XmAb®5574 is an Fc engineered humanized monoclonal antibody (mAb) that binds to the human cell surface antigen CD19 and demonstrates anti-proliferative activity against CD19-positive (CD19+) cell lines. XmAb5574 also has an engineered Fc region to enhance cell killing activity via recruitment of effector cells through increased binding affinity to Fcγ receptors (FcγRs). Consequently, XmAb5574 exhibits superior antibody-dependent cell-mediated cytotoxicity (ADCC) and antibody-dependent cell-mediated phagocytosis (ADCP), when compared to a native IgG1 (non Fc-engineered) or an Fc knock out (Fc KO; engineered to ablate FcγR interaction) version of the anti-CD19 antibody. To evaluate the potential clinical activity of XmAb5574 in CD19+ B cell malignancies such as non-Hodgkin’s lymphoma (NHL), XmAb5574 was tested in murine subcutaneous (sc) xenograft models using the CD19+ Ramos and Raji human lymphoma cell lines. XmAb5574, administered by intraperitoneal injection (ip, 2qw ×2), gave dose-related inhibition of tumor growth with these models. The efficacy against established sc Ramos tumors was shown to be FcγR-dependent and enhanced by Fc engineering. Fc KO antibodies had no effect at the doses used. Preclinical studies were conducted to evaluate the safety and pharmacokinetics of XmAb5574 in non-human primates. In vitro studies demonstrated that the cynomolgus monkey was an appropriate species for study. XmAb5574 bound to a CD19-expressing cynomolgus monkey cell line and CD20+ peripheral lymphocytes from either cynomolgus monkey or human whole blood samples. Binding affinities of XmAb5574 to both human and cynomolgus monkey FcγRs were evaluated by Biacore methods and were found to be similar. Additionally, XmAb5574 gave similar staining patterns in immunohistochemistry cross-reactivity studies with normal human and cynomolgus monkey tissue panels. Single dose administration of XmAb5574 (0, 0.3, 1.0, 3.0, and 10.0 mg/kg, intravenous [iv] infusion) to cynomolgus monkeys gave an immediate and sustained depletion of peripheral B cells in a dose-dependent manner. B cells were reduced in the bone marrow and lymph node with the spleen showing involuted germinal centers and decreased CD20 immunostaining. B cell recovery, peripherally evident after 57 days, was observed in lymphoid tissues after 85 days. The native anti-CD19 IgG1 (non Fc-engineered) did not induce B cell depletion at 3 mg/kg, in contrast to almost complete B cell depletion by XmAb5574 at the same dose. The pharmacokinetics of XmAb5574 were determined in cynomolgus monkeys after a single iv infusion at 0.3, 1, 3, or 10 mg/kg. Blood samples were collected throughout the study, processed to serum, and XmAb5574 concentration determined using an ELISA method. Exposure was approximately dose proportional for the 1–10 mg/kg dose levels but decreased at the 0.3 mg/kg level indicating dose-dependent clearance for XmAb5574 in this species. Among the 1–10 mg/kg dose levels, the clearance and half-life ranged from 4.3–5.8 mL/day and 7.7–10.7 days, respectively. Single iv infusions of XmAb5574 (0.3–10 mg/kg) were well tolerated in cynomolgus monkeys. These preclinical data provide a rationale for the clinical testing of XmAb5574 in patients with B cell malignancies.

Disclosures: Lawrence:Xencor Inc.: Employment, Equity Ownership. Zalevsky:Xencor Inc.: Employment, Equity Ownership. Horton:Xencor Inc.: Employment, Equity Ownership. Leung:Xencor Inc.: Employment, Equity Ownership. Chu:Xencor Inc.: Employment, Equity Ownership. Karki:Xencor Inc.: Employment, Equity Ownership. Zhukovsky:Xencor Inc.: Employment, Equity Ownership. Desjarlais:Xencor Inc.: Employment, Equity Ownership. Carmichael:Xencor Inc.: Employment, Equity Ownership.

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