IMPROVED TARGETING OF AN ANTIEPIDERMAL GROWTH-FACTOR RECEPTOR VARIANT-III MONOCLONAL-ANTIBODY IN TUMOR XENOGRAFTS AFTER LABELING USING N-SUCCINIMIDYL 5-IODO-3-PYRIDINECARBOXYLATE

Monoclonal antibody (mAb) L8A4, specific for the tumor-associated muta nt epidermal growth factor receptor variant III (EGFRvIII), is interna lized and degraded after cell binding, Four paired-label experiments w ere performed in athymic mice bearing EGFRvIII-positive xenografts to determine the suitability of N-succinimidyl 3-iodo-5-pyridinecarboxyla te (SIPC) for labeling this internalizing mAb. In mice with HC2 20 d2 xenografts, tumor uptake reached a maximum of 32.7 +/- 2.0% injected d ose/g when labeled using SIPC, a value significantly higher (P < 0.05, paired t test) than that observed when L8A4 was labeled using Iodogen (24.4 +/- 2.2% injected dose/g). The specificity of mAb uptake in HC2 20 d2 and U87MG Delta EGFR xenografts was measured in separate experi ments by coadministration of L8A4 and nonspecific, isotype-matched P3X 63Ag8 mAb, both radioiodinated using SIPC, Tumor localization indices were approximately 10 or more by 72 h, a degree of specificity 3-4 tim es higher than that reported previously when labeling was performed us ing the tyramine cellobiose (TCB) method, In a final study directly co mparing L8A4 labeled using SIPC and TCB, similar tumor levels were obt ained (SIPC, 33.7 +/- 6.1% injected dose/g at 24 h; TCB, 37.8 +/- 6.7% injected dose/g at 24 h): however, tumor-to-tissue ratios for the liv er, spleen, and kidneys were 3 times higher with SIPC at later time po ints. These results suggest that SIPC is a promising method for labeli ng this anti-EGFRvIII mAb and possibly other mAbs that internalize aft er binding.