Monoclonal antibodies such as L8A4, reactive with the
epidermal growth factor receptor variant III, internalize after receptor binding resulting in proteolytic degradation by lysosomes. Labeling internalizing mAbs requires the use of methodologies that result in the trapping of labeled catabolites in
tumor cells after intracellular processing. Herein we have investigated the potential utility of N-succinimidyl-3-[131I]iodo-4-phosphonomethylbenzoate ([131I]SIPMB), an acylation agent that couples the corresponding negatively charged
acid [131I]IPMBA to the
protein, for this purpose. Biodistribution studies demonstrated that [131I]IPMBA cleared rapidly from normal tissues and exhibited thyroid levels < or =0.1% injected dose, consistent with a low degree of dehalogenation. Biodistribution experiments in athymic mice bearing subcutaneous D-256 human
glioma xenografts were performed to compare L8A4 labeled using [131I]SIPMB to L8A4 labeled with 125I using both the analogous positively charged acylation agent N-succinimidyl-4-guanidinomethyl-3-[125I]iodobenzoate ([125I]
SGMIB) and
Iodogen.
Tumor uptake of [131I]SIPMB-L8A4 (41.9+/-3.5% ID/g) was nearly threefold that of L8A4 labeled using
Iodogen (14.0+/-1.1% ID/g) after 2 days, and
tumor to tissue ratios remained uniformly high throughout with [131I]SIPMB-L8A4. Thyroid uptake increased for the
Iodogen labeled mAb (3.55+/-0.36 %ID at 5 days) whereas that of [131I]SIPMB labeled mAb remained low (0.21+/-0.04% ID at 5 days). In the second biodistribution, L8A4 labeled using [131I]SIPMB and [125I]
SGMIB showed no difference in normal tissue uptake and had nearly identical
tumor uptake ([131I]SIPMB, 41.8+/-14.2% ID/g; [125I]
SGMIB, 41.6+/-15.8% ID/g, at 4 days). These results suggest that [131I]SIPMB may be a viable acylation agent for the radioiodination of internalizing mAbs.