 - At least a 90 day course of azathioprine AZA or 6-mercaptopurine 6-MP prior to Baseline, with a dose of AZA 1.5 mgkgday or 6-MP 1 mgkgday rounded to the nearest available tablet formulation, or a dose that is the highest tolerated by the participant e.g., due to leukopenia, elevated liver enzymes, nausea during that time. Participant must be on a stable dose for at least 28 days prior to Baseline. 