Safety and Tolerability Study of MBS2320 in Healthy Subjects and Subjects With Rheumatoid Arthritis

Principal Inclusion Criteria: Parts A, B, and C. * Healthy males or females between 18 and 60 years of age. * A body mass index (BMI) between 18.0 and 30.0 kg/m2. * Female subjects will be of non-childbearing potential or postmenopausal as defined by the protocol. * Female subjects must not be pregnant. Part D * Subjects will be otherwise healthy males or females with a diagnosis of RA between 18 and 70 years of age. * Subjects will have a BMI between 18.0 and 30.0 kg/m2. * Female subjects must not be pregnant. * Subjects must have been treated with, and tolerated, oral or subcutaneous MTX for a minimum of 3 months prior to screening entry. Principal Exclusion Criteria: Parts A, B, and C. * Male subjects who do not agree to use appropriate contraception. * Female subjects who are receiving HRT who do not agree to use appropriate contraception. * Subjects who have donated blood in the 3 months, plasma in the 7 days or platelets in the 6 weeks prior to screening. * Subjects who consume more than the permitted alcohol requirement, who have a significant history of alcoholism or drug/chemical abuse. * Subjects who are unwilling to abstain from alcohol as required. * A positive urine drug screen, alcohol breath test at screening or first admission. * Subject has received a live virus vaccination within the 30 days prior to first dose administration. * Subjects with a positive test for tuberculosis. Additional Part D Exclusions * Subjects who have received any medication (except MTX) known to chronically alter drug absorption or elimination processes within 30 days prior to the first dose administration. * Subjects currently taking any medications other than those allowed per protocol guidelines.