Therapeutic Effect of Ansamitocin Targeted to Tumor by a Bispecific Monoclonal Antibody

We have constructed a murine hybrid hybridoma that secretes a bispecific monoclonal antibody (mAb) by fusing a hybridoma secreting an anti‐ansamitocins mAb with a hybridoma secreting an anti‐human transferrin receptor (TfR) mAb that binds to human A431 epidermoid carcinoma cells. The bispecific mAb, reactive to both ansamitocins and TfR, was purified by a combination of hydrophobic column chromatography and hydroxyapatite high‐performance liquid chromatography, and evaluated in in vivo experiments using human tumor cell‐implanted nude mice. Ansamitocin P‐3 targeted through one of the antigen combining sites of the bispecific mAb was potentially more effective in suppressing the growth of established A431 tumor xenografts implanted on nude mice than unconjugated ansamitocin P‐3 or the immunoconjugate of ansamitocin P‐3 and monospecific anti‐ansamitocins antibody, and the targeted ansamitocin P‐3 finally eradicated the tumor mass. The bispecific mAb also played an important role in reducing such undesirable side‐effects of ansamitocin P‐3 as the loss of body weight, the damage to liver functions and the decrease in the number of white blood cells.