Monoclonal antibodies in neuro-oncology.

Many studies have suggested the possible existence of tumor-associated antigens in brain gliomas. Strong evidence for the existence of such cell determinants was provided by recent investigations using hybridoma technology. The possibility of obtaining monoclonal antibodies (MAbs) against glioma-associated antigens should help to allow their identification, purification, and characterization. Utilizing MAbs as reagents of predefined specificity, a number of central and peripheral nervous system antigens could be detected. The molecules recognized by MAbs in glioma cells can be subdivided into four categories: [1] biochemical defined proteins, [2] specificities shared by nervous system-lymphoid cells, [3] oncoembryonic-oncofetal determinants, and [4] tumor-restricted antigens. Of greater significance is the heterogeneity of antigen expression among various individual glioma cells observed in frozen sections of tumor biopsies. Using a panel of MAbs, the phenotypic heterogeneity, i.e., the variation in antigen expression can be documented within and among malignant gliomas and cell lines derived from them. In spite of this the characteristic pattern of antibody binding to brain tumors makes MAbs the potentially best reagents for immuno-histochemical application in surgical neuropathology. Moreover, immuno-cytological screening of tumor cells in the cerebrospinal fluid has also proved to be valuable. The localization of radio-labelled MAbs in experimental and human gliomas growing subcutaneously and intracranially in athymic nude mice were explored by radioscintigraphy and autoradiography. Imaging experiments with 131I-labelled MAbs recognizing epitopes on the glioma cell surface showed high levels of specific activity in xenografts. Preliminary data indicate that administration of 131I-MAbs as well as drug conjugates (daunomycin-MAbs) causes a depression of glioma cell proliferation in vitro as well as delayed tumor growth and thus prolonged survival time of tumor-bearing mice. The mechanisms of antibody delivery and transport of "immunotoxins" from the vascular compartment to intracerebral tumor tissue are presently a subject of discussion. The complexity of this area necessitates comprehensive experimental work in order to define the factors involved in the delivery of MAbs to brain to tumor tissue and thus optimize the rate of blood-to-tumor transport. Current investigations have shown that it is possible to image malignant human gliomas using radio-labelled antibodies. The next step will be to attain target immunotherapy. The use of MAbs as carrier molecules for clinical applications might soon be possible.