Inorg. Chem. 1985, 24, 1666-1673
1666
Contribution from the Departments of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221, and University of Oklahoma, Norman, Oklahoma 73019
Comparison of the Chemical and Biological Properties of trans -[Tc( DMPE)2C12]+and trans -[Re(DMPE)2C12]+,Where DMPE = 1,2-Bis(dimethylphosphino)ethane. Single-Crystal Structural Analysis of bans -[Re(DMPE)2C12]PFs J E A N - L U C V A N D E R H E Y D E N , I M A R Y J A N E HEEG,, and E D W A R D DEUTSCH*’ Received November 28, 1984 Recent studies on the monocationic 99mTccomplex r r ~ n s - [ ~ T c ( D M P E ) ~ C 1have ~ 1 +shown that this species accumulates in normal myocardium and allows scintigraphic visualization of the hearts of mongrel dogs and humans. The mechanism of action of the myocardial accumulation is still unclear. Because of the periodic relationship between technetium and rhenium, in vivo evaluation of the analogous complex trans-[Re(DMPE)zC12] can provide insight on the role of the oxidation-reduction potential of complexes in determining biological behavior. Preparation of trans- [ 186Re(DMPE)2C12]+ from ls6Re0.,- is accomplished analogously to the synthesis of ~ ~ ~ ~ S - [ ” ~ T C ( D M Pfrom E ) ~99mTc04-. C ~ ~ ] + The rhenium complex prepared with nonradioactive Re is characterized by single-crystal X-ray structural analysis refined to conventional R factors of 0.030 and 0.037. Yellow crystals of the hexafluorophosphate salt crystallize in the monoclinic space group P 2 , / c with a = 10.661 (5) A,b = 10.214 (6) A,c = 12.835 (6) A, @ = 115.43(4)O, and Z = 2. This octahedrally coordinated complex has essentially the same structure, bond lengths, and bond angles as does the technetium analogue: Re-P = 2.438 (2)8,and Tc-P = 2.436(5) A;Re-C1 = 2.337 (1) 8, and Tc-C1 = 2.323 (3) A. However, cyclic voltammetry shows that the Re complex is 190 mV more difficult to reduce than is the T c analogue. Tissue distribution studies in rats, and qualitative imaging studies in rats and dogs, show a definite difference in the biodistributions of trans-[99mTc(DMPE)2C12]tand tran~-[l~~Re(DMPE),Cl,] +,despite the identical size, shape, charge, and lipophilicity of these complexes. This observation strongly implies that in vivo reduction of the cationic Tc(II1) complex trans-[99mTc(DMPE)2C12]t to the neutral, more lipophilic, Tc(I1) analogue trans-[99mTc(DMPE)2C12]0 determines, at least in part, the biodistribution of trans- [99mTc(DMPE)2C12]t.This in vivo reduction detracts from the utility of trans- [99mT~(DMPE)2Cl,]as a myocardial imaging agent by promoting transfer of radioactivity from the heart to the liver.
Introduction There has recently been a great amount of interest in the preparation and in vivo evaluation of cationic complexes of 9 9 m T ~ as myocardial imaging agents.*I0 In support of this interest there have also appeared several recent publications on the macroscopic Tc(V), Tc(III), and also Tc(1) using the long-lived isotope 99Tc.”-14 Bis(phosphine) ligands, and especially the DMPE ligand (where DMPE represents 1,2-bis(dimethylphosphino)ethane), are the only ligands known to stabilize t h e technetium center in all three oxidation states.I4 In vivo evaluations of t h e three Tc(I), Tc(III), and Tc(V) cationic DMPE complexes were systematically performed in order to develop an animal model t h a t would adequately predict the biochemistry of monocationic complexes of
University of Cincinnati. University of Oklahoma. Deutsch, E.; Glavan, K. A,; Sodd, V. J.; Nishiyama, H.; Ferguson, D. L.; Lukes, S. J. J . Nucl. Med. 1982, 22, 891. Deutsch, E.;Bushong, W.; Glavan, K. A,; Elder, R. C.; Sodd, V. J.; Scholz, K. L.; Fortman, D. L.; Lukes, S. J. Science (Washington, D.C.) 1981, No. 214, 85. Nishiyama, H.;Deutsch, E.; Adolph, R. J.; Sodd, V. J.; Libson, K.; Saenwr. E. L.: Gerson. M.C.: Gabel. M.: Lukes. S. J.: Vanderhevden, J.-L.yFortman, D. L.; Scholz, K. L.; Grossman, L. W.; Williams, C. C. J . Nucl. Med. 1982, 23, 1093. Nishiyama, H.; Adolph, R. J.; Deutsch, E.; Sodd, V. J.; Libson, K.; Gerson, M. C.; Saenger, E. L.; Lukes, S. J.; Gabel, M.; Vanderheyden, J.-L.; Fortman, D. L. J . Nucl. Med. 1982, 23, 1102. Deutsch, E.;Glavan, K. A.; Bushong, W.; Scdd, V. J. Appl. Nucl. Radiochem. 1982, 139 Vanderheyden, J.-L.; Libson, K.; Nosco, D. L.; Ketring, A. R.; Deutsch, E. In?. J . Appl. Radiat. Isot. 1983, 34, 1611. Gerson, M. C.;Deutsch, E. A.; Nishiyama, H.; Libson, K. F.; Adolph, R. J.; Grossman, L. W.; Sodd, V. J.; Fortman, D. L.; Vanderheyden, J.-L.; Williams, C. C.; Saenger, E. L. Eur. J . Nucl. Med. 1983, 8, 371. Gerson, M.C.;Deutsch, E. A,; Libson, K. F.; Adolph, R. J.; Ketring, A. R.; Vanderheyden, J.-L. Williams, C. C.; Saenger, E. L. Eur. J . Nucl. Med. 1984. 9. 403. Deutsch, E.;Libson,’K.;Jurisson, S.; Lindoy, L. F. Prog. Inorg. Chem. 1983. 30.
75.
Abrams,’M. J.; Davison, A.; Jones, A. G.; Costello, C. E.; Pang, H. Inorg. Chem. 1983, 22, 2798. Wester, D. W.; White, D. H.; Miller, F. W.; Dean, R. T. Inorg. Chem. 1984, 23, 1501. Vanderheyden, J.-L.; Ketring, A. R.; Libson, K.; Heeg, M. J.; Roecker, L.; Motz, P.;Whittle, R.; Elder, R. C.; Deutsch, E. Inorg. Chem. 1984, 23, 3184.
logical behavior of cationic complexes in humans.I5 Since the first report concerning trans- [99mT~(DMPE)2C12]+ in 1981,4 the chemistry and biology of this cation have been extensively studied. Acquisition of rat biodistribution data,16 basal and kinetic studies in mongrel dog^,^,^ and evaluation as a myocardial imaging agent in humans9 using the *Tc agent have been accompanied by a full characterization of the 99Tcanalogue using classical chemical t e ~ h n i q u e s . ~ J ~Although ~” there is a definite accumulation of t r ~ n s - [ ~ ~ ~ T c ( D M P E ) ~in C 1the ~ 1 +normal myocardium of many species, t h e mechanism of action of this uptake is not known. In vitro studieslE show that the mechanism of uptake is different from the uptake mechanivm of 43K+, IS9Cst, and zOITl+, the latter being the myocardial imagjng agent currently in clinical use. Further in vitro work involving perfused guinea pig heartsI9has also not defined the biological mechanism of action of t r ~ n s - [ ~ ~ ~ T c ( D M P E ) ~ C 1 ~ 1 + . A totally different approach is presented in this paper. The periodic relationship between technetium a n d rhenium is used t o study very specifically t h e role of reduction potential of trans[M(DMPE)2C12]+(M = Tc, Re) complexes on their in vivo biodistribution. The chemistries of Tc and Re are quite similar due t o their relative positions in the periodic table and the lanthanide contraction. The salient difference between the chemistries of analogous Tc and Re complexes is t h e fact t h a t t h e Re complexes are about 200 mV more difficult t o reduce t h a n t h e Tc analogues. This difference in redox potentials between Tc a n d Re analogues is consistent with several observations m a d e on analogous complexes of second- and third-row transition metals20*21 a n d is large enough t o shift one complex or the other o u t of t h e range of potentials readily accessible to biological systems. The (15) Ketring, A. R.; Deutsch, E.: Libson, K.; Vanderheyden, J.-L. J . Nucl. Med. 1983, 24, P9. (16) Sodd, V. J.; Scholz, K. L.; Grossman, L. W.; Fortman, D. L.; Robbins, P.J.; Deutsch, E.;Nishiyama, H.; Adolph, R. J.; Libson, K. J . Nucl. Med. 1982, 23, P87. (17) Unger, S . Anal. Chem. 1983, 56, 3638. (18) Sands, H.;Delano, M. L.; Shaw, P.;Linder, K.; Camin, L. L.; Gallagher, B. M. J . Nucl. Med. 1983, 24, P28. (19) Pendleton, D. B.; Sands, H.; Gallagher, B. M.; Camin, L. L. J . Nucl. Med. 1984, 25, P24. (20) Warren, L. F.; Bennett, M. A. Inorg. Chem. 1976, 15, 3126. (21) Hurst, R. W.; Heineman, W. R.; Deutsch, E. Inorg. Chem. 1981, 20,
3298.
0020-1669/85/1324-1666$01.50/0 0 1985 American Chemical Society
R e and Tc D M P E Complexes reduction of ~ ~ ~ ~ S - [ M " ' ( D M P E ) ~toC ~trans-[M"~]+ (DMPE)2C12]0will change the net charge of the complex and therefore alter its biodistribution. Since it is felt that only cationic species accumulate in the myocardium, such reduction is expected to lessen the efficacy of potential myocardial imaging agents.' In this paper are presented the preparation and characterization of the trans-[Re(DMPE),Cl,]+ complex, the synthesis and HPLC quality control of its '%Re form, and the structural characterization of ~ ~ ~ ~ S - [ R ~ ( D M P E ) ~as C ~well ~]P asFdata , , comparing the and trans- [99mTcbiodistribution of trans- [186Re(DMPE)2C12]+ (DMPE)2C12]+in dogs and rats.
Experimental Section General Considerations. Standard radiation safety procedures must ~ IE6Reisotopes. Techbe followed at all times when handling 9 9 m Tand netium-99m emits a 141-KeV y ray with a half-life of 6.0 h to decay to technetium-99. Rhenium-I86 emits a 137-KeV y ray associated with a 1.07-MeV j3 particle and decays with a 90-h half-life. Elemental analyses were performed by Galbraith Laboratories, Knoxville, TN. Visible-UV spectra were recorded on a Cary 210 spectrophotometer (Varian) and IR spectra were obtained on a Perkin-Elmer 224 grating infrared spectrophotometer. Octanol/saline partition coefficients were obtained as previously described3 and represent the average of at least three independent determinations. Materials. Unless otherwise noted, all chemicals were of reagent grade. The DMPE ligand was obtained from Strem Chemical Co. and was used without further purification. Dilution and stabilization of the ligand for use in the preparations of 99"k and Is6Re complexes have been previously described.8 Potassium perrhenate was commercially available from Morton Thiokol, Inc. Sephadex S P C-25 was prepared and stored as recommended by the manufacturer (Pharmacia). Tetrabutylammonium Perrhenate, (Bu)~NR&~.KRe04 (600 mg 2.07 mmol) was dissolved in 10 mL of hot water, and 1 mL of 2 M tetrabutylammonium bromide was then added dropwise. The reaction mixture was cooled to room temperature, and the white precipitate was collected and successively washed with water, 10%ethanol/diethyl ether, and diethyl ether; yield 960 mg (94%). I R (KBr pellet) Y, cm-I (intensity): 320 (m), 610 (w), 735 (w), 900 (s), 1390 (m), 1470 (m), 2870 (m), 2960 (s). UV (methanol) A, nm (e, M-I cm-I): 250 sh, 244 (2780), 240 (3430), 235.5 (3750), 231 (3850), 227 (3850), 223 (3770). Tetrabutylammonium Perrhenate (Ig6Re),( B u ) ~ N I ~ ~ R &Enriched + (85.84%) Ia5Remetal (1-2 mg) (Oqk Ridge National Laboratory) was weighed into a quartz vial, which was then sealed by flame under vacuum. The vial was then irradiated in a flux of 1014 neutrons s-I at the University of Missouri research reactor (Columbia, MO) for 24 h. The Re metal was then dissolved by the addition of concentrated H N 0 3 , and the resulting solution was neutralized with ammonia. This solution was then diluted to 4 mL to give a total Re concentration of about 0.004 M and a specific activity of about 30 mCi/mL. To aliquots (0.5-1.0 mL) of this solution were added 0.5 mL of 0.1 M tetrabutylammonium bromide (0.05 mmol). This mixture was passed through a Sep-Pak C18 cartridge (Waters) that had been prepared by successive washings with 3 mL of 95% ethanol and 3 mL of 0.01 M tetrabutylammonium bromide. After loading, the cartridge was washed with 20 mL of water and the activity was then eluted in 99% yield with 2 mL of absolute ethanol. trans -Dichlorobis( 1,Z- bis(dimethylphosphino)ethane)rhenium( 111) Hexafluorophosphate, tran~-[Re(DMPE)~cl,IpF,.Tetrabutylammonium perrhenate (162 mg, 0.33 mmol) dissolved in 5.0 mL of absolute ethanol was placed in a Teflon-lined bomb reactor. A 1.O-mL portion of 12 M HCI (0.012 mol) was added, and the resulting solution was deaerated for 5 min with a stream of nitrogen. The bomb was capped and transferred into an argon-purged glovebox, where 0.55 mL of neat DMPE (0.494 g, 3.3 mmol) was added. The bomb was then sealed and heated at 155 OC for 5 h. After the mixture was cooled to room temperature, 5.0 mL of H 2 0 was added to the reaction solution. Subsequent addition of 0.2 mL of a saturated aqueous solution of ammonium hexafluorophosphate yielded a yellow precipitate of the rhenium(II1) complex that was collected and washed successively with 10% ethanol/O.Ol M KOH, water, cold 95% ethanol, and diethyl ether. Recrystallization of this material from acetonitrile yielded 180 mg (76%) of translucent yellow crystals. The crystals for X-ray structural analysis were obtained by slow evaporation of an acetonitrile solution at room temperature. Anal. Calcd for ReC,2H32PSC12F6:C, 20.52; H, 4.59; CI, 10.09. Found: C , 19.82; H, = 432 4.66; CI, 10.21. Visible-UV (CH3CN) A, nm (e, M-' cm-I): A, (2400); A,, = 421 (2300); A, = 406 (2620); A, = 261 (5520); A, = 236 (3030). IR (KBr pellet) u , cm-' (intensity): 310 (m), 450 (w), 550 (s), 640 (m), 710 (s), 740 (m), 830 (s), 890 (s), 925 (s), 940 (s), 1240 (w), 1290 (m), 1300 (w), 1390 (w), 1420 (s) 2900 (m), 2990 (w).
Inorganic Chemistry, Vol. 24, No. 11, 1985 1667 Table I. Experimental Crystallographic Data for trans-[Re(DMPE),CI,] PF, formula: Rel P5CI2F6CI *HJ2 mol wt: 702.35 cryst dimens: 0.12 X 0.15 X 0.15 mm cryst syst: monoclinic space gp: P 2 , l c cell dimens" a = 10.661 (5) A, b = 10.214 (6) A, c = 12.835 (6) fl = 115.43 (4)O, V = 1262 (1) A3, Z = 2, D(calcd) = 1.848 g cm-3 radiation: Mo K a (A = 0.71069 A) monochromator: graphite crystal 20 range: 2.4-53' max scan time: 45 s scan angle: 0.80 0.20 tan 0 monitor reflcns: 3 measd for orientn check every 200 reflcns; 3 measd for intensity check every 2 h variation in monitors: 1% total no. data collcd: 2836 no. of unique data: 26 15 no. of obsd data with I, 2 3 4 4 : 1737 p(Mo K a ) : 55.9 cm-' transmissn coeff 0.37-0.51 F(000): 684 R: 0.030 R,: 0.037
A,
+
w:
(UJ2
Lattice constants calculated from 48 high-angle reflections measured at f 2 0 . trans -Dichlorobis( 1,Z-bis(dimethylphosphino)ethane)rhenium(III) (Ia6Re), ~ ~ ~ ~ S - [ ~ ~ ~ R ~ ( D M Purified P E ) ~ C( IB~u] )+~. N I ~ ~(1R5-20 ~O~ mCi) in absolute ethanol was diluted to 1.2 mL with absolute ethanol in a 5-mL borosilicate glass reaction vial. A 0.1" portion of 4 M HCI was added, and the mixture was deaerated. A 0.20-mL portion of a 0.54 M DMPE solution in ethanolic HC18 was added, and then the vial was sealed with a Teflon-lined cap and placed in a 150 OC dry heat bath for 2 h. (Caution! This is a potentially explosive procedure. Reaction vials should be pretested to ensure that they will withstand reaction conditions. Reaction vials should be cooled slowly to prevent their breaking. Only borosilicate reaction vials should be used.) After cooling, 0.075 mL of 4 M N a O H was added and the reaction mixture was diluted to 20 mL with water. This solution was loaded onto a Sephadex S P C-25 cationexchange column (1.5 X 6 cm), and the column was successively washed with 20-25 mL of 0.005 M NaOH and 30-35 mL of water. The desired radiopharmaceutical was then eluted with 0.15 M NaCl in 5-mL fractions. The aliquot containing the highest specific activity of trans[186Re(DMPE)2C12]was passed through a sterile 0.22-pm filter (Millex SG) and used for in vivo evaluations. X-ray Characterization and Structure Solution and Refinement of ~ ~ ~ ~ S - [ R ~ ( D M P E ) ~ C IYellow , ] P F ~crystals . of this material were irregular and multifaceted. A sample was mounted on a Nonius CAD-4 automated diffractometer, and axial photos indicated the monoclinic crystal system. Gaussian integration absorption corrections were applied by the method of indexing crystal faces. The collected intensity measurements showed unusually weak hkl reflections for the condition k I = odd, which is consistent with the heavy atom being located on the inversion center. The structure was solved by heavy-atom methods and refined as a full matrix in least-squares calculations.22 Hydrogen atoms were placed in a combination of observed and calculated positions and held invariant with U(H) = 0.09 A2;all non-hydrogen atoms were refined anisotropically. In a final cycle of least-squares refinement, the maximum shift was
