Xencor has dosed the first patient in the Phase IIa, part of its ongoing Phase Ib/IIa clinical trial of XmAb5871 for rheumatoid arthritis, an autoimmune disease.

The Phase IIa trial will assess biweekly doses of XmAb5871 in around 30 rheumatoid arthritis patients with active disease on stable non-biologic DMARD therapy.

Endpoints include clinical markers of disease response.

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

XmAb5871 is the first in a new class of therapeutic antibodies that target the Fc?RIIb pathway in B cells, with the potential to suppress autoimmune disorders but devoid of any side effects caused by B cell depletion.

The Fc engineered monoclonal antibody leverages a dual-targeting mechanism for B cell inhibition by co-engaging CD19 and Fc?RIIb.

Xencor president and CEO Bassil Dahiyat said XmAb5871 represents a novel therapeutic strategy for engaging the Fc?RIIb pathway to suppress autoimmune responses without B-cell depletion.

"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," Dahiyat said.

In December 2010, Amgen and Xencor had entered into a collaboration and option agreement for XmAb5871.

Under that deal, Amgen has the option to acquire an exclusive worldwide license to XmAb5871, exercisable at any time before completion of a data review period following the planned subsequent Phase IIb proof-of-concept clinical trial.

Xencor is responsible for all research, development, and manufacturing activities for XmAb5871 until that time and is eligible for milestone payments.

The company develops new mechanisms of therapeutic action through its XmAb antibody engineering technology that allows small changes to the structure of monoclonal antibodies.

Image: XmAb5871 represents a novel therapeutic strategy for targeting the Fc?RIIb pathway in B cells. Photo: courtesy of Braegel.