MONROVIA, Calif., October 1, 2013—Xencor, Inc. today announced that the first patient has been dosed in the Phase 2a part of its ongoing Phase 1b/2a clinical trial of XmAb®5871 in patients with active rheumatoid arthritis. XmAb5871 is the first in Xencor's class of therapeutic antibodies targeting the Fc?RIIb pathway in B cells, which shows potential to suppress autoimmune disorders without the side effects caused by B cell depletion.

XmAb5871 is an Fc engineered monoclonal antibody that inhibits B cells by dual targeting CD19 and Fc?RIIb. Amgen Inc. and Xencor entered into a collaboration and option agreement for XmAb5871 in December 2010. Under that agreement, Amgen has the option to acquire an exclusive worldwide license to XmAb5871, exercisable at any time before completion of a data review period following our planned subsequent Phase 2b proof-of-concept clinical trial. Xencor leads all research, development, and manufacturing activities for XmAb5871 until that time and is eligible for milestone payments.

"XmAb5871 represents a novel therapeutic strategy for engaging the FcyRIIb pathway to suppress autoimmune responses without B-cell depletion," said Bassil Dahiyat, Ph.D., president and CEO of Xencor. "This Phase 2a rheumatoid arthritis data is intended to inform a larger Phase 2 study and may support the potential use of XmAb5871 in other autoimmune diseases, such as lupus."

The Phase 2a portion of the clinical trial will evaluate biweekly doses of XmAb5871 in approximately 30 rheumatoid arthritis patients with active disease on stable non-biologic DMARD therapy. Endpoints include clinical markers of disease response.

XmAb5871 was well tolerated in the Phase 1a dose escalation part of the trial that enrolled 48 healthy male subjects and inhibited multiple biomarkers of immune function.

About Xencor, Inc.
Xencor is developing engineered monoclonal antibodies for the treatment of autoimmune diseases, asthma and allergic diseases, and cancer. Currently, five antibodies are in clinical development internally and with partners that have been engineered with Xencor'sXmAb technology. Xencor's internally-discovered programs include XmAb5871, in Phase 2a for the treatment of Rheumatoid arthritis and lupus, XmAb7195 in preclinical development for the treatment of asthma, and XmAb5574/MOR208 which has been licensed to Morphosys AG and is in Phase 2 clinical trials for the treatment of acute lymphoblastic leukemia and non-Hodgkin lymphoma. Xencor's XmAb antibody engineering technology enables small changes to the structure of monoclonal antibodies resulting in new mechanisms of therapeutic action. Xencor partners include Amgen, Merck, Janssen R&D LLC and Boehringer Ingelheim.

Xencor Media Contact
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