Monoclonal antibodies coupled to drugs and toxic agents (immunotoxins) or radionuclides (radioimmunoconjugates) represent new tools for immunotherapy of haematological malignancies. Immunotoxins constructed with toxins of either plant or bacterial origin have shown a powerful antitumor activity both in vitro and in mice with severe combined immunodeficiency bearing various kinds of leukaemias and lymphomas. Preliminary clinical trials have shown an activity of these compounds at least in a proportion of patients. However, tumour responses have generally been partial and transient. The main problems with immunotoxin therapy remain the inability of immunotoxins to target tumour cells in the presence of a high burden of disease, the host immune response against both the antibody and the toxin moieties, which precludes repeated administration of immunotoxins, and the vascular leak syndrome. Targeting of tumour cells with specific antibodies armed with radionuclides (usually iodine-131 or yttrium-90) appears to be an even more attractive approach. Preliminary clinical studies have clearly demonstrated the ability of radioimmunoconjugates, especially when administered at high dose followed by bone marrow rescue, to induce durable complete remission in patients with non-Hodgkin's lymphomas refractory to conventional therapies. Radioimmunotherapy also overcomes the antigenic heterogeneity of the tumour cell population, since antigen negative tumour cells will be irradiated by the nearby targeted antigen-positive cells. Efforts should now be focused on defining more precisely the optimal clinical setting for administration of immunotoxin and radioimmunoconjugates (e.g. minimal residual disease), to reduce the immunogenicity of these compounds and solve the problem of vascular leak syndrome.
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