UPTAKE MODIFICATION OF A PROSTATE-REACTIVE MONOCLONAL-ANTIBODY IN PROSTATIC-CARCINOMA SPHEROIDS

Spheroids of human prostatic carcinoma cells (DU 145) were exposed to the I-125-labelled prostatic carcinoma reactive monoclonal antibody, E 4, for varying times in order to study antibody penetration and bindin g. After 2 hours the E4 binding depth was about 85 mu m and after 6 ho urs about 140 mu m The I-125-E4 antibody was distributed throughout sm all spheroids after 24 hours. In spheroids with well-developed central necrosis, no binding appeared in the central region although a penetr ation assay revealed that penetration of antibodies into the center ha d occurred after 24 hours. The binding was specific since incubation o f spheroids with an excess of non-labelled E4 blocked the binding site s. In an attempt to increase the binding depth, for a given antibody e xposure time, pretreatments of spheroids with therapeutic doses of rad iation or the drug estramustine were carried out. Both pretreatments r esulted in small, but significant, increases of antibody binding depth when compared with non-pretreated control spheroids. Furthermore, the penetration assay revealed increased antibody penetration when the an tibody binding sites in the spheroids were blocked, demonstrating that binding delays penetration. In monolayer experiments, a displacement curve and a Scatchard analysis revealed a dissociation constant of abo ut 0.125 nM and the number of binding sites to be in the order of 4x10 (5) per cell.