Upadacitinib in ankylosing spondylitis with cutaneous-limited, seronegative dermatomyositis–compatible interface dermatitis

We report a 43-year-old woman with ankylosing spondylitis (AS) who developed violaceous erythematous papules and plaques over the extensor surfaces of the metacarpophalangeal and interphalangeal joints. Biopsy slides showed lichenoid interface dermatitis with basal vacuolar change compatible with dermatomyositis, though not pathognomonic. Serum autoimmune profiles like myositis-specific autoantibodies and antinuclear antibodies were negative; muscle enzymes were within normal limits; there were no signs of muscle involvement or interstitial lung disease. With immediate hypersensitivity reaction to systemics and intolerance to methotrexate, therapy with the selective Janus kinase (JAK1) inhibitor upadacitinib 15 mg daily was started. The patient showed significant and durable responses in axial symptoms (reduction in BASDAI from 7.1–2.0 and in ASDAS-CRP from 2.46 to 1.13) and near resolution of knuckle joints with no adverse events. This case draws attention to the difficulties in diagnosis caused by seronegative, cutaneous-limited dermatomyositis-compatible interface dermatitis in the setting of spondyloarthritis and proposes JAK1 inhibition as a possible alternative, mechanistically driven therapy in those circumstances when conventional therapy is either contraindicated or not well tolerated. It again underlines the significance of careful surveillance in case there may be evolution toward definite systemic dermatomyositis, interstitial lung disease, or malignancy.