Erbium-169 labeled hydroxyapatite particulates for use in radiation synovectomy of digital joints. A preliminary investigation

Erbium-169 [T1/2 = 9.4 d, Eβ(max) = 342 keV (45%) and 351 keV (55%), Eγ = 110.5 KeV (0.0014%)] is the most preferred radionuclide for radiation synovectomy (RSV) of digital joints. Radiolabeled particulates of appropriate size, wherein the radionuclide is irreversibly attached to the preformed particles, offer distinct advantages as the radiopharmaceuticals for use in RSV over other radiotherapeutic agents in terms of minimal leakage of radioactivity from the joint cavity. Hyroxyapatite (HA) particles, regarded as one of the most suitable carrier moiety for radiosynoviortheses applications, have been envisaged for labeling with 169Er with an aim to developing an agent for RSV of digital joints. Erbium-169 was produced with a specific activity of 415 ± 23 MBq/mg and radionuclidic purity of 99.22 ± 0.17% (both at the end of bombardment) by thermal neutron irradiation of enriched (98.2% in 168Er) erbium target at a flux of ∝ 8 x 1013 n/cm2s for 21 d. HA particles (1-10 μm size), synthesized in-house, were labeled with 169Er in high yield and radiochemical purity (> 99%) using 5 mg of HA particulates at pH 7.5. The radiolabeled particulates showed excellent in vitro stability in normal saline and human serum. Biodistribution and imaging studies (using 171Er-HA) carried out in normal Wistar rats following intra-articular injection of 169Er-HA particulates in the knee joint showed near-complete retention of injected radioactivity within the synovial cavity with almost no activity leaching out to the other organs/tissue. (orig.)