Single isomer technetium-99m tamoxifen conjugates

To produce an imaging agent for breast cancer using a technetium-99m-labeled agent specific for estrogen receptors, an N(2)S(2) bifunctional chelator was conjugated to Z- and E-aminotamoxifens through an amide linker. These bioconjugates have been chelated with both technetium-99m and rhenium. For the Z-isomer, chelation with rhenium in the presence of sodium acetate yields a mixture of two isomers, anti and syn, in a 1:1 ratio and in the presence of hydroxide results in only the anti isomer. Both the Z- and E-tamoxifen conjugates have been chelated with technetium-99m at the tracer level yielding a single isomer product, which is assigned as anti based on chromatographic comparison to the rhenium complexes. Radiochemical yields were consistently greater than 80%, with Sep-Pak column purification yielding a final product with >99% radiochemical purity and no residual starting material. Both in vitro and in vivo biological evaluation of the tamoxifen chelates indicated very limited estrogen receptor binding.     

Synthesis of a diaminedithiol bifunctional chelating agent for incorporation of technetium-99m into biomolecules

The synthesis of a bifunctional chelating agent (BCA), 1, based on the diaminedithiol (DADT) ligand system, is described. The six-step synthetic sequence has been accomplished in 16% overall yield, affording 1, which contains a thiolactone as a reactive moiety, which permits direct coupling to nucleophiles without the formation of byproducts. The reactivity of 1 toward benzylamine and subsequent labeling of the ligand with technetium-99m has been evaluated as a model for preparation of various bioconjugates. Both coupling and exchange labeling occur in high yield under mild conditions, and competition reactions with diethylenetriaminepentaacetic acid (DTPA) indicate the superior stability of the technetium-99m-DADT complex. Preparation of BCA 1 thus provides a new avenue into technetium-labeled radiopharmaceuticals.     

Phosphine-containing HYNIC derivatives as potential bifunctional chelators for (99m)Tc-labeling of small biomolecules

Two prototype phosphine-containing HYNIC chelators, HYNIC-Kp-DPPB and HYNIC-Ko-DPPB (HYNIC = 6-hydrazinonicotinamide; K = lysine; and DPPB = diphenylphosphine-benzoic acid), have been synthesized and characterized by NMR ((1)H, (13)C, and (31)P) and LC-MS. Macrocyclic (99m)Tc complexes, [(99m)Tc(HYNIC-Ko-TPPB)(tricine)] and [(99m)Tc(HYNIC-Kp-DPPB)(tricine)], were prepared by reacting the phosphine-containing HYNIC chelator with (99m)TcO(4)(-) in the presence of excess tricine and stannous chloride. Results from this study clearly demonstrated that both HYNIC-Kp-DPPB and HYNIC-Ko-DPPB are able to form highly stable macrocyclic (99m)Tc complexes, [(99m)Tc(HYNIC-Ko-TPPB)(tricine)] and [(99m)Tc(HYNIC-Kp-DPPB)(tricine)], when tricine is used as the coligand. Radio-HPLC data suggest that the complex [(99m)Tc(HYNIC-Kp-DPPB)(tricine)] exists as only one detectable isomer in solution while the complex [(99m)Tc(HYNIC-Ko-DPPB)(tricine)] has three isomers. It was also found that three isomers of [(99m)Tc(HYNIC-Ko-DPPB)(tricine)] interconvert at elevated temperatures, suggesting that the presence of these isomers might be due conformational changes in the macrocyclic Tc chelate. The LC-MS data for both macrocyclic (99m)Tc complexes are completely consistent with the proposed composition. The phosphine-containing HYNIC chelators described in this study may have the potential as bifunctional chelators for (99m)Tc labeling of small biomolecules.     

First example of well-characterized Re and Tc tricarbonyl complexes of ciprofloxacin and norfloxacin in the development of infection-specific imaging agents

Aiming at the development of ^99mTc-based infection-specific imaging agents, the synthesis and characterization of rhenium and technetium-99m tricarbonyl complexes with derivatized ciprofloxacin and norfloxacin is hereby reported. The ligands were prepared by coupling the tridentate chelator picolylamino-N,N-diacetic acid (PADA) with the piperazinyl (NH) nitrogen of ciprofloxacin or norfloxacin, through the employment of the PADA anhydride. The corresponding rhenium complexes were synthesized using the fac-[NEt₄]₂[ReBr₃(CO)₃] precursor and were fully characterized by elemental analysis and NMR spectroscopy. X-ray crystallography of the ciprofloxacin complex showed that the geometry about rhenium is distorted octahedral defined by the NNO donor atom set of the tridentate chelator and the three carbonyl groups. The analogous technetium-99m complexes were prepared quantitatively through the use of the fac-[^99mTc(H₂O)₃(CO)₃]⁺ precursor and their structure was established by comparative HPLC studies using the well-characterized rhenium complexes as reference. Preliminary studies with the technetium-99m complexes showed high bacterial uptake in vitro.     

Synthesis, characterization, and biological evaluation of neutral nitrido technetium(V) mixed ligand complexes containing dithiolates and aminodiphosphines. A novel system for linking technetium to biomolecules

A new biomolecule labeling method that utilizes the [(99m)Tc(N)(PNP)](2+) metal fragment is presented. Thus, a series of nitrido mixed-ligand M(V) complexes (M = (99m)Tc, (99g)Tc, Re), [M(N)(Ln)(PNP)], where Ln is the dianionic form of a dithiolate or substituted-dithiolate ligand and PNP is an aminodiphosphine, is described. (99m)Tc complexes can be prepared using either a two-step or a three-step procedure starting from generator-eluted pertechnetate through a prereduced mixture of [(99m)Tc(N)]-containing species, followed by sequential or contemporary addition of the relevant dithiolate and aminodiphosphine. The reactions of 2,3-dimercaptopropionic acid (H(2)L1) with [Tc(N)(PNP)](2+) were investigated in detail. It was found that this bidentate ligand coordinated the metal fragment through the [S(-),S(-)] donor atom pair, to yield neutral mixed-ligand complexes [(99m)Tc(N)(L1)(PNP)] in high specific activity. The additional carboxylic functional group was not involved in metal coordination, thus remaining available for conjugation to target-specific molecules. Dithiolates incorporating pendant functional group(s) gave rise to a 1:1 diastereoisomeric mixture of syn-[M(N)(Ln)(PNP)] and anti-[M(N)(Ln)(PNP)] derivatives, depending on the relative orientation of the dithiolate substituent(s) with respect to the terminal nitrido group, and no isomeric conversion was detected. (99m)Tc species had been proven to be identical with the (99g)Tc complexes prepared at the macroscopic level by comparison of the corresponding radiometric and UV/vis HPLC profiles. Challenge experiments with cysteine or glutathione indicated that these physiological agents had no effect on the stability of this class of mixed-ligand (99m)Tc-complexes. Biodistribution studies in rats of selected (99m)Tc-complexes showed a rapid clearance from the blood and tissues after 60 min pi.     

Pyridine-containing 6-hydrazinonicotinamide derivatives as potential bifunctional chelators for 99mTc-labeling of small biomolecules

As a continuation of our interest in novel 99mTc chelating systems, several pyridine-containing HYNIC (6-hydrazinonicotinamide) derivatives (L1-L5) have been synthesized and characterized by NMR (1H and 13C) and LC-MS. 99mTc complexes of L1-L5 were prepared by the reaction of the HYNIC derivative with 99mTcO4- in the presence of excess tricine and stannous chloride. Results from this study show that the attachment site of the linker is critical for the formation of macrocyclic 99mTc complexes. For example, the pyridine-N in L3 is not able to bond to the Tc, because the lysine linker is attached to the 4-position. When the linker is at the 2-position, L1 forms the macrocyclic complex [99mTc(L1)(tricine)], but the radiochemical purity is relatively low. If the linker is attached to the 3-position of the pyridine ring, the HYNIC derivatives form macrocyclic complexes [99mTc(L)(tricine)] (L2, L4, and L5) in high yield (>95%). The HPLC data suggest that the macrocyclic complex [(99m)Tc(L2)(tricine)] exists in solution as four isomers: two diastereomers and two conformational isomers. Diastereomers are due to a combination of the chirality of the lysine linker and of the Tc chelate. Replacing lysine with a pentamethylenediamine linker results in the macrocyclic complex [99mTc(L4)(tricine)] with two conformational isomers, which interconvert rapidly at room temperature. Changing the linker from pentamethylenediamine to hexamethylenediamine did not eliminate the minor isomer; but the percentage of the minor isomer was reduced from approximately 10% for [99mTc(L4)(tricine)] to only 6% for [99mTc(L5)(tricine)]. The linker length is an important parameter to minimize the minor isomer. LC-MS data of complexes [99mTc(L)(tricine)] (L2, L4, and L5) are completely consistent with their proposed compositions. On the basis of these data, it is concluded that pyridine-containing HYNIC derivatives have the potential as bifunctional chelators for 99mTc-labeling of small biomolecules if the linker is attached to the 3-position of the pyridine ring.     

Novel oxorhenium and oxotechnetium MO(NS)(S)2 complexes in the development of 5-HT1A receptor imaging agents

The [NS][S](2) mixed-ligand system was applied to synthesize oxorhenium and oxotechnetium complexes of the general formula MO(o-CH(3)OC(6)H(4)N(CH(2)CH(2))(2)NCH(2)CH(2)S)(p-CH(3)C(6)H(4)S)(2) (M=Re in 1, M=(99)Tc in 2, and M=(99m)Tc in 3). The bidentate [NS] ligand includes the 1-(2-methoxyphenyl)piperazine moiety which is a fragment of the true 5-HT(1A) antagonist WAY 100635. The oxorhenium complex 1 was prepared by a ligand exchange reaction using ReOCl(3)(PPh(3))(2) as precursor while [Bu(4)N][(99)TcOCl(4)] and (99)Tc-gluconate were used as precursors in the synthesis of the oxotechnetium-99 complex 2. Both complexes were characterized by elemental analysis and spectroscopic methods. Crystallographic analysis of 1 showed that the rhenium coordination geometry is trigonal bipyramidal. The basal plane of the trigonal bipyramid is defined by the oxo group and two sulphur atoms, one belonging to the [NS] ligand and the other to an aromatic thiol, while the apical positions are occupied by the nitrogen of the [NS] ligand and the sulphur of the second aromatic thiol. The oxotechnetium-99 complex 2 has almost identical unit cell parameters to those of the oxorhenium complex 1 indicating, in combination with the other analytical data, that the complexes are isostructural. The binding affinity of the oxorhenium complex 1 for the 5-HT(1A) receptor subtype was determined in rat brain hippocampal preparations (IC(50)=106 nM). The oxotechnetium-99m complex 3 was prepared by a ligand exchange reaction using (99m)Tc-glucoheptonate as the precursor. Its structure was established by comparative HPLC studies using the oxotechnetium-99 complex 2 as a reference. Complex 3 was administered by intravenous injection in rats. At 2 min post injection, 0.153% of the injected dose per gram of tissue was measured in rat brain.     

Synthesis and bio-evaluation of a new fatty acid derivative for myocardial imaging

Development of a (99m)Tc-fatty acid analogue is of interest, as (99m)Tc is logistically advantageous over the cyclotron-produced (11)C and (123)I. Synthesis of a 16 carbon fatty acid derivative and its radiolabeling with the novel [(99m)TcN(PNP)](2+) core is described here. Hexadecanedioic acid was conjugated to cysteine in an overall yield of 55%. This ligand could be labeled with (99m)Tc via the [(99m)TcN(PNP)](2+) core, in 80% yield, as a mixture of two isomers (syn and anti). The major isomer isolated by HPLC was used for bioevaluation studies in swiss mice and compared with radioiodinated iodophenyl pentadecanoic acid (IPPA), an established agent for myocardial metabolic imaging. (99m)Tc-labeled complex cleared faster from the non-target organs, namely, liver, lungs, and blood compared to that of [(125)I]-IPPA. However, the complex exhibited lower uptake and faster washout from the myocardium as compared to [(125)I]-IPPA.     

A new tricarbonyl fac-[M(acac)(isc)(CO)3] complex (M = Re, Tc) with acetylacetonate (acac) and isocyanide (isc) in a 2+1 combination

The synthesis and characterization of the neutral 2+1 mixed ligand complex fac-Re(CO)₃(acac)(isc) (4) with acetylacetonate (acac) as the bidentate ligand and an isocyanide (the isocyanocyclohexane, isc) as the monodentate ligand is described. The synthesis of 4 proceeds through the intermediate formation of the fac-Re(acac)(H₂O)(CO)₃ precursor complex 2. Complex 4 was characterized by elemental analysis, spectroscopic methods, and X-ray crystallography showing a distorted octahedral arrangement of the ligands around Re. At technetium-99m level, the corresponding fac-^99mTc(acac)(isc)(CO)₃ complex 5 was obtained in high yield by reacting the fac-^99mTc(acac)(H₂O)(CO)₃ precursor complex 3 with isocyanocyclohexane and its structure was established by chromatographic comparison with the prototypic rhenium complex using high performance liquid chromatography.     

The [(99m)Tc(N)(PNP)](2+) metal fragment: a technetium-nitrido synthon for use with biologically active molecules. The N-(2-methoxyphenyl)piperazyl-cysteine analogues as examples

The incorporation of a bioactive molecule into a nitrido-containing (99m)Tc-complex has been successfully achieved by using the [TcN(PNP)](2+) metal fragment. In this strategy, the strong electrophilic [TcN(PNP)](2+) metal fragment efficiently reacts with bifunctional chelating ligands having a pi-donor atom set, such as N-functionalized O,S-cysteine. The 2-methoxyphenylpiperazine (2-MPP) pharmacophore, which displays preferential affinity for 5HT(1A) receptors, was conjugated to the amino group of cysteine to obtain 2-MPPP-cys-OS, where 2-MPPP is 3-[4-(2-methoxyphenyl)piperazin-1-yl]propionate. The asymmetric Tc(V)-nitrido complexes, [(99g/99m)Tc(N)(PNP)(2-MPPP-cys-OS)] (PNP = PNP3, PNP4), were obtained in high yield (95%), by simultaneous addition of PNP and 2-MPPP-cys-OS ligand to a solution containing a starting (99g)/(99m)Tc-nitrido precursor. A mixture of syn and anti isomers was observed, the latter being the thermodynamically favored species. In vitro challenge experiments using the anti isomers with glutathione and cysteine indicated that no transchelation reaction occurs. Assessment of the in vitro 5HT(1A) receptor-affinity of the technetium complexes revealed that only the anti-PNP4 complex possesses some affinity for the receptor, but displayed negligible brain uptake in biodistribution studies in rats in vivo.     

Benzimidazole derivatives as NSO ligands for the fac-[M(CO)3] (M = Re, Tc)

Two rhenium(I) tricarbonyl complexes with the tridentate monoanionic NSO ligands, 4-(benzimidazol-2-yl)-3-thiabutanoic acid (complex 3) and [1-(11-carboxyundecanyl)-4-(benzimidazol-2-yl)]-3-thiabutanoic acid (complex 4) were synthesized and characterized by spectroscopic methods and elemental analysis. X-ray crystallographic analysis of complex 3 revealed a distorted octahedral geometry around rhenium defined by the three facially bound CO groups and the NSO donor atom set of the tridentate ligand. The analogous technetium-99m complexes (complexes 5 and 6) were also prepared quantitatively by reaction of the NSO ligands with the fac-[^99mTc(H₂O)₃(CO)₃]⁺ synthon and their identity was established by chromatographic comparison to their rhenium congeners. Biodistribution in mice of complex 6 bearing the fatty acid chain showed significant heart uptake (6.26 ± 0.79% ID/g p.i.) at 1 min accompanied, however, with a heart:blood ratio below 1.     

Sequence analysis of twin ATP binding cassette proteins involved in translational control, antibiotic resistance, and ribonuclease L inhibition

The urea-induced unfolding of 'N' isomer (occurring at pH 7.0) and 'B' isomer (occurring at pH 9.0) of human serum albumin was studied by fluorescence and circular dichroism spectroscopic measurements. Urea-induced destabilization in different domains of both the isomers was monitored by using domain specific ligands, hemin (domain-I), chloroform, bilirubin (domain-II), and diazepam (domain-III). Urea-induced denaturation of N and B isomers of HSA showed a two-step, three-state transition with accumulation of intermediates around 4.8-5.2M and 3.0-3.4M urea concentrations, respectively. During first transition (0-4.8M urea for N isomer and 0-3.0M urea for B isomer) a continuous decrease in diazepam binding suggested major conformational changes in domain-III prior to intermediate formation. On the other hand, binding of hemin, a ligand for domain-IB and chloroform, whose binding site is located in domain-IIA remains unchanged up to 5.0M urea for N isomer and 3.0M urea for B isomer. Similarly, fluorescence intensity of Trp-214 that resides in domain-IIA remained unchanged up to the above-said urea concentrations and decreased thereafter. Absence of any decrease in hemin binding, chloroform binding, and Trp-214 fluorescence suggested the non-involvement of domain-IB and domain-IIA in intermediate formation. A significant increase in bilirubin binding prior to intermediate formation showed favorable conformational rearrangement in bilirubin binding cavity formed by loop 4 of domain-IB and loop 3 of domain-IIA. Further, a nearly complete abolishment of bilirubin binding to both isomers around 7.0M and 6.0M urea concentrations, respectively, indicated complete separation of domain-I from domain-II from each other. From these observations it can be concluded that N to B transition of human serum albumin shifted the intermediate formation towards lower urea concentration (3.0-3.4M urea for B isomer as against 4.8-5.2M urea for N isomer). Further both the intermediates were found to possess similar alpha-helical (approximately 39%) content and ligand binding properties.     

DNA-binding and molecular mechanics modelling studies of the bulky chiral platinum(II) complex [PtCl(2)(mepyrr)] (mepyrr=N-methyl-2-aminomethylpyrrolidine)

Detailed studies were carried out on the binding of the enantiomers of [PtCl₂(mepyrr)] (mepyrr = N-methyl-2-aminomethylpyrrolidine) to dG, d(GpG) and a 52-mer oligonucleotide. The pyrrolidine ligand structure was found to be neither sufficiently rigid nor bulky to enforce a single chirality at the exocyclic amine site in this complex, resulting in the presence of diastereomers that complicated the binding studies. Reaction of the (GpG) dinucleotide with R- and S-[PtCl₂(mepyrr)] resulted in formation of four [Pt{d(GpG)}(mepyrr)] isomers for each enantiomer as a consequence of the existence of two orientational isomers and two diastereomers. These isomers formed in different amounts most likely as a consequence of the unequal formation of the diastereomers together with stereoselectivity induced by interactions between the dinucleotide and the mepyrr ligand. The [PtCl₂(mepyrr)] complexes displayed stereoselectivity and enantioselectivity in their reactions with a 52-mer duplex designed to allow formation of only GpG intrastrand adducts. All four bifunctional adducts formed for each enantiomer, providing further evidence of the lack of directing ability of the ligand in formation of the 1,2-intrastrand adduct. Significant amounts of monofunctional species remained in these assays suggesting that the introduction of the methyl substituent to the exocyclic amine inhibited ring-closure to the bifunctional adduct. This was not sufficient to achieve enantiospecificity, but in the case of the R-enantiomer, one of the bifunctional adducts formed in only small amounts.     

Radiosynthesis, biodistribution and micro-SPECT imaging study of dendrimer-avidin conjugate

Partially acetylated generation five polyamidoamine (PAMAM) dendrimer (G5-Ac) was reacted with biotin and 2-(p-isothiocyanatobenzyl)-6-methyl-diethylenetria minepentaacetic acid (1B4M-DTPA), respectively to form the complex Bt-G5-Ac-1B4M which was further conjugated with avidin to give the conjugate Av-G5-Ac-1B4M. Then both of the conjugates were radiolabeled with technetium-99m ((99m)Tc), respectively. Their in vitro cellular uptake study shows that the conjugate of Av-G5-Ac-1B4M-(99m)Tc exhibits much higher cellular uptake in HeLa cells than that of Bt-G5-Ac-1B4M-(99m)Tc. Accordingly the following evaluation such as in vitro/in vivo stability, biodistribution and micro-SPECT imaging was observed only for the conjugate of Av-G5-Ac-1B4M-(99m)Tc.     

Synthesis and biological evaluation of 99mTc, Re-monoamine-monoamide conjugated to 2-(4-aminophenyl)benzothiazole as potential probes for beta-amyloid plaques in the brain

The benzothiazole aniline (BTA) conjugated with monoamine-monoamide (MAMA) was synthesized and then labeled with (99m)Tc. Its corresponding rhenium analogue was synthesized, and the fluorescent staining was performed in brain sections of both Tg mouse and Alzheimer's disease (AD) patient. The fluorescent rhenium complex Re-MAMA-BTA selectively bound to the amyloid aggregates in the brain sections of both APP Tg mouse and AD patient. The analogous (99m)Tc-MAMA-BTA complex could enter the normal mouse brain with high initial uptake. These results are encouraging for further exploration of their derivatives as imaging agents for Abeta plaques in the brain.     

Novel halo-oxo-allenic fatty ester derivatives from epoxidized methyl santalbate (methyl trans-11-octadecen-9-ynoate)

Methyl santalbate (methyl trans-11-octadecen-9-ynoate) from Sandal wood seed oil, Santalbum alum) was epoxidized to methyl trans-11,12-epoxy-octadec-9-ynoate (1). Treatment of compound 1 with tetrabutylammonium dihydrogentrifluoride, and boron trifluoride etherate gave the corresponding anti- (2a) (57%) and syn- (2b) (35%) fluorohydrin derivatives, respectively. These reactions were regio- and stereoselective in nature. The structures of the anti- and syn- isomers were confirmed by NMR spectroscopy. Ring opening of the epoxy system of compound 1 with lithium chloride gave the anti-chlorohydrin derivative (3) (89%). Oxidation of either compound 2a or 2b gave the same fluoro-keto acetylenic fatty ester (4) (75%), and compound 3 on chromic acid oxidation yielded the corresponding chloro-keto acetylene (5) (73%). Isomerization of compounds 4 and 5 with potassium carbonate in dichloromethane furnished the requisite fluoro-allenic (6) (63%, methyl 11-fluoro-12-oxo-9,10-octadecadienoate) and chloro-allenic (7) (80%, methyl 11-chloro-12-oxo-9,10-octadecadienoate) C(18) fatty esters. All products were confirmed by a combination of spectrometric and spectroscopic techniques.     

Synthesis and biological characterization of novel 2-quinolinecarboxamide ligands of the peripheral benzodiazepine receptors bearing technetium-99m or rhenium

Potential receptor imaging agents based on Tc-99m for the in vivo visualization of the peripheral benzodiazepine receptor (PBR) have been designed on the basis of the information provided by the previously published structure-affinity relationship studies, which suggested the existence of tolerance to voluminous substituents in the receptor area interacting with 3-position of the quinoline nucleus of 2-quinolinecarboxamides 5. In the first step of the investigation, the stereoelectronic features of the above-indicated receptor area were also probed by means of 4-phenyl-3-[(1-piperazinyl)methyl]-2-quinolinecarboxamide derivatives bearing different substituents on the terminal piperazine nitrogen atom (compounds 6a-f). The structure-affinity relationship data confirmed the existence of a tolerance to bulky lipophilic substituents and stimulated the design of bifunctional ligands based on the 4-phenyl-3-[(1-piperazinyl)methyl]-2-quinolinecarboxamide moiety (compounds 6h,j,k,m). The submicromolar PBR affinity of rhenium complexes 6j,m suggests that the presence of their metal-ligand moieties with encaged rhenium is fairly compatible with the interaction with the PBR binding site. Thus, in order to obtain information on the in vivo behavior of these bifunctional ligands, (99m)Tc-labeled compounds 6h,k were synthesized and evaluated in preliminary biodistribution and single photon emission tomography (SPET) studies. The results suggest that both tracers do not present a clear preferential distribution in tissues rich in PBR, probably because of their molecular dimensions, which may hamper both the intracellular diffusion toward PBR and the interaction with the binding site.     

Chemistry of sulforhodamine--amine conjugates

Commercially-available sulforhodamine sulfonyl chlorides contain two isomeric monosulfonyl chlorides. Conjugates of these isomers with amines have different properties because the sulfonamide formed from one isomer can undergo ring-closure to a colorless sultam. This chemistry has been examined for a model conjugate with methylamine and for a bioconjugate with 2'(3')-O-[N-(2-aminoethyl)carbamoyl]ATP. The interaction of each isomer of the latter conjugates with myosin subfragment 1 has been characterized. Significant differences between the two isomers are observed in these interactions.     

Asymmetric reduction of ethyl 2-methyl e-oxobutanoate by fungi

Seven fungi, which are found to reduce ethyl 3-oxobutanoate in high yields, were tested for their reducing ability for ethyl 2-methyl 3-oxobutanoate. We obtained some interesting findings. In particular, Penicillium purpurogenum reduced ethyl 2-methyl 3-oxobutanoate to the corresponding alcohols with the diastereomer (anti/syn) ratio of 93/7 with the enantiomeric excess of anti-(2S,3S)- and syn-(2S,3R)- hydroxy esters of 90 and >99 ee%, respectively.     

Synthesis and in vitro characterization of organometallic rhenium and technetium glucose complexes against Glut 1 and hexokinase

A series of nine organometallic technetium-99m and rhenium complexes of glucose are presented and characterized in vitro regarding their potential as surrogates of [18F]-2-fluoro-desoxy glucose ([18F]-FDG). The glucose derivatives are functionalized at positions C-1, C-2, C-3, and C-6. Different spacer lengths and chelating systems have been introduced at these sites. For the (radio)labeling, the organometallic precursors [99mTc(H2O)3(CO)3]+ and [ReBr3(CO)3](2-) respectively have been used. The resulting complexes have been characterized chemically and radiochemically. The formation of uniform products has been observed on the macroscopic (Re) and no-carrier-added level (99mTc). The Tc-99m complexes revealed good inertness against ligand exchange (Cys and His) and excellent stability in physiological buffered saline as well as in human plasma over a period of 24 h at 37 degrees C. The rhenium complexes have been tested for competitive inhibition of the (yeast) hexokinase. Only for C-2 derivatized glucose complexes with extended spacer functionalities Ki values in the millimolar and sub-millimolar range have been observed. In silico molecular docking experiments supported these experimental findings. However, the competitive inhibitors are not recognized as a pseudosubstrate of hexokinase. The cellular uptake of all 99mTc-complexes into HT-29 colon carcinoma cells via Glut1 was generally low and unspecific independent of the position at the hexose ring, the chelating systems, or the overall charge of the corresponding metal complexes. The current results seem to preclude the use of these compounds as [18F]-FDG surrogates primarily due to the low cellular uptake via Glut1.