- Email: [email protected]
Evaluation of the in vitro stability of gadolinium (III) polyoxometalates
Nuclear Copyright
Medicine &. Biology, Vol. 0 1997 Elsevier Science
24, pp. 123-125, Inc.
ISSN
1997
0969-8051/97/$17.00 + 0.00 PII SO969-8051(96)00124-2
ELSEVIER
Evaluation of the In Vitro Stability of Gadolinium (III) Polyoxometalates William J. Crooks III, ’ Gregory R. Choppin, ’ * Buck E. Rogers’ and Michael J. Welch’ ‘DEPARTMENT
OF CHEMISTRY, SCIENCES,
FLORIDA STATE MALLINCKRODT
UNIVERSITY, INSTITUTE
TALLAHASSEE, OF RADIOLOGY,
FL 32306, ST. LOUIS,
AND *DIVISION MO 63 110
OF RADIATION
ABSTRACT.
The gadolinium chelates of lacunary polyoxometalates were evaluated for in vitro stability , endogenous metal cations, and DTPA-doped against rat serum, diethylenetriaminepentaacetic acid (DTPA) rat serum. The chelates dissociated rapidly in rat serum. Challenges by DTPA gave relatively slower dissociation rates, whereas challenges by endogenous metal cations (Fe(W), Zn(II), and C&I)) occurred at a rate comparable to the serum challenge, suggesting the instability in serum is due to a transmetalation mechanism. Challenges by DTPA-doped serum gave slower rates of dissociation than in native serum, verifying the transmetalation mechanism. NUCL MED BIOL 24;2:123-125, 1997. 0 1997 Elsevier Science Inc.
KEY WORDS.
Polyoxometalate,
Gadolinium,
Lanthanide,
INTRODUCTION An investigation of polyoxometalates (POM) as soluble models of clay colloids for studying lanthanide and actinide migration in soils (15, 16) led to this research, whose goal was an evaluation of the sandwich complexes of Gd(POM), as potential contrast agents for magnetic resonance imaging (MRI). Our premise to examine this ligand system was because these anions form uncommonly stable complexes with metal cations. Although their stability constants with metal cations do not approach the magnitude for Gd(EDTA) (5, 6), we were interested in whether Gd(POM), would have kinetic stability in solution due to protection of Gd(II1) by the extremely large ligands. In addition, the hydrophilic character of these complexes may give biodistributions very different from metalorganic ligand complexes. For numerous POM, the in wivo stability, pharmacokinetics, and variable toxicity were established in research on their assessment as anti-HIV agents (8, 9, 11). Our primary interest was to evaluate the in vitro stability of the unusually large, water-soluble gadolinium chelates of POM with regard to their application in imaging and nuclear medicine. POM are inorganic cluster-like compounds composed of transition metals and oxide ions (7, 14). The principal subunits in these structures are the MO, (M = W, MO) octahedra that surround one or more of the central X0, (X = I’, Si, Ge) tetrahedra. The subunits are connected by corner-shared and edged-shared oxo-bridges composed of metal-oxygen-metal bonds, forming discrete species of high symmetry called plenary SrrUctUres (e.g., SiW1z0404and ). The plenary structures may be converted into their ~2w,*Q26defect derivatives, the lacunary structures (e.g., SiW110388and lo-) by loss of a [WO14+ subunit. The defect structures p2w1706, (L) can bind lanthanide and actinide cations (M), forming ML, complexes in the solid state in which the cation is sandwiched between the defect site of two ligands (10, 13, 19).
*Author for correspondence. Received Accepted 24 June 1996.
E-mail:
choppinQchemmail.chem.fsu.edu
Stability,
Transmetalation
MRI agents have been developed using the lanthanide cation Gd(III) based on the difference in the relaxation times of water molecules bonded to the Gd(II1) as a result of the high magnetic moment of Gd(II1). In ho, complexes such as Na,Gd( DTPA)( H,O) at certain sites decrease the electronic relaxation time (T,,) of proton nuclei of the bonded water relative to the unbonded water, resulting in an enhancement of image intensity at that site (14). Most attention has been devoted to organic ligands for complexing gadolinium due to the kinetic stability of the complexes, the high relaxivities (R = l/T,,), and possible target specificity. However, polyoxometalates are inorganic compounds that offer attractive features as ligands and as transport vehicles for gadolinium. For example, POM ligands have multidonor (oxygen) sites that may lead to kinetic stability due to strong bond formation to the Gd(II1). Also, if administered intravascularly, Gd(POM), complexes may experience minimal diffusion into the extravasculature space owing to their large size and hydrophilicity. Such complexes could be useful as blood pool markers for magnetic resonance angiography. The purpose of this study was to investigate the kinetic stability of gadolinium-polyoxometalates and the feasibility of developing these complexes as magnetic resonance imaging agents. In the present paper, we report the in vitro stability of Gd(POM), where POM = laclnnary polyoxometalates.
EXPERIMENTAL Materials The a-isomers of the lacunary ligand series, XM, ,O,,“(X = Si, Ge, P; M,, = W, ,, Mo,W,), and the cy-2-isomers of the Iucunary series, P2M’,,06110(M’ = W,,, Mo,W,~), were synthesized by literature procedures (l-3, 17, 18, 20) and were characterized by Fourier transform infrared spectroscopy (FTIR) and elemental analysis as reported previously (4). In aqueous solution, the lacunary ligands are stable between pH 5 and 8 (14). Working solutions (1.0 mM) of POM and DTPA (diethylenetriaminepentaacetic acid) were prepared by dissolving the solids in 0.1 M NaOAc, pH 5.5 (OAc = acetate). Working solutions of chal-
W. J. Crooks
124
lenger metals (0.125 mM) in 0.1 M NaOAc, pH 5.5, were prepared from CuCl, * 2Hz0, Fe(NO,), * 9Hz0, ZnSO, + 7HzO and GdCl, .6HzO. A working solution of ‘53Gd(OAc)3 (185 kBq/pL) in 0.1 M NaOAc, pH 5.5, was prepared from 153GdC1, stock solution (Amersham). Working solutions of each ‘53Gd(POM)z were prepared by mixing 5 I.LL of ‘53Gd(OAc)3 working solution and 10 ~.LL of POM working solution followed by incubation at room temperature for 30 min. Serum was separated from Sprague-Dawley rat blood using an Integrated Serum Separator Tube (Corvac). The radioactivity was measured with a well-type gamma counter CRC-10 (Capintec). TLC plates (for serum challenges: reversephase KC18 [octadecylsilane bonded, Whatman]; for all other experiments: Cellulose MN300 [Analtech]) were analyzed on a Bioscan Autochanger 400 with a System 200 Imaging Scanner using a high-efficiency collimator. Serum incubations were performed in a Shallow Form Shaking Bath (Precision Scientific) equlibrated at 37.4”C.
III et al.
n WSiWdJ39k
Ed GdW,,‘hk
80
~(p2~1706lh
5
30
60
time (mill) FIG. 1. In serum: Percentage as a function of time.
Procedures Preliminary experiments were performed to assess the chromatographic separation between i5sGd(POM)z and 153Gd(OAc)s for serum and metal challenge studies, and ‘51Gd(POM), and 153Gd(DTPA) for DTPA challenge experiments. For serum challenges, the R, values for 153Gd(OAc)3 and ‘53Gd(POM), were 0 and 0.8, respectively (10% NH,OAc, reverse-phase KC18). For metal challenges, the Rf values for 153Gd(OAc)3 and 153Gd(POM)z were 0.9 and 0.4, respectively (10% NH,OAc, Cellulose MN300). For DTPA challenge experiments, the R, values for iS3Gd(DTPA) and ‘53Gd(POM), were 0.9 and 0.4, respectively (10% NH,OAc, Cellulose MN300). For serum stability studies 15 PL of ‘53Gd(POM)z working solution was added to 150 FL of serum. FOT DTPA challenge experiments, 1 mM, 10 mM, and 100 mM DTPA stock solutions were used. A 5-p,L aliquot of 153Gd(POM), working solution was added to 3.75 FL of each (DTPA), pH 5.5, followed by 10 FL of 0.4 M NaOAc, pH 5.5. Metal challenge experiments were performed by addition of a 20sPL aliquot of 125 mM of Fe(III), Zn(I1) or Cu(I1) solution to a vial containing: 30 FL of ‘53Gd,Gd(SiW1103,)z (100 JLL of Gd(OAc),, and 5 I.LL of ‘53Gd(OAc)3) in 0.1 M NH,OAc, pH 5.5 and 50 FL of 125 mM l&and solution). For stability studies in DTPA-doped serum, the serum was prepared by addition of 10 I.LL of 10 mM DTPA to 200 I.LL of serum followed by vortexing and incubation at 37.4”C for 30 min. To the doped serum was added 5 p,L of 153Gd(SiW,,03,),. The solution of the metal complex in the challenging medium was vortexed for 5 sec. Time zero was marked at the time of mixing. At various time points, 2 I.LL of the reaction mixture was spotted on a lo-cm TLC plate, developed in 10% NH,OAc and scanned for radioactivity as a function of R, value. A total of 10,000 radioactive counts per plate were recorded.
of Gd(POM),
remaining
intact
of the complex remained intact after 3 h. In contrast, when was challenged by Cu(II), Zn(II), and Fe(III), Gd(SiW,,03,)zi3~ demetalation occurred within 5 min. These two results suggest two different mechanisms for the kinetic decomposition of Gd(POM)z: transligation and transmetalation, respectively. In serum, the observed rates of disappearance of the complex are consistent with the transmetalation examples. To verify the transmetalation mechanism in serum, Gd(SiW,1039)z13was evaluated in DTPA-doped serum. DTPA was incubated in serum prior to addition of the complex to reduce the amount of endogenous metal cations available. The significantly slower kinetics in the DTPA-doped serum (Fig. 2) support the hypothesis of transmetalation as the mechanism of instability. The slow decomposition of Gd(POM), in the presence of DTPA demonstrates that Gd(ll1) is effectively sequestered between the two Iucunary ligands; however, the decomposition is fast in the presence of metal cations. These results suggest that the transliga-
n WsiW1+%9k q Gd(PWdbk
80
e .I e
60
40 RESULTS
AND
DISCUSSION
In
serum, Gd(POM), (POM = PW1,03a7-, SiW1103,8-, or were almost quantitatively demetalated within 5 P,w,706iie-) min (Fig. 1). To determine factors responsible for this rapid decomposition, the complexes were challenged by DTPA, endogenous metal cations, and DTPA-doped serum. DTPA, which was used as a representative strong chelator for Gd(III) (log plol = 22), gave relatively slow demetalation kinetics. For example, when by a 5-mole excess of DTPA, 55% Gd(SiW, ,03a)z 13- is challenged
5
30
60
time (mill) FIG. 2. In DTPA-doped serum: Percentage maining intact as a function of time.
of Gd(POM),
re-
Stability
of Gadolinium
125
Polyoxometalates
tion reactions may be impeded by a repulsive DTPA-ligand (anionanion) interaction. In contrast, the transmetalation reaction is accelerated by an attractive metal-&and (cation-anion) interaction. Chelation of the challenging metal to the surface oxygens of one of the ligands in the Gd( POM), complex weakens the Cd-POM bond, which allows faster exchange with the challenger metal. In conclusion, although the magnitude of the thermodynamic stability constants of Eu(III) with lacunary POM (log 13ioz = 11-14) (5) indicates strong interaction, the Gd(POM)z complexes are rapidly dissociated in serum, and the mechanism of instability apparently involves transmetalation by endogenous metal cations. Consequently, applicability of these complexes as MRI agents is severely limited unless this instability can be circumvented by design of more kinetically stabilizing features (e.g., more basic oxyanions, larger cavity sizes). This research was supported at Florida with Mallinckrodt Medical Inc.
State
University
by a contract
References 1. Abessi M., Cadot A., Thouvenot R. and Her& G. (1991) Dawson type heteropolyanions 1. Multinuclear (i’P, 51V, Is’W) NMR structural investigation of octadeca(molybdotungstovanado)-diphosphates o-1,2,3[P,MM’,W,,O,,]“(M, M’ = MO, V, W): Synthesis of new related compounds. Inorg. Chem. 30, 1695-1702. 2. Cadot E., Thouvenot R., T&e A. and Her& G. (1992) Synthesis and multinuclear characterizations of a-SiMoZW,0,,8and cY-SiMo,_,V,W,0,,‘4’“‘(x = 1, 2) h eteropolyoxometalates. Inorg. Chrm. 31, 412884133. 3. Contant R. (1990) Potassium octadecatungstodiphosphates(V) and related lacunary compounds. In: Inorganic Synthesis (Edited by Ginsberg A. P.), pp. 104-111. John Wiley, New York. 4. Crooks W. J. III, Choppin G. R. and Saito A. (1994) A general technique for heteropolyanion analysis using inductively coupled plasmaatomic emission spectroscopy. Anal. Len. 27, 2737-2750. 5. Crooks W. J. III (1995) Studies of lanthanide interactions with polyoxomemlates: Evaluation as magnetic resomrnce imaging agents. Ph.D. dissertation, The Florida State University, Tallahassee. 6. Contant R. and Ciabrini J.-P. (1982) Stabilities of metal (II) alkaline ion complexes of heteropolytungstates: Part 1. Comparative stabilities of isomeric a-metallo-17-tungsto-2-photophates. J. Chem. Res. (S), 50-51.
7. Day V. W. and Klemperer W. G. (1985) Metal oxide chemistry in solution: The early transition metal polyoxoanions. Science 228,533-541. 8. Hill C. L., Hartnup M., Faraj M., Weeks M., Prosser-McCarthy C. M., Brown R. B. Jr., Kadkhodayan M., Sommadossi J-P. and Schinazi R. F. (1990) Polyoxometalates as inorganic anti-HIV-l compounds: Structure-activity relationships. Adu. C&mother. AIDS, 3341. 9. Hill C. L., Weeks M. and Schanazi R. F. (1990) Anti-HIV 1 activity, toxicity and stability studies of representative structural families of polyoxometalates. .J. Med. Chem. 33, 2767-2772. 10. Iball J., Low J. N. and Weakley T. J. R. (1974) Heteropolytungstate complexes of lanthanoid elements: Part 111. Crystal structure of sodium decatungstocerate(IV)-water (l/30). J. Chem. Sot. Dalton Trans. 20212024. 11. Ni L., Boudinot D., Boudinot S. G., Henson G. W., Bossard G. E., Martellucci S. A., Ash P. W., Fricker S. P., Darkes M. C., Theobald B. R., Hill C. L. and Schinazi R. F. (1994) Pharmacokinetics and antiviral polyoxometalates in rats. Antimicrob. Agents Chemother. 38, 504510. 12. Lauffer R. B. (1987) Paramagnetic metal complexes as water proton relaxation agents for NMR imaging: Theory and design. Chem. Reu. 87, 901-927. 13. Molchanov V. N., Kazanskii L. P., Torchenkova E. A. and Simonov V. I. (1979) Crystal structure of potassium cerium phosphotungstate hydrate [K,,Ce(P,W,,O,,), . nH,O] (n = 50). Krismllografiya 24, 167168. 14. Pope M. T. (1983) Heteropoly and Isopoly Oxomemlates. SpringerVerlag, Berlin. 15. Saito A. and Shibukawa J. (1988) Determination of the stability constants of europium(II1) and thorium(lV) with a heteropolyphosphate ion P,W,,06z6m. 1. Radioanal. Nucl. Chem. Lett. 126, 189-197. 16. Saito A. and Choppin G. R. (1991) Interaction of metal cations with heteropolytungstate ions SiW,,0404and P,W,,06,6m. Inorg. Chem. 30, 45634566. 17. T&e A. and Her& G. (1990) (Y-, B- and y-Dodecatungstosilicic acids: Isomers and related lacunary compounds. In: Inorganic Synthesis (Edited by Ginsberg A. P.), pp. 85-96. John Wiley, New York. 18. Tourne C. and Tourne G. (1969) L es series D’heteropolyanions de types XW,,, XZW,,, X’,W,,, X’,ZW,, et leurs homologues molybddiques: 1. Relations g&n&ales et formation. Bull. Sot. Chim. Fr. 4, 1124-1136. 19. Toume C. M., Tourne G. and Brianso M. C. (1980) Bis(undecatungstogermanto)uranate(IV) de cesium: Cs,,[U(GeW,,O,,),] . 13-14H,O. Acm Cryst. B36, 2012-2018. 20. Weakley T. J. R. and Malik S. A. (1967) Heteropolyanions containing two different heteroatoms. .J. Inorg. Nncl. Chem. 29, 2935-2944.
Medicine &. Biology, Vol. 0 1997 Elsevier Science
24, pp. 123-125, Inc.
ISSN
1997
0969-8051/97/$17.00 + 0.00 PII SO969-8051(96)00124-2
ELSEVIER
Evaluation of the In Vitro Stability of Gadolinium (III) Polyoxometalates William J. Crooks III, ’ Gregory R. Choppin, ’ * Buck E. Rogers’ and Michael J. Welch’ ‘DEPARTMENT
OF CHEMISTRY, SCIENCES,
FLORIDA STATE MALLINCKRODT
UNIVERSITY, INSTITUTE
TALLAHASSEE, OF RADIOLOGY,
FL 32306, ST. LOUIS,
AND *DIVISION MO 63 110
OF RADIATION
ABSTRACT.
The gadolinium chelates of lacunary polyoxometalates were evaluated for in vitro stability , endogenous metal cations, and DTPA-doped against rat serum, diethylenetriaminepentaacetic acid (DTPA) rat serum. The chelates dissociated rapidly in rat serum. Challenges by DTPA gave relatively slower dissociation rates, whereas challenges by endogenous metal cations (Fe(W), Zn(II), and C&I)) occurred at a rate comparable to the serum challenge, suggesting the instability in serum is due to a transmetalation mechanism. Challenges by DTPA-doped serum gave slower rates of dissociation than in native serum, verifying the transmetalation mechanism. NUCL MED BIOL 24;2:123-125, 1997. 0 1997 Elsevier Science Inc.
KEY WORDS.
Polyoxometalate,
Gadolinium,
Lanthanide,
INTRODUCTION An investigation of polyoxometalates (POM) as soluble models of clay colloids for studying lanthanide and actinide migration in soils (15, 16) led to this research, whose goal was an evaluation of the sandwich complexes of Gd(POM), as potential contrast agents for magnetic resonance imaging (MRI). Our premise to examine this ligand system was because these anions form uncommonly stable complexes with metal cations. Although their stability constants with metal cations do not approach the magnitude for Gd(EDTA) (5, 6), we were interested in whether Gd(POM), would have kinetic stability in solution due to protection of Gd(II1) by the extremely large ligands. In addition, the hydrophilic character of these complexes may give biodistributions very different from metalorganic ligand complexes. For numerous POM, the in wivo stability, pharmacokinetics, and variable toxicity were established in research on their assessment as anti-HIV agents (8, 9, 11). Our primary interest was to evaluate the in vitro stability of the unusually large, water-soluble gadolinium chelates of POM with regard to their application in imaging and nuclear medicine. POM are inorganic cluster-like compounds composed of transition metals and oxide ions (7, 14). The principal subunits in these structures are the MO, (M = W, MO) octahedra that surround one or more of the central X0, (X = I’, Si, Ge) tetrahedra. The subunits are connected by corner-shared and edged-shared oxo-bridges composed of metal-oxygen-metal bonds, forming discrete species of high symmetry called plenary SrrUctUres (e.g., SiW1z0404and ). The plenary structures may be converted into their ~2w,*Q26defect derivatives, the lacunary structures (e.g., SiW110388and lo-) by loss of a [WO14+ subunit. The defect structures p2w1706, (L) can bind lanthanide and actinide cations (M), forming ML, complexes in the solid state in which the cation is sandwiched between the defect site of two ligands (10, 13, 19).
*Author for correspondence. Received Accepted 24 June 1996.
E-mail:
choppinQchemmail.chem.fsu.edu
Stability,
Transmetalation
MRI agents have been developed using the lanthanide cation Gd(III) based on the difference in the relaxation times of water molecules bonded to the Gd(II1) as a result of the high magnetic moment of Gd(II1). In ho, complexes such as Na,Gd( DTPA)( H,O) at certain sites decrease the electronic relaxation time (T,,) of proton nuclei of the bonded water relative to the unbonded water, resulting in an enhancement of image intensity at that site (14). Most attention has been devoted to organic ligands for complexing gadolinium due to the kinetic stability of the complexes, the high relaxivities (R = l/T,,), and possible target specificity. However, polyoxometalates are inorganic compounds that offer attractive features as ligands and as transport vehicles for gadolinium. For example, POM ligands have multidonor (oxygen) sites that may lead to kinetic stability due to strong bond formation to the Gd(II1). Also, if administered intravascularly, Gd(POM), complexes may experience minimal diffusion into the extravasculature space owing to their large size and hydrophilicity. Such complexes could be useful as blood pool markers for magnetic resonance angiography. The purpose of this study was to investigate the kinetic stability of gadolinium-polyoxometalates and the feasibility of developing these complexes as magnetic resonance imaging agents. In the present paper, we report the in vitro stability of Gd(POM), where POM = laclnnary polyoxometalates.
EXPERIMENTAL Materials The a-isomers of the lacunary ligand series, XM, ,O,,“(X = Si, Ge, P; M,, = W, ,, Mo,W,), and the cy-2-isomers of the Iucunary series, P2M’,,06110(M’ = W,,, Mo,W,~), were synthesized by literature procedures (l-3, 17, 18, 20) and were characterized by Fourier transform infrared spectroscopy (FTIR) and elemental analysis as reported previously (4). In aqueous solution, the lacunary ligands are stable between pH 5 and 8 (14). Working solutions (1.0 mM) of POM and DTPA (diethylenetriaminepentaacetic acid) were prepared by dissolving the solids in 0.1 M NaOAc, pH 5.5 (OAc = acetate). Working solutions of chal-
W. J. Crooks
124
lenger metals (0.125 mM) in 0.1 M NaOAc, pH 5.5, were prepared from CuCl, * 2Hz0, Fe(NO,), * 9Hz0, ZnSO, + 7HzO and GdCl, .6HzO. A working solution of ‘53Gd(OAc)3 (185 kBq/pL) in 0.1 M NaOAc, pH 5.5, was prepared from 153GdC1, stock solution (Amersham). Working solutions of each ‘53Gd(POM)z were prepared by mixing 5 I.LL of ‘53Gd(OAc)3 working solution and 10 ~.LL of POM working solution followed by incubation at room temperature for 30 min. Serum was separated from Sprague-Dawley rat blood using an Integrated Serum Separator Tube (Corvac). The radioactivity was measured with a well-type gamma counter CRC-10 (Capintec). TLC plates (for serum challenges: reversephase KC18 [octadecylsilane bonded, Whatman]; for all other experiments: Cellulose MN300 [Analtech]) were analyzed on a Bioscan Autochanger 400 with a System 200 Imaging Scanner using a high-efficiency collimator. Serum incubations were performed in a Shallow Form Shaking Bath (Precision Scientific) equlibrated at 37.4”C.
III et al.
n WSiWdJ39k
Ed GdW,,‘hk
80
~(p2~1706lh
5
30
60
time (mill) FIG. 1. In serum: Percentage as a function of time.
Procedures Preliminary experiments were performed to assess the chromatographic separation between i5sGd(POM)z and 153Gd(OAc)s for serum and metal challenge studies, and ‘51Gd(POM), and 153Gd(DTPA) for DTPA challenge experiments. For serum challenges, the R, values for 153Gd(OAc)3 and ‘53Gd(POM), were 0 and 0.8, respectively (10% NH,OAc, reverse-phase KC18). For metal challenges, the Rf values for 153Gd(OAc)3 and 153Gd(POM)z were 0.9 and 0.4, respectively (10% NH,OAc, Cellulose MN300). For DTPA challenge experiments, the R, values for iS3Gd(DTPA) and ‘53Gd(POM), were 0.9 and 0.4, respectively (10% NH,OAc, Cellulose MN300). For serum stability studies 15 PL of ‘53Gd(POM)z working solution was added to 150 FL of serum. FOT DTPA challenge experiments, 1 mM, 10 mM, and 100 mM DTPA stock solutions were used. A 5-p,L aliquot of 153Gd(POM), working solution was added to 3.75 FL of each (DTPA), pH 5.5, followed by 10 FL of 0.4 M NaOAc, pH 5.5. Metal challenge experiments were performed by addition of a 20sPL aliquot of 125 mM of Fe(III), Zn(I1) or Cu(I1) solution to a vial containing: 30 FL of ‘53Gd,Gd(SiW1103,)z (100 JLL of Gd(OAc),, and 5 I.LL of ‘53Gd(OAc)3) in 0.1 M NH,OAc, pH 5.5 and 50 FL of 125 mM l&and solution). For stability studies in DTPA-doped serum, the serum was prepared by addition of 10 I.LL of 10 mM DTPA to 200 I.LL of serum followed by vortexing and incubation at 37.4”C for 30 min. To the doped serum was added 5 p,L of 153Gd(SiW,,03,),. The solution of the metal complex in the challenging medium was vortexed for 5 sec. Time zero was marked at the time of mixing. At various time points, 2 I.LL of the reaction mixture was spotted on a lo-cm TLC plate, developed in 10% NH,OAc and scanned for radioactivity as a function of R, value. A total of 10,000 radioactive counts per plate were recorded.
of Gd(POM),
remaining
intact
of the complex remained intact after 3 h. In contrast, when was challenged by Cu(II), Zn(II), and Fe(III), Gd(SiW,,03,)zi3~ demetalation occurred within 5 min. These two results suggest two different mechanisms for the kinetic decomposition of Gd(POM)z: transligation and transmetalation, respectively. In serum, the observed rates of disappearance of the complex are consistent with the transmetalation examples. To verify the transmetalation mechanism in serum, Gd(SiW,1039)z13was evaluated in DTPA-doped serum. DTPA was incubated in serum prior to addition of the complex to reduce the amount of endogenous metal cations available. The significantly slower kinetics in the DTPA-doped serum (Fig. 2) support the hypothesis of transmetalation as the mechanism of instability. The slow decomposition of Gd(POM), in the presence of DTPA demonstrates that Gd(ll1) is effectively sequestered between the two Iucunary ligands; however, the decomposition is fast in the presence of metal cations. These results suggest that the transliga-
n WsiW1+%9k q Gd(PWdbk
80
e .I e
60
40 RESULTS
AND
DISCUSSION
In
serum, Gd(POM), (POM = PW1,03a7-, SiW1103,8-, or were almost quantitatively demetalated within 5 P,w,706iie-) min (Fig. 1). To determine factors responsible for this rapid decomposition, the complexes were challenged by DTPA, endogenous metal cations, and DTPA-doped serum. DTPA, which was used as a representative strong chelator for Gd(III) (log plol = 22), gave relatively slow demetalation kinetics. For example, when by a 5-mole excess of DTPA, 55% Gd(SiW, ,03a)z 13- is challenged
5
30
60
time (mill) FIG. 2. In DTPA-doped serum: Percentage maining intact as a function of time.
of Gd(POM),
re-
Stability
of Gadolinium
125
Polyoxometalates
tion reactions may be impeded by a repulsive DTPA-ligand (anionanion) interaction. In contrast, the transmetalation reaction is accelerated by an attractive metal-&and (cation-anion) interaction. Chelation of the challenging metal to the surface oxygens of one of the ligands in the Gd( POM), complex weakens the Cd-POM bond, which allows faster exchange with the challenger metal. In conclusion, although the magnitude of the thermodynamic stability constants of Eu(III) with lacunary POM (log 13ioz = 11-14) (5) indicates strong interaction, the Gd(POM)z complexes are rapidly dissociated in serum, and the mechanism of instability apparently involves transmetalation by endogenous metal cations. Consequently, applicability of these complexes as MRI agents is severely limited unless this instability can be circumvented by design of more kinetically stabilizing features (e.g., more basic oxyanions, larger cavity sizes). This research was supported at Florida with Mallinckrodt Medical Inc.
State
University
by a contract
References 1. Abessi M., Cadot A., Thouvenot R. and Her& G. (1991) Dawson type heteropolyanions 1. Multinuclear (i’P, 51V, Is’W) NMR structural investigation of octadeca(molybdotungstovanado)-diphosphates o-1,2,3[P,MM’,W,,O,,]“(M, M’ = MO, V, W): Synthesis of new related compounds. Inorg. Chem. 30, 1695-1702. 2. Cadot E., Thouvenot R., T&e A. and Her& G. (1992) Synthesis and multinuclear characterizations of a-SiMoZW,0,,8and cY-SiMo,_,V,W,0,,‘4’“‘(x = 1, 2) h eteropolyoxometalates. Inorg. Chrm. 31, 412884133. 3. Contant R. (1990) Potassium octadecatungstodiphosphates(V) and related lacunary compounds. In: Inorganic Synthesis (Edited by Ginsberg A. P.), pp. 104-111. John Wiley, New York. 4. Crooks W. J. III, Choppin G. R. and Saito A. (1994) A general technique for heteropolyanion analysis using inductively coupled plasmaatomic emission spectroscopy. Anal. Len. 27, 2737-2750. 5. Crooks W. J. III (1995) Studies of lanthanide interactions with polyoxomemlates: Evaluation as magnetic resomrnce imaging agents. Ph.D. dissertation, The Florida State University, Tallahassee. 6. Contant R. and Ciabrini J.-P. (1982) Stabilities of metal (II) alkaline ion complexes of heteropolytungstates: Part 1. Comparative stabilities of isomeric a-metallo-17-tungsto-2-photophates. J. Chem. Res. (S), 50-51.
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