XmAb®5592 Fc-Engineered Humanized Anti-HM1.24 Monoclonal Antibody Has Potent in Vitro and In Vivo Efficacy against Multiple Myeloma.
Authors: Tai Yu-Tzu , Muchhal Umesh , Li Xian-Feng , Nahar Sabikun , Song Weihua , Horton Holly , Nguyen Hanh , Karki Sher , Desjarlais John , Munshi Nikhil C. , Richardson Paul , Anderson Kenneth C. 
Published: October 2019

Abstract
Abstract 609
HM1.24/CD317 or BST2, a cell surface protein highly expressed on malignant plasma cells, represents a potential target of immunotherapy for multiple myeloma (MM). Here we characterized XmAb®5592, a novel Fc-engineered and humanized anti-HM1.24 antibody (Ab), and studied mechanisms of its anti-MM activity. XmAb®5592, with double amino acid substitution in Fc region of the wild type IgG1, has approximately 40-fold and 10-fold increases in affinity for Fc gamma receptor III (FcγRIIIa) and (FcγRIIa), respectively, expressed on effector cells including NK cells. The Fv region of XmAb®5592 was humanized and engineered to achieve high affinity and specificity of binding to HM1.24-expressing target cells. XmAb®5592 reacts against a panel of MM cell lines (n=19) which are both sensitive or resistant to current anti-MM conventional and novel therapies. Importantly, it triggers 10-100-fold higher antibody-dependent cell-mediated cytotoxicity (ADCC) against these MM cell lines than a native/non Fc-engineered version (anti-HM1.24 IgG1) of the Ab. Specifically, the maximum specific lysis of MM1S, MM1R, and RPMI8226 target cells induced by XmAb®5592 is at a concentration of 0.001-0.01 μg/ml, whereas the IgG1 analog did not induce maximum cell lysis until 0.1 μg/ml. The maximum 100% specific lysis of INA-6 target cells occurred at 0.1 μg/ml of XmAb5592, in contrast to 60% maximum lysis induced by10 μg/ml of the IgG1 analog. Since the bone marrow (BM) microenvironment induces resistance in MM cells to conventional therapies, we next asked whether XmAb®5592 induced ADCC against MM cells even in the presence of BM stromal cells (BMSCs). Importantly, XmAb®5592 triggered significant ADCC against MM1S, MM1R, and INA-6 MM cells in the context of BMSCs. XmAb5592 also reacts against patient MM cells, and triggers robust ADCC against CD138-purified patient MM cells in assays using NK effector cells from normal donors. Furthermore, cross-linked XmAb5592 inhibited RMPI 8226 cell growth in the absence of effector cells. The in vivo efficacy of XmAb®5592 was next evaluated in murine subcutaneous (sc) xenograft murine models using RPMI 8226 cells. Administration of XmAb5592 (9mg/kg, ip, 2x/week for 4 weeks) led to a significant reduction in growth of established tumors in vivo compared to a non-engineered IgG1 anti-HM1.24 analog. At termination of the study. 7/15 mice were tumor free in the XmAb- treated group versus only 1/15 tumor free mice in the IgG1 analog treated group. An anti-HM1.24 antibody with Fc region engineered to completely ablate binding to FcγRs (knock-out) behaved equivalent to the PBS vehicle control in these studies, again underlining the significance of interaction with FcγR for anti-tumor efficacy. These results therefore suggest that XmAb5592, an anti-HM1.24 antibody engineered for improved effector function and antitumor potency in vitro and in vivo, is a promising next-generation immunotherapeutic for MM.

Disclosures:
Muchhal: Xencor: Employment. Horton:Xencor: Employment. Nguyen:Xencor: Employment. Karki:Xencor: Employment. Desjarlais:Xencor: Employment. Munshi:Millennium Pharmaceuticals: Honoraria, Speakers Bureau. Richardson:Keryx Biopharmaceuticals: Honoraria. Anderson:Millennium Pharmaceuticals: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau.