Radioiodinated peanut lectin: A potential radiopharmaceutical for immunodetection of carcinoma expressing the T antigen

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Abstract
The Thomsen-Friedenreich (T) antigen, β-D-Gal-(1→3)-α-D-GalNAc, is exposed in reactive form on many human adenocarcinomata, but not on corresponding benign tissues. Peanut lectin (PNA) has a strong binding affinity for the T antigen and reportedly binds preferentially to certain malignant tissues. We investigated the potential of radiolabelled PNA as a tumour localising agent in an animal model system using a mouse lymphoma (RI) shown to bind fluorescein-labelled PNA in vitro. The radioiodinated lectin showed good tumour localization and rapid blood clearance. Clear images of tumours were obtained, in serial scintigraphic imaging, by 24 and 48 h. No blood background subtraction was necessary. Biodistribution studies revealed tumour to blood ratios in mice were 6:1 (at 24 h) and 17:1 (at 48 h), and tumour to muscle ratios were 34:1 (at 24 h) and 40:1 (at 48 h). Rapid in vivo breakdown of 125I-PNA led to some localization of free iodide in the kidneys, stomach, thyroid and salivary glands.