Open-Label Crossover Study Comparing CTP-543 to Jakafi®

Inclusion Criteria: * Healthy adult males and females between 18 and 50 years of age, inclusive * Body weight ≥ 50 kg and BMI within the range of 18 to 30 kg/m2, inclusive Exclusion Criteria: * History of clinically significant central nervous system (eg, seizures), cardiac, pulmonary, metabolic, renal, hepatic, or gastrointestinal (GI) conditions * PR interval \> 220 msec or QRS duration \> 120 msec or QTcF interval \> 450 msec obtained at screening visit or prior to the first dose of study drug * History of herpes zoster * Hemoglobin, white blood cell, or platelet levels below the lower reference limit at screening or prior to the first dose of study drug * Liver function tests greater than the upper limit of normal * Positive blood screen for human immunodeficiency virus (HIV antibody), hepatitis B virus surface antigen, or hepatitis C virus antibody * Urinalysis positive for protein or glucose * A positive screen for alcohol, drugs of abuse, or tobacco use * Donation of blood, plasma or other blood products prior to screening * A positive tuberculosis test