Cocktails of ricin a‐chain immunotoxins against different antigens on hodgkin and sternberg‐reed cells have superior anti‐tumor effects against H‐RS cells in vitro and solid Hodgkin tumors in mice

Three ricin A‐chain immunotoxins (ITs) recognizing different antigens on Hodgkin‐Reed/Sternberg (H‐RS) cells were evaluated for their anti‐tumor effects when used in combination as “cocktails”. These ITs, BB10.dgA (CD2S), HRS3.dgA (CD30), and IRac.dgA (70 kDa), strongly inhibited the growth of L540Cy H‐RS cells in vitro. The protein synthesis of this cell line was reduced more efficiently by the combination of 2 of these ITs than by BB10.dgA, HRS3.dgA or IRac.dgA alone. A cocktail of all 3 ITs was most effective in vitro. This was at least in part due to the non‐homogeneous distribution of CD25, CD30 or IRac on the L540Cy H‐RS target cells and to the fact that sub‐populations deficient in one antigen nevertheless expressed appreciable levels of the other target antigens. IT cocktails were also superior as anti‐tumor agents in nude mice with solid L540Cy tumors. Ninety percent of mice treated with cocktails containing 2 or 3 ITs had continuous complete remissions (CCR), as compared with only 40% of mice treated with the same dose of a single IT. Analysis of 7 L540Cy sub‐lines re‐established ex vivo from mice that relapsed after having achieved complete remission (CR) after therapy with a single IT showed that the surviving tumor cells were antigen‐deficient variants which were resistant to the original IT, but which could be killed by ITs directed against other target antigens. Thus, IT cocktails give superior results against human H‐RS cells, both in vitro and in vivo.