Five Prime Therapeutics Inc., of South San Francisco, said healthy volunteers in its phase I trial of FPA008, a monoclonal antibody that inhibits colony stimulating factor-1 receptor, was well tolerated at doses associated with modulation of key biomarkers. The company said it is now ready to begin dosing patients with rheumatoid arthritis (RA) who are on on methotrexate to further assess tolerability and preliminary efficacy measures.