Radiometal complexes: characterization and relevant in vitro studies

Radiometals are, and will continue to be, very important to diagnostic and therapeutic nuclear medicine applications as they predominantly possess the most suitable nuclear properties for these types of applications. This article attempts to give the reader an overview of key aspects that need to be considered in the design and synthesis of a radiopharmaceutical using the commonly known and employed radionuclides, such as technetium, rhenium, the lanthanides and copper. While it is important to understand each radiometal ion has its own specific coordination chemistry requirements, there are several issues that are critical to all radiometal ions for their incorporation into a radiopharmaceutical. 1) The route of production and the presence of long lived contaminating radionuclides and or of naturally occurring metal ions that will interfere with the efficient and optimum radiolabelling of their ligand of choice as well as the final specific activity of the product; 2) the significant differences between the chemistry at the macroscopic (mM and higher concentrations) and radiotracer levels (uM and lower concentrations for the high specific activity radionuclides); 3) the rate of complexation and of dissociation of the radiometal ion vs the competing reaction of radiometal hydrolysis; 4) natural biological pathway of the radio-metal ion and therefore the design of the appropriate and relevant in vitro tests to assess the stability of the radiometal complex. These are a selection of critical factors that need to be considered in the design of a successful radiopharmaceutical, whether it is used for imaging or therapy. However, one should consider tailoring their investigations to suit the radiometal under investigation, and to be mindful where the technology is to be applied (e.g. imaging organs or disease).

Nanocompatible Chemistry toward Fabrication of Target-Specific Gold Nanoparticles

Nanocompatible chemistry which utilizes a novel nontoxic phosphino amino acid as a reducing agent has resulted in the development of therapeutically useful gold nanoparticles under biologically benign media. Stabilization of gold nanoparticles by the edible gum arabic matrix has provided an effective pathway toward in vivo stable target-specific gold nanoparticles.

Sorption of 99mTc radiopharmaceutical compounds by soils

Study of the sorption of 99mTc radiopharmaceutical compounds by soils has assessed the fate of these compounds in the event of a surface spill and examined the potential of these compounds as hydrologic tracers. Sorption from deionized water, filtered Missouri River water, and artificial seawater by five surface soils was investigated. For all water types, the Tc radiopharmaceutical compounds showed greater sorption than the uncomplexed pertechnetate. The most lipophilic complexes showed the highest sorption on soils.

Exceptional kinetic propensity of hydroxymethyl phosphanes toward Rh(III) stabilization in water

The reactions of (HOCH2)2P(C6H4)P(CH2OH)2 (HMPB) and P(CH2OH)3 (THP) with RhCl3.xH2O in aqueous media gave water-soluble complexes cis-[RhCl2{eta2-(HOCH2)2P(C6H4)P(CH2OH)2}2]Cl (3) and fac-[RhCl3(P(CH2OH)3)3] (4) respectively in good yields, X-ray crystal structures of 3 and 4 confirmed their molecular constitution. These reactions provide the first examples demonstrating the kinetic propensity of hydroxymethyl phosphanes to stabilize Rh in +3 oxidation state in water.

Rhodium-105 tetrathioether complexes: radiochemistry and initial biological evaluation

105Rhodium(III) complexes with three different acyclic tetrathioether ligands containing pendant carboxylic acid groups have been synthesized and characterized. The complexes were evaluated for stability under physiological conditions and the most promising complexes were evaluated in vivo in normal mice. The primary route of clearance for these complexes was the renal/urinary system, consistent with the presence of pendant carboxylate groups. The results indicate that the cis-[Rh(III)Cl2(2,5,8,11-tetrathiadodecane-1,12-dicarboxylic acid)]+ complex shows the most promising in vivo characteristics on which to base a potential therapeutic radiopharmaceutical.

Radiochemical studies of 99mTc complexes of modified cysteine ligands and bifunctional chelating agents

The synthesis of four novel ligands using the amino-acid cysteine and its ethyl carboxylate derivative is described. The synthetic method involves a two-step procedure, wherein the intermediate Schiff base formed by the condensation of the amino group of the cysteine substrate and salicylaldehyde is reduced to give the target ligands. The intermediates and the final products were characterized by high resolution nuclear magnetic resonance spectroscopy. Complexation studies of the ligands with 99mTc were optimized using stannous tartrate as the reducing agent under varying reaction conditions. The complexes were characterized using standard quality control techniques such as thin layer chromatography, paper electrophoresis, and paper chromatography. Lipophilicities of the complexes were estimated by solvent extraction into chloroform. Substantial changes in net charge and lipophilicity of the 99mTc complexes were observed on substituting the carboxylic acid functionality in ligands I and II with the ethyl carboxylate groups (ligands II and IV). All the ligands formed 99mTc complexes in high yield. Whereas the complexes with ligands I and II were observed to be hydrophilic in nature and not extractable into CHCl3, ligands III and IV resulted in neutral and lipophilic 99mTc complexes. The 99mTc complex with ligand II was not stable and on storage formed a hydrophilic and nonextractable species. The biodistribution of the complexes of ligands I and II showed that they cleared predominantly through the kidneys, whereas the complexes with ligands III and IV were excreted primarily through the hepatobiliary system. No significant brain uptake was observed with the 99mTc complexes with ligands III and IV despite their favorable properties of neutrality, lipophilicity, and conversion into a hydrophilic species. These ligands offer potential for use as bifunctional chelating agents.

Synthesis of heptadentate (N4O3) amine-phenol ligands and radiochemical studies with technetium-99m

Heptadentate amine-phenol ligands with N4O3 donor atoms for coordination were synthesized by condensing tris(2-aminoethyl)amine with salicylaldehyde or acetophenone and reducing the Schiff bases formed with NaBH4. The ligands were characterized by 1H and 13C nuclear magnetic resonance spectroscopy. Radiochemical studies were carried out with no-carrier-added 99mTc and 99mTc spiked with 0.1-100 microM of 99Tc. Complexation yields were estimated from thin layer chromatography, paper electrophoresis, and solvent extraction studies. 99mTc complexes were formed in yields better than 90% with the amine-phenol ligands. The complexes were found to be neutral and lipophilic. Biodistribution studies of the 99mTc complexes showed that clearance was mainly through the hepatobiliary system.

Technetium-99m complexes of polydentate amine-pyrrole and amine-thiophene ligands

Novel polydentate amine-pyrrole and amine-thiophene ligands were synthesized and characterized by 1H and 13C NMR spectroscopy. Radiochemical studies with 99mTc were carried out at 0.1-100 microM of technetium. Complexation yields were estimated from thin layer chromatography (TLC), paper electrophoresis, and solvent extraction studies. The 99mTc complexes formed were found to be neutral and lipophilic. Complexes with the corresponding imine-ligands were formed in lower yields. Biodistribution studies of the 99mTc complexes of these ligands showed no significant uptake in brain or heart, and the clearance was mainly through the hepatobiliary system.

An Rh-105 complex of tetrathiacyclohexadecane diol with potential for formulating bifunctional chelates

1,5,9,13-Tetrathiacyclohexane-3,11-diol (16S4-diol), a sulfur crown ether analog, was studied as a potential chelating agent to complex no-carrier-added (NCA) grade 105Rh(III) in high yield at low ligand concentrations. trans-[RhCl2(16S4-diol)]chi (chi = Cl, PF6) was prepared using nonradioactive RhCl3.3H2O and characterized by UV-Vis, nuclear magnetic resonance (NMR) and X-ray crystallography. It was shown to have a +1 charge with the Rh(III) metal center coordinated to the four S atoms equatorially and two Cl atoms in trans axial positions. The 105Rh-16S4-diol complex prepared with NCA 105Rh(III)-chloride reagent was found to exhibit identical chromatographic properties as trans-[Rh(III)Cl2(16S4-diol)]+ (including silica and C-18 thin-layer chromatography [TLC] and electrophoresis). The preparation of 105Rh-16S4-diol complex formation optimized for conditions of pH, temperature, time, % ethanol and quantity of 16S4-diol resulted in yields > 90%. Very low quantities of 16S4-diol (3 nmol) complex NCA 105Rh(III) under relatively mild reaction conditions (heating at 64 degrees C for 90 min) in the presence of ethanol (10%), yielded the high specific activity 105Rh-16S4-diol complex as a single cationic species. The 105Rh-16S4-diol complex was shown to be stable for > or = 4 days in physiological buffers at room temperature and in human serum at 37 degrees C.

[99mTc] Teboroxime and [99mTc]Cl(DMG)3B2MP: binding characteristics andmetabolism of two [99mTc]BATOs in blood and tissues

Studies were performed in vitro and in vivo to evaluate the binding properties and metabolism of [99mTc]Cl(CDO)3BMe (Teboroxime) and [99mTc]Cl(DMG)3B2MP in blood and target tissues of rats. Both radiopharmaceuticals displayed rapid binding (within 1-3 min) with high affinity to plasma proteins and blood cells. The amounts of radioactivity associated with blood components became progressively greater with time of exposure to either compound. There was a higher proportion of the radiopharmaceuticals associated with blood components during in vivo conditions, likely due, at least in part, to clearance of the free fraction from the plasma pool. Exposure of both compounds to blood results in axial ligand exchange of the chloro atom to a hydroxyl. The results suggest that it is the free species that is extracted primarily by tissues.

The single-pass cerebral extraction and capillary permeability-surface area product of several putative cerebral blood flow imaging agents

We have determined cerebral blood flow (CBF) and the single-pass cerebral extraction (E) of several putative agents for external imaging of CBF. Simultaneous measurements of blood flow and extraction were performed in 106 rats. For all agents, comparison of linear and exponential regressions of E on CBF indicates that this relationship can be described as linear over the range of flows studied. Analysis of covariance indicates that the extraction of 123I-IMP, 67Cu-PTSM and 99mTc-HMPAO is higher than that of 99mTc-Cl(DMG)3(2MP) and 99mTc-ECD, particularly at flows above the normal range. Accordingly, for 123I-IMP, 67Cu-PTSM and 99mTc-HMPAO, the slope of the linear regression equation for the relationship between brain capillary permeability surface area product (PS) and CBF is higher than that for 99mTc-Cl(DMG)(3)2MP and 99mTc-ECD. PS varies as a linear function of CBF over the range of flows studied. At a CBF level that corresponds to normal regional CBF for human cortex, 0.5 ml/g/min, all the agents have a single-pass extraction of approximately 70% or greater. While all the agents detected changes in CBF in the normal to ischemic range, at higher flows 123I-IMP, 67Cu-PTSM and 99mTc-HMPAO showed substantially greater fidelity to true CBF than 99mTc-Cl(DMG)(3)2MP and 99mTc-ECD.

Direct analysis of whole blood by internal surface reversed-phase chromatography: an examination of the binding and metabolism of technetium dioxime complexes

We have developed a method using internal surface reversed-phase (ISRP) packing for rapid on-line separation of small hydrophobic compounds from cellular whole blood components. This is achieved by the use of 75-microns ISRP chromatographic material packed into a small high-performance liquid chromatographic (HPLC) column, in conjunction with column switching. We have applied this analytical method to study the in vitro metabolism of 99mTc-BATO (boronic acid adducts of technetium dioxime) cerebral and myocardial perfusion tracers in whole blood. The results from the ISRP procedure were compared with a conventional centrifugation method of analysis. This novel HPLC methods provides a rapid, convenient and reliable method for the analysis of radioactive and non-radioactive lipophilic components in whole blood.