Structural studies of ReO(V) mixed ligand [SNS][Cl] and [SNS][S] complexes

Potential use of drug carried-liposomes for cancer therapy via direct intratumoral injection

Liposomes have recognized advantages as nano-particle drug carriers for tumor therapy. In this study, the pharmacokinetics and distribution of intratumorally administered liposomes were investigated as drug carriers for treating solid tumors via direct intratumoral administration. 99mTc-liposomes were administered intratumorally to nude rats bearing human head and neck squamous cell carcinoma xenografts. Planar gamma camera images were analyzed to evaluate the local retention of the intratumorally administered liposomes. Co-registered pinhole micro-single photon emission computed tomography (SPECT)/computed tomography (CT) images were acquired of the whole animal as well as the dissected tumors to determine intratumoral distribution of the 99mTc-liposomes. For 99mTc-liposomes, there was an initial retention of 47.4 +/- 11.0% (n = 4) in tumors and surrounding tissues. At 20 h, 39.2 +/- 10.6% (n = 4) of 99mTc-activity still remained in the tumor. In contrast, only 18.7 +/- 3.3% (n = 3) of the intratumoral 99mTc-activity remained for unencapsulated 99mTc-complex at 20 h. Pinhole micro-SPECT images demonstrated that 99mTc-liposomes also have a superior intratumoral 99mTc-activity diffusion compared with unencapsulated 99mTc-complex. Higher intratumoral retention of 99mTc-liposomes accompanied by an improved intratumoral diffusion suggests that intratumorally administered liposomal drugs are potentially promising agents for solid tumor local therapy.

Direct 99mTc labeling of pegylated liposomal doxorubicin (Doxil) for pharmacokinetic and non-invasive imaging studies

Pharmacokinetic and organ distribution studies of liposomal drugs in humans are a challenge. A direct labeling method using (99m)Tc-N,N-bis(2-mercaptoethyl)-N',N'-diethyl-ethylenediamine (BMEDA) complex to label the commercially available pegylated liposomal doxorubicin, Doxil, has been introduced. Biodistributions of (99m)Tc-Doxil in normal rats were performed to evaluate the feasibility of using it for monitoring the pharmacokinetics of liposomes encapsulating drugs. Labeling efficiency of (99m)Tc-Doxil was 70.6 +/- 0.8% (n = 3). In vitro incubation of (99m)Tc-Doxil in 50% fetal bovine serum or 50% human serum at 37 degrees C showed good labeling stability with 72.3 +/- 3.6% or 78.6 +/- 1.8% of activity associated with Doxil at 24 h, respectively (n = 3). There was a two-phase blood clearance with half-clearance times of 2.2 and 26.2 h after bolus intravenous injection in normal rats. Distribution of (99m)Tc-Doxil at 44 h after injection had 19.8 +/- 1.3% of injected dose in blood, 14.1 +/- 1.7% in liver, 2.6 +/- 0.3% in spleen, 9.0 +/- 0.8% in bone with marrow, 6.0 +/- 0.5% in skin, and 15.3 +/- 4.3% in bowel (n = 5). Unencapsulated (99m)Tc-BMEDA had a very rapid blood clearance with a half-clearance time of only 0.12 h (n = 4). By using this (99m)Tc labeling method, biodistribution and pharmacokinetics of ammonium gradient liposomes encapsulating drugs can be determined by noninvasive scintigraphic imaging. This labeling method may be extended to (186)Re and (188)Re labeling to combine chemotherapy and radionuclide therapy for tumor treatment.

Synthesis and characterization of novel oxotechnetium (99Tc and 99mTc) and oxorhenium complexes from the 2,2'-bipyridine (NN)/thiol (S) mixed-ligand system

The synthesis and characterization of oxotechnetium and oxorhenium mixed-ligand complexes of the general formula MO[NN][S](3) (M = (99)Tc and Re), where NN represents the bidentate ligand 2,2'-bipyridine and S represents a monodentate thiophenol, is reported. The complexes were prepared by ligand exchange reactions using (99)Tc-gluconate and ReOCl(3)(PPh(3))(2) as precursors for the oxotechnetium and oxorhenium complexes, respectively. Compound 1 (M = (99)Tc, S = 4-methylthiophenol) crystallizes in the monoclinic space group P2(1)/a, a = 23.12(1) A, b = 14.349(6) A, c = 8.801(4) A, beta = 94.81(2) degrees, V = 2918(2) A(3), Z = 4. Compound 3 (M = Re, S = 4-methylthiophenol) crystallizes in the monoclinic space group P2(1)/a, a = 23.018(9) A, b = 14.421(5) A, c = 8.775(3) A, beta = 94.78(1) degrees, V = 2903(2) A(3), Z = 4. Compound 4 (M = Re, S = 4-methoxythiophenol) crystallizes in the orthorhombic space group Pbca, a = 16.32(1) A, b = 24.55(2) A, c = 16.94(1) A, V = 6788(9) A(3), Z = 8. In all cases, the coordination geometry around the metal is distorted octahedral with the equatorial plane being defined by the three sulfur atoms of the thiophenols and one nitrogen atom of 2,2'-bipyridine, while the apical positions are occupied by the second nitrogen atom of 2,2'-bipyridine and the oxygen of the M=O core. The complexes are stable, neutral, and lipophilic. Complete (1)H and (13)C NMR assignments are reported for all complexes. The analogous oxotechnetium complexes have been also synthesized at tracer level ((99m)Tc) by mixing the 2,2'-bipyridine and the corresponding thiol with Na(99m)TcO(4) generator eluate using NaBH(4) as reducing agent. Their structure was established by chromatographic comparison with authentic oxotechnetium and oxorhenium complexes using high performance liquid chromatography techniques.

A novel liposome radiolabeling method using 99mTc-"SNS/S" complexes: in vitro and in vivo evaluation

Liposomes are important carriers for controlled drug release, for gene or antisense therapy, and for immunization. Radiolabeled liposomes can be used to evaluate the in vivo behavior of different liposome formulations, as well as for diagnostic imaging and radionuclide therapy. A novel method for radiolabeling liposomes with (99m)Tc-"SNS/S" complexes is introduced. This labeling method can be applied to liposome radiolabeling with not only (99m)Tc but also two therapeutic radionuclides, (186)Re and (188)Re. Liposomes encapsulating glutathione (GSH) were studied for (99m)Tc labeling. N,N-bis(2-mercaptoethyl)-N',N'-diethyl-ethylenediamine (BMEDA), N,N-bis(2-mercaptoethyl)-1-butylamine (BMBuA), and benzene thiol (BT) were investigated to make (99m)Tc-"BMEDA", (99m)Tc-"BMEDA + BT", (99m)Tc-"BMBuA", and (99m)Tc-"BMBuA + BT", for liposome labeling. The labeling efficiencies of (99m)Tc-GSH liposomes were from 36.9 to 69.2%. After incubation in serum, (99m)Tc-GSH liposomes labeled with (99m)Tc-"BMEDA" or (99m)Tc-"BMEDA + BT" had the best labeling stability of the formulations tested. Distribution studies after intravenous injection of (99m)Tc-liposomes composed of distearoyl phosphatidylcholine (DSPC) and cholesterol had a slow blood clearance and a high spleen accumulation demonstrating the in vivo labeling stability of the radiolabeled liposomes. The (99m)Tc-liposomes have great potential as a radiopharmaceutical system for evaluating various kinds of liposomes with different lipid composition, for evaluating in advance a subsequent radionuclide therapy using (186)Re or (188)Re labeled liposomes and for diagnostic imaging.

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.

Synthesis and characterization of oxorhenium(V)-'3+1' mixed thiolate [SNS]/[S] and [ONS]/[S] complexes. Crystal and molecular structures of [ReO(eta-SCH(2)C(5)H(3)NCH(2)S)(eta-C(6)H(4)Br-4-S)], [ReO(eta-SCH(2)C(5)H(3)NCH(2)O)(eta-C(6)H(4)X-4-S)] (X=Cl, OMe), [ReO(eta-SCH(2)C(5)H(3)NCH(2)O)(eta-C(6)H(4)OCH(3)-4-CH(2)S)] and [ReO(eta-SCH(2)C(5)H(3)NCH(2)S)(eta-C(5)H(4)NH-2-S)][Cl]

The reaction of [ReOCl(3)(PPh(3))(2)] with the tridentate ligands 2,6-dithiomethylpyridine (4) or 6-thiomethyl-2-pyridinemethanol (5) and the appropriate monothiols, such as para-substituted benzenethiols (C(6)H(4)X-4-SH) (where X=F, Cl, Br and OCH(3)) and para-substituted benzylmercaptans (C(6)H(4)X-4-CH(2)SH) (where X=F, Cl and OCH(3)) in the presence of Et(3)N leads to the isolation of a series of integrated '3+1' oxorhenium(V) complexes. Similarly, reaction of [ReOCl(3)(PPh(3))(2)] with 2-mercaptopyridine and 4 afforded a cationic oxorhenium(V) complex, [ReO(eta(3)-SCH(2)C(5)H(3)NCH(2)S)(eta(1)-C(5)H(4)NH-2-S)][Cl] (22). Crystal data for 10, C(13)H(11)BrNOS(3)Re: triclinic, P1, a=7.2909(5), b=14.1978(9), c=15.991(1) A, alpha=77.184(1), beta=86.588(1), gamma=75.507(1) degrees , V=1562.69(18) A(3), Z=4. For 16, C(13)H(11)ClNO(2)S(2)Re: orthorhombic, P2(1)2(1)2(1), a=6.9728(6), b=13.477(1), c=15.761(1) A, V=1481.1(2) A(3), Z=4. For 18, C(14)H(14)NO(3)S(2)Re: monoclinic, P2(1)/c, a=15.6512(14), b=7.6678(7), c=13.732(1) A, beta=112.489(1) degrees , V=1522.7(2) A(3), Z=4. For 21, C(15)H(16)NO(3)S(2)Re: monoclinic, P2(1)/c, a=13.929(1), b=7.741(1), c=15.583(2) A, beta=103.600(2) degrees , V=1633.1(3) A(3), Z=4. For 22, C(12)H(11)ClN(2)OS(3)Re: monoclinic, C2(1)/(c), a=27.827(3), b=8.529(1), c=14.957(2) A, beta=119.314(2) degrees , V=3095.3(6) A(3), Z=8.

Synthesis and characterization of complexes of the {ReO} core with SNS and S donor ligands

The reaction of [ReOCl(3)(PPh(3))(2)] with N,N-bis(2-mercaptoethyl)benzylamine and 4-bromobenzenethiol allowed for the isolation of [ReO{eta(3)-(SCH(2)CH(2))(2)N(CH(2)C(6)H(5))}-(eta(1)-C(6)H(4)Br-4-S)] (1). The reaction of [ReOCl(3)(PPh(3))(2)] with [(HSCH(2)CH(2))(2)N(CH(2)C(5)H(4)N)] and the appropriate thiol in chloroform treated with triethylamine has led to the isolation of a series of neutral rhenium complexes of the type [ReO{eta(3)-(SCH(2)CH(2))(2)N(CH(2)C(5)H(4)N)}(eta(1)-C(6)H(4)X-4-S)] (X = Br (2), Cl (3), F (4), and OCH(3) (5)) and [ReO{eta(3)-(SCH(2)CH(2))(2)N(CH(2)C(5)H(4)N)}(eta(1)-C(6)H(4)OCH(3)-4-CH(2)S)] (6). Likewise, under similar reaction conditions, the use of the related tridentate ligand, [(HSCH(2)CH(2))(2)N(CH(2)CH(2)C(5)H(4)N)], has led to the isolation of a series of rhenium complexes of the type [ReO{eta(3)-(SCH(2)CH(2))(2)N(CH(2)CH(2)C(5)H(4)N)}(eta(1)-C(6)H(4)X-4-S)] (X=Br (7), Cl (8), OCH(3) (9)), as well as [ReO{eta(3)-(SCH(2)CH(2))(2)N(CH(2)CH(2)C(5)H(4)N)}(eta(1)-C(6)H(4)Cl-4-CH(2)S)].0.5CH(3)(CH(2))(4)CH(3) (10). These compounds are extensions of the '3+1' approach to the synthesis of materials with the {MO}(3+) core (M=Tc and Re), which have applications in nuclear medicine. The ligands chosen allow systematic exploration of the consequences of para-substitution on the monodentate thiolate ligand [S] and of derivatization of the substituent R on the tridentate aminodithiol ligand [SNS] of the type (HSCH(2)CH(2))(2)NR. Such modifications can influence lipophilicity, charge, size and molecular weight of the complex and consequently the biodistribution.

Trends in NMR chemical shifts and ligand mobility of TcO(V) and ReO(V) complexes with aminothiols

Detailed 1H and 13C NMR studies have been conducted in a series of oxotechnetium and oxorhenium complexes with aminothiol ligands ([SNS][S], [SNN][S], [SNNS]) designed as potential radiopharmaceuticals. The results of these studies in combination with others in the literature show that the oxometal core creates an anisotropic environment and affects the chemical shifts of the coordinated ligandbackbone in a consistent way. Protons oriented towards the oxygen appear deshielded relative to their geminals oriented away from the oxygen. In addition, the direction of a side chain (towards or away from the oxometal core) on the ligand backbone is shown to have a major effect on chemical shifts. The fluxional mobility of the ligand in complexes of the [SNS][S] type was also studied by NMR and the free energy of activation delta G(C)double dagger for the conformational inversion of the ligand was calculated from the temperature dependence of the carbon chemical shifts. delta G(C)double dagger was found to depend on the orientation of the side chain present on the coordinated nitrogen. The energy barrier for the inversion is larger for the oxorhenium complexes than for the analogous oxotechnetium complexes.