Preparation and in vivo evaluation of novel linkers for 211At labeling of proteins.

The syntheses, radiolabeling, antibody conjugation and in vivo evaluation of new linkers for (211)At labeling of monoclonal antibodies are described. Syntheses of the N-succinimidyl esters and labeling with (211)At to form succinimidyl 4-methoxymethyl-3-[(211)At]astatobenzoate (9) and succinimidyl 4-methylthiomethyl-3-[(211)At]astatobenzoate (11) from the corresponding bromo-aryl esters is reported. Previously reported succinimidyl N-{4-[(211)At]astatophenethyl}succinamate (SAPS) is employed as a standard of in vivo stability. Each agent is conjugated with Herceptin in parallel with their respective (125)I analogue, succinimidyl 4-methoxymethyl-3-[(125)I]iodobenzoate (10), succinimidyl 4-methylthiomethyl-3-[(125)I]iodobenzoate (12) and succinimidyl N-{4-[(125)I]iodophenethyl}succinamate (SIPS), respectively, for comparative assessment in LS-174T xenograft-bearing mice. With 9 and 11, inclusion of an electron pair donor in the ortho position does not appear to provide in vivo stability comparable to SAPS. Variables in radiolabeling chemistry of these three agents with (211)At are notable. Sequential elimination of acetic acid and oxidizing agent, N-chlorosuccinimide (NCS), from the (211)At radiolabeling protocol for forming SAPS improves yield, product purity and consistency. NCS appears to be critical for the radiolabeling of 6 with (211)At. Formation of 11, however, is found to require the absence of NCS. Elimination of acetic acid is found to have no effect on radiolabeling efficiency or yield for either of these reactions.