Inorg. Chem. 2003, 42, 2818−2820
Toward Novel DNA Binding Metal Complexes: Structure and Basic Kinetic Data of [M(9MeG)2(CH3OH)(CO)3]+ (M ) 99Tc, Re) Fabio Zobi, Bernhard Spingler, Thomas Fox, and Roger Alberto* Institute of Inorganic Chemistry, UniVersity of Zu¨ rich, Winterthurerstrasse 190, CH-8057 Zu¨ rich, Switzerland Received January 23, 2003
To study the interaction of the fac-[M(CO)3]+ moiety (M ) 99mTc, 188 Re) with DNA bases, we reacted [M(OH2)3(CO)3]+ with 9-methylguanine (9-MeG), guanosine (G), and 2-deoxyguanosine (2dG). Two bases bind to the metal center via the N7 atoms. X-ray structure analysis of [99Tc(CH3OH)(9-MeG)2(CO)3]+ (4) (monoclinic, I2/a, a ) 28.7533(14) Å, b ) 8.0631(4) Å, c ) 32.3600(15) Å, β ) 91.543(6)°, V ) 7499.6(6) Å3, Z ) 8) and [Re(OH2)(9-MeG)2(CO)3]+ (7) (monoclinic, P21/n, a ) 12.2873(11) Å, b ) 16.0707(13) Å, c ) 14.1809(16) Å, β ) 103.361(12)°, V ) 2724.4(5) Å3, Z ) 4) reveals that the two bases are in a head-to-tail (HT) orientation. Kinetic studies show that the rates of substitution of the purine bases are comparable to that of one of the active forms of cisplatin. The bis-substituted complexes are generally less stable than the platinum adducts, and metalation of the bases is reversible.
DNA is generally agreed to be the major biological target of platinum compounds, and their cytotoxicity is correlated with the formation of 1,2-intrastrand adducts between the N7 atoms of two adjacent purine residues.1 The products of this interaction are d(GpG) cross-links and less frequently d(ApG). Not only have these adducts been observed both in vitro and in vivo, but also clinically inactive compounds fail to form such cross-links.1a Mononuclear and dinuclear tricarbonylrhenium(I) complexes of the type [ReBr(CO)3(Ph2PCH2)2NR] and [Re2(µ-OR)3(CO)6] have recently been described to efficiently suppress the growth of solid and suspended tumor cell lines.2 Substitution of the alkoxide or hydroxide ligands and coordination to N7 in purine bases in a fashion similar to cisplatin was anticipated to be a possible mode of action for some of these complexes.2a * To whom correspondence should be addressed. E-mail: ariel@aci. unizh.ch. (1) (a) Jamieson, E. R.; Lippard, S. J. Chem. ReV. 1999, 99, 2467-2498. (b) Spingler, B.; Whittington, D. A.; Lippard, S. J. Inorg. Chem. 2001, 40, 5596-5602. (c) Takahara, M. P.; Frederick, C. A.; Lippard, S. J. J. Am. Chem. Soc. 1996, 118, 12309-12321. (d) Gelasco, A.; Lippard, S. J. Biochemistry 1998, 37, 9230-9239. (2) (a) Yan, Y.-K.; Cho, S. E.; Shaffer, K. A.; Rowell, J. E.; Barnes, B. J.; Hall, I. H. Pharmazie 2000, 55, 307-313. (b) Zhang, J.; Vittal, J. J.; Henderson, W.; Wheaton, J. R.; Hall, I. H.; Hor, T. S. A. Yan, Y.-K. J. Organomet. Chem. 2002, 650, 123-132.
2818 Inorganic Chemistry, Vol. 42, No. 9, 2003
Scheme 1
In our work on novel [M(OH2)3(CO)3]+ (M ) 99mTc, Re) based radiopharmaceuticals, we have shown that the d6 “fac-[M(CO)3]+” moiety binds a wide variety of monoand multidentate ligands to yield highly robust complexes. Aromatic amines in particular are potent ligands, and there exists a rhenium structure with a single N7 bound 9-ethylguanine.3 Hence, it is likely that [M(OH2)3(CO)3]+ would also bind to two bases, isolated or in DNA, despite the octahedral geometry and therefore exhibit potential cytostatic properties based on a mechanism comparable to that of cisplatin. To assess this possibility, we studied the basic coordination chemistry of [ReBr3(CO)3]2- (in water [Re(OH2)3(CO)3]+, 1) and [TcCl3(CO)3]2- (in water [Tc(OH2)3(CO)3]+, 2) with 9-methylguanine (9-MeG), guanosine (G), and 2-deoxyguanosine (2dG). We present in this paper the first X-ray structures of technetium and rhenium complexes bound to two guanines via the N7 atoms together with kinetic and thermodynamic data of the interaction of 1 and 2 with G and 2dG. The comparison of these data with those of [Pt(NH3)2(H2O)2]2+ implies that 1 or 2 is a potential cytotoxic agent affecting DNA like cisplatin. The complexes with 99m Tc or 188Re could be used as novel radiodiagnostic or -therapeutic agents. The reaction of 1 with 2 equiv of 9-MeG, G, or 2dG in CH3OH or in a CH3OH/H2O mixture (Scheme 1) can conveniently be monitored by 1H NMR spectroscopy or HPLC. The mono- and the bis-purine base substituted complexes [M(CH3OH)2(9-MeG)(CO)3]+ and [M(CH3OH)(9-MeG)2(CO)3]+ (3, M ) Re; 4, M ) 99Tc) formed stepwise. This was also observed in the formation of [Re(CH3OH)(G)2188
(3) Oriskovich, T. A.; White, P. S.; Thorp, H. H. Inorg. Chem. 1995, 34, 1629-1631.
10.1021/ic030028m CCC: $25.00
© 2003 American Chemical Society Published on Web 04/05/2003
COMMUNICATION
Figure 1. ORTEP view of [99Tc(CH3OH)(9-MeG)2(CO)3]+ (4) with 50% probability for thermal ellipsoids.
(CO)3]+ (5) and [Re(CH3OH)(2dG)2(CO)3]+ (6). The reaction in the presence of water gave the corresponding complexes, i.e., [Re(OH2)(9-MeG)2(CO)3]+ (7). The Re and the Tc complexes could be obtained as analytically pure white microcrystalline solids although the formation of the bis-substituted complexes 3-7 is not complete at a 1:2 stoichiometry (Supporting Information). Diffraction-quality crystals of 4(ClO4) as well as 7(ClO4) were grown from methanol and hexane,4 and the X-ray structure of 4 is given in Figure 1.5 It has been pointed out that the carbonyl oxygen of coordinated guanine becomes sterically demanding in octahedral complexes;6 thus, it might be expected that the structure is sterically too crowded to accommodate two guanine bases, but this is obviously not the case. In all the complexes, the guanines coordinate through N7 only while the carbonyl oxygens (O6 and O16) are not involved in coordination as encountered with other metals.7 The two bases are in a head-to-tail (HT) orientation which is the most common solid-state conformation of cis-bis(ligand) complexes of purines with PtII, CoIII, CuII, and ZnII.8 The O6 carbonyl group of one guanine bisects the plane between two CO ligands with average O6-CO oxygen distances of 3.211 Å. It is possible that steric interaction with the CO ligands determines the relative orientation of this 9-MeG ligand. In addition, the orientation of the other 9-MeG is stabilized by a hydrogen bond between the carbonyl oxygen (4) CAUTION: Perchlorate salts can detonate explosiVely and without warning. Although we haVe encountered no incidents with the reported compounds, all due precautions should be taken. (5) Crystals suitable for X-ray diffraction were obtained by vapor diffusion of pentane into a methanolic solution of 4. Crystal data: C24H38Cl2N11O14Tc, MW ) 873.55, colorless block, monoclinic, I2/a, a ) 28.7533(14) Å, b ) 8.0631(4) Å, c ) 32.3600(15) Å, β ) 91.543(6)°, V ) 7499.6(6) Å3, Z ) 8, Fcalcd ) 1.55 Mg/m3, µ(Mo KR) ) 0.60 mm-1, Stoe IPDS diffractometer, Mo KR radiation (λ ) 0.71073 Å), 11284 reflections, 7224 with I > 2σ(I) used for refinement [R ) 0.0692, wR2 ) 0.1857, hydrogens calculated, except H1 (OH of bound CH3OH)]. (6) van Vliet, P. M.; Haasnoot, J. G.; Reedijk, J. Inorg. Chem. 1994, 33, 1934-1939. (7) (a) Dunbar, K. R.; Matonic; J. H.; Saharan, V. P.; Crawford, C. A.; Christou, G. J. Am. Chem. Soc. 1994, 116, 2201-2202. (b) Smith, D. P.; Baralt, E.; Morales, B.; Olmstead, M. M.; Maestre, M. F.; Fish, R. H. J. Am. Chem. Soc. 1992, 114, 10647-10649. (c) Abbott, D. W.; Woods, C. Inorg. Chem. 1983, 22, 2918-2923. (d) Gellert, R. W.; Fisher, B. E.; Bau, R. J. Am. Chem. Soc. 1980, 102, 7812-7815. (e) Szalda, D. J.; Kistenmacher, T. J.; Marzilli, L. G. J. Am. Chem. Soc. 1976, 98, 8371-8377. (f) Cozak, D.; Mardhy, A.; Olivier, M. J.; Beauchamp, A. L. Inorg. Chem. 1986, 25, 2600-2606. (g) Lorberth, J.; Massa, W.; Essawi, M.-E.; Labib, L. Angew. Chem., Int. Ed. Engl. 1988, 27, 1160-1161. (8) Xu, Y.; Natile, G.; Intini, F. P.; Marzilli, L. G. J. Am. Chem. Soc. 1990, 112, 8177-8179 and references therein.
Figure 2. ORTEP view of [Re(OH2)(9-MeG)2(CO)3]+ (7) with 50% probability for thermal ellipsoids.
(O16 in Figure 1) and the hydroxyl proton of CH3OH, which is coordinated to the sixth site. Replacing CH3OH by H2O affords a better model of the complexes in aqueous solution. X-ray quality crystals of 7 were obtained from 3 in water. An ORTEP plot of 7 is given in Figure 2.9 No reorientation took place, and the structural features resemble closely those of 4 with no additional hydrogen bond being formed. Obviously, the HT orientation is thermodynamically preferred which was confirmed by the reaction of 1 with 9-MeG in water yielding exclusively the same compound 7. Although in H2O and CH3OH one single stereoisomer is found, this is not the case in aprotic solvents. The existence of distinct stereoisomers in other solvents such as DMSO parallels the chemistry of 1 and cisplatin. Dissolving [6]Br (prepared from aqueous methanol) in DMSO-d6 shows initially one single species as indicated by a single set of two sharp H8 signals for the independent guanosines. Within days, however, three different isomers are in stable equilibrium, and they do not exhibit any cross-peaks between the H8 protons of coordinated purines (2D exchange-NOESY, Supporting Information). This indicates that in the single species the two H8’s are separated by more than 3 Å. Similarly, platinum complexes of purines are known to exist in different arrangements,10 and as in these squareplanar compounds, the absence of NOE cross-peaks is indicative of HT atropisomers in octahedral rhenium complexes.11 The rates of substitution and the stability constants K1 and K2 of the purine bases G and 2dG are important for comparing the behavior of 1 with cisplatin. The species distribution at equilibrium, measured by 1H NMR, allowed an estimation of the stability constants according to Scheme 1. (9) Crystals suitable for X-ray diffraction were obtained by vapor diffusion of pentane into a methanolic solution of 3 containing 3% H2O (v/v). Crystal data: C16H22ClN10O12Re, MW ) 768.07, colorless block, monoclinic, P21/n, a ) 12.2873(11) Å, b ) 16.0707(13) Å, c ) 14.1809(16) Å, β ) 103.361(12)°, V ) 2724.4(5) Å3, Z ) 4, Fcalcd ) 1.75 Mg/m3, µ(Mo KR) ) 4.62 mm-1, Stoe IPDS diffractometer, Mo KR radiation (λ ) 0.71073 Å), 6490 reflections, 3708 with I > 2σ(I) used for refinement [R ) 0.0446, wR2 ) 0.0955, hydrogens calculated]. (10) (a) Sullivan, S. T.; Saad, J. S.; Fanizzi, F. P.; Marzilli, L. G. J. Am. Chem. Soc. 2002, 124, 1558-1559. (b) Williams, K. M.; Scarcia, T.; Natile, G.; Marzilli, L. G. Inorg. Chem. 2001, 40, 445-454. (c) Sullivan, S. T.; Ciccarese, A.; Fanizzi, F. P.; Marzilli, L. G. Inorg. Chem. 2000, 39, 836-842. (d) Ano, S. O.; Intini, F. P.; Natile, G.; Marzilli, L. G. J. Am. Chem. Soc. 1998, 120, 12017-12022. (11) Wong, H. C.; Intini, F. P.; Natile, G.; Marzilli, L. G. Inorg. Chem. 1999, 38, 1006-1014.
Inorganic Chemistry, Vol. 42, No. 9, 2003
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COMMUNICATION Table 1. Rate Constants for the Formation of the 1:1 (k1/10-2 M-1 s-1) and 1:2 (k2/10-2 M-1 s-1) Complexes between cis-[Pt(NH3)2(H2O)2]2+ or [M(H2O)3(CO)3]+ Ions (M ) Re, 99Tc) with G and 2dG complex ]2+ a
[Pt(NH3)2(OH2)2 [Re(OH2)3(CO)3]+ b [Re(OH2)3(CO)3]+ b [99Tc(OH2)3(CO)3]+ b
base
k1
k2
T (K)
G G 2dG G
23.7 ( 0.3 28.5 ( 1.6 14.5 ( 2.2 104.1 ( 9.3
15.8 ( 0.2 4.40 ( 0.27 6.01 ( 0.15
298.2 310.2 298.2 298.2
a Unbuffered aqueous solution: pH ) 3.85-4.15.13 aqueous solution.
b
Unbuffered
The values are 1.04 × 103 ( 0.46 M-1 (K1) and 4.31 × 102 ( 0.45 M-1 (K2) for G and 1.92 × 103 ( 0.98 M-1 (K1) and 4.48 × 102 ( 0.31 M-1 (K2) for 2dG. The rate of formation k1 of [M(OH2)2(G)(CO)3]+ was determined under pseudofirst-order conditions by monitoring the growth of the monoligated species with 1H NMR in D2O. The rates of decomposition k-2 for 5 and 6 were received from the decrease of the corresponding 1H signals in water considering the back and forward reaction according to literature.12 From K1, K2, k1, and k-2, the remaining k-1 and k2 could be calculated (Table 1 and Supporting Information). Although the conditions for the determination of the rate constants for Re(I)-G differ slightly from those of Pt(II), it is obvious that they are in the same order of magnitude. The value of k1 is larger for Re than for Pt, but the reverse is true for k2. This might reflect the sterically more demanding octahedral Re complex. Binding of G to the Tc complex is about 5 times faster in k1, a behavior that is expected going one group higher in the periodic table. Comparison of stability constants K1 and K2 is difficult since platination of a base can be considered irreversible.
2820 Inorganic Chemistry, Vol. 42, No. 9, 2003
Rate constants exclusively refer to the interaction of Re with N7 in G or 2dG. Although the nucleosides offer multiple binding sites, the Re(I) coordinates selectively to N7, and no evidence for coordination to, i.e., carbonyl oxygen, or to the ribose, was seen. Steric hindrance from the sugar excludes binding to N3, and direct binding of a metal to the exocyclic C2NH2 group has never been observed under physiological conditions.14 In conclusion, we have shown that two guanine bases can coordinate to a Re(I) or a Tc(I) center yielding reasonably stable complexes with slow on and off rates, the on rates being comparable to that of an active form of cisplatin. We could elucidate the first X-ray structure of a Tc complex with purine bases. They are in a HT orientation in the solidstate. These results rationalize that the mechanism of cytotoxicity exhibited by certain rhenium compounds might parallel that of cisplatin. Clearly, a perspective of these results could be a novel kind of chemo- or radiotherapeutic metal-based drug. Cytotoxicity studies are currently underway. Acknowledgment. We wish to thank Mallinckrodt Med. Inc Petten NL for financial support. Supporting Information Available: Crystallographic data of 4 and 7. Experimental preparation of 3-7. Kinetic data. NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org. IC030028M (12) King, E. L. Int. J. Chem. Kinet. 1982, 14, 1285-1286. (13) Arpalahti, J.; Lippert, B. Inorg. Chem. 1990, 29, 104-110. (14) Lippert, B. Coord. Chem. ReV. 2000, 200-202, 487-516.
