Monoclonal antibody-ricin conjugate cytotoxic to cells expressing the common acute lymphoblastic leukemia antigen (CALLA).

The monoclonal antibody PHM-6, which is specific for the common acute lymphoblastic leukemia antigen (CALLA), was conjugated to the plant toxin ricin. Binding of the PHM-6-ricin conjugate to cells via the ricin molecule was blocked by the presence of 100 mM lactose. The IC50 (concentration resulting in 50% inhibition) of the PHM-6-ricin conjugate for the CALLA-positive KM-3 cell line was 280-fold greater than for bone marrow stem cells, indicating the potential of this conjugate for immunological purging of autologous remission marrow.