Comparison of the potency and therapeutic efficacy of the anti-CD7 immunotoxin HB2-saporin constructed with one or two saporin moieties per immunotoxin molecule.

Immunotoxins that carry two toxin molecules to the target cell should in theory have a greater anti-tumour effect than those that carry just one. We have investigated the therapeutic efficacy of two anti-CD7-saporin immunotoxins constructed with one saporin (HB2-Sap 1-mer) or two saporin molecules (HB2-Sap 2-mer) per immunotoxin molecule. In vitro, the 2-mer immunotoxin was 5.6 times more effective than the 1-mer immunotoxin at inhibiting protein synthesis in the CD7+ human T-cell acute lymphoblastic leukaemia (T-ALL) cell line HSB-2 and was also more effective at inhibiting HSB-2 cell proliferation. Flow cytometry revealed that the 2-mer immunotoxin had a reduced binding capacity to HSB-2 cells compared with the 1-mer immunotoxin or native HB2 antibody. In therapy studies in SCID mice with disseminated HSB-2 human leukaemia, the 2-mer immunotoxin performed marginally better than the 1-mer immunotoxin, but log-rank analysis did not reveal any significant differences between the two therapy groups. We therefore conclude that, although the 2-mer immunotoxin performed better than the 1-mer immunotoxin against target HSB-2 cells in vitro, this improved performance was not reflected as an improved in vivo therapeutic outcome in the SCID mouse model.