The Development and Evaluation of Novel Reagents for Pre-Targeted Radioimmunotherapy

Abstract

In a pretargeting radioimmunotherapy system, monoclonal antibodies (MAbs) derivatised with a high affinity streptavidin receptor are used in a localisation step. Once tumour localisation has taken place and active or passive MAb clearance has been completed the final step is the administration of a radiotoxic radiopharmaceutical, which is a ligand to a high affinity receptor on the MAb. This technique maximises the tumour to background ratio and facilitates the use of much higher tumour radiation doses. Four pretargeting effector molecules were developed with a view to selecting the best performer for further pretargeting assays. When analysed for key characteristics, two of the novel.. DOTA-biotin conjugated effector molecules were selecxed for advanced testing in a pretargeting context. The other two effector molecules were not suitable for in-vivo use. Mouse biodistribution studies of indium-111 and yttrium-90 labelled effector molecules showed minimal organ retention and very rapid clearance. Herceptin was conjugated with streptavidin in the presence of stannous ions using UV irradiation to produce free suiphydryl groups. The free sulphydryls of the irradiated antibody were calibrated and stability of the conjug9te was tested. The Herceptin-streptavidin system was evaluated for cell binding potential with human breast cancer cells. Pretargeting studies were performed both in-vitro and in-vivo as were internalisation and externalisation assays. The two biotin derivatisede ffector moleculesla bel well, have been shown to be very stable, and have a favourable biodistribution in murine models. The Herceptinstreptavidin conjugate has been produced with a 1: 1 stoichiometry to greater than 99 % purity. Binding studies have been successful and show a very similar nature to Herceptin with albeit slower antigen association kinetics. Biodistribution and in-vivo pretargeting results are pending. With the results collected, it appears that the effector . fl moleculesa re promisinga gentsf or pretargeting,t hough final results must be collated befored rawingc onclusionso n the antibodyc onjugate.