The Radiolabelling of Monoclonal Antibodies with In^III and their Application In Vivo for Detection of Thyroid Metastases

Abstract

Most primary differentiated thyroid carcinomas and their metastases synthesize and release into circulation thyroglobulin (Tg) where its measurement is a good indicator of disease activity (1). Localization of these thyroid metastases in thyroidectomized patients is normally achieved by whole body I¹³¹ scans after replacement thyroxine (T₄) therapy has been discontinued for one month. It was to circumvent the problem of induced hypothyroidism that led us to use a radiolabelled monoclonal antibody specific for human T_g to externally image its binding to Tg on the cell surfaces of metastases in patients on T₄ (2). In our patient study using I¹²³ labelled antibody it was shown that approximately half the injected dose remained in circulation at 24 hours, and to allow a greater uptake of antibody on to tumour would require external imaging at 48 hours. Imaging with I¹²³ labelled antibody is not feasable at 48 hours and requires a longer acting physical half-life isotope such as In¹¹¹ (2.8 days). The objectives of this present study were to label the monoclonal antibody with In¹¹¹ and investigate the effects of this labelling procedure on its structure and biological function as an in vivo imaging agent.