Targeting of saporin to Hodgkin's lymphoma cells by anti‐CD30 and anti‐CD25 bispecific antibodies

CD25 and CD30 represent suitable target molecules for bispecific antibody (bimAb)‐driven toxin delivery to lymphoid tumour cells. We describe two new anti‐CD30/anti‐saporin bimAbs (termed CD30 × sap1 and CD30 × sap2), produced by hybrid hybridomas, which react against non‐cross‐reactive epitopes of the saporin molecule, and compared their effect with a bimAb reacting with saporin and with CD25 (CD25 × sap1). In a protein synthesis inhibition assay these bimAbs were able to enhance saporin toxicity (IC 50 8.5 × 10 −9 M in the absence of mAbs) with a similar activity: in the presence of 10 −9   M CD30 × sap1 bimAb the IC 50 was 2.75 × 10 −11   M , whereas with CD30 × sap2 bimAb the IC 50 was 6.5 × 10 −11   M and CD25 × sap1 bimAb displayed an IC 50 of 3 × 10 −11   M (as saporin). The combined use of the two anti‐CD30 bimAbs further increased cytotoxicity by 100‐fold, resulting in an IC 50 of 1.9 × 10 −13   M . A slightly less efficient improvement was obtained by combining the CD25 × sap1 bimAb with the CD30 × sap2 bimAb directed against a different toxin epitope (saporin IC 50 to 7 × 10 −13   M ). In contrast, no synergistic effect was observed using the combination of the anti‐CD25 bimAb with the anti‐CD30 bimAb reacting with the same epitope of saporin (IC 50  = 4.5 × 10 −11   M ). Analysis of FITC–saporin binding to L540 cells by flow cytometry demonstrated that the appropriate combinations of the two anti‐CD30/anti‐saporin bimAbs or of the anti‐CD30/anti‐saporin and anti‐CD25/anti‐saporin bimAbs had a cooperative effect on the binding of the ribosome‐inactivating protein (RIP) to the cells, when compared with single bimAbs.