Extensive studies have demonstrated the effectiveness of treatment with radiolabeled monoclonal antibodies, particularly for lymphoma. However, the maximal radioimmunoconjugate dose that can be used is limited by the non-specific delivery of radiation to normal tissue. To decrease the radiation delivered to normal organs, we propose to rapidly remove circulating non-bound radioisotope by using linkers than can be rapidly cleaved upon exposure to the appropriate enzyme. In preliminary studies, we have synthesized an I-131 labeled anti-CD20 cleavage of the circulating radioimmunoconjugate following infusion of beta-lactamase, and rapid clearance following infusion of beta-lactamase, and rapid clearance of the released isotope-containing moiety by the kidney, thereby reducing non-specific background radiation. However, as antibodies that remain on the tumor cell surface are both susceptible to enzymatic cleavage and to clearance from the tumor site as a result of decreasing local concentration of labeled antibody, the use of antibodies that are internalized by the target tumor cells will be require. Rapid targeting of the tumor by the radioimmunoconjugate may also may also be required for optimization of this approach, allowing early administration of the cleaving agent before the normal organs receive substantial amounts of radiation. Thus, in Aim 1 we will prepare beta-lactamase-sensitive linkers than can be radiolabeled using methods that minimize loss of the radionuclide after internalization by the target cells and are thus suitable for use with internalizing antibodies.
In Aim 2, using a murine xenograft model, we will determine the in vivo biodistribution of beta-lactamase-sensitive radioimmunoconjugates prepared from non-internalizing beta-lactamase sensitive anti-CD20 radioimmunoconjugates, as well as radioimmunoconjugates prepared from internalizing anti-CD19 or CD22 radioimmunoconjugates, as well as radioimmunoconjugates prepared from internalizing anti-CD19 or CD22 antibodies.
In Aim 3, we will determine whether there is an advantage to utilizing constructs with more rapid tumor uptake by evaluating the biodistribution of novel antibody constructs radiolabeled using enzymatically cleavable linkers. Using beta- lactamase sensitive radioimmunoconjugates determined optimal in murine studies we will evaluate their biodistribution in non-human primates in Aim 4. The overall goal of this project is to improve the delivery of radiation to tumor compared to normal tissues.
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