The Janus kinase (JAK) inhibitor, tofacitinib, has shown efficacy for the treatment of psoriasis in a phase IIb trial (A3921047; NCT00678210).

To report haematology data from the phase IIb trial, given the importance of JAK‐dependent signalling in haematopoiesis.

Patients with moderate‐to‐severe chronic plaque psoriasis were randomized to receive tofacitinib 2, 5 or 15 mg, or placebo, twice daily over 12 weeks. Blood samples were collected at screening, baseline, weeks 2, 4, 8 and 12 during treatment, and weeks 14 and 16 during off‐treatment follow‐up.

Baseline haematology was similar across patients receiving tofacitinib 2 mg (n = 49), 5 mg (n =49) or 15 mg (n =49), or placebo (n =50). Tofacitinib conferred dose‐dependent decreases in haemoglobin, haematocrit and red blood cell counts, while reticulocyte counts initially declined, before recovering by week 8, and exceeding baseline levels after treatment cessation. With regard to white blood cells, tofacitinib had no clear dose‐dependent effects on basophils or monocytes, but appeared to be associated with transient or reversible dose‐dependent decreases in neutrophil and eosinophil counts and transient increases in lymphocyte counts, which were primarily attributable to increases in B‐cell counts. Natural killer cell counts declined with tofacitinib.

Tofacitinib conferred tolerable, dose‐dependent changes in haematological parameters during short‐term administration in patients with psoriasis. The effects did not appear to be progressive, and were often transient or reversible.