Imaging of
cancer by radioimmunoscintigraphy (RIS) following administration of radiolabeled
monoclonal antibodies (mAbs) or fragments has been under development for two decades. Efficacy is a function of many variables:
antigen expression on
tumor cells relative to normal tissues and body fluids, affinity, specificity, pharmacokinetics and immunogenicity of the mAb and properties of the
radionuclide and imaging techniques.
CEA-Scan (a 99mTclabeled
Fab' fragment of a mouse mAb directed against CEA) has several advantageous properties, and has been approved for use in
colorectal cancer SPECT imaging in North America and Europe, and is also under investigation in Japan. It has been shown to
complement CT in detecting recurrent or metastatic disease, even in the liver where 111In mAbs frequently fail. In
breast cancer, it provides clearer identification of malignant disease in patients with indiscriminate mammography. Recently, it has also been used intra-operatively in radio-immunoguided surgery in both colorectal and
breast cancer patients. These new applications are currently under active investigation and clinical trials are planned. Clinically useful information is obtained in a relatively high percentage of patients, which can alter treatment decisions. In most cases the benefits are in reducing unnecessary surgery or biopsies in inoperable patients, but major benefits (especially in
colorectal cancer) would be obtained if new
therapies effective against the RIS-detected
minimal residual disease were forthcoming.