Modifications in Synthesis Strategy Improve the Yield and Efficacy of Geldanamycin−Herceptin Immunoconjugates

Geldanamycin (GA) was modified with N-tert-butyloxycarbonyl-1,3-diaminopropane to introduce a latent primary amine. After deprotection, this primary amine provided a site for introduction of a maleimide group that enabled linkage to proteins. This maleimido derivative of geldanamycin (GMB-APA-GA) was linked to the monoclonal antibody Herceptin after the antibody had been modified with Traut's reagent to introduce thiol groups. By this sequence, a new immunoconjugate (H:APA-GA) was generated that showed greater antiproliferative activity than the previously reported analogous immunoconjugate created with a 1,4-diaminobutane spacer derivative of geldanamycin to form an immunoconjugate, H:ABA-GA. Both immunoconjugates inhibited in vitro the growth of MDA-361/DYT2 cells, a cell line overexpressing the HER2 antigen, while Herceptin alone was ineffective. However, H:APA-GA showed better efficacy than H:ABA-GA (IC(50) = 0.2 vs 0.58 mg/mL and cell doubling time >12 vs 6 days, respectively). Results of the in vivo therapy experiments in a xenograft model were consistent with the in vitro findings. Treatment with Herceptin prolonged the survival of the tumor-bearing mice when compared with the control group, but H:ABA-GA and H:APA-GA were each more efficacious than unmodified Herceptin. However, unlike H:ABA-GA, the immunoconjugate H:APA-GA caused stable tumor regression (in 25% of the recipients), showing a qualitative improvement with potential clinical relevance.