Yun Hee Jang, Mario Blanco, Siddharth Dasgupta,
William A. Goddard, III
MSC, Beckman Institute, Caltech
David A. Keire, John E. Shively
The Beckman Research Institute of the City of Hope
Chelating ligand (DOTA)
COO-
-OOC
N
N
90
3+
Y
N
N
-OOC
• Therapy: 90Y3+(64h)
• Diagnosis: 111In3+(2.8d)
64Cu2+(12.8h)
• MRI contrast agent: Gd3+
H
C
N
Antibody
O
b-emitting
targeting
(2.25 MeV, t1/2=64h)
Tumor cell
D. Parker,
Chem. Soc. Rev.
19, 271 (1990)
-O
O
O
NH+
COO-
-OOC
N
N
OO-
COO-
-OOC
+HN
N
N
N
O
COO-
O-
DOTA
DTPA
(log K=24.8)
(log K=22.1)
Thermodynamic
stability
O
Kinetic inertness
at pH 2~8 w.r.t. acidpromoted dissociation
<0.5% dissociated
over 18 days in serum
(pH 7.4, 37oC): inert
Rapid complexation
x1600 slower than
Y-DTPA formation
Not inert
leading to
bone-marrow toxicity
Lewis, Raubitschek and Shively, Bioconjugate Chem. 5, 565 (1994)
O
-O
Y3+
+
-O
O
O
NH+
O-
O
NH+
E.T. Clark and A.E. Martell,
Inorg.Chim.Acta 190, 27 (1991)
X.Y. Wang, et al.
Inorg.Chem. 31, 1095 (1992)
N
Y3+
N
O-
O-
+HN
N
O-
+HN
N
O
OO
fast
O
or
O-
O
-O
O
slow
O
-O
N
O-
N
Y3+
O
O
ON
N
O
NH+
N
O-
O-
Y3+
O-
O
N
N
O
OO
Y3+ + H2(DOTA)2-
Type I: labile
Type II: stable/inert
• Calculate structure/energy change occurring during complex-formation
• Identify the rate-determining step:
Deprotonation or conformation change?
• Design new chelating agent and predict its energetics/kinetics
• B3LYP/LACVP* // HF/LACVP* (6-31g* for C/H/O/N; Hay-Wadt ECP for Y)
• Vibration analysis  ZPE / thermodynamic quantity  Gibb’s free energy
• Continuum solvation calculation by solving Poisson-Boltzmann equation
• Jaguar 3.5 (Schrodinger Inc.)
-H+
YH2(DOTA)+
Y3+ outside the cage
-H+
YH(DOTA)
Y(DOTA)the same as x-ray structure
of final complex
• Y3+ moves into the cage spontaneously with deprotonation.
• RMS deviation between ring conformations < 0.5 Å.
• Deprotonation is the rate-determining step.
Direct attack of outside base on the ring proton? No room for it.
top view
side view
bottom view
Conformation change to the one favorable to attack?
Too high cost, especially, for YH(DOTA)
4-coordinate
YH2(DOTA)+
YH(DOTA)
3-coordinate
2-coordinate
16.6* (12.1)** kcal/mol 42.7* (34.5)** kcal/mol
21.6* (24.6)** kcal/mol
* 1.807 Å for r(Y)
** 1.673 Å for r(Y)
in solvation calculation
Proton transfer from ring NH to COO (more accessible to outside base)?
reactant (NH...COO)
Relative energy (kcal/mol)
Gas-phase E
Aqueous-phase
DGactivation(aq)
TS (N..H..COO)
reactant
0
0
TS
2.0
12.2 *,**
8.4 *,**
product (N...COOH)
Product
-25.2
-8.8* (-6.0)**
expt'l ***
8.1~9.3
***experimental DG for Eu,Gd,Ce,Ca-complexes (Inorg.Chem. 32, 4193 (1993))
• Proton transfer is easier than conformation change.
• Calculated activation free energy is in agreement with experimental value.
Structural change leading to more stable TS:
6-membered ring of DO3A1Pr rather than 5-membered ring of DOTA
-O
O
O
NH+
N
O-
-O
OO
O
+HN
N
one more CH2
DO3A1Pr (Pr=propionate)
TS (DO3A1Pr)
TS (DOTA)
B3LYP//HF in aqueous phase (kcal/mol)
DO3A1Pr
DOTA
Energy barrier
4.5
12.2
Activation free energy
3.9
8.4
-O
O
O
one more CH2
-O
O
N
N
O-
-O
O
O
N
N
+HN
O-
+HN
N
N
OO
OO
one more CH2
Hpr(DO3A1Pr): 0.0 kcal/mol
O
O-
Hac(DO3A1Pr): 7.8 kcal/mol
Protonation at propionate site is more stable.  6-membered ring TS
• Deprotonation from ring nitrogen is the rate-determining step.
• Deprotonation occurs by proton transfer from ring nitrogen to carboxylate.
• Adding CH2 to one carboxylate arm can improve the incorporation rate.
• Explicitly-coordinated water molecules
How many water molecules?
Effect on structure/energetics
• Introduction of amide linkage
-O
O
O
NH+
O-
N
?
O
+HN
N
N
OO
H
Caltech
City of Hope
William A. Goddard, III
John E. Shively
Siddharth Dasgupta
David Keire
Mario Blanco
Daniel Mainz
Sungu Hwang
Supported by
NSF