99mTc-Anti-ED-B fibronectin single-chain antibody fragment L19-His

Excerpt

^99mTc-Anti-ED-B fibronectin single-chain antibody fragment L19-His (^99mTc-L19-His) is a radiolabeled molecular imaging agent developed for single-photon emission computed tomography (SPECT) imaging of tumor angiogenesis and guidance for antiangiogenic treatment (1). ^99mTc is a gamma emitter with a half-life (t_½) of 6.01 h.

Angiogenesis is a process of development and growth of new blood vessels from pre-existing vessels (2). Tumor growth depends on the formation of new blood vessels from this process. Normal angiogenesis is orderly and highly regulated, whereas tumor angiogenesis is chaotic and irregular. Abnormal angiogenesis is important in the carcinogenesis, growth, and progression of solid and hematologic tumors in humans (3). Fibronectins (FNs) are a family of universal cell-adhesion molecules that are widely distributed (1). FN is a polymorphic glycoprotein of ~2500 amino acids and has a high molecular mass of 250–280 kDa. FN occurs in soluble form in plasma and other body fluids and in insoluble form in the extracellular matrices (4, 5). Both forms are dimers composed of a series of repeating units of three types and joined by two disulfide bonds at the C-terminus of the molecule. FN polymorphism arises from alternative splicing patterns of the pre-mRNA or post-translational modifications of FN itself (5). Alternative splicing in three regions [extra domain A (ED-A), extra domain B (ED-B), and type III homology connecting segment (IIICS)] may generate 20 different FN subunit isoforms. The splice variant ED-B FN, which is highly expressed during angiogenesis in both neoplastic and normal tissues (6), is an oncofetal antigen expressed at different levels in the stroma associated with the neovasculature of solid tumors. High levels of ED-B expression have been found in primary and metastatic tumors in breast, colorectal, and non-small cell lung cancers (1, 7-9).

Molecular imaging of angiogenesis offers serial non-invasive evaluation of both location and growth dynamics of tumors (10). SPECT or positron emission tomography imaging with an appropriate radiolabeled tracer targeted to angiogenic pathways may allow the evaluation of specific aspects of tumor vascular biology (9). A molecular probe that targets ED-B FN can be both an early tumor marker and a tool to monitor the success of antiangiogenic cancer therapy. The single-chain antibody fragment (scFv) L19, which has a high affinity for ED-B FN, was developed by Pini et al. (11). Radioiodinated L19 showed specific accumulation around tumor neovasculature and tumor stroma with high ED-B expression (12, 13). In an effort to prepare a stable ^99mTc-labeled L19, Berndorff et al. (1) genetically introduced a (His)₆ peptide sequence at the C-terminus of L19 to produce L19-His molecules. Two other L19 derivatives, AP39 and L19-Hi20, were also prepared for radiolabeling. These ^99mTc-labeled L19 derivatives appeared to have favorable biodistribution and imaging properties in mice bearing murine embryonal teratocarcinomas (F9). However, the study did not provide data to confirm that the binding was a result of angiogenesis.