Optimizing Radiolabeled Engineered Anti-p185HER2 Antibody Fragments forIn vivoImaging

Abstract

We have recently described the in vivo properties of an iodinated anti-p185^HER2 engineered antibody fragment [minibody (scFv-C_H3)₂; 80 kDa], made from the internalizing 10H8 monoclonal antibody. Although the 10H8 minibody showed excellent binding to the target in vitro, only modest tumor uptake [5.6 ± 1.7% injected dose per gram (ID/g) of tissue] was achieved in nude mice bearing MCF7/HER2 breast cancer tumors. Here, in an attempt to improve targeting, the 10H8 minibody was conjugated to 1,4,7,10-tetraazacyclododecane-N, N′, N′′, N′′′-tetraacetic acid (DOTA), radiometal labeled, and evaluated in vivo. The tumor uptake of ¹¹¹In-DOTA 10H8 minibody was 5.7 ± 0.1% ID/g, similar to the radioiodinated 10H8 minibody. However, in addition to the expected liver clearance, the kidneys had unexpectedly high activity (34.0 ± 4.0% ID/g). A minibody derived from a second anti-p185^HER2 antibody (trastuzumab; hu4D5v8) was also made. Tumor uptakes, evaluated by quantitative microPET using ⁶⁴Cu-DOTA hu4D5v8 minibody, were 4.2 ± 0.5% ID/g. Furthermore, in non-tumor-bearing mice, ¹¹¹In-DOTA hu4D5v8 minibody exhibited similar elevated uptake in the kidneys (28.4 ± 6.5% ID/g). Immunohistochemical staining of kidneys from non-tumor-bearing mice showed strong specific staining of the proximal tubules, and Western blot analysis of kidney lysate confirmed the presence of cross-reactive antigen. To further improve tumor uptake and normal tissue distribution, a larger hu4D5v8 fragment [(scFv-C_H2-C_H3)₂; 105 kDa] was made, engineered to exhibit rapid clearance kinetics. This fragment, when evaluated by microPET, exhibited improved tumor targeting (12.2 ± 2.4% ID/g) and reduced kidney uptake (13.1 ± 1.5% ID/g). Thus, by manipulating the size and format of anti-p185^HER2 antibody fragments, the kidney activity was reduced and high or low expression of p185^HER2 in xenografts could be distinguished by microPET imaging.