“2+1” Dithiocarbamate–isocyanide chelating systems for linking (M=99mTc, Re) fragment to biomolecules

2 + 1 Tricarbonyl Complexes of Technetium(I) with a Combination of N,N-Bidentate Ligands and Ethyl Isocyanoacetate: How Strong Is the Interfering Effect of Chloride Ions on Their Formation?

Technetium(I) 2 + 1 tricarbonyl complexes with a combination of N,N-bidentate ligands (2,2'-bipyridine, bipy; 1,10-phenanthroline, phen) and ethyl isocyanoacetate were prepared and characterized by NMR, IR, UV/visible, and luminescence spectroscopies and by high-performance liquid chromatography (HPLC). The crystal structures of [99Tc(CO)3(bipy)(CNCH2COOEt)](ClO4) (in the form of a solvate with 0.5CH2Cl2) and [99Tc(CO)3(phen)(CNCH2COOEt)](ClO4) (in the form of an adduct with an outer-sphere phen molecule) were determined by single-crystal X-ray diffraction. To evaluate the interfering effect of chloride ions on the formation of the 2 + 1 complexes, the kinetics of the replacement of labile monodentate ligand X in the complexes [MX(CO)3(N∧N)] (M = Re, 99Tc; N∧N = bipy, phen; X = Cl-, ClO4-) by CNCH2COOEt in ethanol were compared. The 99Tc bipy complexes with X = ClO4- (according to the IR data, perchlorate anion in ethanol is displaced from the coordination sphere by the solvent molecule) and X = Cl- are characterized by close ligand replacement rates. In the case of the 99Tc complexes with phen and Re complexes with both phen and bipy, the chloride complexes are appreciably less reactive than the chloride-free complexes. The technetium complexes are considerably more reactive in ligand replacement than their rhenium analogues. In the chloride-containing medium (saline), the complex [99mTc(CO)3(bipy)(CNCH2COOEt)]+ can be prepared under the conditions acceptable for nuclear medical applications, although higher isonitrile concentrations are required as compared to the chloride-free system.

Synthesis and evaluation of new mixed "2 + 1" Re, 99mTc and 186Re tricarbonyl dithiocarbamate complexes with different monodentate ligands

A series of "2 + 1" mixed ligand tricarbonyl complexes of the general formula fac-[Re/^99mTc/¹⁸⁶Re(CO)₃(DDTC)(L)] containing diethyldithiocarbamate (DDTC) as a monoanionic bidentate ligand and a series of monodentate ligands L was synthesized, characterized and evaluated. The impact of ligand L on the radiochemical yield (RCY) and biodistribution of the final compounds was also investigated. DDTC and the appropriate L ligand [cyclohexyl isocyanide (cisc), tert-butyl isocyanide (tbi), triphenylphosphine (PPh₃), methyldiphenylphosphine (PPh₂Me), triphenylarsine (AsPh₃), imidazole (im), and 4-aminopyridine (4AP)] readily reacted in equimolar amounts with the [Et₄N]₂[Re(CO)₃Br₃] precursor to afford fac-[Re(CO)₃(DDTC)(cisc)], Re1, fac-[Re(CO)₃(DDTC)(tbi)], Re2, fac-[Re(CO)₃(DDTC)(PPh₃)], Re3, fac-[Re(CO)₃(DDTC)(PPh₂Me)], Re4, fac-[Re(CO)₃(DDTC)(AsPh₃)], Re5, fac-[Re(CO)₃(DDTC)(im)], Re6 and fac-[Re(CO)₃(DDTC)(4AP)], Re7, complexes in high yields (>80%). All Re complexes were fully characterized by IR, NMR, and in addition Re4, Re5, and Re7 with X-ray crystallography. Analogous reactions as performed with Re were subsequently explored on the ^99mTc and ¹⁸⁶Re-tracer levels using the corresponding fac-[^99mTc/¹⁸⁶Re(CO)₃(H₂O)₃]⁺ precursor. Complexes ^99mTc1 - ^99mTc5, ¹⁸⁶Re1 and ¹⁸⁶Re3 were obtained in high radiochemical yield (>91%), while the complexes ^99mTc6, ^99mTc7 and ¹⁸⁶Re7 formed with radiochemical yields of 55%, 28%, and 75%, respectively. The ^99mTc and ¹⁸⁶Re-complexes were characterized by comparative HPLC analysis using the analogous Re complexes. During histidine and cysteine challenge experiments at 37 °C through 6 h, complexes ^99mTc1 - ^99mTc5 remained > 92% stable, while complexes ^99mTc6 and ^99mTc7 remained only 8% stable through 3 h. Similar studies for ¹⁸⁶Re-complexes showed that ¹⁸⁶Re1 and ¹⁸⁶Re3 remained > 95% stable for up to 48 h, while ¹⁸⁶Re7 had decreased to 7% after 3 h. LogD_7.4 data of ^99mTc1 - ^99mTc5, ¹⁸⁶Re1, and ¹⁸⁶Re3 complexes, which ranged from 2.59 to 3.39, suggested high lipophilicity. Biodistribution studies in healthy Swiss albino mice showed hepatobiliary excretion for ^99mTc1, ^99mTc2, and ^99mTc4, fast blood clearance for ^99mTc4, while high liver uptake and retention for ^99mTc3 and ^99mTc5 were measured. Moreover, ^99mTc2 showed high accumulation in the lungs with sustained retention (52.80% ID/g at 4 h p.i.) and significant brain uptake at 2 min p.i. (1.89% ID/g). The study showed the great influence of monodentate ligand in the synthesis and biodistribution of the mixed ligand complexes.

Introduction of Re(CO)3+/99mTc(CO)3+ Organometallic Species into Vinylpyrrolidone-Allyliminodiacetate Copolymers

N-vinylpyrrolidone-co-allylamine copolymers (VP-co-AA) containing iminodiacetic (IDA) chelation units were prepared in the range of molecular masses of the copolymers from 9000 to 30,000 Da depending on polymerization conditions. Non-radioactive organometallic species Re(CO)₃⁺ were introduced into polymeric carriers under mild conditions; the prepared metal–polymeric complexes were characterized by IR, NMR, ESI-MS and HPLC. IR spectra data confirmed the coordination of M(CO)₃⁺ moiety to the polymeric backbone via IDA chelation unit (appearance of characteristic fac-M(CO)₃⁺ vibrations (2005, 1890 cm⁻¹), as well as the appearance of group of signals in ¹H NMR spectra, corresponding to those inequivalent to methylene protons CH₂COO (dd, 4.2 ppm), coordinated to metal ions. The optimal conditions for labeling the PVP-co-AA-IDA copolymers with radioactive ^99mTc(CO)₃⁺ species were determined. The radiochemical yields reached 97%. The obtained radiolabeled polymers were stable in blood serum for 3 h. In vivo distribution experiments in intact animals showed the high primary accumulation of technetium-99m MPC (MM = 15,000 Da) in blood with subsequent excretion via the urinary tract.

Aquation and Anation Kinetics of Rhenium(I) Dicarbonyl Complexes: Relation to Cell Toxicity and Bioavailability

The aquation reactions of four rhenium(I) dicarbonyl complexes, [Re(CO)₂(NN)(PR₃)(Cl)], where NN = 1,10-phenanthroline (Phen) and 2,9-dimethyl-1,10-phenanthroline (DMPhen) and PR₃ = 1,3,5-triaza-7-phosphaadamantane (PTA) and 1,4-diacetyl-1,3,7-triaza-5-phosphabicylco[3.3.1]nonane (DAPTA). Additionally, the anation reactions of the corresponding aqua complexes with Cl^- were investigated. Single crystals of [Re(CO)₂(DMPhen)(PTA)(Cl)]·DMF and [Re(CO)₂(DMPhen)(DAPTA)(Cl)] were obtained, and their structures were determined using X-ray diffraction. The Re-Cl interatomic distances are 2.4991(13) and 2.4922(6) Å, respectively, indicating a mild trans influence effect of the phosphine ligands. The rate constants, k_aq, for the aquation reactions of these complexes spanned a range of (3.7 ± 0.3) × 10^-4 to (15.7 ± 0.3) × 10^-4 s^-1 with the two Phen complexes having rate constants that are 2.5 times greater than those of the DMPhen complexes at 298 K. Similarly, the second-order anation rate constants (k_Cl) of the resulting aqua complexes, [Re(CO)₂(NN)(PR₃)(H₂O)]⁺, with Cl^- ions at 298 K varied between (2.99 ± 0.05) × 10^-3 and (6.79 ± 0.09) × 10^-3 M^-1 s^-1. Likewise, these rate constants for the Phen complexes were almost 2 times faster than those of the DMPhen complexes. The pK_a values of the four aqua complexes were determined to be greater than 9.0 for all of the complexes with [Re(CO)₂(Phen)(PTA)(H₂O)]⁺ having the highest pK_a value of 9.28 ± 0.03. From the pK_a values and the ratios of the aquation and anation rate contants, which give thermodynamic Cl^- binding constants, the speciation of the rhenium(I) complexes in blood plasma, the cytoplasm, and the cell nucleus were estimated. The data suggest that the aqua complexes would be the dominant species in all three environments. This result may have important implications on the potential biological activity of these complexes.

Technetium-99m radiochemistry for pharmaceutical applications

Technetium-99m (^99m Tc) is a widely used radionuclide, and the development of ^99m Tc imaging agents continues to be in demand. This overview discusses basic principles of ^99m Tc radiopharmaceutical preparation and design and focuses on the ^99m Tc radiochemistry relevant to its pharmaceutical applications. The ^99m Tc complexes are described based on the most typical examples in each category, keeping up with the state-of-the-art in the field. In addition, the main current strategies to develop targeted ^99m Tc radiopharmaceuticals are summarized.

Activation of the manganese(i) tricarbonyl core by selective variation of bidentate ligands (L,L′-Bid = N,N′ and N,O donor atom sets) in fac-[Mn(CO)3(L,L′-Bid)(CH3OH)]n complexes

A range of fac-[Mn(CO)₃(L,L′-Bid)(H₂O)]ⁿ complexes has been synthesized and the methanol substitution has been investigated for the first time.

Synthesis, characterization and pre-clinical evaluation of (99m) Tc-tricarbonyl complexes as potential myocardial perfusion imaging agents

Myocardial perfusion imaging is an established Nuclear Medicine investigation. Current myocardial perfusion imaging agents sestamibi and tetrofosmin have number of drawbacks; low heart uptake coupled with uptake into the surrounding tissues leads to a poorer image quality. There is a need for continued research into designing and evaluating potentially superior myocardial imaging agents. Tri-carbonyl-technetium and rhenium complexes were prepared by combination with mono-dentate and bi-dentate ligands. Complexes were characterized by HPLC, MAS, nuclear magnetic resonance, infrared, single-crystal X-ray diffraction and partition coefficient determinations. (99m) Tc(CO)3 complexes were administered intravenously to Sprague Dawley rats, and tissue distribution studies were carried out at 15 min and 1 h p.i. Radiochemical purity was assessed as >90%. 1-10-phenanthroline, 2,2'-bipyridine and imidazole complexes gave the highest heart uptake. The percentage injected dose per gram (n = 3) at 1 h for 1-10-phenanthroline/imidazole was blood 0.21 ± 0.01, heart 1.12 ± 0.11, kidney 3.61 ± 1.13, liver 0.62 ± 0.06, lung 0.28 ± 0.12, spleen 0.24 ± 0.05, small intestine contents 1.87 ± 0.92; and for 2,2'-bipyridine /imidazole was blood 0.23 ± 0.02, heart 1.07 ± 0.18, kidney 3.31 ± 1.28, liver 0.56 ± 0.09, lung 0.14 ± 0.02, spleen 0.2 ± 0.1, small intestine content 1.05 ± 0.48. Further investigation to evaluate more complexes based on 1,10-phenanthroline, 2,2'-bipyridine and imidazole derivatives could potentially lead to agents with an increased heart uptake and faster clearance from the liver and gastrointestinal tract.