The 99mTc-labeled anti-receptor for advanced glycation endproducts (RAGE) monoclonal antibody F(ab’)2 fragment, abbreviated as 99mTc-anti-RAGE mF(ab’)2 or 99mTc-anti-RAGE F(ab’)2, is a radiotracer developed by Tekabe et al. for imaging atherosclerotic lesions by targeting highly expressed RAGE with single-photon emission computed tomography (SPECT) (1). RAGE is a 35-kDa transmembrane receptor of the immunoglobulin (Ig) superfamily (2, 3).
RAGE has one V domain, two C domains, one transmembrane domain, and one cytoplasmic tail. The V domain consists of two N-glycosylation sites and is responsible for extracellular ligand binding. RAGE exists in three forms: full-length, membrane-bound, and soluble (2-4). Under physiological conditions, RAGE is expressed at low levels in a variety of cells in a regulated manner, but RAGE is highly expressed in a series of age- and diabetes-related chronic inflammatory diseases and cancer (5-8). The advanced glycation endproducts (AGEs) are the major ligands for RAGE. AGEs are a heterogeneous group of peptides and proteins derived from non-enzymatic glycosylation processes (3, 5). Large amounts of AGEs is formed through metabolism and aging, and this establishes a positive feedback cycle under pathological conditions such as diabetes (2, 4, 9). The interaction between AGEs and RAGE affects almost all types of cells and molecules and results in pro-inflammatory gene activation (8, 10). The pathological effects induced by the AGEs/RAGE interaction include increasing vascular permeability, inhibiting vascular dilation, inducing cytokine secretion, enhancing oxidative stress, and modulating cell response to exogenous growth factors. Understanding of the AGEs/RAGE interaction is crucial to develop new treatment regimens for age- and diabetes-related conditions and cancer (4, 10).
In the case of atherosclerosis, AGEs are formed in both diabetic and nondiabetic conditions, but to a greater extent in diabetes. RAGE itself is expressed in nearly all cell types pertinent for the development and progression of atherosclerotic plaque. The AGEs/RAGE interaction leads to diabetic vascular complications and augments atherosclerotic plaque development and progression (8, 9). In a proof-of-concept study of whether the expression level of RAGE could be detected and thus used as a marker of atherosclerosis, Tekabe et al. developed a 99mTc-labeled polyclonal antibody F(ab’)2 fragment against RAGE and further demonstrated that uptake of the fragment in the atherosclerotic plaques could be visualized with planar γ imaging in a nondiabetic apolipoprotein E–null (ApoE–/–) mouse model (2). To reduce the nonspecific binding associated with the polyclonal antibody, Tekabe et al. developed a monoclonal antibody against the extracellular domain of RAGE (1). The intact monoclonal antibody was then digested with pepsin, and the generated F(ab’)2 fragment was radiolabeled to produce the radiotracer 99mTc-anti-RAGE mF(ab’)2. SPECT imaging showed that the radiotracer identified early accelerated lesions in diabetic ApoE–/– mice to a better degree than in nondiabetic mice. The results indicated that SPECT with 99mTc-anti-RAGE mF(ab’)2 may be used to assess novel therapies in experimental animals and possibly in humans (1).