Recent Advances in Technetium Chemistry: Bridging Inorganic Chemistry and Nuclear Medicine

Kinetic and high-pressure mechanistic investigation of the aqua substitution in the trans-aquaoxotetracyano complexes of re(v) and tc(v): some implications for nuclear medicine

A kinetic study of the aqua substitution in the [TcO(OH(2))(CN)(4)](-) complex by different thiourea ligands (TU = thiourea, NMTU = N-methyl thiourea, NNDMTU = N, N'-dimethylthiourea) yielded second-order formation rate constants (25 degrees C) as follows [NNDMTU, NMTU, TU, respectively]: k(f) = 11.5 +/- 0.1, 11.38 +/- 0.04, and 7.4 +/- 0.1 M(-1)s(-1), with activation parameters: Delta H(#) (k(f) ) : 55 +/- 2, 42 +/- 3, 35 +/- 5 kJ mol(-1); DeltaS(#) (k(f) ) : - 40 +/- 8, - 84 +/- 11, - 110 +/- 17 J K(-1)mol(-1). A subsequent high-pressure investigation of the aqua substitution in the [ReO(OH(2))(CN)(4)](-) and [TcO(OH(2))(CN)(4)](-) complexes by selected entering ligands yielded DeltaV(#) (k(f) ) values as follows: Re(V): -1.7 +/- 0.3(NCS(-)), -22.1 +/- 0.9 (TU) and for Tc(V): -3.5 +/- 0.3(NCS(-)), -14 +/- 1 (NNDMTU), and -6.0 +/- 0.5 (TU) cm(3)mol (-1), respectively. These results point to an interchange associative mechanism for the negative NCS(-) as entering group but even a pure associative mechanism for the neutral thiourea ligands.

New directions in the coordination chemistry of 99mTc: a reflection on technetium core structures and a strategy for new chelate design

Bifunctional chelates offer a general approach for the linking of radioactive metal cations to macromolecules. In the specific case of 99mTc, a variety of technologies have been developed for assembling a metal-chelate-biomolecule complex. An evaluation of these methodologies requires an appreciation of the coordination characteristics and preferences of the technetium core structures and oxidation states, which serve as platforms for the development of the imaging agent. Three technologies, namely, the MAG3-based bifunctional chelates, the N-oxysuccinimidylhydrazino-nicotinamide system and the recently described single amino acid chelates for the {Tc(CO)3}1+ core, are discussed in terms of the fundamental coordination chemistry of the technetium core structures. In assessing the advantages and disadvantages of these technologies, we conclude that the single amino acid analogue chelates (SAAC), which are readily conjugated to small peptides by solid-phase synthesis methods and which form robust complexes with the {Tc(CO)3}1+ core, offer an effective alternative to the previously described methods.

Calix[4]arene rhenium(V) complexes as potential radiopharmaceuticals

The calix[4]arene platform was used for the syntheses of novel rhenium(V) complexes, that may have potential applications as radiopharmaceuticals. The reaction of ReO(PPh3)2Cl3 with tetradentate N2O2-calix[4]arene ligand 8 in ethanol gave the novel mixed-ligand rhenium complex 9 with the structure ReO(N2O2-calix)OEt. The configuration was elucidated by using a number of 1H NMR techniques. In 9, the ethoxy ligand could be easily and quantitatively exchanged for another monodentate ligand to give complex 12. Tetradentate N2S2-calix[4]arene ligand 15 formed the rhenium complex 16 either via reaction with ReO(PPh3)2Cl3 in an organic solvent or by reaction with rhenium gluconate in an aqueous solution. Complex 16 showed good stability in phosphate-buffered saline solution (37 degrees C, 5 d). The crystal structures of a mono- and a bimetallic complex were determined. The bimetallic N2O2-calixarene complex dimer 11 crystallized in the monoclinic space group C2/c, with a = 38.963(5) A, b = 23.140(6) A, c = 27.382(6) A, beta = 128.456(10) degrees, V = 19,333(7) A3, Z = 8, and final R = 0.0519. The monometallic N2S2 model complex 17 crystallized in the monoclinic space group Cc, with a = 15.715(2) A, b = 12.045(2) A, c = 20.022(3) A, beta = 94.863(12) degrees, V = 3776.3(10) A3, Z = 4, and final R = 0.0342.

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.

Quantitative study of the structure-stability relationship of Tc complexes

By studying structural parameters (van der Waals radius and bond length from x-ray crystallography) of more than 100 known Tc compounds and using the cone packing model, a stability indicator of the Tc compounds was derived. The indicator is based on the solid angle factor sum (SAS), which is calculated from van der Waals radii and bond lengths between all coordinating atoms and Tc. The average SAS value is 0.97 +/- 0.13. The SAS value reflects the limits of the packing of ligand around Tc. The quantitative calculation of the SAS is useful for predicting stability and designing new Tc radiopharmaceuticals.

Technetium-99m complexes of dimethylaminomethylene diphosphonate (DMAD)--I. Anion exchange HPLC characterization of 99mTc(NaBH4)-DMAD mixtures

Radiopharmaceutical analogs prepared by sodium borohydride reduction of 99mTcO4- in the presence of DMAD have been characterized by anion exchange HPLC (high performance liquid chromatography) separation of the resulting mixture into component 99mTc-DMAD complexes. The distribution of complexes within a radiopharmaceutical formulation can be manipulated by controlling technetium concentration, pH, presence or absence of air, and time post reaction.

A formate based precursor for the preparation of technetium complexes

Tc-formate complexes have been prepared by electrolytic, stannous, and borohydride reduction of pertechnetate in formate buffer. Individual Tc-formate complexes were isolated from reaction mixtures with silica bonded phase anion-exchange HPLC. The predominate air stable red Tc-formate complex has a negative charge, an absorption maximum at 536 nm, and a molecular weight between 100 and 300 dalton. The isolated major Tc-formate complex was found to react with isocyanide, diphosphonate, and various nitrogenous ligands. The relevant use of Tc-formate complexes as synthetic precursors to other technetium complexes is seen as being beneficial to the understanding of technetium ligand substitution.

Quantitative HPLC determination of [99mTc]-pertechnetate in radiopharmaceuticals and biological samples--I. Technique development

Techniques have been developed which allow HPLC (high performance liquid chromatography) to be used for the quantitative determination of [99mTc]pertechnetate in radiopharmaceuticals and biological samples. An instrumental technique accounts for 99mTc species which do not elute from the HPLC column, while a chemical technique obviates interferences caused by Sn(II). These two techniques are incorporated into an anion exchange HPLC procedure which is applied to the determination of [99mTc]pertechnetate in 99mTc-diphosphonate radiopharmaceuticals and biological samples.

153Sm-EDTMP and 186Re-HEDP as bone therapeutic radiopharmaceuticals

Two new radiopharmaceuticals, 186Re-1-hydroxyethylidenediphosphonate (186Re-HEDP) and 153Sm-ethylenediaminetetramethylenephosphonate (153Sm-EDTMP), have been proposed as palliative treatments for metastatic bone cancer. Biolocalization properties of these chelates in animals as well as the physical decay and production properties of the respective radionuclides are consistent with those of a therapeutic agent. Subtherapeutic doses of both agents have been administered to human cancer patients to determine their biokinetics and skeletal localization. The 186Re-HEDP studies were conducted at the University of Cincinnati while the 153Sm-EDTMP studies were conducted at the University of Missouri-Columbia. The pharmacokinetics of these agents in humans were consistent with those found in animals. Imaging studies show that the retained activity localizes primarily in the skeleton with high selective uptake in skeletal lesions; there is no visualization of other organs or soft tissue. This paper will review the development and preparation procedures for these radiopharmaceuticals and briefly summarize the animal and patient data.

The chemistry of rhenium and technetium as related to the use of isotopes of these elements in therapeutic and diagnostic nuclear medicine

The beta emitting isotopes 186Re and 188Re are logical choices on which to base therapeutic radiopharmaceuticals that might be expected to be analogous to diagnostic radiopharmaceuticals based on 99mTc. However, the chemistry of rhenium is sufficiently different from that of technetium so that the development of Re radiopharmaceuticals often cannot be predicated on the known chemistry and biological behavior of 99mTc radiopharmaceuticals. The relevant chemical differences involve the greater stability of the higher oxidation states of Re (and thus the greater tendency of reduced Re radiopharmaceuticals to undergo re-oxidation to perrhenate), and the greater substitution inertness of reduced Re complexes. These differences are illustrated in the preparation and use of 186Re (Sn)-HEDP and 99mTc(Sn)-HEDP diphosphonate radiopharmaceuticals designed, respectively, for palliative therapy and diagnosis of metastatic cancer to bone, and in the preparation and biodistribution of tr[186Re(DMPE)2Cl2]+ and [186Re(DMPE)3]+, analogs to the potential myocardial perfusion imaging agents tr-[99mTc(DMPE)2Cl2]+ and [99mTc(DMPE)3]+. [HEDP = (1-hydroxyethylidene)diphosphonate; DMPE = 1,2-bis(dimethylphosphino)ethane].