Molecular Targeting of Carbonic Anhydrase IX in Mice with Hypoxic HT29 Colorectal Tumor Xenografts

BackgroundCarbonic anhydrase IX (CAIX) is a membrane spanning protein involved in the enzymatic regulation of tumor acid-base balance. CAIX has been shown to be elevated in a number of hypoxic tumor types. The purpose of this study was to determine the efficiency of intact and IgG fragments of cG250 to target CAIX in vivo in a hypoxic tumor model.
Methodology/Principal FindingsConventional biodistribution studies were performed with 111In-DO3A-cG250, 111In-DO3A-F(ab')2-cG250 and 111In-DO3A-Fab-cG250. Additional ex vivo analysis of the tumor was performed with markers for tumor hypoxia, blood perfusion and endogenous CAIX expression. All four data sets were digitally correlated to determine the optimal agent for determining hypoxia in a HT29 colon cancer xenograft. The HT29 human colorectal tumor xenografts show strong CAIX expression in hypoxic areas of poor blood perfusion. The intact IgG had an initial high focal uptake at the periphery of these hypoxic regions and penetration into the areas of highest CAIX expression over the 7-day study period. The lower molecular weight antibody fragments had a faster uptake into areas of high CAIX expression, but had a much lower absolute uptake at the optimal imaging times.
Conclusions/SignificanceFor the clinical detection of hypoxia induced CAIX using cG250 antibody based agents, imaging with the intact IgG at 7 days post injection would allow for the most sensitive and accurate detection of CAIX.

背景
碳酸酐酶IX（Carbonic anhydrase IX, CAIX）是一种跨膜蛋白，参与肿瘤酸碱平衡的酶促调控。研究证实，CAIX在多种缺氧肿瘤类型中表达上调。本研究旨在探究完整型及IgG片段形式的cG250在缺氧肿瘤模型体内靶向CAIX的效率。
方法与主要结果
采用¹¹¹In-DO3A-cG250、¹¹¹In-DO3A-F(ab')₂-cG250及¹¹¹In-DO3A-Fab-cG250开展常规生物分布实验。额外采用肿瘤缺氧、血液灌注及内源性CAIX表达相关标志物对肿瘤进行离体分析。对上述四组数据集进行数字化关联分析，以筛选HT29结肠癌异种移植模型中用于检测缺氧的最优试剂。HT29人结直肠癌异种移植模型在血液灌注不良的缺氧区域呈现较强的CAIX表达。完整IgG在这类缺氧区域的边缘初始呈现高局灶摄取，并在为期7天的实验周期内逐步渗透至CAIX高表达区域。分子量较低的抗体片段虽能更快地富集至CAIX高表达区域，但在最优成像时间点的绝对摄取量显著更低。
结论与意义
采用基于cG250抗体的试剂临床检测缺氧诱导的CAIX时，于注射后7天使用完整IgG进行成像，可实现对CAIX最灵敏、最精准的检测。