Antitumor Effects of Ricin A Chain Immunotoxins Prepared from Intact Antibodies and Fab′ Fragments on Solid Human Hodgkin's Disease Tumors in Mice

Three monoclonal antibodies which strongly bind to Hodgkin and Reed-Sternberg cells and two corresponding Fab′ fragments were linked to deglycosylated ricin A chain (dg A) to evaluate their potential as immunotoxins for the treatment of Hodgkin's disease. Two of the antibodies, Ber-H2 and HRS-3, were shown to bind to the same epitope on the CD30 antigen, whereas the third antibody, IRac, bound to a different antigen. None of the antibodies significantly cross-reacted with normal human tissues as judged by indirect immunofluorescence and immunoperoxidase analyses on frozen sections from 28 normal tissues. All three antibodies formed potent and specific immunotoxins. They inhibited protein synthesis of the L540 Hodgkin's disease cell line in vitro by 50% at concentrations of 1 x 10^-11 M for IRac·dgA, 9 x 10-11 m for HRS-3-dgA, and 2 x KT10 m for Ber-H2 dgA. HRS-3 Fab′ and IRac Fab′ immunotoxins were 7.8- and 60-fold less cytotoxic, respectively, than their intact counterparts in vitro. In vivo, a single i.v. injection of a dose of Ber-H2 � dgA, HRS-3 � dgA, or IRac � dgA corresponding to 40% of the LD50 induced lasting complete remissions in 38, 44, and �0%, respectively, of mice with solid s.c. L540 tumors of 60 to 80 mm3 size (0.5-cm diameter). At equivalent dosage (40% of the LD50), the HRS-3 Fab′ � dgA and the IRac Fab′ dgA both induced lasting complete remissions in 25% of the mice, although the HRS-3 Fab′ dgA was significantly superior to IRac Fab′ dgA at retarding tumor growth in the remaining animals. The effectiveness of the immunotoxins depended on the size of the tumor at the time of injection, since IRac·dgA treatment induced complete remissions in 100% of mice with small tumors (10 to 20 mm3, approximately 0.3 cm in diameter) but only 13% of mice with larger tumors of 400 to 600 mm3 (approximately 1 cm in diameter). Tumors which regrew after IRac·dgA treatment mainly consisted of antigen-deficient mutants having reduced sensitivity to IRac·dgA but normal sensitivity to HRS-3 dgA. It is concluded that HRS-3 dgA, HRS-3 Fab' dgA, and IRac·dgA are candidates for the treatment of Hodgkin's disease in humans.