Xencor ($XNCR) has reacquired the rights to an Amgen ($AMGN)-optioned antibody, walking away from the Big Biotech and an arthritis indication with hopes of a new path in rare disease. Xencor is nixing a 2010 deal that gave Amgen the option of buying into the company's XmAb5871 after it completed a Phase II trial in rheumatoid arthritis, and the company is calling off work in that indication altogether, canceling plans to start a Phase IIb study next year and forgoing up to $425 million in milestone payments. Instead, Xencor intends to try the treatment in IgG4-related disease, a fibro-inflammatory autoimmune disorder that affects up 20,000 people in the U.S. and has no approved therapies. The company expects to kick off its new clinical program for XmAb5871 in 2015, CEO Bassil Dahiyat said, and it has enough cash to pay the drug's way into 2016. More