Radioimmunoscintigraphy using an anti-prostate monoclonal antibody (E4): adosimetric evaluation

The aim of this study was to evaluate different strategies to increase the tumour radiation dose for experimental radioimmunotherapy using I-125-label led monoclonal antibody (MAb) E4 in a nude mice model xenografted with DU-1 45 tumours. The effects from a single injection of the I-125-labelled MAb E 4, the same total amount of radiolabelled MAb E4 divided into three repeate d injections, and the effect of pre-targeting with non-labelled MAb E4 for reducing the amount of shed antigen were investigated. Based on repetitive quantitative radioimmunoscintigraphies, calculation of the tumour radiation dose delivered from the I-125-nuclide was performed for each strategy. The single injection strategy without pretargeting rendered the highest mean t umour radiation dose, i.e. 0.23 Gy/MBq. Pretargeting with non-labelled MAb E4 before a single injection of [I-125]E4 resulted in a slightly lower mean tumour radiation dose, i.e. 0.19 Gy/MBq, compared to the single injection alone. An even lower mean tumour radiation dose, i.e. 0.14 Gy/MBq, was obta ined when the same total administered amount of activity was divided into t hree separate injections given in 10-day intervals. We concluded that the s ingle injection strategy is the most efficient when using MAb E4 in this tu mour model. The tumour radiation doses were not increased by dividing the s ame amount of activity into three injections or by pretargeting with non-la belled MAb E4.