OPTIMIZATIONS OF RADIOLABELING OF IMMUNOPROTEINS WITH BI-213

Carrier-free Bi-213 ((t(1/2) = 45.6 m, an alpha-emitter) is available from the Ra-225/Ac-225/Bi-213 generator system with Ra-225 (t(1/2) = 1 4 d) and Ac-225 (t(1/2) = 10 d) being the daughters of 7340-y Th-229 w hich, in turn, is the decay daughter of long-lived U-233. In the curre nt generator system, Ra-225/Ac-225 is adsorbed on an organic cation ex change resin (AG 50X4, X8 or highly cross-linked AG MP-50) and Bi-213 is eluted with 0.1 M HI. We have found the lower cross-linked resin to be more suitable for rapid elution of Bi-213 from the generator with Ac-225 breakthrough of <1%. Generators were typically made from Ac sep arated from Ra as the breakthrough of Ra is significantly higher than that of Ac. Generator elution of Bi-213, labeling of MAb and HPLC puri fication were accomplished within 35-40 min. Radiolabeling yields of C HX-b-DTPA-conjugated MAbs (34A and 14) were >50% with specific activit y of similar to 200 mu Ci of Bi-213 per 150 mu g of conjugated protein .