OBJECTIVE: Baricitinib is an orally administered inhibitor of JAK1 and JAK2 that 
has been shown to be effective in treating rheumatoid arthritis (RA). This study 
was undertaken to analyze changes in lymphocyte cell subsets during baricitinib 
treatment and to correlate these changes with clinical outcomes.
METHODS: An integrated analysis was conducted by pooling data from 3 completed 
phase III trials comparing placebo with baricitinib treatment (RA-BEAM, 
RA-BUILD, and RA-BEACON) and 1 ongoing long-term extension study (RA-BEYOND) in 
patients with active RA (n = 2,186).
RESULTS: Baricitinib treatment was associated with an early transient increase 
in total lymphocyte count at week 4, which returned to baseline by week 12. 
Transient changes within normal reference ranges in T cells and subsets were 
observed with baricitinib treatment, up to week 104. B cells and relevant 
subpopulations increased after 4 weeks of baricitinib treatment, with no further 
increases noted through 104 weeks of treatment. Natural killer (NK) cells 
temporarily increased after 4 weeks of baricitinib treatment, before decreasing 
below baseline levels and then stabilizing over time. With baricitinib 
treatment, few correlations were observed between changes in lymphocyte subsets 
and clinical end points, and most correlations were also observed within the 
placebo group. A modest potential association between low NK cell numbers and 
treatment-emergent infections was observed in the baricitinib 4 mg/day treatment 
group, but not for serious infections or herpes zoster.
CONCLUSION: Overall, these findings demonstrate that changes in lymphocyte 
subsets were largely within normal reference ranges across the baricitinib phase 
III RA clinical program and were not associated with increased risk of serious 
infections.