Preparation and in vivo evaluation of linkers for 211At labeling of humanized anti-Tac.

The syntheses, radiolabeling, antibody conjugation, and in vivo evaluation of new linkers for 211At labeling of humanized anti-Tac (Hu-anti-Tac), an antibody to the alpha-chain of the IL-2 receptor (IL-2Ralpha) shown to be a useful target for radioimmunotherapy are described. Synthesis of the organometallic linker precursors is accomplished by reaction of the corresponding bromo- or iodoaryl esters with bis(tributyltin) in the presence of a palladium catalyst. Subsequent conversion to the corresponding N-succinimidyl ester and labeling with 211At of two new linkers, N-succinimidyl 4-[211At]astato-3-methylbenzoate and N-succinimidyl N-(4-[211At]astatophenethyl)succinamate (SAPS), together with the previously reported N-succinimidyl 4-[211At]astatobenzoate and N-succinimidyl 3-[211At]astato-4-methylbenzoate, are each conjugated to Hu-anti-Tac. The plasma survival times of these conjugates are compared to those of directly iodinated (125I) Hu-anti-Tac. The N-succinimidyl N-(4-[211At]astatophenethyl)succinamate compound (SAPS) emerged from this assay as the most viable candidate for 211At-labeling of Hu-anti-Tac. SAPS, along with the directly analogous radio-iodinated reagent, N-succinimidyl N-(4-[125I]astatophenethyl)succinamate (SIPS), are evaluated in a biodistribution study along with directly iodinated (125I) Hu-anti-Tac. Blood clearance and biological accretion results indicate that SAPS is a viable candidate for further evaluation for radioimmunotherapy of cancer.

Preparation of 211At-labeled humanized anti-Tac using 211At produced in disposable internal and external bismuth targets.

These studies describe the production and purification of 211At as well as the procedure for labeling humanized anti-Tac, the antibody to the alpha-chain of the IL-2 receptor (IL-2R alpha), which has been shown to be a useful target for immunotherapy. The optimized protocol combines the advantages of the two-stage dry distillation procedure with the astatination of trialkylstannyl substances as labeling compounds for proteins. The 211At was produced by bombarding either an external or a recently developed disposable internal bismuth target with alpha-particles from a Cyclotron Corporation CS-30 cyclotron. The 211At was found to contain less than 0.01% 210At. The production rate for the external target was 0.15 mCi +/- 0.056 microA(-1) h(-1) (n = 9) (5.55 MBq mcroA[-1] h[-1]). The production rate for the internal target was 0.44 +/- 0.14 mCi microA(-1) h(-1) (n = 16) (16.28 MBq mcroA[-1] h[-1]).