Evaluation of Ricin A Chain-containing Immunotoxins Directed against the CD30 Antigen as Potential Reagents for the Treatment of Hodgkin's Disease

Five monoclonal CD30 antibodies and two Fab' fragments were linked to deglycosylated ricin A chain (dgA), and their potential as immunotoxins for the treatment of Hodgkin's disease was evaluated. Cross-blocking experiments demonstrated that HRS-1, HRS-3, HRS-4, and Ber-H2 recognize the same epitope on the CD30 antigen and that Ki-1 binds to a different epitope. Scatchard analyses showed that HRS-3, HRS-4, and Ber-H2 bound strongly to L540 Hodgkin cells (Kd 15, 7, and 14 nM, respectively), whereas HRS-1 and Ki-1 bound more weakly (Kd 160 and 380 nM, respectively). The different affinities of the antibodies correlated closely with their cytotoxic potency as immunotoxins. HRS-3.dgA, HRS-4.dgA, and Ber-H2.dgA inhibited the protein synthesis of L540 cells by 50% at concentrations of 0.9–2.0 × 10−10 M., whereas HRS-1.dgA and Ki-1.dgA were about 100 times less potent with 50% inhibitory concentrations of 0.8–1.0 × 10−8 M. The most effective immunotoxins, HRS-3.dgA and HRS-4.dgA, were only 15 times less toxic than ricin itself. HRS-3 Fab.dgA and HRS-4 Fab'.dgA were 7.8 and 3 times less potent than their IgG.dgA counterparts with 50% inhibitory concentrations of 7 × 10−10 and 3 × 1010 M., respectively. Staining of human tissues revealed an unexpected cross-reactivity of HRS-4 with pancreatic cells of malignant and nonmalignant origin. HRS-1, HRS-3, Ber-H2, and Ki-1 showed very little cross-reactivity with any normal human tissues. It is concluded that HRS-3.dgA and HRS-3 Fab'.dgA are the immunotoxins of choice for in vivo therapy.