Arming An Anti-CD38, Myeloma-Targeting Antibody With An Attenuated IFNα Provides >10,000-Fold Enhanced Tumor-Specific Activity Compared To Native IFNα

Interferon-α (IFNα) exerts profound apoptosis-inducing activity on myeloma cells, but only at concentrations 10-100-fold above those clinically achievable due to dose-limiting toxicities. In colony forming assays, all multiple myeloma (MM) samples tested (8 primary myelomas and 2 cell lines) were sensitive to high doses of IFNα. Furthermore, very high doses of IFNα prevented MM tumor growth in 2 primary MM xenograft models (LAGk-2 and LAGk-1A). In order to exploit this potential of IFNα, we sought to improve its therapeutic index by increasing its activity towards myeloma cells relative to normal cells. Tumor-specificity can be modestly increased by attaching IFNα to an anti-CD38 antibody, thus targeting it to MM cells. Here we demonstrate that by introducing an attenuating mutation into the IFNα portion of this fusion protein, the IFNα activity is reduced by >10,000 fold on CD38 negative cells while it exerts similar potency on CD38⁺ MM cells compared to native IFNα. We show that direct in vitro killing activity of this anti-CD38-attenuated IFNα fusion protein on CD38⁺ myeloma cells is comparable to free IFNα, however, the activity of this fusion protein is reduced by ∼10,000 fold if the CD38 antibody is replaced by an antibody of irrelevant specificity, or if CD38^- cells are used in the assay. In addition, we tested activity of the anti-CD38-attenuated INFα fusion protein in the H929 MM xenograft model and found that a single dose at 10mg/kg was sufficient to completely eradicate established tumors (150mm³) in 10/10 mice. Furthermore, when mice with starting tumor volumes of >700mm³ were treated for 3 weeks, we saw complete regression of these large tumors, an effect not observed with other MM agents tested. Our findings suggest that by administering this fusion protein to patients, it may be possible to target the robust tumor-killing activity of IFNα to the myeloma cells, with minimal toxic effects resulting from IFN’s activity on normal cells.