The invention relates to the use of a JAKl kinase- selective inhibitor that has minimal inhibitory activity towards Jak2 kinase for treating a disease, such as an inflammatory disease (e.g., moderate to severe Rheumatoid Arthritis) and/or bone loss, either alone or in combination with a DMARD (disease modifying anti-rheumatic drug), such as methotrexate. The invention also provides pharmaceutical composition, dosage formulation, administration route, and dosage schedule thereof.