Radioisotopes of Rhenium

Two radioisotopes of rhenium, 186Re (Tj/2 = 3.72 d; Emax beta = 1.07 Mev) and 188Re (Ti/2 = 16.98 h; Emax beta = 2.12 Mev) have found use in nuclear medicine therapy. On the other hand, 186Re is short-lived and has rapid uptake in target and clearance from normal organs. Convenient physical half-life (89 hours) with an Emax of 1.07 MeV beta emission and 137 keV (9%) photon, 186Re delivers more radiation than 188Re in terms of rad/mCi. When chelated to hydroxyethylidene diphosphonate (HEDP), etidronate is stable and shows uptake similar to Technetium [99m] medroxy diphosphonate (99mTcMDP). It has an uptake pattern in normal skeleton and metastatic foci. Like other bone seekers, it is rapidly excreted via urine and shows optimal retention in the skeleton at 24 hours. Methods are available for the production of high purity compounds (25). 186Re can be produced to very high specific activity by neutron irradiation of enriched 185Re. In practical terms, production of 188Re is usually ioo 188

achieved using a W/ Re generator system and is able to produce carrier-free 188Re (26). The main advantages of rhenium are its chemical similarities to technetium and the fact that proteins can be labeled by either direct methods (to the sulfhydryl group) (26) or indirect methods (by using bifunctional chelates to biological compounds) (27).

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