Antitumour activity of a melanoma-specific immunotoxin, ME20-LysPE40.

An immunotoxin conjugate has been prepared by linking an internalizing antibody with melanoma selectivity, ME20, with a binding-defective form of Pseudomonas exotoxin A, LysPE40. ME20-LysPE40 binds to a 105,000 Da cell-surface antigen present on melanoma cells (ME20-M) within twofold of unmodified ME20 and was cytotoxic to two human melanoma cell lines, H3606 and MALME-3M, with EC50 values of 100 and 200 pM, respectively. Immunotoxin treatment, initiated 1 day following subcutaneous implantation of H3606 melanoma cells into mice, prevented outgrowth of tumour xenografts in > 50% of the mice. In contrast, only a modest inhibition in tumour growth was observed if the immunotoxin was administered 5 days after implantation of in vivo passaged H3606 tumour fragments in mice. This study shows that the internalizing monoclonal antibody ME20 IgG can be used for targeting a toxin toward melanoma cells displaying the ME20-M antigen.