CYTOREDUCTIVE EFFECTS OF ANTITRANSFERRIN RECEPTORS IMMUNOTOXINS IN A MULTICELLULAR TUMOR SPHEROID MODEL

We have evaluated the sensitivity to immunotoxins (IT) of monolayer an d of 200-250 mu m multicellular tumor spheroid (MTS) cultures obtained with human breast (MCF7) and glioblastoma (U118) tumor cells and with rat glioblastoma (9L) cells. Monolayer MCF7 and U118 cells were highl y sensitive to anti-transferrin receptor (anti-TfnR) ricin A chain (RT A)-IT (Tfn-RTA and MAb OKT9-RTA) treatment in the presence of the intr acellular RTA-IT enhancing agent human serum albumin-monensin (HSA-Mo) conjugate. A 790- to 2000-fold higher concentration of anti-TfnR IT w as instead required to reduce by 50% the volume of individually treate d MCF7 spheroids, as evaluated by applying the Gompertz growth model. Monolayer 9L cells showed 230- to 5700-fold lower sensitivity to Tfn-R TA IT than MCF7 and U118 monolayers, yet 9L spheroid cells were almost as sensitive to anti-TfnR IT as monolayer 9L cultures. Binding studie s performed with [I-125]-Tfn and FITC-labelled anti-TfnR MAb revealed that 9L monolayers and MTS expressed 4.1-fold and 8.8-fold lower amoun ts of TfnR than MCF7 monolayers and MTS, respectively. However, Tfn bo und to TfnR sites of 9L and of MCF7 cells with comparable affinity. Ex periments carried out with the diphtheria toxin mutant CRM107 linked t o Tfn confirmed the pattern observed with RTA-IT. Monolayers and spher oids showed no considerable differences in sensitivity to ricin toxin. Collectively, these results indicated that the efficacy of IT against 3-D tumors is heavily influenced by the number of target Ag expressed by the tumor cells, as well as by the affinity of IT/toxin-cell inter action. (C) 1994 Wiley-Liss, Inc.