Microwave-Assisted Synthesis of Organometallic Complexes of 99mTc(CO)3 and Re(CO)3: Its Application to Radiopharmaceuticals

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Abstract

99mTc-tricarbonyl [99mTc(CO)3] complexes have been conventionally synthesized by heating [99mTc(CO)3(H2O)3]+ and a tridentate chelating ligand under atmospheric pressure; however, this method is poor in terms of chemical yield and reproducibility. Moreover, since the half-life of 99mTc is very short (6 h), the development of facile and rapid methods of synthesizing 99mTc-labeled compounds, which could be used as radioactive tracers for single photon emission computed tomography (SPECT), is required. Thus, we initiated a study on the application of a microwave reaction to the synthesis of 99mTc(CO)3–2-picolylamine monoacetic acid (PAMA) [99mTc(CO)3–PAMA] complexes on the basis of the fact that synthesis of metal complexes proceeds rapidly by microwave irradiation owing to an efficient exothermic phenomenon and heat conduction effect. Formation of by-products could be markedly suppressed by comparison with that in conventional methods. In the present study, rhenium (Re), an element belonging to the same group in the periodic table as technetium (Tc), and which also forms bipyramidal complexes, was first used to investigate the synthetic reaction because no stable isotopes exist for Tc. As a result, when water was used as the solvent under the irradiation of microwaves within 1 min, the Re(CO)3-PAMA complex could be directly synthesized from ethyl ester of PAMA (PAMAEE) and [Re(CO)3(H2O)3]Br in one step and with a high yield (94%). Finally, the 99mTc(CO)3–PAMA complex was successfully synthesized at a high radiochemical yield (>99%) within 1 min of reaction using 99mTc instead of Re under the same conditions.

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