Saporin 6 conjugated to monoclonal antibody selectively kills human melanoma cells

A novel human melanoma specific immunotoxin is described, which has been produced utilizing the murine monoclonal antibody (mAb) Ep2, IgG2a isotype, recognizing an epitope of the glycoprotein/proteoglycan high molecular weight-melanoma associated antigen. mAb Ep2 has been chemically conjugated by a disulphide bond, using the bifunctional reagent SPDP, to the plant toxin Saporin 6 (SAP) extracted from seeds of Saponaria officinalis. Cytotoxicity studies performed in vitro on melanoma cells have shown that Ep2/SAP immunotoxin efficiently kills antigen expressing cells and that its IC50 is approximately 1 x 10(-10) M, while not affecting the viability of antigen-negative melanoma cells at doses as high as 1 x 10(-7) M, therefore indicating a therapeutic index of Ep2/SAP immunotoxin higher than 1000. Kinetic studies have demonstrated that protein synthesis inhibition by Ep2/SAP is rapidly achieved, since a 90% reduction is observed within 3.1 h, and that this inhibitory activity is apparently first order with time. Furthermore, the cytotoxic activity of the immunoconjugate is not dependent, and is not influenced by, the presence in the culture medium of the lysosomotropic agent chloroquine.