Synthesis of tailored ligands for radiopharmaceutical applications

Abstract

An important aspect of clinical imaging techniques involves the use of complexed gamma or positron-emitting radionuclides, e.g. (^99m)Tc(γ, t1/2 = 6.02 h, 141 keV) for Single Photon Emission Tomography (SPET), (^62)Cu (β(^+),t1/2 = 9.74 min, 1.315 MeV) for Positron Emission Tomography (PET). With such radiopharmaceuticals in mind, two new classes of acyclic tetradentate ligands have been synthesised, featuring an N(_2)S(_2) donor system. This array of donor atoms is particularly attractive for its 'soft’ metal preference, favouring the complexation of radionuclides such as (^64)Cu and (^99m)Tc in diagnostic imaging of disease states, and (^186/188)Re (β(^-), t1/2 = 90h) in targeted radiotherapy. In both ligand systems the sulfur donor is adjacent to pentavalent phosphorus. Substitution at phosphorus allows easy modification of aryl or alkyl groups to vary the lipophilicity of the complex without affecting the binding properties. The first class of new ligands presented incorporate thiophosphinic acid groups which are more acidic than their corresponding phosphinates and carboxylates, increasing the stability of the complexes in acidic media. The synthetic routes to series of such ligands have been investigated and optimised. Initial attempts to access the aza- thiophosphinates via treatment of the sulfur-containing P(III) species with the diamine and paraformaldehyde failed to show any significant reaction. The best method of P=S bond formation was found to be via sulfur transfer to the corresponding P(V) phosphinate. The other class of ligands synthesised incorporate the dialkylthiophosphoryl group in conjunction with hydrazide or aminopyridyl moieties. The solid state structures of the ligands were determined by X-ray crystallography. The solution complexation behaviour of the ligands with a variety of metals, particularly copper and rhenium was studied. The thiophosphinic acid ligands form 1:1 complexes with copper(II) under dilute conditions and oligomeric complexes at high solution concentrations. The complexes formed with copper(II) and oxorhenium(V) appear to be charge neutral. Several of the dialkylthiophosphoryl ligands formed copper(I) complexes by spontaneous reduction of a copper(II) source, and redox behaviour was also shown towards oxorhenium(V). Some of the ligands radiolabel efficiently with technetium-99m and copper-64 and their preliminary evaluation shows promise for further development.