To determine the efficacy and safety of pamapimod (a selective inhibitor of the α‐isoform of p38 MAP kinase) as monotherapy in comparison with methotrexate (MTX) treatment in adult patients with active rheumatoid arthritis (RA).

Patients were randomly assigned to 1 of 4 treatment groups and received 12 weeks of double‐blind treatment. One group received MTX (7.5 mg/week with planned escalation to 20 mg/week), and 3 groups received pamapimod (50, 150, or 300 mg) once daily. The primary efficacy end point was the proportion of patients meeting the American College of Rheumatology 20% improvement criteria (achieving an ACR20 response) at 12 weeks. Secondary end points included ACR50 and ACR70 responses, change from baseline in the Disease Activity Score in 28 joints (DAS28), categorical analyses of DAS28/European League Against Rheumatism response, and change from baseline in each parameter of the ACR core set of measures. Safety monitoring included recording of adverse events (AEs), laboratory testing, immunology assessments, administration of electrocardiograms, and assessment of vital signs.

Patients assigned to receive MTX and pamapimod had similar demographics and baseline characteristics. At week 12, fewer patients taking pamapimod had an ACR20 response (23%, 18%, and 31% in the 50‐, 150‐, and 300‐mg groups, respectively) compared with patients taking MTX (45%). Secondary efficacy end points showed a similar pattern. AEs were typically characterized as mild and included infections, skin disorders, and dizziness. Pamapimod was generally well tolerated, but the 300‐mg dose appeared to be more toxic than either the 2 lower doses or MTX.

The present results showed that pamapimod was not as effective as MTX in the treatment of active RA.