Special Seminar: Alpha emitting radionuclides from natural thorium irradiated with protons
Speaker: Aleksandr Vasiliev, INR/Moscow State U
Alpha emitting radionuclides (Ac-225, Ra-223, Bi-213, Th-226) are of considerable interest for radioimmunotherapy applications. Due to short range in tissue (< 100 micron) and high linear energy transfer (about 100 keV/micron) alpha-emitters are especially suited for targeted radiotherapy of cancer. Ac-225 and Ra-223 may be used either directly or as isotope generators of the short-lived alpha-emitting isotopes: 225Ac/213Bi and 223Ra/211Pb. Among the existing methods irradiation of natural thorium with 200-60 MeV protons is one of the most prospective. It can provide curie amounts of Ac-225 and Ra-223 during a 10-day irradiation. The method for simultaneous recovery of 225Ac and 223Ra from an irradiated thorium target was proposed. According to the procedure of 225Ac recovery, irradiated thorium was extracted with di(2-ethylhexyl) phosphoric acid (HDEHP). Aqueous phase containing 225Ac as well as 223Ra was passed through the column with DGA resin (N,N,N',N' tetroctyldiglicolamide as extracting agent). Ac and lanthanides were concentrated on the column in 6M HNO3, whereas 1A and 2A group cations, ruthenium and antimony passed through. Combination of cation exchange chromatography with Dowex 50x8 and extraction chromatography with Sr-resin (4,4'(5')-di-t-butylcyclohexano 18-crown-6 (crown ether) in 1-octanol as extracting agent) was studied for isolation of pure Ra fraction containing 223Ra and some impurity of short-lived 224Ra. A column with Dowex 50x8 was used for separation of Ra-Ba fraction from other radionuclides. Elution with HNO3, H2SO4 and HClO4 solutions of different concentration was tested. Gradient elution with HClO4 provided the purest Ra-Ba solution. After Dowex 50x8, the Ra-Ba fraction was loaded onto a column with Sr Resin. Ra was absorbed from 0.5-2 M perchloric acid solution and desorbed with 2-3 M nitric acid while Ba remained on the column. The total chemical yields of Ra and Ac over 80% were achieved. The same target may be used for production of other prospective radionuclides: Pa-230 (T1/2= 18.7 d) which decays to U-230 (T1/2 = 20.8 d), Ag-111 (beta emitter) and others. Using of HAP as carriers is regarded to be prospective for 223Ra alpha-therapeutic applications due to similarity of properties of radium and calcium, as well as bioavailability and biocompatibility of hydroxyapatite.