Crystal structures of technetium compounds

Synthesis and characterization of rhenium(iii) complexes with (Ph2PCH2CH2)2NR diphosphinoamine ligands

The synthesis and characterization of a new series of neutral, six-coordinated compounds [Re^IIIX₃(PNPR)], where X is Cl or Br and PNPR is a diphosphinoamine having the general formula (Ph₂PCH₂CH₂)₂NR (R = H, CH₃, CH₂CH₃, CH₂CH₂CH₃, CH₂CH₂CH₂CH₃ and CH₂CH₂OCH₃) are reported. Stable [Re^IIIX₃(PNPR)] complexes were synthesized, in variable yields, starting from precursors where the metal was in different oxidation states (iii and v), by ligand-exchange and/or redox-substitution reactions. The compounds were characterized by elemental analysis, proton NMR spectroscopy, cyclic voltammetry, UV/vis spectroscopy, positive-ion electrospray ionization mass spectrometry (ESI(+)-MS) and X-ray diffraction analysis. Although the formulation of the complexes allows either meridional or facial isomers, the latter arrangement was prevalent both in the solid and solution states. Only [ReCl₃(PNPH)] showed a meridional configuration both in solution and in the crystalline state. [ReBr₃(PNPme)] prefers the meridional configuration in the crystalline state and the facial one in solution. While ESI(+)-MS and voltammetric data seem to indicate some dependency from the nature of the alkyl substituent at the nitrogen, the available structural data of the complexes show only slight differences both for angles and bond lengths upon change of the alkyl chain tethered to the nitrogen.

Synthesis, metal complexation and biological evaluation of a novel semi-rigid bifunctional chelating agent for 99mTc labelling

A novel bifunctional chelating agent bearing an aromatic ring has been synthesised and characterised. This ligand formed well-defined oxorhenium complexes. The analogous 99mTcO-complex was obtained in an excellent yield with high radiochemical purity (>95%). The biodistribution of the 99mTc-complex after intravenous injection studied in normal rats showed that the activity was excreted mainly via renal-urinary pathway indicating its use for labelling peptides with 99mTc.

The crucial role of the diphosphine heteroatom X in the stereochemistry and stabilization of the substitution-inert [M(N)(PXP)]2+ metal fragments (M = Tc, Re; PXP = diphosphine ligand)

The nature of the heteroatom X incorporated in the five-membered PXP-diphosphine bridging chain was found to play a primary unit role both in the overall stability and in the stereochemical arrangement of nitrido-containing [M(N)(PXP)](2+) metal fragments (M = Tc, Re). Thus, by mixing PXP ligands with labile [Re(N)Cl(4)](-) and Tc(N)Cl(2)(PPh(3))(2) nitrido precursors in CH(2)Cl(2)/MeOH mixtures, a series of neutral M(N)Cl(2)(PXP) complexes (M = Tc, 1-5; M = Re, 8, 9) was collected. In the resulting distorted octahedrons, PXP adopted facial or meridional coordination, and combination with halide co-ligands produced three different stereochemical arrangements, that is, fac,cis, mer,cis, and mer,trans, depending primarily on the nature of the diphosphine heteroatom X. When X = NH, mer,cis-Tc(N)Cl(2)(PNP1), 1, was the only isomer formed. Alternatively, when a tertiary amine nitrogen (X = NR; R = CH(3), CH(2)CH(2)OCH(3)) was introduced in the bridging chain, fac,cis-M(N)Cl(2)(PN(R)P) complexes (M = Tc, 2, 3; M = Re, 8f) were obtained. Isomerization into the mer,cis-Re(N)Cl(2)(PN(R)P), 8m, species was observed only in the case of rhenium when the tertiary amine group carried the less encumbering methyl substituent. fac,cis-Tc(N)Cl(2)(PSP), 4f, was isolated in the solid state when X = S, but a mixture of fac,cis-Tc(N)Cl(2)(PSP) and mer,trans-Tc(N)Cl(2)(PSP), 4m, isomers was found in equilibrium in the solution state. A similar equilibrium between fac,cis-M(N)Cl(2)(POP) (M = Tc, 5f; M = Re, 9f) and mer,trans-M(N)Cl(2)(POP) (M = Tc, 5m; M = Re, 9m) species was detected in POP-containing complexes. The molecular structure of all of these complexes was assessed by means of conventional physicochemical techniques including multinuclear NMR spectroscopy and X-ray diffraction analysis of representative mer,cis-Tc(N)Cl(2)(PN(H)P), 1, fac,cis-Tc(N)Cl(2)(PSP), 4f, and mer,cis-Re(N)Cl(2)(PN(Me)P), 8m, compounds.

Synthesis and structural characterization of new oxorhenium and oxotechnetium complexes with XN2S-tetradentate semi-rigid ligands (X = O, S, N)

Twelve novel oxo-technetium and oxo-rhenium complexes based on N2S2-, N2SO- or N3S-tetradentate semi-rigid ligands have been synthesised and studied herein. By reacting the ligands with a slight excess of suitable [MO]3+ precursor (ReOCl3(PPh3)2 or [NBu4][99gTcOCl4]), the monoanionic complexes of general formula [MO(Ph-XN2S)]- could be easily produced in high yield. The complexes have been characterized by means of IR, electrospray mass spectrometry, elemental analysis, NMR and conductimetry. The crystal structures of [PPh4][ReO(Ph-ON2S)] 1b and [NBu4][99gTcO(Ph-ON2S)] 1c have been established. The [MO]3+ moiety was coordinated via the two deprotonated amide nitrogens, the oxygen and the terminal sulfur atoms in 1b and 1c. In both compounds, the ON2S coordination set is in the equatorial plane, and the complexes adopted a distorted square-pyramidal geometry with an axial oxo-group. The chemical and structural identity of the different prototypic complexes (rhenium, 99gTc complexes and their corresponding 99mTc radiocomplexes) have been also established by a comparative HPLC study.

Technetium-99m DTPA dimethyl ester: a renal function imaging agent. Comparative studies in animals with technetium-99m mercaptoacetyl triglycine and 131I-ortho-iodohippurate

The dimethyl ester of diethylene triamine penta-acetic acid (DMDTPA) (I) can be easily and efficiently labelled with 99mTc. This method can be readily adapted for kit formulations to produce a highly stable and very pure chelate, as shown by paper electrophoresis and reverse-phase high performance liquid chromatography experiments. In mice, this chelate was excreted unchanged in the urine, and the amount of renal excretion was much higher than that of 99mTc-DTPA and comparable with that of 99mTc-mercaptoacetyl triglycine (99mTc-MAG(3)) at two different time points. The renal excretion of co-injected 131I-ortho-iodohippurate (131I-OIH), however, was significantly greater than that of the 99mTc chelates. The renal clearance values of 99mTc-DMDTPA and 99mTc-MAG(3) were also similar and exceeded the corresponding value for 99mTc-DTPA, but were only half that of the 131I-OIH value in the rat. The renograms for 99mTc-DMDTPA and 99mTc-MAG(3) showed overall similarity in a dog model. The diethyl ester (III) and monoethyl ester (II) of DTPA, after labelling with 99mTc, produced the same chelate, as shown by analytical results and biological data, indicating that one of the ester groups in the DTPA diester is dealkylated during the chelation procedure. To confirm this, two more ligands, diethylene triamine 1,4,7,7-tetra-acetic acid (IV) and diethylene triamine 1,4,7-triacetic acid (V), were synthesized, resembling DTPA monoalkyl ester (II) and dialkyl ester (I, III), respectively, in the arrangement of the donor atoms. Ligand IV but not ligand V, on 99mTc chelation, can generate the specific pharmacophore for renal tubular transport that is also present in the ester chelates 99mTc-I, 99mTc-II and 99mTc-III, as shown by their decreased renal excretion in mice pretreated with a renal tubular transport inhibitor such as probenecid.

Synthesis and characterization of rhenium(V) oxo complexes with N-[N-(3-diphenylphosphinopropionyl)glycyl]cysteine methyl ester. X-ray crystal structure of (ReO[Ph(2)P(CH(2))(2)C(O)-Gly-Cys-OMe(P,N,N,S)])

The PN(2)S chelate N-[N-(3-diphenylphosphinopropionyl)glycyl]-S-tritylcysteine methyl ester [PN(2)S(Trt)-OMe] was synthesized and reacted with ReOCl(3)(PPh(3))(2) and Ph(4)P[ReOCl(4)]. The reactions of both tritylated and detritylated ligands with Re(V)O precursors gave two diastereomers, 9a and 9b, of the ReO(PN(2)S-OMe) complex. The two isomers, produced in a 1:1 molar ratio, are stable and do not interconvert. They were separated by reverse-phase HPLC and characterized by NMR, FT-IR, and UV-visible spectroscopy and electrospray mass spectrometry. X-ray analysis established for 9a the presence in the solid of the syn isomer. Compound 9a, C(21)H(23)N(2)O(5)PSRe, crystallized from warm acetonitrile in the triclinic space group Ponemacr;, a = 9.828(2) A, b = 11.163(2) A, c = 11.641(2) A, alpha = 106.48(3) degrees, beta = 109.06(3) degrees, gamma = 102.81(3) degrees, V = 1085.7(4) A(3), Z = 2. The PN(2)S coordination set is in the equatorial plane, and the complex shows a distorted square pyramidal coordination. The anti configuration assigned to 9b is consistent with all the available physicochemical data. Follow-up of the reaction of the detritylated ligand with Ph(4)P[ReOCl(4)] in ethanol or acetonitrile indicated that the phosphorus atom of the chelate binds first to the metal and that this bond acts as the driving force for coordination.

Evolution of Tc-99m in diagnostic radiopharmaceuticals

The progress in diagnostic nuclear medicine over the years since the discovery of 99mTc is indeed phenomenal. Over 80% of the radiopharmaceuticals currently being used make use of this short-lived, metastable radionuclide, which has reigned as the workhorse of diagnostic nuclear medicine. The preeminence of 99mTc is attributable to its optimal nuclear properties of a short half-life and a gamma photon emission of 140 keV, which is suitable for high-efficiency detection and which results in low radiation exposure to the patient. 99mTcO4-, which is readily available as a column eluate from a 99Mo/99mTc generator, is reduced in the presence of chelating agents. The versatile chemistry of technetium emerging from the 8 possible oxidation states, along with a proper understanding of the structure-biologic activity relationship, has been exploited to yield a plethora of products meant for morphologic and functional imaging of different organs. This article reviews the evolution of 99mTc dating back to its discovery, the development of 99Mo/99mTc generators, and the efforts to exploit the diverse chemistry of the element to explore a spectrum of compounds for diagnostic imaging, planar, and single photon emission computed tomography. A brief outline of the 99mTc radiopharmaceuticals currently being used has been categorically presented according to the organs being imaged. Newer methods of labeling involving bifunctional chelating agents (which encompass the "3 + 1" ligand system, Tc(CO)3(+1)-containing chelates, hydrazinonicotinamide, water-soluble phosphines, and other Tc-carrying moieties) have added a new dimension for the preparation of novel technetium compounds. These developments in technetium chemistry have opened new avenues in the field of diagnostic imaging. These include fundamental aspects in the design and development of target-specific agents, including antibodies, peptides, steroids, and other small molecules that have specific receptor affinity.

Novel oxorhenium and oxotechnetium complexes from an aminothiol[NS]/thiol[S] mixed-ligand system

The simultaneous action of a bidentate aminothiol ligand, LnH, (n = 1: (CH3CH2)2NCH2CH2SH and n = 2: C5H10NCH2CH2SH) and a monodentate thiol ligand, LH (LH: p-methoxythiophenol) on a suitable MO (M = Re, 99gTc) precursor results in the formation of complexes of the general formula [MO(Ln)(L)3] (1, 2 for Re and 5. 6 for 99gTc). In solution these complexes gradually transform to [MO(Ln)(L)2] complexes (3, 4 for Re and 7, 8 for 99gTc). The transformation is much faster for oxotechnetium than for oxorhenium complexes. Complexes 1-4, 7, and 8 have been isolated and fully characterized by elemental analysis and spectroscopic methods. Detailed NMR assignments were made for complexes 3, 4, 7, and 8. X-ray studies have demonstrated that the coordination geometry around rhenium in complex 1 is square pyramidal (tau = 0.06), with four sulfur atoms (one from the L1H ligand and three from three molecules of p-methoxythiophenol) in the basal plane and the oxo group in the apical position. The L1H ligand acts as a monodentate ligand with the nitrogen atom being protonated and hydrogen bonded to the oxo group. The four thiols are deprotonated during complexation resulting in a complex with an overall charge of zero. The coordination geometry around rhenium in complex 4 is trigonally distorted square pyramidal (tau = 0.41), while in the oxotechnetium complex 7 it is square pyramidal (tau = 0.16). In both complexes LnH acts as a bidentate ligand. The NS donor atom set of the bidentate ligand and the two sulfur atoms of the two monodentate thiols define the basal plane, while the oxygen atom occupies the apical position. At the technetium tracer level (99mTc), both types of complexes, [99mTcO(Ln)(L)3] and [99mTcO(Ln)(L)2], are formed as indicated by HPLC. At high ligand concentrations the major complex is [99mTcO(Ln)(L)3], while at low concentrations the predominant complex is [99mTcO(Ln)(L)2]. The complexes [99mTcO(Ln)(L)3] transform to the stable complexes [99mTcO(Ln)(L)2]. This transformation is much faster in the absence of ligands. The complexes [99mTcO(Ln)(L)2] are stable, neutral, and also the predominant product of the reaction when low concentrations of ligands are used, a fact that is very important from the radiopharmaceutical point of view.

The syntheses and structures of '3+2' and '2+2+1' oxorhenium mixed-ligand complexes employing 8-hydroxy-5-nitroquinoline as the bidentate N,O donor ligand

The syntheses and structural characterizations of a series of novel '3+2' and '2+2+1' mixed-ligand complexes carrying 8-hydroxy-5-nitroquinoline (HL) as the bidentate N,O donor atom system are reported. Thus one-pot reactions of [ReOCl(3)(PPh(3))(2)] with dianionic tridentate ligands H(2)L(n) (where H(2)L(1)=HOC(6)H(4)-2-CH=NC(6)H(4)-2-OH; H(2)L(2)=HOC(6)H(4)-2-CH=N-C(6)H(4)-2-SH; H(2)L(3)=HOC(6)H(4)-2-CH=NN=C(NHC(6)H(5))-SH; H(2)L(4)=2-CH(2)OH-C(5)H(3)N-6-CH(2)SH; and H(2)L(5)=2-CO(2)H-C(5)H(3)N-6-CO(2)H) and HL afforded a series of '3+2' oxorhenium complexes of the type [ReO(H(2)L(n))(L)] 2-6, which exhibit distorted octahedral geometries. Crystals of 1 are monoclinic space group C2/c, a=16.702(1), b=14.275(1), c=22.363(2) A, beta=108.083(1) degrees V=5068.2(7) A(beta) and Z=8; those of 2 are monoclinic space group P2(1)/n, a=8.5093(4) b=38.518(2), c=11.6092(5) A, beta=97.708(1) degrees , V=3770.7(3) A(3) and Z=8; those of 5 are triclinic, space group P1-, a=7.5899(8), b=10.322(1), c=11.905(1) A, alpha=78.636(2) degrees , beta=74.229(2) degrees , gamma=71.391(2) degrees , V=844.3(2) A(3), and Z=2. Upon reaction of 2,6-pyridinedimethanol (H(2)L(6)) with the intermediate complex [ReOCl(2)(L)(PPh(3))] (1), only one of the two methylene hydroxy group was deprotonated and a new '2+2+1' complex [ReO(OCH(3))(HL(6))(L)].CH(3)OH (7) was obtained. Crystal data for 7: monoclinic P2(1)/n, a=12.0579(6), b=11.0993(6), c=14.9262(8) A, beta=107.872(1) degrees , V=1901.2(2) A(3), and Z=8. In the preparation of complex 3, cleavage of the C=N bond of the Schiff base H(2)L(2) was observed and the '2+2+1' complex 8 [ReO(PPh(3))(eta(2)-NHC(6)H(4)-2-S)(L)].CH(2)Cl(2) having 2-aminothiophenol as a dianionic bidentate ligand was isolated. Crystals of 8 are monoclinic space group C2/c, a=25.627(2), b=8.1305(6) c=31.404(2) A beta=96.147(1) degrees , V=6505.7(8) A(3), and Z=8. Reduction of HL to 8-hydroxy-5-aminoquinoline was realized during the formation of complex 6, and a new complex 9 was thus isolated involving the coordination of two sets of N,O donor atoms from L and 8-hydroxy-5-aminoquinoline while a methoxy oxygen atom completes the octahedral coordination geometry. Crystals of 9 are monoclinic space group P2(1)/n, a=7.5330(6), b=15.095(1), c=16.394(1) A, beta=99.690(2) degrees , V=1837.7(3) A(3), and Z=4.

Oxorhenium(V) complexes containing tridentate Schiff-base and monothiol coligands

The reaction of [n-(C(4)H(9))(4)N][ReOBr(4)(OPPh(3))] with the tridentate Schiff-base [HOC(6)H(4)C(H)NC(6)H(4)SH] allows for the isolation of [ReOBr{eta(3)-(OC(6)H(4)C(H)NC(6)H(4)S)}] (1). The reaction of [n-(C(4)H(9))(4)N][ReOBr(4)(OPPh(3))] with [HOC(6)H(4)C(H)NC(6)H(4)SH] and the appropriate benzenethiol (C(6)H(4)X-4-SH) where X=H, Br, Cl, F, and OCH(3) in methanol-acetonitrile treated with triethylamine has led to the isolation of a series of rhenium complexes of the type [ReO{eta(3)-(OC(6)H(4)C(H)NC(6)H(4)S)} (eta(1)-C(6)H(4)X-4-S)] (X=H (2), Br (3), Cl (4), F (5), and OCH(3) (6)). Likewise, under similar reaction conditions, the use of benzylmercaptan ligands of the type (C(6)H(4)X-4-CH(2)SH) where X=H, Cl, F, and OCH(3) has led to the isolation of a series of rhenium complexes of the type [ReO{eta(3)-(OC(6)H(4)C(H)NC(6)H(4)S)} (eta(1)-C(6)H(4)X-4-CH(2)S)] (X=H (7), Cl (8), F (9), and OCH(3) (10)). The incorporation of the appropriate amine functionality into the substituent R of the monodentate ligand allows for the isolation of a cationic oxorhenium(V) species, namely, [ReO{eta(3)-(OC(6)H(4)C(H)NC(6)H(4)S)} (eta(1)-C(5)H(4)NH-2-S)][Br] (11).

Oxorhenium(V) and Oxotechnetium(V) Complexes of Cysteine

Earlier attempts to obtain technetium complexes with cysteine always resulted in the formation of a product contaminated with polymeric species. A pure product, which could be chemically characterized and adopted for radiopharmaceutical preparation, has now been obtained by using cystine as the precursor of cysteine. This method has been extended to prepare the corresponding rhenium chelate, isolated as the tetraphenylphosphonium salt [Ph(4)P](+)[{ReO(Cys)(2)}(-){HReO(Cys)(2)}].4H(2)O. The X-ray crystal structure of this compound revealed the presence of both neutral and anionic chelated species. In [HReO(Cys)(2)], the cysteine carboxylate moiety is unidentatedly bound to rhenium, while the carboxylic acid of the second cysteine remains as free COOH. The coordination environment around rhenium in the anionic species [ ReO(Cys)(2)(-)] is similar, the only difference being that the uncoordinated carboxylate moiety is present as a COO(-) anion. The thiolate, amine coordination of the ligand with the metal is present in both the chelate units. The compound crystallized in an orthorhombic system with the space group P2(1)2(1)2(1), and having four formula units in each cell. The crystal data are a = 9.700(2) Å, b = 12.836(3) Å, and c = 36.228(3) Å. The rhenium chelate has been structurally correlated with the technetium chelates through comparable spectroscopic and chromatographic data. The technetium-99m analogue of this rhenium chelate exhibited renal tubular transport and renal retention, which makes this radiopharmaceutical useful for evaluation of the clinical status of renal patients.

A New Donor Atom System [(SNN)(S)] for the Synthesis of Neutral Oxotechnetium(V) Mixed-Ligand Complexes

A new approach to the "3 + 1" mixed ligand oxotechnetium complexes of the general formula TcOL1L2, with ligands (L1H(2)) containing the SNN donor set and various monodentate thiols as coligands (L2H) is reported. The ligands L1H(2) (1-3, general formula R(1)CH(2)CH(2)NHCH(2)C(R(2))(2)SH where R(1) = N(CH(3))(2) and R(2) = H in 1, R(1) = pyrrolidin-1-yl and R(2) = H in 2, and R(1) = piperidin-1-yl and R(2) = CH(3) in 3) act as tridentate SNN chelates to the TcO(3+) core, leaving open one coordination site cis to the oxo group. In the presence of a monodentate thiol (L2H) and using (99)Tc(V)-gluconate as precursor, the vacancy is filled by the thiol which acts as the coligand. With this approach four neutral oxotechnetium complexes (4-7, general formula TcO[R(1)CH(2)CH(2)NCH(2)C(R(2))(2)S][SR] where RSH = p-methoxybenzenethiol, or p-methylbenzenethiol or benzyl mercaptan) were prepared in high yield by reacting L1H(2) and L2H with Tc(V)-gluconate in a ratio 1:1:1. The complexes were characterized by elemental analysis and spectroscopic methods. Complete assignments of (1)H and (13)C NMR resonances were made for all complexes. X-ray crystallographic studies of 5 (R(1) = pyrrolidin-1-yl, R(2) = H, RSH = p-methylbenzenethiol) and 7 (R(1) = piperidin-1-yl, R(2) = CH(3), RSH = benzyl mercaptan) showed that the complexes crystallize in the monoclinic space group P2(1)/n (a = 10.223(1) Å, b = 9.283(1) Å, c = 18.337(2) Å, beta = 97.262(2) degrees, V = 1726.3(4) Å(3), Z = 4; a = 11.876(2) Å, b = 10.470(2) Å, c = 17.098(3) Å, beta = 105.990(4) degrees, V = 2043.8(6) Å(3), Z = 4, for 5 and 7, respectively). Complexes5 and 7 have distorted square pyramidal coordination geometry with the oxo ligand in the axial position. The steric requirements of the oxo group cause the Tc atom to be displaced 0.68 Å out of the mean equatorial plane of the NNSS donor atoms in both complexes.

Rhenium complexes ofP,P,P′,P′-tetrakis(o-hydroxyphenyl)diphosphinoethane

Preparation of the hydrochloride salt of a new potentially hexadentate ligand precursor P,P,P′,P′-tetrakis(o-hydroxyphenyl)diphosphinoethane dihydrochloride (abbreviated H4P2O4•2HCl) is described. From H4P2O4•2HCl, neutral [Re2O2Cl2(PPh3)2(μ-P2O4)] and dianionic [Re2O2Br4(μ-P2O4)]2−dinuclear rhenium(V) complexes were synthesized. The complexes have been characterized by elemental analysis, infrared spectroscopy, mass spectrometry, and1H/31P{1H} NMR spectra.31P{1H} NMR revealed that only one isomer, presumably the anti, was present for [Re2O2Cl2(PPh3)2(μ-P2O4)], and that two isomers, both anti and syn isomers, were observed for [Re2O2Br4(μ-P2O4)]2−. The coligands (PPh3for the former, Br−for the latter) of both complexes underwent ligand exchange with pyridine. Key words: rhenium, P,P,P′,P′-tetrakis(o-hydroxyphenyl)diphosphinoethane, dimer, isomers.

Synthesis and Characterization of Oxotechnetium(V) Mixed-Ligand Complexes Containing a Tridentate N-Substituted Bis(2-mercaptoethyl)amine and a Monodentate Thiol

A series of 22 mixed-ligand complexes of the general formula TcOL(1)L(2), where L(1)H(2) are N-substituted bis(2-mercaptoethyl)amine ligands, [SN(R)S], and L(2)H are monodentate thiols as coligand, is reported. The complexes were prepared by the ligand exchange method using Tc-gluconate as precursor and equimolar quantities of the two ligands. In all cases the syn stereoisomer was formed in high yield and isolated as a crystalline product. In four cases HPLC analysis demonstrated the presence of the anti stereoisomer in the reaction mixture. Although the yield was less than 1%, one anti isomer, 4a, was successfully isolated as brown crystals. The isolated complexes were characterized by spectroscopic methods and elemental analysis. The formation of the two diastereomers, syn and anti, was expected due to the configuration of the nitrogen substituent (R) with respect to the central TcO core. The X-ray crystallography showed that the coordination geometry of the syn isomers 9, 11, and 18 is trigonal bipyramidal while for the anti isomer 4a it is distorted square pyramidal. This is the first documentation of syn/anti isomerism in N-substituted TcO[SN(R)S][S] mixed-ligand complexes.

Cysteine, a chelating moiety for synthesis of technetium-99m radiopharmaceuticals--Part IV. Benzyl cysteine and derivatives

To explore the possibility of utilizing cysteine derivatives for technetium-99m radiopharmaceutical preparation with clinical potential, we synthesized two benzyl substituted cysteine compounds, namely, S-benzyl cysteine 1 and cysteine benzyl ester 3. It was expected, from our previous studies on benzoyl cysteines, that the above two ligands after chelation with 99mTc would be excreted by the hepatobiliary pathway. Although for 99mTc-3 the above expectation was realized, 99mTc-1 behaved in a most unexpected way by affixing itself with kidney and selecting the renal tubular secretory pathway for its excretion. It is anticipated that the affinity of 99mTc-1 for kidney is due to its interaction with the kidney sulphhydryl group and it also formed an adduct with other sulphydryl containing compounds like thiophenol. In terms of the kidney-to-background ratio, 99mTc-1 showed some superiority over other kidney structure agents, like 99mTc-dimercaptosuccinic acid and 99mTc-glucoheptanoic acid and, therefore, the chelate (99mTc-1) may have the potential to replace the above two radiopharmaceuticals in clinical use.