Abstract 4456: Molecular events required for the induction of lethal iron deprivation in malignant hematopoietic cells via an antibody-avidin fusion protein specific for human transferrin receptor 1

Proceedings: AACR 101st Annual Meeting 2010‐‐ Apr 17‐21, 2010; Washington, DC Iron is an essential nutrient for cell growth and division. The transferrin receptor 1 (TfR1 or CD71) is a major gateway for iron import, and as such is expressed on a variety of cells; it is over-expressed in a number of tumors including hematopoietic malignancies. We have previously shown that an antibody-avidin fusion protein (ch128.1Av) specific for TfR1 is toxic to cancer cells, capable of inducing lethal iron deprivation in some malignant B-cells such as IM-9, but not in others such as U266. Importantly, ch128.1Av induces iron deprivation without directly preventing the binding of transferrin (Tf) to TfR1. In an attempt to identify the molecular events behind the iron deprivation induced by ch128.1Av, we have found that like Tf, internalization of ch128.1Av bound to TfR1 is clathrin, dynamin, and temperature dependent. Additionally, recycling of TfR1-bound ch128.1Av at short timescales (30min) is also similar to that of Tf and does not significantly alter the rate of Tf uptake. Long-term incubation (>6h) of IM-9 cells with ch128.1Av precludes their uptake of Tf and induces degradation of TfR1. This is not the case with U266 cells, in which Tf uptake remains unaffected and degradation of TfR1 is negligible in the presence of ch128.1Av. Clustering of cell surface TfR1 by ch128.1Av, given its multivalent nature, was previously thought to cause degradation of the receptor in sensitive cells such as IM-9. We now show that clustering of TfR1 occurs both in cells that are sensitive and resistant to treatment with the antibody (IM-9 and U266), indicating that clustering of TfR1 induced by ch128.1Av is not sufficient to induce receptor degradation and subsequent iron deprivation. Instead, resistance to treatment with ch128.1Av may be attributed to elements downstream of receptor binding/clustering; possibly related to those involved with TfR1 recycling and degradation. Further studies are needed to identify these resistance factors and potentially enhance the efficacy of ch128.1Av as well as other antibodies and/or small molecules that similarly target TfR1 for the treatment of hematologic malignancies. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4456.