RADIOIMMUNOTHERAPY OF PROSTATIC ADENOCARCINOMAS - EFFECTS OF I-131-LABELED E4 ANTIBODIES ON CELLS AT DIFFERENT DEPTH IN DU-145 SPHEROIDS

Spheroids of the human prostatic adenocarcinoma cell line DU 145 were used to study experimental radioimmunotherapy. Spheroids were incubate d with the I-131-labelled monoclonal E4 antibody until the radionuclid e immunoconjugate had bound the 5 to 6 outermost cell layers of the sp heroids. A set of 50 spheroids were exposed, either immediately or 48 hr after antibody incubation and washings, to a dilute trypsin solutio n with the aim of stripping off cells from the spheroid surface. Strip ped cells were collected in fractions corresponding to defined spheric al shells. Cells were subsequently plated for clonogenic growth. The t echnique of automated sequential trypsinization of spheroids followed by a clonogenic survival assay permits studies on therapeutic efficacy for radionuclide immunoconjugates on cells from different layers of s pheroids. In addition, the absorbed doses throughout a spheroid were c alculated. The binding and retention kinetics of the radionuclide immu noconjugate and the excess of I-131-E4 in the culture medium during in cubation are factors that were all accounted for in the calculations. If the calculated absorbed doses were inserted into the linear-quadrat ic survival model and the low dose rate was taken into account, surviv al values were well in accordance with the experimentally obtained val ues. The results demonstrate that the I-131-labelled E4 antibody is ca pable of sterilizing cultured tumour cells that have bound the radionu clide immunoconjugate and, by means of radiation ''cross-fire'', those cells located in close proximity. (C) 1995 Wiley-Liss, Inc.