Development of new ricin A-chain immunotoxins with potent anti-tumor effects against human Hodgkin cells in vitro and disseminated Hodgkin tumors in SCID mice using high-affinity monoclonal antibodies directed against the CD30 antigen.

The lymphocyte activation marker CD30 has been shown to be an excellent target for the immunotherapy of human Hodgkin's lymphoma. In order to develop new potent immunotoxins (ITs) against CD30, we chemically linked 6 recently described monoclonal antibodies (MAbs) via SMPT to deglycosylated ricin A-chain (dgA). Cross-blocking experiments demonstrated that these MAbs, termed Ki-2 to Ki-7, recognize 3 different clusters on the CD30 antigen: Ki-2, Ki-4, Ki-5 and Ki-7 recognize cluster A; Ki-6 recognizes cluster B; Ki-3 binds to cluster C. Staining of 29 sections of normal human organs revealed no major cross-reactivity of any MAbs tested. Binding to the CD30 antigen on L540Cy Hodgkin cells was assessed by flow cytometry, and demonstrated high affinities for Ki-2, Ki-3 and Ki-4. The concentration giving 50% of the mean fluorescence intensity (MFI50) was 0.58 micrograms/ml to 0.78 micrograms/l. MAbs Ki-5, Ki-6, and Ki-7 bound much more weakly. The staining intensity of the MAbs correlated with the cytotoxicity of the corresponding ITs. Ki-2.dgA, ki-3.dgA and Ki-4.dgA inhibited the protein synthesis of L540Cy cells by 50% at concentrations (IC50) of 3.5 x 10(-10)M to 4.0 x 10(-11)M. The most effective IT, Ki-4dgA, is 5-fold more potent than previously reported CD30 ricin A-chain ITs. Ki-4.dgA was subsequently used for the treatment of disseminated human Hodgkin's lymphoma in a SCID mouse model. The mean survival time (MST) of lymphoma-bearing SCID mice was extended from 42 days in untreated controls to more than 132 days when Ki-4.dgA was applied one day after tumor challenge. Ki-4.dgA is a new potent IT suitable for further evaluation against Hodgkin's lymphoma in man.