight under H2, and then Pd/C was removed by
filter paper, which was washed alternately with ACN (3 × 30 mL) and
3 M aqueous HCl (2 × 10 mL). The solution was evaporated and the
dark orange oil dissolved in minimal ACN and then run through a
small Celite plug to remove residual Pd/C. The solvent was removed
by rotary evaporator with EtOH added and evaporated twice to
remove residual acetic acid. Following in vacuo solvent removal, the
dark orange oil solidified (712 mg, 90%) and was used without further
purification. 1H NMR (400 MHz, 298 K, D2O): δ 8.20 (d, J = 7.4 Hz,
[
112Cd(p,2n)111In; no carrier added; Fe, Cd, Cu, Pb, Zn, Ni each
≤100 ng/mCi] and purchased from BWX Technologies as 0.05 M
J
Inorg. Chem. XXXX, XXX, XXX−XXX
Jaraquemada-Pelaéz, Mariá De Guadalupe
