Download Chelate chemistry for molecular imaging Ligands versus Chelates

9/28/2012
Take home lessons: What you should learn
from todays lecture
Chelate chemistry for molecular imaging
• Choose the proper ligand for a particular metal ion.
Why is this important?
A. Dean Sherry, PhD
• Macrocyclic ligands versus acylic ligands: pros & cons
Professor of Chemistry, University of Texas at Dallas
Director, Advanced Imaging Research Center, University of Texas Southwestern
Medical Center
• Which is more important for metal chelate
applications in vivo? Thermodynamics or kinetics
Professor of Radiology, UT Southwestern Medical Center
WMIC, Dublin, Sept 5, 2012
Ligands
Some common ligands used for chelating metal ions
Chelates
versus
A ligand is defined as any anion or organic molecule
that donates a pair of electrons to a metal ion.
Acyclic ligands
Macrocyclic ligands
HO2C
N
N
N
N
HOOC
CO2H
HO2C
N
N
CO2H
HO2C
N
O2C
HO2C
-
O2C
CO2H
HO2C
Ethylenediaminetetraacetate
(EDTA) a hexadentate ligand
Nitrilotriacetate (NTA) classic
example of a tetradentate ligand
• The resulting ML complexes are called
chelates.
HN R
N
N
N
N
CO2H
DO3A-monoamide
N
HO2C
N
N
O
HO2C
DOTA side-arm
version
N
N
N
N
CO2H
H
N R
HO2C
NOTA backbone
version
HO2C
N
HO2C
HO2C
CO2H
N
N
N
N
N
N
CO2H
H
R N
HN R
S
PCTA backbone
version
O
HN R
HOOC
HOOC
COOH
N
HOOC
OH
N
H2N
O
O
O
N
H
N
OH
desferrioxamine
HO2C
N
N
O
N
OH
N
O
N
N
HO2C
NETA
Question: So, with all these choices, how does one
choose the best ligand or bifunctional ligand for your
particular molecular imaging application?
Answer:
NH
N R
H
CB-TETA side-arm
version
CHX-DTPA
CO2H
PCTA
HO2C
S
HO2C
HN
N
N
CO2H
DOTA backbone
version
N
HO2C
CO2H
Bifunctional ligands are widely used to attach a ligand to
a targeting moiety  bifunctional chelates (BFC)
NH
CO2H
N
HO2C
N
N
HO2C
NOTA
N
CB-TE2A
Typical chelates
Gd(DTPA)268Ga(NTA)
Fe(EDTA)-
O
HO2C
N
COOH
DTPA
CO2H
NH
H
N R
N
N
N
COOH
S
H
R N
HO2C
HO2C
O
N
N
CO2H
Bifunctional ligands are widely used to attach a ligand to
a targeting moiety  bifunctional chelates (BFC)
N
CO2H
N
Multidentate ligands bind to a metal ion by displacing
solvent molecules (often water) to form a ML complex
• Multidentate ligands are called chelating agents (claw).
N
TETA
CO2This version of “cis-Pt”
has one bidentate ligand
(en) and two monodentate
ligands (Cl-)
N
COOH
N
N
HO2C
DOTA
N
“cis-platinum” has two
types of monodentate
ligands, :NH3 and Cl-
HO2C
HO2C
CO2H
N
HOOC
-
N
N
COOH
N
HOOC
COOH
DTPA backbone
version
N
N
COOH
O
COOH
•
•
•
•
It depends!
On size matching of metal ion and ligand cavity
Thermodynamics (how stable does the chelate need to be?)
Kinetics of ML complex formation
Charge on resulting chelate; perhaps it needs to be charged
or neutral
DTPA monoamide
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9/28/2012
Some common radionuclides and ligands
used in molecular imaging
Imaging
Modality
Most
common
metal ion
Most common ligand(s)
MRI
Gd3+
DOTA-like, DTPA-like
8-coordinate, needs
one water site, prefers
“hard” donor atoms
PET
64Cu2+
cyclen, DOTA, TETA,
bridged TETA, NOTA
Prefers N donors, only
4 coordinate necessary
68Ga3+
DOTA, NOTA
Prefers octahedral and
“hard” donor atoms
SPECT
Ligand
characteristics
• Selection of ligand donor atoms
(coord #, donor types)
86Y3+
DOTA-like
8-9 coordinate like
Gd3+
89Zr2+
Desferrioxamine-like
“hard” oxygen donors
99mTc2+
Methoxyisobutylisonitrile
(Sestamibi)
N donor atoms
111In3+
DTPA-like
8-coordinate,
somewhat “softer” ion
Factors to consider in the design of
ligands for various metal ions
• HSAB principles, matching coordination numbers
• Role of ligand architecture (cyclic versus acyclic)
• Limited to size-match selectivity
• Kinetics of ML complex formation & dissociation
• Ligands with flexible backbone structures form
complexes more quickly but also dissociate faster
• Macrocyclic ligands form complexes more slowly but also
dissociate more slowly
Hard-Soft Acid-Base Principles (HSAB)
Size does matter!
Ligand donor types (from softest to hardest): P < S < N < O < F
For 3d transition metal ions
21
Sc
22
Ti
23
V
24
Cr
25
Mn
26
Fe
27
Co
28
Ni
29
Cu
30
Zn
HO2C
1) M2+ ions get smaller and harder from left to right (increasing nuclear charge)
N
N
N
N
HO2C
Ti2+ (86 pm) > V2+ (79 pm) > Cr2+ (73 pm) > Mn2+ (67 pm) > Fe2+ (61 pm)
CO2H
HO2C
N
N
CO2H
N
CO2H
HO2C
DOTA
NOTA
2) M+ (softer) < M2+ (intermediate) < M3+ (harder)
V2+ (79 pm) >> V3+ (64 pm) >> V4+ (53 pm)
Gd3+ (4f7) ionic radius ~1.0Å
log Kst = 24.7a
Ga3+ (3d10) ionic radius ~0.6Å
log Kst = 21b, 26c
As one progresses down a series (3d  4d  5d) for ions of the same charge
22
Ti
hardest
25
Mn
hardest
30
Zn
hardest
40
Zr
intermed.
43
Tc
intermed.
48
Cd
intermed.
72
Hf
softest
75
Re
softest
80
Hg
softest
aWP
kf,HL
kd,HL
Potentiometry is considered the“gold standard”
12
12
LnL + H+
K ST

K HL
[ L]  [ H ]

[ HL]
et al., Inorg. Chem., 49, 10960-10969 (2010)
Thermodynamic Stability Constant Determinations:
100
10.5
10.5
Kcond = conditional stability constant (defined by conditions, i.e., pH)
[ LnL]

[ Ln 3 ]  [ L]
& Martell, Inorg. Chimica Acta, 181, 273-280 (1991)
cKubicek,
Fraction of Y3+ species
Ln3+ + HL
99
pH
7.5
7.5
Y3+
80
Y3+L
60
p-NO22-Bz-DOTA
-Bz-DOTA
p-NO
40
typical curve
curve for
for fast
fast
typical
ML formation
formation
ML
2+
M2+
p-NO22-Bz-DOTA
-Bz-DOTA
M
:: p-NO
(log K
KML
=23.5)
(log
ML=23.5)
20
0
1.5
2
2.5
pH
66
4.5
4.5
33
Thermodynamic ML
stability constant
Highest pKa of the ligand
(most basic site)
the reverse rxn is referred to as the first
protonation step or constant
log Kst = 31b
Cacheris, SK Nickle & AD Sherry, Inorg Chem, 26, 958-960 (1987)
bClark
Some basic definitions: thermodynamic constants,
kinetic constants, pKa’s and basicity
log Kst = 14.3a
3+
Y : slow kinetics
(log KML=23.9)
1.5
1.5
00
22
44
66
88
10
10
Base equivalents
2
9/28/2012
CO2-
-O2C
N
N
N
N
Ligand charge also plays a role
CO2-
-O2C
CO2-
-O2C
N
N
N
-O
CO2-
-O
-O2C
CO2N
2C
N
N
N
N
2C
CO2 -
CO2 -
-O
-O
2C
N
log Kst (Gd3+)
N
N
N
N
CNHCH2 CH 2 CH 3
CO2 -
-O
-O
2C
N
N
N
CO2 -
CO2 -
DO3A-SA
20.1b
24.7a
N
2C
DO3A-Npropylamide
CO2-
-O2C
N
2C
DOTA
N
-O2C
CO2-
O
CO2-
-O2C
-2 O
-2 O
3P
N
N
N
N
3P
PO3 -2
PO3 -2
DOTP
27.2c
28.8d
CO2-
N
aWP
Cacheris, SK Nickle & AD Sherry, Inorg Chem, 26, 958-960 (1987)
Sherry, RD Brown, CFGC Geraldes, SH Koenig, K-T Kuan & M Spiller, Inorg Chem., 28, 620-622 (1989)
Andre, E Brucher, R Kiraly, RA Carvalho, H Maecke, CFGC Geraldes, Helv. Chim. Acta, 88, 633 (2005).
Sherry, J Ren, J. Huskens, E Brucher, E. Toth, et al. Inorg. Chem. 1996,35, 4604 (1996)
bAD
cJP
-O2C
dAD
WP Cacheris, SK Nickle & AD Sherry, Inorg
Chem, 26, 958-960 (1987)
Correlation of kf,HL, kd,H, and log KML with the first
protonation constant of the ligand (log KHL)
kf,HL
Keq 
K ST 
LnL + H+
kd,HL
[ LnL]  [ H  ] k f , HL

 K ST  K HL
[ Ln 3 ]  [ HL] k d , H
k f , HL
log K ST 
k d , H  K HL
k f , HL
kd ,H
log KST
Ln3+ + HL
log KST vs ΣpKa’s for a series of GdL complexes
 pK a
From: WP Cacheris, SK Nickle & AD Sherry, Inorg Chem, 26, 958-960 (1987)
A more recent compilation: Brücher, et al., “Stability and Toxicity of Contast
Agents”, Chapter 5, in The Chemistry of Contrast Agents in Medical Magnetic
Resonance Imaging, edited by Merbach, Toth & Helm, Wiley, 2012.
Importance of kinetics:
rates of ML formation
30
DO3AP
DOTA-SA
26
DOTA
log K Gd(L)
p-NO2 -Bz-DOTA
DO3A-nBuAm
2HE-DO2A
PC2A
kd,HL
LnL + H+
DOTEP
Ln3+aq
DOTAM
DTMA
DOTA-4Gly
NOTA
fast
10
15
kf,HL
TRITA
BT-DO3A
BP2A
Ln3+ + HL
Why do macrocyclic ligands form complexes with metal ions so slowly?
DO2A
p-NO 2 -Bz-PCTA
14
DOTP
DOA3P
DO2A2P
DO3A
PCTA
18
DO3MA
DO3APABn
HP-DO3A
2HP-DO2A
DO3A-Pic
22
DO3APPrA
17
19
21
H
log K 1 + log K 2
23
H
slow, requires
deprotonation
25
H2DOTA (pre-organized)
Intermediate LnH2DOTA
Fully formed LnDOTA
3
9/28/2012
Correlation of kf,HL with the first protonation
constant of the ligand (log KHL)
The three bifunctional ligands were conjugated to trastuzumab
log kf,HL
8
4
10,5
oxo & pcta, 0.5 mg
HP-DO3A
HE-DO3A
The oxo and pcta derivatives
form complexes with 64Cu2+
much more quickly than dota
HIP-DO3A
DO3A
dota, 0.1 mg
12-membered ring, acetate systems
11
dota, 0.5 mg
oxo, 0.1 mg
pcta, 0.1 mg
DOTA
6
5
CL Ferreira, DTTT Yapp, S Crisp, et al., Eur J Nucl Med Mol Imaging 37: 2117-2126 (2010)
CuDOTA only moderately stable in vivo; CuNOTA and Cu-CB-TETA more stable but they also form
complexes too slowly for efficient antibody labeling
9
7
Comparison of bifunctional chelates for 64Cu2+-antibody imaging
11,5
12
12,5
13
DO3MA
13,5
log
pKKaHL
Transverse PET images at 24 h (top) and 40 h (bottom) post-injection for each 64Curadiolabeled BFC-trastuzumab conjugate in LCC6Vector or LCC6HER2 tumor-bearing mice.
K. Kumar and M. F. Tweedle, Inorg. Chem., 1993, 32, 4193-4199
Increased backbone rigidity improves the
kinetic stability of complexes
Are LnDOTA-tetraamide
complexes safe for in vivo use?
Gd(AZEP-DTPA)
Gd(PIP-DTPA)
AZEP-DTPA
Gd(DTPA)
PIP-DTPA
H-S Chong, K Garmestani, LH Bryant, Jr., MW Brechbiel, J Org Chem, 66, 7745-7750 (2001)
Take home lessons:
• Why it is important to choose the proper ligand for a particular
metal ion for molecular imaging.
• Ligand size and donor type should match the characteristics
of the metal ion
• Macrocyclic ligands versus acylic ligands:
• Macrocyclic ligands form more stable ML chelates but they
also form more slowly (this kinetic barrier may be important in
radiolabeling of antibodies which cannot be heated)
• Which is more important for metal chelate applications in vivo?
• Thermodynamics is important but kinetics is likely even
more important for in vivo applications!
Thanks
• To my many students, postdocs & collaborators
Prof. Zoltan Kovacs, Prof. Mark Woods, Bill Cacheris, Garry Kiefer,
Prof. Gyula Tirsco, Won-dae Kim, Prof. Jimen Ren, Prof. Istvan
Lazar, Shanrong Zhang, Prof. Carlos Geraldes, Prof. Jurriaan
Huskens, Flavio Chavez, Prof. Navin Bansal, Yunkou Wu and,
especially, Prof. Erno Brücher (Hungary) who taught me how to do
potentiometry well.
• Acknowledgements: Financial support from NIH grants RR-02584,
CA-115531, CA-126608, EB-004582 and the Robert A. Welch
Foundation (AT-584).
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