IL22RA1 and rheumatoid arthritis: However, fletikumab was a human antibody (IgG4) with a high affinity (Kd: 37 pM) and a very high potency in blocking IL-20 mediated activity on IL-20RA/IL-20RB and IL-22RA1/IL20R2 expressing cells (IC50: 0.27 nM).It had a serum half-life of ~3 weeks in RA patients (70).