Anti-EGFRvIII monoclonal antibody armed with 177Lu: in vivo comparison of macrocyclic and acyclic ligands

Abstract

Introduction: Monoclonal antibody (mAb) L8A4 binds specifically to the epidermal growth factor receptor variant III (EGFRvIII) that is present on gliomas but not on normal tissues, and is internalized rapidly after receptor binding. Because of the short range of its β-emissions, labeling this mAb with (177)Lu would be an attractive approach for the treatment of residual tumor margins remaining after surgical debulking of brain tumors.

Materials and methods: L8A4 mAb was labeled with (177)Lu using the acyclic ligands [(R)-2-amino-3-(4-isothiocyanatophenyl)propyl]-trans-(S,S)-cyclohexane-1,2-diamine-pentaacetic acid (CHX-A″-DTPA) and 2-(4-isothiocyanatobenzyl)-6-methyldiethylene-triaminepentaacetic acid (1B4M-DTPA), and the macrocyclic ligands S-2-(4-isothiocyanatobenzyl)-1,4,7,10-tetraazacyclododecane-tetraacetic acid (C-DOTA) and α-(5-isothiocyanato-2-methoxyphenyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (MeO-DOTA). Paired-label tissue distribution experiments were performed in athymic mice bearing subcutaneous EGFRvIII-expressing U87.ΔEGFR glioma xenografts over a period of 1 to 8 days to directly compare (177)Lu-labeled L8A4 to L8A4 labeled with (125)I using N-succinimidyl 4-guanidinomethyl-3-[(125)I]iodobenzoate ([(125)I]SGMIB).

Results: Except with C-DOTA, tumor uptake for the (177)Lu-labeled mAb was significantly higher than the co-administered radioiodinated preparation; however, this was also the case for spleen, liver, bone and kidneys. Tumor/normal tissue ratios for (177)Lu-1B4M-DTPA-L8A4 and, to an even greater extent, (177)Lu-MeO-DOTA-L8A4 were higher than those for [(125)I]SGMIB-L8A4 in most other tissues.

Conclusions: Tumor and normal tissue distribution patterns for this anti-EGFRvIII mAb were dependent on the nature of the bifunctional chelate used for (177)Lu labeling. Optimal results were obtained with 1B4M-DTPA and MeO-DOTA, suggesting no clear advantage for acyclic vs. macrocyclic ligands for this application.

Figure 1

Structures of the bifunctional chelators CHX-A″-DTPA, 1B4M-DTPA, C-DOTA and MeO-DOTA used in this study.

Figure 2

Ratio of ¹⁷⁷Lu to ¹²⁵I activity in U87. EGFR glioma xenografts and selected normal tissues following four paired label experiments. [¹²⁵I]SGMIB-L8A4 co- administered with L8A4 mAb labeled with ¹⁷⁷Lu using either 1B4M-DTPA, CHX-A″-DTPA, C-DOTA, or MeO-DOTA.

Figure 3

Tumor:normal tissue uptake ratios in athymic mice with subcutaneous U87. EGFR glioma xenografts from four paired label experiments. [¹²⁵I]SGMIB-L8A4 co-administered with L8A4 mAb labeled with ¹⁷⁷Lu using either 1B4M-DTPA, CHX-A″-DTPA, C-DOTA, or MeO-DOTA.