Immunoconjugates of geldanamycin and anti-HER2 monoclonal antibodies: Antiproliferative activity on human breast carcinoma cell lines

Background: HER2 is a membrane receptor whose overexpression is strongly as sociated with poor prognosis in breast carcinomas. Inhibition of HER2 activ ity can reduce tumor growth, which led to the development of Herceptin, an anti-HER2 monoclonal antibody (MAb) that is already in clinical use, Howeve r, the objective response rate to Herceptin monotherapy is quite low. HER2 activity can also be inhibited by the highly cytotoxic antibiotic geldanamy cin (GA), However, GA is not used clinically because of its adverse toxicit y. Our purpose was to enhance the inhibitory activity of anti-HER2 MAb by c oupling it to GA. Methods: We synthesized 17-(3-aminopropylamino)GA (17-APA -GA) and conjugated it to the anti-HER2 MAb e21, to form e21:GA, The nonint ernalizing anti-HER2 MAb AE1 was used as a control. Internalization assays and western blot analyses were used to determine whether the anti-HER2 MAbs and their immunoconjugates were internalized into HER2-expressing cells an d reduced HER:! levels. All statistical tests were two-sided. Results: The immunoconjugate e21:GA inhibited the proliferation of HER2-overexpressing c ell lines better than unconjugated e21 (concentration required for 50% inhi bition = 40 versus 1650 mu g/mL, respectively). At 15 mu g/mL, e21:GA reduc ed HER2 levels by 86% within 16 hours, whereas unconjugated e21, 17-APA-GA, or AE1:GA reduced HER2 levels by only 20%, These effects were not caused b y release of 17-APA-GA from the immunoconjugate because immunoconjugates co ntaining [H-3]GA were stable in serum at 37 degrees C, Furthermore, e21:GA did not significantly inhibit proliferation of the adult T-cell leukemia ce ll line HuT102, which is HER2 negative yet highly sensitive to GA. Conclusi ons: Our findings suggest that conjugating GA to internalizing MAbs enhance s the inhibitory effect of the MAbs, This approach might also be applied in cellular targeting via growth factors and may be of clinical interest.