A polar substituent containing acylation agent for the radioiodination of internalizing monoclonal antibodies: N-succinimidyl 4-guanidinomethyl-3-[I-131]iodobenzbate ([I-131]SGMIB)

The objective of this study was to develop an acylation agent for the radio iodination of monoclonal antibodies that would maximize retention of the la bel in tumor cells following receptor- or antigen-mediated internalization. The strategy taken was to add a polar substituent to the labeled aromatic ring to impede transport of labeled catabolites across lysosomal and cell m embranes after antibody degradation. Preparation of unlabeled N-succinimidy l 4-guanidinomethyl-3-iodobenzoate (SGMIB) was achieved in six steps from 3 -iodo-4-methylbenzoic acid. Preparation of 4-guanidinomethyl-3-[I-131]-iodo benzoic acid from the silicon precursor, 4-(N-1,N-2-bis-tert-butyloxycarbon yl)guanidinomethyl-3-trimethylsilylbenzoic acid proceeded in less than 5% r adiochemical yield. A more successful approach was to prepare [I-131]SGMIB directly from the tin precursor, N-succinimidyl 4-(N-1,N-2-bis-tert-butylox ycarbonyl)guanidinomethyl-3-trimethylstannylbenzoate, which was achieved in 60-65% radiochemical yield. A rapidly internalizing anti-epidermal growth factor receptor variant III antibody L8A4 was labeled using [I-131]SGMIB in 65% conjugation efficiency and with preservation of immunoreactivity. Pair ed-label in vitro internalization assays demonstrated that the amount of ra dioactivity retained in cells after internalization for L8A4 labeled with [ I-131]SGMIB was 3-4-fold higher than that for L8A4 labeled with I-125 using either Iodogen or [I-125]SIPC. Catabolite assays documented that the incre ased retention of radioiodine in tumor cells for antibody labeled using [I- 131]- SGMIB was due to positively charged, low molecular weight species. Th ese results suggest that [I-131-SGMIB warrants further evaluation as a reag ent for labeling internalizing antibodies.