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

The objective of this study was to develop an acylation agent for the radioiodination of monoclonal antibodies that would maximize retention of the label 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 membranes after antibody degradation. Preparation of unlabeled N-succinimidyl 4-guanidinomethyl-3-iodobenzoate (SGMIB) was achieved in six steps from 3-iodo-4-methylbenzoic acid. Preparation of 4-guanidinomethyl-3-[I-131]-iodobenzoic acid from the silicon precursor, 4-(N-1,N-2-bis-tert-butyloxycarbonyl)guanidinomethyl-3-trimethylsilylbenzoic acid proceeded in less than 5% radiochemical 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-butyloxycarbonyl)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. Paired-label in vitro internalization assays demonstrated that the amount of radioactivity 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 increased retention of radioiodine in tumor cells for antibody labeled using [I-131]- SGMIB was due to positively charged, low molecular weight species. These results suggest that [I-131-SGMIB warrants further evaluation as a reagent for labeling internalizing antibodies.