Pembrolizumab-induced psoriatic arthritis treated with disease-modifying anti-rheumatic drugs in a patient with gastric cancer: A case report

BACKGROUND Immune checkpoint inhibitor (ICI)-induced rheumatic immune-related adverse events (irAEs) have been infrequently reported, and the treatment of severe or refractory arthritis as irAEs has not been established yet. CASE SUMMARY The patient was a 67-year-old man with a history of well-controlled foot psoriasis who presented with polyarthralgia. He had received pembrolizumab for metastatic gastric adenocarcinoma 2 mo previously. Physical examination revealed erythematous swelling in the distal interphalangeal joints, left shoulder, and both knees. He had plaque psoriasis with psoriatic nail dystrophy and dactylitis in the distal joints of the fingers and toes. Inflammatory markers including C-reactive protein and erythrocyte sedimentation rate were elevated but rheumatoid factor and anticyclic citrullinated peptide antibody were negative. The patient was diagnosed with psoriatic arthritis (PsA) and started on methylprednisolone 1 mg/kg/day after pembrolizumab discontinuation. However, despite 1 wk of methylprednisolone treatment, PsA worsened; hence, leflunomide and methotrexate were started. After 4 wk of steroid treatment, PsA worsened and improved repeatedly with steroid tapering. Therefore, the therapy was intensified to include etanercept, a tumor necrosis factor inhibitor, which ultimately resulted in adequate PsA control. CONCLUSION This is the first report of ICI-induced PsA in a gastric cancer patient. Some rheumatic irAEs with refractory severe arthritis may require disease-modifying anti-rheumatic drugs and long-term management.