Relationship between lymphocyte count and risk of infection in Japanese rheumatoid arthritis patients treated with tofacitinib.

Objectives: We characterised changes in absolute lymphocyte counts (ALCs) and lymphocyte subset counts (LSCs), and their relationship to incidence of serious infection events (SIEs) and herpes zoster (HZ) events in Japanese patients with moderate to severe rheumatoid arthritis enrolled in the tofacitinib clinical programme.

Methods: Data included 765 patients receiving tofacitinib in Phase 2, Phase 3, and long-term extension studies. ALCs/LSCs and incidence rates (patients with events/100 patient-years) of SIEs and HZ were analysed over 75 months.

Results: Median ALCs were generally stable over 75 months of treatment. Transient numerical increases from baseline in median LSCs were observed at Month 3; LSCs were generally lower than baseline for Months 36-75. SIE/HZ incidence rates were higher in patients with ALC <0.5 × 103 cells/mm3 versus those with ALC ≥0.5 × 103 cells/mm3 during tofacitinib treatment. Baseline LSCs were similar in patients with/without SIEs or HZ events.

Conclusions: SIE/HZ risk was highest in patients with ALC <0.5 × 103 cells/mm3, supporting this threshold as clinically relevant for defining increased SIE/HZ risk in Japanese patients with rheumatoid arthritis receiving tofacitinib. However, SIEs and HZ events did not necessarily occur simultaneously with confirmed lymphopenia, preventing conclusions on possible causal relationships being drawn.