In-Labeled DTPA conjugated anti-c-kit monoclonal antibody 12A8 Fab fragment

The c-kit (CD 117 antigen or stem cell factor ligand receptor) is a 145-kDa type III glycoprotein receptor tyrosine kinase (TK; encoded by the proto-oncogene) that has a TK at the juxtamembrane position, which is activated when a ligand binds to the extracellular ligand-binding domain of the receptor (1). Almost 70% of gastrointestinal stromal tumors (GISTs) are known to overexpress a c-kit that has a mutation in exon 11 and produces a constitutively activated TK that promotes the survival and proliferation of cells and leads to the development of a malignant phenotype in the cells of the GISTs (1, 2). The biology and mutations of the c-kit gene and the detection, diagnosis, and chemotherapy of the cancerous GISTs are discussed in detail elsewhere (3, 4). An immunohistochemical method is commonly used to detect the overexpression of c-kit in GIST cancers, but this invasive technique is known to generate variable results (3, 5). In addition, investigators often use positron emission tomography (PET) with F-fluoro-deoxyglucose to detect, monitor, and assess the response of GISTs to chemotherapy with imatinib, but this radiolabeled probe does not distinguish between GISTs and other cancerous tumors that have an epithelial or lymphatic origin (6). Recently, the In-labeled anti–c-kit monoclonal antibody (mAb) 12A8 ([In]12A8) was developed and evaluated for the detection of GISTs with single-photon emission computed tomography (SPECT) in nude mice bearing xenograft tumors that express c-kit (6). However, with [In]12A8 the tumors were first visible on the animals at 48 h postinjection (p.i.) and became clearly visible only at 96 h p.i. This indicated that it was necessary to develop a radiolabeled immunoreagent that would have superior pharmacokinetic properties and yield faster results compared to the In-labeled mAb. It is well known that mAb fragments such as Fab (50 kDa) and single-chain antibody fragments (27 kDa) have superior pharmacokinetic properties compared to intact mAbs (~150 kDa) (7, 8). In an effort to achieve this goal, a Fab fragment of mAb 12A8 was labeled with In (to obtain [In]12A8 Fab) and with Cu (to obtain [Cu]12A8 Fab), and the two probes were evaluated for the detection of tumors that express c-kit in nude mice (2). This chapter describes the SPECT studies performed with [In]12A8 Fab. The detection of tumors that express c-kit with [Cu]12A8 Fab is discussed in a separate chapter of MICAD (www.micad.nih.gov) (9).