Jamal Zweit

Copper radionuclides and radiopharmaceuticals in nuclear medicine

The chemistry, radiochemistry, radiobiology, and radiopharmacology of radiopharmaceuticals containing copper radionuclides are reviewed. Copper radionuclides offer application in positron emission tomography, targeted radiotherapy, and single photon imaging. The chemistry of copper is relatively simple and well-suited to radiopharmaceutical application. Current radiopharmaceuticals include biomolecules labelled via bifunctional chelators primarily based on cyclic polyaminocarboxylates and polyamines, and pyruvaldehyde-bis(N4-methylthiosemicarbazone) (PTSM) and its analogues. The chemistry of copper, of which only a fraction has yet been exploited, is likely to be applied more fully in the future.

Evaluation of copper(II)-pyruvaldehyde bis (N-4-methylthiosemicarbazone) for tissue blood flow measurement usina a trapped tracer model

Copper(II)-pyruvaldehyde bis (N-4-methylthiosemicarbazone) (Cu-PTSM) labelled with 62,64Cu is a promising radiotracer for the study of blood flow using positron emission tomography (PET). We have investigated the application of a simple trapped tracer model to measurements of tissue 64Cu-PTSM uptake combined with continuous arterial sampling. A dual-tracer method was used to compare blood flow estimated by 64Cu-PTSM with values derived from measurements using cobalt-57 microspheres in the rat. Prolonged retention of 64Cu-PTSM following intravenous administration was initially confirmed in both normal tissues and tumours. After intraventricular 64Cu-PTSM infusion, cumulative arterial 64Cu activity increased progressively, and after extraction in n-octanol was found to plateau to levels corresponding with those reached following administration of 57CO microspheres. Rapid and species-dependent rates of 64Cu-PTSM decomposition to non-extractable 64Cu complexes were found in rat and human blood in vitro (70%±6% and 43%±5% respectively at 16 min), demonstrating the need for immediate processing of arterial samples. Close agreement was found between blood flow estimated by 64Cu-PTSM and 57CO microsphere methods in tissues of low to moderate flow: muscle (0.01, 0.08, 0.07 ml/min per gram; mean difference, mean 64Cu, mean 57Co), brain (0.09, 0.52, 0.43 ml/min per gram) and kidney (−0.16, 2.29, 2.45 ml/min per gram). Estimates of cardiac output also compared favourably between the two methods (5.7, 59.8, 54.1 ml/min). We conclude that a simple tissue trapping model may be suitable for the derivation of blood flow estimates using 62,64Cu-PTSM, PET imaging and continuous arterial blood sampling.

Development of a high performance zinc-62/copper-62 radionuclide generator for positron emission tomography

Clinical utilisation of positron emission tomography could be enhanced by the availability of shortlived radionuclides derived from generator systems. The zinc-62/copper-62 combination is one such system which could be used as a source for a number of copper-62 radiopharmaceuticals. We have developed and optimised a high activity (5.6 GBq, 150 mCi) zinc-62/copper-62 generator to provide 62CU in a form that is suitable for direct labelling of pyruvaldehyde-bis-(N4-methylthiosemicarbazone)-copper(II), Cu(PTSM). The distribution coefficients of Zn(II) and Cu(II) between anion-exchange resin and various hydrochloric acid/organic solvent mixtures were measured. Based on these measurements a generator eluent of 0.3 M HCl/40% ethanol provided 62CU in >90% yield in a 3-ml volume. A very low 62Zn breakthrough of less than 3×10−7% was achieved. Copper-PTSM was successfully labelled with the no-carrier-added 62Cu eluent directly from the generator with 94% radiochemical yield.

Copper bis(diphosphine) complexes: radiopharmaceuticals for the detection of multi-drug resistance in tumours by PET.

Abstract.Experience with imaging of the multi-drug resistance (MDR) phenotype in tumours using technetium-99m sestamibi, a substrate of the P-glycoprotein (Pgp) transporter, suggests that better quantification of images and separation of MDR from other variables affecting tracer uptake in tumours are required. One approach to these problems is the development of short half-life positron-emitting tracers which are substrates of Pgp. Several lipophilic cationic copper(I) bis(diphosphine) complexes labelled with copper-64 have been synthesised and evaluated in vitro as substrates for Pgp. The synthesis is rapid and efficient with no need for purification steps. The chemistry is suitable for use with very short half-life radionuclides such as copper-62 (9.7 min) and copper-60 (23.7 min). Incubation of the complexes with human serum in vitro showed that they are sufficiently stable in serum to support clinical imaging, and the more lipophilic members of the series are taken up rapidly by cells (Chinese hamster ovary and human ovarian carcinoma) in vitro with great avidity. Uptake in human ovarian carcinoma cells is significantly reduced after several months of conditioning in the presence of doxorubicin, which induces increased Pgp expression. Uptake in hooded rat sarcoma (HSN) cells, which express Pgp, is significantly increased in the presence of the MDR modulator cyclosporin A. Biodistribution studies in hooded rats show rapid blood clearance, excretion through both kidneys and liver, and low uptake in other tissues. The one complex investigated in HSN tumour-bearing rats showed uptake in tumour increasing up to 30 min p.i. while it was decreasing in other tissues. We conclude that diphosphine ligands offer a good basis for development of radiopharmaceuticals containing copper radionuclides, and that this series of complexes should undergo further evaluation in vivo as positron emission tomography imaging agents for MDR.

Metal complexes of bleomycin: evaluation of [Rh-105]-bleomycin for use in targeted radiotherapy.

Bleomycin has been used as a carrier for several radioisotopes; however, its potential for clinical use has been limited either by the in vivo stability of the complexes or the half-life of the isotope used. The chemical, biological, and radiological properties of 105Rhodium appear to make it an ideal choice for targeted radiotherapy. The synthesis and purification of a hereto unreported 105Rhodium-bleomycin (105Rh-BLM) complex is described. The stability of this complex in plasma is sufficient to allow targeted delivery of the radioisotope. 57Cobalt-bleomycin was studied under identical conditions for comparative purposes. The suitability of 105Rh-BLM for targeted therapy, which appears to be limited by the renal clearance of this agent, is discussed.

Effect of ligand and solvent on chloride ion coordination in anti-tumour copper(I) diphosphine complexes: Synthesis of [Cu(dppe)2]Cl and analogous complexes (dppe = 1,2-bis(diphenylphosphino)ethane)

Abstract Complexes formed between copper(I) and 1,2-bis(diphenylphosphino)ethane (L1) have previously been isolated as salts of the [CuL21]+ cation if only non-coordinating anions are present, or as [Cu2Cl2L31] if chloride is present. We describe the synthesis of [CuL21]Cl, the stoichiometry of which is confirmed by elemental analysis, FAB mass spectroscopy and conductivity. Polar solvents (water-ethanol mixtures) lead to isolation of the latter, whereas solvents of lower polarity (CHCl3, CH2Cl2) lead to isolation of the complexes containing coordinated chloride. Related ligands such as cis-1,2-bis(diphenylphosphino)ethene, 1,2-bis(diethylphosphino) ethane, 1,2-bis(dimethylphosphino)ethane and 1,2-bis(hydroxymethylphosphino)ethane, give salts [CuL2]Cl. This behaviour, and that of other 1,2-bisphosphine ligands, is rationalised in terms of competition between chloride and phosphine ligands for binding sites on the metal, with the equilibrium position determined by solvent polarity, ligand structure and rigidity, and steric and electronic properties of the ligands.

Diphosphine bifunctional chelators for low-valent metal ions. Crystalstructures of the copper(I) complexes [CuClL12]and [CuL12][PF6][L1 = 2,3-bis(diphenylphosphino)maleicanhydride]

The chelating diphosphine 2,3-bis(diphenylphosphino)maleic anhydride (L 1 ) reacted with CuCl to give [CuClL 1 2 ] 1a in which the copper is bound to three phosphorus atoms and one chlorine in a pseudo-tetrahedral structure, as demonstrated by X-ray crystallography. Compound L 1 reacted with [Cu(MeCN) 4 ][PF 6 ] to give the red salt [CuL 1 2 ][PF 6 ] 1b in which the copper is bound by four phosphorus atoms of two chelating bidentate ligands, as demonstrated by X-ray crystallography. Addition of chloride ions to 1b quantitatively affords 1a. Complex 1a can be converted quantitatively into [CuL 1 2 ][NO 3 ] 1c by treatment with AgNO 3 . These complexes reacted quantitatively with water, benzylamine and methanol to give the corresponding [CuL 2 2 ] + 2, [CuL 3 2 ] + 3 and [CuL 4 2 ] + 4 where L 2 = 2,3-bis(diphenylphosphino)maleic acid and L 3 and L 4 are the monobenzylamide and monomethyl ester derivatives, respectively, of L 2 . Uncomplexed L 1 can also be hydrolysed to L 2 or derivatised with benzylamine or methanol to give L 3 and L 4 respectively. Compounds L 2 , L 3 and L 4 reacted with copper-(I) or -(II) salts to yield salts of 2, 3 and 4. This chemistry provides versatile routes to the synthesis of stable bioconjugates containing copper or radiocopper and of derivatised solids containing pendant diphosphine ligands.