Multi-center, multi-national, double-blind, randomized, active-controlled (MTX), 3-month study with 4 doses of CH-4051 (0.3 mg, 1.0 mg, 3.0 mg and 3.0 mg with 1mg of folic acid supplementation p.o. daily) compared to a "standard" dose of MTX at 20 mg per week with 1 mg of folic acid supplementation p.o. daily. This study will be conducted in two parts: