Characterization of changes in lymphocyte subsets in baricitinib-treated patients with rheumatoid arthritis in a phase 3 study (RA-beam)

Background: Baricitinib (bari; an oral JAK 1/JAK 2 inhibitor) is in development for patients (pts) with active RA. In healthy subjects, absolute lymphocyte counts (ALC) increased after bari administration, returning to baseline (BL) by 24 hrs.1 Objectives: To examine changes in ALC and lymphocyte (LYM) subsets in pts with active RA treated with bari (4 mg QD), placebo (PBO), or adalimumab (ADA, 40 mg Q2W). Methods: 1305 pts with active RA despite MTX treatment were randomized 3:3:2 to PBO, bari, or ADA. ALC, T, and B cell subsets (T: Th1, Th17, CD4, CD8, T reg, CD3+CD4+CD127-/loCD25+; B: switched/nonswitched memory, mature naive, immature transitional), and natural killer (NK) cells were quantified by flow cytometry at BL and wks 4, 12, and 24. Wk 4 phlebotomy was postdose bari or PBO; wks 12 and 24 were predose. Results: ALC and T cells/subsets increased with bari and ADA at wk 4 (generally within normal ranges), returning to near BL at wks 12 and 24 in bari but remaining elevated in ADA (Table). B cells/subsets increased at wks 4 in bari and ADA and remained elevated through wk 24. NK cells were increased at wk 4 in bari and were below BL but within the normal range at wks 12 and 24; NK cells were increased at wks 12 and 24 in ADA. The percentages of pts with ≥1 low NK cell count were 20.5%, 32.6%, and 20.6% in PBO, bari, and ADA, respectively; percentage of pts with ALC CTCAE grade ≥2 were 14.1%, 9.9%, and 10.0%. Through wk 24, serious infection rates were 1.4%, 1.0%, and 0.6% for PBO, bari, and ADA; rates were 1.0%, 1.3%, and 0%, respectively, in pts with ≥1 low NK cell count and 2.9%, 4.2%, and 0% in pts with ≥1 ALC CTCAE Grade ≥2 value. Rates of herpes zoster (HZ) were 0.4%, 1.4%, and 1.2% for PBO, bari, and ADA; rates were 0%, 0.6%, and 0%, respectively, in pts with ≥1 low NK cell count and 1.4%, 2.1%, and 0% in pts with ≥1 ALC CTCAE Grade ≥2 value (no statistically significant differences between treatment groups). Conclusions: Changes in ALC and subpopulations with bari in RA-BEAM were largely within normal ranges and are consistent with previous data.2 Sustained increases in B cells were observed in bari and ADA. Sustained increases in ALC/T cells were only seen in ADA. A modest reduction in NK cells at wks 12 and 24 with bari was not associated with serious infection or HZ. (Table Presented) .