Surfactant protein B labelled with [(99m)Tc(CO)3(H20)3](+) retains biological activity in vitro

Optimising the radiolabelling properties of technetium tricarbonyl and His-tagged proteins

BACKGROUND

To date, the majority of protein-based radiopharmaceuticals have been radiolabelled using non-site-specific conjugation methods, with little or no control to ensure retained protein function post-labelling. The incorporation of a hexahistidine sequence (His-tag) in a recombinant protein can be used to site-specifically radiolabel with 99mTc-tricarbonyl ([99mTc(CO)3]+). This chemistry has been made accessible via a technetium tricarbonyl kit; however, reports of radiolabelling efficiencies and specific activities have varied greatly from one protein to another. Here, we aim to optimise the technetium tricarbonyl radiolabelling method to produce consistently >95% radiolabelling efficiencies with high specific activities suitable for in vivo imaging.

METHODS

Four different recombinant His-tagged proteins (recombinant complement receptor 2 (rCR2) and three single chain antibodies, α-CD33 scFv, α-VCAM-1 scFv and α-PSMA scFv), were used to study the effect of kit volume, ionic strength, pH and temperature on radiolabelling of four proteins.

RESULTS

We used 260 and 350 μL [99mTc(CO)3]+ kits enabling us to radiolabel at higher [99mTc(CO)3]+ and protein concentrations in a smaller volume and thus increase the rate at which maximum labelling efficiency and specific activity were reached. We also demonstrated that increasing the ionic strength of the reaction medium by increasing [Na+] from 0.25 to 0.63 M significantly increases the rate at which all four proteins reach a >95% labelling efficiency by at least fourfold, as compared to the conventional IsoLink® kit (Covidien, Petten, The Netherlands) and 0.25 M [Na+].

CONCLUSION

We have found optimised kit and protein radiolabelling conditions suitable for the reproducible, fast, efficient radiolabelling of proteins without the need for post-labelling purification.

Design and synthesis of a bombesin peptide-conjugated tripodal phosphino dithioether ligand topology for the stabilization of the fac-[M(CO)3]+ core (M=(99 m)Tc or Re)

A new tumor-seeking tridentate topology consisting of a phosphino dithioether ((HOCH(2))(2)PCH(2)CH(2)S(CH(2))(n)CH(2)SR; PS(2)) ligand framework for the production of kinetically inert and in vivo stable facial [(99m)Tc(CO)(3)(PS(2))](+) or [Re(CO)(3)(PS(2))](+) is described. The X-ray crystal structure of fac-Re(CO)(3)(PS(2))PF(6) is reported. The bioconjugation strategies for incorporating bombesin (BBN) peptides on to the PS(2) tripodal framework and, thereby, de novo designing of GRP receptor-seeking Tc(PS(2)-BBN)(CO)(3) are developed.

Radiolabeling of rituximab with (188)Re and (99m)Tc using the tricarbonyl technology

INTRODUCTION

The most successful clinical studies of immunotherapy in patients with non-Hodgkin's lymphoma (NHL) use the antibody rituximab (RTX) targeting CD20(+) B-cell tumors. Rituximab radiolabeled with β(-) emitters could potentiate the therapeutic efficacy of the antibody by virtue of the particle radiation. Here, we report on a direct radiolabeling approach of rituximab with the (99m)Tc- and (188)Re-tricarbonyl core (IsoLink technology).

METHODS

The native format of the antibody (RTX(wt)) as well as a reduced form (RTX(red)) was labeled with (99m)Tc/(188)Re(CO)(3). The partial reduction of the disulfide bonds to produce free sulfhydryl groups (-SH) was achieved with 2-mercaptoethanol. Radiolabeling efficiency, in vitro human plasma stability as well as transchelation toward cysteine and histidine was investigated. The immunoreactivity and binding affinity were determined on Ramos and/or Raji cells expressing CD20. Biodistribution was performed in mice bearing subcutaneous Ramos lymphoma xenografts.

RESULTS

The radiolabeling efficiency and kinetics of RTX(red) were superior to that of RTX(wt) ((99m)Tc: 98% after 3 h for RTX(red) vs. 70% after 24 h for RTX(wt)). (99m)Tc(CO)(3)-RTX(red) was used without purification for in vitro and in vivo studies whereas (188)Re(CO)(3)-RTX(red) was purified to eliminate free (188)Re-precursor. Both radioimmunoconjugates were stable in human plasma for 24 h at 37 °C. In contrast, displacement experiments with excess cysteine/histidine showed significant transchelation in the case of (99m)Tc(CO)(3)-RTX(red) but not with pre-purified (188)Re(CO)(3)-RTX(red). Both conjugates revealed high binding affinity to the CD20 antigen (K(d) = 5-6 nM). Tumor uptake of (188)Re(CO)(3)-RTX(red) was 2.5 %ID/g and 0.8 %ID/g for (99m)Tc(CO)(3)-RTX(red) 48 h after injection. The values for other organs and tissues were similar for both compounds, for example the tumor-to-blood and tumor-to-liver ratios were 0.4 and 0.3 for (99m)Tc(CO)(3)-RTX(red) and for (188)Re(CO)(3)-RTX(red) 0.5 and 0.5 (24 h pi).

CONCLUSION

Rituximab could be directly and stably labeled with the matched pair (99m)Tc/(188)Re using the IsoLink technology under retention of the biological activity. Labeling kinetics and yields need further improvement for potential routine application in radioimmunodiagnosis and therapy.

Thiol- and thioether-based bifunctional chelates for the {M(CO)3}+ core (M = Tc, Re)

By analogy to the recently described single amino acid chelate (SAAC) technology for complexation of the {M(CO)3}+ core (M = Tc, Re), a series of tridentate ligands containing thiolate and thioether groups, as well as amino and pyridyl nitrogen donors, have been prepared: (NC5H4CH2)2NCH2CH2SEt (L1); (NC5H4CH2)2NCH2CH2SH (L2); NC5H4CH2N(CH2CH2SH)2 (L3); (NC5H4CH2)N(CH2CH2SH)(CH2CO2R) [R = H (L4); R = -C2H5 (L5). The {Re(CO)3}+ core complexes of L1-L5 were prepared by the reaction of [Re(CO)3(H2O)3]Br or [NEt4]2[Re(CO)3Br3] with the appropriate ligand in methanol and characterized by infrared spectroscopy, 1H and 13C NMR spectroscopy, mass spectrometry, and in the case of [Re(CO)3(L2)] (Re-2) and [Re(CO)3(L1)Re(CO)3Br2] (Re-1a) by X-ray crystallography. The structure of Re-2 consists of discrete neutral monomers with a fac-Re(CO)3 coordination unit and the remaining coordination sites occupied by the amine, pyridyl, and thiolate donors of L2, leaving a pendant pyridyl arm. In contrast, the structure of Re-1a consists of discrete binuclear units, constructed from a {Re(CO)3(L1)}+ subunit linked to a {Re(CO)3Br2}- group through the sulfur donor of the pendant thioether arm. The series of complexes establishes that thiolate donors are effective ligands for the {M(CO)3}+ core and that a qualitative ordering of the coordination preferences of the core may be proposed: pyridyl nitrogen approximately thiolate > carboxylate > thioether sulfur > thiophene sulfur. The ligands L1 and L2 react cleanly with [99mTc(CO)3(H2O)3]+ in H2O/DMSO to give [99mTc(CO)3(L1)]+ (99m)Tc-1) and [99mTc(CO)3(L2)] (99mTc-2), respectively, in ca. 90% yield after HPLC purification. The Tc analogues 99mTc-1 and 99mTc-2 were subjected to ligand challenges by incubating each in the presence of 1000-fold excesses of both cysteine and histidine. The radiochromatograms showed greater than 95% recovery of the complexes.

Developing the {M(CO)3}+ Core for Fluorescence Applications: Rhenium Tricarbonyl Core Complexes with Benzimidazole, Quinoline, and Tryptophan Derivatives

Tridentate ligands derived from benzimidazole, quinoline, and tryptophan have been synthesized, and their reactions with [NEt4]2[Re(CO)3Br3] have been investigated. The complexes 1-4 and 6 and 7 exhibit fac-{Re(CO)3N3} coordination geometry in the cationic molecular units, while 5 exhibits fac-{Re(CO)3N2O} coordination for the neutral molecular unit, where N3 and N2O refer to the ligand donor groups. The ligands bis(1-methyl-1H-benzoimidazol-2-ylmethyl)amine (L1), [bis(1-methyl-1H-benzoimidazol-2-ylmethyl)amino]acetic acid ethyl ester (L2), [bis(1-methyl-1H-benzoimidazol-2-ylmethy)amino]acetic acid methyl ester (L3), [bis(quinolin-2-ylmethyl)amino]acetic acid methyl ester (L4), 3-(1-methyl-1H-indol-3-yl)-2-[(pyridin-2-ylmethyl)amino]propionic acid (L5), 2-[bis(pyridin-2-ylmethyl)amino]-3-(1-methyl-1H-indol-3-yl)propionic acid (L6), and 2-[bis(quinolin-2-ylmethyl)amino]-3-(1-methyl-1H-indol-3-yl)propionic acid (L7) were obtained in good yields and characterized by elemental analysis, 1D and 2D NMR, and high-resolution mass spectrometry (HRMS). The rhenium complexes were obtained in 70-85% yields and characterized by elemental analysis, 1D and 2D NMR, HRMS, IR, UV, and luminescence spectroscopy, as well as X-ray crystallography for [Re(CO)3(L1)]Br (1), {[Re(CO)3(L2)]Br}2.NEt4Br . 8.5H2O (3(2).NEt4Br . 8.5H2O), [Re(CO)3(L4)]Br (4), and [Re(CO)3(L6)]Br (6). Crystal data for C21H19BrN5O3Re (1): monoclinic, P2(1)/c, a = 13.1851(5) A, b = 16.1292(7) A, c = 10.2689(4) A, beta = 99.353(1) degrees , V = 2154.8(2) A3, Z = 4. Crystal data for C56H73Br3N11O18.50 Re2 (3(2).NEt4Br . 8.5H2O): monoclinic, C2/c, a = 34.7760(19) A, b = 21.1711(12) A, c = 20.3376(11) A, beta = 115.944(1) degrees , V = 13464.5(1) A3, Z = 8. Crystal data for C26H21BrN3O5Re (4): monoclinic, P2(1)/c, a = 16.6504(6) A, b = 10.1564(4) A, c = 14.6954(5) A, beta = 96.739(1) degrees , V = 2467.9(2) A3, Z = 4. Crystal data for C27H24BrN4O5Re (6): monoclinic, P2(1), a = 8.7791(9) A, b = 16.312(2) A, c = 8.9231(9) A, beta = 90.030(1) degrees , V = 1277.8(2) A3, Z = 2.

Stability, protein binding and thiol interaction studies on [2-acetoxy-(2-propynyl)benzoate]hexacarbonyldicobalt

Cobalt-alkyne complexes are a new class of potent cytotoxic drugs. The lead compound [2-acetoxy-(2-propynyl)benzoate]hexacarbonyldicobalt (Co-ASS) showed high effects on several human cancer cell lines. In order to evaluate further the in-vitro properties and reactivity of this substance we performed stability and protein binding studies and investigated the interaction of this complex with 1,2-ethanedithiol, L-cysteine and glutathione. UV-Vis, HPLC and AAS studies showed that the compound was sufficiently stable under in-vitro conditions. Binding to human serum albumin increased from approximately 25% at the beginning to over 50% after 48 h of incubation as determined by ethanol preciptation and size exclusion chromatography. The interaction with thiols resulted in disulfide bond formation of the thiols.

New directions in the coordination chemistry of 99mTc: a reflection on technetium core structures and a strategy for new chelate design

Bifunctional chelates offer a general approach for the linking of radioactive metal cations to macromolecules. In the specific case of 99mTc, a variety of technologies have been developed for assembling a metal-chelate-biomolecule complex. An evaluation of these methodologies requires an appreciation of the coordination characteristics and preferences of the technetium core structures and oxidation states, which serve as platforms for the development of the imaging agent. Three technologies, namely, the MAG3-based bifunctional chelates, the N-oxysuccinimidylhydrazino-nicotinamide system and the recently described single amino acid chelates for the {Tc(CO)3}1+ core, are discussed in terms of the fundamental coordination chemistry of the technetium core structures. In assessing the advantages and disadvantages of these technologies, we conclude that the single amino acid analogue chelates (SAAC), which are readily conjugated to small peptides by solid-phase synthesis methods and which form robust complexes with the {Tc(CO)3}1+ core, offer an effective alternative to the previously described methods.

Carbohydrate-Appended 2,2‘-Dipicolylamine Metal Complexes as Potential Imaging Agents

Three discrete carbohydrate-appended 2,2'-dipicolylamine ligands were complexed to the {M(CO)(3)}(+) (M = (99m)Tc/Re) core: 2-(bis(2-pyridinylmethyl)amino)ethyl-beta-d-glucopyranoside (L(1)()), 2-(bis(2-pyridinylmethyl)amino)ethyl-beta-D-xylopyranoside (L(2)()), and 2-(bis(2-pyridinylmethyl)amino)ethyl-alpha-d-mannopyranoside (L(3)). An ethylene spacer is used to separate the carbohydrate moiety and the dipicolylamine (DPA) function in all three ligands. The Re complexes [Re(L(1-3))(CO)(3)]Br were characterized by (1)H and (13)C 1D/2D NMR spectroscopies, which confirmed the pendant nature of the carbohydrate moieties in solution. NMR measurements also established the long-range asymmetric effect of the carbohydrate functions on the chelating portion of the ligand. One analogue, [Re(L(1))(CO)(3)]Cl, was characterized in the solid state by X-ray crystallography. Further characterization was provided by IR spectroscopy, elemental analysis, conductivity, and mass spectrometry. Radiolabeling of L(1)-L(3) with [(99m)Tc(H(2)O)(3)(CO)(3)](+) afforded high yield compounds of identical character to the Re analogues. The radiolabeled compounds were found to be stable toward ligand exchange in the presence of a large excess of either cysteine or histidine over a 24-h period.

Rhenium Tricarbonyl Core Complexes of Thymidine and Uridine Derivatives

Thymidine and uridine were modified at the C2' and C5' ribose positions to form amine analogues of the nucleosides (1 and 4). Direct amination with NaBH(OAc)3 in DCE with the appropriate aldehydes yielded 1-{5-[(bis(pyridin-2-ylmethyl)amino)methyl]-4-hydroxytetrahydrofuran-2-yl}-5-methyl-1H-pyrimidine-2,4-dione (L1), 1-{5-[(bis(quinolin-2-ylmethyl)amino)methyl]-4-hydroxytetrahydrofuran-2-yl}-5-methyl-1H-pyrimidine-2,4-dione (L2), and 1-[3-(bis(pyridin-2-ylmethyl)amino)-4-hydroxy-5-(hydroxymethyl)tetrahydrofuran-2-yl]-1H-pyrimidine-2,4-dione (L5), while standard coupling procedures of 1 and 4 with 5-(bis(pyridin-2-ylmethyl)amino)pentanoic acid (2) and 5-(bis(quinolin-2-ylmethyl)amino)pentanoic acid (3) in the presence of HOBT-EDCI in DMF provided a second novel series of bifunctional chelators: 5-(bis(pyridin-2-ylmethyl)amino)pentanoic acid [(3-hydroxy-5-(5-methyl-4-oxo-3,4-dihydro-2H-pyrimidin-1-yl)tetrahydrofuran-2-yl)methyl] amide (L3), 5-(bis(quinolin-2-ylmethyl)amino)pentanoic acid [(3-hydroxy-5-(5-methyl-4-oxo-3,4-dihydro-2H-pyrimidin-1-yl)tetrahydrofuran-2-yl)methyl] amide (L4), 5-(bis(pyridin-2-ylmethyl)amino)pentanoic acid [2-(2,4-dioxo-3,4-dihydro-2H-pyrimidin-1-yl)-4-hydroxy-5-(hydroxymethyl)tetrahydrofuran-3-yl] amide (L6), and 5-(bis(quinolin-2-ylmethyl)amino)pentanoic acid [2-(2,4-dioxo-3,4-dihydro-2H-pyrimidin-1-yl)-4-hydroxy-5-(hydroxymethyl)tetrahydrofuran-3-yl] amide (L7). The rhenium tricarbonyl complexes of L1-L4, L6, and L7, [Re(CO)3(LX)]Br (X=1-4, 6, 7: compounds 5-10, respectively), have been prepared by reacting the appropriate ligand with [NEt4][Re(CO)3Br3] in methanol. The ligands and their rhenium complexes were obtained in good yields and characterized by common spectroscopic techniques including 1D and 2D NMR, HRMS, IR, cyclic voltammetry, UV, and luminescence spectroscopy and X-ray crystallography. The crystal structure of complex 6.0.5NaPF6 displays a facial geometry of the carbonyl ligands. The nitrogen donors of the tridentate ligand complete the distorted octahedral spheres of the complex. Crystal data: monoclinic, C2, a = 24.618(3) A, b = 11.4787(11) A, c = 15.5902(15) A, beta = 112.422(4) degrees , Z = 4, D(calc) = 1.562 g/cm3.

Design and synthesis of site directed maleimide bifunctional chelators for technetium and rhenium

A new family of heterobifunctional linkers (L1-L9) containing a terminus consisting of a tridentate donor set for coordination of the {M(CO)(3)}(+) core (M = Tc, Re), and a thiol reactive maleimide group has been prepared conveniently and in high yield under Mitsunobu reaction conditions by the coupling of an appropriate alcohol derivative with maleimide. The rhenium complexes [Re(CO)(3)(Lx)]Br (x= 1-9) were prepared in good yields from the reactions of the ligands and (NEt(4))(2)[Re(CO)(3)Br(3)] in refluxing methanol. The ligands and their Re complexes were characterized by (1)H and (13)C NMR, IR, and ESI-MS. Ligand L4 and [Re(CO)(3)(L5)]Br have been structurally characterized by X-ray crystallography. Photoexcitation of solutions of the complexes [Re(CO)(3)(Lx)]Br (x= 4-6) gives rise to intense and prolonged luminescence at room temperature (fluorescence lifetimes of ca. 16 micros). The ligands and their Re complexes react smoothly at the maleimide linker with sulfhydryl groups of peptides and proteins at room temperature in phosphate-buffered saline (PBS, pH 7.4) to form stable thioether bioconjugates. The photoluminescence properties of the labeled conjugates are similar to those of the parent complexes, but with even longer lifetimes. The ligands can also be labeled at room temperature with (99m)Tc to give chemically robust complexes. The corresponding hydrazinonicotinamide derivative N-[5-(6'-hydrazinopyridine-3'-carbonyl)aminopentyl]maleimide (L10) was also prepared. While coupling of L10 to cysteine ethylester and synthesis of the rhenium derivative [ReCl(3)(HYNIC-maleimide)(2)] were successfully accomplished, attempts to couple [ReCl(3)(HYNIC-maleimide)(2)] to glutathione or BSA yielded intractable mixtures.

Mono-, bi-, or tridentate ligands? The labeling of peptides with 99mTc-carbonyls

The labeling of targeting peptides with (99m)Tc is a useful concept for the diagnosis of various diseases such as cancer. Although in research for at least one decade, only a very few radiopharmaceuticals based on peptides are in clinical use. The difficulty of labeling, and the resulting authenticity of the new vector, is largely responsible for this observation. In this overview, we present an alternate strategy based on the organometallic fac-[(99m)Tc(CO)(3)](+) core for introducing (99m)Tc in biomolecules in general and in peptides in particular. The three coordination sites available in [(99m)Tc(OH(2))(3)(CO)(3)](+) can be occupied with many different ligand types, pendant to a biomolecule and serving as the anchor group for labeling. This makes the appropriate choice difficult. We intend to present some useful concepts for the practice. Monodentate chelators are robust but bear the risk of multiple binding of biomolecules. Coordinating a bidentate ligand of choice prior to labeling bypasses this problem and enables a systematic drug discovery by variation of the bidentate ligand. Bidentate ligands attached to the biomolecule are stronger but occasionally require protection of the remaining site by a monodentate ligand. Both approaches refer to a mixed-ligand [2+1] approach. Tridentate chelators are the most efficient but need some protecting group chemistry in order to achieve selectivity for the coupling process. Examples with cysteine and histidine are presented. This article aims to provide versatile and reproducible approaches for the labeling of biomolecules while not focusing on particular systems. It should be left to the readers to derive a strategy for their own peptide.

Unusual Reactivity of the {ReVO}3+ Core: Syntheses and Characterization of Novel Rhenium Halide Complexes with N-Methyl-o-diaminobenzene

The reactions of 1 or 2 equiv of N-methyl-o-diaminobenzene with trans-[ReOX(3)(PPh(3))(2)] (X = Cl, Br) in refluxing chloroform gave oxo-free rhenium complexes [Re(VI)X(4)(NC(6)H(4)NHCH(3))(OPPh(3))] (X = Cl, 3; X = Br, 6), [Re(V)X(2)Y(NC(6)H(4)NHCH(3))(PPh(3))(2)] (X, Y = Cl, 4; X = Br, Y = Cl, 7), [Re(IV)Cl(2)(NHC(6)H(4)NCH(3))(2)] (5), and [Re(IV)Br(3)(NHC(6)H(4)NCH(3))(PPh(3))] (8). All complexes were characterized by elemental analysis, (1)H NMR and IR spectroscopy, cyclic voltammetry, EPR spectroscopy, and X-ray crystallography. The complexes all display distorted octahedral coordination geometry. For Re(IV) complexes 5 and 8, the ligands coordinate in the benzosemiquinone diimine form. In Re(VI) complexes 3 and 6 and the Re(V) complexes 4 and 7, the ligands coordinate in the dianionic monodentate imido form. The EPR spectra of Re(VI) species 3 and 6 in dichloromethane solution at room temperature exhibit the characteristic hyperfine pattern of six lines, with evidence of strong second-order effects. The IR spectra of the complexes are characterized by Re=N and Re-N stretching bands at ca. 1090 and 540 cm(-)(1), respectively. The Re(IV) and Re(V) complexes display well-resolved NMR spectra, while the Re(VI) complexes exhibit no observable spectra, due to paramagnetism. The cyclic voltammograms of complexes 3 and 6 display Re(VII)/ Re(VI) and Re(VI)/Re(V) processes, those of 4 and 7 exhibit Re(VI)/Re(V) and Re(V)/Re(IV) couples, and those of 5 and 8 are characterized by Re(V)/Re(IV) and Re(IV)/Re(III) processes.

Bifunctional single amino acid chelates for labeling of biomolecules with the [Tc(CO)(3)](+) and [Re(CO)(3)](+) cores. Crystal and molecular structures of [ReBr(CO)(3)(H(2)NCH(2)C(5)H(4)N)], [Re(CO)(3)[(C(5)H(4)NCH(2))(2)NH]]Br, [Re(CO)(3)[(C(5)H(4)NCH(2))(2)NCH(2)CO(2)H]]Br, [Re(CO)(3)[X(Y)NCH(2)CO(2)CH(2)CH(3)]]Br (X = Y = 2-pyridylmethyl; X = 2-pyridylmethyl, Y = 2-(1-methylimidazolyl)methyl; X = Y = 2-(1-methylimidazolyl)methyl), [ReBr(CO)(3)[(C(5)H(4)NCH(2))NH(CH(2)C(4)H(3)S)]], and [Re(CO)(3)[(C(5)H(4)NCH(2))N(CH(2)C(4)H(3)S)(CH(2)CO(2))]]

The reactions of a series of potentially tridentate ligands, derived from single amino acids or amino acid analogues, with [NEt(4)](2)[ReBr(3)(CO)(3)] have been investigated. The model compounds [Re(CO)(3)Br[(2-pyridylmethyl)NH(2)]] (1) and [Re(CO)(3)[(2-pyridylmethyl)(2)NH]]Br (2) were also prepared and structurally characterized. With ligands possessing two pyridyl appendages, (2-pyridylmethyl)(2)NX (X = -CH(2)CO(2)H, -CH(2)CO(2)Et, -CH(2)CH(2)CO(2)H, -CH(2)CH(2)CO(2)Et, -CH(2)CH(2)CH(2)CH(2)CH(NHCO(2)(t)Bu)CO(2)H), complexes of the type [Re(CO)(3)(ligand)]Br (3-6) were isolated. All possess the fac-[Re(CO)(3)N(3)] coordination geometry in the cationic molecular unit. Similarly, the ligands with the imidazolyl arms (2-pyridylmethyl)[2-(1-methylimidazolyl)methyl]NCH(2)CO(2)Et and [2-(1-methylimidazolyl)methyl](2)NCH(2)CO(2)Et, complexes 7 and 8 of the same [Re(CO)(3)(ligand)]Br type, were prepared. Replacement of one pyridyl arm with a thiophene group yielded the complex [Re(CO)(3)[(2-pyridylmethyl)N(CH(2)CO(2))(2-thiophenemethyl)]] (9), while additional substitution of X = -H for -CH(2)CO(2)H yielded [Re(CO)(3)Br[(2-pyridylmethyl)NH(2-thiophenemethyl)]] (10). In both 9 and 10, the thiophene is uncoordinated and pendant, and the derivatives display fac-[Re(CO)(3)N(2)O] and fac-[Re(CO)(3)N(2)Br] coordination geometries, respectively. Crystal data: C(9)H(8)BrN(2)O(3)Re (1), triclinic P1, a = 8.156(1) A, b = 12.077(1) A, c = 12.945(2) A, alpha = 92.183(3) degrees, beta = 107.848(3) degrees, gamma = 100.955(7) degrees, V = 1185.1(3) A, Z = 4; C(15)H(13)BrN(3)O(3)Re (2), tetragonal P4(1), a = 8.6095(3) A, c = 22.228(1) A, V = 1646.9(1) A(3), Z = 4; C(17)H(14)BrN(3)O(5)Re.CH(3)OH (3), monoclinic P2(1)/m, a = 7.4425(3) A, b = 9.7596(4) A, c = 14.0646(6) A, beta = 97.753(1) degrees, V = 1012.26(7) A(3), Z = 2; C(19)H(19)BrN(3)O(5)Re (4), tetragonal P42(1)c, a = 16.895(3) A, c = 15.042(3) A, V = 4293.7(13) A(3), Z = 8; C(18)H(20)BrN(4)O(5)Re.CH(3)OH.H(2)O (7), monoclinic P2(1)/c, a = 10.2816(4) A, b = 30.386(1) A, c = 14.5810(6) A, beta = 99.868(1) degrees, V = 4488.03(3) A(3), Z = 8; C(17)H(21)BrN(5)O(5)Re.0.5CH(2)Cl(2).0.5H(2)O (8), triclinic P1, a = 11.5363(6) A, b = 13.1898(6) A, c = 16.4933(8) A, alpha = 89.356(1) degrees, beta = 74.907(1) degrees, gamma = 76.216(1) degrees, V = 2349.8(2) A(3), Z = 4; C(16)H(13)N(2)O(5)ReS (9), monoclinic P2(1)/c, a = 17.2072(7) A, b = 8.5853(4) A, c = 11.5607(5) A, beta = 101.73(1) degrees, V = 1672.2(1) A(3), Z = 4; and C(14)H(12)N(2)O(3)BrReS (10), triclinic P1, a = 7.5585(3) A, b = 9.7713(4) A, c = 11.7103(4) A, alpha = 109.566(1) degrees, beta = 98.298(1) degrees, gamma = 100.925(1) degrees, V = 779.73(5) A(3), Z = 2.